Abstract:
A heat treatment process to improve the strength of weldable near alpha titanium alloys comprising solution treating the alloy in the beta field and stress relief treating the alloy at two different temperatures, one of which is at 535° C.±100° C., particularly suitable for the titanium alloy containing 5.5% aluminium, 3.5% tin, 3% zirconium, 1% niobium, 0.25% molybdenum, 0.3% silicon.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to the heat treatment of metals and has particular reference to the heat treatment of titanium near alpha alloys. 
     The search for improved mechanical properties in titanium alloys has normally taken the route of modifying the composition of the alloy to improve the balance of properties available. Titanium alloys have been in existence commercially for a little over 30 years and it is becoming increasingly difficult to design new titanium alloys with improved properties. 
     Initial improvements were made quite rapidly, but the rate of development has slowed down as the law of diminishing returns takes effect. Undoubtedly improvements will occur in the future. However, even small improvements in properties are valuable in that they enable aero engines to be designed so as to be lighter and hence more fuel efficient. The need for fuel efficiency in aero engines is so great that aero engine designers are looking to use titanium alloys in ever hotter regions of the engine to enable weight savings to be obtained. There is, therefore, a great deal of pressure on the metallurgist to improve the balance of metallurgical properties present in the alloy. 
     As mentioned above most of the emphasis on improving properties has gone towards modifying the composition of the alloy. Little practical evaluation has been given to modifications to the heat treatment to be used on the alloys. This invention is, however, concerned with the improvement in titanium alloys by modifying the heat treatment given to them during their processing. 
     As in the case of many metals titanium exists mainly in two distinct phases, a so-called alpha phase and a so-called beta phase. The beta phase is more stable at elevated temperatures and the proportions of alpha and beta in various titanium alloys are defined by the composition and heat treatment of the alloys. Certain alloying elements used in titanium stabilise the alpha phase and these are frequently referred to as alpha stabilisers. Other alloying elements stabilise the beta phase and these are frequently referred to as beta stabilisers. Certain titanium alloys consist almost completely of alpha titanium when in equilibrium at room temperature with a trace of beta--less than 5% beta. These alloys are sometimes referred to as near alpha alloys and certain of the alloys are properly regarded as weldable. A near alpha titanium alloy may also be regarded as one containing not more than about 2% by weight of beta stabilisers such as molybdenum copper silicon etc. A more complete definition of a near alpha titanium alloy is an alpha stabilised alloy, that is an alloy containing alpha stabilising elements, with an amount of beta stabiliser which gives a small volume fraction (less than about 5%) of retained beta and which can be beta processed and/or beta heat treated and give acceptable ductility and fracture resistance. 
     The term &#34;weldable&#34; as used herein is not intended merely to refer to the ability of the metal to be welded directly to itself but is intended to refer to the metal being useable in an aircraft engine in the welded condition. The only two weldable near alpha beta heat treated alloys in existence at the present time are the alloys known as IMI 685, namely the alloy 6% aluminium, 5% zirconium, 0.5% molybdenum, 0.25% silicon, balance titanium and 5331S, namely the alloy 5.5% aluminium, 3.5% tin, 3% zirconium, 1% niobium, 0.25% molybdenum, 0.3% silicon, balance titanium. All percentages as used herein are weight percentages. The near alpha alloys are conventionally used in the solution treated and stress relieved condition. The solution treatment of the alloy 5331S conventionally comprises a treatment at 1050° C. for a time depending on section size--one hour per 2.5 cm. The alloy is then oil quenched and is given a stress relief treatment for two hours at 625°  C. although the exact stress relief time may vary with section. The solution treatment modifies the metallurgical structure of the alloy and the stress relieving treatment stress relieves the alloy from the stresses built up in the alloy during the quenching phase. 
     It will be appreciated that different types of titanium alloys have different types of heat treatment. Thus a conventional heat treatment for a near alpha alloy has been solution treatment in the beta field followed by a stress relieving treatment at a temperature typically in the region 525°-625° C. for a time of about 24 hours. By comparison, however, other types of titanium alloys are given a very different type of heat treatment. Thus an age hardenable titanium alloy, such as titanium plus 21/2% copper, would be given an alpha solution treatment at about 800° C. followed by a nucleation treatment at 400° C. for 8 hours to nucleate the typical &#34;Duralumin&#34; type precipitate and then a further heat treatment at 475° C. for 8 hours to grow the precipitate. The alloy titanium plus 21/2% copper is one which contains only beta stabilisers and is normally treated in the alpha plus beta or alpha plus compound regions of the phase diagram. In effect alloys of this precipitation hardening type rely on forming, at room temperature, a supersaturated solution of copper in the alpha phase. Subsequently the age hardening heat treatments result in the diffusion of copper to precipitation sites and then further precipitation on these sites during subsequent heat treatment. 
     There are believed to be no commercially used fully beta stable titanium alloys. Experimental alloys such as titanium plus 20% molybdenum plus 10% vanadium are fully beta stabilised. The only heat treatment given to such alloys is to beta solution heat treat. No further heat treatment is given. 
     A typical metastable beta titanium alloy, such as titanium plus 15% molybdenum would be given a beta solution treatment at a temperature above 25° C. above the beta transus, i.e. 815° C. for the Ti+15% Mo alloy and it would then be water quenched to room temperature. The alloy would then be composed of 100% beta phase. It would then be given a single or duplex ageing to precipitate out from the beta phase either an omega phase or an alpha phase. 
     Alpha plus beta titanium, such as the alloy titanium plus 6% aluminium plus 4% vanadium is typically heat treated in one of two ways. In one way, the alloy is annealed at a temperature low in the alpha plus beta phase field--i.e. 700° C. to give equiaxed alpha plus retained beta. In the other heat treatment the alloy is solution treated in the alpha plus beta field, air cooled to room temperature and then stress relieved at a single temperature in the range 500° C. to 700° C. to give an equiaxed alpha plus transformed beta structure. 
     Plain alpha titanium such as commercial purity titanium is simply stress relieved with a single heat treatment in the range 600° C. to 700° C. to given an equiaxed primary alpha structure. 
     However, it is not possible to equate the heat treatment used for one type of alloy, such as an age hardenable alloy of the titanium plus 21/2 copper type, with that required for another type of alloy, such as a metastable beta or near alpha alloy. 
     Although practical heat treatments have been developed for near alpha alloys and have been shown to work well it is not certain what is happening in the near alpha alloy when it is heat treated. During the solution treatment it is clear that the alloy is converted into the beta phase and during cooling converts mainly to the alpha phase. However the heat treatment given to stress relieve the alloy after cooling gives rise to numerous types of reactions within the alloy itself. 
     Thus during the stress relieving process it is quite probable that some form of ordering is taking place within the alpha matrix and furthermore some amount of precipitation of very fine particles of material is taking place within the matrix. Once precipitated the morphology of the precipitate is altered as the heat treatment persists. Furthermore subcells are formed within the alloy. In addition to changes in relation to the precipitate there are also changes in the composition of the matrix. 
     The relative speeds of the various reactions alter as the temperature of heat treatment changes and furthermore vary with the time at a given temperature. This makes the prediction of the outcome of a variation in heat treatment very difficult when it is considered on a detailed and practical scale. 
     A subcell of the type referred to above is basically a subgrain in which there is a small difference in the angle of the atomic planes between one cell and another of the order of 5°, whereas for a true grain boundary the angular differences between the atomic planes would normally be 30° or more. A subcell may be regarded as a sign of partial recovery within the alloy caused by small movements of dislocations in the alloy. As the amount of precipitate and the morphology of the precipitate changes, the ability of the precipitate to lock up dislocations also changes, and this again gives rise to variations in the properties of the material. 
     An important part of the stress relieving treatment given to near alpha alloys is to stress relieve the internal stresses built up in the alloy during the quenching from the solution treatment temperature. These stresses are conventionally relieved by the movement of dislocations within the material and by the reformation of grain boundaries, and consequently the effect of the type of precipitate and its morpholoy on stress relief is a further complication. 
     Although extending the time of the stress relief treatment or increasing the temperature of the heat treatment reduces the amount of internal stress, it has been found that in near alpha alloys this reduces the creep strength of the alloy very considerably. Thus from Table I it can be seen increasing the temperature of the stress relief treatment from 500° C. to 600° C. whilst keeping the duration of the treatment constant at 24 hours led to a doubling of the creep extension a marginal fall in the strength of the alloy and a significant reduction in ductility. The alloy being tested was the near alpha alloy IMI 685. All the material was solution treated at 1050° C. and oil quenched. 
     
                                           TABLE I__________________________________________________________________________Stress Relief   Creep T.P.S., 520° C. 310 N · mm.sup.-2                     0.2% PS                          UTS  EL5D                                   R in ATreatment   100 hrs, %        Nmm.sup.-2                          Nmm.sup.-2                               %   %__________________________________________________________________________24 hrs/500° C.   --                888  1016 12  23&#34;       0.063             *922 1013 11.5                                    19*24 hrs/575° C.   --                900  1013 7   16&#34;       0.119             *936 1018 5    7*24 hrs/600° C.   --                883   999 8   13&#34;       0.124             *939 1008 3    8*__________________________________________________________________________ *All post creep tensile test samples had their surfaces retained. 
    
     For each heat treatment pair, the upper line refers to material which has not been creep tested the lower line for material which has been creep tested. 
     The same effect of a fall in the creep strength was observed when the time of the stress relief treatment was increased at constant temperature. 
     Table II, below shows that increasing the stress relief time at a constant temperature gives an increase in strength but a marked reduction in creep strength. The alloy tested was 5331S which had been solution treated at 1050° C. for 2 hours and then oil quenched. 
     
                                           TABLE II__________________________________________________________________________   Creep T.P.S.,Stress Relief   540° C./300 Nmm.sup.-2             0.1% PS                  0.2% PS                       UTS  EL5D                                R in AHeat Treatment   100 hr %        300 hr %             Nmm.sup.-2                  Nmm.sup.-2                       Nmm.sup.-2                            %   %__________________________________________________________________________2 hrs/625° C.   --   --   845  865   999 14  17.5&#34;       0.084        0.256             *913 932  1027 7.5 10*4 hrs/625° C.   --   --   843  867   995 12  14&#34;       0.135        0.305             *917 937  1030 8.5  8.5*8 hrs/625° C.   --   --   861  881  1001 11  16&#34;       0.164        0.351             *926 945  1038 6    7*__________________________________________________________________________ *All post creep tensile test samples had their surfaces retained. 
    
     It will be appreciated that an alloy which has a good creep resistance is one which will extend as little as possible under creep loading conditions, i.e. the value of creep T.P.S. (total plastic strain) should be as low as possible. 
     It has now been discovered that the properties of near alpha alloys, and in particular 5331S, can be improved by modifying the heat treatment given heretofor to alloys of this type. In particular it has been found that the strength and creep resistance of the alloy can be improved by modification to the known heat treatment. 
     SUMMARY OF THE INVENTION 
     By the present invention there is provided a method of heat treating a near alpha titanium alloy which includes the steps of solution treating the alloy at a temperature in excess of 900° C. and then heat treating the alloy at a temperature in the region of 400° C. to 750° C. or 450° C. to 750° C. for a time in excess of 30 minutes wherein the improvement comprises carrying out two or more heat treatments at different temperatures with the first heat treatment taking place at a temperature lower than the or a subsequent heat treatment. 
     The alloy may be solution treated at a temperature in the beta field, preferably at a temperature in the range 990° C. to 1,100° C. dependent on the beta transus temperature of the alloy. The temperature may be 1 030° to 1 070° C. for 5331S. 
     One of the heat treatments, preferably the first, may take place at a temperature of 535° C.±100° C. or ±75° C. or ±50° C. or ±35° C. for a time between one and 168 hours. Preferably the said temperature of heat treatment may be 535° C.±30° C. or ±25° C. or ±20° C. or ±15° C. or ±10° C. or ±5° C. or 535° C. exactly. The duration of heat treatment may be 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 30, 36, 48, 50, 72, 100 or 168 hours. 
     The second heat treatment temperature may be 650° C.±50° C. or 600° C.±30° C. preferably 625° C. The duration of the second heat treatment may be in the range 1 to 168 hours. 
     The alloy may be cooled to ambient temperature between the solution treatment and the heat treatments. The alloy may be air cooled or may be quenched. The quenching may be by oil quenching. Alternatively the alloy may be cooled from the solution treatment temperature to the temperature of the first heat treatment. The latter cooling may be by quenching into a bath of molten material at or near the temperature of the first heat treatment, or may be effected by moving the alloy from a furnace at the solution temperature to a furnace at the temperature of the first heat treatment, or by cooling the alloy in the furnace from the solution temperature to the temperature of the first heat treatment, or by a combination of the methods. 
     Unexpectedly it has been found that using the multiple heat treatments of the present invention has enabled an increase in the time/temperature of the stress relief treatment to be effected--with its accompanying lowering of internal stress but with not only no reduction in creep strength but an actual improvement in creep strength. In view of previous knowledge and experience of the effect of increasing the time and or temperature of the stress relief treatment these results are most unexpected and it could not have been predicted that such an improvement in creep properties could have been obtained in this manner. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     By way of example embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings of which: 
     FIG. 1 is a graph of total plastic strain TPS against primary heat treatment hrs/°C.; 
     FIG. 2 is a graph of reduction in area percentage against primary heat treatment hrs/°C.; 
     FIG. 3 is a graph of 0.2% proof stress and elongation against primary heat treatment hrs/°C.; 
     FIG. 4 is a graph of 0.2% proof stress and elongation against secondary heat treatment hrs/°C.; and 
     FIG. 5 is a graph of reduction in area against secondary heat treatment hrs/°C. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Samples of titanium alloy bar of a composition 5.5% aluminium, 3.5% tin, 3% zirconium, 1% niobium, 0.25% molybdenum, 0.3% silicon, balance titanium (ie 5331S) were cut to shape. The samples were of 50 mm diameter and were of a sufficient length to permit conventional tensile test samples to be cut from them. A first set of four specimens were prepared and were solution treated for 2 hours at 1040° C. The samples were oil quenched from temperature and were subsequently heat treated in four different ways. The tensile properties of the four treatments is given in Table III. 
     
                                           TABLE III__________________________________________________________________________Effect of Prolonged Heat Treatment and Duplex Heat Treatment onTensile Properties of 5331S 50 mm Bar SolutionTreated 1040° C./2 hr OQ (Oil Quenched)Specimen       0.1% PS               0.2% PS                    UTS  EL5D                             R in ANumberHeat Treatment          Nmm.sup.-2               Nmm.sup.-2                    Nmm.sup.-2                         %   %__________________________________________________________________________1    625° C./2 hours          852  867   996 13  222    560° C./100 hours          871  909  1026 11  16.53    560° C./100 hours          887  907  1010  9  14+ 650° C./24 hours4    580° C./100 hours          892  912  1019  9  13+ 650° C./24 hours__________________________________________________________________________ 
    
     In the table the &#34;0.1%PS&#34; refers to the 0.1% proof strength. The &#34;Nmm -2  &#34; means Newtons per mm 2 . The term &#34;UTS&#34; means ultimate tensile strength. The term &#34;EL5D%&#34; refers to the elongation on a gauge length of 5 times the diameter of the sample section. The &#34;R in A%&#34; refers to the reduction in area measured at the break. It can be seen that reducing the stress relief treatment temperature and increasing the stress relief treatment time gives an improvement in the proof strength and tensile strength of the materials and that the duplex stress relief treatment given to Samples 3 and 4 gives further increases in the tensile strength at the expense of ductility as measured by the elongation and reduction in area. 
     A further thirteen samples of 5331S were taken and solution treated at 1050° C. for 2 hours and then oil quenched. After the solution treatment the samples were given a duplex heat treatment and the results are given in Table IV. 
     
                                           TABLE IV__________________________________________________________________________Effect of Duplex Heat Treatment on Tensile Properties of5331S 50 mm .0. Bar - Solution Treated at 1050° C./2 hour OilQuenchSample              0.1% PS                    0.2% PS                         UTS  EL5D                                  R in ANumberHeat Treatments               Nmm.sup.-2                    Nmm.sup.-2                         Nmm.sup.-2                              %   %__________________________________________________________________________5    425° C./24 hr       625° C./8 hr               850  867  1002 12  176           625° C./24 hr               869  887  1002 5   87           625° C./48 hr               868  888  1002 8.5 138    475° C./24 hr       625° C./2 hr               826  850   987 11  209           625° C./8 hr               861  875   998 11  1510          625° C./24 hr               858  880   998 10  1311   525° C./2 hr       625° C./2 hr               840  863   998 11  1512          625° C./8 hr               853  868  1002 12.5                                  1713          625° C./24 hr               861  883  1004 10  1214   525° C./24 hr       625° C./2 hr               848  867  1004 13.5                                  2215          625° C./8 hr               855  878  1006 13  2216          625° C./24 hr               873  891  1012 12  1917   525° C./48 hr       600° C./100 hr               896  918  1016 6.5 918          625° C./24 hr               884  904  1002 6   9__________________________________________________________________________ 
    
     It can be seen from Table IV that within any group 567, 8910, 11 12 13, and 14 15 16, that increasing the length of time of the second heat treatment gives an increase in strength of the alloy. It is particularly noticeable in samples 14 15 and 16 that this increase in strength is not accompanied by any significant loss of ductility. 
     It can also be seen that optimum results appear to follow the duplex heat treatment given to Sample 17 insofar as the tensile strength is concerned. However, when comparing both tensile and ductile properties the optimum results appear to be those obtained with Sample 16. 
     Following the preliminary investigation outlined above further investigation took place to establish the effect of duplex heat treatment using a lower temperature first heat treatment followed by extended times at and around 625° C. In a second further stage duplex heat treatments using extended times at 625° C. were followed by a further set of heat treatments at lower treatment temperatures. All treatments were carried out on 50 mm diameter bars solution treated in full section at 1 050° C. for 2 hours and then oil quenched. 
     The test pieces for the treatments were cut from the bar with the majority of the exterior of the bar being rejected during the machining operation. It was not possible to carry out the entire programme on material from one batch and the material used for the investigation of lower temperatures for the primary treatment followed by extended heat treatments at 625° C. had a beta grain size of approximately 0.5 mm compared to a rather coarser beta grain size for the second set of experiments (the grain size in that case being approximately 1 mm). As a result it is not possible to compare directly the results between the two parts although this in itself is not an essential requirement. The range of heat treatments is illustrated in Tables V to X. 
     
                                           TABLE V__________________________________________________________________________          Creep @          600° C./200 Nmm.sup.-2          TPSStress Relief  100 hr               300 hr                    0.1% PS                         0.2% PS                              UTS  EL5D                                       R in AHeat Treatment(s)          %    %    Nmm.sup.-2                         Nmm.sup.-2                              Nmm.sup.-2                                   %   %__________________________________________________________________________625° C./2 hrs          --   --    836 859  971  13.5                                       26.5(5331S STD (standard)          0.571               1.525                    .sup.# 898                         916  994  5.5 10.5.sup. #500° C./24 hr + 625° C./8 hr          --   --    850 867  972  14  23.5          0.481               1.480                    .sup.# 903                         921  1003 5   9.5.sup.#500° C./24 hr + 625° C./24 hr          --   --    861 881  993  12.5                                       21          0.597               1.839                    .sup.# 899                         914  999  6.5 11.5.sup.#500° C./24 hr + 625° C./48 hr          --   --    861 885  982  13  19          0.550               1.717                    .sup.# 893                         908  982  6.5 8.5.sup.#500° C./24 hr + 600° C./24 hr          --   --    844 861  967  13.5                                       23          0.510               1.408                    .sup.# 891                         905  991  5.5 9.5.sup.#500° C./24 hr + 650° C./24 hr          --   --    847 861  959  14  19          0.568               1.854                    .sup.# 899                         912  984  4   8.5.sup.#Average (Excl STD)          --   --    853 871  975  13.4                                       21.1          0.541               1.660                    .sup.# 897                         912  992  5.5 9.5.sup.#__________________________________________________________________________ .sup.# All Post Creep Tensile Test Samples had their Surfaces Retained. 
    
     
                                           TABLE VI__________________________________________________________________________          Creep @          600° C./200Nmm.sup.-2          TPSStress Relief  100 hr               300 hr                    0.1% PS                         0.2% PS                              UTS  EL5D                                       R in AHeat Treatments(s)          %    %    Nmm.sup.-2                         Nmm.sup.-2                              Nmm.sup.-2                                   %   %__________________________________________________________________________625° C./2 hrs          --   --   836  859  971  13.5                                       26.5(5331S STD)    0.571               1.525                    .sup.# 898                         916  994  5.5 10.5.sup.#510° C./24 hr + 625° C./8 hr          --   --   839  856  963  13  22.5          0.491               1.444                    .sup.# 901                         920  998  5.5 8.5.sup.#510° C./24 hr + 625° C./24 hr          --   --   854  872  969  15  19.5          0.572               1.845                    .sup.# 889                         910  976  5.5 8.5.sup.#510° C./24 hr + 625° C./48 hr          --   --   866  883  980  14  21          0.476               1.635                    .sup.# 899                         914  998  6   10.sup.#510° C./24 hr + 600° C./24 hr          --   --   851  871  978  13.5                                       23          0.611               1.840                    .sup.# 898                         913  995  5   10.sup.#510° C./24 hr + 650° C./24 hr          --   --   855  874  971  9   14.5          0.580               1.964                    .sup.# 898                         908  979  5   10.sup.# *Average (Excl STD)          --   --   853  871  972  12.9                                       20.1          0.546               1.746                    .sup.# 897                         913  989  5.4 9.4.sup.#__________________________________________________________________________ .sup.# All Post Creep Tensile Samples had their Surfaces Retained. *Extra heating of 4 hrs/600° C. on loading for 300 hr creep. 
    
     
                                           TABLE VII__________________________________________________________________________          Creep @          600° C./200Nmm.sup.-2          TPSStress Relief  100 hr               300 hr                    0.1% PS                         0.2% PS                               UTS EL5D                                       R in AHeat Treatment(s)          %    %    Nmm.sup.-2                         Nmm.sup.-2                              Nmm.sup.-2                                   %   %__________________________________________________________________________625° C./2 hrs          --   --   836  859  971  13.5                                       26.5(5331S STD)    0.571               1.525                    .sup.# 898                         916  994  5.5 10.5.sup.#520° C./24 hr + 625° C./8 hr          --   --   843  857  957  14.5                                       23          0.485               1.592                    .sup.# 901                         917  989  5.5 8.5.sup.#520° C./24 hr + 625° C./24 hr          --   --   866  882  987  10  21          0.532               1.731                    .sup.# 896                         911  991  6   10.5.sup.#520° C./24 hr + 625° C./48 hr          --   --   874  894  991  10.5                                       21          0.530               1.774                    .sup.# 891                         906  994  6   12.5.sup.#520° C./24 hr + 600° C./24 hr          --   --   852  870  980  13  21          0.625               1.880                    .sup.# 892                         900  991  3   11.sup.#520° C./24 hr + 650° C./24 hr          --   --   864  883  985  9   12          0.505               1.508                    .sup.# 889                         912  999  4.5 8.sup.#Average (Excl STD)          --   --   860  877  980  11.4                                       19.6          0.535               1.697                    .sup.# 894                         909  993  5   10.1.sup.#__________________________________________________________________________ .sup.# All Post Creep Tensile Samples had their Surfaces Retained. 
    
     
                                           TABLE VIII__________________________________________________________________________          Creep @          600° C./200Nmm.sup.-2          TPSStress Relief  100 hr               300 hr                    0.1% PS                         0.2% PS                              UTS  EL5D                                       R in AHeat Treatment(s)          %    %    Nmm.sup.-2                         Nmm.sup.-2                              Nmm.sup.-2                                   %   %__________________________________________________________________________625° C./2 hrs          --   --   836  859  971  13.5                                       26.5(5331S STD)    0.571               1.525                    .sup.# 898                         916  994  5.5 10.5.sup.#530° C./24 hr + 625° C./8 hr          --   --   848  869  973  11  18.5          0.485               1.352                    .sup.# 901                         919  1002 4   7.sup.#530° C./24 hr + 625° C./24 hr          --   --   850  877  992  14  17          0.426               1.226                    .sup.# 903                         917  1008 4.5 7.5.sup.#530° C./24 hr + 625° C./48 hr          --   --   871  890  985  13.5                                       19.5          0.462               1.456                    .sup.# 901                         918  1007 3   8.5.sup.#530° C./24 hr + 600° C./24 hr          --   --   860  881  992  13.5                                       22          0.511               1.435                    .sup.# 905                         921  1003 4   8.5.sup.#530° C./24 hr + 650° C./24 hr          --   --   852  872  974  11.5                                       15.5          0.470               1.663                    .sup.# 900                         916  991  3   7.5.sup.#Average (Excl STD)          --   --   856  878  983  12.7                                       18.5          0.471               1.426                    .sup.# 902                         918  1002 3.7 7.8.sup.#__________________________________________________________________________ .sup.# All Post Creep Tensile Samples had their Surfaces Retained. 
    
     
                                           TABLE IX__________________________________________________________________________          Creep @          600° C./200Nmm.sup.-2          TPSStress Relief  100 hr               300 hr                    0.1% PS                         0.2% PS                              UTS  EL5D                                       R in AHeat Treatment(s)          %    %    Nmm.sup.-2                         Nmm.sup.-2                              Nmm.sup.-2                                   %   %__________________________________________________________________________625° C./2 hrs          --   --   836  859  971  13.5                                       26.5(5331S STD)    0.571               1.525                    .sup.# 898                         916  994  5.5 10.5.sup.#540° C./24 hr + 625° C./8 hr          --   --   844  865  973  14  20.5          0.477               1.430                    .sup.# 897                         919  1014 5   8.5.sup.#540° C./24 hr + 625° C./24 hr          --   --   860  876  974  12  16          0.419               1.250                    .sup.# 905                         920  994  4   6.sup.#540° C./24 hr + 625° C./48 hr          --   --   863  883  984  13  17.5          0.424               1.447                    .sup.# 915                         931  1013 4   7.5.sup.#540° C./24 hr + 600° C./24 hr          --   --   856  874  982  13  20          0.475               .sup. 1.480°                    .sup.# 914                         930  1016 5   7.sup.#540° C./24 hr + 650° C./24 hr          --   --   856  873  975  11  16.5          0.518                1.615*                    .sup.# 897                         918  1009 5.5 11.sup.#Average (Excl STD)          --   --   856  874  978  12.6                                       18.1          0.463               1.444                    .sup.# 906                         924  1009 4.7 8.sup.#__________________________________________________________________________ .sup.# All Post Creep Tensile Samples had their Surfaces Retained. °Temperature drop during 300 hr creep test to a minimum of 440° C. for 6 hours. *Extra heating of 4 hrs/600° C. on loading for 300 hr creep. 
    
     
                                           TABLE X__________________________________________________________________________          Creep @          600° C./200Nmm.sup.-2          TPSStress Relief  100 hr               300 hr                    0.1% PS                         0.2% PS                              UTS  EL5D                                       R in AHeat Treatment(s)          %    %    Nmm.sup.-2                         Nmm.sup.-2                              Nmm.sup.-2                                   %   %__________________________________________________________________________625° C./2 hrs          --   --   836  859  971  13.5                                       26.5(5331S STD)    0.571               1.525                    .sup.# 898                         916  994  5.5 10.5.sup.#550° C./24 hr + 625° C./8 hr          --   --   845  868  975  12  17          0.453               1.379                    .sup.# 904                         917  1022 5.5 8.sup.#550° C./24 hr + 625° C./24 hr          --   --   855  876  979  12  17.5          0.515               1.528                    .sup.# 907                         926  1002 6.5 6.5.sup.#550° C./24 hr + 625° C./48 hr          --   --   872  891  995  10  14          0.393               1.132                    .sup.# 915                         934  1011 2   5.5.sup.#550° C./24 hr + 600° C./24 hr          --   --   859  881  998  15  17.5          0.357               .sup. 1.032°                    .sup.# 915                         934  1014 5   8.sup.#550° C./24 hr + 650° C./24 hr          --   --   858  881  994  10  13          0.384               1.224                    .sup.# 928                         937  1031 5.5 7.sup.#Average (Excl STD)          --   --   858  879  988  11.8                                       15.8          0.420               1.259                    .sup.# 914                         930  1016`                                   4.9 7.sup.#__________________________________________________________________________ .sup.# All Post Creep Tensile Samples had their Surfaces Retained. °Temperature drop during 300 hr creep test to a minimum of 400° C. for 6 hours. 
    
     Tensile room temperature tests were carried out as were creep tests to measure the total plastic strain after 100 hours and 300 hours at 600° C. under a stress of 200N/mm 2 . In addition post creep tensile tests of samples having had 300 hours at 600° C. were carried out with the surface retained. The test results for the first part of the investigation are given in Tables V to X and the results are averaged for particular primary or secondary treatments and given in Table XI. The results for the second series of heat treatments are given in Tables XII to XIV. The average of the results for particular primary or secondary treatments is given in Table XV. 
     
         __________________________________________________________________________                      Creep Data                      600° C./200Nmm.sup.-2                                Tensile Data After 300                                hr/600° C.      Unexposed Tensile Data                      TPS       (Surface Retained)Average of all      0.2% PS           UTS              EL5D                  R in A                      100 hr                           300 hr                                0.2% PS                                     UTS EL5D                                             R in AResults Given      Nmm.sup.-2              %       %    %    Nmm.sup.-2                                         %__________________________________________________________________________TABLE XI(a)A Primary  24 hr/      871  975              13.4                  21.1                      0.541                           1.660                                912  992 5.5 9.5Treatment of  500° C.A Primary  24 hr/      871  972              12.9                  20.1                      0.546                           1.746                                913  989 5.4 9.4Treatment of  510° C.A Primary  24 hr/      877  980              11.4                  19.6                      0.535                           1.697                                909  993 5.0 10.1Treatment of  520° C.A Primary  24 hr/      878  983              12.7                  18.5                      0.471                           1.426                                918  1002                                         3.7 7.8Treatment of  530° C.A Primary  24 hr/      874  978              12.6                  18.1                      0.463                           1.444                                924  1009                                         4.7 8Treatment of  540° C.A Primary  24 hr/      879  988              11.8                  15.8                      0.420                           1.259                                930  1016                                         4.9 7Treatment of  550° C.__________________________________________________________________________TABLE XI(b)A Secondary  8 hr/      864  969              13.1                  20.8                      0.479                           1.442                                919  1005                                         5.1 8.3Treatment at  625° C.A Secondary  24 hr/      877  982              12.6                  18.7                      0.510                           1.570                                916  995 5.5 8.4Treatment at  625° C.A Secondary  48 hr/      888  986              12.3                  18.7                      0.473                           1.527                                919  1001                                         4.6 8.8Treatment at  625° C.A Secondary  24 hr/      873  983              13.6                  21.1                      0.515                           1.513                                917  1002                                         4.6 9Treatment at  600° C.A Secondary  24 hr/      877  982              12.6                  18.7                      0.510                           1.570                                916  995 5.5 8.4Treatment at  625° C.A Secondary  24 hr/      874  976              10.8                  15.1                      0.504                           1.638                                917  999 4.6 8.7Treatment at  650° C.STD        859  971              13.5                  26.5                      0.571                           1.525                                916  994 5.5 10.5__________________________________________________________________________ 
    
     
                                           TABLE XII__________________________________________________________________________          Creep @          600° C./200Nmm.sup.-2          TPSStress Relief  100 hr               300 hr                    0.1% PS                         0.2% PS                              UTS  EL5D                                       R in AHeat Treatment(s)          %    %    Nmm.sup.-2                         Nmm.sup.-2                              Nmm.sup.-2                                   %   %__________________________________________________________________________625° C./2 hrs          --   --   833  853   979 10.5                                       17(5331S STD)    0.578               1.757                    .sup.# 912                         922  1010 5.5 8.5.sup.#625° C./8 hr + 500° C./24 hr          --   --   889  907  1010 7.5 13.5          0.403               1.251                    .sup.# 921                         937  1025 2   6.5.sup.#625° C./8 hr + 510° C./24 hr          --   --   891  904  1008 7.5 9          0.470               1.444                    .sup.# 917                         937  1010 1.5 5.sup.#625° C./8 hr + 520° C./24 hr          --   --   888  902  1006 7   10          0.407               1.267                    .sup.# 922                         936  1011 1   3.sup.#625° C./8 hr + 530° C./24 hr          --   --   895  909  1015 8.5 14          0.462                1.423*                    .sup.# 907                         923  1027 2   7.sup.#625° C./8 hr + 540° C./24 hr          --   --   889  908  1014 6.5 10.5          0.409               1.223                    .sup.# 919                         933  1013 2.5 4.sup.#625° C./8 hr + 550° C./24 hr          --   --   887  902  1008 7.5 13.5          0.393               1.376                    .sup.# 912                         931  1013 3   5.sup.#Average (Excl STD)          --   --   890  905  1010 7.4 11.8          0.424               1.331                    .sup.# 916                         933  1017 2   5.1.sup.#__________________________________________________________________________ .sup.# All Post Creep Tensile Samples had their Surfaces Retained. *Extra heating of 8 hrs/600° C. on loading for 300 hr creep. 
    
     
                                           TABLE XIII__________________________________________________________________________          Creep @          600° C./200Nmm.sup.-2          TPSStress Relief  100 hr               300 hr                    0.1% PS                         0.2% PS                              UTS  EL5D                                       R in AHeat Treatment(s)          %    %    Nmm.sup.-2                         Nmm.sup.-2                              Nmm.sup.-2                                   %   %__________________________________________________________________________625° C./2 hrs          --   --   833  853   979 10.5                                       17(5331S STD)    0.578               1.757                    .sup.# 912                         922  1010 5.5 8.5.sup.#625° C./24 hr + 500° C./24 hr          --   --   885  904  1002 4.5 10          0.475               1.483                    .sup.# 911                         927  1017 3.5 4.sup.#625° C./24 hr + 510° C./24 hr          --   --   900  915  1012 4.5 7.5          0.463               1.466                    .sup.# 910                         925  1020 2.5 6.5.sup.#625° C./24 hr + 520° C./24 hr          --   --   897  913  1015 5.5 11          0.459               1.398                    .sup.# 902                         922  1019 4   6.sup.#625° C./24 hr + 530° C./24 hr          --   --   897  915  1012 6   9          0.401               1.206                    .sup.# 917                         936  1022 3.5 8.sup.#625° C./24 hr + 540° C./24 hr          --   --   897  918  1020 6   11          0.418               1.427                    .sup.# 913                         928  1008 3.5 7.5.sup.#625° C./24 hr + 550° C./24 hr          --   --   904  920  1027 4   8          0.513               1.668                    .sup.# 913                         930  1017 3.5 6.5.sup.#Average (Excl STD)          --   --   897  914  1015 5.1 9.4          0.455               1.441                    .sup.# 911                         928  1017 3.4 6.4.sup.#__________________________________________________________________________ .sup.# All Post Creep Tensile Samples had their Surfaces Retained. 
    
     
                                           TABLE XIV__________________________________________________________________________          Creep @          600° C./200Nmm.sup.-2          TPSStress Relief  100 hr               300 hr                    0.1% PS                         0.2% PS                              UTS  EL5D                                       R in AHeat Treatment(s)          %    %    Nmm.sup.-2                         Nmm.sup.-2                              Nmm.sup.-2                                   %   %__________________________________________________________________________625° C./2 hrs          --   --   833  853   979 10.5                                       17(5331S STD)    0.578               1.757                    .sup.# 912                         922  1010 5.5 8.5.sup.#625° C./48 hr + 500° C./24 hr          --   --   901  913  1009 5   7          0.494               1.498                    .sup.# 913                         930  1018 4   7.sup.#625° C./48 hr + 510° C./24 hr          --   --   905  917  1021 6   8.5          .sup. 0.489°               1.480                    .sup.# 899                         920  1003 3.5 8.5.sup.#625° C./48 hr + 520° C./24 hr          --   --   901  917  1020 3.5 7           0.481*               1.695                    .sup.# 909                         927  1011 4   7.5.sup.#625° C./48 hr + 530° C./24 hr          --   --   898  916  1018 5.5 6.5          0.483               1.732                    .sup.# 899                         922  1006 5   11.sup.#625° C./48 hr + 540° C./24 hr          --   --   901  917  1014 5.5 6.5          0.469                1.561**                    .sup.# 910                         926  1009 3.5 8.5.sup.#625° C./48 hr + 550° C./24 hr          --   --   900  916  1020 6   10          0.452               1.407                    .sup.# 917                         932  1017 3.5 6.sup.#Average (Excl STD)          --   --   901  916  1017 5.3 7.6          0.478               1.562                    .sup.# 908                         926  1011 3.9 8.1.sup.#__________________________________________________________________________ .sup.# All Post Creep Tensile Samples had their Surfaces Retained. °Value at 117 hours. *Extra heating of up to 24 hrs/600° C. on loading for 300 hr creep test. **Temperature dropped down to 592° C. for up to 17 hours. 
    
     
                                           TABLE XV__________________________________________________________________________                      Creep Data                      600° C./200Nmm.sup.-2                                Tensile Data After 300                                hr/600° C.      Unexposed Tensile Data                      TPS       (Surface Retained)Average of all      0.2% PS           UTS              EL5D                  R in A                      100 hr                           300 hr                                0.2% PS                                     UTS EL5D                                             R in AResults Given      Nmm.sup.-2              %       %    %    Nmm.sup.-2                                         %__________________________________________________________________________A Primary  8 hr/      905  1010              7.4 11.8                      0.424                           1.331                                933  1017                                         2.0 5.1Treatment of  625° C.A Primary  24 hr/      914  1015              5.1 9.4 0.455                           1.441                                928  1017                                         3.4 6.4Treatment of  625° C.A Primary  48 hr/      916  1017              5.3 7.6 0.478                           1.562                                926  1011                                         3.9 8.1Treatment of  625° C.A Secondary  24 hr/      908  1007              5.7 10.2                      0.457                           1.411                                931  1020                                         3.2 5.8Treatment of  500° C.A Secondary  24 hr/      912  1014              6   8.3 0.474                           1.463                                927  1011                                         2.5 6.7Treatment of  510° C.A Secondary  24 hr/      911  1014              5.3 9.3 0.449                           1.453                                928  1014                                         3.0 5.5Treatment of  520° C.A Secondary  24 hr/      913  1015              6.7 9.8 0.449                           1.454                                927  1018                                         3.5 8.7Treatment of  530° C.A Secondary  24 hr/      914  1016              6   9.3 0.432                           1.404                                929  1010                                         3.2 6.7Treatment of  540° C.A Secondary  24 hr/      913  1018              5.8 10.5                      0.453                           1.484                                931  1016                                         3.3 5.8Treatment of  550° C.STD        853  979              10.5                  17  0.578                           1.757                                922  1010                                         5.5 8.5__________________________________________________________________________ 
    
     FIG. 1, which is a graph of total plastic strain TPS against the primary heat treatment, shows averaged results for secondary heat treatment at a number of temperatures for different times. The reference point STD shows the TPS for solution treated material which is treated at 625° C. for 2 hours the so called standard treatment. It can be seen that increasing the primary temperature from 500° C. to 550° C. results in a general improvement in creep strength as measured by TPS from an average of approximately 0.575% to approximately 0.45%. It is worth noting that the use of a primary treatment irrespective of temperature leads to a general improvement in creep strength irrespective of the time or temperature of the secondary treatment used. 
     FIG. 2 shows that the primary treatment has little effect on the post creep ductility of the material compared to material given the so called standard treatment. In FIG. 2 the upper series of lines corresponds to the ductility as measured by R in A percentage of unexposed material. The lower series of lines corresponds to R in A measurements on samples tested in the post creep state having had 300 hours creep at 600° C. at a stress of 200N/mm 2 . Although it can be seen that there is a fall off in the unexposed ductility there is very little fall off in the post creep ductility for material given primary treatment at a series of temperatures between 500° C. and 550° C. It can also be seen that there is very little difference in post creep ductility in the particular secondary treatment whether it be 8 hours at 625° C. or 24 hours at 600° C. or 24 hours at 650° C. 
     The effects of varying the primary treatment on the 0.2% proof stress and the elongation as measured by percentage elongation at break is illustrated in FIG. 3. The upper series of lines corresponds to the 0.2% proof stress measurements and the lower series of lines corresponds to the elongation at break measured in percentage. These figures illustrate that compared to the so called standard heat treatment the 0.2% proof stress can be increased from approximately 860N/mm 2  to about 890N/mm 2  whilst the elongation falls only slightly from about 13% to about 121/2%. It is interesting to note that there is only a slight loss of elongation whereas the reduction in area is more significantly affected. 
     The information given above and illustrated in FIGS. 1 to 3 shows, therefore, that in general after creep exposure there is little effect on ductility between the so called standard heat treatment and the duplex treatments whereas there are significant improvements in strength to be obtained and the best compromise of results appears to be present in material given a primary heat treatment of 530° C. to 540° C. for 24 hours. 
     Considering the effects of the secondary treatment it can be seen that basically improvements in strength and creep resistance have been achieved at the expense of a slight loss of unexposed ductility. 
     Considering FIG. 4 this shows the effect of increasing the secondary treatment time at 625° C. in the left hand side and on the right hand side shows the effect of increasing the secondary treatment temperature at a constant time of 24 hours. The two upper graphs illustrate the 0.2% proof stress and the two lower graphs are of elongation in percentage. Considering first the graph in the upper left hand corner this shows that increasing the duration of the secondary treatment has a beneficial effect on the 0.2% proof stress. The average rises from approximately 863 to about 887 N/mm 2 . There is a small reduction in elongation (the lower left hand graph) as measured in the unexposed condition. The graphs on the left hand side relate to material which has had an initial treatment at 500° C., 510° C., 520° C., 530° C., 540° C. and 550° C. as illustrated by the individually identified lines. The average is shown as a solid line between the x&#39;s. Thus although it can be seen that increasing the duration of the secondary treatment is beneficial, increasing the temperature at a constant time of 24 hours is less beneficial-see the right hand pair of graphs. The right hand upper graph shows that increasing the temperature of the secondary heat treatment has no significant effect on the proof stress although the proof stress at 625° C. is slightly better than at any other temperature on average. By comparison, however, there is a steady fall in the elongation as is indicated by the lower right hand graph. 
     FIG. 5 shows the effect of the secondary treatments on the ductility of the alloy in the creep tested and non-creep tested conditions. The lower two graphs relate to alloys which are given tensile tests in the post creep condition whereas the two upper graphs relate to alloys tested in the non-creep tested condition. The two graphs on the left hand side illustrate the effects of increasing the duration of the secondary treatment from 8 to 24 to 48 hours whilst keeping the temperature of the heat treatment constant at 625° C. It can be seen that there is little effect on the post creep ductility of the alloy whereas there is a slight fall of in the non-creep tested material. Similarly the effects of holding the time constant at 24 hours but testing at different temperatures shows that the measurements illustrated in the right hand pair of graphs mean the post creep properties are constant whereas there is a fall off in non-creep tested material. 
     The information given above shows, therefore, that the use of duplex heat treatment enables significant increases in the 0.2% proof stress to be obtained without any serious loss of ductility. There will also be significant improvements in internal stress levels resulting from the use of extended heat treatments. Unexpectedly, however, it has also been discovered that extending the time of the secondary heat treatment at a temperature of 625° C. gives an improvement in creep strength if the original treatment is carried out at a temperature of 530° C. or 540° C. Thus from Table VIII it can be seen that the 100 hour creep strength has not been adversely affected being 0.485 total plastic strain after an 8 hour secondary treatment compared to 0.462 total plastic strain after a 48 hour treatment. The effect is even more significant in material heat treated at 540° C. as shown in Table IX. Even given a 300 hour creep exposure at 600° C. the total plastic strain remains substantially constant at  1.43% after an 8 hour secondary treatment and 1.447% after a 48 hour treatment. These figures are within the normal scatter that is to be found in any experimental evidence. By comparison it can be seen that both the 0.1% and 0.2% proof strengths are improved for the 48 hour treated material, that there is very little effect on the elongation at 5D or in the R in A figures. 
     By comparison, however, for material given a single stress relief treatment for 2 hours at 625° C. and then creep tested at 540° C. the total plastic strain was 0.084% after 100 hours at 300N/mm 2 . For material treated at 625° C. for 8 hours the total plastic strain was found to be 0.164% under the same conditions. Logically, therefore, it would have been expected that the same degradation would have occurred for duplex heat treated material. It is not known why this improvement in creep strength is obtained with duplex heat treatment. 
     The work carried out has also shown that the increase in properties required are more significant when the second treatment is carried out at a higher temperature than the first treatment. Tables XII XIII and XIV show that increasing the temperature from 500° C. to 550° C. as a secondary age has no significant effect on any of the properties, the implication of this is that it is the primary heat treatment which dominates if the primary heat treatment is at a higher temperature than the secondary heat treatment. 
     It is also becoming apparent that in the particular alloy 5331S secondary treatment at temperatures of about 650° C. appear to cause a reduction in properties, possibly resulting from annealing out of dislocations or some form of spheroidisation of the precipitate within the alloy. 
     Although the work indicated above has all been carried out on the alloy 5331S it is believed that similar results would be obtained with other near alpha alloys, such as IMI 685 or other such near alpha alloys to be developed in the future.