Abstract:
A high temperature article, for example a rocket nozzle suitable for liquid-fuelled rocket motors for satellites, is formed from an alloy which is a binary or tertiary alloy from the Pt-Ir-Rh system. Such alloys exhibit a good balance between ease and reliability of manufacture, cost of alloy and high temperature strength and oxidation resistance.

Description:
This is a continuation of application Ser. No. 08/529,102, filed on Sep. 15, 1995, which was abandoned upon the filing hereof. 
    
    
     FIELD OF THE INVENTION 
     The present invention concerns improved high temperature articles, such as rocket nozzles. 
     Space vehicles, such as satellites, require many rocket motors and nozzles for positioning. These structures are usually operated at temperatures in excess of 2000° C. and are required to sustain substantial structural loads. At these temperatures, oxidation of the material generally occurs resulting in a decrease in efficiency. In general, materials capable of withstanding such high temperatures with minimal oxidation, do not have the strength to withstand substantial loads. Conversely, materials capable of withstanding substantial loads at those temperatures are generally subject to considerable oxidation. Consequently, rocket motors have been operated at below optimum temperatures in order to maintain structural strength with minimal oxidation. Even so, the life of such structures was generally limited. 
     DESCRIPTION OF THE PRIOR ART 
     Attempts have been made to overcome these problems. UK patent application GB 2,020,579A proposes the use of 10% by weight rhodium/platinum alloy for use in high-velocity gas streams, but this alloy has a markedly lower ability to withstand high operating temperatures. U.S. Pat. No. 4,917,968 uses an iridium/rhenium bi-layer composite, formed by chemical vapour deposition (CVD) of iridium onto a molybdenum mandrel followed by deposition of rhenium and dissolution of the molybdenum. A CVD process by its nature is generally limited to the application of pure metals and therefore gives no real opportunity to use the advantages of alloying. 
     There remains concern, however, within the aerospace industry about the reliability of the manufacturing process and the reliability of the nozzles formed by the above process. The investment in a satellite and its launch is such that there must be complete confidence in all parts. 
     Consequently there remains a need in the industry for alternative rocket nozzles having reliable and acceptable manufacturing methods combined with acceptable high temperature properties. It is desirable to be able to operate the rocket motor at as high a temperature as possible, since this equates to using less fuel for a given thrust, in turn permitting one or more of an increased payload, fuel load and the ability to maintain the satellite in position for an increased life. 
     SUMMARY OF THE INVENTION 
     The present inventors have found an alloy system which can withstand the high temperatures and loads required by the various applications. These alloy systems show good oxidation resistance and have the added benefit of greater ductility which gives improved fabricability, and more predictable failure mode. 
     Accordingly, the present invention provides a high temperature article prepared from an alloy capable of sustaining substantial temperatures and loads wherein said alloy is a binary or tertiary alloy from the system platinum/iridium/rhodium, provided that if the alloy is a binary rhodium/platinum alloy, the rhodium content is greater than 25% and that if the alloy is a binary platinum/iridium alloy, the iridium content is greater than 30%. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a triangular compositional diagram of alloys according to the invention. 
    
    
     Examples of suitable binary alloys are: 
     a) Rh/Ir in which the content of Rh is up to 60wt %, more preferably up to 40wt %; 
     b) Rh/Pt in which the content of Rh is from 25 to 40wt %, more preferably 25 to 30wt %; 
     c) Ir/Pt in which the content of Ir is 30 to 99.5wt %, preferably 30 to 40wt % or 60 to 99.5wt %. 
     Preferably the article is prepared from a Rh/Ir binary alloy, in which the Rh content is from 0.5 to 10 wt %, for example 2.5 to 5wt %. 
     Preferred tertiary alloys are those represented on the attached triangular compositional diagram (FIG. 1) as falling within the total hatched and cross-hatched area, and more preferred tertiary alloys are those falling within the cross-hatched area of the diagram. 
     The invention also encompasses modifications of the above alloys by the incorporation of a refractory metal such as rhenium or zirconium in an amount of up to 5% by wt, or the incorporation of other metal components providing that high temperature strength and oxidation resistance are not excessively adversely affected. 
     The invention further includes high temperature articles manufactured from the specified alloys and coated with a refractory metal or alloys thereof such as rhenium or tungsten/rhenium, for example by vacuum plasma spraying using conventional equipment, followed by hot isostatic pressing, or by a chemical or electrochemical deposition route. 
     Alternatively, the high temperature article may not be made completely from the above alloys, but may be a ceramic or metal article coated with one of the above alloys. Accordingly, an alternative embodiment of the present invention provides a coating for applying to a ceramic or metal, eg a refractory metal, substrate of a binary or tertiary alloy from the system platinum/iridium/rhodium, provided that if the alloy is a binary rhodium/platinum alloy, the rhodium content is greater than 25% and that if the alloy is a binary platinum/iridium alloy, the iridium content is greater than 30%. 
     The alloys specified form solid solutions and may be cast into ingots, forged, rolled, swaged, machined and/or drawn into tube, providing that robust tooling is used. For example, the alloy components may be melted in a vacuum furnace, although air furnaces may be used. Joining techniques used in platinum group metal metallurgy may be used. 
     Depending upon the properties of the alloy chosen, the high temperature article may be manufactured from tube or by forming sheet into the appropriate shape, by joining different shaped cone and tube shapes, by progressively forming (rolling) a flared cone from a tube, or possibly by die casting or machining from a casting. In all cases, a final shape may be achieved by machining. Alternatively, the article may be manufactured by coating a substrate with the alloy using plasma spraying, particularly vacuum plasma spraying, followed by removal of the substrate, for example by dissolving the substrate, oxidising or machining out the substrate. The particular wall thicknesses will depend upon the particular article being formed, but may be of the order of 0.040 in (approximately 1 mm) or less. 
     The high temperature articles of the invention show a good balance of oxidation resistance, high temperature strength and relative ease of manufacture, leading to reliability combined with acceptable production costs. 
     Suitable articles according to the present invention include rocket nozzles, spark plug electrodes, electrodes eg for glass melting applications, glass melting and forming apparatus eg crucibles, stirrers, fibrising equipment, core pinning wire for investment casting eg turbine blade manufacture, and lead-outs for halogen bulbs. 
     Preferably the articles of the present invention are rocket nozzles, which may be used for main thrusters or subsidiary thrusters (positioning rockets), and are preferably used with liquid fuel rockets. 
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention will now be described by way of Example only. 
     Experimental Procedures 
     Ir metal and Ir-2.5%Rh and ir-5% Rh alloys were melted and alloyed in air before electron beam melting into ingots. Each of the wire-bar ingots were then hot forged, hot swaged and hot drawn to wire. The sheet ingots were hot forged and hot rolled to size. 
     Oxidation Tests 
     Furnace oxidation tests were performed on samples cut from sheet. Dimensional and weight measurements were performed before and after exposing these samples for 8 hours at 1450° C. This data was used to calculate oxidative weight loss per unit area per unit time for Ir, Ir-2.5%Rh and Ir-5% Rh. Results (in mg/cm 2  /hr) (Table 1) clearly show that a Rh addition of only 2.5% is sufficient to more than halve the oxidation rate of Ir at 1450° C. Further improvement is achieved with an addition of 5% Rh. Microstructural analysis of cross sections through the tested samples did not reveal resolvable differences in oxide layer thickness. 
     Resistance heating of wire samples in flowing air was also performed to obtain comparative oxidation resistance at very high temperatures. This involved connecting a length of wire, nominally 1 mm diameter and 50mm long, between the terminals of a variable electrical supply. Distance between the electrical terminals was fixed to ensure that each test was performed under similar conditions. Current flowing through each wire sample was increased slowly until the desired test temperature was achieved. Temperature was measured using an optical pyrometer focused on the hottest section of the wire. Tests were conducted at temperatures of 1650°-1700° C. for 5-6 hours, 2050°-2100° C. for 40 minutes and 2200°-2250° C. for 20 minutes. Weight measurements were performed before and after each test. Size (surface area) of the hot zone was not known though was probably similar for each test condition. Results (Table 1) are therefore presented in the form of weight loss per unit time in order to illustrate comparative performance of the three materials under similar extreme conditions. Tests performed at 1650°-1700° C. corroborate the findings from the furnace oxidation tests, clearly demonstrating a halving of the oxidation rate of Ir by alloying with 2.5%Rh. Tests performed at 2025 -2100° C. demonstrate that improvements, albeit smaller, in oxidation resistance can be obtained until, at 2200°-2250° C., no difference in oxidation resistance was measured. 
     
                                           TABLE 1__________________________________________________________________________Ir/Rh Oxidation Behaviour            Ir  Ir-2.5% Rh                      Ir-5% Rh                            units__________________________________________________________________________Furnace oxidation tests8 hours at 1450° C.            12.5                5.6   4.3   mg/cm.sup.2 /hrResistance heating of wire samples1700° C.  21  10    11    mg/hr2050-2100° C.            77  58    64    mg/hr2200-2250° C.            132 132   133   mg/hr__________________________________________________________________________ 
    
     Hardness Tests 
     Vickers hardness tests were performed on polished microsections removed from sheet in the as-rolled condition and after 8 hours at 1450 ° C. The results are shown in Table 2. 
     
                       TABLE 2______________________________________Hardness          Ir    Ir-2.5% Rh                          Ir-5% Rh______________________________________As-rolled        536     530       566After 8 hours at 1450° C.            309     309       294______________________________________ 
    
     Sheet Tensile Data 
     Tests were performed on dumbell samples using a servo-hydraulic tensometer. The test specimens were machined from as-rolled sheet using spark and wire erosion. Tests performed at strain rates of 0.016min -1  and 15.8min -1  at 20° C. clearly demonstrated the significant increase in tensile strength and ductility that can be achieved through minor additions of Rh to Ir (Table 3). The retention of this high strength and ductility under high strain rate conditions is even more remarkable. At 1150° C. very large deformation was obtained in both of the Ir/Rh alloys (Table 4). 
     Wire Tensile Tests 
     Tensile tests were performed on as-drawn wire samples of Ir, Ir-2.5%Rh and Ir-5% Rh at room temperature. Wire diameter was nominally 1 mm and strain rate was 0.01 min -1 . Results (Table 5) for tensile elongation and reduction in area demonstrate significant improvement in the ductility of Ir by alloying with 5% Rh. 
     
                                           TABLE 3__________________________________________________________________________Ir/Rh Sheet Tensile Data at 20° C. Strain Rate       Yield Strength                   Tensile Strength                                ElongAlloy min.sup.-1       MPa   psi                tsi                   MPa psi   tsi                                %__________________________________________________________________________Ir    0.016 approx 740  714 107,735                             48 2.8Average     740         743          2.8 15.8              761 110,345                             49 1.9 15.8              713 103,385                             46 1.7Average                 737          1.82.5% Rh/Ir 0.016 931         1097                       159,065                             71 5.3 0.016 938         1088                       157,760                             70 4.1Average     935         1093         4.7 15.8              1314                       190,630                             85 10.5 15.8              1177                       170,665                             76 6.8Average                 1246         8.75% Rh/Ir 0.016 1080        1307                       189,515                             85 8.5 0.016 1107        1425                       206,625                             92 12.7 0.016 1093        1395                       202,276                             90 12.3       1093        1376         11.2 15.8              1431                       207,495                             93 13.8 15.8              1431                       207,495                             93 12.6Average                 1431         13.2__________________________________________________________________________ Strain rate = Variable; Specimens = sheet dumbell; As rolled 
    
     
                       TABLE 4______________________________________Ir/Rh Sheet Tensile Data at 1150° C.      Strain Rate                Tensile Strength                                ElongAlloy      min.sup.-1                MPa     psi   tsi   %______________________________________Ir         0.016     315     45,675                              20    17Average              315                 172.5% Rh/Ir 0.016     215     31,175                              14    57      0.016     193     27,985                              12    70Average              204                 645% Rh/Ir   0.016     191     27,695                              12    59      0.016     205     29,725                              13    73      0.016     220     31,900                              14    54                205                 62______________________________________ Strain rate = 0.016 min.sup.-1 ; Specimens = sheet dumbell; Asrolled. 
    
     
                                           TABLE 5__________________________________________________________________________Ir/Rh - Wire Tensile Data    Yield Strength         Tensile Strength                     Elong                         R of AAlloy    MPa psi      tsi         MPa psi  tsi                     %   %   Comments__________________________________________________________________________Ir  BRO712 0.89 mm diameter as drawn wire         1869             271,005                  121                     13.2                         13  fracture at 45 degrees         1835             266,075                  119                     10  10         1869             271,005                  121                     10.3                         11  broke in jaws         1906             276,370                  123                     16.2                         17         1872             271,440                  121                     7.8 1   broke in jawsAverage    1648   238,960      107         1870             271,179                  121                     11.5                         10.4Standard       25         3.2 5.9dev__________________________________________________________________________ 
    
     
         __________________________________________________________________________    Yield Strength         Tensile Strength                     Elong                         R of AAlloy    MPa psi      tsi         MPa psi  tsi                     %   %   Comments__________________________________________________________________________2.5%    BRO888 1.05 mm diameter, as drawn wireRh/Ir         1483             215,035                  95 3.7 11  flat, 0 degree brittle type fracture         1511             219,095                  97.8                     5.6 13         1560             226,200                  101                     7.1 14         1565             226,925                  101                     6.9 15  broke in jaw         1623             235,335                  105                     12.2                         19         1518             220,110                  98.3                     8.1 15  broke in jaws         1560             226,200                  101                     7.7 14  broke in jaws         1536             222,720                  99.5                     7.3 15  broke in jaws         1527             221,415                  98.9                     10.9                         16  broke in jaws         1567             227,215                  101                     7.5 14Average    1363   197,635      88.3         1545             224,025                  100                     7.7 14.6Standard       39         2.4 2.1dev__________________________________________________________________________ 
    
     
         __________________________________________________________________________    Yield Strength         Tensile Strength                     Elong                         R of AAlloy    MPa psi      tsi         MPa psi  tsi                     %   %   Comments__________________________________________________________________________5%  BR2489 1.06 mm diameter as drawn wireRh/Ir         1784             258,680                  116                     28.1                         40  Notable necking with fibrous         cup-cone type fracture         1837             266,365                  119                     34.9                         45  broke in jaws         1840             266,800                  119                     16.5                         22  broke in jaws         1764             255,780                  114                     26.9                         35         1804             261,580                  117                     24.2                         37  broke in jawsAverage    1501   217,645      97.2         1806             261,841                  117                     26.1                         35.8Standard       33         6.7 8.6dev__________________________________________________________________________