A beta titanium alloy

Abstract: A beta titanium alloy comprises 25wt% vanadium, 15wt% chromium, 2wt% aluminium, up to 0.15wt% oxygen, 0.1 to 0.3wt% carbon and the balance titanium plus incidental impurities. The carbon is present in the form of titanium carbide precipitates distributed throughout the beta titanium alloy matrix, the titanium carbide precipitates refine the grain size of the beta titanium alloy matrix and remove oxygen from the beta titanium alloy matrix to reduce precipitation of alpha titanium in the beta titanium alloy matrix to increase the ductility of the beta titanium alloy. The alloy is useful for gas turbine engine compressor blades (10), compressor vanes, compressor casings etc.

Description: The present invention relates to beta titanium alloys, particularly to burn resistant beta titanium alloys. Titanium alloys are used in gas turbine engines, particularly for compressor blades and compressor vanes in the low pressure compressor and the high pressure compressor. A problem associated with titanium alloys is that titanium is a highly reactive metal and may burn in appropriate circumstances. For example if the tip of a titanium alloy compressor blade rubs on the compressor casing, during operation of the gas turbine engine, the friction may lead to ignition of the titanium alloy compressor blade. Thus there is a requirement for a titanium alloy which is burn resistant, preferably it does not burn, if friction occurs between a titanium alloy compressor blade and a compressor casing, during operation of the gas turbine engine. Additionally there is a requirement for such a titanium alloy to be ductile and there is a requirement for such a titanium alloy to be relatively cheap in terms of raw products and processing requirements. A non burning beta titanium alloy is known from published UK patent application GB2238057A, which comprises at least 20wt% vanadium, at least 10wt% chromium and at least 40wt% titanium. This alloy may comprise up to 2.5wt% carbon and up to 0.3wt% oxygen. This mentions that the carbon addition improves the post creep ductility of the alloy and the carbon forms carbides. There is no discussion of the oxygen in the alloy. None of the alloy examples comprise oxygen. The alloy does not comprise any aluminium. Thus this alloy is relatively expensive to produce because the vanadium is added as an element rather than as a vanadium-aluminium master alloy. A beta titanium alloy is known, from UK patent GB1175683 which, comprises 25-40wt% vanadium, 5-15wt% chromium, 0-10wt% aluminium and the balance titanium and impurities. This alloy may comprise up to 2wt% carbon and up to 0.3wt% oxygen. The carbon is added to increase the strength of the alloy and the oxygen is an impurity. None of the alloy examples comprise oxygen. This alloy is relatively cheap to produce because the vanadium is added in the form of vanadium-aluminium master alloy. A paper entitled "Structure and Stability of precipitates in 500 degree C exposed Ti-25V-15Cr-xAl alloy", by Li, Y. G. et al., discloses that alpha titanium precipitation is influenced by temperature, oxygen content and aluminium content in the alloy. Accordingly the present invention seeks to provide a novel beta titanium alloy which has improved ductility. Accordingly the present invention provides beta titanium alloy comprising at least 10wt% of one or more beta stabilising elements, the beta stabilising element are selected from the group comprising vanadium, molybdenum, tantalum, niobium, chromium, tungsten, manganese, copper, nickel and iron, 1.0 to 3.0 wt% aluminium 0.1 to 0.4wt% carbon up to 0.2wt% oxygen and the balance titanium and incidental impurities, wherein the carbon is present in the form of titanium carbide precipitates distributed throughout the beta titanium alloy matrix, the titanium carbide precipitates consisting of Ti 2 C titanium carbide precipitates, the titanium carbide precipitates refine the grain size of the beta titanium alloy matrix and remove oxygen from the beta titanium alloy matrix to reduce precipitation of alpha titanium in the beta titanium alloy matrix to increase the ductility of the beta titanium alloy. Preferably the present invention provides a beta titanium alloy comprising 20 to 30wt% vanadium, 13 to 17wt% chromium, 1.0 to 3.0wt% aluminium, 0.1 to 0.4wt% carbon, up to 0.2wt% oxygen and the balance titanium plus incidental impurities. Preferably the beta titanium alloy comprises 1.5 to 2.5wt% aluminium. Preferably the beta titanium alloy comprises 0.15 to 0.3wt% carbon. Preferably the beta titanium alloy comprises less than 0.15wt% oxygen. Preferably the beta titanium alloy comprises 23-27wt% vanadium, 13-17wt% chromium, 1-3wt% aluminium, up to 0.15wt% oxygen, 0.1 to 0.3wt% carbon and the balance titanium plus incidental impurities. Preferably the beta titanium alloy comprises 25wt% vanadium, 15wt% chromium, 2wt% aluminium, up to 0.15wt% oxygen, 0.1 to 0.3wt% carbon and the balance titanium plus incidental impurities. Preferably the beta titanium alloy is used to make an article comprising a beta titanium alloy. Preferably the article comprises a component for a gas turbine engine. Preferably the component comprises a compressor blade or a compressor vane. The component may comprise a tip portion for a compressor blade. The present invention will be more fully described by way of example with reference to the accompanying drawings in which:- Figure 1 shows a compressor blade comprising a beta titanium alloy according to the present invention. Figure 2 shows a compressor blade having a tip portion comprising a beta titanium alloy according to the present invention. Figure 3 is a graph of elongation against oxygen content for beta titanium alloys with varying degrees of carbon addition. A gas turbine engine compressor blade 10, as shown in figure 1, comprises an aerofoil 12, a platform 14 and a root 16. The compressor blade 10 comprises a beta titanium alloy, preferably a burn resistant beta titanium alloy, according to the present invention. The beta titanium alloy compressor blade may be forged, or cast, or produced by other thermomechanical processes. A gas turbine engine compressor blade 20, as shown in figure 2, comprises an aerofoil 22, a platform 24 and a root 26. The compressor blade 10 also comprises a tip portion 28 on the extremity of the aerofoil 22 remote from the platform 24 and root 26. The tip portion 28 comprises a beta titanium alloy, preferably a burn resistant titanium alloy according to the present invention. The tip portion 28 may comprise weld filler deposited onto the aerofoil 22 by using the burn resistant beta titanium alloy as the weld filler during welding, e.g. tungsten inert gas (TIG) welding. The weld filler subsequently being machined to size and shape. Alternatively the tip portion 28 may comprise a block of the burn resistant beta titanium alloy which is welded onto the aerofoil, e.g. tungsten inert gas (TIG) welding, laser welding, electron beam welding etc. The block subsequently being machined to size and shape. The burn resistant titanium alloy according to the present invention comprises 20 to 30wt% vanadium, 13 to 17w% chromium, 1.0 to 3.0wt% aluminium, 0.1 to 0.4wt% carbon, up to 0.2wt% oxygen and the balance titanium plus incidental impurities. Preferably the beta titanium alloy comprises 23-27wt% vanadium, 13-17wt% chromium, 1-3wt% aluminium, up to 0.15wt% oxygen, 0.1 to 0.3wt% carbon and the balance titanium plus incidental impurities. Preferably the beta titanium alloy comprises 25wt% vanadium, 15wt% chromium, 2wt% aluminium, up to 0.15wt% oxygen, 0.1 to 0.3wt% carbon and the balance titanium plus incidental impurities. The burn resistant beta titanium alloy in particular has a favourable combination of carbon and oxygen which enhances the ductility of the burn resistant titanium alloy. It has been found that there is a synergy between the oxygen and carbon levels. In particular it has been found that the carbon reacts with the titanium to form titanium carbide (Ti 2 C) precipitates which refine the grain size of the beta titanium alloy matrix. Furthermore the titanium carbide (Ti 2 C) precipitates have an affinity for the oxygen and the oxygen becomes attached to the titanium carbide (Ti 2 C) precipitates and thus the oxygen is removed from the beta titanium alloy matrix. The presence of oxygen in the beta titanium alloy matrix has the effect of promoting the precipitation of alpha titanium in the beta titanium alloy matrix. The presence of alpha titanium in the beta titanium alloy reduces the ductility of the beta titanium alloy. Thus because the titanium carbide precipitates remove oxygen from the beta titanium alloy matrix there is less oxygen available to promote the precipitation of the alpha titanium, and thus the precipitation of alpha titanium in the beta titanium alloy matrix is reduced. Therefore this increases the ductility of the beta titanium alloy. It is to be noted that the carbon does not remove all the oxygen from the beta titanium alloy matrix. It has been found that titanium carbide (Ti 2 C) precipitates are formed when more than 0.1wt% carbon is present in the beta titanium alloys mentioned above. These titanium carbide precipitates getter the oxygen and refine the grains. The carbon addition improves the stability of the beta titanium alloys. The increase in the ductility of the beta titanium alloy provided by the synergy between the oxygen and the carbon enables aluminium to be added to the beta titanium alloy, and this enables the use of cheaper master alloys, e.g. vanadium aluminium master alloys. EXAMPLES Alloys with the composition listed in table 1 were produced using a plasma melter from mixtures of master alloys and elemental raw materials. Either titanium sponge with 0.04wt% oxygen or titanium granules with 0.086wt% oxygen were used according to the desired oxygen levels. The base level of carbon with no deliberate addition of carbon is 0.02wt% carbon which was brought in by impurities in the raw materials. (Composition in weight%) Alloy Elements Code Ti V Cr Al C O A8 Balance 25 15 2 0.02 0.065 A14 Balance 25 15 2 0.02 0.095 A12 Balance 25 15 2 0.02 0.135 A17 Balance 25 15 2 0.10 0.115 A18 Balance 25 15 2 0.20 0.110 A11 Balance 25 15 2 0.30 0.095 A13 Balance 25 15 2 0.09 0.165 A19 Balance 25 15 2 0.21 0.15 A20 Balance 25 15 2 0.31 0.15 The alloy samples were all forged at 1050°C to produce pancakes about 16mm thick. The samples were then heat treated at 850°C for 2 hours air cooled, or heat treated at 1050°C for 0.5 hours air cooled followed by ageing at 700°C for 4 hours air cooled or heat treated at 1050°C for 0.5 hours air cooled then followed by ageing at 700°C for 4 hours air cooled and then followed by heat treatment at 550°C for 500 hours air cooled. The alloy samples were cut, polished and etched for conventional optical microscopy and scanning electron microscopy. Additionally X-ray diffraction, EDX and transmission electron microscopy were performed on the alloy samples. All the alloy samples were tested in tension at room temperature, and the results are listed in table 2 and illustrated graphically in figure 3. Condition 1:- 850°C/2 hours air cooled. Condition 2:- 1050°C/0.5 hours air cooled and 700°C/4 hours air cooled. Condition 3:- 1050°C/0.5 hours air cooled and 700°C/4 hours air cooled and 550°C/500 hours air cooled. The carbon in alloys A8, A14 and A12 are impurities in the alloy rather than deliberate addition of carbon. (Tensile Properties) Alloy Code Heat Treat Conditions 0.2% Proof Stress (MPa) Ultimate Tensile Strength (MPa) Elongation (%) A8 1 828 858 21.0 A14 1 805 842 1.5 2 892 892 1.4 3 953 955 0.6 A12 1 835 853 0.5 2 902 0.1 3 949 949 0.3 A17 1 916 921 24.0 2 878 891 16.4 3 887 896 4.9 A18 1 899 939 20.3 2 894 923 15.0 3 849 891 11.8 A11 1 867 905 16.6 2 866 882 14.0 3 849 891 4.6 A13 1 891 0 2 938 944 0.5 A19 1 900 914 8.4 2 882 903 8.7 3 890 911 9.9 A20 1 935 964 10.9 2 903 915 1.0 3 885 927 10.5 It is clear from table 2, and figure 3, that when there is no deliberate carbon addition the trend is for the elongation, the ductility, to decrease with increasing oxygen levels. It is also clear that when carbon is added the elongation, ductility, is improved. It is seen that there is a significant increase in ductility by adding over 0.1wt% carbon to beta titanium alloys with 0.095 to 0.115wt% oxygen, see alloys A14, A17, A18 and A11. The improvement in ductility for alloys with 0.15 to 0.165wt% oxygen and over 0.2wt% carbon is also significant for most of the heat treatments. It is seen that the ductility of the beta titanium alloys without carbon addition deteriorates after heat treatment condition 3. This is due to precipitation of alpha titanium in the beta titanium alloy matrix. The ductility of the beta titanium alloys with carbon addition also deteriorates after heat treatment. However, some ductility is retained. Also the ductility of alloys A19 and A20 with high carbon and high oxygen levels have greater ductility than alloy A17 with lower carbon and oxygen levels after heat treatment condition 3. Examination showed that the alloy samples without carbon failed by cleavage fracture, whereas alloy samples with carbon failed by a ductile, or by a mixture of ductile/brittle, manner. The addition of carbon overcomes the detrimental effects that oxygen and alpha titanium have on the room temperature ductility of the beta titanium alloy and the metallurgical stability of the beta titanium alloy after high temperature exposure. The titanium carbide precipitates formed are stable to heat treatment and these titanium carbide precipitates refine the as forged and heat treated microstructure and as cast microstructure. The refined microstructure may deform more uniformly and may have an effect on the ductility of the beta titanium alloy. The titanium carbide precipitates getter oxygen, increase the ductility of the beta titanium alloy matrix and suppress the formation of the alpha titanium in the beta titanium alloy matrix. The refined beta titanium alloy matrix has smaller grains and thus there are more grain boundaries. The amounts of alpha titanium precipitation present on each grain boundary is less and this further increases ductility by reducing embrittlement due to alpha titanium. The carbon level must not be too high in the beta titanium alloys, since the precipitation of too much titanium carbide is detrimental to ductility. It is well known in the art that the addition of carbon to beta titanium alloys produces titanium carbides. It is also well known that beta titanium alloys become brittle due to titanium carbide precipitation. Thus this improvement in ductility of the beta titanium alloy due to the higher than normal addition of carbon in the presence of the oxygen is completely unexpected. Although the invention has been described with reference to a narrow range of beta titanium alloys it is believed that it is applicable to all beta titanium alloys with more than 10wt% of one or more beta stabilising elements and oxygen present, which decreases the ductility of the beta titanium alloy by stabilising alpha titanium, or alpha 2 titanium, in the beta titanium alloy matrix. The beta stabilising element may be one or more of the elements vanadium, molybdenum, tantalum, niobium, chromium, tungsten, manganese, copper, nickel and iron. The advantages provided by the present invention are an increase in the ductility of the beta titanium alloy provided by the synergy between the oxygen and the carbon. This enables aluminium to be added to the beta titanium alloy, and this enables the use of cheaper master alloys, e.g. vanadium aluminium master alloys. There may also be an improvement in the processing temperature range. The prior art mentioned above does not disclose, or suggest, that there is a synergy between carbon and oxygen in beta titanium alloys which increases the ductility of the beta titanium alloy. Although the invention has been described with reference to the use as compressor blades and compressor vanes, it may also be used to make compressor casings and other suitable components for gas turbine engines or other engines and for other applications.

Claims: A beta titanium alloy comprising at least 10wt% of one or more beta stabilising elements, the beta stabilising element is selected from the group comprising vanadium, molybdenum, tantalum, niobium, chromium, tungsten, manganese, copper, nickel and iron, 1.0 to 3.0 wt% aluminium, 0.1 to 0.4wt% carbon up to 0.2wt% oxygen and the balance titanium and incidental impurities, wherein the carbon is present in the form of titanium carbide precipitates distributed throughout the beta titanium alloy matrix, the titanium carbide precipitates consisting of Ti 2 C titanium carbide precipitates, the titanium carbide precipitates refine the grain size of the beta titanium alloy matrix and remove oxygen from the beta titanium alloy matrix to reduce precipitation of alpha titanium in the beta titanium alloy matrix to increase the ductility of the beta titanium alloy. A beta titanium alloy as claimed in claim 1 wherein the beta titanium alloy comprises 20 to 30wt% vanadium, 13 to 17wt% chromium, 1.0 to 3.0wt% aluminium, 0.1 to 0.4wt% carbon, up to 0.2wt% oxygen and the balance titanium plus incidental impurities A beta titanium alloy as claimed in claim 1 or claim 2 wherein the beta titanium alloy comprises 1.5 to 2.5wt% aluminium. A beta titanium alloy as claimed in claim 1, claim 2 or claim 3 wherein the beta titanium alloy comprises 0.15 to 0.3wt% carbon. A beta titanium alloy as claimed in any of claims 1 to 4 wherein the beta titanium alloy comprises less than 0.15wt% oxygen. A beta titanium alloy as claimed in any of claims 1 to 5 wherein the beta titanium alloy comprises 23-27wt% vanadium, 13-17wt% chromium, 1-3wt% aluminium, up to 0.15wt% oxygen, 0.1 to 0.3wt% carbon and the balance titanium plus incidental impurities. A beta titanium alloy as claimed in claim 6 wherein the beta titanium alloy comprises 25wt% vanadium, 15wt% chromium, 2wt% aluminium, up to 0.15wt% oxygen, 0.1 to 0.3wt% carbon and the balance titanium plus incidental impurities. An article (10)comprising a beta titanium alloy as claimed in any of claim 1 to 7. An article (10) as claimed in claim 8 wherein the article (10) comprises a component for a gas turbine engine. An article as claimed in claim 9 wherein the component (10) comprises a compressor blade or a compressor vane. An article as claimed in claim 9 wherein the component (28) comprises a tip portion for a compressor blade (20).

Inventors: BLENKINSOP PAUL ADDYMAN, LI YUE GANG, BLENKINSOP, PAUL ADDYMAN, LI, YUE GANG
Applicants: ROLLS ROYCE PLC, ROLLS-ROYCE PLC
Application year: 1999
Main classification: C22C14
IPC codes: C22C14