Method of heat treating titanium aluminide

Abstract

A gamma titanium aluminide alloy consisting of 46 at % aluminium, 8 at % niobium, up to 0.07 at % carbon and the balance titanium plus incidental impurities has an alpha transus temperature Tα=1335° C. The gamma titanium aluminide alloy was heated to a temperature T1=1360° C. and was held at T1=1360° C. for 1 hour or longer. The gamma titanium aluminide alloy was fluidised bed, or salt bath, quenched to a temperature T2, where 900° C.<T2<1200° C., and was held at temperature T2 for a sufficient time to allow the massive transformation to go to completion. The gamma titanium aluminide alloy was heated to a temperature T3=1300° C. or 1320° C. and was held at T2 for 4 hours. The gamma titanium aluminide alloy was air cooled to ambient temperature. The gamma titanium aluminide alloy has a fine duplex microstructure comprising differently orientated alpha plates in a massively transformed gamma matrix. The heat treatment reduces quenching stresses, allows larger castings and a broader range of titanium aluminide alloys to be grain refined.

Description

The present invention relates to a method of heat-treating titanium aluminide and in particular to a method of heat-treating gamma titanium aluminide.

There is a requirement to refine the microstructure of a titanium aluminide alloy, in particular cast titanium aluminide alloy, which does not involve hot working of the titanium aluminide alloy.

Our European patent application no. 03253539.5 filed 4 Jun. 2003 discloses a method of heat-treating a titanium aluminide alloy having a single alpha phase field and being capable of producing a massively transformed gamma microstructure. In that method of heat-treating the titanium aluminide alloy is heated to a temperature above the alpha transus temperature, is maintained above the alpha transus temperature in the single alpha phase field for a predetermined time period, is cooled from the single alpha phase field to ambient temperature to produce a massively transformed gamma microstructure, is heated to a temperature below the alpha transus temperature in the alpha and gamma phase field, is maintained at the temperature below the alpha transus temperature for a predetermined time period to precipitate alpha plates in the massively transformed gamma microstructure such that a refined microstructure is produced and is then cooled to ambient temperature.

A problem with this heat-treatment is that the cooling, quenching, of the titanium aluminide from above the alpha transus to ambient temperature induces quenching stresses in the titanium aluminide. A further problem is that the heat-treatment is only suitable for relatively thin castings. Another problem is that the heat-treatment is only applicable to compositions of titanium aluminide with a particular range of aluminium.

Accordingly the present invention seeks to provide a novel method of heat-treating titanium aluminide alloy which reduces, preferably overcomes, the above-mentioned problems.

Accordingly the present invention provides a method of heat-treating titanium aluminide alloy, the titanium aluminide alloy having a single alpha phase field and being capable of producing a massively transformed gamma microstructure, the method comprising the steps of:-

• (a) heating a titanium aluminide alloy to a temperature above the alpha transus temperature,
• (b) maintaining the titanium aluminide alloy at a temperature above the alpha transus temperature in the single alpha phase field for a predetermined time period,
• (c) cooling the titanium aluminide alloy from the single alpha phase field to a temperature in the range of 900° C. to 1200° C.,
• (d) maintaining the titanium aluminide alloy at the temperature in the range of 900° C. to 1200° C. for a predetermined time period to produce a massively transformed gamma microstructure,
• (e) heating the titanium aluminide alloy to a temperature below the alpha transus temperature in the alpha and gamma phase field,
• (f) maintaining the titanium aluminide alloy at the temperature below the alpha transus temperature for a predetermined time period to precipitate alpha plates in the massively transformed gamma microstructure such that a refined microstructure is produced in the titanium aluminide alloy,
• (g) cooling the titanium aluminide alloy to ambient temperature.

Preferably in step (b) the predetermined time period is up to 2 hours.

Preferably in step (f) the predetermined time period is up to 4 hours.

Preferably step (e) comprises heating the titanium aluminide alloy to a temperature about 30° C. to 60° C. below the alpha transus temperature.

Preferably step (a) comprises heating the titanium aluminide alloy to a temperature of about 20° C. to 30° C. above the alpha transus temperature.

Preferably step (g) comprises air-cooling or furnace cooling.

Preferably step (c) comprises fluidised bed cooling or salt bath cooling.

It may be possible to cool the titanium aluminide to ambient temperature after step (d) and before step (e) The titanium aluminide may be cooled to ambient temperature by air-cooling or oil cooling

The titanium aluminide alloy may comprise 48 at % aluminium, 2 at % chromium, 2 at % niobium and the balance titanium and incidental impurities.

The alpha transus temperature is about 1360° C., step (a) comprises heating to a temperature of 1380° C., step (b) comprises maintaining the titanium aluminide alloy at a temperature of about 1380° C. for about 1 hour, step (c) and (d) comprise salt bath, or fluidised bed, cooling the titanium aluminide alloy from a temperature of 1380° C. to a temperature between 900° C. and 1200° C. and maintaining the titanium aluminide alloy at the temperature in the range of 900° C. to 1200° C. for a predetermined time period to produce a massively transformed gamma microstructure, steps (e) and (f) comprise heating the titanium aluminide alloy to a temperature of about 1320° C. for about 2 hours to precipitate alpha plates in the massively transformed gamma microstructure such that a refined microstructure is produced in the titanium aluminide alloy, and step (g) comprises air cooling the titanium aluminide alloy to ambient temperature.

The titanium aluminide alloy may comprise 46 at % aluminium, 8 at % niobium, up to 0.07 at % carbon and the balance titanium and incidental impurities.

The alpha transus temperature is about 1335° C., step (a) comprises heating to a temperature of 1360° C., step (b) comprises maintaining the titanium aluminide alloy at a temperature of about 1360° C. for about 1 hour, steps (c) and (d) comprise salt bath cooling, or fluidised bed cooling, the titanium aluminide alloy from a temperature of 1360° C. to a temperature between 900° C. and 1200° C. and maintaining the titanium aluminide alloy at the temperature in the range of 900° C. to 1200° C. for a predetermined time period to produce a massively transformed gamma microstructure, steps (e) and (f) comprise heating the titanium aluminide alloy to a temperature of about 1300° C. to about 1320° C. for about 4 hours to precipitate alpha plates in the massively transformed gamma microstructure such that a refined microstructure is produced in the titanium aluminide alloy, and step (g) comprises air cooling the titanium aluminide alloy to ambient temperature.

The titanium aluminide alloy may consist of 45-46 at % aluminium, 8 at % niobium, up to 0.07 at % carbon and the balance is titanium and incidental impurities.

The titanium aluminide alloy may consist of 45-46 at % aluminium, 2-6 at % niobium, 2-6 at % hafnium and the balance is titanium plus incidental impurities.

The titanium aluminide alloy may be a cast titanium aluminide component.

The method may comprise hot isostatic pressing of the cast titanium aluminide alloy component.

Preferably the hot isostatic pressing of the cast titanium aluminide alloy component is concurrent with step (f).

Preferably the hot isostatic pressing comprises applying a pressure of about 150 MPa for about 4 hours.

The titanium aluminide alloy may be a compressor blade or a compressor vane.

The present invention will be more fully described by way of example with reference to the accompanying drawings in which:

FIG. 1 is graph of temperature versus time illustrating the method of heat-treating a titanium aluminide alloy according to the present invention.

FIG. 2 is a gamma titanium aluminide alloy gas turbine engine compressor blade heat treated according to the present invention.

A method of heat-treating a titanium aluminide alloy according to the present invention is described with reference to FIG. 1. The present invention is concerned with heat-treating gamma titanium aluminide alloys with at least 46 at % aluminium and a single alpha phase field.

The heat treatment process comprises heating the gamma titanium aluminide to a temperature T₁ above the alpha transus temperature T_α. The gamma titanium aluminide alloy is then maintained at a temperature T₁ above the alpha transus temperature T_α in the single alpha phase field for a predetermined time period t₁. The gamma titanium aluminide is quenched, for example fluidised bed cooled, or slat bath cooled, from the single alpha phase field at temperature T₁ to a temperature T₂. The gamma titanium aluminide alloy is maintained at temperature T₂ for a predetermined time period t₂ to produce a massively transformed gamma microstructure. The gamma titanium aluminide alloy is then heated to a temperature T₃ below the alpha transus temperature T_α. The gamma titanium aluminide alloy is maintained at the temperature T₃ in the alpha and gamma phase field for a predetermined time period t₃ to precipitate alpha plates in the massively transformed gamma microstructure such that a refined microstructure is produced in the titanium aluminide alloy. The gamma titanium aluminide is cooled, for example air cooled, or furnace cooled, to ambient temperature.

In particular, the gamma titanium aluminide is heated to a temperature T₁ about 20° C. to 30° C. above the alpha transus temperature T_α. The gamma titanium aluminide alloy is maintained at the temperature T₁ for up to 2 hours. The gamma titanium aluminide alloy is then quenched, for example fluidised bed cooled, or salt bath cooled, to a temperature T₂ about 900° C. to 1200° C. and maintained for a predetermined time period to induce a massively transformed gamma microstructure. The gamma titanium alloy is heated to a temperature T₃ about 30° C. to 60° C. below the alpha transus temperature T_α. The gamma titanium aluminide alloy is maintained at the temperature T₃ for up to 4 hours to precipitate fine alpha plates with different orientations in the massively transformed gamma microstructure due to the massive gamma to alpha+gamma phase transformation. This gives rise to a very fine duplex microstructure. The differently orientated alpha plates precipitated in the massive gamma phase matrix effectively reduce the grain size of the gamma titanium aluminide. The gamma titanium aluminide alloy is then cooled, for example air cooled, or furnace cooled, to ambient temperature.

The holding at temperature T₁ for a time period t₁ also acts a homogenisation process for cast titanium aluminide alloys.

EXAMPLE

A gamma titanium aluminide alloy consisting of 46 at % aluminium, 8 at % niobium, up to 0.07 at % carbon and the balance titanium plus incidental impurities was heat treated according to the present invention. This gamma titanium aluminide alloy has an alpha transus temperature T_α=1335° C. The gamma titanium aluminide alloy was heated to a temperature T₁=1360° C. and was held at T₁=1360° C. for 1 hour for small components and longer for larger components. The gamma titanium aluminide alloy was fluidised bed, or salt bath, quenched to a temperature 900° C.<T₂<1200° C. and was held at temperature T₂, where 900° C.<T₂<1200° C., for a sufficient time to allow the massive transformation to go to completion. The gamma titanium aluminide alloy was heated to a temperature T₃=1300° C. or 1320° C. and was held at T₂=1300° C. or 1320° C. for 4 hours. The gamma titanium aluminide alloy was air cooled to ambient temperature.

As an alternative the gamma titanium aluminide alloy is air-cooled or oil cooled from temperature T₂ to ambient temperature before the gamma titanium aluminide alloy is heated to the temperature T₃.

The present invention is applicable to a gamma titanium aluminide alloy consisting of 46 at % aluminium, 5 at % niobium, 0.3 at % boron, 0.2 at % carbon and the balance titanium plus incidental impurities. The present invention is applicable to a gamma titanium aluminide alloy consisting of 47 at % aluminium, 2 at % niobium, 1 at % tungsten, 1 at % chromium, 1 at % boron, 0.2 at % silicon and the balance titanium plus incidental impurities. The present invention is applicable to gamma titanium aluminide alloy consisting of 47 at % aluminium, 2 at % tantalum, 1 at % chromium, 1 at % manganese, 1 at % boron, 0.2 at % silicon and the balance titanium plus incidental impurities. The present invention is also applicable to gamma titanium aluminide alloy consisting of 46 at % aluminium, 5 at % niobium, 1 at % tungsten and the balance titanium plus incidental impurities. The present invention is applicable to a gamma titanium aluminide alloy consisting of 45-46 at % aluminium, 8 at % niobium, up to 0.07 at % carbon and the balance is titanium and incidental impurities. The present invention is also applicable to a gamma titanium aluminide alloy consisting of 45-46 at % aluminium, 2-6 at % niobium, 2-6 at % hafnium and the balance is titanium plus incidental impurities. The present invention is also applicable to a gamma titanium aluminide alloy consisting of 48 at % aluminium, 2 at % chromium, 2 at % niobium and the balance titanium and incidental impurities.

The advantages of the present invention are that the cooling, quenching, of the titanium aluminide from above the alpha transus to an intermediate temperature induces reduced levels of quenching stresses compared to cooling, quenching, to ambient temperature as described in our European patent application no. 03253539.5. A further advantage is that at temperatures above about 1000° C. the titanium aluminide is relatively ductile and the quenching stresses do not cause fracture. Another advantage is that the heat-treatment is suitable for relatively thin castings and for larger castings so that they all have improved ductility and high strength. Also the heat-treatment is applicable to compositions of titanium aluminide with a broader range, a lower level, of aluminium and hence it is applicable to stronger titanium aluminide alloys. It is believed that the lower level of aluminium may be 45 at % and possibly 44 at %. Thus, the present invention provides a heat treatment for gamma titanium aluminide alloy components, which provides grain refinement. It is particularly suitable for relatively large and complex shaped cast components where the previous heat treatment would induce high residual stresses and possibly cracking of the gamma titanium aluminide alloy components. The heat treatment also permits grain refinement throughout relatively large and complex shaped components rather than just the surface regions of the component.

It may be possible to heat the titanium aluminide alloy component to a temperature of about 1300° C. and to maintain the titanium aluminide alloy component at about 1300° C. to allow the temperature to equilibrate in the titanium aluminide alloy component so that the titanium aluminide alloy component needs to be maintained at temperature T₁ for a shorter time period.

In the case of cast gamma titanium aluminide alloy components it may be necessary to remove porosity from the cast gamma titanium aluminide alloy component. In this case the cast gamma titanium aluminide alloy component may be hot isostatically pressed (HIP) to remove the porosity. The hot isostatic pressing preferably occurs at the same time as the heat treatment temperature T₂ and for the time period of about 4 hours at a pressure of about 150 MPa and this is beneficial because this dispenses with the requirement for a separate hot isostatic pressing step.

The present invention is particularly suitable for gamma titanium aluminide gas turbine engine compressor blades as illustrated in FIG. 2. The compressor blade 10 comprises a root 12, a shank 14, a platform 16 and an aerofoil 18. The present invention is also suitable for gamma titanium aluminide gas turbine engine compressor vanes or other gamma titanium aluminide gas turbine engine components. The present invention may also be suitable for gamma titanium aluminide components for other engine, machines or applications.

Claims

1. A method of heat-treating titanium aluminide alloy, the titanium aluminide alloy having a single alpha phase field and being capable of producing a massively transformed gamma microstructure, the method comprising the steps of

(a) heating a titanium aluminide alloy to a temperature above the alpha transus temperature,

(b) maintaining the titanium aluminide alloy at a temperature above the alpha transus temperature in the single alpha phase field for a predetermined time period,

(c) cooling the titanium aluminide alloy from the single alpha phase field to a temperature in the range of 900° C. to 1200° C.,

(d) maintaining the titanium aluminide alloy at the temperature in the range of 900° C. to 1200° C. for a predetermined time period to produce a massively transformed gamma microstructure,

(e) heating the titanium aluminide alloy to a temperature below the alpha transus temperature in the alpha and gamma phase field,

(f) maintaining the titanium aluminide alloy at the temperature below the alpha transus temperature for a predetermined time period to precipitate alpha plates in the massively transformed gamma microstructure such that a refined microstructure is produced in the titanium aluminide alloy,

(g) cooling the titanium aluminide alloy to ambient temperature.

2. A method as claimed in claim 1 wherein in step (b) the predetermined time period is up to 2 hours.

3. A method as claimed in claim 1 wherein in step (f) the predetermined time period is up to 4 hours.

4. A method as claimed in claim 1 wherein step (e) comprises heating the titanium aluminide alloy to a temperature about 30° C. to 60° C. below the alpha transus temperature.

5. A method as claimed in claim 1 wherein step (a) comprises heating the titanium aluminide alloy to a temperature of about 20° C. to 30° C. above the alpha transus temperature.

6. A method as claimed in claim 1 wherein step (g) comprises air-cooling or furnace cooling.

7. A method as claimed in claim 1 wherein step (c) comprises fluidised bed cooling or salt bath cooling.

8. A method as claimed in claim 1 comprising cooling the titanium aluminide to ambient temperature after step (d) and before step (e).

9. A method as claimed in claim 8 wherein the titanium aluminide is cooled to ambient temperature by air-cooling or oil cooling.

10. A method as claimed in claim 1 wherein the titanium aluminide alloy comprises 48 at % aluminium, 2 at % chromium, 2 at% niobium and the balance titanium and incidental impurities.

11. A method as claimed in claim 10 wherein the alpha transus temperature is about 1360° C., step (a) comprises heating to a temperature of 1380° C., step (b) comprises maintaining the titanium aluminide alloy at a temperature of about 1380° C. for about 1 hour, step (c) and (d) comprise salt bath, or fluidised bed, cooling the titanium aluminide alloy from a temperature of 1380° C. to a temperature between 900° C. and 1200° C. and maintaining the titanium aluminide alloy at the temperature in the range of 900° C. to 1200° C. for a predetermined time period to produce a massively transformed gamma microstructure, steps (e) and (f) comprise heating the titanium aluminide alloy to a temperature of about 1320° C. for about 2 hours to precipitate alpha plates in the massively transformed gamma microstructure such that a refined microstructure is produced in the titanium aluminide alloy, and step (g) comprises air cooling the titanium aluminide alloy to ambient temperature.

12. A method as claimed in claim 1 wherein the titanium aluminide alloy comprises 46 at % aluminium, 8 at % niobium, up to 0.07 at % carbon and the balance titanium and incidental impurities.

13. A method as claimed in claim 12 wherein the alpha transus temperature is about 1335° C., step (a) comprises heating to a temperature of 1360° C., step (b) comprises maintaining the titanium aluminide alloy at a temperature of about 1360° C. for about 1 hour, steps (c) and (d) comprise salt bath cooling, or fluidised bed cooling, the titanium aluminide alloy from a temperature of 1360° C. to a temperature between 900° C. and 1200° C. and maintaining the titanium aluminide alloy at the temperature in the range of 900° C. to 1200° C. for a predetermined time period to produce a massively transformed gamma microstructure, steps (e) and (f) comprise heating the titanium aluminide alloy to a temperature of about 1300° C. to about 1320° C. for about 4 hours to precipitate alpha plates in the massively transformed gamma microstructure such that a refined microstructure is produced in the titanium aluminide alloy, and step (f) comprises air cooling the titanium aluminide alloy to ambient temperature.

14. A method as claimed in claim 1 wherein the titanium aluminide alloy consists of 45-46 at % aluminium, 8 at % niobium, up to 0.07 at % carbon and the balance is titanium and incidental impurities.

15. A method as claimed in claim 1 wherein the titanium aluminide alloy consists of 45-46 at % aluminium, 2-6 at % niobium, 2-6 at % hafnium and the balance is titanium plus incidental impurities.

16. A method as claimed in claim 1 wherein the titanium aluminide alloy is a cast titanium aluminide component.

17. A method as claimed in claim 1 wherein comprising hot isostatic pressing of the cast titanium aluminide alloy component.

18. A method as claimed in claim 17 wherein the hot isostatic pressing of the cast titanium aluminide alloy component is concurrent with step (f).

19. A method as claimed in claim 17 wherein the hot isostatic pressing comprises applying a pressure of about 150 MPa for about 4 hours.

20. A method as claimed in claim 1 wherein the titanium aluminide alloy is a compressor blade or a compressor vane.