GB2440334A

GB2440334A - A method of controlling the microstructure of a metal

Info

Publication number: GB2440334A
Application number: GB0611585A
Authority: GB
Inventors: Aijun Huang; Dawei Hu; Michael Loretto; Xinhua Wu; Junfa Mei
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2006-06-13
Filing date: 2006-06-13
Publication date: 2008-01-30
Also published as: GB0611585D0

Abstract

A method of controlling the microstructure of a metal having at least two phases, a first phase having a lower molar volume than a second phase. The metal is heated to a predetermined temperature, and a predetermined pressure is applied for a predetermined time such that the proportion of the first phase of the metal with the lower molar volume is increased and the proportion of the second phase of the metal is decreased. The predetermined temperature is the same as, or within a predetermined range of the transformation temperature of the first and second phases. The method may be used on gamma titanium aluminide alloys involving a gamma-alpha phase transformation and on beta titanium alloys which involve a beta-alpha phase transformation.

Description

A METHOD OF CONTROLLING THE MICROSTRUCT[JRE OF A METAL

ARTICLE

The present invention relates to a method of controlling the microstructure of a metal article and in particular to a method of controlling the microstructure of a titanium alloy article.

The microstructure of metal articles is conventionally controlled by heat-treating the metal articles at predetermined temperatures for predetermined times.

A titanium alloy Ti64 comprises 6wt% aluminium, 4wt% vanadium and the balance titanium plus incidental impurities. Ti 64 comprises alpha phase and beta phase and is normally heat-treated a 945 C to 985 C for 1 hour, water quenched and aged at 700 C for 2 hours and then air-cooled.

Another titanium alloy Ti6246 comprises 6wt% aluminium, 2wt% tin, 4wt% zirconium, 6wt% molybdenum and the balance titanium plus incidental impurities. Ti6246 comprises alpha phase and beta phase and is normally heat treated at 30 C below the beta transus for 2 hours, fan air * .* ** * cooled and aged at 595 C for 8 hours and air cooled. S...

A further titanium alloy Ti834 comprises 5.8wt% aluminium, 4wt% tin, 3.5wt% zirconium, 0.7wt% niobium and the balance titanium plus incidental impurities. Ti834 S..

* comprises alpha phase and beta phase and is normally heat-treated at 1020 C for 2 hours, oil cooled and aged at 625 C for 2 hours.

Accordingly the present invention seeks to provide a novel method of controlling the microstructure of metal articles.

Accordingly the present invention provides a method of controlling the microstructure of a metal article, the method comprising providing a metal having at least two phases, a first phase of the metal having a lower molar volume than a second phase of the metal, heating the metal article to a predetermined temperature, the predetermined temperature is the same as, or within a predetermined range of, the transformation temperature at which the metal transforms between the second phase of the metal and the first phase of the metal and applying a predetermined pressure for a predetermined time such that the proportion of the first phase of the metal with the lower molar volume is increased and the proportion of the second phase of the metal is decreased.

Preferably the volume change per mole of the phase of the metal multiplied by the predetermined pressure applied is of the same order as the energy change per mole involved in the transformation from the second phase to the first phase.

Preferably the metallic article comprises a titanium alloy article, an iron alloy article or an aluminium alloy IS article.

Preferably the titanium alloy article comprises a gamma titanium aluminide alloy article.

The gamma titanium aluminide alloy article may comprise at least 46at% aluminium. The gamma titanium aluminide alloy article comprises 46at% aluminium, 8at%

S

niobium and the balance titanium plus incidental impurities.

Preferably the gamma titanium aluminide alloy article comprises gamma phase and alpha phase, the gamma phase having a lower molar volume than the alpha phase, the method comprising heating the gamma titanium aluminide alloy to a predetermined temperature, the predetermined temperature is the same as, or within a predetermined range of, the transformation temperature between the alpha phase and the gamma phase and applying a predetermined pressure to the gamma titanium aluminide alloy article for a predetermined time such that the proportion of the gamma phase is increased and the proportion of alpha phase is reduced. Preferably the temperature is 1280 C and the pressure is 15OMPa. Preferably the predetermined time is 4 hours to 6 hours.

Alternatively the titanium alloy article comprises a beta titanium alloy article.

The titanium alloy article may comprise 6wt% aluminium, 4wt% vanadium and the balance titanium plus incidental impurities.

The titanium alloy article may comprise beta phase and alpha phase, the beta phase having a lower molar volume than the alpha phase, the method comprising heating the titanium alloy to a predetermined temperature, the predetermined temperature is the same as, or within a predetermined range of, the transformation temperature between the alpha phase and the beta phase and applying a predetermined pressure to the titanium alloy article for a predetermined time such that the proportion of the beta IS phase is increased and the proportion of alpha phase is reduced. The predetermined pressure is 100MPa and the predetermined temperature is 920 C. The predetermined time is 4 hours to 6 hours.

Preferably the pressure is applied by hot isostatic : ,. 20 pressing. Alternatively the pressure is applied uni-directionally. * 0*

Preferably the metal article is a compressor disc, a turbine disc, a compressor blade, a compressor vane, a turbine blade or a turbine vane.

Preferably the metal article has been thermo-mechanically processed, the metal article has been cast or the metal article has been formed by powder metallurgy.

Alternatively the present invention provides a method of controlling the microstructure of a metal article, the method comprising providing a metal having at least two phases, a first phase of the metal having a lower molar volume than a second phase of the metal, heating the metal article to a predetermined temperature and applying a predetermined pressure such that the predetermined pressure changes the transformation temperature at which the metal transforms between the second phase of the metal and the first phase of the metal, the predetermined temperature is the same as, or within a predetermined range of, the transformation temperature and the pressure is applied for a predetermined time such that the proportion of the first phase of the metal with the lower molar volume is increased and the proportion of the second phase of the metal is decreased.

The present invention will be more fully described by way of example with reference to the accompanying drawings in which:-Figure 1 shows a turbofan gas turbine engine having a metal article according to the present invention.

Figure 2 shows an enlarged view of a metal article according to the present invention.

Figure 3 is a back-scattered scanning electron micrograph of a gamma titanium aluminide article according

to the prior art.

Figure 4 is a back-scattered scanning electron micrograph of a gamma titanium aluminide article according to the present invention.

Figure 5 is a back-scattered scanning electron " micrograph of a gamma titanium aluminide article according

to the prior art.

Figure 6 is a back-scattered scanning electron micrograph of a gamma titanium aluminide article according to the present invention.

Figure 7 is an optical micrograph of a Ti64 titanium alloy article according to the present invention.

A turbofan gas turbine engine 10, as shown in figure 1, comprises in axial flow series an intake 12, a fan section 14, a compressor section 16, a combustion section 18, a turbine section 20 and a core exhaust 22. The turbine section 20 comprises a high-pressure turbine (not shown) arranged to drive a high-pressure compressor (not shown) in the compressor section 16, an intermediate pressure turbine (not shown) arranged to drive an intermediate pressure compressor (not shown) in the compressor section 16 and a low pressure turbine (not shown) arranged to drive a fan (not shown) in the fan section 14.

The compressor section 16 comprises compressor blades 24, as shown more clearly in figure 2. The compressor blade 24 comprises a root 26, a shank 28, a platform 30 and an aerofoil 32. The compressor section 16 also comprises a compressor disc 34, as shown more clearly in figure 2. The compressor disc 34 comprises a cob 36, a web 38 and a rim 40. The rim 40 has a plurality of circumferentially spaced slots 42, or a circumferentially extending slot, to receive the roots 26 of the compressor blades 24.

The compressor blades 24 comprise a gamma titanium IS aluminide comprising at least 46at% aluminium for example 46at% aluminium, 8at% niobium and the balance titanium plus incidental impurities. Alternatively the compressor blades 24 comprise a gamma titanium aluminide comprising 45-46at% * aluminium, 2-6at% niobium, 2-6at% hafnium and the balance *.SI titanium plus incidental impurities. As a further alternative the compressor blades 24 comprise an alpha-beta titanium alloy, Ti64. Ti64 comprises 6wt% aluminium, 4wt% * * vanadium and the balance titanium plus incidental :.: * impurities. S.S * I

The compressor disc 34 comprises Ti6246, Ti834 or Ti64. Ti6246 comprises 6wt% aluminium, 2wt% tin, 4wt% zirconium, 6wt% molybdenum and the balance titanium plus incidental impurities. Ti834 comprises 5.8wt% aluminium, 4wt% tin, 3.5wt% zirconium, 0.7wt% niobium and the balance titanium plus incidental impurities.

The present invention is concerned with a method for controlling the microstructure of metal articles. The method of controlling the microstructure involves the application of pressure during the heat treatment of the metal articles.

The application of pressure during the heat treatment favours the formation of phases with a lower molar volume, or lower atomic volume, in preference to phases with a higher molar volume, or higher atomic volume. Thus, by heating the metal article substantially to a predetermined temperature, the predetermined temperature is the same as, or within a predetermined range of, the transformation temperature at which there is a transformation between the two metal phases and by applying a predetermined pressure it is possible to increase the proportion of the phase with the lower molar volume and decrease the proportion of the phase with the higher molar volume. The molar volumes AV of the phases must differ sufficiently. The pressure P required is within the range of pressures normally used for hot isostatic pressing. The volume change per mole multiplied by the pressure applied must be of the same order of magnitude as the energy change per mole E involved in the phase transformation, e.g. P x = E. Precisely at *.SS S * S'S the actual phase transformation temperature the energies of :. 20 the phases are identical, so the energy change is zero, and a very small value of P x AV is able to alter the phase balance in the metal article. However, in order to have a reasonably wide process window it is necessary that the *SS.

rate of change of the difference in energies of the phases is not very rapid with temperature change, so that typical values of P x AV are able to provide the energy to change the proportions of the phases present in the metal article.

The temperature used during the hot pressing, e.g. hot isostatic pressing, must allow diffusion of the atoms to occur during the hot pressing. The temperature used during hot pressing is at, or close to, the relevant phase transformation temperature and therefore diffusion of the atoms is possible.

All of these conditions can be calculated for most alloys, by a person skilled in the art, and it is straightforward for a person skilled in the art to assess the potential of hot pressing, hot isostatic pressing, for control of the phases in a metal article.

Considered another way, if there is a change in molar volume during a transformation from one phase to another in a metal, or an alloy, the transformation temperature is a function of the pressure, i.e. the pressure changes the transformation temperature. Hence, for a given transformation from one phase to another phase of a metal, or alloy, the extent to which the transformation temperature can be changed depends upon AV, known for a specific phase transformation, and upon the pressure applied. The precise extent of any processing window thus depends upon P x AV and the window extends over a temperature range where P x AV is sufficiently large to IS influence significantly the new equilibrium proportion of the two phases of the metal, or alloy. If at the : ** processing temperature the energy difference between the two phases of the metal, or alloy, is small, comparable S. with P x AV, then it is possible that only the phase with the smaller molar volume will be in equilibrium at the hot pressing pressure and temperature and a single phase structure may be obtained even though the standard phase :.:. diagram suggests that the metal, or alloy, is at a *S*S temperature where it should be a two phase metal or alloy.

As the temperature is changed to be further from the high-pressure transformation temperature so the proportion of the phases of the metal, or alloy, will change. When the temperature is sufficiently far away from the transformation temperature the influence of the pressure is negligible. The magnitude of the observed changes in phase proportion will be determined (a) by the relative molar volumes of the phases at the hot isostatic pressing temperature, (b) by the value of P x LW and (c) by the rate of change of the phase proportions with temperature, i.e. by the slope of the phase boundaries. These three factors are totally general and consideration of these will allow the effectiveness of pressure in influencing the phase balance to be assessed for all alloy systems.

The present invention is applicable to the alloys used for making the compressor blades 24 and also to the alloys used for making the compressor discs 34.

Example 1

Samples of a gamma titanium aluminide alloy comprising 46at% aluminium, 8at% niobium and the balance titanium plus incidental impurities were heated to a temperature above the alpha transus, alpha transus is 1335 C, for example at a temperature of 1360 C, and held at that temperature for about 1 hour. Then the samples were cooled in a bath of molten salt at a temperature of 850 C to form a massively transformed gamma microstructure. All of the samples were then heat treated within the temperature range of the alpha * .

plus gamma two-phase field. The transformation

temperature, the alpha transus, between the alpha phase and the gamma phase is 1335 C at atmospheric pressure. Some of * the samples were heat treated at a temperature of 1280 C for : * 4 hours in a vacuum tube furnace and some of the samples were hot isostatically pressed at a pressure of 15OMPa, at *.S * S a temperature of 1280 C for 4 hours in a HIP vessel. Other samples were heat treated at a temperature of 1280 C for 6 hours in a vacuum tube furnace and some of the samples were hot isostatically pressed at a pressure of l5OMPa, at a temperature of 1280 C for 6 hours in a HIP vessel. All samples were cooled to room temperature from 1280 C at the same rate of about 10 C/hour. The pressure decreases in phase with the temperature in the HIP vessel.

The microstructure of the samples heat treated at a temperature of 1280 C for 4 hours in a vacuum tube furnace is shown in figure 3 and the microstructure of the samples hot isostatically pressed at a pressure of 15OMPa, at a temperature of 1280 C for 4 hours in a HIP vessel is shown in figure 5. It is seen from figures 3 and 5 that the hot isostatically pressed samples have significantly less alpha phase than the other samples. The alpha phase is the lighter phase and the gamma phase is the darker phase in the figures. Thus, the effect of the pressure during the heat treatment is to reduce the amount of alpha phase present in the samples. The microstructure of the samples heat treated at a temperature of 128000 for 6 hours in a vacuum tube furnace is shown in figure 4 and the microstructure of the samples hot isostatically pressed at a pressure of 15OMPa, at a temperature of 1280 C for 6 hours in a HIP vessel is shown in figure 6. It is seen from figures 4 and 6 again that the hot isostatically pressed samples have significantly less alpha phase than the other samples. Thus, the effect of the pressure during the heat : ** treatment is to reduce the amount of alpha phase present in the samples.

S

It is believed that this is due to the fact that the :. 20 alpha phase has a higher molar volume than the gamma phase * (at room temperature) . The molar volume of the alpha phase is about 2% greater than the molar volume of the gamma phase at room temperature and the different coefficients of expansion increase this to about 2.5% at a temperature of 1280 C. Hence, at higher pressures the proportion of alpha phase present in the samples decreases and the proportion of gamma phase increases.

In further tests samples, which had previously been hot isostatically pressed, were heat treated at 1280 C for 2 hours in a vacuum tube furnace and this resulted in an increase in the proportion of alpha phase and a decrease in the proportion of gamma phase in the samples. Samples, which had previously been heat-treated only, were hot isostatically pressed at 1280 C for 4 hours and this resulted in a decrease in the proportion of the alpha phase and an increase in the proportion of the gamma phase in the samples.

The room temperature mechanical properties of the samples following the heat treatments are illustrated in

Table 1.

Treatment 0.2% Proof Stress UTS % Elongation (MPa) (MPa) HIP 4Hrs, 1280 C, 15OMPa 383 460 1 Heat Treat 4Hrs, 1280 C 523 567 0.5 It is clear from Table 1 that the hot isostatic pressing of the gamma titanium aluminide alloy has increased the ductility but reduced the 0.2% proof stress and the ultimate tensile strength of the gamma titanium aluminide.

: .. In the gamma titanium aluminide alloy comprising 46at% * ** S *** aluminium, 8at% niobium and the balance titanium plus *.S* incidental impurities the width of the process window is determined by (a) the value of P x AV, where P is the hot isostatic pressing (HIP) pressure and AV is the volume * * change per mole of the alloy and (b) by the energy difference between the alpha and gamma phases at the processing temperature. This latter factor changes slowly as the temperature is decreased below the alpha transus of 1335 C. The energy change available is about 40J/mole and this corresponds to an effective change in processing temperature of about 15 C at 1280 C but of only about 6 C at 1000 C, which in turn leads to the change in proportion of the two phases, as is evident from the phase diagram (at atmospheric pressure) . It is cleat that the effect of hot isostatic pressing is more pronounced at temperatures closer to the transus temperature.

Example 2

The volume difference between alpha and beta phases in Ti64 is 4.2%, i.e. twice the difference between the gamma and alpha phases in Example 1. Two different samples of Ti64 alloy were hot isostatically pressed at a pressure of 100MPa, at a temperature of 920 C for 4 hours in a HIP vessel to diffusion bond them together. The first sample of Ti64 was in the form of a 20mm thick plate with an oxygen content of about 2000ppm and an aluminium content of 6.5wt% and this has a beta transformation temperature of around 1010 C. The second sample of Ti64 was in the form of 100mm diameter bar-stock with an oxygen content of about l000ppm and an aluminium content of 6wt% and this has a beta transformation temperature of around 970 C. The first sample of Ti64 retained its fine alpha/beta phase grain structure, as shown on the left hand side of figure 7, S.S* *... showing that it was not treated above its beta transus

S *555

temperature. The second sample of Ti64 was clearly raised above its transus temperature because all the alpha grains had been dissolved to permit beta grain growth that subsequently transformed to lamellar alpha/beta structure, as shown on the right hand side of figure 7. This demonstrates that the pressure had reduced the beta transformation temperature of the second sample from 970 C to below the hot isostatic pressing (HIP) temperature of 920 C. The extent of the pressure effect is revealed by the first sample of Ti64 beta transformation not being reduced from 1010 C to below 920 C.

The consequences of this effect is that the alpha phase nucleation and growth temperature is reduced to produce a finer precipitation in a wide range of titanium alloys and in particular beta stabilised titanium alloys.

The present invention is applicable to virtually all alloys as a way of controlling the microstructure, and hence the properties of the metal article. The use of hot pressing, hot isostatic pressing, to control the microstructure of alloys may be used for thermo-mechanically processed, e.g. forged metal articles, powder metallurgy metal articles or cast metal articles. The temperature of the hot pressing, hot isostatic pressing, is determined by the phase diagram, e.g. the phase transformation temperature between the phases. The temperature of hot isostatic pressing is generally much lower than those conventionally used for hot isostatic pressing.

The only alloys where hot pressing, hot isostatic pressing, can not be used to control the microstructure of the alloy is those alloys in which the molar volumes, or atomic volumes, of the two phases are the same or are too similar.

Generally pressures of 5OMPa to 25OMPa, more preferably 100MPa to 200MPa may be used for 2 to 8 hours, more preferably 4 to 6 hours.

:. 20 The present invention is particularly suitable for * titanium alloys, iron alloys e.g. steels or aluminium *** alloys.

: * * The present invention may be used to form phases in S..

alloys, which are at present unknown. Such unknown phases would again have to have small molar volumes, so that P x AV would tend to result in their formation rather than the known equilibrium phases, and predictions concerning them would require significant theoretical insight and fundamental calculations of the molar volume of the hypothetical new phases.

Claims

Claims: - 1. A method of controlling the microstructure of a metal

article, the method comprising providing a metal having at least two phases, a first phase of the metal having a lower molar volume than a second phase of the metal, heating the metal article to a predetermined temperature, the predetermined temperature is the same as, or within a predetermined range of, the transformation temperature at which the metal transforms between the second phase of the metal and the first phase of the metal and applying a predetermined pressure for a predetermined time such that the proportion of the first phase of the metal with the lower molar volume is increased and the proportion of the second phase of the metal is decreased.

2. A method as claimed in claim 1 wherein the volume change per mole of the phase of the metal multiplied by the predetermined pressure applied is of the same order as the energy change per mole involved in the transformation from the second phase to the first phase.

3. A method as claimed in claim 1 or claim 2 wherein the metallic article comprises a titanium alloy article, an . * iron alloy article or an aluminium alloy article.

: * 4. A method as claimed in claim 3 wherein the titanium alloy article comprises a gamma titanium aluminide alloy article.

5. A method as claimed in claim 4 wherein the gamma titanium aluminide alloy article comprises at least 46at% aluminium.

6. A method as claimed in claim 5 wherein the gamma titanium aluminide alloy article comprises 46at% aluminium, 8at% niobium and the balance titanium plus incidental impurities.

7. A method as claimed in any of claims 3 to 6 wherein the gamma titanium aluminide alloy article comprises gamma phase and alpha phase, the gamma phase having a lower molar volume than the alpha phase, the method comprising heating the gamma titanium aluminide alloy to a predetermined temperature, the predetermined temperature is the same as, or within a predetermined range of, the transformation temperature between the alpha phase and the gamma phase and applying a predetermined pressure to the gamma titanium aluminide alloy article for a predetermined time such that the proportion of the gamma phase is increased and the proportion of alpha phase is reduced.

8. A method as claimed in claim 7 wherein the predetermined temperature is 128000 and the predetermined pressure is 15OMPa.

9. A method as claimed in claim 8 wherein the predetermined time is 4 hours to 6 hours.

10. A method as claimed in claim 3 wherein the titanium alloy article comprises a beta titanium alloy article.

11. A method as claimed in claim 10 wherein the titanium alloy article comprises 6wt% aluminium, 4wt% vanadium and

I Ills

the balance titanium plus incidental impurities.

12. A method as claimed in claim 10 or claim 11 wherein the titanium alloy article comprises beta phase and alpha * phase, the beta phase having a lower molar volume than the alpha phase, the method comprising heating the titanium : . * alloy to a predetermined temperature, the predetermined * I s', temperature is the same as, or within a predetermined range of, the transformation temperature between the alpha phase and the beta phase and applying a predetermined pressure to the titanium alloy article for a predetermined time such that the proportion of the beta phase is increased and the proportion of alpha phase is reduced.

13. A method as claimed in claim 12 wherein the predetermined pressure is lOOMPa and the predetermined temperature is 920 C.

14. A method as claimed in claim 13 wherein the predetermined time is 4 hours to 6 hours.

15. A method as claimed in any of claims 1 to 14 wherein the pressure is applied by hot isostatic pressing.

16. A method as claimed in any of claims 1 to 15 wherein the pressure is applied uni-directionally.

17. A method as claimed in any of claims 1 to 16 wherein the metal article is a compressor disc, a turbine disc, a compressor blade, a compressor vane, a turbine blade or a turbine vane.

18. A method as claimed in any of claims 1 to 17 wherein the metal article has been thermo-mechanically processed, the metal article has been cast or the metal article has been formed by powder metallurgy.

19. A method of controlling the microstructure of a metal article substantially as hereinbefore described with reference to the accompanying drawings.

20. A method of controlling the microstructure of a metal article, the method comprising providing a metal having at least two phases, a first phase of the metal having a lower molar volume than a second phase of the metal, heating the metal article to a predetermined temperature and applying a predetermined pressure such that the predetermined pressure changes the transformation temperature at which the metal transforms between the second phase of the metal and the * first phase of the metal, the predetermined temperature is the same as, or within a predetermined range of, the transformation temperature and the pressure is applied for a predetermined time such that the proportion of the first phase of the metal with the lower molar volume is increased and the proportion of the second phase of the metal is decreased.