GB2085029A - Heat treatment of titanium alloys - Google Patents

Description

1

GB 2 085 029 A 1

SPECIFICATION Heat treatment

This invention relates to the heat treatment of metals and has particular reference to the heat treatment of titanium near alpha alloys.

5 The search for improved mechanical properties in titanium alloys has normally taken the route of 5

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 10 the law of diminishing returns takes effect. Undoubtedly improvements will occur in the future. 10

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 even hotter regions of the engine to enable weight savings to be obtained. There is, therefore, a great deal of pressure on the 15 metallurgist to improve the balance of metallurgical properties present in the alloy. 15

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.

20 As in the case of many metals titanium exists mainly in two distinct phases, a so-called alpha 20

. phase and a so-called beta phase. The beta phase is more stable at elevated temperatures and the proportions of aipha 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 25 are frequently referred to as beta stabilisers. Certain titanium alloys consist almost completely of alpha 25 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 30 near alpha titanium alloy is an alpha stabilised alloy, that is an alloy containing alpha stabilising 30

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 "weldable" as used herein is not intended merely to refer to the ability of the metal to be 35 welded directly to itself but is intended to refer to the metal being useable in an aircraft engine in the 35 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 40 used herein are weight percentages. The near alpha alloys are conventionally used in the solution 40

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 45 structure of the alloy and the stress relieving treatment stress relieves the alloy from the stresses built 45 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 50 of about 24 hours. By comparison, however, other types of titanium alloys are given a very different type 50 of heat treatment. Thus an age hardenable titanium alloy, such as titanium plus 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 "Duralumin" type precipitate and then a further heat treatment at 475°C for 8 hours to grow the precipitate. The alloy titanium plus 2\% copper is one which contains only beta 55 stabilisers and is normally treated in the alpha plus beta or alpha plus compound regions of the phase 55 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.

60 There are believed to be no commercially used fully beta stable titanium alloys. Experimental 60

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

2

GB 2 085 029 A 2

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 5 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 then stress relieved at a single temperature in the range 500°C to 700°C to give an equiaxed alpha plus transformed beta structure.

10 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 2\% copper type, with that required for another type of alloy, such . as a metastable beta or near alpha alloy.

15 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.

20 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. 25 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 30 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.

35 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.

40 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 45 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.

5

10

15

20

25

30

35

40

45

TABLE I

Stress Relief Treatment

Creep T.P.S., 520°C 310 N.mm"2 100 hrs, %

0.2% PS Nmm"2

UTS Nmm*2

EL5D

%

R in A

%

24 hrs/500°C

888

1016

12

23

11

0.063

*922

1013

11.5

19*

24 hrs/575p C

__

900

1013

7

16

* i

0.119

*936

1018

5

7*

24 hrs/600°C

__

883

999

8

13

l}

0.124

05 CO

o> *

1008

3

8*

*A11 post creep tensile test samples had their surfaces retained.

4

GB 2 085 029 A 4

For each heat treatment pair, the upper line refers to material which has not been creep tested the lower line for material whichhas 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.

5 Table II, below shows that increasing the stress relief time at a constant temperature gives an 5

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

Stress Relief Heat Treatment

Creep T.P.S., 540°C/300 Nmm"2

0.1% PS Nmm"2

0.2% PS Nmm"2

UTS Nmm"2

EL5D

%

R in A

%

100 hr %

300 hr %

2 hrs/625°C

_

845

865

999

14

17.5

j»

0.084

0.256

*913

932

1027

7.5

10*

4 hrs/625°C

_

_

843

867

995

12

14

1 i

0.135

0.305

*917

937

1030

8.5

8.5*

8 hrs/625°C

861

881

1001

11

16

I J

0.164 |

0.351

*926

945

1038

6

7*

* All post creep tensile test samples had their surfaces retained.

6

GB 2 085 029 A 6

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 5 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.

By the present invention there is provided a method 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 10 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 15 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 + 1 5°C or + 10°C or ± 20 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 25 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 temperture 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 30 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 35 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.

By way of example embodiments of the present invention will now be described by way of 40 example only with reference to the accompanying drawings of which:

Figure 1 is a graph of total plastic strain TPS against primary heat treatment hrs/°C;

Figure 2 is a graph of reduction in an area percentage against primary heat treatment hrs/°C;

Figure 3 is a graph of 0.2% proof stress and elongation against primary heat treatment hrs/°C;

Figure 4 is a graph of 0.2% proof stress and elongation against secondary heat treatment hrs/°C;

45 and

Figure 5 is a graph of reduction in area against secondary heat treatment hrs/°C.

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 50mm diameter and were of a sufficient length to permit conventional tensile test 50 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.

5

10

15

20

25

30

35

40

45

50

TABLE III

Effect of Prolonged Heat Treatment and Duplex Heat Treatment on Tensile Properties of 5331S 50mm Bar Solution Treated 1040°C/2hr OQ (Oil Quenched)

Specimen Number

Heat Treatment

0.1% PS Nmm"2

0.2% PS Nmm"2

UTS Nmm"2

EL5D

%

R in A

%

1

625°C/2 hours

852

867

996

13

22

2

560°C/100 hours

871

909

1026

11

16.5

3

560°C/100 hours + 650°C/24 hours

887

907

1010

9

14

4

580°C/100 hours + 650°C/24 hours

892

912

1019

9

13

8

GB 2 085 029 A 8

In the table the "0.1 %PS" refers to the 0.1% proof strength. The "Nmm-2" means Newtons per mm2. The term "UTS" means ultimate tensile strength. The term "EL5D%" refers to the elongation on a gauge length of 5 times the diameter of the sample section. The "R in A%" refers to the reduction in area measured at the break. It can be seen that reducing the stress relief treatment temperature and 5 increasing the stress relief treatment time gives an improvement in the proof strength and tensile 5

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 elongation and reduction in area.

A further thirteen samples of 5331S were taken and solution treated at 1050°C for 2 hours and 10 then oil quenched. After the solution treatment the samples were given a duplex heat treatment and the 10 results are given in Table IV.

TABLE IV

Effect of Duplex Heat Treatment on Tensile Properties of 5331S 50mm </> Bar — Solution Treated at 1050°C/2 hour Oil Quench

Sample

0.1% PS

0.2% PS

UTS

EL5D

R in A

Number

Heat Treatments

Nmm*2

Nmm"2

Nmm"2

%

%

5

625°C/8hr

850

867

1002

12

17

6

425°C/24hr

„ /24hr

869

887

1002

5.

8

7

,, /48hr

868

888

1002

8.5

13

8

625°C/2hr

82Q

850

987

11

20

9

475°C/24hr

,, /8hr

861

875

■'998

11

15

10

,, /24hr

858

880

,998

10

13

11

625°C/2hr

840

863

•998

11

15

12

525°C/2hr

,, /8hr

853

868

1002

12.5

17

13

,, /24hr

861

883

1004

10

12

14

625°C/2hr

848

867

1004

13,5

22

15

525°C/24hr

„ /8hr

855

878

1006

13

22

16

,, /24hr

873

891

1012

12

19

17

525°C/48hr

600°C/100hr

896

918

1016

6.5

9

18

625°C/24hr

884

904

1002

6

9

10

GB 2 085 029 A 10

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.

5 It can also be seen that optimum results appear to follow the duplex heat treatment given to 5

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 solution treatment using a lower temperature first heat treatment followed by 10 extended times at and around 625°C. In a second further stage duplex heat treatments using extended 10 times at 625°C were followed by a further set of heat treatments at lower treatment temperatures. All treatments were carried out on 50mm 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 15 being rejected during the machining operation. It was not possible to carry out the entire programme on 15-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 0.5mm 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 20 the results between the two parts although this in itself is not an essential requirement. The range of 20 heat treatments is illustrated in Tables V to X.

TABLE V

Stress Relief Heat Treatment(s)

Creep @ _ 600° C/200N/mm"2 TPS

0.1% PS Nmm"2

0.2% PS Nmm"2

UTS Nmm"2

EL 5D

%

R in A

%

100hr

%

i*

CO

625°C/2hrs

__

836

859

971

13.5

26.5

(5331S STD (standard)

0.571

1.525

#898

916

994

5.5

10.5#

500°C/24hr + 625°C/8hr

...

850

867

972

14

23.5

0.481

1.480

#903

921

1003

5

9.5#

500°C/24hr + 625°C/24hr

861

881

993

12.5

21

0.597

1.839

#899

914

999

6.5

11.5#

500°C/24hr + 625°C/48hr

_

861

885

982

13

19

0.550

1.717

#893

908

982

6.5

8.5#

500°C/24hr + 600°C/24hr

844

861

967

13.5

23

0.510

1.408

#891

905

991

5.5

9.5 #

500°C/24hr + 650°C/24hr

847

861

959

14

19

0.568

1.854

#899

912

984

4

8.5#

Average (Excl STD)

_

853

871

375

13.4

21.1

0.541

1.660

#897

912

992

5.5

9.5#

#A|| Post Creep Tensile Test Samples had their Surfaces Retained.

TABLE VI

Stress Relief Heat Treatments)

Creep @ 600°C/200N/mm"2 TPS

0.1% PS Nmm"2

0.2% PS Nmm"2

UTS Nmm"2

EL 5D

%

R in A

%

100hr

%

300hr

%

625°C/2hrs

_

836

859

971

13.5

26.5

(5331S STD)

0.571

1.525

#898

916

994

5.5

10.5#

5"IO°C/24hr + 625°C/8hr

...

839

856

963

13

22.5

0.491

1.444

#901

920

998

5.5

8.5#

5lOaC/24hr + 625°C/24hr

854

872

969

15

19.5

0.572

1.845

#889

910

976

5.5

8.5#

5lO°C/24hr -i 625°C/48hr

—

866

883

980

14

21

0.476

1.635

#899

914

998

6

10#

5lO°C/24hr + 600"C/24hr

851

871

978

13.5

23

0.611

1.840

#898

913

995

5

10#

510°C/24hr + 650°C/24hr

855

874

971

9

14.5

0.580

1.964

#898

908

979

5

10#*'

Average (Excl STD)

_

_

853

871

972

12.9

20.1

0.546

1.746

#897

913

989

5.4

9.4#

#A|| Post Creep Tensile Samples had their Surfaces Retained. *Extra heating of 4hrs/600°C on loading for 300hr creep.

TABLE VII

Stress Relief Heat Treatment(s)

Creep @ 600°C/200N/mm"2 TPS ,

0.1% PS Nmm"2

0.2% PS Nmm"2

UTS Nmm*2

EL 5D

%

R in A

%

100hr

%

i*

CO

625°C/2hrs

836

859

971

13.5

26.5

(5331S. STD)

0.571

1.525

#898

916

994

5.5

10.5#

520°C/24hr + 625°C/8hr

_

843

857

957

14.5

23

0.485

1.592

#901

917

989

5.5

8.5#

520°C/24hr + 625°C/24hr

866

882

987

10

21

0.532

1.731

#896

911

991

6

10.5#

520°C/24hr + 625°C/48hr

874

894

991

10.5

21

0.530

1.774

#891

906

994

6

12.5#

520°C/24hr + 600°C/24hr

852

870

980

13

21

0.625

1.880

#892

900

991

3

11#

520°C/24hr+650°C/24hr

864

883

985

9

12

0.505

1.508

#889

912

999

4.5

8#

Average (Excl STD)

.

860

877

980

11.4

19.6

0.535

1.697

#894

909

993

5

10.1#

#AII Post Creep Tensile Samples had their Surfaces Retained.

TABLE VIII

Stress Relief Heat Treatment(s)

Creep § 600°C/200N/mm"2 TPS

0.1% PS Nmm-2

0.2% PS Nmm'2

UTS Nmm"2

EL 5D

%

R in A

%

100hr

%

300hr

%

625°C/2hrs

_

__

836

859

971

13.5

26.5

(5331S STD)

0.571

1.525

#898

916

994

5.5

10.5#

530°C/24hr + 625°C/8hr

—

848

869

973

11

18.5

0.485

1.352

#901

919

1002

4

7#

530°C/24hr + 625°C/24hr

_

850

877

992

14

17

0.426

1.226

#903

917

1008

4.5

7.5#

530°C/24hr + 625°C/48hr

871

890

985

13.5

19.5

0.462

1.456

#901

918

1007

3

8.5#

530°C/24hr h- 600°C/24hr

_

860

881

992

13.5

22

0.511

1.435

#905

921

1003

4

8.5#

530°C/24hr + 650°C/24hr

852

872

974

11.5

15.5

0.470

1.663

#900

816

991

3

7.5#

Average (Excl STD)

856

878

983

12.7

18.5

0,471

1.426

#902

918

1002

3.7

7.8#

#A|| Post Creep Tensile Samples had their Surfaces Retained.

TABLE IX

Creep @ 600° C/200N/mm "z TPS

Stress Relief Heat Treatment(s)

100hr

%

300hr

%

0.1% PS Nmm"2

0.2% PS Nmm"2

UTS Nmm"2

EL 5D

%

R in A

%

625°C/2hrs (5331S STD)

0.571

1.525

836 #898

859 916

971 994

13.5 5.5

26.5 10.5#

540°C/24hr + 625°C/8hr

0.477

1.430

844 #897

865 919

973 1014

14 5

20.5 8.5#

540°C/24hr + 625°C/24hr

0.419

1.250

860 #905

876 920

974 994

12 4

16 6#

540°C/24hr + 625°C/48hr

0.424

1.447

863 #915

883 931

984 1013

13 4

17.5 7.5#

540°C/24hr + 600°C/24hr

0.-475

1.480°

856 #914

874 930

982 1016

13 5

20 7#

540°C/24hr + 650°C/24hr

0.518

1.615*

856 #897

873 918

975 1009

11 5.5

16.5

11#

Average. (Excl STD)

0.463

1.444

886 #906

874 924

978 1009

12.6 4.7

18.1 8#

#A|| Post Creep Tensile Samples had their Surfaces Retained.

"Temperature drop during 300hr creep test to a minimum of 440°C for 6 hours. * Extra heating of 4hrs/600°C on loading for 300hr creep.

TABLE X

Stress Relief Heat Treatment(s)

Creep @ 600 ° C/200N/mm"2 TPS

0.1% PS Nmm"3

0.2% PS Nmm"2

UTS Nmm"2

EL 5D

%

R in A

%

lOOhr

%

300hr

%

625°C/2hrs

___

836

859

971

13.5

26.5

(5331S STD)

0.571

1.525

#898

916

994

5.5

10.5#

550°C/24hr + 625°C/8hr

—

845

868

975

12

17

0.453

1.379

#904

917

1022

5.5

8#

550°C/24hr + 625°C/24hr

__

855

876

979

12

17.5

0.515

1.528

#907

926

1002

6.5

6.5#

550°C/24hr + 625°C/48hr

872

891

995

10

14

0.393

1.132

#915

934

1011

2

5.5#

550°C/24hr + 600°C/24hr

__

_

859

881

998

15

17.5

0.357

1.032°

#915

934

1014

5

8#

550°C/24hr + 650°C/24hr

_

—

858

881

994

10

13

0.384

1.224

#928

937

1031

5.5

7#

Average {Excl STD)

858

879

988

11.8

15.8

0.420

1.259

#914

930

1016

4.9

7 ft

#A|| Post Creep Tensile Samples had their Surfaces Retained.

"Temperature drop during 300hr creep test to a minimum of 440°C for 6 hours.

17

GB 2 085 029 A 17

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/Mmm2. 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 average for 5 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.

TABLE XI(a)

Average of all Results Given

Unexposed Tensile Data

Creep Data 600°C/200N/mm'2 TPS

Tensile Data After 300hr/600"C (Surface Retained)

0.2% PS UTS N/mm"2

EL5D R in A

%

100hr

%

300hr

%

0.2% PS UTS N/mnT2

EL5D R in A

%

A Primary 24hr/ Treatment of 500°C

871

975

13.4

21.1

0.541

1.660

912

992

5.5

9.5

A Primary 24hr/ Treatment of 510°C

871

972

12.9

20.1

0.546

1.746

913

989

5.4

9.4

A Primary 24hr/ Treatment of 250°C

877

980

11.4

19.6

0.535

1.697

909

993

5.0

10.1

A Primary 24hr/ Treatment of 530°C

878

983

12.7

18.5

0.471

1.426

918

1002

3.7

7.8

A Primary 24hr/ Treatment of 540°C

874

978

12.6

18.1

0.463

1.444

924

1009

4.7

8

A Primary 24hr/ Treatment of 550°C

879

988

11.8

15.8

0.420

1.259

930

1016

4.9

7

TABLE XI(b)

Average of all Results Given

Unexposed Tensil&Data

Creep Data 600°C/200N/mm"2 TPS

Tensile Data After 300hr/600°C (Surface Retained)

0.2% PS UTS N/mm"2

EL5D R in A

%

100hr

%

300hr

%

0.2% PS UTS N/mm"2

EL5D R in A

%

A Secondary 8hr/ Treatment at 625°C

864

969

13.1

20.8

0.479

1.442

919

1005

5.1

8.3

A Secondary 24hr/ Treatment at 625°C

877

982

12.6

18.7

0.510

1.570

916

995

5.5

8.4

A Secondary 48hr/ Treatment at 625°C

888

986

12.3

18.7

0.473

1.527

919

1001

4.6

8.8

A Secondary 24hr/ Treatment at 600°C

873

983

13.6

21.1

0.515

1.513

917

1002

4.6

9

A Secondary 24hr/ Treatment at 625° C

877

982

12.6

18.7

0.510

1.570

916

995

5.5

8.4

A Secondary 24hr/ Treatment at 650 °C

874

976

10.8

15.1

0.504

1.638

917

999

4.6

8.7

STD

859

971

13.5

26.5

0.571

1.525

916

994

5.5

10.5

TABLE XII

Stress Relief Heat Treatment(s)

Creep @ 600°C/200N/mm"2 TPS

0.1% PS Nmm-2

0.2% PS Nmm"2

UTS Nmm"2

EL 5D

%

R in A

%

100hr

%

300hr

%

625°C/2hrs

833

853

972

10.5

17

(5331S. STD)

0.578

1.757

#912

922

1010

5.5

8.5#

625°C/8hr + 500°C/24hr

_

889

907

1010

7.5

13.5

0.403

1.251

#921

937

1025

2

6.5#

625°C/8hr+ 5lO°C/24hr

_

891

904

1008

7.5

9

0.470

1.444

#917

937

1010

1.5

5#

625°C/8hr + 520°C/24hr

888

902

1006

7

10

0.407

1.267

#922

936

1011

1

3#

625°C/8hr + 530°C/24hr

895

909

1015

8.5

14

0.462

1.423*

#907

923

1027

2

7#

625°C/8hr + 540°C/24hr

889

908

1014

6.5

10.5

0.409

1.223

#919

933

1013

2.5

4#

625°C/8hrf 550°C/24hr

887

902

1008

7.5

13.5

0.393

1.376

#912

931

1013

3

5#

Average (Excl STD)

890

905

1010

7.4

11.8

0.424

1.331

•#916

933

1017

2

5.1#

#A|| Post Creep Tensile Samples had their Surfaces Retained. 'Extra heating of 8hrs/600°C on loading for300hr creep.

TABLE XIII

Creep @ 600°C/200N/mm*2 TPS

Stress Relief Heat Treatments)

100hr

%

300hr

%

0.1% PS Nmm-2

0.2% PS Nmm'2

UTS Nmm"2

EL 5D

%

R in A

%

625°C/2hrs (5331S.STD)

0.578

1.757

833 #912

853 922

979 1010

10.5 5.5

17

8.5#

625°C/24hr + 500°C/24hr

0.475

1.483

885 #911

904 927

1002 1007

4.5 3.5

10

4#

625°C/24hr+ 5l0°C/24hr

0.463

1.466

900 #910

915 925

1012 1020

4.5 2.5

7.5 6.5#

625°C/24hr+ 520°C/24hr

0.459

1.398

897 #902

913 922

1015 1019

5.5 4

11

6#

625°C/24hr + 530°C/24hr

0.401

1.206

897 #917

915 936

1012 1022

6

3.5

9

8#

625°C/24hr + 540°C/24hr

0.418

1.427

897 #913

918 928

1020 1008

6

3.5

11

7.5%

625°C/24hr + 550°C/24hr

0.513

1.668

904 #913

920 930

1027 1017

4

3.5

8

6.5#

Average (Excl STD)

0.455

1.441

897 #911

914 928

1015 1017

5.1

3.4

9.4 6.4#

#Ail Post Creep Tensile Samples had their Surfaces Retained.

TABLE XIV

Creep § 600°C/200N/mm"2 TPS

Stress Relief Heat Treatment(s)

lOOhr

%

300hr

%

0.1% PS Nmm"2

0.2% PS Nmm "2

UTS Nmm"2

EL 5D

%

R in A

%

625°C/2hrs (5331S STD)

0.578

1.757

833 #912

853 922

979 1010

10.5 5.5

17 8.5%

625°C/48hr + 500°C/24hr

0.494

1.498

901 #913

913 930

1009 1018

5 4

7

7#

625°C/48hr+ 5lO°C/24hr

0.489"

1.480

905 #899

905 920

1021 1003

6

3.5

8.5 8.5#

625°C/48hr + 520°C/24hr

0.481*

1.695

901 #909

917 927

1020 1011

3.5 4

7

7.5#

625°C/48hr + 530°C/24hr

0.483

1.732

898 #899

916 922

1018 1006

5.5 5

6.5 11#

625"C/48hr + 540"C/24hr

0.469

1.561**

901 #910

917 926

1014 1009

5.5 3.5

6.5 8.5#

625°C/48hr+ 550°C/24hr

0.452

1.407

900 #917

916 932

1020 1017

6

3.5

10 6#

Average (Excl STD)

0.478

1.562

901 #908

916 926

1017 1011

5.3 3.9

7.6 8.1#

#A|| Post Creep Tensile Samples had their Surfaces Retained. "Value at 117 hours.

* Extra heating of up to 24hrs/600°C on loading for 300hr creep test. ** Temperature dropped down to 592°C for up to 17 hours.

to OJ

TABLE XV

Average of all Results Given

Unexposed Tensile Data

Creep Data 600°C/200N/mm"2 TPS

Tensile Data After 300hr/600°C (Surface Retained)

0.2% PS UTS N'mm"2

EL5D R in A

%

100hr

%

300hr

%

0.2% PS UTS N/mm"2

EL5D R in A

%

A Primary 8hr/ Treatment of 625°C

905

1010

7.4

11.8

0.424

1.331

933

1017

2.0

5.1

A Primary 24hr/ Treatment of 625°C

914

1015

5.1

9.4

0.455

1.441

928

1017

3.4

6.4

A Primary 48hr/ Treatment of 625°C

916

1017

5.3

7.6

0.478

1.562

926

1011

3.9

8.1

A Secondary 24hr/ Treatment of 500°C

908

1007

5.7

10.2

0.457

1.411

931

1020

3.2

5.8

■A Secondary 24hr/ Treatment of 510°C

912

1014

6

8.3

0.474

1.463

927

1011

2.5

6.7

A Secondary 24hr/ Treatment of 520°C

911

1014

5.3

9.3

0.449

1.453

928

1014

3.0

5.5

A Secon dary 24hr/ Treatment of 530°C

913

1015

6.7

9.8

0.449

1.454

927

1018

3.5

8.7

A Secondary 24hr/ Treatment of 540°C

914

1016

6

9.3

0.432

1.404

929

1010

3.2

6.7

A Secondary 24hr/ Treatment of 550°C

913

1018

5.8

10.5

0.453

1.484

931

1016

3.3

5.8

STD

853

979

10.5

17

0.578

1.757

922

1010

5.5

8.5

O 00

w o

00 U1

o

N>

CD >

NJ CO

24

GB 2 085 029 A 24

Figure 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 5 550°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.

Figure 2 shows that the primary treatment has little effect on the post creep ductility of the 10 material compared to material given the so called standard treatment. In Figure 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/mm2. 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 15 ' 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 Figure 3. The upper series of lines 20 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/mm2 to about 890N mm2 whilst the elongation falls only slightly from about 13% to about 124-%. It is interesting to note that there is only a slight loss of elongation whereas the reduction in area is more significantly 25 affected.

The information given above and illustrated in Figures 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 appear to be present in material given a primary heat treatment of 530°C to 30 540°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 Figure 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 35 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/mm2. There is a small reduction in elongation (the lower left hand graph) as measured in the unexposed condition. The graphs 40 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'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 — seethe right hand pair of graphs. The right hand upper graph shows that "increasing the temperature of 45 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.

Figure 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 relates to alloys which are given tensile 50 tests in the post creep conditions whereas the two upper graphs relates 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 off in the non-creep tested material. Similarly the effects of 55 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 60 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 65 after an 8 hour secondary treatment compared to 0.462 total plastic strain after a 48 hour treatment.

5

10

15

20

25

30

35

40

45

50

55

60

65

25

GB 2 085 029 A 25

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 5 the 0.1% and 0.2% proof strengths are improved for the 48 hour treated material, that there is very little 5 effect on the elongation at 5D or in the R in A figures.

By comprison, 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/mmz. For material treated at 625°C for 8 hours the total plastic strain was found to be 0.164% under the same

10 conditions. Logically, therefore, it would have been expected that the same degradation would have 10 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

15 and XIV show that increasing the temperature from 500°C to 550°C as a secondary age has no 15

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

20 temperatures of about 650°C appear to cause a reduction in properties, possibly resulting from 20

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.

Claims (10)

25 CLAIMS 25

1. 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 400°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

30 place at a temperature lower than the or a subsequent heat treatment. 30

2. A method as claimed in Claim 1 in which the heat treatment takes place at a temperature in the range 450°C to 750°C.

3. A method as claimed in Claim 1 or Claim 2 in which the solution treatment occurs in the beta field.

35

4. A method as claimed in Claim 3 in which the temperature of the solution treatment is in the 35 range 990°C to 1 100°C dependent on the beta transus temperature of the alloy.

5. A method as claimed in Claim 4 in which the alloy is a titanium base alloy containing by weight 5.5% aluminium, 3.5% tin, 3% zirconium, 1 % niobium, 0.25% molybdenum, 0.3% silicon and the solution treatment takes place at a temperature in the range 1 030°C to 1 070°C.

40

6. A method as claimed in Claim 5 in which first heat treatment takes place at a temperature 40 chosen from the group 535°C ± 100°C, 535°C ± 75°C, 535°C ± 50°C, 535°C ± 30°C, 535°C ± 25°C, 535°C ± 20°C, 535°C ± 15°C, 535°C ± 10°C, 535°C ± 5°C and 535°C.

7. A method as claimed in Claim 6 in which the duration of the first heat treatment is selected from the group 2,4, 6, 8, 10, 12, 14, 16, 18,20, 22, 24, 30, 36,48, 50, 72, 100 or 168 hours.

45

8. A method as claimed in Claim 6 or claim 7 in which the second heat treatment takes place at a 45

temperature selected from the group 650°C + 50°C, 650°C ± 25°C, 625°C ± 30°C, 625°C ± 10°C, 625°C, 600°C ± 30°C, 600°C ± 20°C 600°C ± 10°C and 600°C.

9. A method as claimed in Claim 8 in which the duration of the second heat treatment is selected from the group, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 30, 36, 48, 50, 72, 100 or 1 68 hours.

50

10. A method as claimed in Claim 9 in which the alloy is cooled to ambient temperature between 50

the solution treatment and the first of the heat treatments.

Printed for Her Majesty's Stationery Office by the Courier Press, Leamington Spa, 1982. Published by the Patent Office, 25 Southampton Buildings, London, WC2A 1AY, from which copies may be obtained.