NiTi-base intermetallic alloys strengthened by Al substitution

MATERIALS SCIENCE & EKGIllEERING

l

ELSEVIER

Materials Science and Engineering A223 (1997) 36-41

NiTi-base intermetallic alloys strengthened by A1 substitution Y. Koizumi*, Y. Ro, S. Nakazawa, H. Harada Nationa! Research Institute for Metals, i-2-i Se~zgen, Tsz&uba Science City, Ibara/d 305, Japan

Abstract A series of NiTi-base alloys with A1 additions substituting the Ti were designed and evaluated in terms of the microstructure and mechanical properties. It was found that the compression strength is improved drastically by the A1 additions, especialIy when the A1 amount is high enough to precipitate Ni,TiAI (Heuslar compound) phase which is coherent to the NiTi(B2) phase matrix; an alloy with 8.4 tool.% A1 showed compressive yield strengths as high as 2300 and 200 MPa at room and high (1000°C) temperatures, respectively. When the A1 content exceeds 11 tool.%, however, Ni2TiA1 phase started to deposit in a dendritic manner to reduce the strength. Although compressive ductility declined with the increase in A1 content, 5.2% deformation was achieved with the 8.4 moI.% Al-containing alloy at room temperature. © i997 Elsevier Science S.A. Ke).words: NiTi-base alloys, AI substitution: Microstructure

1. Introduction The materials used in jet engine blades and disks, as well as fuselage materials, are subject to extreme temperature gradients. For example, the outer disk material experiences temperatures exceeding 600°C while the inner material may be a relatively cool 200°C. Therefore, the alloys used in these areas must be strong in a wide temperature range. Materials used up to now have been nickel-base superalloys which have shown a high strength at high temperatures but, relatively speaking, have low strength at low temperatures. In particular, their specific weight of 7.9-9.0 is high, causing tremendous centrifugal stress in materials rotating at low temperatures in inner areas; therefore, the use of nickelbase superalloys has reached the limits of its strength. At the same time, these materials also tend to make jet engines heavier. In terms of volume, turbine disks are particularly large, so it is necessary to reduce the specific weight to achieve a reduction in weight. The purpose of this study was to investigate the feasibility of designing and developing new alloys to replace nickel-base superalloys, particularly in turbine disks. This was done by examining the high-temperature strength of nickel-titanium (NiTi) alloys, which have been known as shape-memory alloys. Particular

* Corresponding author. 0921-5093/97/$17.00 Published by Elsevier Science S.A. P[[ S0921-5093(96) 10508-6

attention was paid to the coherent precipitation of Ni~A1Ti [1,2] in alloys produced by substituting a portion of the Ti in NiTi alloys with AI.

2. Experimental A total of seven types of alloys were used in the experiments. Fig. 1 shows the NiA1-NiTi pseudo-binary phase diagram, where the chemical compositions of the alloys examined are shown by arrows; a portion of the Ti in NiTi alloys was replaced with A1 in these alloys. Alloys with higher AI contents are in the twophase region of NiTi-type intermetallic compound (B2 phase or fl, phase) and Ni2A1Ti compound (Heuslertype phase or t3' phase). The actual chemical compositions of each of these alloys are shown in Table 1. Fig. 2 shows a cast specimen that is 50 m m in diameter and approximately 220 m m in length. From about 50 m m from the bottom of the conical section of the ingot, a spark cutter was used to cut off lengthwise (i.e., parallel) sections of (/)3.5 and (}6.5 mm. To remove the processed layers, the surfaces were ground, then two sizes of compression-test specimens were produced, ~b3 × 6 and ~b6 x 12 mm. Compression testing was conducted in the open air with as-cast materials. The compression tests were performed under the following conditions: the ¢3 x 6 m m specimens were at room temperature, while the ~b6 x 12 m m specimens

37

K Koizutni et al. / Materials Science and Eng#zeering A227 (i997) 76-4I

were at 800 and 1000°C. The initial strain rate was 2.8 X 10 - 4 S - i . In addition, parallel sections of the q54 x 20 m m 0% A1 and 4.4% A1 tensile test specimens were made with as-cast materials, and were tested at r o o m temperature and at 1000°C. Furthermore, to investigate the effect of heat treatment, a Micro-Vickers hardness test was conducted after solution treatment in an Ar-sealed quartz tube at 1200°C for 4 h, followed by aging treatment at 8000C for either 10 or 100 h. Microstructural observations were made with a scanning electron microscope (SEM) using as-cast and heattreated materials etched with aqua regia.

Alloys

Ni

Ti

A1

0A1 4.4A1 7.1A1 8.4A1 11.0A1 13.9AI 21.0AI

49.82 50.10 50.14 50.7I 50.81 59.49 49.20

50 18 45.49 42.80 40.86 38.24 35.62 29.82

0.00 4.41 7.06 8.43 10.95 I3.89 20.98

ering the previously mentioned cutting characteristics; when the amount of fl' phase is large, processing methods should be carefully considered.

3. Results and discussion

3.2. Characteristics o f mechanical property

7.I. 3,fachinability andmicrostructure o f as-cast materials

Fig. 2 shows marks from the lathe cutting at the tip of the specimen. As we can see from the photograph, lathe processing is possible for a material containing up to 8.4% A1; however, above 11% A1, cutting becomes difficult. Therefore, about 8.4% is the limit for A1 content in as-cast materials when they are used after machining by a lathe. The processing of specimens in this experiment, such as threaded tensile-test specimens, was made difficult by these types of restrictions. Therefore, a spark cutter was used to make cylindrical compression-test specimens. Fig. 3 shows the microstructure of the as-cast materials. As we can see, the fl' phase deposits as the primary dendrite when the A1 content exceeds 1 i%, and the fl' phase deposition, which makes materials rather brittle, increases as the A1 content is further increased. Therefore, there is a suitable range for the A1 content consid-

18oof

Table I Chemical composition of alloys examined (moI.%)

'

The stress-strain curves obtained from the compression tests at room temperature are shown in Fig. 4. These curves show that the 0% A1 alloy has ductility but not much strength. However, with the increase of the A1 content from 7.1 to 8.4%, strength increased but ductility decreased. Even so, the 8.4% A1 alloy showed 5.2% ductility. The X symbol on the curve in Fig. 4 shows when rupture occurred, while the arrows show when the compression test was suspended. Finally, at room temperature, the 0% A1 alloy in the tensile test showed 5.6% tensile ductility and the 4.4% A1 alloy showed 1.0%. Considering the existence of casting defects in these specimens, as shown in Fig. 3, we can say that this NiTi/Ni2A1Ti alloy system, especially at low A1 content, has basically tensile ductility. According to the results of the compression test at I000°C (see Fig. 5), an increase in A1 content was accompanied by an increase in strength; however, strength declined as A1 content exceeded 11.0%. To compare the results of these compression tests with conventional alloys, the yield stress values of 0.2% for the tensile test for reference alloys are included at the

@ 16oo ,

..

"~

.,_~

~,

,

+~ 1400 NiAI ga x...

&

~- 1200 1000 NiAI

,

/

20 at. % T i

80

40

50 NiTi

Fig. 1. NiAI-NiTi pseudo-binary phase diagram, with the chemical compositions of alloys examined as arrows.

t. I A ±

Fig. 2. Cast specimen, showing cutting marks made by a lathe.

Y. Koizmnz et al. / Materials Science and Etzg#wering A223 (1997) 36-4I

38

7.1A1 =

a~:

!':Kf
o

~ :" :,~:-

.

: -

11 OA1

N

Fig. 3. Microstructures of" as-cast materials.

far right of the figure. Data for the 0% A1 and 4.4% A1 alloys for this yield stress value were obtained (and included in the figure) to compare compression and tensile tests. The yield stress values of 0.2% for the 0% A1 and 4.4% A1 alloys corresponded well with one another. From this we can see that there is about the same degree of strength in these test alloys as there is in

3000 at 293 K

//~#~

mid-grade superalloys, such as U500 and U700. Fig. 6 shows the effect of A1 substitution on strength in the NiTi alloys. As we can see, a yield stress peak of 0.2% appeared in the 1000°C compression test. The region in which this peak appeared was the two-phase region of fl~ + fl'. This closely resembles the strength peak shown by nickel-base superatloys in the two-phase region of 7 + 7 ' [3]. In all of these phenomena, the coherent two-phase structure strengthened depending on the existence of phase boundaries; by comparing

8,4At Tensile property

v

~" 3001a 1273K _

~2000

110A[

U700

z

2°°1/f

0A{

'

~1000

.~_~ o /

O U

~0 I# E /fv o 0

5

10

15

Compressive strain(%)

20

25

0

Aslroloy USO0

/

-"--~21.oAt 4.4A[ ;

T

0 A[;

T

,

,

2 4 6 8 10 12 Compressive strain (%)

W&spaf foy -4.4AI 0AI 14

Fig. 4. Stress-strain curves from compression tests at r o o m tempera-

Fig. 5. Stress-strain curves from the compression test at 1000°C. The

ture.

0.2% tensile strength of commonly used alloys is at the far right.

39

Y, Koizumi et al. / Materials Science and Engineet #zg A223 (i997) 3d-4I 300

400

Compression t e s t at 1000 ° C

Specific E

m 200

I I

P

J I

\

300

8.4AI \

~r

I

P~

100 r'.J o

p~p'

Ip' I

I I

i 10

T 20

-- ~ ~ 4 . 4 A I ....... -'22-"-- z~

P

,\

Rene 95

/

x

X "~. # \ \\ _ 4~

\

o

x xq

-

%

"'J
,

0 0

(at%)

p 200

,

t 400

,

-I 600

each a11oy of single phase constituting these consistent two-phase structures, we can interpret this as an increase in strength. Fig, 7 illustrates the relationship between 0.2% compression strength and temperature. The broken line represents results from our experiments. For comparison, the tensile test data for three reference alloys are shown ( ): (1) fla single phase of an anti-corrosion and anti-abrasion alloy, N T alloy, whose composition is 48.0% Ni, 49.5% Ti, 1.5% Fe and 1.0% Mo [4]; (2) TiA1 alloy, whose composition is Ti and 48 tool.% A1 [5]; and (3) a nickel-base superaltoy, Rene95 [6]. At high temperatures, the test alloys were equal to the TiA1 alloy, while at low temperatures, the test alloys were stronger than any of the other alloys. At room temperature, the

I

0.2% yield stress

(MPa)

\\

2000

\

8.4AI

Z ~ ~

"7.1AI ~

R

,:\

e

n

e

95

1000 4.4A1 "--A --. --. X \ \ x x , , / T i 4 8 A i ~NT(Suzuki

0

0

200

e t a l ) " ,,xx',/~xx,(Huang and Hall)

400

600

800

Temperature

, ~'?~'~ 800

~ 1000

,

1200

02)

Fig. 8. Temperature dependence of specificstrength.

Fig. 6. Effect of AI substitution on compression yieId stress at IO00°C.

A.

,~Al

•

Temperature

AI c o n t e n t

Ti48AI (Huang and Hall)

100

o co

l I

~o

.

200

[]

o

strenqth

1000

12_00

03)

Fig. 7. Temperature dependence of compression yield stress.

strength of the test alloys was twice as high as that of Rene95. Since the TiA1 alloy has a lower specific weight than the test alloy, a comparison of specific strengths (see Fig. 8) shows that the strength of the TiA1 alloy shows a relative increment; nevertheless, at low temperatures, the test alloys are 2.5 times stronger. The figure also indicates that the f12 singlephase 4.4% A1 alloy is stronger than the nickel-base superalloy Rene95 at low temperatures. These data suggest that the test alloys have superior properties at low temperatures. 3.3. Effects o f heat treamzent

The data from compression tests in this study are all from as-cast materials. Therefore, to further improve the strength, the effects of heat treatment were investigated, specifically the effects of solution and aging treatments. Hardness experiments were conducted on alloys that were heated at 1200°C for 4 h and then air cooled (solution-treated alloys), and on those heated at 800°C for either 10 or 100 h and then air cooled (aged alloys). The results are shown in Fig. 9. At 800°C, the 4.4% A1 alloy, which became f12 single phase, did not show any effect from the heat treatment. At the same temperature, the 7.1 and 8.4% AI materials in the two-phase-region of fi2 + fi' did exhibit some effect; however, these effects tended to decrease with aging. Examining the differences in microstructures caused by different aging times using an SEM, we could not confirm any structural differences in the fi2 single-phase region of the heat-treated 4.4% A1 alloy. However, in the heattreated 7.1% A1 and 8.4% AI alloys of the f l a + f l ' two-phase region, the fl' phase began to grow coarse with aging, as shown in Fig. 10. This is believed to have had an effect on hardness. Based on these re-

40

Y. Koizumi et al. / Materials Science and Engineering A223 (1997) 36-41

4. Conclusions

700

u

~

~

8 . 4 A I (~2+~

600 ,....

o

500

6

4.4A1 (~2)

¢o

400

300

t

Ascast

I

I

T

Solution +800~C10h +80~C100h -> A.C. ->A.C. -> A.C.

Heat treatment Fig. 9. Relation between heat treatment and hardness. suits, we can postulate that aging at 800°C for a shorter time may be effective in improving strength. However, the effects of aging should also be investigated at lower temperatures. Finally, thermomechanical treatment, powder metallurgy, and general production methods of superalloys and TiA1 alloys may also be effective in increasing both strength and ductility.

800°C, IOh, AC

An investigation was conducted of the microstructure and strength at high temperatures of a series of alloys made by substituting a portion of the Ti in NiTi intermetallic compound phases (/32 phase) with A1. The results are as follows: (i) By coherent precipitation, the NiTi intermetallic compound (/?2 phase) resulted in a drastic increase in compression strength at both high and room temperatures. In other words, at 1000°C, its compression strength was the same level ( ~ 2 5 0 MPa) as that of such mid-grade superalloys as U500 and U700. Furthermore, at room temperature, indications were that its compression strength ( ~ 2 3 0 0 MPa) exceeded that of the strongest superalloy, Rene95. (2) Ductility at room temperature (compression deformability) declined gradually as A1 content increased, but even in the 8.4% AI alloy, which had a large amount of /?' phase precipitation, compression deformation of up to 5.2% was achieved. (3) When AI content exceeded 11 tool.%, the /?' phase began to produce large amounts of primary dendrites. This was accompanied by a decline in strength at high temperatures.

800°C, IOOh, AC

Fig. 10. Microstructures of solutioned (i.e., 1200°C, 4 h, air-cooled)and aged materials.

Y. Koizumt et al. / Materials Science and Engineering A223 (I997) 36-4I

(4) H e a t treatment through solution and aging may be effective in improving the microstructure and mechanical properties of/72 + f l ' two-phase alloys. (5) F r o m r o o m temperature to about 400°C, the /?2 single-phase alloy (4.4% A1) had a specific strength that was equal to or greater than that for both the TiA1 and nickel-base superalloys. At the same time, there were indications of tensile ductility.

Acknowledgements We would like to express our sincerest gratitude to M r H o r i k a w a and all the other researchers at the F u r u k a w a Denko, Y o k o h a m a Laboratories, for all their help in producing the alloys, We would also like to thank everyone at the Atomic Arrangement: Design

41

and Control for new Materials Project, a J a p a n - U K joint project between the Research Development Corporation of Japan (JRDC) and the University of Cambridge, who cooperated with us in undertaking this study.

References [I] P. Nash and W.W. Liang, Metall. Trans. A, I6 (I985) 319, [2] W.J. Boettinger, L.A. Bendersky, F.S. Biancaniello and J.W. Cahn, Mater. Sci Eng., 9g (1988) 273. [3] H. Harada, M. Yamazaki and Y. Koizumi, Tetsu to Hagane, 65 (1979) 1049. [4t Y. Suzuki. K. Takayanagi, Y. Fujii, T. Kuroyanagi and T. Tsutsui, Titanium 80 Science and Technology: Proc. 4th Int. C o ~ Titanium, 1980, p. 497. [5] S.C. Huang and E,L. Hall, Metall. Trans. A, 22 (199I) 427. [6] H. Hattori and N. Oh1 (IHI), private communication.