Decomposition of sigma phase in a NiCrW system

Scripta

METALLURGICA

V o l . 17, pp. 4 7 5 - 4 7 8 , 1983 Printed in t h e U . S . A .

DECOMPOSITION

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Pergamon P r e s s Ltd. All rights reserved

A Ni-Cr-W

SYSTEM

Makoto KIKUCHI,

Masanori KAJIHARA*, Yoshikuni KADOYA*, and Ryohei TANAKA Department of Metallurgical Engineering Tokyo Institute of Technology, Tokyo, Japan * Graduate Student, Tokyo Institute of Technology (Received (Revised

December February

6, i,

1982) 1983)

Introduction Superalloys based on a Ni-Cr-W ternary system have been developed as a government project in Japan to fulfill the requirement of heat exchanger tubings used at around i O00°C in a very high temperature gas-cooled reactor (1-3). The detailed knowledge of the phase equilibria in the Ni-Cr-W ternary system has been needed to give a sound basis for the superalloy development. Unfortunately, however, only very limited information is available on the phase equilibria in this ternary system (4-6). We have, therefore, continued a series of studies on the phase equilibria in both Ni-Cr-W and Ni-Cr-W-C systems to satisfy the urgent need for the better selection of composition of Ni-Cr-W base superalloys (7-15). The existence of ~ phase in the Ni-Cr-W system was first reported by Kuo (16). Since then, a few papers referred to the observation of ~ phase in this system (4,17,18). The phase field of ~ phase, however, has not yet been determined. During the course of the experimental determination of the phase equilibria in this system in the temperature range between i 000 and 1 200 °C, we have determined the phase field of ~ phase in this system (19,20). The temperature dependence of the ~ phase field in this temperature range strongly suggests that ~ phase becomes unstable below 950°C. This report presents a preliminary peratures lower than i O00°C.

observation

of the decomposition

of @ phase at the tem-

Experimental A Ni-Cr-W alloy was prepared from three elemental metals with purity higher than 99.9 per cent. The nominal composition of the alloy was 28 wt pct Ni - 47 wt pct Cr - 25 wt pct W. Argon arc melting was carried out to produce a 20 g button ingot. Arc melting process was divided into two steps. In the first step, nickel and tungsten were melted to make a binary master ingot. In the second step, the master alloy together with chromium were melted to produce the ternary alloy. Ingot yield was 99.95 per cent. The button ingot was homogenized in purified hydrogen gas at 1 250°C for 50 h followed by furnace cooling. Plate specimens with 5 mm in thickness were cut from the homogenized ingot, sealed into evacuated silica capsules and aged at 800 and 900°C for 1 780 h. The distribution of phases in the aged specimens was examined by optical as well as back scattered electron metallography. Phase identification was carried out by means of powder X-ray diffraction method. Results and Discussion Figures l(a) and (b) show the single phase field of ~ phase determined experimentally in the temperature range between i 000 and 1 200°C (19,20). Figure l(a) shows the overlapped projection of the ~ phase field at 1 000, 1 i00 and 1 200°C. Figure l(b) shows a vertical section of the phase diagram cut out in the position indicated in Fig. l(a). In these figures, the three terminal solid solution of nickel, chromium and tungsten are designated as y, al and ~2, respectively, according to Kornilov and Budberg (4). The ~ phase field becomes narrower with decreasing temperature. Extrapolation of the phase boundary to a temperature range lower than i 000aC predicts decomposition of @ phase into three other phases at a temperature slightly higher than 950°C.

475 0036-9748/83/040475-04503.00/0 Copyright (c) 1 9 8 3 P e r g a m o n Press

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The nominal composition of the 28 wt pct Ni - 47 wt pct Cr - 25 wt pct W alloy is indicated in Fig. l(a). From these figures, the alloy is expected to become a single phase of o at 1 250°C. The slight decrease in chromium and tungsten, however, might bring the alloy into the o + y region at this temperature. This is actually the case. Figure 2 shows a back scattered electron micrograph of the specimen homogenized at 1 250°C for 50 h, followed by furnace cooling. The microstructure consists of o phase with a small amount of y phase having a black contrast. It should be noted that o phase is retained even after furnace cooling. No evidence of decomposition of o phase was found in this microstructure. The composition of o and y phases was measured by means of an electron probe microanalyzer. The composition of o and ¥ phases was 28.5 Ni - 46.5 Cr - 25.0 W and 48.6 Ni - 41.4 Cr - i0.0 W, respectively, in weight per cent. After 1 780 h aging at 800 and 900°C, o phase decomposed into three other phases as predicted from the phase diagram. The X-ray powder diffraction pattern of the aged specimens showed ~, ~I and ~2 phases. No trace of o phase was found in the diffraction pattern. The lattice parameters of these phases are tabulated in Table 1. TABLE i Lattice Parameters Temperature

of Decomposition Lattice Parameter

Products (nm)

(°C)

Y

~,

800

0.3591

0.2897

0.3153

900

0.3597

0.2900

0.3152

~2

The lattice parameter of nickel ( a = 0.3524 n m ) increases with increasing chromium and tungsten concentration (21). The dilatation of the lattice due to tungsten is substantially larger than that due to chromium. The lattice parameter of chromium ( a = 0.28829 nm ( 2 2 ) ) increases with increasing tungsten, but decreases with increasing nickel concentration (21). The lattice parameter of tungsten ( a = 0.316522 nm ( 2 3 ) ) decreases with increasing chromium (21). The effect of nickel on the lattice parameter of tungsten has not been reported. The solute concentration in each phase can be qualitatively estimated from the above observations. The lattice parameters of Y nickel phase formed both at 800 and 900°C are substantially larger than that 6f pure nickel. This is mainly due to a large concentration of tungsten. The lattice parameter of ~ phase at 800°C is smaller than that at 900°C. This is due to lower concentrations of both chromium and tungsten at lower temperature. The lattice parameter of ~I chromium phase formed at lower temperature is again smaller. This is due to a lower concentration of tungsten. The lattice parameter of ~2 tungsten phase does not depend on the reaction temperature, but it is substantially smaller than that of pure tungsten, This is mainly due to solution of chromium in tungsten. Figure 3 shows a microstructure decomposed at 900°C for 1 780 h. This micrograph is a back scattered image taken at 25 kV. White, gray and black contrast phases are ~2, Y and ~l, respectively. Most of the area in this micrograph, which had been originally o phase before aging, was composed of three phases intermixed with one another. The size of each phase is of the order of i ~m. In some parts, the y, ~I and ~2 phases are arranged to form three alternate layers. The circular area about i0 ~m in diameter, where no ~2 phase was observed, was originally y phase. Only ~I chromium phase precipitated in this area. The microstructure developed at 800°C was finer than that shown in Fig. 3. The size of each phase was about 0. i ~m, that is, about an order of magnitude smaller than the size of the phases in Fig. 3. Conclusions In heat-resisting steels and superalloys, formation of o phase should be avoided, because it causes severe embrittlement. In the early stage of development of Ni-Cr-W base superalloys, it had been suspected that increasing tungsten concentration for higher strength led to o phase formation. Actually, however, it turned out that ~2 tungsten phase instead of O phase formed in high tungsten alloys, The ~2 tungsten phase precipitated at grain boundaries was reported to increase both high temperature creep strength and rupture elongation (8). As shown exper-

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imentally in this paper, no o phase exists as an equilibrium phase in the Ni-Cr-W system below 950°C. The alloys without O phase above 950°C are not prone to o phase formation below 950°C. Acknowledsement The authors are grateful to Dr. T. Naito and Mr. M. Ogata at Komatsu Manufacturing Co. for their provision of arc-melting facilities. This work is partially supported by the Grant-inAid for Scientific Research from the Ministry of Education of Japan. References i. 2. 3. 4. 5. 6. 7. 8. 9. i0. ii. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.

T.Sugeno, K.Shimokawa and K.Tsuruoka, Iron Steel Engineer 53-1, 40 (1976). K.Shimokawa, Trans. Iron Steel Inst. Japan 19, 291 (1979). R.Tanaka and T. Matsuo, Tetsu-to-Hagane (J. Iron Steel Inst. Japan) 68, 226 (1982). I.I.Kornilov and P.B.Budberg, Zhur. Neorg. Khim. 2, 860 (1957). H.Buckle, Rev. Met. 54, 9 (1957). T.Margaria, C.Allibert ~nd J.Driole, J. Less-Common Metals 53, 85 (1977). M.Kikuchi, S.Takeda, M.Kajihara and R. Tanaka, Tetsu-to-Hagane (J. Iron Steel Inst. Japan) 64, 1622 (1978). R. Tanaka, M. Kikuchi, T.Matsuo, S.Takeda, H.Nishikawa, T.Ichihara and M.Kajihara, Superalloys Proc. 4th International Symposium Superalloys, 481 (1980). M. Kikuchi, S.Takeda, M.Kajihara and R.Tanaka, Report of the 123rd Committee on Heat-Resisting Metals and Alloys 19, 141 (1978). M.Kajihara, M.Kikuchi and R. Tanaka, ibid. 21, 147 (1980). M.Kikuchi, M.Kajihara, Y.Kadoya and R. Tanaka, ibid. 22, 31 (1981). M.Kajihara, M. Kikuchi and R. Tanaka, ibid. 22, 339 (1981). M. Kikuchi, M.Kajihara, Y.Kadoya, H. Usuki and R.Tanaka, ibid. 23, 181 (1982). S.Takeda, M.Kajihara, M.Kikuchi and R. Tanaka, ibid. 19, 243 (1978). M.Kajihara, Y.Kadoya, S.Takeda, M.Kikuchi and R. Tanaka, ibid. 22, 165 (1981). K.Kuo, Acta Met. i, 720 (1953). H.J.Goldschmidt, J. Less-Common Metals 2, 138 (1960). F.D. Lemkey, United Technologies Research Center Report, R75-912046-4 (1975). Y.Kadoya, Master of Engineering Thesis, Tokyo Institute of Technology (1982). H.Usuki, Bachelor of Engineering Thesis, Tokyo Institute of Technology (1982). W.B.Pearson, Handbook of Lattice Spacings and Structures of Metals and Alloys, Pergamon Press, London Vol.l (1958), Vol. 2 (1967). S.Mueller and P.Duenner, Z. Naturforsch. 20a, 1225 (1965). W.Parrish, Acta Cryst. 13, 838 (1960).

~ Ni

W\ ~

FIG. l(a) Single phase field of o phase determined experimentally in the temperaturf range between i 000 and 1 200°C. (a) Overlapped projection of o phase field.

-25W

~(

Cr

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o+~,, a, '

'

'

I

'~ i

(i, 2 -

FIG. l(b)

I I

Single phase field of ~ phase determined experimentally in the temperature range between 1 000 and 1 200°C. (b) Vertical section of the phase diagram indicated in (a).

1200 +

c~2+o

o +

11oo o o.

E

/,

1000

,',.,965 oC

",,27 e2+m 9o0

i

A

j

+ Y

i

I

100

L

I

I

I\

50 W(wt%)

FIG. 3

FIG. 2 Back scattered electron micrograph of 28Ni47Cr-25W alloy homogenized at 1 250°C for 50 h, followed by furnace cooling.

Back scattered 47Cr-25W alloy 50 h, followed at 900°C for 1

electron micrograph of 28Nihomogenized at 1 250°C for by furnace cooling, and aged 780 h.