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THE CREEP BEHAVIOR OF CR MODIFIED Ni3 Al SINGLE CRYSTALS S.E. Hsu; C.H. Tong; T.S. Lee and T.S. Liu Chung Shan Institute of Science and Technology P.O.BOX 90008-8, Lung-Tan, Taiwan 32526, R.O.C.

ABSTRACT Ni-Al-Cr single crystals with 4 at%Cr and different levels of aluminum content are grown in a directional solidification facility. The crystals are creep tested at temperatures from 7600C to 8600C. Steady state, as well as transient, creep rates are measured. The results show that Cr modified Ni3 A1, when tested along the direction, exhibit remarkably high values of activation energy for creep. Along with evidence from dislocation structures, it is likely that there is a change in the dislocation core structure upon Cr addition.

INTRODUCTION The creep properties of binary and ternary nickel aluminides based on Ni 3 A1 have been studied extensively[1-8]. Ni 3 Al + B was found to have a creep behavior similar to pure metals.[1]. Some ternary Ni-Al-X[4,7,8], on the other hand, have shown large deviations in creep parameters, such as the stress exponent or activation energy, from those of pure metals. It is well known that two phase alloys can have much higher creep parameters than single phase ones. However, it is not clear that other factors such as the dislocation cross-slip tendency, stacking fault or antiphase boundary (APB) energies, which are affected by alloying elements, would also contribute to changes in creep parameters of ordered nickel aluminides. Chromium is of special interest among ternary elements which affect the creep behavior of Ni 3 Al. Additions of Cr were found capable of improving the tensile strengths[9], the rupture strengths [7,10], and also the ductility[10] of Ni 3 Al. However, its presence in nickel base superalloys were found to offset the creep strength improvement by other alloying elements. In earlier reports [7,11], it was found that although there were no apparent differences in stress exponents, 7 - 7' two phase nickel aluminides containing 8 at% Cr had much higher activation energies for creep than that of binary ones. No satisfactory explanation of the results has been proposed so far. In view of the beneficial effects of Cr on tensile properties of nickel aluminides, it is worthwhile investigating the high temperature creep deformation mechanism of Cr modified Ni3 Al.

EXPERIMENTS

Cr

Plate type single crystalline nickel aluminides with 4 at% and different aluminum contents were produced by directional

Mat. Res. Soc. Symp. Proc. Vol. 133. t1989 Materials Research Society

276

solidification process. Chemical compositions of the specimens are listed in Table I. Creep test specimens with gauge dimensions of 15mm x 3mm x 2mm were machined from these plates along directions after a 72 hours homogenization treatment at 10500C. Creep tests were run in air under constant load conditions. Strain rates were measured by LVDTs with a sensitivity better Sample for TEN were taken from tested specimens after than 10-5. 2% deformations along the primary slip plane as indicated by the sli