Ductility at high temperature and creep behaviour of directionally solidified Ni-Cr-base alloys

Abstract

Tests have been carried out at several constant plastic strain rates in the temperature range 500°–900° C on directionally solidified Nimonic 80A and on MAR M 246 in the directionally solidified, single-crystal, and conventionally cast forms. Creep tests have also been completed on these alloys at 750°C for Nimonic 80A and at 850°C for MAR M 246 at stresses between 185 and 309 N mm⁻² and 300 and 500 N mm⁻², respectively. The results show that the ductility of Nimonic 80A was substantially improved by directional solidification compared with the values expected from the wrought material under the same conditions of test. In the case of MARM 246, the directionally solidified material was, in general, more ductile than the conventionally cast, the margin being greatest for the higher temperatures and the slowest rate of strain. The single-crystal material showed only a slight further improvement in ductility at the lowest temperature of test. The creep properties of all three forms of MARM 246 were similar, but the stress dependence of the minimum creep rate was unusually high for all the cast alloys including the Nimonic 80A. In the absence of transverse grain bounderies, fracture in the Nimonic 80A appeared to originate from irregularities in the cast structure, i.e. kinked grain boundaries and micro pores. In the MARM 246, on the other hand, there was little evidence of cavitation, and cracking was often associated with carbide particles. Cracks tended to propagate through the matrix in directionally solidified material or through carbide particles in the interdendritic regions in the conventionally cast form.