PRECIPITATION AND GROWTH OF DELTA-PHASE (NI3NB) IN A NI-15CR-8FE-6NB ALLOY

The precipitation and growth behaviors of delta phase in Ni-base super alloy, modified Inconel X-750 type alloy (X-750M), were investigated m ainly by optical and transmission electron microscopy. The strengtheni ng phase in the aged X-750M was metastable gamma'' phase. The gamma'' phase changed gradually into the stable delta phase during aging. The orientation relationship between gamma and delta phase in the cellular and the Widmanstatten {111}(gamma)//(010)(delta), [1(1) over bar0$](7 )//[100](delta). The interlamellar spacing of the delta phase in each precipitation type was inversely proportional to the degree of underco oling below the equilibrium transformation temperature of gamma phase to delta+gamma phases. According to the theory of diffusion controlled eutectoidal growth, the equilibrium transformation temperatures of th e cellular and the Widmanstatten precipitation were estimated to be 1 279 and 1 313 K, respectively. These temperatures were consistent with the experimental tendency.