Mosaic structure formation due to oriented precipitation of Ni 4W around W fibers in a Ni-W directionally solidified eutectic alloy

Abstract

Systematic dark-field electron microscopie observations have been carried out to study precipitation behavior of an ordered Dl a-Ni 4W (β) phase from a supersaturated Mi-rich solid solution (α) matrix in a Ni-45.5 wt% W directionally solidified eutectic alloy. Three pairs of six orientational variants of the β phase, each pair having the Dl a c-axis parallel to one of the three cube axes of the parent f.c.c. lattice and undergoing a contractive transformation strain of 2% in that direction, are biased upon precipitation by the presence of W fibers that are set in array in an [001] x direction. This biasing is characterized by (1) suppression of a pair of anti-parallel twin related variants with their common c-axis lying along the fiber axis and (2) mosaic-like distribution of the remaining two pairs of variants in domains that fan out from individual W fibers in [100] x and [010] x directions. On the assumption that the ordering transformation proceeds by means of nucleation and growth, the results are explained in terms of elastic interaction between a misfitting β nucleus and an internal stress developed around individual W fibers due to differential thermal shrinkage of the fiber to the matrix. Geometric and crystallographic features of the observed mosaic structure are shown to be correlative to an expected distribution of the thermally-induced stress around a single ‘elliptic’ fiber which is derived from anisotropic elasticity calculations.