Morphology of W fibers and kinetic undercooling in directionally solidified NiAl–W eutectic alloy

Abstract

The relationship between the cross-sectional shape of W fibers and kinetic undercooling in directionally solidified (DS) NiAl–W eutectic alloys was investigated. When the growth rate was less than 8 µm/s, the cross-sectional shape of W fibers was hexagonal (faceted); conversely, when the growth rate was more than 8 µm/s, their cross-sectional shape was elliptical (nonfaceted). Meanwhile, the NiAl matrix and W fibers in DS NiAl–W eutectic alloys presented a particular crystallographic orientation. The crystallographic orientation between NiAl matrix and W fibers was [ $$ \bar{1} $$ 11]NiAl//[200]W in the growth rate of 6 µm/s, and [ $$ \bar{1} $$ 11]NiAl//[ $$ \bar{1} $$ 11]W in the growth rate of 8 µm/s. A critical kinetic undercooling $$ \Delta T_{k,W}^{c} $$ can be used to predict the transition from the faceted to nonfaceted growth of W fibers in DS NiAl–W eutectic alloys. When the kinetic undercooling $$ \Delta T_{k} $$ of the W phase was less than $$ \Delta T_{k,W}^{c} $$ , the W fibers’ growth was faceted with a hexagonal shape, whereas when $$ \Delta T_{k} > \Delta T_{k,W}^{c} $$ , the W fibers exhibited nonfaceted growth with elliptical shape.