Microstructural studies of a Ni-W directionally solidified eutectic composite

Abstract

The influence of heat treatment on the microstructure of a Ni-45.5 wt pct W directionally solidified eutectic composite has been studied. Four types of microstructural conditions have been examined using light and thin foil electron microscopy techniques. The as-grown composite consists of W fibers in a matrix which contains some WNi₄ precipitates. Quenching the composite from above the peritectoid temperature results in W fibers in a Ni-W solid solution matrix. Subsequent aging results in precipitation of coherent WNi₄. Long time aging at temperatures just under the peritectoid temperature promotes the peritectoid reaction leading to envelopment of the W fibers by a layer of WNi₄. The envelopment reaction is accompanied by a change from faceted to round fiber morphology. The presence of W fibers in the Ni-W solid solution influences aging in two ways when compared to a solid solution alloy. First, the fiber/matrix interfaces act as dislocation sources on quenching; these dislocations are absorbed into the WNi₄ precipitate/matrix interfaces leading to early loss of coherency. Second, the WNi₄ precipitates undergo a preferential coarsening due to elastic interaction between the fibers and the WNi₄ precipitates; this leads to a mosaic structure, the occurrence of which is uniquely dependent on the presence of W fibers.