Microstructure and crystallography of unidirectionally solidified Ni-W eutectic alloy

A Ni-W eutectic alloy was subjected to a process of unidirectional solidification (UDS) by the Bridgman-Stockbarger technique. Three phases were identified by transmission electron microscopy, namely: W fibers, a solid solution matrix of W in Ni, and Ni₄W precipitates of the Dla structure in the matrix. The growth axis of the W-fibers was found to be <111> and the orientation relationship between them and the Ni(N) matrix was identified as the Bain type, so that (100)_bcc ‖ (100)_fcc. The shape of the Ni₄W precipitates varies from equiaxial at high solidification rates to elongated plates at low rates. The orientation relationship between the precipitates and the matrix is the same for all solidification rates. The microstructure of specimens subjected to creep deformation was studied and the deformation modes were identified. These include dislocations and microtwins that originate mainly at the boundary between the Ni(W) matrix and the W-fibers. In some specimens the creep test was carried out after solution treatment at 1030 °C followed by quenching, which resulted in a Ni(W) matrix reinforced with W-fibers without Ni₄W precipitates. The microstructural changes during this creep process and the fracture surface were studied by SEM and TEM.