Design of heat-treatment and its effect on Ni-rich matrix of a W-Ni-Co alloy

Abstract

The aim of this investigation is to optimize the heat-treatment for tailoring a suitable microstructure, especially that of the matrix phase in a W-Ni-Co alloy. Different microstructural features that have been focused on are grain size, fine W particles, intermetallics and short-range ordering. 92W-5.3Ni-2.7Co alloy prepared through powder metallurgy route was imparted a cyclic heat-treatment to obtain a microstructure comprising finely distributed tungsten particles in an FCC matrix that envelopes the large tungsten grains. While the fine W particle influences the mechanical behaviour of W-Ni-Co alloys, the grain size of the matrix and short-range ordering are also expected to play a significant role. The effect of different heat-treatments on the microstructure of the matrix was studied using transmission electron microscope (TEM) and electron back scatter diffraction (EBSD). It is observed that increasing the quenching rate diminishes short-range ordering in the matrix. The formation of intermetallic phase at intermediate temperature range (between 800 °Cand 900 °C) and its subsequent dissolution at higher temperature influences the final microstructure of the alloy. In the present study, the effect of the intermetallic phase on the formation of randomly orientated fine equiaxed grains in the matrix has been elucidated.