Nickel-Tungsten Composite-Like Microstructures Processed by Spark Plasma Sintering for Structural Applications

Abstract

In order to specifically improve their microstructures and enhance their mechanical properties, spark plasma sintering (SPS) has been employed to fabricate tungsten compacts and nickel-tungsten alloys, designated as Ni-xW where x corresponds to the weight percent of W. Pure dense α-W samples having relative densities above 94% and fine-grained microstructures have been processed by SPS at 1400 °C from a monomodal ultrafine-grained W powder. Taking advantage of the tungsten oxide melting which induces a fast densification and the cleaning effect that occurs during SPS, fully dense α-W compacts (relative density of 99%), having a multimodal microstructure, have then been successfully fabricated from a nanopowder obtained by self-propagating high-temperature synthesis. Another way to process dense W (>97%) with enhanced mechanical properties is to alloy W with Ni (4 wt%, Ni-96W) in order to activate its sintering. Besides, in the richer Ni part of Ni-W alloys, a wide range of full dense samples were obtained: Ni-10W, Ni-30W, Ni-50W, and Ni-65W. The mass density enhancement resulted from plastic flow of Ni at high temperature and enhanced diffusion processes. SPS is fast enough to avoid the formation of brittle intermetallic compounds usually observed during conventional sintering routes, and it also promotes W diffusion which leads to the formation of a Ni(W) solid solution matrix in which ultrafine-grained W powder are scattered, thus enhancing mechanical properties.