Effect of Silicon on Densification, Mechanical Properties and Isothermal Oxidation Behavior Study of W-10Ni-3Co alloys Fabricated by Powder Metallurgy

Abstract

Tungsten heavy alloys are two-phase alloys consisting of tungsten and a low melting point binder phase such as nickel, iron, copper, and cobalt. These alloys are optimal candidates for industrial, military and high temperature applications due to their high density, strength, stiffness and oxidation resistance. The high rate of oxidation of pure tungsten at elevated temperatures (>1000 C) creates problem especially for applications in gas-metal environments. The addition of oxide-forming alloying elements like silicon (Si), chromium (Cr), and aluminum (Al), result in the growth of a stable, protective oxide scale that prevents oxidation at high temperatures.

In the present study, 1, 5, 10, 15, and 20 wt.% of Si were introduced into W, W-10Ni, and W-10Ni-3Co alloys through mechanical alloying followed by conventional sintering in H2 atmosphere at 1500 ⁰C for 2 hours. The addition of Si into tungsten results in the formation of WSi2 and W5Si3 intermetallic compounds after sintering. The percentage of these silicides increases with increasing the amount of Si addition. Due to the presence of these brittle compounds, the hardness of the material increases, and the compression strength decreases at higher Si percentages (<1 wt. %). The addition of Ni/Co, along with Si lowers the formation of intermetallics, and the compression strength of the tungsten has been retained in alloys up to 1 wt.% of Si. Further, 0.3 wt.% of nano Y2O3 particles are dispersed into W-1Si, W-10Ni-1Si, and W-10Ni-3Co-1Si alloys, and the alloys show higher strength and hardness due to the combined effect of Ni, Co, Si, and Y2O3.

The isothermal oxidation tests were performed on the sintered alloys at 800, 1000 and 1200 ⁰C for 10 hours. The oxidation resistance increases with the silicon percentage in the tungsten due to the formation of SiO2 layer on the surface of the tungsten. The presence of Ni/Co in the alloys provide more resistance to oxidation by forming NiWO4 and CoWO4 oxide layers in addition to SiO2. The addition of up to 1 wt.% Si along with Ni, Co, and Y2O3, show the optimum mechanical properties and oxidation resistance.