Effect of Silicon and Y2O3 on Densification, Mechanical Properties and Isothermal Oxidation Behaviour of W-10Ni-3Co alloys Fabricated by Powder Metallurgy

Abstract

Tungsten heavy alloys are highly sought after for industrial and military applications due to their exceptional density, strength, and stiffness. They find utility as kinetic energy penetrators, counterweights, and radiation shields. To enhance densification through liquid phase sintering and reduce the sintering temperature, nickel (Ni) and cobalt (Co) are commonly incorporated into tungsten-heavy alloys. Additionally, mechanical alloying can introduce nano Y2O3 particles into the alloy matrix. This results in a uniformly dispersed fine oxide strengthening (ODS) and an extremely refined grain structure, ultimately leading to increased strength at high temperatures and inhibiting recrystallization. The main objective behind the development of ODS tungsten alloys is to raise the maximum operating temperature. Nonetheless, the susceptibility of tungsten to oxidation at elevated temperatures poses a significant challenge. One potential solution to this critical safety concern involves incorporating oxide-forming alloying elements such as silicon (Si), chromium (Cr), and aluminum (Al), which promote the growth of a stable and protective oxide scale. This oxide scale effectively hinders oxidation at high temperatures. In the present study, we have introduced 1, 5, 10, 15, and 20 wt.% of Si into W, WNi, and WNiCo alloys through planetary milling and conventional sintering in hydrogen gas atmosphere at 1500 ⁰C for 2 hours. The alloy phase evolution and microstructure development have been characterized using XRD, SEM, and TEM, and the hardness and compression strength of the alloys are examined using UTM. The addition of Si into tungsten results in the formation of WSi2 and W5Si3 compounds after sintering. The percentage of these silicides increases with more 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. 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 5 wt.% of Si. However, in the case of high wt.% of Si alloys, even in the presence of Ni/Co due to the more pronounced effect of intermetallics, the alloys become more brittle and subjected to early failure under compression test. Hence, alloys up to 5 wt.% of silicon has been selected as the optimum value. Further, 0.3 wt.% of Y2O3 particles are dispersed into WS1, WNS1, and WNCS1 alloys, and the alloys showed 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 ⁰C, 1000 ⁰C, and 1200 ⁰C for 10 hours. The activation energy of oxidation is increased with the silicon percentage in the tungsten due to the formation of SiO2 layer on the top 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, showed the optimum mechanical properties and better oxidation resistance. Selective alloys are synthesized using SPS at 1300 C for 10 min at 50 MPa pressure to observe the effect of consolidation technique on densification, hardness and oxidation properties of alloys.