Effect of Si addition on Curie temperature and thermal expansion coefficient of (Fe71.2B24Y4.8)96Nb4 Invar bulk metallic glasses

The ultra-low thermal expansion coefficient alpha makes the Fe-Ni Invar alloys useful in various applications. Their low strength and low Curie temperature T-c are, however, limiting factors. Interestingly, some Fe-based bulk metallic glasses (BMGs), with inherent high strength, exhibit the clear Invar effect. In particular, the (Fe71.2B24Y4.8)(96)Nb-4 BMG has the lowest alpha among Fe-based BMGs, but it unfortunately also has the lowest T-c. In this work, silicon was added into this alloy with the aim to elevate T-c while maintaining a low alpha. It was found that when silicon partially substituted boron, T-c did not increase significantly but alpha did, which is not ideal. On the other hand, when silicon partially substituted yttrium and niobium and especially niobium, T-c increased significantly while alpha did not, which is close to the ideal scenario. When 3% of niobium was substituted by silicon, T-c reached the maximum value of 296 degrees C while alpha remained a low value of 7.4 x 10(-6)/degrees C. Comparing to the Fe-Ni Invar alloy, although this BMG has an inferior alpha, it has much higher T-c (+115 degrees C) and strength (similar to 9 times), presenting a potential for application as a new Invar material with moderate (low) thermal expansion, high operating temperature, and high strength