Development of a new thermo-mechanical processing route for Nb-5Mo-1Zr-0.1C (wt%) alloy

Abstract

In the present work, a new thermo-mechanical processing route with reduced deformation temperature has been developed for Nb-5Mo-1Zr-0.1C (wt%) alloy. For this purpose, the as-cast Nb-5Mo-1Zr-0.1C alloy was processed through two different processing routes: 1) Warm Rolling (WR) and 2) Cold Rolling (CR) routes. In the WR route, the as-cast alloy was deformed by warm rolling (WR) and subsequently recrystallized by heat treating at 1400 °C for 4 h. In the CR route, prior to deformation by cold rolling (CR), the as-cast alloy was heat treated at 1400 °C for 16 h. Detailed microstructural characterization of heat treated samples by electron microscopy revealed that Nb2C carbides, which were present in the as-cast microstructure got dissolved during the heat treatment process and in place excess carbon reprecipitated as (Nb,Zr)C carbides. This change in the carbide phase led to the successful deformation of the heat treated alloy by CR up to 75% of thickness reduction. The CR samples were heat treated at the same temperature of WR samples to get the recrystallized microstructure. The mechanical testing of the recrystallized samples obtained through both the routes (WR & CR) showed similar mechanical properties and it is attributed to nearly identical microstructure consisting of spherical morphology of (Nb,Zr)C carbides in the recrystallized Nb grains. Therefore, the developed low temperature processing route (CR route) can be used as alternative to the WR processing route for bulk scale of fabrication of components of Nb alloy. This study also shows that by addition of 5 wt% Mo in Nb-1Zr-0.1C alloy, the strength of the alloy can be increased to twofold by solid solution strengthening. In addition to the processing, the carbide transformation mechanism in cold rolled route from Nb2C to needle morphology of (Nb,Zr)C and to spherical morphology of (Nb,Zr)C has been studied in detail.