Precipitation behavior in a Nb-5W-2Mo-1Zr niobium alloy fabricated by electron beam selective melting

Refractory niobium-based alloy Nb521 (Nb-5 W-2Mo-1Zr) was fabricated by electron beam selective melting (EBSM). The microstructure feature of as-built Nb521 alloy was systematically studied in combination with the unsteady thermal cycling inherent in the EBSM process. The content of precipitates gradually increased from the top region to the bottom region for EBSM-built Nb521 samples. Dot-like or rod-like precipitates with different morphologies were distributed inside grains or along grain boundaries in the as-built sample, whereas needle-like precipitates with a large aspect ratio existed in Nb521 alloy ingot. All precipitates in the EBSM-built Nb521 sample mainly contain (Nb, Zr)C and Nb₂C. With prolonging the thermal balance holding time, the elongated precipitates were partially broken, and became fine and uniform. It is expected that the results obtained in this study can lay a solid theoretical and technological basis for the fabrication of refractory Nb-based alloy by the EBSM process.