Preparation of fine-grained/ultrafine-grained Nb521 alloy with superior mechanical property by friction stir processing

High strength-ductility synergy is difficult to achieve in Nb alloys. Although high strength has been achieved through severe plastic deformation (SPD) technology, led to low ductility in alloys. In this work, FSP technology was applied to treat Nb–5W–2Mo–1Zr-0.1C (Nb521) alloys in preparation of fine-grained (FG)/ultrafine-grained (UFG) Nb521 with excellent strength and ductility. The microstructure evolution and mechanical property improvement mechanism were systematically studied for Nb521 alloy through various characterization pathways. The research results indicated that UFG Nb521 alloy with a grain size of 0.63 ± 0.41 μm can be prepared using low shoulder plunge depth FSP (LPD-FSP), which is the first report of UFG Nb521 alloy. The main reason for the formation of onion rings structure in SZ is the periodic wear of the stirring tool, and the onion rings structure does not cause mechanical damage. The texture formed by Nb521 alloy under different processing parameters is off-axis shear texture, which matches the ideal shear texture of D2 $\left(11\bar{2}\right)\left[111\right]$ after rotation. In addition, this study also elaborated on the refinement mechanism of the second phase particles (Nb, Zr) C in Nb521 alloy during FSP. This study also indicated that the increase in yield strength of FSP samples at room temperature is mainly determined by grain refinement. These findings provided new ideas for the development of high-performance niobium alloys.