Microstructure transition from lamellar eutectic to anomalous eutectic of Nb-Si based alloy powders by heat treatment and spark plasma sintering

With a nominal composition of Nb-18Si-24Ti-2Cr-2Al (at%), pre-alloyed Nb-Si based alloy powders were fabricated by plasma rotating electrode processing (PREP). The phases and microstructures of the pre-alloyed powders subjected to spark plasma sintering (SPS) and heat treatments (HT) at 900 °C, 1000 °C, 1200 °C and 1400 °C for 1 h were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed that main phases of the pre-alloyed powders were Nbss, βNb₅Si₃and Nb₃Si in the as-received state, but evolved into Nbss and αNb₅Si₃after either SPS or HT at >1000 °C for 1 h. The as-received powders processed by PREP were distinguished by novel refined lamellar eutectic structures. The microstructure transition from lamellar eutectic to anomalous eutectic was triggered by either SPS or HT at > 1000 °C for 1 h, accompanied by a significant drop in hardness. The silicides in the anomalous eutectics were coarsened and smoothened after HT at a higher temperature. The corresponding microstructure transition mechanism was discussed.