The microstructure evolution of directionally solidified Nb-22Ti-14Si-4Cr-2Al-2Hf alloy during heat treatment

To obtain a homogenous and optimizing microstructure, non-equilibrium directionally solidified (DS) samples at high withdrawal rate were heat treated (HT). The heat treatments were carried out at 1500 degrees C for 3-100 h. The DS and HT samples consisted of Nb-SS, alpha-Nb5Si3 and gamma-Nb5Si3, and the DS morphology altered significantly after heat treatment. The well developed Nb-SS dendrites were compromised and the size decreased from more than 100 mu m to less than 60 mu m when the heat treated time was 6 h. On the other hand, the Nb-SS in eutectic interconnected and meanwhile connected with the dendrites, and the frequency of small size got lower with the heat treated time increasing. When heat treated for 100 h, the size of Nb-SS was comparatively uniform and concentrate in the region of 5-20 mu m with a value of 63% for Ostwald ripening. The dendric and eutectic morphologies exited in the DS sample disappeared, turning into a net-work structure. The components of primary Nbss dendrites and Nbss in the intercellular regions were homogenized after heat treated for 3 h. However the enrichment of Ti, Cr and Hf in gamma-Nb5Si3 compared to alpha-Nb5Si3 could not be eliminated by homogenizing treatment even for 100 h. (c) 2013 Elsevier Ltd. All rights reserved.