The microstructures of Nb–18Si–5Ge–5Al and Nb–24Ti–18Si–5Ge–5Al in situ composites

Abstract

The microstructures and hardness of the as cast and heat treated alloys (at.%) Nb–18Si–5Ge–5Al (ZF8) and Nb–24Ti–18Si–5Ge–5Al (ZF5) were studied. The Ti and Al additions individually or simultaneously had a weak effect on the macrosegregation of Si. The as cast microstructures of both alloys consisted of Nbss and βNb5Si3 with the two phases forming a eutectic. The formation of the latter was sensitive to melt undercooling. Titanium in synergy with Al decreased the Si+Ge+Al concentration of the Nbss+βNb5Si3 eutectic. After the heat treatment (1400°C/100h), in both alloys the βNb5Si3 was partially transformed to αNb5Si3, and small particles of Nbss were dispersed in some of the Nb5Si3 grains. Aluminium and Ge had a strong effect on alloy macrohardness and Nb5Si3 microhardness and Ti on the retention of macrohardness. Titanium and Al in synergy with Ge increased the hardness of the Nbss and Nb5Si3.