Effects of grain size and test temperature on ductility and fracture behavior of A B-doped Ni 3Al alloy

Abstract

Effect of grain size on ductility and fracture behavior of boron-doped Ni 3Al (Ni-23A1-0.5Hf, at.%) was studied by tensile tests using a strain rate of 3.3 × 10 −3 S −1 at temperatures to 1000°C under a high vacuum of < 1 × 10 −4 Pa. At temperatures below 700°C, the alloy showed essentially ductile transgranular fracture with more than 30% elongation whereas it exhibited ductile grain-boundary fracture in the temperature range from 700 to 800°C. In both cases, the ductility was insensitive to grain size. On the other hand, at temperatures above 800°C, the ductility decreased from about 17 to 0% with increasing grain size. The corresponding fracture mode changed from grain-boundary fracture with dynamic recrystallization to brittle grain-boundary fracture. The ductile transgranular fracture at lower temperatures is explained by stress concentration at the intersection of slip bands. The grain-size dependence of ductility is interpreted in terms of stress concentration at the grain boundaries. Finally, it is suggested that the temperature dependence of ductility in this alloy might be related to the thermal behavior of boron segregated to the grain boundaries.