EFFECTS OF GRAIN-SIZE AND TEST TEMPERATURE ON DUCTILITY AND FRACTURE-BEHAVIOR OF A B-DOPED NI3AL ALLOY

Effect of grain size on ductility and fracture behavior of boron-doped Ni3Al(Ni-23Al-0.5Hf, at.%) was studied by tensile tests using a strain rate of 3.3×10-3s-1 at temperatures to 1000°C under a high vacuum of -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 sensitive 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.