Ductile-brittle transition of polycrystalline ni3al with varying grain size

It is known that in B-(un)doped NijAl polycrystals, the dependence of yield strength on grain size follows the Hall-Petch relationship: ay= a0 + Kyd~1/2, and the slope Kycan be reduced by B-doping owing to the lowering of grain boundary resistance to slip transmission. If the intergranular cracking in polycrystalline NijAl occurs from the microcavity along the grain boundaries, the effective external tensile stress for the propagation of the crack-like microcavity along the grain boundaries can be deduced as: 01= a-t + Kud~l/2, where Ku reflects the effects of such factors as environment, strain rate, boron doping and the orientation of the grain boundary on the trend of intergranular cracking. For loaded polycrystalline NI3.A1, it should be competitive between the intergranular cracking and slip transmission across the grain boundary. Therefore, comparing the varieties of both ay and GI with grain size, the dependence of ductile-brittle transition on grain size, and the effects of the above factors on ductile-brittle transition can be expected. The model also predicts that there exists a critical grain size for the ductile-brittle transition of polycrystalline NijM alloys, and B-doping can increase the critical grain size due to the reduction of the slope Ky and the increase of Ku. The reported experimental results verified the above model.