The antimony segregation at grain boundary was observed and the temper embrittlement in titanium-doped nickel-chromium steel was analyzed.It is concluded that the antimony segregation at grain boundary is nonequilibium and the kinetics of temper embrittlement agrees well with those of nonequilibrium antimony segregation at grain boundary.Besides,the mechanism of nonequilibrium antimony segregation at grain boundary proved to be the most satisfactory one among the existing mechanisms to interpret the antimony-induced embrittlement kinetics in the nickel-chromium steel.Based on these,the activation energy and frequency factor of diffusion of antimony-vacancy complexes were obtained according to the concept of critical time in nonequilibrium grain boundary segregation theory.
Intermediate temperature embrittlement(ITE)is a general phenomenon in Ni alloys and recently was interpreted by dynamic strain aging(DSA).The relationship between ITE and DSA was studied by a binary Ni-Bi alloy.The experimental alloy of well-controlled purity was produced by vacuum induction melting and then heat-treated properly.Tensile tests were performed at various tensile temperatures,and the elongation at fracture was used to indicate the ductility.In order to identify the mechanisms of fracture and ITE,fracture morphologies of the samples of low ductility were observed by scanning electron microscopy.According to the tensile ductility,Ni-Bi alloy shows an obvious embrittlement behavior in the intermediate temperature range(700-750 ℃).However,the stress-strain curves of Ni-Bi alloy and the fracture morphologies indicate that DSA does not exist over the whole temperature range.Based on the experimental results and literatures,the interpretation of DSA was then discussed and proved to be invalid for elucidating the general feature of ITE in Ni-Bi alloy and Ni-based superalloys.
