The grain boundary state (the activation enthalpy for grain boundary self-diffusion and free energy) and its effect on deformation behaviour have been investigated in ultrafine-grained (grain size, 1.0-1.8 μm) type 316 austenitic stainless steel produced by repeated cold-rolling and annealing treatments, without and with the addition of about 200 ppm B. It is shown that the grain boundary state is not affected by the boron addition, probably because the total volume fraction of carbides formed is so large that most of the boron atoms are tied up as M23(C, B)6carbides. Thus the segregation effects of boron at grain boundaries cannot be responsible for the deformation behaviour and mechanical properties of ultrafine-grained steel. The Lüders-type deformation is observed to occur in boron-free type 316 stainless steel. Experimental data suggest that the occurrence of this non-homogeneous deformation is related to the number of particles (carbides) precipitated at triple points and grain boundaries and the transition for non-homogeneous deformation lies somewhere between 1.0 × 106and 1.5 × 106precipitates mm-2of a grain boundary surface. A framework for the model of non-homogeneous deformation based on the mechanism of the locking of dislocation sources at triple points and grain boundaries by the precipitates is discussed. © 1985.
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