Deformation-induced nonequilibrium grain-boundary phase in submicrocrystalline iron

The shapes of Mössbauer spectra of coarse-grained annealed armco iron and submicrocrystalline (SMC) iron with a grain size of 0.22 μm were compared. The SMC was made from the armco iron by pressure shear and subsequent annealing at 420 °C. We intended to separate an additional subspectrum, which appears as a result of the grain refinement. That subspectrum was attributed to grain-boundary atoms or the “grain-boundary phase” (GBP). The volume fraction of that phase was determined. On the average, the GBP interatomic spacing was suggested to be 1% larger than that of the grain phase (GP) and the presence of considerable compressive stresses in the GBP were deduced. A possible reason for the satellite subspectrum appearing after the grain refinement was that it was attributed to a sudden change in the fundamental characteristics of the solid at a certain critical value of the lattice parameter, which varies continuously in going from the geometrical boundary to the grain center.