Plastic yielding and tensile strength of near-micrometer grain size pure iron

In order to investigate the influence on mechanical properties of grain size in the near-micrometer regime, samples of Fe with nearly full density were prepared using the spark plasma sintering (SPS) technique with average grain sizes of 1.0 mu m, 2.6 mu m and 4.0 mu m. In each of these samples the resulting sintered material was in a nearly fully-recrystallized condition with a random texture. The mechanical properties of the samples were examined via tensile testing. A strong dependence of the tensile flow characteristics on the average grain size was observed in this size regime, with the 1.0 mu m grain-size sample showing a very high strength, but no uniform elongation and a large yield drop developed in the 2.6 mu m grain-size sample. Analysis of the grain size dependence of yield strength suggests a positive Hall-Petch deviation from the expected grain boundary strengthening in the SPS Fe samples, assuming a typical range of values for the grain boundary strength (Hall-Petch coefficient). Pre-compression of the 2.6 mu m grain-size prior to tensile testing sample results in a removal of the yield drop and a corresponding large decrease in the yield stress by as much as 145 MPa. The observations suggest that the positive Hall-Petch deviation seen previously in the near-micrometer regime for fully recrystallized Al and Cu may also be present in Fe, and highlight the likely importance of dislocation source hardening in this grain size regime.