Further Studies of Grain-Boundary Hardening in Iron and Iron Alloys

Abstract

Hardening at grain boundaries in irons of various purities and in certain iron-base alloys has been investigated by means of a microhardness technique. An increase in grain-boundary hardness with increasing solute content occurred in Fe–Sn, Fe–Si, and Fe–(P + S) alloys based on zone-refined iron. The boundary hardness approached a saturation value for each system at ∼ 500 ppm (at.) solute and was overwhelmed by solid-solution-hardening at higher solute contents. Similar behaviour was observed with Fe–W alloys (using a less-pure base iron) but in this case the grain-boundary hardening increased to a maximum at ∼ 200 ppm (at.) of Wand then decreased to the matrix value with further solute additions, the normal solid-solution-hardening being detected only at still higher solute additions. Fe–Cr alloys showed no change in grain-boundary hardness with increasing solute content, but the grain-interior hardness decreased slightly to a minimum value at ∼ 1 at.- % Cr. The changes in micro-hardness have been correlated with recrystallization behaviour and electrical resistivity; anomalies found in the measurement of these two properties appear to be associated with the variation of grain-boundary hardening with composition. The results have been used to explain certain features of the boundary-hardening phenomenon and to test the validity of ideas that have been proposed to account for the effect.