Effect of manganese on the grain boundary network of lath martensite in precipitation hardenable stainless steels

Abstract The fully martensitic microstructures of a manganese bearing precipitation hardenable (PH) stainless steel and a commercial 17-4 PH steel were compared to investigate the influence of chemical composition on the grain boundary network of the lath martensitic microstructure. The martensitic transformation in both steels led to the bimodal misorientation angle distribution having multiple maxima at misorientations in the range of 10–15° and 50–60°. This closely matched with the misorientations between variants associated with the Kurdjumov-Sachs orientation relationship (K-S OR). However, the Mn addition appeared to relatively reduce the area of boundaries with misorientations near 60°. This was a result of the reduction in the area of 60°/[011] intervariant boundaries in the Mn bearing steel. The phenomenological theory of martensite revealed that the Mn addition altered the 3-variant clustering (i.e., V 1 V 3 V 5 ) in 17-4PH steel to the 4-varaint clustering (i.e., V 1 V 2 V 3 V 5 ) arrangement to minimize the strain associated with the displacive martensitic transformation, promoting the population of 60°/[111] intervariant boundaries at the expense of 60°/[011] intervariant boundary. The changes in local variant selection affected the connectivity of the grain boundary network, suggesting that grain boundary network characteristics (i.e., population and connectivity) of the lath martensite microstructure can be manipulated by altering the chemical composition of the steel. The five-parameter grain boundary analysis for both martensitic steels, however, revealed similar grain boundary plane distributions for all boundaries associated with the K-S OR, being terminated on {011} planes due to the constraint that results from the displacive phase transformation.