The effect of the elastic energy on the shape and orientation relations of η-Ni3Ti precipitates in lath martensite

Abstract

Understanding the elastic interaction between the precipitates may assist in controlling their distribution and help design new steels with enhanced mechanical properties. Custom 465® stainless steel contains only η-Ni3Ti precipitates, making it an ideal steel to study the interaction and orientation relation (OR) between precipitates, which are known to grow in arrays parallel to the four closed-packed axes,< 111 >bcc, and closed-packed planes, {110}bcc, of the matrix. This forms 144 possible orientation pairs, raising the question whether there are preferred relations between neighboring precipitates.In this work we tackle this question by calculating the elastic fields and interaction between η-Ni3Ti precipitates in bcc lath martensite in order to study the effect of elastic energy on the shape and orientation of the precipitates. The results of the calculations are compared to transmission electron microscopy (TEM) observations of Custom 465® stainless steel in different aging conditions. The calculations predict that the η-Ni3Ti precipitates will favor an ellipsoidal morphology with a high aspect ratio. Experimentally, needle-shaped precipitates of b/a = 3 are observed in the steel in the peak-aged condition. Careful examination of the 144 possible orientation pairs reveals that they may be classified into five groups of crystallographically independent pairs of precipitates. The interaction energy of two precipitates reaches up to ~ 7% of the self-energy, higher than the typical interaction energy of precipitates in Ni-based superalloys. Here, the ORs between the precipitate is expected to occur in a more sophisticated manner along the four< 111 > branches. We provide evidence that effective interactions exist inside branches and between interpenetrating branches inclined at 109.5°, although they are not very clear since not all of the favored sites are occupied.