Interaction between η-Ni3Ti and reversed austenite within Custom 465 stainless steel: experimental evidence and related patents investigation

After various aging treatment conditions, precipitated nano-sized intermetallic compound particles, i.e., eta-Ni3Ti, with a rod-like shape and reversed austenite with various shapes, i.e. lath, blocky, and granular morphologies, were distributed within martensite laths of Custom 465 stainless steel in the present study. Identified via STEM mapping and corresponding diffraction patterns, the precipitates of eta-Ni3Ti had a hexagonal closed-packed (HCP) crystal structure and adopted a Burgers orientation relationship, i.e., {011}(alpha ') // {0001}(eta) ; < 1 (11) over bar >(alpha ') // < 11 (2) over bar0 >eta, with the martensite matrix. Furthermore, the size of the tiny eta-Ni3Ti particles increased with the isothermal aging temperature and time, indicating that the precipitation behavior obeyed the thermal activation mechanism. Despite forming simultaneously upon aging treatment, the formations of reversed austenite and eta-Ni3Ti particles were mutually independent due to the excess content of Ni in the precipitation hardened (PH) stainless steel. In addition, the existence of such reversed austenite also led to the inhomogeneous distribution of dislocations due to the discrepancy in the thermal expansion coefficients of reversed austenite and martensite, which further resulted in the inhomogeneous distribution of eta-Ni3Ti particles and a wider Vickers hardness distribution, especially after aging at low temperature.