Crystallographic model for bcc-to-9R martensitic transformation of Cu precipitates in ferritic steel

Abstract

A crystallographic model, which can calculate lattice invariant shear, habit plane and orientation relationship for bcc-to-9R martensitic transformation of Cu precipitates in low-alloyed transformation-induced plasticity (TRIP) steel, is suggested. At the early stage of ageing at 500°C, coherent bcc Cu clusters were formed and characterized by a periodic array of distinguished striations perpendicular to [001]α direction. On prolonged ageing for 10 h, spherical precipitates with banded contrast formed within ferrite matrix and high-resolution transmission electron microscopy (HRTEM) observations confirmed that these bands corresponded to the intervariant boundary of twinned 9R Cu precipitates lying parallel to the {110}α planes and showed an orientation relationship with α: , . Analytical equations for lattice invariant shear and habit plane were derived, and calculated to be 0.09669 and , respectively. The orientation relationship between 9R Cu precipitates and ferrite matrix, calculated using transformation matrix, was and. The deviation angles between the calculated and measured by HRTEM were approximately 0.1° in direction and 3.7° in plane, respectively. The present model successfully described the crystallographic features of bcc-to-9R martensitic transformation of Cu precipitates in ferritic steel.