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

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 degrees C, coherent bcc Cu clusters were formed and characterized by a periodic array of distinguished striations perpendicular to [001](alpha) 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}(alpha) planes and showed an orientation relationship with alpha: (011)(bcc)//(11 (4) over bar (9R), [1 (1) over bar1](bcc)//[(1) over bar 10](9R). Analytical equations for lattice invariant shear and habit plane were derived, and calculated to be 0.09669 and ( 0: 34179 +/- 0: 80472 0: 48539 )(bcc), respectively. The orientation relationship between 9R Cu precipitates and ferrite matrix, calculated using transformation matrix, was [(1) over bar 10](9R)//[1:00131-1: 00379 1:00186](bcc) and ((11) over bar4)(9R)//(0.07769 1.07769 1.00008)(bcc). The deviation angles between the calculated and measured by HRTEM were approximately 0.1 degrees in direction and 3.7 degrees in plane, respectively. The present model successfully described the crystallographic features of bcc-to-9R martensitic transformation of Cu precipitates in ferritic steel.