Variant selection in heterogeneous nucleation on defects in diffusional phase transformation and precipitation

In heterogeneous nucleation on lattice defects in the matrix during diffusional phase transformation and precipitation reactions, the variant of product phase with the specific orientation relationship is strongly selected by the nature of the defects. For dislocations, effective accommodation of the transformation strain by the strain field of dislocations occurs, leading to the variant selection in which the direction of the maximum misfit is nearly parallel to the Burgers vector of the dislocations. For high-angle grain boundaries and subgrain boundaries, precipitates tend to select the variant with a low-energy interface (often the parallel close packed planes) inclined at the smallest angle to the grain boundary plane. On the other hand, precipitates that nucleate on the incoherent inclusions embedded in the matrix do not hold any specific orientation relationship with respect to the matrix. It is important to produce many variants locally for the effective refinement of microstructures that improves the mechanical properties of steel and titanium alloys. (C) 2001 Elsevier Science B.V. All rights reserved.