Correlation between imposed deformation and transformation lattice strain on α variant selection in a metastable β-Ti alloy under isothermal compression

Abstract

Many phase transformations produce crystallographic variants. Under a mechanical constraint, certain variants could be selected. Although efforts have been made on resolving the selection rule, the transformation lattice deformation associated selection mechanism has not been well addressed. Thus in the present work, α variant selection of the β to α transformation in a metastable β-Ti alloy under compression was studied. Results show that the selection of the α variants is strongly affected by the imposed strain and the applied load with dependence on the local crystal perfection of the β grains. In the slightly deformed β grains, 2 Burger orientation relationship (BOR) variants forming ‘cross-shaped’ clusters and interrelated by a 90° rotation around the <1 1.38 0>α axis are selected and form in large quantities (group I variants). Such variants consume the maximum deformation work by the applied load. This energy consumption is rooted from the maximum strain contribution of the selected variants to the macroscopic deformation and the maximum shear stress from the external load resolved on their systems for transformation. In the heavily deformed β grains occupied by dislocation slip bands, several numbers (2–4) of BOR variants are selected but form in much less quantities (group II variants). The selection energy criterion is still obeyed by the group II variants but with restriction from the local deformation. The present work provides clear information on the interweaving of the imposed compression with the internal lattice deformation and its impact on the β to α transformation variant selection.