Determination of the structure of α-β interfaces in metastable β-Ti alloys

The crystallography and structure of the interfaces formed between refined hexagonal close packed (hcp) alpha precipitates and the body centered cubic (bcc) beta matrix in a solution-treated and quenched metastable beta titanium alloy, Ti-5Al-5Mo-5V-3Cr (wt%, Ti-5553), were investigated after heating to 600 degrees C and subsequently aging for 0 and 30 min, respectively. The two phases adopted the classical Burgers orientation, and the observed interfacial structures were analyzed using a topological model, i.e. they contained disconnections, as well as beta crystal lattice dislocations acting as a lattice invariant deformation (LID). The structure of the alpha/beta interface was observed along two mutually perpendicular directions, in high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Viewed along the [0001](alpha):[110](beta) directions, an array of disconnections was observed to accommodate the misfit between the alpha and beta crystals. While the spacing of these disconnections varied, the step height associated with them was always twice the {112}(beta) interplanar spacing, and their Burgers vector b was [-0.0190.019 0.034](beta). When the interface was viewed along the [(11) over bar 20](alpha) : [110](beta) direction, in the sample aged for 0 min at 600 degrees C the misfit between the two lattices along [0001](alpha):[110](beta) was so small that no misfit or lattice invariant dislocations were observed within the field of view, in contrast with the interface structure reported in other more conventional titanium alloys. After aging the sample for 30 min, however, the interfacial structure equilibrated, and crystal lattice dislocations were observed on the terrace planes, accommodating the LID, with b = 1/2[(111) over bar](beta). Growth of the alpha precipitates is shown to occur principally by the emission of disconnections from the tip of the existing alpha precipitates into the beta matrix, and its resulting conversion to alpha. These results, combined with analysis using the topological model, show that the nucleation and growth of the hcp alpha phase from the bcc beta matrix in this alloy can be described in terms of the motion of disconnections, accompanied by the displacive diffusion necessary to accommodate the differences in composition between the alpha and beta phases. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.