Crystallography of surface precipitates associated with shape change in a Ti-5.26 wt.% Cr alloy

The crystallographic features of surface alpha precipitates accompanied by surface tilt(s) in a Ti-5.26 wt.% Cr alloy have been comprehensively studied by transmission electron microscopy of samples prepared using a focused ion beam. For comparison, the bulk precipitates formed far below the free surface in the same alloy have also been examined. It is found that both the surface and the bulk alpha precipitates exhibit a lath-shaped morphology and their habit plane always contains a single set of misfit dislocations with the Burgers vector [1 (1) over bar 1](beta)/2 vertical bar[2 (1) over bar (1) over bar 3](alpha)/6. However, the surface precipitates differ from the bulk ones in terms of their orientation relationship with the matrix, the habit plane and the long axis direction. As a result, the interphase interface between the surface precipitates and matrix contains glissile dislocations and the interface of bulk precipitates is associated with sessile dislocations. Such a glissile interface is one of the major common features of displacive-diffusional and martensitic transformations and can be used to further understand the mechanism of bainitic transformation in steels and other alloy systems.