Phase transformation induced twinning in commercially pure titanium: An in-situ study

In this work, in-situ high temperature electron back-scatter diffraction (HT-EBSD) was designed to investigate the phase transformation in Grade 1 commercially pure titanium (CP Ti). The transformation from α-Ti to β-Ti during the heating process mostly followed the Burgers orientation relationship (BOR). However, {112¯1}<1¯1¯26>tensile twinning was found to occur during the β-Ti to α-Ti transformation in the continuous cooling process, which led to the formation of Type 2 α-variants that defied the BOR. This was not reported before for CP Ti, while similar previous observations were all associated with Ti alloys containing β-stabilizer(s). We attribute this observation to the relatively fast cooling process (16 °C/s) where the lattice mismatch between the β-Ti and α-Ti phases has introduced a sufficiently large internal stress within a short time. Diffusional-displacive mixed transformation was proposed for the β-Ti to α-Ti transformation, which can reasonably explain the observations in this work.