Formation of face-centered cubic titanium in laser surface re-melted commercially pure titanium plate

Micron-scale face-centered cubic titanium phase (named as δ phase) were noticed in the re-melted zone of laser surface re-melted commercially pure titanium plate. The morphology, sub-structure, orientation and distribution of δ phase were investigated by scanning electron microscopy, electron back-scattered diffraction and transmission electron microscopy. Three kind formation processes of δ phase were put forward based on the investigation. The first one is α′ → δ transformation which takes place in single α′ grains and leads to the orientation relationship {001}δ//{0001}α′ <110>δ//<112¯0>α^′. The second one is β → α′ + δ transformation which takes place at α′/α′ interfaces and leads to the orientation relationship {001}δ//{11¯0}β 〈110〉δ//〈111〉β. The third one is another kind of β → α′ + δ transformation that takes place at α′/α′ interfaces and leads to the orientation relationship {11¯1}δ//{11¯0}β < 110 > δ// < 111 < β. It is believed that the transformations of δ phase are stress assistant ones and in the present investigation, the phase transformation stress of β → α′ transformation acts as the assistant driving force for the formation of δ phase.