Microstructure Evolution Of TA2 Commercial Pure Titanium At High Temperature

TA2 commercial pure titanium has broad applications such as ship building and chemical industry for its outstanding corrosion resistance, good synthetic mechanical performance and excellent welding property. Its microstructure can be adjusted through heat treatment so as to get a good balance of strength, plasticity, and toughness. So it is very important to study the microstructure evolution behaviour at high temperature.The microstructure evolution behaviour of TA2 commercial pure titanium in heating, insulation and cooling processes were investigated by Confocal Laser Scanning Microscope, Optical Microscope, and Differential Scanning Calorimetry.During heating process, a part of curvingαgrain boundaries migrated and tend to be straighten. Short-time heating had little effect onαgrain size. As the temperature was higher than Tα/α+β, several parallel"lense-shaped"βplates nucleated in the inside of anαgrain and grew by consuming the surroundingαphase, being accompanied by"tent-shaped"surface reliefs. Theβ→αtransformation ended as the temperature was higher than Tα+β/β. The parallelβplates in anαgrain formed a cluster. The clusters devoured each other and formed equiaxedβgrains. The kinetics of theβgrain growth during heating and insulation processes was measured and fitted.Microstructures were different when cooled from different state. When cooled fromα+βtwo-phase region, the microstructure restored to be equiaxed and the surface reliefs disappeared. When cooled from the state thatα→βtransformation just accomplished, it formedαgrains with irregular boundaries. When cooled from equiaxedβgrain state, it was found thatβ→αtransformation proceeded through two successive steps, the nucleation and growth of grain boundaryαphase and the following nucleation and growth of widmanstattenαphase. Severalαnucleated and grew atβgrain boundaries firstly and form grain boundaryαfilm alongβgrain boundaries. Then widmanstattenαnucleated at the phase boundaries of grain boundaryαandβmatrix and grew intoβphase in the shape of parallel plates colonies and finally formed widmanstatten microstructure.It was distinct the effect that the cooling rate acted onβ→αtransformation when cooled from equiaxedβgrain state. The higher the cooling rate was, the thinner the grain boundaryαand the widmanstattenαwas, the fast the widmanstattenαlengthening rate andβ→αtransformation speed was. The widening of grain boundaryα, the lengthening and the width of widmanstattenα, the kinetics ofβ→αtransformation were measured. The experiential formula of the kinetics ofβ→αtransformation was given according to the experimental measurements.