Study On The Variant Selection And Detwining Behavior Of {332} <113>Twin In ?-type Ti-15Mo Alloy

Titanium and titanium alloys are widely applied in aerospace and marine engineering because of their excellent corrosion resistance,toughness and specific strength.The strength and plasticity of metastable ?-type titanium alloys can be improved by twinning-induced plasticity(TWIP).{332} <113> deformation twinning is one of the main deformation mechanisms for metastable ?-type titanium alloys.However,currently the research on {332} <113> twins is still limited.To study variant selection and transfer behavior of {332} <113> deformation twins,Ti-15 Mo titanium alloy was characterized by SEM and EBSD combined with the calculation of composite Schmid factor(CSF).Meanwhile,the quasi-in-situ test was conducted to investigate the twinning and detwinning of {332} <113> twins.It reveals the effect of the twinning and detwinning on the mechanical properties of Ti-15 Mo titanium alloy.Firstly,many {332} <113> twins are produced in the Ti-15 Mo alloy by uniaxial tensile test along the rolling direction(RD).The results show that only 47.8% of twins are at the first place in the Schmid factor(SF)Rank.Hence,it can be deduced that Schmid's law cannot be applied to all twins.Most of the twins formed at the grain boundaries with the form of twin pairs.Among the 54 pairs of observed twins,24 are sequential twinning and 30 are simultaneous twin nucleation.For sequential twinning,the actual activated twin pairs have the largest CSF value(Rank 1).For the simultaneous twin nucleation,the activated twin pairs possess the CSF Ranks of Rank1 or Rank 2 with a percentage of 80% and 20%,respectively.These results indicate that CSF can effectively predict the variant selection of twins at grain boundaries.In addition,by comparing m? with CSF,CSF was proved to better analyze and predict the selection of twin variant.During the compression test after pre-tension,profuse {332} <113> twinning and detwinning were observed.Moreover,the yield strength of the pre-tension sample is significantly improved comparing with the sample without pre-tension.It was because the lattice distortion produced at the intersection of twins-twins and twins-grain boundaries remained during the de-twinning process.Additionally,abundant dislocations were generated at the original twin boundaries after de-twining.These defects can effectively hinder the motion of dislocations and the migration of twins,which thereby improve the strength.The analysis of dislocation slip show that a lot of parallel slip traces were found in the parent grains and ended at the early {332} twin boundary(formed during pre-tension)during the compression test(reverse loading process after pre-tension).It suggests that the detwinning takes place because of the interaction of original twin boundaries and dislocation slip.