Twinning Induced Plastic Deformation Behaviors In Body-Centered Cubic Ti-Mo Alloy

Dislocation slip is the main plastic deformation mode in the currently applied titanium alloys,whereas the inverse relationship between strength and plasticity under the guidance of dislocation motion theory restricts their further development.Recently,the body-centered cubic（BCC）structured metastableβ-type titanium alloys with{332}<113>twinning-induced plasticity（TWIP）effect show the high product of strength and plasticity,and impact work,exhibiting the wide application prospect in many fields of armour protection,deep-sea exploration and ultralow temperature.The high strength and plasticity are needed at quasi-static/dynamic loadings and at the certain temperature environments for the TWIP titanium alloys,to satisfy the requirement of processing forming and safe service.However,the{332}<113>twinning behavior,the flow stress,and the strain hardening mechanism under the complex service conditions are still not systematically understood.In this study,the effect of grain size,deformation temperature and strain rate on the twinning behavior and flow stress was systemically studied in the Ti-15Mo alloy with{332}<113>TWIP effect and a comparison material of Ti-15Mo-1Fe alloy deformed by dislocation slip.According to the dislocation thermal activation theory,the strain rate sensitivity of flow stress was examined in the TWIP titanium alloy.Based on the evolution of deformation microstructures,the flow stress model was further established,and the strain hardening mechanism of this type alloys was clarified.This study is of great significance for guiding the design,microstructure control and processing formation of high-performance titanium alloys.The main conclusions of this study are summarized as follows.The twinning behavior and flow stress of TWIP titanium alloys exhibited the significant grain size dependency.As the grain size decreased,the yield strength of twinning type alloy increased,whereas the strain hardening rate and tensile strength decreased.Based on the coupling effect of static grain refinement and dynamic grain refinement caused by twinning during deformation,the dynamic Hall-Petch relation was established.At yield point,the Hall-Petch coefficient of twinning was 1112 MPa·μm^1/2,which was around 5 times larger than that for dislocation slip.In the coarse-grained specimens,the twins were formed at elastic-plastic deformation stage,and their interactions with grain boundaries induced the significant local stress concentration,which promoted the subsequent formation of twins and satisfied the macroscopic yield.In the fine-grained specimens,the weak local stress concentration inhabited the formation of twins,and the extra external stress was needed to promote the formation of twins to achieve the macroscopic yield.The modified dynamic Hall-Petch coefficient calculated by the effective grain size exponentially decreased as the strain increased,which was resulted from the successive occurrence of the twin formation,the suppression of twin formation,the twin-dislocation and dislocation-dislocation interactions.The twinning behavior and flow stress of TWIP titanium alloys depended on the deformation temperature and strain rate.As the deformation temperature increased from 298 K to 673 K,the yield strength of twinning type alloy remained relatively constant,whereas it decreased in the dislocation slip type alloy.Theβ-phase stability enhanced resulting from the precipitation of Mo-depleted isothermalω-phase,consequently,the deformation mode changed from twinning to dislocation slip.The contribution of dislocation slip with higher critical resolution shear stress to plastic deformation increased,which compensated the decrease in yield strength due to the thermal softening.As the deformation temperature increased,the strain hardening rate and tensile strength of twinning type alloy decreased due to the reduction of twins and geometrically necessary dislocations.At 298 K and 573 K,as the strain rate increased,the yield strength of two alloys increased,and its increase values were lower in the twinning type alloy than those in the dislocation slip type alloy.The strain hardening rate decreased,resulting in the abnormally negative strain rate sensitivity（SRS）at large strains in the twinning type alloy.Except for the dislocation thermal activation behavior,the effect of deformation temperature and strain rate on flow stress in the twinning type alloy was mainly attributed to the change of deformation mode.TWIP titanium alloys exhibited the increasingly positive instantaneous strain rate sensitivity（ISRS）and decreasingly negative strain rate sensitivity of strain-hardening（SRSS）.The ISRS and SRSS were related to dislocation activation volume and deformation microstructure evolution,respectively.The ISRS coefficient of twinning type alloy was positive under all strains,and it increased as the strain increased,which was different from the basically unchanged ISRS coefficients in the conventional metallic materials.At the yield point,the thermal activation behavior in this alloy was controlled by the solution strengthening of interstitial oxygen atoms,rather than the Peierls-Nabarro stress in the conventional BCC structured metals.Consequently,the dislocation activation volume was 284b³,which was around 4-10 times larger than those in the common BCC structured metals.As the strain increased,a large number of geometrically necessary dislocations piled-up at twin boundaries impeded the dislocation motion,resulting in the decrease in dislocation activation volume.As the strain rate increased,the reduction of dislocation density and twin area fraction led to the decrease in strain hardening rate and flow stress,which resulted in the decrease in the SRSS coefficient.Although the strain rate strengthening gradually enhanced as the strain increased,the reduced strain hardening resulted in the decrease in flow stress as the strain rate increased.The strain hardening of TWIP titanium alloys significantly depended on the back stress strengthening for dislocation motion induced by twin boundaries.Based on the cyclic loading-unloading tensile tests and microstructural characterizations,the contribution of back stress to flow stress gradually increased with the increase of twins.Compared to the grain boundaries,the twin boundaries exhibited the greater capacity for the storage of geometrically necessary dislocations,leading to the back stress strengthening for dislocation motion.During the deformation,the density of geometrically necessary dislocations was larger than the density of statistically stored dislocations,and their interactions induced the dislocation strengthening.The flow stress model of TWIP titanium alloys was established by combining the evolution of dislocations and twins during the deformation.At 298 K,the back stress strengthening gradually replaced dislocation strengthening and became the dominant strengthening mode due to the rapid formation of twins as the strain increased.As the strain rate increased,their contributors were significantly reduced due to the suppression of dislocations and twins.However,the flow stress of this alloy was mainly contributed by dislocation strengthening at573 K.The significant strain hardening of the TWIP titanium alloys was clarified experimentally and theoretically by the synergistic effect of back stress strengthening and dislocation strengthening caused by the pile up of a large number of geometrically necessary dislocations at the twin boundaries.