| Titanium alloys are widely used in aerospace and deep space exploration due to high specific strength and good low temperature performance.The strength of titanium alloy increases with the reduction of service temperature,but its plasticity decreases significantly,and often accompanied by plastic instability phenomenon,which limits the application of titanium alloys in ultra-low temperature.This study employed the molecular dynamics method to analyze the plastic deformation behavior of titanium alloy at extremely low temperature and to reveal the mechanism of its low-temperature ductile-brittle transition.The generalized stacking-fault energy(SFE)of{0001}and{10-10}of Ti and Ti-Al alloys in the ground state were calculated by molecular statics.The prismatic SFE of Ti and Ti-Al alloys at finite temperature(280～40 K)are calculated by the thermodynamic integration method.The results show that the basal 1/3<-1100>Shockley partial dislocation and the prismatic 1/3<11-20>perfect dislocation have a low stacking-fault barrier.The addition of Al element(6at.%)reduces the stacking-fault barrier of 1/3<-1100>dislocations from 218.6 m J/m~2 to 191.9 m J/m~2.The prismatic SFE of Ti increases from 315.5 m J/m~2 to 341.3 m J/m~2as the temperature decreases(300-20K),whereas the SFE of Ti-Al increases from 330.9 m J/m~2 to 347.8 m J/m~2.The impact of temperature on the SFE is lessened by the Al.Four common twin models({10-11},{10-12},{11-21}and{11-22})in Ti and Ti alloys were established to compute the twin boundary energy(TBE),and the effects of Al element(6at.%)and temperature(300～0 K)on the TBE were analyzed.The TBE changes by less than 6%from 300 to 4 K,but the Al element has a strong influence on the TBE.The addition of Al(6at.%)significantly decreases the TBE of{10-11}and{10-12}and increases the TBE of{11-21}and{11-22}.The Ti-Al-V ternary alloy model was simulated under uniaxial tensile deformation by molecular dynamics simulation from 300 K to 2 K.The effects of crystal orientation,element content,and deformation temperature are contrasted.Analysis was done on the tensile deformation process’microstructure,dislocation evolution,slip/twin initiation,and other properties.The results show that as the temperature is lowered from 300 K,the axial ratio of the Ti6Al4V alloy rapidly decreases,however,more slowly under 50K than at higher temperatures.Similarly,the yield strain increases slowly and even decreases when stretched along the directions of[11-20]and[-1100].As for the influence of load direction on the deformation mechanism,the simple prismatic slip results in the model’s fracture rather than an increase in dislocation.Twins nucleate at the late deformation stage and in regions of severe deformation.The{10-11}compression twins are found when stretched along[11-20],and{10-12}twins are found when stretched along[0001].Regarding the effect of alloyed element content on the deformation mechanism,it is evident that as the Al and V element content of the Ti-Al-V alloy varies,strength increases while plasticity reduces from 300 K to 50 K.However,the strength and plasticity of Ti6.5Al6.2V increased with the decrease of temperature.Dislocation analysis at 50 K showed that Al promotes dislocation growth at 6.2 wt.%V content,whereas it hinders dislocation growth at 0.4 wt.%V content.More 1/3<-1100>dislocations and 1/3<11-20>dislocations were activated during stretched at medium and high V content(3.3-6.2 wt.%).Polycrystalline models of Ti and Ti6Al4V were established to investigate the microstructure and dislocation evolution under uniaxial compression.Under uniaxial compression of Ti and Ti6Al4V models,the stacking-fault caused by 1/3 of the basal plane<-1100>Shockley partial dislocation were firstly formed at the grains boundary,and then extended into the grains.The prismatic 1/3<11-20>perfect dislocation is decomposed into the basal 1/3<-1100>Shockley partial dislocation,which increases the slip deformation mode of the Ti alloy.The addition of Al and V elements in the Ti6Al4V model reduces the initial total grain boundary dislocation length to 1/3.Under uniaxial compression,the Ti6Al4V model forms a relatively stable layer dislocation,which increases the dislocation density and the strength of the Ti alloy.In addition,Al and V elements can inhibit HCP-BCC phase transition. |
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