Study On The Phase Transformation And Twinning Behaviors In Ti And Ti Alloys During Shock Deformation And Heat Treatment

Titanium（Ti）and its alloys,with the highest strength-to-weight ratio among all alloys,have attracted extensive attentions in aerospace and military fields such as the landing gear,engines and armor materials due to their excellent properties including low density,high strength,good resistance against corrosion and great high temperature properties.Specifically,Ti alloys used for armor materials normally need to resist deformation at high strain rate.The structural Ti alloys normally exist in dual phase structure including hexagonal closed packedαphase and body-centered cubicβphase.The size,content and morphology ofαcan be adjusted by heat treatment or thermomechanical treatment,which determines the mechanical performance of Ti alloys.However,unlike austenitic steels,aluminum alloys and other materials with face-centered cubic structure,the Ti and Ti alloys possess relatively poor crystal structure symmetry and fewer independent slip systems of dislocations.Thus,deformation twinning and martensite phase transformation also play important roles during deformation.Therefore,exploring the phase transformation and twinning behavior of Ti and Ti alloys during impact deformation and heat treatment,clarifying the dominant deformation mechanism and its relationship with mechanical properties under different conditions can provide a theoretical basis for the development of high strength and toughness Ti alloys.In this study,pure Ti,Ti-x Al（x=3,6 wt.%）alloys,Ti-x Mo（x=4,7,12 wt.%）alloys and TC18（Ti-5Al-5Mo-5V-1Cr-1Fe）alloy were chosen to investigate the phase transformation and twinning behaviors during heat treatment and shock deformation.The main contents and conclusions are as follows:（1）The twinning behavior of pure Ti during shock load was studied.Besides the common{101 2}<101 1>,{112 1}<112 6>,{112 2}<112 3>,{101 1}<101 2>,{112 4}<224 3>twinning,a new type compressive twinning{112 3}<112 2>with orientation relationship 87°<101 0>and a type of high angle kink band with misorientation 61°<7 10 1 7 3>were also observed.Moreover,deviation of some twins from the ideal twinning relationship was also observed due to the deformation induced dislocation-twin interactions.These results indicated that high-speed impact,as a kind of severe plastic deformation mode with high strain rate,can trigger some abnormal deformation mechanisms which are difficult to be activated under conventional deformation conditions.In addition,the effects of strain rate and grain size on twinning behaviors of pure Ti were systematically studied.The results show that the fraction of twined grains is positively related to strain rate.While the fraction of twined grains shows nonlinear relationship with grain size which firstly increased and eventually decreased with the increasing of grain size.（2）The effect of Al content on twinning behaviors in Ti-x Al（x=0,3,6 wt.%）alloys were systematically investigated.With the increment of Al content,the fraction of twined grains containing{112 2}<112 3>twins decreased rapidly and completely disappeared in the Ti-3Al alloy.The fraction of twined grains containing{101 2}<101 1>twins also decreased with the increment of Al content,but the decrement was less obvious than the{112 2}<112 3>twins.However,the opposite effect was observed for{112 1}<112 6>twins,that is,the fraction of{112 1}<112 6>twined grains,increased with the increment of Al content.The first principles calculation confirmed the addition of Al significantly increased the formation energy of{101 2}<101 1>and{112 2}<112 3>twinning,which suppressed the formation of these twins.While for{112 1}<112 6>twinning,the formation energy decreased with the increment of Al content,which favored the formation of{112 1}<112 6>twins.（3）The effect of Mo addition on the microstructure of Ti-x Mo（x=4,7,12 wt.%）alloy was investigated.The Ti-4Mo alloy exhibited a basket weave structure,while Ti-7Mo exhibited a lamellar structure and Ti-12Mo exhibited a singleβphase.With the increment of Mo content,the stability ofαphase decreased leading to a significant reduction of the quantities and variants ofα.Therefore,the microstructure of the alloy changed from the basket-weave structure to lamellar structure and finally to a singleβphase with the increase of Mo content.（4）The martensite phase transformation and twinning behaviors of shock load Ti-12Mo alloy were investigated.The results indicated that the{332}<113>twinning dominated the deformation.The{332}<113>twinning is closely related to theα{130}<310>twinning,which can be described asβ→α→α_{}→β_{}.The intermediate process of{130}<310>twinning inαphase has been characterized by electron backscattered diffraction.Besides,three types of twin-twin intersections were observed and summarized,including retaining phase,secondary{332}<113>twinning and formation of kink bands in the intersecting areas.Further analysis indicated that the formation of such kink bands strongly depended on the primary twin variants selection,which only occurred when two primary twin variants with an misorientation of～36°<575>interacted with each other.Meanwhile,theβ→αandβ→ωphase transitions were also characterized,and the phase transition mechanisms were also discussed.（5）The effects of subtransus triplex heat treatments on the microstructure,phase transformation and mechanical properties of TC18alloy were investigated.By adjusting the temperature and time of the first and third stage aging heat treatment,it is found that the comprehensive mechanical properties are the best after hot rolling at 830°C for 0.5 h+750°C for 2 h+450°C for 8-16 h.The yield strength and total elongation increased from 1029±13 MPa,2±0.8%to 1117±15 MPa,15.9±1.9%and1192±24 MPa,11.7±1.5%.The functions of three stage heat treatment are as follows.The first step is recrystallization annealing to reduce the casting defects.In the second step,the morphology ofαphase is adjusted to a near-spherical short-rod shape to improve the plasticity.The last step is to precipitate the agingαphase in theβmatrix and induce the growth of the primaryαphase to further increase the strength.