| Beta-Titanium(?-Ti)alloys have some excellent properties such as high strength,good corrosion resistance and biocompatibility,which are potentially attractive for many applications in biomedicine and aerospace.However,low ductility and weak work hardening hinder their wider applications in high-performance and safety-critical fields.Recently,the strategies of transformation-induced plasticity(TRIP)and twinning-induced plasticity(TWIP)are developed to improve the strength and ductility in ?-Ti alloys concurrently.The ?" martensite,? phases,{332}(113)? and{112}<111>? twins are the major products of TRIP and TWIP,which are clearly observed in a variety of ?-Ti alloys.At present,formation mechanisms of the above four deformation products in ?-Ti alloys have been studied extensively.However,there are concerns about the relationship between the deformation products due to the complexity of the TRIP/TWIP process in ?-Ti alloys,especially between the formed phase and the parent phase.There is a lack of systematic and in-depth research on issues such as the interface structure,the evolution of the local structure of the interface and the twinning mechanism.As is known,the two-phase interface structure and local structure evolution behavior in ?-Ti alloys are affected by multiple coupling factors such as alloy element composition,temperature and deformation stress,which also explains why it is difficult to complete the above-mentioned research under the combined effect of multiple coupling factors in just one step.Fortunately,it is generally considered that the alloying element composition as an intrinsic factor plays the most dominant role in the formation and evolution of interface structure.Therefore,this work mainly focuses on this intrinsic influence of internal alloying element composition on the formation and evolution of interface structure,which can be regarded as the first step in the systematic study of the above multi-factor coupling complex problems In this paper,aiming at the three ?-Ti alloys,i.e.Ti-Mo,Ti-Nb and Ti-V,we have systematically studied the effects of alloying elements on the interfacial structure and local structure evolution behavior of two phases by combining first-principles calculation and Virtual Crystal Approximation,and the following research results have been obtained.(1)(110)?/(001)?",(112)?/(110)?",(332)?/(130)?",(111)?/(0001)?,(112)?/(10(?)0)?,and(332)?/(1(?)00)? interfaces were screened out from the viewpoints of crystallography and energy.With the addition of Mo,Nb and V elements,the effects of alloying elements on ?/?" and ?/? interfaces were revealed.The(112)?/(110)?" interface energy is the lowest when the alloying element concentration is low.When the concentration is high,the(110)?/(001)?"interface energy decreases,which is lower than those of the other two interfaces.It is indicated that the ?/?" interface prefers to form(112)?/(110)?" interface at the low concentration,while the(110)?/(001)?" interface is easily formed at the high concentration.No matter how the type and concentration of alloying elements change,the(112)?/(10(?)0)? interface energy is always the lowest among the three ?/? types.It is found that the ?/? interface prefers to form the(112)?/(10(?)0)?,interface structure.(2)The structure of(332)[11(?)]? twin boundary in ?-Ti alloys is closely related to the type and concentration of alloying elements.And the structure of(332)[11(?)]?TB is symmetry twin boundary when the Mo,Nb and V concentration is greater than?12 at.%,?30 at.%and?23 at.%,respectively.The structure of(332)[11(?)]? TB changes into an interface "sandwich" configuration containing interfacial twin boundary(ITB)?" phase when the alloying element concentration is less than the critical value above.The existence of ITB ?" phase can stabilize the(332)[11(?)]? TB.It explains the reason that why ITB ?" phase is always observed in the vicinity of the(332)[113]? TB experimentally.When the alloying element concentration reaches a certain range(-7.8 at.%-27 at.%Mo,>15 at.%Nb and>15 at.%V),the structure of(112)[11(?)]? TB is the symmetrical TB coupled with the ITB ? phase.The ITB ?phase can improve the stability of the symmetrical(112)[11(?)]? TB,which explains the experimental phenomenon that the ITB ? phase always accompanies the formation of(112)[11(?)]? TB.(3)It is revealed that the metastable ? phase can assist the(112)[11(?)]? twinning mechanism on the basis of the(112)?/(10(?)0)? interface model in Ti-Mo alloys.During the twinning process,the partial dislocations slip first on successive(10(?)0)?planes adjacent to the(112)?/(10(?)0)? interface and lead to the formation of(112)[11(?)]? twin nucleus.Once the twinning nucleation finishes,the(112)[11(?)]?twin can easily grow up through "shuffle" mechanism.The mechanism is also applicable for Ti-Nb and Ti-V alloys.The metastable ?-assisted(112)[11(?)]?twinning mechanism is more applicable at the high ?-stabilizing element concentration.(4)The nucleation and the intrinsic microstructure evolution of martensite from(332)[11(?)]? twin boundary(TB)were investigated at the atomic scale,and it is also seen a transformation of(332)[11(?)]? TB?(130)[(?)10]?" TB?? phase.From the viewpoints of crystallography and kinetics,it is found that the martensite easily nucleates at the(332)[11(?)]? TB and subsequently grows inwards the grain interior.When Mo,Nb and V are added into ?-Ti alloys,the ?" martensite still prefers to nucleate and grow at the twin boundary.If there is no(332)[11(?)]? TB in ?-Ti alloys,the ?" martensite is more likely to form in the alloys with high Mo concentration,or Nb and V in the concentration range of?25 at.%-35 at.%. |
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