Election Microscopy Characterization Of The Interaction Behaviors For {332}<113> Deformation Twins In A Metastable ?-Ti Alloy

Among metallic biomaterials,the Ti-25Nb-25 Ta ?-alloy is potentially attractive for medical applications.The main efforts of the previous studies on this metastable ?-Ti alloy have been focused on the improvement of its mechanical properties,however,microstructural investigation which can offer valuable information for establishing the intrinsic relationship between microstructure feature and mechanical properties is still quite lacking.The {332}<113> deformation twin is a unique deformation mode which is suggested to be responsible for the low yield strength and high uniform elongation of metastable ?-Ti alloys.As deformation products which can be formed extensively in the deformed Ti-25Nb-25 Ta alloy,it is inevitable that {332}<113> twins can encounter and interact with various obstacles(such as various twin variants and grain boundaries)during the deformation process.In general,a growing deformation twin is usually stressed at its tip region.In particular,extremely high stress concentration will occur when the twinning is blocked by a boundary,and this can further result in microcracking or even fracturing along the interaction region.Due to the lattice instability of metastable ?-Ti alloys,activation of other stress accommodation manners such as phase transformation and secondary twinning is also expectable if the local stress associated with the interaction region is significantly high.Nevertheless,there are few studies to deal with this issue and to clarify the related complex structural variations so far.In this study,tensile deformation and TEM structural characterization have been carried out for Ti-25Nb-25 Ta alloy samples,through which the stress accommodation manners and the associated local structural variations for the twin-twin and the twin-grain boundary interactions have been studied carefully.The purposes of this study are to deepen the understanding of the interaction process associated with the {332} deformation twinning,to explain some relevant fundamental scientific questions and to provide valuable information for the design and development of metastable ?-Ti alloys.The main results obtained are summarized as follows:1.TEM observations clearly reveal that a {332}<113> deformation twin plate interacting with a small-angle grain boundary(SAGB)by being blocked or by crossing it can result in significantly high local stress concentration and complex local structure variations.It is found that a unique stress-induced ? platelet with unusual habit plane(//{111}?T)can be formed facing close to the SAGB in the twin plate due to the twin-SAGB interaction.The twin-SAGB interaction configuration could further undergo complex structural variation during continuous formation.Its unilaterally interacting with deformation dislocations in the matrix is responsible for the formation of the characteristic step and dislocation wall at only one side of the {332} twin plate.The node at the step can be identified to be the site of higher stress concentration from which the {112} secondary twinning in the {332} twinned region can be activated.2.It has been observed that several {332}<113> deformation twin plates can interact with a SAGB to form a quite complex interaction configuration.A {332} twin plate in the deformed Ti-25Nb-25 Ta alloy usually exhibits a sharp growth front,where a high stress concentration can occur.However after its crossing through the SAGB,its growth front turns to be a flat interface which can exhibit an orientational relationship close to that for a {112} twin.This characteristic alteration associated with the twin-SAGB interaction can be attributed to the existence of much higher stress concentration at the noncoherent front interface of the SAGB-crossing twin plate,such that this extremely high stress field will cause local interface adjustment at the twin tip to form a kind of special flat front interface orientationally close to a lower energy ?3 type interface with the {112} twinning relationship when the applied stress is unloaded.3.It has been observed that lots of {332}<113> twins are terminated at high angle grain boundaries(HAGBs).It has been found that abundant geometrically necessary dislocations can be generated to form a deformation band in the adjacent grain due to the blocking interaction of its HAGB.This indicates that the {332} twinning shear blocked by a HAGB can be transferred by inducing the formation of a dislocation band.4.The interaction of different {332}<113> twin variants can form a terminated configuration.The results that the incident twin impinging on an obstacle twin can produce a strong interaction which can bring about obvious local variations in both orientation and structure.It has been observed that the unilaterally curved boundary caused by the local shrinkage of the incident twin exhibits a gradual transition in orientation from a {332} twinning relationship to near a {112} one,and that a transition region having a misorientation of 13°-16° with respect to the matrix can be formed due to the shrinkage.On the other hand,{332}<113> and {112}<111> types of secondary twins along with kink bands can also be formed in the twinned regions close to the twin-twin interaction interface to accommodate high local stress occurring there.