Microstructure And Strain Recovery Characteristics Of TiB/Ti-V-Al Shape Memory Composites

To meet the requirements of light weight,high strength and large recoverable strain of shape memory alloys for intelligent interference sealing connections in aerospace and weapon fields,a lightweight TiB/Ti-V-Al shape memory composite with high strength and large recovery strain was obtained by introducing the TiB phase into the Ti-V-Al alloy.The effects of TiB on the martensitic transformation,microstructure,mechanical properties and strain recovery characteristics of the TiB/Ti-V-Al shape memory composites were systematically studied by XRD,SEM,TEM,DSC,HE-XRD and tensile tests.The microstructure evolution during the deformation process and the corresponding deformation mechanism were clarified.The results show that the orientation of the TiB whiskers in the Ti-V-Al matrix evolves from a random distribution to an orientation after hot rolling or hot extrusion and they remain intact.The optimal hot working process is determined to be hot extrusion（extrusion ratio 16:1）and quenching after annealing at 900℃for 0.5 h.As shown by the microstructure observation results,the phase constitution of the Ti-V-Al matrix composites at room temperature changes from the soleα’’phase to the mixture ofα’’,β,and TiB phases gradually with the increase in the addition of TiB₂.The introduction of the in-situ generated TiB results in a unique gradient microstructure:the coupling of the short-range ordered martensite nanodomain close to TiB and the long-range ordered martensite variant away from TiB.The region adjacent to TiB（approximately 2-3μm）is composed of theβparent phase and the nanoscale martensite domain.Then fineα’’martensite plates appear slightly away from TiB.Finally,the microstructure evolves to the V-shaped accommodation of martensite in further areas.The formation of short-range ordered martensite domains around TiB in TiB/Ti-V-Al shape memory composites is mainly related to the lattice distortion formed by B interstitial atoms and the gradient stress field around TiB.These two factors together introduce local energy barriers on the Landau free energy landscape of the martensitic transformation close to TiB,which restricts the dynamic process of the martensitic transformation,and thus results in short-range ordered martensitic domains at the nanoscale.However,the effect of the stress field gradually disappears away from TiB,while the effect of B atoms is insufficient,so long-range ordered martensitic laths with self-accommodation morphology similar to bulk materials are formed.The DSC results show that the reverse martensitic transformation temperature of the TiB/Ti-V-Al shape memory composites decreases first and then remains unchanged with increasing amount of in-situ generated TiB.The change in the transformation temperature originates from the variation in the composition of the matrix due to B addition,the decrease in grain size,and the introduction of irreversible defects around TiB.The strength,elongation and recovery strain of the TiB/Ti-V-Al shape memory composites first increased and then decreased with increasing TiB content.When the addition of TiB₂ was 1 wt.%,the tensile strength and elongation of the TiB/Ti-V-Al shape memory composite reached their maximum values,which were 961 MPa and23%,respectively.A large recovery strain of about 5.6%is obtained under 6%tension.It is worth mentioning that TiB/Ti-V-Al shape memory composites do not show superelastic characteristics when the parent phase content around TiB reaches more than 20%.The martensitic nanodomains in the parent phase grow upon loading,but the elastic energy released during unloading can not overcome the additional phase transition barrier caused by the stress field around TiB and the B atoms.Therefore,the grown martensitic nanodomains are retained and only recovered during heating.The structural evolution of TiB/Ti-V-Al shape memory composites during deformation can be divided into three stages.First,in the region close to TiB,the short-range ordered martensite domain grew,and the parent phaseβgradually transformed toα’’martensite;while in the area away from TiB,twinning occurred and formed the{111}type I,<211>type II,and{011}compound twins.Then reorientation twinning appears in the grown martensite plate around TiB,while stress inducedωphase and deformation twinning with{110}<110>and{130}<310>twin modes take place at some distance from TiB.Finally,most areas of the matrix are{110}<110>twins,with aωthin layer existing in the twinning interfaces,and most of the TiB whiskers were fractured.Combined with the in-situ tensile TEM and in-situ tensile XRD results,it can be found that the growth of nanodomains around TiB can effectively coordinate the deformation in the TiB/Ti-V-Al shape memory composite.The compatible deformation realizes the redistribution of internal stress from the matrix to the reinforced phase,giving full play to the high bearing capacity of TiB whiskers.Hence irreversible crystal defects such as dislocations and deformation twins are effectively avoided.In conclusion,the strain recovery characteristics and tensile properties of the material are improved simultaneously,thus realizing the integration of shape memory properties and structural bearing properties.