Behavior And Mechanism Of Thermal And Stress-induced Martensitic Transformation And Plastic Deformation Mechanism Of Domain Containing TiNb(Mo) Alloys

Shape memory alloys are widely used because of their excellent functional and structural properties.The memory effect and superelasticity of shape memory alloys result from the reversible transformation between parent phase and martensite phase.In recent years,a domain structure between the parent phase and martensitic phase has been found in a variety of shape memory alloys,and it is pointed out that this kind of microstructure has a crucial influence on the phase transformation and mechanical behavior of the alloy.In this paper,Ti Nb（Mo）alloy containing nano domains is taken as the research object.The microstructure,phase transformation and mechanical behavior of the alloy are systematically studied by non in-situ and in-situ means,focusing on the microstructure evolution in the process of phase transformation and plastic deformation,and trying to understand the correlation between Nano domains and defects such as phase and dislocation.The main contents of this paper are as follows:Ti Nb（Mo）alloys with 12 components were prepared by vacuum arc melting,rolling and solid solution treatment.It is found that Nb and Mo are the stabilizing elements of the parent phase.When the content of Nb is low or there is no Mo element,the Ti Nb（Mo）alloy is martensitic at room temperature;When the content of Nb is high and a small amount of Mo is added,it is the parent phase at room temperature.It is observed that most of the Ti Nb（Mo）in the hexagonal alloy theωphase nano domains are evenly dispersed in the parent phase,and the domain size is between 1 nm-18 nm.Synchrotron radiation XRD results show that the content of phaseωnano domain increases with the decrease of temperature,and vice versa.The results of in-situ cooling TEM show that the martensitic phase nucleates and grows preferentially at the grain boundary of the parent phase when the domain containing Ti Nb（Mo）alloy undergoes martensitic transformation theωphase nano domain has a significant effect on martensite nucleation.The tensile test results at room temperature show that the super elastic strain recovery rate of Ti₈₄Nb₁₂Mo₄,Ti₈₄Nb₁₃Mo₃,Ti₈₂Nb₁₅Mo₃,Ti₇₈Nb₁₉Mo₃,Ti₇₆Nb₂₂Mo₂,Ti₇₇Nb₂₃ and Ti₇₃Nb₂₇ is more than 2%.The critical transformation stress of Ti Nb Mo alloy containing Mo element is higher than that of Ti Nb alloy without Mo element,which may be related to the high density of nano domains in Mo alloy.In situ synchrotron radiation XRD tensile results show that martensitic transformation and martensitic phase diffraction peak occur when the strain of Ti₈₄Nb₁₂Mo₄ alloy is greater than 1%,and reverse martensitic transformation occurs after unloading.In this process,when loading theωphase content gradually decreases when unloading theωphase content increases gradually.The microstructure characterization results show that the density of nano domains near the dislocation and phase interface is higher than that in other regions,which is speculated to be related to the reorientation of nano domains during plastic deformation or phase transformation.The results of variable temperature tensile test show that the relationship between the transformation critical stress and deformation temperature of Ti₇₃Nb₂₇,Ti₇₇Nb₂₃,Ti₈₂Nb₁₅Mo₃ and Ti₈₄Nb₁₂Mo₄ alloys is inconsistent with Clausius-Clapeyron the transformation critical stress does not decrease but increases with the decrease of temperature（below 60℃）,which may be due to the high density at lower temperature theωblocking effect of phase nano domain on phase transition.