Study Of Martensitic Transformation Under The Coupling Action Of Thermal And Stress In TiNi Alloy

The TiNi shape memory alloys(SMA), are currently much accounted of material scientists, industry and the medicine field because of their excellent mechanic performance, corrosion resistance, unique shape memory effect, biocompatibility and abundant martensitic transformation. When stress or strain is applied to TiNi alloy, the Ms, the martensitic transformation dynamics and the pattern of martensite are changed. In the thermodynamics of thermoelastic martensite, stress is used to study the effect on the transformation. Meanwhile, A lot of characters of the stress-induced transformation are explained, but it is difficult to describe the growth of the martensite variant and pseudo-elastic.Aiming at some aspects mentioned above, martensitic transformation under thermal and stress coupling by the Landau theory is investigated , comparison is made a to the experiment result. At the same time, the Two-Way Shape Memory Effect (TWSME) is also studied. The microstructures of the stress-induced and strain-induced martensite are observed. The influence of the once over-deformation to the TWSME is analysed. The primary conclusions are listed as follows:Ms can be increased with the increment of stress. When the stress is low, the thermalelastic martensitic transformation under thermal and stress coupling can be divided into two processes: variant self-accommodation formation and one orientation growth, and with the increment of stress, variant self accommodation formation is restrained, there is only variant one orientation growth. The effect of the stress field was added into the Landau theory, the free energy, which is related to stress, is composed of linearity term of transformation strain and quadratic term. The result of this mode is the same to the result from experiment; In the process of transformation, the main metastructure of the transformation are < 011 >_M II twins. Meanwhile, the (001)_M compound twins are discovered.There are three microstructure of the strain-induced martensite in the TiNi alloy: strip, acerose, massive, and their deformation mechanism and microstructure are different from stress-induced martensite. When once over-deformation of TiNi alloy is less than 12%,the recoverable deformation of the TWSME is increased with the increment of once over-deformation. When once over-deformation of TiNi alloy is 12%, the recoverable deformation of the TWSME reachs the max . When once over-deformation of TiNi alloy is more than 12% , the recoverable deformation of the TWSME gets small. And the TEM bright field images of martensite explain the regularity.