Grain Size Stability And Martensitic Transformation Behavior Of An Ultrafine Grained TiNi Alloy

In this paper, the effect of annealing on microstructure of the as-ECAP processed Ti49.2Ni50.8 alloy was systematically investigated using scanning electron microscope (SEM),transmission electron microscope(TEM),electron backscattering diffraction (EBSD),energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) analysis. Martensitic transformation behavior of the initially coarse-grained and ultrafine grained Ti49.2Ni50.8 alloys were compared by using differential scanning calorimetry (DSC). The emphasis was laid on the influence of annealing treatment on martensitic transformation.The results show that the microstructure of Ti49.2Ni50.8 alloy mainly consists of parent phase with B2 structure and Ti2Ni phase. TEM observation reveals that ECAP treated Ti49.2Ni50.8 alloy by 6 passes has an average grain size of 259 nm, while ECAP treated Ti49.2Ni50.8 alloy by 8 passes has an average grain size of 219 nm. The grain size distribution of ECAP treated Ti49.2Ni50.8 alloy by 8 passes satisfies the Gauss distribution and the grain size increases with the rising annealing temperature and the annealing time. There are a large numbers of {111} grain orientation in the alloy after annealing at 600 ℃,and the grain size grows up to a micron level rapidly.A B2(?)B19’ one step martensitic transformation and reverse transformation, occurs respectively, in the coarse grained Ti49.2Ni50.8 alloy during cooling and heating. Compared with its coarse grain counterpart, the martensitic transformation temperature of ECAP treated ultra-fine grain Ti49.2Ni50.8 alloy shows a sharp decrease,while the transformation hysteresis increases.A transformation sequence of B2→R→B19’ occurs during cooling. At 200℃, for coarse grained Ti49.2Ni50.8 alloy, when annealing time is lower than 20 hours, single-stage transformation B2(?)B19’occurred upon cooling and heating; when the annealing time is higher than 20 hours, two-stage transformation B19’→R→B2 is observed.Meanwhile, when annealing time is lower than 10 hours,ultrafine grained Ti49.2Ni50.8 alloy showes B19’→-B2 single-stage transformation;when the annealing time is higher than 10 hours,two-stage transformation B19’R→B2 is observed.There are no difference about the transformation sequences of martensitic between coarse grain and fine grain with different annealing time in 300℃～50℃.At the same annealing temperature,R phase transformation and its reverse transformation have an increasing trend as the annealing time increases,and the transformation hysteresis of reverse transformation of R phase transformation remains unchanged basically. Martensitic transformation and its reverse transformation have a decreasing trend with the change of annealing time in 200℃, which has an increasing trend in 300℃～500℃.