Effect Of Initial Microstructure On Martensitic Transformation Behavior Of ECAP-processed TiNi Alloy

In present work, microstructure, phase constituent, martensitic transformation behavior and mechanical properties of Ti_49.2Ni_50.8 alloy processed by equal channel angular pressing?ECAP? with different initial microstructure were studied by means of optical metallographic microscope?OM?, transmission electron microscope ?TEM?, X-ray diffraction?XRD? analysis,differential scanning calorimetry?DSC? and mechanical testing.The solution treated and aged Ti_49.2Ni_50.8 alloy shows the almost same grain size which is about 70 ?m. Ti3Ni4 precipitates was observed in the matrix after aging treatment, and the size of Ti3Ni4 phase increases with the rise of aging temperature and time.After 1 pass pressing, microstructure of the alloy is characterized by elongated grains.The grain width of the sample initially aged at different aging temperatures decreased with the rise of aging temperature. The grain width of the sample initially aged at 450? with different durations increased with the increase of aging time. After 8 passes, the microstructure of the alloy mainly consisted of equalaxial grains. The grain size of sample initially aged at 450?for 10min and 1h are 400nm and 247nm respectively. After aged at 450? for 10min and processed by ECAP for 1 pass, Ti3Ni4 precipitates were not found in the matrix. After aged at 450? for 1h,then processed by ECAP for 8 passes, Ti3Ni4 precipitates were not found in the matrix.A B2?B19' one step martensitic transformation occurs in the solution treated alloy.When aged at a temperature lower than 550?, martensitic transformation occurs in the sequence of B2?R?B19' phase. With the increase of aging time, the alloys aged at 450?shows B2?R?B19' martensitic transformation.The initial aging treatment has a significant influence on the martensitic transformation behavior and mechanical properties of ECAPed Ti_49.2Ni_50.8 alloy. After 1 pass of ECAP, the alloys initially aged at different temperatures for 1h shows different transformation behaviors,i.e. the sample aged at 450? shows B2?R?B19',B2?B19' transformation,the other samples show B2?R?B19' martensitic transformation.After processed by ECAP for 1 pass, the sample aged at 450? for 10min shows B2?R?B19' martensitic transformation and B19'?B2 reverse transformation. The other passes processed samples show B2?R?B19' martensitic transformation and its reverse transformation. When aged for 1h, and processed by ECAP for 8 passes, the sample shows B2?R?B19' martensitic transformation and its reverse transformation. When aged for 10h,and processed by ECAP, the sample shows B2?R?B19' martensitic transformation and B19'?B2 reverse martensitic transformation. After 1 pass, the critical stress to induced martensitic transformation of the ECAPed specimen is larger than that of the initial sample.