Study Of Microstructure, Phase Transformation And Canned Compression Process Of Nimncuga Shape Memory Alloy

In the present paper,Ni₅6Mn_25-xCu_xGa₁9?x=0,2,4, 6? high temperature shape memory alloys were prepared and some of them were subjected to severe plastic deformation by canned compression method. Optical microscope, scanning electron microscope, X-ray diffraction technique, differential scanning calorimeter, Vickers hardness instrument,compression testing were employed to investigate the influence of Cu element and canned compression deformation on the martensitic transformation, microstructure, mechanical property and shape memory effect of Ni₅6Mn_25-xCu_xGa₁9?x=0, 2,4, 6? alloy systematically.The results demonstrated that a second phase ?y? forms due to the addition of Cu. At room temperature,the microstructure of Ni₅6Mn₂5Ga₁9 alloy is tetragonal martensite alone but that of Ni₅6Mn_25-xCu_xGa₁9 ?x=2, 4, 6? consists of martensite matrix and the second phase,the volume fraction and size of which increase with higher content of Cu. With increasing Cu content, transformation temperatures firstly drop a little and then shift to higher temperature.An appropriate addition of Cu favors to improve the alloy's mechanical property and maintain a good shape memory effect.The optimum processing parameters are as follows: room temperature, cylinder alloy samples with 6mm in height and 4mm in diameter,3-mm-thick can,and lubricant consisting of graphite and engine oil.It was indicated that canned compression is capable of improving the plastic deformation ability of the NiMnCuGa alloys, as evidenced by the dimple-featured fracture surface. Larger deformation by canned compression causes the increase of martensite reverse transformation temperature. When the deformation exceeds 25%, no phase deformation is observed.However, annealing at 900? for half an hour could recover the microstructure and phase transformation. Canned compression plus subsequent heat treatment could obviously refine the grain size. For the post-annealed at 900? Ni₅6Mn₁9Cu₆Ga₁9alloy subjected to the canned compression,not only the mechanical property is improved but also shape memory effect is maintained.