Transformation And Mechanical Behavior Of Aged Ni-Mn-Ga-Ti Alloys

The aging precipitation behavior, martensitic transformation characteristics, mechanical properties, magnetic properties of Ni53Mn23.5Ga23.5-xTix (x=0, 0.5, 2, 3.5, 5, 8) alloys have been systematically investigated by means of SEM, TEM, XRD, DSC and compression test as well as ac susceptibility measurements and MFIS tests. The precipitation growth behavior of the second phase has also been illuminated. Moreover, the effect of the aging second phase on the martensitic transformation, mechanical properties and magnetic properties has been revealed. In addition, the mechanism of improving the mechanical property by the aging second phase has been studied.The results show that the crystal structure of solution-treated Ni-Mn-Ga-Ti alloys is 5M martensite at room temperature. The second phase of Ni3(Ti,Mn,Ga) with hexagonal structure is precipitated in the matrix of Ni53Mn23.5Ga23.5-xTix alloys between 773K and 1173K.Ni-Mn-Ga-Ti alloys exhibit one-step thermoelastic martensitic transformation during the heating and cooling procedure. The martensitic transformation temperatures of the studied alloys increase with increasing Ti content. Moreover, it is found that aging treatment has obvious effect on the martensitic transformation temperatures. For Ni-Mn-Ga-Ti alloys with 3 h aging time, the martensitic transformation temperatures first decrease and then increase with the increase of aging temperature. When the aging temperature is fixed at 973 K, the martensitic transformation temperatures reach its minimum value. When the aging temperature is 873 K, the martensitic transformation temperatures decrease with the increase of aging time.The experimental results show that the fracture strength and strain first increase and then decrease with increasing the Ti content, and reaching the maximum value when Ti content is 5 at.%. In addition, it is noted that proper aging treatment improves the mechanical property of Ni-Mn-Ga-Ti alloys significantly. When the studied alloys aging between 873 K and 1173K, the fracture strength and strain increases first increase then decrease with the increase of aging temperture.When the aging temperature is fixed at 873K, the fracture strength and strain decrease with the increase of aging temperature. In particular, the fracture strain of Ni53Mn23.5Ga18.5Ti5 alloy aged at 873 K for 0.5 h is 11.9%, which is two times higher than that of Ni-Mn-Ga alloy. Ni53Mn23.5Ga18.5Ti5 alloy after constraint aging at 873 K for 3h, the fracture strength of it reaches 1403 MPa.The aging-treatment changes the fracture type of Ni-Mn-Ga-Ti studied alloys. The fracture morphology of solution-treated Ni-Mn-Ga alloys exhibit brittle intergranulal fracture. With the increase of aging temperature and time, the fracture type gradually becomes ductile intragranular fracture. However, as the aging temperature is higher than 973 K, the fracture morphology changes to peeling along the grain boundary due to high ratio of precipitation in grain boudary, resulting in the increases of brittle.The Curie temperature and saturation magnetization decrease linearly with the increase of Ti content in solution-treated Ni-Mn-Ga-Ti alloys. The Curie temperature first decreases and then increases with increasing aging temperature, reaching the minimum value at 973 K. While, the saturation magnetization decreases slightly with increasing aging temperature. In addition, it is found that the MFIS decreases with increasing Ti content. Both the aging and thermo-mechanical training can raise the saturated MFIS significantly. The mechanism for the effect mentioned above can be attributed to the coherent stress-field in the matrix, which is generated by the dispersed coherent Ni3(Ti,Mn,Ga) precipitation. When the martensitic transformation occurs during cooling, the martensitic variant with the highest orientation factor would grow first and become the preferred orientation martensite at room temperature, which leads to the significant raise of the MFIS.