Investigation Of Training Treatments And Its Effect On Mechanical Properties Of Ni-Mn-Ga Foams

Ni-Mn-Ga foams with one dimensional and three dimensional pores werefabricated by replication method. The pore structures were observed by scaningelectron microscope (SEM) and optical microscope (OM). The phase and phasetransformation process were analyed by X-ray diffraction (XRD), differentialscanning calorimetry (DSC) and vibrating sample magnetometer (VSM). Threedimensional pore structure Ni-Mn-Ga foams with monomodal pores and bimodalpores were investigated after different training treatments, mainly about twinningstress and porosity. At last, shape memory effect and superelastic of Ni-Mn-Gafoams were measured after different training treatments.Both DSC and VSM measurements indicated that martensitic transformationtemperatures of the Ni-Mn-Ga foams with three dimentional pores were aroundroom temperature (RT) and the XRD results confirmed a five-layer modulatedmartensite at RT. According to OM results, twins can span from one pore across theentire strut to the next pore after heat treatment.Mechanical training process favored the growth of the variants along the stressdirection at the expense of others, thus, reduced the number of variants as well asthe twining stresses. A significant reduction of twining stress from6.12MPa to0.75MPa after5times mechanical training was observed in Ni-Mn-Ga foam withbimodal pores, which was comparable to Ni-Mn-Ga single crystals. In comparisonwith Ni-Mn-Ga foam with monomodal pores, Ni-Mn-Ga foam with bimodal poreshave lower twining stress because of its unique pore structures. In addition, one wayshape memory in Ni-Mn-Ga foams can be improved after mechanical training. Oneway shape memory recovery ratio as well as recovery strain were increased withincreasing times of mechanical training.Internal stress and irreversible defects were introduced afterthermal-mechanical training which led to the occurrence of two way shape memoryeffects. Cooling during thermal-mechanical training favored the variants with theshort axis along the compressive stress direction and eventually a single variant orrare variant martensite can be formed. The twining boundary mobility was improvedwith increasing training stress, which can be observed from a reduction of modulusof the foams. Better two way shape memory effect existed after properthermal-mechanical training while the recovery rate was reduced if too manytraining cycles were applied. Thermal cycling treatment during training process canimprove the effects of mechanical training while counteracts the effect of thermal-mechanical due to the irreversible defects. After40times ofthermal-mechanical training, the transformation strain of two way shape memory ofNi-Mn-Ga foam decreased from70%to32.1%.Superelastic tests were carried out in Ni-Mn-Ga foams with bimodal pores. Thecritical stress required for inducing martensitic transformation increased withincreasing temperature above As. The stress induced martensitic transformationdidn’t exist at15MPa at a temperature slightly above Ms, while the superelasticrecovery rate and revovery strain both increased with increasing temperature.