Influence of Multi-Axial Isothermal Forging on the Stability of Martensitic Transformation in a Heusler Ni-Mn-Ga Alloy

The Heusler alloys demonstrate magnetically induced strain and magnetocaloric properties, but the mechanical properties are poor. Therefore, in this work, the influence of thermomechanical treatment on the properties of Ni-Mn-Ga-Si Heusler alloys is considered. The effect of multi-axial isothermal forging on functional properties of the Ni2.30Mn0.73Ga0.90Si0.07 alloy at 973 K and true strain (e) of 3.9 is presented. It is shown that a unique two-phase microstructure is formed as a result of forging. The large grains in the size range of 100–200 µm are surrounded by a fine-grained structure. The study of the thermomechanical properties by the three-point bend test has shown that the alloy demonstrates a single-stage reversible deformation of 3.1% at a constant stress of 860 MPa as compared to the same alloy in the as-cast condition which shows 2% at 380 MPa. The specimen demonstrates a reversible deformation of 5% without any degradation during thermal cycling (with a base of up to 4000 thermal cycles) under a stress of 550 MPa and up to 5% with degradation occurring at 700 thermal cycles under a stress of 650 MPa. Thus, forging makes it possible to obtain a material with higher operational properties and greater resistance to fracture during multiple cycles of martensitic transformation. In this case, it is possible to obtain anisotropy of properties equal or close to that of the specimen in the as-cast state.