Ferromagnetic shape memory alloys, including Ni-Mn-Ga and Ni-Mn-X (X=In, Sn, Sb), exhibited multifunctional properties such as magnetic-field-induced strain, magnetic shape memory effect, magnetoresistance, magnetocaloric effect and elastocaloric effect. These materials have great potential applications for actuators, sensors, transducers and refrigerants. However, they limited by their high intergranular fracture tendency and were generally brittle at room temperature, making them difficult to be plastically deformed at the ordered L21 state. It has long been revealed that stoichiometric Ni2MnGa alloy exhibited chemical ordering transition from ordered L21 phase to partial ordered B2 phase at high temperatures over ~1000 K. In such B2 state, the alloy may show a significant enhancement in the plastic deformation capacity, which may probably provide a feasible strategy for shaping engineering components for various applications. Unfortunately, the underlying mechanism governing the high temperature plastic deformation processes are not yet well understood.
Researchers led by Professor Xuexi Zhang from Harbin Institute of Technology in China, investigated the high temperature behavior near chemical ordering transition temperatures to reveal the plastic deformation ability and mechanism of L21 and B2 phases. Their work was recently published in peer-reviewed journal, Materials & Design.
The researcher begun their studies by using isothermal compression method for the Ni-Mn-Ga alloys from 473 K to 1273 K. They then generated the processing map based on the compressive curves and applied the hot extrusion of Ni-Mn-Ga alloy to verify the validity of processing map. Then, they investigated the microstructures and grain orientation evolution after compressive deformation. Finally, the research team discussed the plastic deformation mechanism and plastic workability of Ni-Mn-Ga alloys.
The authors confirmed that Ni-Mn-Ga alloys displayed different deformation abilities near chemical ordering transition temperature. The ordered L21 phase exhibited brittle fracture below 473 K and the plastic deformation occurred in low strain rate above 573 K. At temperatures higher than the chemical ordering transition temperature, the intrinsically brittle polycrystalline alloy displayed strikingly plastic deformation capacity in the partial ordered B2 phase. The deformation working window for Ni-Mn-Ga alloys was temperature 1223 – 1273 K and stain rate 0.02 – 0.3 s-1, and dynamic recrystallization was the dominating deformation mechanism.
The team observed the microstructure evolution under different compressive conditions. It was revealed that the mechanism of the compressive deformation in Ni-Mn-Ga alloys may be described by the Zener-Hollomon parameter Z. The dynamic recrystallization was the dominant mechanism at the low Z region in the B2 state then with increasing Z the dynamic recovery and ordering transition from B2 to L21 occurred, finally only dynamic recovery existed at the high Z region in the L21 state.
Since this is the first study on the hot deformation capacity of Ni-Mn-Ga alloy, the authors anticipate that it will attract the attention of many researchers and provide an avenue for future developments in this field.
L.S. Wei, X.X. Zhang, M.F. Qian, P.G. Martin, L. Geng,T.B. Scott, H.X. Peng. Compressive deformation of polycrystalline Ni-Mn-Ga alloys near chemical ordering transition temperature. Materials & Design, 142 (2018) Pages 329-339.
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