Compressive deformation of polycrystalline Ni-Mn-Ga alloys near chemical ordering transition temperature

Highlights: • Plasticity in a Ni_51.3Mn_27.6Ga_21.1 alloy was demonstrated over L2₁ → B2 transition temperatures. • DRV process occurred around the L2₁ → B2 transition temperature range. • DRX and DRV/DRX mixed processes existed in B2 phase. • Partially ordered crystal structure and DRX contributed to the good plasticity of B2 phase. Ferromagnetic shape memory Ni-Mn-Ga alloys exhibit an ordered L2₁ to partially ordered B2 transition at temperatures 873–1073 K. Based on the isothermal compressive results, it is demonstrated that L2₁ phase was brittle near ordering temperature, but the B2 phase was excellent above the chemical ordering temperature. In the compressive mode at temperatures of 473–1273 K and strain rates of 0.001–1 s⁻¹, the plastic deformation mechanism was found to be strongly dependent on 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. Based on the processing map constructed from the isothermal compressive curves, hot extrusion of Ni-Mn-Ga alloy with ratio as high as 9:1 was achieved at 1273 K. This proved that polycrystalline Ni-Mn-Ga alloys are strikingly facile for large plastic deformation in the B2 state by dynamic recrystallization at temperatures around 1273 K and strain rate 0.03 s⁻¹, which affords a viable temperature and strain rate processing window for this intrinsically brittle alloy.