Effect of solution treatment on the martensitic transformation behavior of a Ni₄₃Co₇Mn₃₉Sn₁₁ polycrystalline alloy

The effect of solution treatment on the martensitic transformation behavior of a Ni₄₃Co₇Mn₃₉Sn₁₁ polycrystalline alloy fabricated by an arc melting method was investigated by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and differential scanning calorimetry (DSC). The examination indicates the presence of severe chemical segregation in the dendritic as-cast structure because of solidification. This chemical segregation completely impedes the intrinsic martensitic transformation. Annealing at 1223 K for 24 h is identified as the threshold annealing condition to eliminate the microstructural segregation and begin the martensitic transformation, as indicated by a broad and obscure feature. Annealing at 1273 K for 24-48 h is found to be effective at promoting notably the martensitic transformation, but the martensitic transformation exhibits a multiple-step feature. Complete homogeneity is achieved by annealing at 1273 K for 72 h, which produces a sharp, single-step martensitic transformation. The microstructural evolution and the valence electron concentrations of alloys (e/a ratio) are evaluated, which are reflective of the degree of compositional homogeneity of alloys, confirming that high annealing temperature and long holding time are vital to reveal the intrinsic martensitic behavior of this alloy. The adequately homogenized alloy displays a martensitic transformation at 292 K and an enthalpy of 11.2 J/g.