The mechanism clarification of Ni-Mn-Fe-Ga alloys with excellent and stable functional properties

The functional properties, thermomechanical stability, ductility and shape memory effect of four types of two-phase NiMnGa-based high-temperature shape memory alloys were investigated, including Ni_56+xMn ₂₅Ga_19-x (x = 1, 2, 3, 4), Ni ₅₆Mn_25-yFe_yGa₁₉ (y = 4, 8, 12, 16), Ni₅₆Mn_25-zCo_zGa ₁₉ (z = 4, 6, 8) and Ni₅₆Mn_{25- w}Cu_wGa₁₉ (w = 2, 4, 8) alloys. It is found that Ni₅₆Mn_25-yFe_yGa₁₉ alloys (y > 4) exhibit the most excellent thermomechanical stability with no γ′ precipitates after thermomechanical cycling. Results further show that different alloying elements (Ni/Co/Cu/Fe) have different effects on the mechanical properties of both γ phase and corresponding two-phase NiMnGa-based alloys, which are closely related to the thermomechanical stability, ductility and shape memory effect of these alloys. The strength of Ni-Mn-Ga alloys decreases obviously with the addition of Fe, and the ductility of the alloys is effectively improved due to the formation of the most ductile γ phase with the lowest strength in Ni₅₆Mn_{25- y}Fe_yGa₁₉ alloys. Particularly, Fe addition significantly improves shape memory effect of Ni-Mn-Ga alloys. As a result, two-phase Ni₅₆Mn_25-yFe_yGa ₁₉ alloys exhibit the most excellent and stable functional properties as potential high-temperature shape memory materials.