Transformation strain and volume effect associated with martensitic transformation in polycrystalline Ni49+x+yMn23-xGa28-y Heusler alloys.

Combined thermal strain and magnetization measurements were performed in polycrystalline alloys Ni 49.31 Mn 22.76 Ga 27.93 (Ni 49), Ni 53.00 Mn 21.04 Ga 25.96 (Ni 53) and Ni 55.62 Mn 17.52 Ga 26.86 (Ni 55.5) to understand the influence of magnetic field (B) on transformation strain (Δλ) and relative volume change (ω) at martensitic transformation (MT). It is found that during cooling and heating both Ni 49 and Ni 53 samples undergo a MT while the Ni 55.5 sample experiences a coupled magnetostructural transformation (MST). Such a MST can enhance the Δλ and ω whereas thermal hysteresis weakens them, which results in a non-monotonic variation in Δλ: 0.211% (Ni 49) → 0.166% (Ni 53) → 0.185% (Ni 55.5) under B = 0 T. Similar tendency was observed in ω. Besides, the values of Δλ and ω increase with increasing field, partially due to the contribution of magnetic field-induced strain. The maximum value ω = 0.674% was obtained in Ni 49 sample under 3 T, indicating that an extra volume space of ∼1% should be needed to avoid the destruction of refrigeration chamber. These findings provide key information for future use of Ni–Mn-Ga alloys in refrigeration engineering field. • Ni–Mn-Ga alloys exhibit a large transformation strain (Δλ) at martensitic transformation (MT). • Magnetostructural transformation can enhance transformation strain and relative volume change (ω). • Thermal hysteresis weakens the magnitude of Δλ and ω. • Magnetic field has a positive effect on the increase in Δλ and ω due to the magnetic field-induced strain. • A large relative volume change of 0.674% was obtained in Ni 49.31 Mn 22.76 Ga 27.93 under 3 T. [ABSTRACT FROM AUTHOR]