Martensitic transformation and abnormal thermal expansion in Ni-Mn-Ga alloy magnetic refrigeration materials

Abstract: Ni-Mn-Ga alloy is a kind of typical magnetic refrigeration material. The thermal expansion coefficient of the alloy is one of the most important parameters in the engineering design. The mismatch of thermal expansion coefficient between the various materials used in magnetic refrigerators can lead to thermal stresses that can trigger complex failure mechanisms like component distortion, stress rupture, and thermomechanical fatigue, and thereby seriously degrade device reliability and lifetime. Therefore, minimizing the mismatch between the materials is desirable if for no other reason than to significantly improve overall device reliability. In this work, the magnetic refrigeration materials Ni54+xMn19-xGa27 (x=0, 0.2, 0.4, 0.6) with nominal composition were fabricated by conventional arc melting in an argon atmosphere using high purity elements Ni (99.98%), Mn (99.98%) and Ga (99.999%) to reveal the thermal expansion properties of Ni-Mn-Ga alloy. The actual compositions, crystal structures, martensitic transformation temperatures, magnetic transition and zero field thermal strain of the alloys were experimentally investigated by a scanning electron microscope equipped with an energy-dispersive spectrometer (SEM-EDS), an x-ray diffractometer (XRD), a differential scanning calorimetry (DSC), a vibrating sample magnetometer (VSM) and a standard strain-gauge connected with physical properties measurement system (PPMS). The room temperature XRD patterns of Ni54+xMn19-xGa27 indicate that the alloys (x≤0.4) possess an ordered L21 (cubic) austenite structure and the sample (x=0.6) predominately possesses a 7M modulated martensitic structure with small residual traces of non-modulated martensitic structure. The results from DSC and magnetic transition measurements show that the martensitic transformation temperatures gradually increase with the Ni content increasing. Moreover, the peak temperatures of direct and reverse martensitic transformations increase from 257, 266 (x=0.0) to 292, 300 K (x=0.6), respectively, and they have a distinct thermal hysteresis of about 6-11 K. Zero field thermal strain curves illustrate that the Ni-Mn-Ga alloys exhibit an isotropic thermal expansion property. The thermal expansion coefficients range from 8.87×10-6 to 16.60×10-6 /K and from 5.21×10-6 to 6.26×10-6 /K at martensite phase and austenite phase, respectively, for Ni54+xMn19-xGa27 alloys. More importantly, the alloys exhibit an obvious and abnormal negative thermal expansion behavior in the vicinity of the martensitic transformation. The negative thermal expansion coefficients are in the range from -147.54×10-6 to -17.60×10-6 /K and from -133.30×10-6 to -88.72×10-6 /K respectively for heating and cooling the alloys. The amplitude of thermal expansion coefficient near the martensitic transformation is approximately 1.47-16.63 (heating) and 6.63-12.89 (cooling) times those of the martensite phase. This abnormal negative thermal expansion behavior may well be related to the nucleation and orientation of martensitic variants during the martensitic transformation. The mechanism behind the negative thermal expansion phenomenon is not clear and requires a further study with experimental techniques on microscopy. The results in this work are very meaningful for the adjustment and control of thermal expansion coefficient in Ni-Mn-Ga magnetic refrigeration material.