Importance of the study of thermal properties in the research of Ni-Mn-Ga shape memory alloys, pp. 181-230
Authors: Alexandra Rudajevova
Abstract: The thermal properties of Ni-Mn-Ga based shape memory alloys were studied in the temperature range from room temperature up to 650 K. Both studied polycrystalline alloys, Ni53.6Mn27.1Ga19.3 and Ni54.2Mn29.4Ga16.4, are high-temperature shape memory alloys. The thermal expansion of the Ni53.6Mn27.1Ga19.3 alloy was investigated for samples with equiaxed grains and columnar grains. The thermal expansion characteristics of both structural types differ in the transient temperature range. Preferential orientations of the martensitic variants in the columnar grains make positive change on the temperature dependencies of the thermal expansion characteristics in the transformation temperature range. Only very small changes of the dilatation characteristics were found in the alloy with equiaxed grains. The thermal expansion characteristics of the Ni53.6Mn27.1Ga19.3 alloy were studied also after compression deformation. Two types of deformation can be distinguished by the dilatation measurements. Thermal arrest memory effect was also studied by the dilatation technique for both Ni-Mn-Ga alloys. Analysis of these results shows that the thermal arrest memory effect is a consequence of both the shape memory effect and the stress focusing affect in the temperature range where the phase transformation occurs. The temperature dependence of the thermal diffusivity, specific heat, and density were determined experimentally. The thermal conductivity was obtained as a product of these parameters for the shape memory of the Ni53.6Mn27.1Ga19.3 alloy. The thermal diffusivity and thermal conductivity of the martensite was about 40% lower than for the austenite. Thermal diffusivity was measured by the flash method, where the time dependence of the temperature rise after irradiation was also determined. The latent heat of the phase transformation substantially lowers the temperature rise after irradiation in the transient temperature range. Martensitic phase transformation in Ni-Mn-Ga-based shape memory alloys is an athermal phase transformation that starts practically immediately after reaching a certain transient temperature. This temperature is given in each point of the sample by two processes: the thermal conductivity and the phase transformation. Both processes take place in cooperation and their dynamics determines properties of the shape memory material in the transient temperature range. The complex process of the propagation of the phase transformation in the sample is analyzed. The influence of the heat transfer in the sample as well as in the measuring system on the transformation temperatures is also discussed.