Abstract
Measurements have been carried out to compare stress-induced martensite stabilization aged (SIM-aged) and as grown shape memory Ni53Mn25Ga22 single crystals with the means of simultaneous differential scanning calorimetry (DSC) and acoustic emission (AE). Contrary to expectations, the position of the hysteresis practically did not change, whilst the width of the hysteresis increased, and the forward and reverse transitions became sharper in the SIM-aged sample. The energy distributions of acoustic hits showed regular power law behaviour and the energy exponents were slightly different for heating and cooling; this asymmetry had different signs for the SIM-aged and as grown samples. During heating, in accordance with the sharper transitions observed in the DSC runs, two well-marked jumps could be seen on the plot of cumulative number of the acoustic emission events. Therefore, these were attributed to high sudden jumps in the phase transition during heating observed in the DSC. The effect of the SIM-aging on the transformation entropy was also investigated and it was found that it was about 36% less in the case of the SIM-aged sample.
Highlights
Martensite stabilization heat treatments under uniaxial stress, large enough to produce a single variant martensite structure, can result in stabilization of this martensite variant [1,2,3,4]
We found in Ni51 Fe18 Ga27 Co4 single crystals [15] that the transformation entropy decreased by about 36% after stress-induced martensite (SIM)-aging and it was practically unchanged after austenite stabilization treatment
The signs of the changes in our previous results [15] were in accordance with the data given in [24], we observed about a three times larger decrease due to SIM-aging. This fact calls for further measurements and we investigated the change of the transformation entropy in the Ni53 Mn25 Ga22 alloy too
Summary
Martensite stabilization heat treatments under uniaxial stress, large enough to produce a single variant martensite structure, can result in stabilization of this martensite variant [1,2,3,4]. The main features of the above stabilization can be understood on the basis of the so-called symmetry conforming short range ordering process [4,10,11,12] While this plausibly predicts the shift of transformation temperatures, To , to higher values, the SIM-aging, by giving preference to one martensite variant, should have important additional effects (related to dissipative and elastic energies) on the martensitic transformation. In the present paper we report comparative investigations of the martensitic transformation in SIM-aged and as grown (not SIM-aged) shape memory Ni50 Mn28.5 Ga22 single crystals using DSC and AE measurements These results, together with the results obtained in [15] will provide a good set of experimental data for better understanding of the effects of SIM-aging. This fact calls for further measurements and we investigated the change of the transformation entropy in the Ni53 Mn25 Ga22 alloy too
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