Acoustic emission during approaching the critical point on stress-temperature diagram of martensitic transformation in Ni48Fe20Co5Ga27 (at.%) single crystal

Recently, the existence of martensite-austenite critical point, CP, on the stress-strain curves in the single crystalline Ni-Fe(Co)-Ga ferromagnetic shape memory alloys has been discovered by disappearance of hysteresis. It was also argued that the shear modulus at the CP and in the postcritical phase should be zero. In the present work we continue the investigation of the above new features on the "compression stress-temperature" phase diagram of the martensitic transformation (MT) of Ni48Fe20Co5Ga27 (at.%) single crystals in the vicinity of the CP by acoustic emission (AE). It is found that the AE activity (amount of events and their amplitudes) drastically dropped down by approaching the CP and tended to stabilize in a postcritical region. The above decrease is mainly due to the decrease of the number of large avalanches, while the exponents of the probability distribution of amplitudes and energies remained approximately the same, illustrating no change in the mechanism of acoustic emission events. The coordinates of the CP were estimated to be (340 +/- 50) MPa and (327 +/- 40) K. The elastic modulus was reduced with increasing temperature from 20 GPa down to 2 GPa in the critical region, whereby a huge total deformation of about 13 % was achieved. The stress-induced MT also shifted as a function of temperature and its slope decreased by about a factor of two upon approaching the CP. All features of the critical state determined have an obvious practical interest, as well as present a solid basis for theoretical understanding of critical and postcritical states in solids.