Change of Acoustic Emission Characteristics during Temperature Induced Transition from Twinning to Dislocation Slip under Compression in Polycrystalline Sn.

In this study, acoustic emission (AE) measurements on polycrystalline tin as a function of temperature at different driving rates under compression were carried out. It is shown that there is a definite difference between the acoustic emission characteristics belonging to twinning (low temperatures) as well as to dislocation slip (high temperatures). The stress averaged values of the exponents of the energy probability density functions decreased from ε = 1.45 ± 0.05 (-60 °C) to ε = 1.20 ± 0.15 (50 °C) at a driving rate of ε=0.15 s-1, and the total acoustic energy decreased by three orders of magnitude with increasing temperature. In addition, the exponent γ in the scaling relation SAE~DAEγ (SAE is the area and DAE is the duration) also shows similar temperature dependence (changing from γ = 1.78 ± 0.08 to γ = 1.35 ± 0.05), illustrating that the avalanche statistics belong to two different microscopic deformation mechanisms. The power law scaling relations were also analyzed, taking into account that the detected signal is always the convolution of the source signal and the transfer function of the system. It was obtained that approximate values of the power exponents can be obtained from the parts of the above functions, belonging to large values of parameters. At short duration times, the attenuation effect of the AE detection system dominates the time dependence, from which the characteristic attenuation time, τa, was determined as τa ≅ 70 µs.

Acoustic Emission Characteristics and Change the Transformation Entropy after Stress-Induced Martensite Stabilization in Shape Memory Ni53Mn25Ga22 Single Crystal.

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.