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Significance
The mechanical behavior of numerous solid materials is governed by the twining process – a vital plastic deformation mode that occurs through nucleation and twin boundary motion. Besides, the twinning reorientation mechanism is responsible for the attenuation of seismic waves in the Earth’s lower mantle. Recently, it has been discovered that twinning reorientation in shape memory alloys (SMAs), ferroelectric materials and ferromagnetic SMA (FSMA) produce significant straining effects that provide a fundamental mechanism for facilitating the transformation between mechanical, electrical, thermal and magnetic energy. This interesting property finds many applications in actuation, energy harvesting and sensing.
Twinning boundaries propagate in a jerky motion consisting of avalanches when driven at slow rates. At higher speeds, the propagation of the twin boundary motion also displays a jerky motion that follows kinetic relations dependent on the boundary lattice barriers. Nevertheless, the magnitude of the jerks in both cases remains unclear. Also, the connection between the avalanches at slow rates and the changes in velocity at higher rates, if any, remains largely underexplored. Different methods have been used to measure and study the twin boundary motion at slow and high rates. However, most have several drawbacks that prevent them from being used to study jerky motion at micro- and nano-scale.
Recently, the potential application of magnetic emission measurements in the study of avalanches has been demonstrated. Its sensitivities to small-scale avalanches are comparable to acoustic emission (AE). It also captures the overall magnetization change rate and can directly measure the displacement of a moving interface. This is often difficult to achieve due to the material behavior complexities. In FSMA, for instance, the twinning reorientation-induced magnetization changes are often coupled with that of ordinary domain switching, hindering the ability to relate the motion of a particular twin boundary to the measured magnetic emission (ME) events.
To overcome these challenges, PhD candidate Emil Bronstein, Professor Ronen Talmon and Professor Doron Shilo from the Technion – Israel Institute of Technology in collaboration with Dr. László Tóth, Dr. Lajos Daróczi and Prof. Dezsö Beke from the University of Debrecen studied the jerky motion of twin boundaries in the FSMA Ni-Mn-Ga during slow compression. The generated avalanche events were simultaneously measured by means of force and ME and sensors. The experiment was carefully designed to eliminate the possible process complexity to permit the study of single twin boundary motion. Their work is currently published in the journal, Advanced Functional Materials.
The research team observed numerous small and rapid ME avalanches with features akin to jerky twin boundary motion during and between stress drop avalanches. For each stress drop, the overall ME correlated well with the stress drop amplitude, suggesting that the measured ME was directly related to the twin boundary motion. Moreover, the ME measurements were shown to capture avalanches that occur at nanometer and microsecond scales. Further statistical analysis of the ME events during and between the stress drops showed that they were created through the same process, with the difference between them being the delay times between the events. This implied that the avalanches observed during the slow rate twin boundary motion and the subsequent changes in the velocity during high-rate motion represent similar behaviors that can be explained using the same theory.
In summary, the authors successfully used the simultaneous stress and ME measurements to study the twin boundary jerky motion in FSMA Ni-Mn-Ga. The values of the transformed volumes calculated by the ME signals agreed well with those calculated based on the stress drops. Although the same process was observed during and between stress drops, the avalanche occurrence was much greater during stress drops. The results also revealed the possibilities of unexplored avalanche hierarchies with much smaller sizes and durations that could not be detected via ME detection sensor. In a statement to Advances in Engineering, the authors said their findings contribute to developing a unified theory for describing the twin boundary motion at different rates and scales.
Credit Adv Funct Materials, Volume: 31, Issue: 50, First published: 17 September 2021, DOI: (10.1002/adfm.202106573)
Reference
Bronstein, E., Tóth, L., Daróczi, L., Beke, D., Talmon, R., & Shilo, D. (2021). Tracking Twin Boundary Jerky Motion at Nanometer and Microsecond Scales. Advanced Functional Materials, 31(50), 2106573.
