In spite of the very good performance (i.e. especially high twin boundary mobility and actuating efficiency) of MSMAs in laboratory experiments with individual twin boundaries, the practical performance (e.g. actuating strain, efficiency, force and fatigue) is typically far from optimum. This originates from the fact that self-accommodated twin microstructures are not optimal for MIR. If there is no control of twin microstructure, the behavior of the material may be poor. The performance is slightly better with type 1 twins and the best with type 2 twins. The importance of type 2 twins is best seen when considering the typical temperature dependencies of twin boundary mobility, see Figure below [StrakaT1xT2, Straka_IMT, Heczko1.7K]. While type 2 twins may perform very well till 1.7 K, type 1 twins are usable only in quite narrow temperature interval below martensite transformation [Straka_Actuator_2014].
So-called switching field HSW(T) as a measure of twin boundary mobility for type 1 and for type 2 twins [Straka_IMT, Heczko1.7K]. Lower HSW is better. High efficiency and zero efficiency levels are indicated by blue and red lines, respectively.
The problem to adress is that insofar nobody knows how to effectively force the material to maintain permanently the highly mobile (type 2) twins and, consequently, to obtain the best practical performance. More generally considered and formulated, there has been numerous previous studies related to the behavior of twinned structures in SMAs, e.g. work of Roytburd et al. on polydomain heterostructures and crystals [1999_Roytburd], and MSMAs. However, the understanding is still far from complete in the field of MSMAs, hindering the practical exploitation of this promising material. There is an urgent need and demand from applied research to better control the twin boundary mobility, and especially to find methods how to create and maintain permanently the twinned microstructures with optimum performance [Actuator_conference]. There are still too many unanswered significant scientific questions, such as what are the physical reasons behind specific temperature dependencies of mobility, whether the dependencies can be modified, and what are the ultimate limits for the twin boundary mobility. The problem and discussion are not limited to MIR-related effects, but are relevant also for magneto/elasto-calorics or other effects employing the phase transformations. The (twin) microstructure has very strong influence on the transformations (e.g. on thermal hysteresis) and on the consequent magneto/elasto-caloric behavior and on the performance of other transformation-including effects.
Reflecting the above described problem, the aim of the project is to build a comprehensive understanding and theoretical description of formation mechanisms of twins and of twin-twin and twin-defect interactions in Ni-Mn-Ga based magnetic shape memory alloys. Further we aim to apply these new findings for development and further study of permanent twin microstructures with optimal functional properties. The proposed project can be characterized as experimentally oriented basic research supported by the relevant theory, but is motivated by issues close to applied research.