Analysis of size effects during martensitic transitions in epitaxial films and microparticles of the Ni–Mn–Sn alloy

The available literature data on the effect of the thickness of the epitaxial films and size of the crystallites in the microparticles of the Ni–Mn–Sn alloy powder on the parameters of the martensitic transitions in this alloy have been analyzed in the framework of the theory of defuse martensitic transitions (DMT), which is based on thermodynamic and kinetic ratios. The purpose of the analysis is to establish an explicit (functional) dependence of these parameters on the film thickness $D$ and the size of nanocrystallites $d$ in the film and microparticles. The results of the $R$-diffraction analysis show that internal elastic microdeformations and stresses arise in films and microparticles of the alloy powder due to the coherent coupling of the epitaxial film with a solid substrate or as a result of severe plastic deformation of the alloy during its grinding in a ball mill. The analysis shows that the local microstresses significantly affect the type of the dependence of the temperature interval (diffusity) $\Delta T$ of the transition on the size factors $D$ or $d$. In the absence of microstresses, these dependences have the form $\Delta T\sim 1/D^{2}$ or $\Delta T\sim 1/d^{2}$. In the presence of microstresses, the temperature transition interval depends on the size factors as $\Delta T\sim 1/D$ или $\Delta T\sim 1/d$.