Mechanical Behavior of Superelastic ThCr₂Si₂-Structured Intermetallic Compounds and Their Derivatives

ThCr₂Si₂-structured intermetallic compounds and their derivatives are of interest in the field of solid-state physics due to their unique electronic and magnetic properties, including high-temperature superconductivity and pressure-dependent magnetism. Recently, micro-mechanical studies have revealed that they exhibit superelasticity through a lattice collapse-expansion mechanism. Under c-axis compression, Si-Si type bonds are formed, leading to the sudden reduction of c lattice parameter. Then, if the applied stress is relaxed, the crystal restores the original c lattice parameter by breaking Si-Si type bonds. This lattice collapse-expansion mechanism produces recoverable strain higher than 10%. Thus, ThCr₂Si₂-structured intermetallic compounds and their derivatives are considered as a new class of superelastic materials.<br/>This presentation will discuss the mechanical behavior of various ThCr₂Si₂-structured intermetallic compounds and their derivatives (CaFe₂As₂, (CaK)Fe₄As₄, LaRu₂P₂, and SrNi₂P₂) and their dependence on temperature and loading orientation. These crystals exhibit superelasticity via making and breaking As-As and P-P bonds. The corresponding stress-strain response with hysteresis loops resembles that of shape memory alloys. Due to the significant change in the c lattice parameter under compression, the extremely high recoverable strain (10~17%) are achieved. As a result, the modulus of resilience is orders of magnitude higher than that of most engineering materials. Unlike CaFe₂As₂, (CaK)Fe₄As₄, and LaRu₂P₂ that show superelasticity only under compression, SrNi₂P₂ exhibits superelasticity under both compression and tension because P atoms in SrNi₂P₂ are partially bonded at the stress-free state. C-axis compression forms a bond between unbonded P atoms and causes lattice collapse while c-axis tension breaks a bond between bonded P atoms and causes lattice expansion. As a result, SrNi₂P₂ exhibits tension-compression asymmetry in mechanical response, and this asymmetry leads to the elastocaloric effect comparable with conventional shape memory alloys such as Nitinol. For all four crystals, as the temperature decreases, the lattice collapse occurs more easily because thermal contraction makes bond formation easier while the lattice expansion becomes more difficult.<br/>In addition, we recently discovered that CaFe₂As₂ exhibits a unique hysteresis behavior in the load-depth curve for a-axis nanoindentation, which does not cause lattice collapse-expansion. Transmission electron microscopy revealed that many new grain boundaries are created through multiple instances of atomic layer buckling and the nucleated dislocations are piled-up near these grain boundaries. These piled-up dislocations cause a reversed plastic flow due to the back stress (the Bauschinger effect), leading to a large hysteresis loop in the nanoindentation load-depth curve. Density Functional Theory calculation confirmed that CaFe₂As₂ has an anisotropic layered structure, where atomic layer buckling and dislocation nucleation can occur easily.<br/>All results in this study provide an important insight into the fundamental understanding of the mechanical properties of ThCr₂Si₂-structured intermetallic compounds and their derivatives under different loading conditions and at different temperatures. Considering the existence of more than ~1000 possible ThCr₂Si₂-structured intermetallic compounds, our results will be greatly useful in identifying those with superelastic properties.