Observation of Non-Hookean Huge Elastic Deformation in a Bulk Heusler-Type Cu–Al–Mn Alloy

The familiar metals and alloys in our daily lives are mostly crystalline materials, and they elastically deform because of temporary stretching or contracting of bonds between atoms. The ideal elastic strain for a crystalline metal is the strain at which the lattice itself disintegrates and hence set a firm upper bound of the elastic strain of the material. Theoretically, this strain value can be in the order of 10% for most crystalline metals with absolutely no defects. However, a macroscopic block of conventional crystalline metals practically suffers a very limited elastic deformation of <0.5% with a linear stress–strain relationship obeying Hooke’s law. In this presentation, we present our recent results on the experimental observation of a large tensile elastic deformation with an elastic strain of >4.3% in a Cu–Al–Mn single-crystalline alloy with an L2₁-ordered structure at its bulk scale at room temperature. The large macroscopic elastic strain that originates from the reversible lattice strain of a single phase is demonstrated by in-situ microstructure and neutron diffraction observations. Furthermore, the tensile elastic reversible deformation, which is nonhysteretic and quasilinear, is associated with a pronounced elastic softening phenomenon. The increase in the stress gives rise to a reduced Young’s modulus, unlike the traditional Hooke’s law behavior. This non-Hookean huge tensile elastic deformation behavior is discussed in terms of the strong lattice anharmonicity in the present bulk Heusler-type Cu–Al–Mn crystalline alloy. The experimental discovery of a non-Hookean huge elastic deformation offers the potential for the development of bulk crystalline metals as high-performance mechanical spring materials for use as metallic seals and connectors, or for new applications via “elastic strain engineering.”