Atomistic insight into ideal strength, fracture toughness, deformation, and failure mechanisms of magnetocaloric Fe2AlB2

The thermal and magnetic cycling of a magnetocaloric material degrades its mechanical properties and device performance. We used ab initio tensile and shear simulations to investigate the mechanical properties such as ideal strength, fracture toughness and deformation and failure mechanisms of Fe2AlB2 at finite strain. The weakest direction of Fe2AlB2 is [010], and the weakest slip system is (010)[100]. The ideal tensile strength (sigma(m) = 12.51 GPa) of Fe2AlB2 is less than its ideal shear strength (tau(m) = 13.32 GPa). The strain energy difference (Delta E = -13 eV/f.u.) of Fe2AlB2 confirms cleavage fracture as its most plausible failure mode. The concomitant changes in the c-lattice parameter and Al-Al bond along the c-axis determine the ideal tensile strength of Fe2AlB2. Likewise, the subtle changes in the a-lattice parameter and Al-Al bond along the a-axis specify its ideal shear strength. The tensile strain induces a magnetic to nonmagnetic transition in Fe2AlB2 at the critical tensile strain (epsilon(c) = 0.08). A similar transition occurs at the critical fracture strain (epsilon(cf) = 0.48) due to shear deformation. The brittle nature of Fe2AlB2 is predicted by its anisotropic Poisson's ratios, strength ratio, and failure mode. The fracture toughness of Fe2AlB2 for mode I fracture is (K-Ic = 2.17 MPa m(1/2)), mode II fracture is (K-IIc = 1.33 MPa m(1/2)), and mode III fracture is (K-IIIc = 1.16 MPa m(1/2)). The failure mechanism of Fe2AlB2 due to the tensile deformation is marked by the sharp and appreciable changes in the lattice parameters, bonding characteristics, and magnetic moment of Fe at the critical fracture strain (epsilon(cf) = 0.44). This study provides a fundamental understanding of the mechanical behavior of Fe2AlB2 at the finite strain relevant to the cycling stability of the magnetocaloric Fe2AlB2.