Elastic constants of equiatomic L1 0-ordered FePd single crystals

Abstract

The elastic properties of a γ 1-FePd single crystal have been measured using the pulse–echo overlap method. The anisotropic elastic constants of this tetragonal L1 0 structure are explained in part by the different bond strength between Fe–Pd neighbors compared with Fe–Fe and Pd–Pd neighbors. The tetragonal elastic anisotropy as measured for the single crystal appears to depend on the remaining fraction of misoriented order variants. The anisotropy parameters for γ 1-FePd are ( c 44/ c 66) = 0.93 ± 0.04, A 1 = 2 c 66/( c 11 − c 12) = 2.78 ± 0.14 and A 2 = 4 c 44/( c 11 + c 33 − 2 c 13) = 2.53 ± 0.40, implying a shear stiffness along [1 0 0] that is smaller in (0 0 1) than in (0 1 0). The magnitude of the elastic anisotropy (e.g., A 1 and A 2) of γ 1-FePd is similar to that of some face-centered cubic metals (e.g., Au with A = 2 c 44/( c 11 − c 12) = 2.90 and Ni with A = 2.51) and considerably larger than that of γ-TiAl (e.g., A 1 = 1.44 and A 2 = 1.98). However, the deviations of the considerably anisotropic elastic properties of the tetragonal γ 1-FePd from that expected for an equivalent elastically anisotropic crystal of cubic symmetry are less pronounced than for γ-TiAl. In contrast to γ-TiAl, the elastic constants indicate that shearing in the (1 1 1) plane is easiest along [−1 1 0] and increases in difficulty as the shear direction changes via [−1 0 1] towards [−1 −1 2] for γ 1-FePd.