The Effect of Secondary-Phase Fraction on the Deformation of Olivine + Ferropericlase Aggregates (DATA)

Abstract

To study the mechanical and microstructural evolution of polymineralic rocks, we performed deformation experiments on two-phase aggregates of olivine (Ol) + ferropericlase (Per). Two-phase samples were prepared with periclase fractions (fPer) between 0.1 to 0.8. Additionally, single-phase samples of both Ol and Per were prepared to facilitate comparison between the mechanical and microstructural behavior of two-phase and single-phase materials under the same experimental conditions. Each sample was deformed in torsion at T = 1523 K, P = 300 MPa at a constant strain rate up to a final shear strain of γ = 6 to 7. Microstructural developments indicate differences in both grain size and crystalline texture between single- and two-phase samples. During deformation, grain size approximately doubled in our single-phase samples of Ol and Per but remained unchanged or decreased in two-phase samples. Zener-pinning relationships fit to the mean grain sizes in each phase demonstrate that the grain size of the primary phase is controlled by phase boundary pinning. The stress-strain data and calculated values of the stress exponent, n, indicate that Ol in our samples deformed by dislocation-accommodated sliding along grain-grain interfaces while Per deformed via dislocation creep. At shear strains of γ < 1, the strengths of samples with fPer ≥ 0.5 match those modeled by assuming both phases deform at the same stress, while the strengths of samples with fPer ≤ 0.5 are greater than predicted by instead assuming both phases deform at the same strain rate. Above γ = 4, however, sample strengths are greater than those predicted by either the uniform stress or the uniform strain rate bound. We hypothesize that these high strengths are due to the presence of phase boundaries throughout our two-phase samples, for which deformation is rate-limited by dislocation motion along interfacial boundaries.