Superplasticity and ductile fracture of synthetic feldspar deformed to large strain

Abstract

We performed high‐strain torsion experiments on fine‐grained (≈4 μm) anorthite aggregates in a Paterson‐type gas deformation apparatus. The dense hydrous (≈0.1 wt% H2O) samples contain < 3 vol% Si‐enriched residual glass located at triple junctions. Specimens were twisted at constant rate to a maximum shear strain of about 5 at experimental conditions of 100–400 MPa confining pressure, temperatures of 950°C–1200°C, and shear strain rates of ≈2 × 10−5, 5 × 10−5, and 2 × 10−4 s−1. Resulting maximum shear stresses at the sample periphery were in the range of ≈2–80 MPa. The samples showed strain hardening at slow deformation rate and strain weakening at fast strain rate, respectively. Fitting the stress strain rate data to a power law yields linear viscous behavior. Microstructural analysis shows locally enhanced dislocation density, suggesting diffusion‐assisted and/or dislocation‐assisted grain boundary sliding as the dominant deformation mechanism. Deformed samples exhibit abundant cavities, nucleated mostly at grain triple junctions and at grain boundaries in response to cooperative grain boundary sliding. At shear strains ≥ 2, growth and coalescence of the cavities form an anastomozing network of regularly spaced strings oriented at about 30° to the direction of maximum compressive stress and ∼15° to the shear plane. Strain is localized along these shear bands associated with a shape‐preferred orientation of high‐aspect ratio feldspar grains and with segregation of initial pore fluids (residual glass) into the bands. More than one third of the samples were deformed to terminal failure that occurred suddenly at shear strains of ≈3–5. The experiments indicate that cavitation damage may facilitate fluid flow and deep seismicity in highly strained shear zones.