Creep of polycrystalline anorthite and diopside

Abstract

We investigated the creep strength of fine‐grained aggregates of single‐phase anorthite and diopside at temperatures ranging from 1253 to 1553 K at flow stress between 5 and 250 MPa and at 0.1 MPa pressure. Powders of crushed natural, iron‐bearing diopside and of pure anorthite and diopside glasses were hot pressed at temperatures of 1373–1423 K and 300 MPa confining pressure in a gas‐medium apparatus and at 1423–1473 K and 2200 MPa in a piston‐cylinder device. Grain size of the synthetic materials was 3.4 μm for anorthite and between 1.5 and 7.5 μm for diopside. Creep tests were performed using two uniaxial creep machines in Toulouse and Potsdam. Synthetic anorthite and diopside aggregates showed a stress exponent n ≈ 1 and a grain size exponent m ≈ −3, suggesting grain boundary diffusion‐controlled creep. Creep rates were independent of oxygen fugacity. Coarser‐grained (43 μm) iron‐bearing samples showed a transition from diffusion‐controlled creep to dislocation creep with n ≈ 5. Activation energies for diffusion and dislocation creep of iron‐bearing diopside polycrystals were Q = 364–468 and 719 kJ/mol, respectively. The activation energies for diffusion‐controlled creep of synthetic fine‐grained anorthite and diopside aggregates were Q = 362–383 and 558 kJ/mol, respectively. At similar thermodynamic conditions, diffusion creep rates of diopside are nearly 10 times lower than for anorthite. Irrespective of processing route, the data for anorthite and diopside aggregates from both laboratories are in good agreement.