Mechanical properties and atomistic deformation mechanism of γ-Y2Si2O7 from first-principles investigations

The theoretical mechanical properties and atomistic shear deformation mechanisms Of gamma-Y2Si2O7, one of the most refractory silicates and potentially useful as a high-temperature structural ceramic, were investigated using first-principles calculations. The material shows low shear moduli to bulk modulus ratios, as well as a low ideal shear strength to tensile strength ratio. The unusual low shear deformation resistance of gamma-Y2Si2O7 originates from the inhomogeneous strength of its chemical bonds. The Y-O bond is weaker and readily stretches and shrinks; and Si-O bond is stronger and more rigid. The relative softer YO6 octahedron positively accommodates shear deformation by structural distortion, while the Si2O7 pyrosilicate unit is more resistant to deformation. The reported shear-load-bearing mechanism is quite similar to those found in the "quasi-ductile" LaPO4 monazite and ternary layered carbides (the so-called MAX phases), and can endow gamma-Y2Si2O7 with quasi-ductility and damage tolerance. (C) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.