Effects of grain orientations and pre-existing defects on mechanical properties and deformation mechanisms of polycrystalline yttria-stabilized tetragonal zirconia

Abstract

Experimental and computational studies have revealed that the mechanical properties and phase transformation behavior of single crystalline zirconia-based shape memory ceramics are significantly dependent on their crystal orientation with respect to the loading direction. However, such effects have not been studied in polycrystalline zirconia-based ceramics. In this work, we utilize molecular dynamics simulations to investigate the effects of grain orientations and manufacturing-produced sub-surface defects on the mechanical properties and deformation mechanisms of polycrystalline yttria-stabilized tetragonal zirconia (YSTZ). Polycrystalline models are generated according to the dominant plastic deformation mechanisms of each individual grain utilizing the single crystal deformation data, and the polycrystalline models mainly comprise phase transformation-dominated grains (P-grain), combined phase transformation- and dislocation-dominated grains (P-D-grain), and dislocation-dominated grains (D-grain). Grain orientations are noted to play a significant role in determining the strength of polycrystalline YSTZ. Martensitic phase transformation is the dominant deformation mechanism in P-grain dominated polycrystals, while in P-D-grain and D-grain dominated cases amorphization and sliding of grain boundaries become the main deformation mechanisms, which can significantly degradate the shape memory behavior. The strength values of polycrystals are considerably decreased due to the addition of a void. Dislocations and phase transformation are observed to nucleate around the void, which accordingly alter the deformation mechanisms observed in defect-free polycrystals.