Direct observation of mechanical twinning in pure iridium

Abstract

Mechanical twinning in crystalline solids is understood according to a recent study to occur via two distinct mechanisms; i.e. a step-wise (athermal) and a dislocation pole (thermal) mechanism. In f.c.c. structures, the two twinning mechanisms are both operational. Twinning by a dislocation pole mechanism normally takes place at elevated temperatures since thermal activation is needed in structures requiring a high passing stress. When twinning occurs by the step-wise mechanism, the selection of {111} planes as twin boundaries to form a coherent interface between the twin and the matrix lattice is limited. In the past twenty years, Field Ion Microscopy (FIM) has successfully been used to analyze many microstructural defects. However, there has been no detailed analysis of mechanical twin morphology using FIM techniques. We present here the FIM observations of deformation twins in pure Ir and compare the results with predictions of twin thickness and boundary structures according to the two twinning mechanisms.