Study of effect of annealing on micro-hardness of molybdenum single crystals

Abstract

Molybdenum (Mo) crystals exhibit excellent mechanical properties such as high yield strength, high elastic modulus, very high melting point, and excellent corrosion resistance, making them a suitable choice for various industrial applications. The mechanical behaviour of these crystals has been investigated through tension, compression, and indentation experiments. The indentation experiments on annealed Mo single-crystal thin film show an increase in hardness with an increase in annealing temperature from 400-650˚C. However, it is not clear whether this behaviour is specific to the Mo thin films, or these are applicable to the bulk samples of single crystals also. In order to address these issues, the Vickers indentation experiments are performed on as-received and annealed samples of (100)-, (110)- and (111)-oriented Mo single crystals. The results show that the low-temperature annealing leads to an increase in micro-hardness by 9% for (100)-oriented crystals. The X-ray Diffraction (XRD), Electron Backscatter Diffraction (EBSD) and energy dispersive X-ray spectroscopy (EDS) studies have shown that there is a drop in the number of dislocation sources or dislocation density (𝜌) with no high-angle grain boundary formation or any microstructural changes or any oxide formation. This necessitates to enhance the applied stress required for significant yielding (or yield strength) which results in an increase in the hardness. Based on experimental observations, a hardening mechanism governed by dislocation annihilation during annealing has been proposed in the present study.