Indentation response of body centered cubic molybdenum single crystals

Abstract

Molybdenum (Mo), a body-centered cubic (BCC) metal, has been found to be a potential candidate for various applications including semiconductor, defense, and medical industries, owing to their high yield (~324 MPa) and compressive strength (~400 MPa), superior fracture strength (), very high melting point (~2890 K), good thermal conductivity (~138 W/(mK) at 300 K) and exceptional corrosion resistance. Tension and compression experiments have shown that the mechanical response of Mo crystal depends on the applied strain rate, ambient temperature, and crystallographic orientations. Further, experimental and numerical studies performed on other BCC single crystals show that the hardness and pile-up patterns depend on their crystallographic orientation. It must be mentioned that most of the studies are performed on Ta, W, BCC Fe and Ti alloys, and little attention has been given to the Mo crystals. Consequently, deformation response of Mo crystals is not well understood. Therefore, nano- and micro-indentation experiments on Mo single crystals oriented along (100), (110) and (111) are performed to understand their indentation response, in this thesis. Atomic force microscopy is used to analyze the impression of the indents. Results show that nano as well as micro-hardness depends of the orientation, and (110) orientation offers the highest resistance to plastic deformation. Most importantly, nano-hardness decrease with increase in load. The micro-hardness is found to be lower than the nano-hardness which is attributed to the larger strain gradients during nanoindentation in contrast to the micro-indentation. Further, the micro hardness also decreases with increasing indentation load up to 1000 mN. The nanoscale and microscale intrinsic material lengths marginally depend on crystal orientation and are found to be around 0.55-0,65 and 10.89-12.45, respectively. The pile-ups patterns produced through micro indentation on the surfaces of (100), (110), and (111) oriented Mo single crystals have shown four- , two-, and three-fold symmetry, respectively. The crystal plasticity model proposed by Daphalapurkar et al. (2018) is implemented in commercially available software package Abaqus (6.17) by writing user element (UEL) subroutine