Plasticity length-scale effects fundamentals

: dislocation generation and mobility as a function of applied stress distribution and stacking fault energy

Abstract

Over a range from nano to micro, metals display size-dependence when deformed non-uniformly into the plastic range. This is a behavior which cannot be characterized by conventional plasticity theories because they incorporate no material length scale.This body of work is dedicated to advancing our comprehension of the mechanisms influencing the strength of metallic materials. The focus is on experimental investigations aimed at understanding how the physical dimensions of a structure impact the mechanisms that dictate strength, particularly when the scale of the structure is on the order of a few micrometres and nanometres.Instrumented Indentation testing (nanoindentation) enables us to obtain valuable information on the mechanical properties of materials, including hardness and modulus and with that various informationabout microstructure can be disclosed. The test can be done not only bulk materials but also grains, inclusions, or phases too small to be probed by other techniques.A Berkovich indenter was employed to apply deformation to single crystal metals with high purity (>99.9%) in two crystal structures: body-centred cubic (BCC) and face-centred cubic (FCC) to reduce the number of factors affecting the test results, such as grain boundaries and microstructural defects.In various studies, only a restricted amount of data has been collected and a model based on the collected data in that regime has been proposed. Later on, the model had to go through modification due to accessible wider range of data or collected data from another material which has shown different response to the performed test. In this work, an attempt was made to cover a wide range single crystal material, and the data was collected in a wide range of indentation depth in order to succeed to give a comprehensive definition of hardness.The analysis of the collected data was done based on slip-distance theory and using Hou and Jennett[1] model. The analysed data leads to a linear function fitted to the data. The constants of that linear function are indicators of plastic zone size and dislocation density.It was found that there is a consistent trend in all tested materials that shows a change in plasticity mechanism in regard to length scale. In FCC materials, more than one linear function can be fitted to the data which demonstrates the variety of governing plasticity mechanisms at different length scales,while BCC materials are found to be less pronounced to that. The change can be realized based on change in the contribution of plastic zone size and dislocation density. The ratio of those parameters, which in combination give the hardness, varies in different regimes.

Date of Award	May 2024
Original language	English
Awarding Institution	Coventry University
Supervisor	Nigel Jennett (Supervisor), Vit Janik (Supervisor) & Mingwen Bai (Supervisor)