Date of Award

8-2015

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

T. G. Nieh

Committee Members

Yanfei Gao, James Morris, Dayakar Penumadu

Abstract

The incipient plasticity indicates the nucleation or activation of defects in materials and enables us to study the intrinsic mechanical properties of materials. In this dissertation, instrumented indentation technique was employed to study the incipient plasticity of metals (bcc-Cr, fcc-Ni, fcc-Au) and a fcc-structured high entropy alloy (HEA) NiFeCoMnCr. The critical shear stresses for pop-in in these materials were all within the range of theoretical strength of materials, indicating the nucleation of dislocations in perfect crystals.

In Chapter 3 and 4, indentation tests were conducted at elevated temperatures to study the pop-in behavior in bcc-Cr and fcc-Au. The pop-in load was found to drop with increasing temperature. Activation energy for pop-in was found too low to form a dislocation loop homogeneously in the lattice. The heterogeneous nucleation of dislocations at point defects in the lattice might be responsible for the pop-in events. Atomistic simulations of indentations were also performed in the Au(100) lattice with/without point defects at different temperatures. A good agreement was found between simulations and the experiments. Using an atomistic model, the activation parameters for the incipient plasticity were calculated from the simulation results, which were comparable with experimental results.

In Chapter 5, a broad range of tip radii of indenters were used to investigate the effect of indentation volume on pop-in behavior in the indentation experiments on bcc-Cr. The critical shear stress was found to increase with decreasing the tip radius. The cumulative pop-in probability on load was successfully described by a combined model over the full range of tip radius, indicating the incipient plasticity might be triggered either by the nucleation of dislocation or the multiplication of existing dislocations underneath the indenter.

In Chapter 6, I found the combined model can also well described the pop-in behavior of fcc-Ni, and fcc-NiFeCoMnCr under three different tip radii (80, 255, 759 nm). The effect of tip radius on the elasic/plastic responses of Ni and HEA were evaluated quantitatively.

From the above studies, I tentatively make a conclusion in Chapter 7 and also presented a future perspective based on my research experience in the past five years.

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