Date of Award

2020-01-01

Degree Name

Doctor of Philosophy

Department

Material Science and Engineering

Advisor(s)

Misra Devesh

Abstract

Austenitic stainless steels and cobalt-chrome alloys are used to fabricate biomedical devices with good mechanical strength, excellent wear and corrosion resistance. Cellular activity of Zr-modified Co-Cr-Mo alloys and osteoblast functions on Cu-containing and Cu-free austenitic stainless steel were studied. Experiments on the influence of Zr addition to Co-Cr-Mo alloys and Cu-containing austenitic stainless steel clearly demonstrated that cell adhesion, proliferation and cell-substrate interactions were favorably modulated in the presence of Zr and Cu. Additionally, stronger vinculin focal adhesion contact and signals associated with actin stress fibers together with extracellular matrix protein, fibronectin, were observed. Furthermore, comparative studies on the effect of grain size (nanograined/ultrafine-grained- NG/UFG: ~200-400 nm, coarse-grained-CG: ~55±20 µm) indicated higher cell attachment, proliferation and higher expression level of prominent proteins, fibronection, actin and vinculin on the NG/UFG surface and was in striking contrast with the CG counterpart. This behavior is attributed to the differences in the fraction of grain boundaries and physicochemical properties. A concept of a phenomenological parameter, microstructure entropy, S*, was developed using austenitic stainless steels as an example to understand the yield strength in alloy systems with bimodal grain size distribution obtained from large sets of experimental data. Six factors emerged from statistical data analysis in terms of grain size and grain numbers from 60 sets of data. Microstructure entropy (S*) was obtained from the bimodal structure data in a self-similar regime. Inverse of the square root of microstructure entropy (1/√S*) and yield strength exhibited a linear relationship. The proposed conceptual methodology has wide acceptance for any grain size distribution to obtain microstructure entropy (S*) of a specific grain structure and predict yield strength. A generic equation similar to Hall-Petch relationship, but involving microstructure entropy is proposed to predict and understand yield strength of metallic systems characterized by grain size distribution or microstructural evolution.

Language

en

Provenance

Received from ProQuest

File Size

131 pages

File Format

application/pdf

Rights Holder

Na Gong

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Engineering Commons

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