MICROMECHANICAL ANALYSIS AND CHARACTERIZATION OF MATERIALS WITH SPATIALLY DISTINCT MICROSTRUCTURAL FEATURES
thesisposted on 14.01.2021, 18:38 by Raheleh Mohammad Rahimi
Correlations between a materials microstructure and mechanical behavior are important for materials development. As materials characterization methods must consider instrument accessibility, sample dimensions and economical aspects, developing functional techniques in order to obtain better understanding of materials behavior in micro and nano scale is crucial. Procedures for assessing and interpreting the mechanical responses at small scales, combined with investigating the microstructure, are considered as significant steps to design and develop the effective frameworks for evaluating bulk properties. This research demonstrates how fundamental understanding of microstructures can assist interpreting of mechanical performance of bulk materials. Testing of materials at small scales is very important because the mechanical failure of any bulk material starts with the formation, extension, or local accumulation of initially small defects, leading finally to a catastrophic fracture by an expanding crack. Thus, any bulk material profits from an in-depth understanding of its deformation and mechanical phenomena at the nano- and micrometer length scale.
This thesis shows how the micro constituents’ interactions and grain boundaries reactions to dislocations in alloys and thin films contribute to understanding material flow behavior and differences in the mechanical properties of these materials in a wide range of material systems with variations in appropriate sizes which need to be probed. Among other things, this work shows that sources of variation can be specified and quantified as predictive tools for designing materials. Several examples are presented. First, the strength and strain hardening of martensite and ferrite in a dual phase steel with a grain size less than 5 μm were determined using an inverse technique. The yield strength of the ferrite and martensite phases are obtained as 370 MPa and 950 MPa respectively. The calculated hardening exponent of the alloy was exactly the same as experimental tensile test results (0.11). The constraint phenomena was effective in restricting deformation of this elastic-plastic alloy. Secondly, the differences in hardness and pop-in behavior were used to understand of the influences of different types of grain boundaries, high density dislocations, and twins in Al thin films before and after plasticity. The third example assesses the strength of several species of diatom frustules for the first time using a combination of indentation techniques. Lightweight materials with densities well below 1000 kg/m3 demonstrated strengths on the order of 100’s of MPa. Finally, conditions for laser grown oxides and laser shock peening on a commercial steel which lead to an optical marking without a change in strength around the marking have been identified.