Experimental investigation of microstructure and properties in structural alloys through image analyses and multiresolution indentation

This work addresses the challenges in the investigation of structural alloy microstructures and their mechanical properties at multiple length scales. The investigations are performed on small volume ferrite-pearlite steel samples that were excised from in-service gas turbine components after prolonged exposure (up to 99,000 hours) to elevated temperatures, which promotes microstructural changes (spheroidization of pearlite and graphitization) as well as their yield strengths. Recent advances in spherical indentation protocols are combined for the first time to investigate the mechanical response of microscale ferrite-pearlite constituents and estimates of bulk properties on macroscale. It is shown that indentation yield strength captured with large indenter tips on an ensemble of ferrite-pearlite grains correlate strongly to the bulk yield strength evaluated with tensile measurements. Measurements on the individual ferrite and pearlite constituents follow a similar trend of decreasing yield strength as the bulk measurements. Second, to advance the reliability and accuracy of microstructure characterization, an image segmentation framework is developed that consists of five main steps designed to achieve systematic image segmentation on broad classes of microstructures utilizing widely available image processing tools. The flexibility and modularity of the framework was demonstrated on various types of microstructures images. The developed framework was used to segment the microstructures of ferrite-pearlite samples. The extracted microstructure statistics from the segmented images and multiresolution indentation yield strength measurements were used to evaluate established composite theory estimates and have demonstrated highly consistent estimates for these material systems.Ph.D

MULTISCALE CHARACTERIZATION OF ΑLPHA-BETA TITANIUM ALLOYS USING HIGH THROUGHPUT SPHERICAL INDENTATION TEST PROTOCOLS

Traditionally, new materials follow well stablished paths from their manufacturing beginnings to their final application. Development, evaluation, certification and deployment are some of the steps in these processes. However, some of these stages are characterized for very specialized protocols, resulting in prolonged timelines from beginning to end. The design process of materials can sometimes be defined as ambiguous due to the lack of fundamental knowledge at salient length-scales. This can be attributed to the absence of trustworthy testing methods that provide meaningful and reliable knowledge on the behavior of materials at length-scales over different orders of magnitude. In addition to this characteristic, cost and time efficiency are also crucial attributes in the materials design field, as large amounts of data covering wide ranges or parameters provide stronger bases for physics-based models that can reverse-engineer the whole process. The work presented here, evaluates spherical nano-indentation protocols as a high-throughput approach for the mechanically characterization of several α- and α/β titanium alloys at the grain-scale level. We start by the exploring the mechanical response of primary-α grains, and their dependence on the HCP lattice orientation and the corresponding grain chemical composition. Next, we move into the mechanical behavior of single grains in fully basket-weave titanium microstructures. By looking into the grain responses, a reduction on the multiple microstructural features in this type of morphologies is accomplished, leading to better statements of the influence of lath-microstructure and α-lath orientations on the indentation properties. This work is accompanied by a thorough microstructural characterization/quantification of the basket-weave morphology that is later subjected to a dimensionality reduction for a simplified understanding. And finally, we aim to bridge multiresolution indentation measurements form a bimodal titanium microstructure for the subsequent evaluation of composite theories in the prediction of the effective indentation yield.Ph.D