Thesis Defense: John Rotella
|Event Date:||December 3, 2021|
|School or Program:||Materials Engineering
"Preferential Microstructural Pathways of Strain Localization within Nickel and Titanium Alloys"
John Rotella, MSE PhD Candidate
Advisor: Professor Michael Sangid and Professor Rodney Trice
Modern structural materials utilize tailored microstructures to retain peak performance within the most volatile operating conditions. Features such as grain size, grain boundary (GB) character and morphology and secondary phases are just a few of the tunable parameters. By tailoring these types of microstructural features, the deformation behavior of the material is also altered. The localization of plastic strain is directly correlated to material failure. Thus, a systematic approach was utilized to understand the effect of microstructural features on the localization of plastic deformation utilizing digital image correlation (DIC). First, at the macroscopic scale, strain accumulation is known to form parallel to the plane of maximum shear stress. The local deviations in the deformation pathways at the meso-scale are investigated relative to the plane of maximum shear stress. The deviations in the deformation pathways are observed to be a function of the accumulated local plastic strain magnitude and the grain size. Next, strains characterized via DIC were used to calculate a value of incremental slip on the active slip systems and identify cases of slip transmission. The incremental slip was calculated based on a Taylor-Bishop-Hill algorithm, which determined a qualitative assessment of deformation on a given slip system, by satisfying compatibility and identifying the stress state by the principle of virtual work. Inter-connected slip bands, between neighboring grains, were shown to accumulate more incremental slip (and associated strain) compared with slip bands confined to a single grain, where slip transmission did not occur. These results rationalize the role of grain clusters leading to intense strain accumulation and thus serving as potential sites for fatigue crack initiation. Lastly, at GB interfaces, the effect of GB morphology (planar or serrated) on the cavitation behavior was studied during elevated temperature dwell-fatigue at 700 °C. The resulting γ′ precipitate structures were characterized near GBs and within grains. Along serrated GBs coarsened and elongated γ′ precipitates formed and consequently created adjacent regions that were denuded of γ′ precipitates. Cyclic dwell-fatigue experiments were performed at low and high stress amplitudes to vary the amount of imparted strain on the specimens. Additionally, the regions denuded of the γ′ precipitates were observed to localize strain and to be initial sites of cavitation. These results present a quantitative strain analysis between two GB morphologies, which provides the micromechanical rationale to assess the increased proclivity for serrated GBs to form cavities.