UC San Diego

Dynamic deformation occurs when bodies are subjected to rapidly changing loads and can differ significantly from deformation that occurs under static or quasi-static situations. It is of great significance to understand the deformation and failure mechanisms of advanced materials, and there are potential applications in which dynamic deformation and failure can occur. Two classes of advanced materials, ultrafine-grained (UFG) (~500 nm and ~100 nm) titanium and high-entropy alloys (HEAs) (Al0.3CoCrFeNi and CoCrFeMnNi) are the focus of this doctoral investigation.

The deformation and adiabatic shear localization at cryogenic temperatures (173 K and 77 K) in ultrafine-grained (100 and 500 nm) titanium are investigated. In comparison with conventionally-grained titanium, the strength of ultrafine-grained titanium is higher due to the classic Hall-Petch effect while the strain-hardening rate approaches zero. Our results show that shear localization in dynamic deformation is also altered. The width of the shear band of coarse-grained titanium decreases from 30 µm at 293 K to 18 µm at 77 K (a 40% decrease). In contrast, for 100 nm titanium, the width of shear band decreases more significantly from 4 µm at room temperature to 1 µm (a 75% decrease) at cryogenic temperature (77 K). This difference is attributed to the combined effects of a decrease in the thermal conductivity and the specific heat capacity, and an increase in the thermal softening, which can lead to a band with thickness of 1 µm. These changes agree with the predictions of the Grady and Bai-Dodd theories. The dislocation evolution and the subgrain rotation mechanisms responsible for forming ultrafine- and nano- recrystallized grains are modeled. In addition, the Zener-Hollomon parameter is incorporated in the analysis to predict the critical dislocation density for shear localization and the recrystallized grain size in titanium .

The mechanical behavior of three single-phase face-centered-cubic (fcc) Al0.3CoCrFeNi, annealed CoCrFeMnNi and as-processed CoCrFeMnNi high-entropy alloys (HEAs) was studied in both quasi-static and high strain-rate regimes. Based on Hall-Petch strengthening, solid-solution strengthening, order hardening, cutting forest dislocations, and twinning hardening mechanisms, a constitutive equation was proposed to describe the flow of the annealed CoCrFeMnNi high-entropy alloy under dynamic impact. The resistance to shear localization is being established by dynamically-loading hat-shaped specimens that induce forced shear localization. Adiabatic shear band formation required an imposed shear strain of ~7 for the annealed CoCrFeMnNi HEA and cannot be observed at a strain of 1.1 for the Al0.3CoCrFeNi HEA. The structural and mechanical response that give rise to a remarkable resistance to shear localization are characterized by a combination of (1) a high strain-hardening ability, enabled by solid solution hardening, forest dislocation hardening, order hardening, and twinning hardening, (2) a high strain-rate sensitivity and (3) modest thermal softening; these combination effects give rise to the remarkable resistance to shear localization. First, the low stacking-fault energies in as-received high-entropy alloys lead to the formation of twinned segments inside the coarse grains. Then, when the thermal softening overcomes strain hardening, the shear bands would form, and dynamic recrystallization occurs inside the segments for the further break-up of the grains. Classical Straker equation is applied to predict the critical shear strain for shear localization, which was quite comparable to the experimental values in the high-entropy alloys. It was revealed that the as-processed CoCrFeMnNi HEA was prone to shear localization due to the initially high dislocation density which results in a relatively low work-hardening effect.

The dynamic deformation of these two metallic materials leads to adiabatic shear band formation at extreme shear strains. The resultant of the ultrafine grain structure observed in these two materials with diverse structures (HCP for Ti and FCC for HEAs) is remarkably similar and reinforces the concept of rotational dynamic recrystallization as the mechanism responsible for localization.