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Dislocation dynamics in chemically and microstructurally complex metallic materials


Dislocations are the main carriers of the plasticity and are the dominate deformation mechanism in metallic materials. With recent innovations in manufacturing, a number of novel metallic materials with high strength have been produced. Compared to the conventional metals, these metallic materials are more chemically and microstructurally complex. To best tailor these features for optimal performnace, it is necessary to understand how these complex factors affect dislocation dynamics, such as nucleation and propagation. Using atomistic simulations, this thesis research aims to reveal the dislocation dynamics responsible for their superior strength in three types of metallic materials, i.e., multi-principal element alloys (MPEAs) with local chemical fluctuations, metallic nanolaminates with interfaces, and irradiated metals with helium (He) nanobubbles.

First, in CoCrNi MPEA, local chemical fluctuations, i.e., lattice distortion (LD) and chemical short-range order (CSRO), play an important role in the nucleation and evolution of dislocations. Under uniaxial tensile loading, LD not only lowers the Young’s modulus and strain for nucleation of Shockley partial (SP) dislocations, but also promotes the nucleation of SP dislocations and reduces their mobility, providing enough space and time for the nucleation of nanotwinning. By contrast, CSRO enhances the Young’s modulus and critical strain to nucleate SP dislocations. Similarly, dislocations are also resisted by CSRO clusters, resulting in CSRO strengthening.

Second, in metallic nanolaminates consisting of alternating metallic layers, confined layer slip (CLS) has been proposed as the main dislocation mode. CLS involves a moving dislocation confined between the parallel interfaces. It has also been postulated that this dislocation dynamics process is affected by interface structure and layer thickness. Via atomistic simulation, it is shown that compared to coherent interfaces, the CLS of dislocations between incoherent interfaces is much more difficult. Notably the key obstruction originates from the misfit dislocations within the incoherent interfaces and it is shown that the dislocation may invoke climb to continue the CLS process. It is also found that in Nb/Nb nanolaminates with coherent interfaces, the CLS stress scales inversely with the layer thickness, as proposed by analytical models. A modified CLS model is proposed that agrees with simulation and treats the influence of the interface as an additional, layer-size-independent resistance.

Last but not least, in irradiated Cu with He nanobubbles, it is well known that these nanobubbles significantly influence both material strength and ductility. In studying the interaction between the gliding dislocation and nanobubble, it is found that instead of the conventional dislocation bypass over a He bubble, i.e., bubble cutting or dislocation climb, a new multi-step-bypass (MSB) maneuver occurs. It is demonstrated that this MSB mechanism operates even at room temperature when the He atom density in the bubble is sufficiently large and the ratio of the bubble spacing to its diameter is sufficiently low. For MSB, the entire dislocation changes its glide plane to overcome the bubble, which is promoted by higher temperatures and larger He atom density in the bubble. Compared to the conventional bypass modes, MSB is more energetically favorable.

The dislocation dynamics mechanisms discovered in this research can help to deepen understanding of microstructure/performance relationships and guide the microstructure design of these high performance structural materials.

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