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Note: Thesis (Ph.D. -- Brown University (2012)
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Abstract: In most engineering metal alloys, the overall plastic resistance is contributed by two or more strengthening mechanisms operating simultaneously. Two major sources of strengthening in metal alloys are substitutional solutes and forest dislocations. Theoretical, experimental and numerical studies have been carried out and extensively explored the strengthening mechanism due to solid-solution or forest dislocation, whereas a clear understanding of how different strengthening mechanisms operate together to generate a macroscopically measured strength is missing. In this thesis, the hardening behavior containing solid-solution strengthening and forest dislocations strengthening is studied both numerically and theoretically. In order to study the basic scaling law proposed, $\tau^k = \tau_{\rm 1}^k + \tau_{\rm 2}^k$, in previous theories, with $1\le k\le 2$, where $\tau_{\rm 1}$ and $\tau_{\rm 2}$ are the strengths of the two individual strengthening mechanisms, line-tension simulations of dislocation glide through two types of point obstacles are firstly performed to examine the friction limit. Our results show clearly that the applicability of the linear rule $k=1$ is dependent on the relative density difference of two different strengthening mechanisms. When the obstacle spacing ratio is larger than 67, the friction limit ($k=1$) is ensured. Because the studies of dislocation-obstacle interaction rely greatly on an accurate value of line tension of dislocation, a robust atomistic simulation approach is then proposed in this thesis to compute the line tension in FCC and HCP metals. Using this method, the requirements for an accurate line tension calculation at atomic scale are listed and the line tension for dislocation of edge and screw character in FCC aluminum and basal edge dislocation in HCP magnesium are computed, providing a reliable data set for further studies. The results and remaining unclarity in simulations on continuum scales inspire further studies of solute/dislocation-junction interaction at atomistic resolution. Therefore, we in this thesis realize massive atomistic simulations to investigate the formation and strength of dislocation junctions in FCC Al/Mg alloys. Compared with previous dislocation dynamics results, the shielding effect due to solute atoms on dislocation-dislocation interaction is not observed at atomistic scale. More importantly, once the space between dislocation-junction remains fixed, the linear supposition rule ($k=1$) has the ability to account for the combination of solute strengthening and dislocation-junction strengthening in metal alloys at atomistic scale, under which the individual strengthening contribution can be well defined by previous solute strengthening model. Once the overall strength due to multiple strengthening operating together is well evaluated at zero temperature, the thermally-activated plastic flow in the presence of multiple obstacle types is studied both numerically and theoretically in a regime of relative obstacle densities for which the zero-temperature stress is additive. The numerical methods consider the low-density ``forest" obstacles first as point obstacles and then as extended obstacles having a finite interaction length with the dislocation, while the high-density ``solute" obstacles are treated as point obstacles. Results show that the linear summation fails when a finite interaction width between dislocation and obstacles is considered. An analytical model for the activation energy versus flow stress is then proposed to account for the effect of the finite interaction length. The model predictions agree well with numerical results for a wide range of obstacle properties, showing clearly the effect due to the finite interaction between dislocation and the obstacles.
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Subject (FAST) (authorityURI="http://id.worldcat.org/fast", valueURI="http://id.worldcat.org/fast/1134850")
Subject (FAST) (authorityURI="http://id.worldcat.org/fast", valueURI="http://id.worldcat.org/fast/805706")
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Identifier: DOI: 10.7301/Z01G0JKF
Access Condition: rights statement (href="http://rightsstatements.org/vocab/InC/1.0/"): In Copyright
Access Condition: restriction on access: Collection is open for research.
Type of Resource (primo): dissertations