Dislocation behaviors under extreme and complex conditions
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The increasing demand for better structural materials creates many extreme and complex conditions under which the relevant dislocation physics have not been well understood. This thesis computationally investigates the fundamental dislocation physics under several extreme and complex conditions such as small lengthscale, short timescale, high stresses, deleterious impurity environment and convoluted alloying effects. These extreme and complex conditions are widely encountered in materials for nano-technology, space exploration, and advanced structural applications. Using atomistic simulations and experimentally-relevant modeling, this thesis work offers several important insights into the mechanical behaviors of nanoscale metals, body-centered-cubic metals and complex concentrated alloys. First, in the sub-50 nm regime, surface stresses impose significant influences on the thermally activated nucleation of dislocations, leading to material strengths following either the “smaller is stronger” or “smaller is weaker” trend. Second, strongly overdriven high-speed dislocations, created under ultra-high stresses, often show counter-intuitive kinetic-energy-dominated behaviors, defeating the textbook-described potential-energy controlled dislocation reactions and leading to strongly correlated plasticity. Third, the core machinery of screw dislocations plays a crucial role in creating, transporting and accumulating vacancies underlying the damage initiation of embrittlement for body-centered-cubic niobium containing low levels of oxygen. Last but not least, dislocations in complex concentrated alloys show a wide range of distinct features from that in elemental metals and conventional alloys, demonstrating pronounced strengthening potential by means of tunable local chemical order, a tell-tale feature of complex concentrated alloys. These uncovered dislocation physics under typical extreme and complex conditions are expected to expand our understanding on various mechanical properties such as yielding, strain hardening, plasticity and damage.