Fundamental Understanding of Deformation in High Entropy Structural Alloys

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Alloying of metals has been the method of producing considerable strength to withstand high service loads in engineering applications. However, the resistance to fracture (often termed the toughness), i.e. the ability to tolerate cracks without catastrophic fracture, is often compromised upon increase in strength. This is especially true at low temperatures where toughness is substantially lower. Another important measure, the ductility - the ability of the material to sustain large elongations or deflections - also suffers upon raising the strength in conventional alloys. This award supports fundamental research to provide understanding towards the development of a new class of alloys called the high entropy alloys (HEAs) which have the capability of possessing both a high strength, high toughness, and high ductility. These alloys involve multiple elements but derive strength through interactions of elements at small scales, and circumvent the loss of ductility even at very low temperatures, broadening their potential use. These alloys can enable wide-scale deployment in areas critical to the US economy including mechanical, civil, and materials science sectors. The work will educate graduate students and also help broaden participation of underrepresented groups through outreach at the high school level, including summer programs with demonstrations of alloy behavior and hands-on activities making useful devices with alloys.Two of the most important mechanisms in plastic deformation of metals are slip and twinning. Exceptional properties can be achieved for cases where slip deformation and twinning deformation can occur simultaneously with unprecedented strain hardening. Their mutual synergism can produce superior properties. In this work, first principles simulations of potential new high entropy alloy compositions will consider the energy landscapes for slip and twin deformation, and develop models that encompass both continuum and atomistic effects. The most promising alloys will then be manufactured and tested. Experimental methodologies will then be developed to measure the onset of slip and twin events at low temperatures where the tremendous benefits of these materials will be realized. The experiments will also consider finite temperatures and aim to advance digital image correlation methods (a technique for measuring local displacements and strains) to subgrain-submicron scales to understand precisely the deformation phenomenon in these alloys.