Materials’ mechanical behaviors under extreme conditions have been a subject of intensive research. At high temperatures, plastic deformation is dictated by atomic diffusion, which causes degradation of strength, elongation, phase transformation, and precipitation. At low temperatures, a ductile-to-brittle transition is expected, because the atomic mobility vanishes limiting plastic deformation through, e.g., movement of dislocations. The tragedy of space shuttle Challenger underscores the harm that low temperature can cause to the integrity of materials. In complex materials, however, other deformation mechanisms become competitive at low temperatures, providing alternative mechanisms to deform. High entropy alloy (HEA) is a case in point, where the activation of twinning was identified as a source of the increased ductility at 77 K. 

HEAs involves a novel alloy design and consist of multi-principal elements. Despite the complex chemistry, HEAs are capable of forming a single-phase solid-solution with an incredibly simple lattice. For example, CrMnFeCoNi HEA has a face-centered-cubic (fcc) structure. At room temperature, the deformation in the quinary CrMnFeCoNi and the quaternary CrFeCoNi alloys is dominated by dislocation activities. At liquid nitrogen temperature, they show increased strength and ductility. Although the activation of twinning, in addition to dislocation slip, is considered the main reason for the unusual ductility; it is unclear if other mechanisms are also present or contribute, especially when the temperature is further lowered. The possibility of the co-existence of multiple deformation mechanisms raised an important question on how individual deformation mechanisms compete or synergize during the deformation process. While transmission electron microscopy (TEM) is a major method for the study of deformation behaviors, in-situ observations with TEM are difficult, especially at temperatures near absolute zero. In addition, the field of view by TEM is limited. 

We answered this challenge with in-situ neutron diffraction measurements of HEAs, revealing multiple stages of deformation at ultralow temperatures, beginning with dislocation slip, followed by stacking faults and twinning, before transitioning to inhomogeneous deformation by serrations. Quantitative analysis demonstrated that the cooperation of these different deformation mechanisms led to the extreme work hardening, with extraordinary strength of 2.5 GPa and excellent ductility of 62% for CrMnFeCoNi HEA at 15 K. The low stacking fault energy plus the stable fcc structure at low temperature, enabled by the high entropy alloying, played a pivotal role in bridging dislocation slip and serration. Insights from the in-situ experiments point to the possibility of using entropy in the design of structural materials with superior properties. For this thesis, in-situ neutron diffraction measurements were carried out on three representative fcc equiatomic multicomponent alloys, i.e., quinary CrMnFeCoNi, quarternary CrFeCoNi, and ternary CrCoNi, which all showed a multi-stage deformation process at low temperatures.