Fundamental mechanisms of discontinuous deformation in metals for cryogenic-environment applications

Metallic materials are known to exhibit low-temperature discontinuous deformation (i.e., low-temperature serrated deformation, LTSD) at cryogenic temperatures, which can lead to sudden failures or catastrophic accidents. Therefore, understanding LTSD is crucial for ensuring material stability and reliability in a cryogenic environment. Thus far, the widely accepted explanations for the origins of LTSD can be categorized into two mechanisms: (i) dislocation-based mechanical instability and (ii) thermomechanical instability. However, interpreting LTSD using each theory independently has limitations in clearly elucidating the LTSD mechanism. Therefore, the current understanding of LTSD remains insufficient and is still subject to debate because it is challenging to prove experimentally. To address this issue, we suggest a novel LTSD mechanism, namely a thermally induced dislocation dynamics model, based on the experimental evidence that considers both the dislocation dynamics and thermomechanical characteristics at cryogenic temperatures. Furthermore, we present a modified deformation-mechanism map of a SS316L that incorporates the newly proposed LTSD mechanisms. The origin of LTSD is considered in the unique framework of dislocation behavior under severely limited thermal-vibration energy at cryogenic temperatures, leading to the dislocation avalanches and development of hierarchical dislocation networks, including multiple lattice defects. Therewith, the localized heating generated from dislocation avalanches induces multiple types of LTSD and gives rise to transitions from the heterogeneous to homogeneous deformation. Our findings highlight the rate-dependent nature of LTSD and negative strain-rate sensitivity in the strength-elongation relationship and include the first observation of changes in small stress fluctuations and their relationship to the changes in larger serrations.