Serrated flow in alloy systems

This chapter presents the problem of the complexity of plastic flow in alloys, which is manifested by serrated deformation curves and transient plastic strain localizations. This phenomenon, which uncovers an inherently collective nature of the dynamics of crystal defects, is well known for conventional alloys. Surprisingly, despite the particular microstructure of HEAs, which strongly impacts the microscopic dynamics of defects, not only the HEAs are prone to the macroscopically jerky flow, but the serration patterns display many similar features, thus bearing evidence to similar laws for the collective dynamics. To provide a comprehensive approach to this problem, the chapter is subdivided into two parts. The first part presents the state-of-the art of this research under the angle of distinct regimes of collective dislocation dynamics found in conventional alloys, e.g., self-organized criticality, deterministic chaos, or synchronization. The second part summarizes recent investigations into serrated flows in HEAs, uncovering common and specific types of behaviors. Both parts conclude with the formulation of diverse open questions concerning these strongly coupled fields of research. Abstract for Section 11.1 This part presents a review of investigations into the complexity of the plastic flow associated with the Portevin-Le Chatelier effect, or jerky flow, in traditional alloys that have basic elements determining the crystal lattice of the material. The main accent is put on the illustration of the state-of-the art of this research under the angle of distinct regimes of collective dislocation dynamics, e.g., self-organized criticality, deterministic chaos, or synchronization. Finally, an extension of the research toward finer scales, which are resolved using acoustic emission and image correlation techniques, is discussed. Abstract for Section 11.2 Expanding upon the previous chapter, this chapter presents a literature review of the serrated flow phenomenon in high-entropy alloys (HEAs). The serrated flow is important, as it can result in permanent macroscopic and microstructural changes in HEAs. The literature reveals several important findings regarding the effect of strain rate and test temperature on the serrated flow. Furthermore, this chapter explores the relationship among the composition, microstructure, testing condition, and serration behavior. Toward the end of the chapter, a concise summary is presented for the temperature, strain rate, mechanical-testing type (compression/tension/nanoindentation), and serration type for HEAs. This chapter also provides an overview of the different types of analytical methods that have been successfully implemented to model and analyze the serration behavior in HEAs. Such techniques include the mean-field theory (MFT) formalism, complexity analysis method, and multifractal technique. Finally, future research topics in this area are presented, such as the effects of twinning and irradiation on the serration behavior.