Study on Dynamic Compressive Properties and Fracture Behavior of Metastable High-Entropy Alloys at room- and cryogenic-temperature

Abstract

Over the past decade, research on high-entropy alloys (HEAs) having excellent properties at cryogenic temperatures has been actively conducted, according to various industrial needs. HEA is a newly developed type of alloy and exhibits excellent mechanical properties as a structural material. The recently developed face-centered-cubic (FCC)- based HEA has attracted much attention as it exhibits TWinning-Induced Plasticity (TWIP) behavior by deformation at cryogenic temperatures, with having excellent strength, ductility and fracture toughness. Starting from an alloy having TWIP as a deformation behavior, alloy research in various compositions beyond the original definition of HEA is being actively conducted. In addition to the traditional strengthening mechanisms of alloys such as solid solution strengthening, precipitation strengthening and grain refinement, the scope of research is expanding to utilizing behaviors of FCC to hexagonal-close-packed (HCP) Transformation-Induced Plasticity (TRIP) and FCC to body-centered-cubic (BCC) TRIP applied metastability-engineering strategy as a strengthening mechanism. However, most of the researches on HEA have been focused on the relationship between mechanical properties and deformation behavior under static or quasi-static conditions. In order for HEAs with excellent mechanical properties to be industrially used in extreme conditions such as military defense and aerospace industries, it is essential to apply actual metal forming processes such as rolling and forging for precision machining. These machining processes typically involve high strain rates of 1–1×104 s-1, it is important to understand the mechanical properties and deformation behavior under high-speed strain conditions as well as securing excellent mechanical properties under quasi-static conditions. Therefore, the purpose of this thesis is as follows. First, the quasi-static and dynamic compression characteristics of FCC-based metastable HEAs exhibiting both TRIP and TWIP were investigated at room- and cryogenic-temperature. Electron back-scatter diffraction (EBSD) analysis was utilized to evaluate detailed deformation behaviors in terms of microstructural evolution and to confirm interrelations between microstructural evolution processes and compressive properties. Decrease of the test temperature activated both TWIP and TRIP behaviors. The increased flow stress under dynamic loading condition activated the TWIP behavior, and the increase in temperature by adiabatic heating decreased the TRIP behavior. Also, shear cracking was observed under the condition where BCC-TRIP was actively occurring. Second, in relation to the shear cracking observed in the active condition of BCC-TRIP, the dynamic compression characteristics and fracture behavior of metastable HEAs having BCC-TRIP as deformation behavior were investigated. The deformation and fracture behavior according to composition, test temperature, and strain rate were investigated. The more active the BCC-TRIP condition (higher TRIP rate), the lower the critical strain for adiabatic-shear-band (ASB) and shear cracking. The ASB formed by shear localization due to the low plastic accommodation capacity of the BCC acted as a crack initiation and propagation site, leading to fracture of the material by shear cracking. This finding suggests the delicate control of FCC stability would be significant considering the applications of the metastable alloys toward cryogenic or dynamic environment.