Bottom-up fundamental approach for characterizing plasticity and deformation in BCC and FCC high entropy alloys

Johns Hopkins University, Baltimore MD

Investigators

Abstract

NON-TECHNICAL SUMMARY: Recently, it has been shown that when five or more elements are combined in nearly equal atomic concentrations, a new class of metals emerges. These metals are typically referred to as multi-component concentrated solid solution alloys, or more commonly high entropy alloys (HEAs). These new complex alloys have drawn substantial attention in recent years due to the vast available composition space for tuning their mechanical, thermal, electrical, and magnetic properties. Over the past decade, several such alloys have been shown experimentally to demonstrate very promising fracture toughness behavior, low temperature ductility, high temperature strength retention, and tensile ductility. These superior properties, as compared to traditional metallic alloys, suggest immense potential for HEAs in a wide range of functional and structural applications. Nevertheless, the fundamental mechanisms that control such properties are not yet fully characterized. Accordingly, this award will support research to fundamentally quantify the mechanisms controlling the properties of these complex alloys, with specific focus on their high temperature mechanical properties for extreme environment applications. This will be accomplished by utilizing a bottom-up multiscale modeling approach (i.e. from atoms to continuum scale). This modeling approach is of high scientific and engineering interest especially with respect to the materials genome initiative. The integration of research, education and outreach in this project is also a central component and will focus on: (1) improve STEM achievement in a predominately African American elementary schools in Baltimore; (2) involve under-represented students from a local historically black college through internships on research in mechanics and materials; and (3) develop an education portfolio that will increase the knowledge of undergraduate and graduate students in fundamentals of state-of-the-art multiscale modeling. TECHNICAL SUMMARY: The primary objectives of this research are to fundamentally identify the underlying mechanisms controlling the ductility at low temperatures, the work-hardening response, and the retention of strength at high temperatures in face-centered cubic (FCC) and body-centered cubic (BCC) multicomponent concentrated solid solution alloys. Primarily, the focus will be on FCC Cantor-like alloys, such as CoCrNi, CoCrFeNi and CoCrFeMnNi alloys, which show very promising fracture toughness behavior and low temperature ductility, as well as refractory BCC HEAs alloys, such as HfNbTiZr, HfNbTaTiZr, and NbTiZr with V, Mo, Ta, and Al additions, which show significant high temperature strength retention and tensile ductility. This will be achieved through a bottom-up coupled approach, which combines molecular dynamics (MD), Kinetic Monte Carlo (KMC), and three-dimensional (3D) discrete dislocation dynamics (DDD) simulations to predict dislocation and twinning mediated plasticity in both FCC and BCC HEAs. The 3D DDD simulations proposed here will be informed by the MD and KMC simulations to also incorporate twinning mediated plasticity and fluctuations in screw dislocation cross-slip activation energy due to statistical variations in local atomic concentrations. The results of these multiscale simulations will also be used to develop an analytical model to predict the high temperature yield stress of BCC multi-component concentrated solid solution alloys. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.