Understanding the Fundamental Deformation Processes of BCC Refractory High Entropy Alloys using Experimentally-Validated Kinetic Monte Carlo Simulations

University Of California-Los Angeles, Los Angeles CA

Investigators

Abstract

NON-TECHNICAL SUMMARY: Developing new materials is at the heart of technology advancements such as cellphones, earthquake-resistant structures, or advanced satellites and space probes. One area where new materials are sorely needed is that of energy generation based on standard steam cycles. There, one way to reduce greenhouse gas emissions is to increase the thermodynamic efficiency of the plants. This can be done by developing materials that can sustain higher operating temperatures than the current standard set by nickel-based superalloys. High-entropy alloys are one such class of materials that hold the promise of increasing the operating temperature by up to 300~500 degrees. High-entropy alloys are made up of four or more chemical elements (specifically transition metal elements) in equal proportions, and thus have a very complex chemistry. This project focuses on understanding the deformation behavior and strength of these alloys at high temperatures using the most advanced computational and experimental tools at the atomic scale. The goal is to discover the mechanisms that make these systems strong at high temperature so that we can design yet better alloys and use them to replace current materials in power generation plants to increase their efficiency. For this, a diverse group of students and scientists will be engaged, including women, latino students from the Los Angeles area, and military veterans, which bring discipline, focus, and familiarity with high-precision machinery and computers. This project will be able to advance our goals towards reducing the carbon footprint of existing power plants, and contribute to other applications such as improved jet and rocket engines and safer nuclear power plants. TECHNICAL SUMMARY: Refractory high entropy alloys (RHEA) are a class of materials consisting of four or more refractory metal elements in equiatomic proportions. These alloys show great promise for high temperature applications due to their high strength and ductility in a wide temperature range, potentially superior to even Ni-based superalloys. These systems usually crystallize in a body-centered cubic (bcc) alloy, which suggests that their plastic response is controlled by thermally activated motion of screw dislocations. However, the high strength of these systems at high temperature does not fit standard theories of lattice resistance and solid solution hardening. In this project, a kinetic Monte Carlo (kMC) model of screw dislocation glide will be developed. The alloy will be represented as an effective medium characterized by an atomic averaging of all the alloy elements, and where each atom then is treated as a solute in this effective environment. The project focuses on the NbMoTaW system as a representative RHEA. The computational approaches will be validated using in-situ transmission electron microscopy nanomechanical tests of single-crystal specimens, which will be used to study the temperature, orientation and strain rate dependence of the alloy. Ultimately, the tools developed under this proposal will be useful to assess how refractory high entropy alloys deform as a function of temperature and strain rate, with the goal of improving the high temperature behavior of these systems and evaluating their potential to increase the efficiency of power plants. The proposal contains a plan to involve both undergraduate students and students of underrepresented minorities in the research activities. As well, the proposal investigators will reach out to minority students and Armed Forces veterans pursuing undergraduate degrees in science and engineering. The results of this proposal will be used to enhance the content of several courses at UCLA where dislocations, strengthening mechanisms, and metals plasticity are a central part of the syllabus. 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.