New Rules for Coupled Severe Plastic Deformations, Phase Transformations, and Structural Changes in Metals under High Pressure

Iowa State University, Ames IA

Investigators

Abstract

NON-TECHNICAL ABSTRACT Processes that require extreme stretching, bending and forming of metals into useful parts typically involve using very high pressures to do so. These very high-pressure methods are used widely to create materials with very specific properties on the inside and at their surface. However, all these operations are generally studied after the events have been completed. This award supports a fundamental quantitative study of these processes while they are occurring and is focused on finding new laws that relate the severe stretching of metal, their evolution at a very fine scale, on the order of a human hair, and the accompanying changes in the metal, called phase transformations. In this project, Titanium, a mixture of Titanium and Zirconium, and an alloy of Aluminum-Iron-Cobalt-Nickel-Copper are being studied under high pressures and strain rates typical of current and future materials technologies. In addition, the project provides opportunities to educate and train undergraduate students, graduate students and a postdoc in the areas of materials, high-pressure sciences and materials processing. This is being accomplished through special courses and research at the PI’s institution, experiments at an extremely high-powered x-ray facility called a “synchrotron” and interaction between experimental and computational efforts, all with an emphasis on underrepresented students. TECHNICAL ABSTRACT The goal of the project is to perform a fundamental in-situ quantitative study and find new laws for coupled severe plastic deformation, nanostructure evolution, and phase transformations in Ti, a mixture of Ti and Zr, and a AlFeCoNiCu high entropy alloy. These metals will be explored over a broad range of straining programs under pressures up to 65 GPa, and strain rates in the range 10-5-103/s. Experiments will be conducted using a dynamic rotational diamond anvil cell and the intellectual merit will be derived from the quantitative checking of our hypotheses, including: (a) Are crystallite size and dislocation density of all phases getting pressure-, strain- and strain-path-independent, steady-state values before and after phase transformations, and does this depend on the volume fractions during phase transformations and/or the strain rate? (b) Does each phase behave like a perfectly plastic, isotropic, and strain-path-independent material for each strain rate and what is the pressure and strain rate dependence of the yield strength? (c) Are phase transformation kinetics independent of strain path? (d) Does a high strain rate promote phase transformations due to increased yield strength? And (e) Will phase transformations in each material in the Ti-Zr mixture be promoted in comparison to single material studies due to additional obstacles for dislocation pileups? Methods to determine the evolution of highly heterogeneous fields of stress, plastic strain, strain rate tensors, volume fraction of phases, crystallite size, dislocation density, and concentration of species in a dynamic rotational diamond anvil cell will be developed, all in real time, using in-situ X-ray diffraction and other diagnostics in a feedback loop. In addition, simulations including a microscale phase field and physics-based macroscale model as well as a finite-element simulation of the experiments are being developed. Parameter identification, machine learning, model refinement, and all material properties (e.g. viscoplastic, evolution of phase transformations, crystallite size, dislocation density) are being determined, and quantitative models are also being finalized. For broader impacts beyond the technical contributions, a graduate course is being developed, and mentoring opportunities in research for undergraduate students, graduate students and a post-doc are being carried out in conjunction with this project. This project is jointly funded by the Metals and Metallic Nanostructures Program and the Established Program to Stimulate Competitive Research (EPSCoR). 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.