Understanding Transformation Superplasticity, High Temperature Deformation and Manufacturing of Entropy Stabilized Oxides

University Of California-Irvine, Irvine CA

Investigators

Abstract

The inherent brittleness of ceramic materials often makes them difficult to form during manufacturing, limiting them to applications that do not require complex shapes. The recently discovered class of ceramics, entropy stabilized oxides (ESOs), exhibit a unique reversible phase transformation behavior that provides significant control over the properties via heat treatment, thus allowing for an efficient means of manufacturing these materials into complex shapes without fracture or failure. This award supports fundamental research to explore the mechanisms underlying transformation superplasticity and uncover new methods of manufacturing ESOs. These materials show promise as supercapacitors, battery cathodes, catalysts, and electrolytes, which impacts U.S. industry and economy. The project explores the fields of advanced manufacturing and materials science and engineering, with educational, training and outreach activities that provide exposure to interdisciplinary topics for undergraduate, graduate and K-12 students, especially those from underrepresented minorities. Entropy stabilized oxides (ESOs) are ceramic materials in which configurational disorder is compositionally engineered into a single phase with multiple cations randomly populating sublattice locations. ESOs display a reversible phase transformation behavior when heat treated within a particular temperature window. This phase transformation manifests as a controllable phase heterogeneity, giving unprecedented control over the microstructure of ESOs. Such a dramatic transformation could be leveraged to enhance ductility during high temperature deformation, allowing for efficient forming through transformation superplasticity and cyclic deformation. Transformation superplasticity allows for mechanical deformation of more than 100% due to the internal strain that arises from the mismatch between co-existing phases, which can accumulate during thermal cycling. The research involves the following tasks: 1) synthesis and consolidation of bulk ESO samples with different grain sizes; 2) heat treatment to achieve the desired phase state (amount and composition of the secondary phase(s)); 3) high temperature deformation and superplastic forming; and 4) evaluation of microstructure and properties. Experiments are complemented with theoretical modeling to analyze the influence of grain size, applied stress, pressure, strain rate and temperature on diffusion, and consequently deformation mechanisms and superplasticity. This research establishes a relationship between starting microstructure and deformation behavior, thus advancing the understanding of deformation mechanisms in ESOs and their potential for superplastic forming using techniques such as forging and extrusion. 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.