Solid Solution Enhanced Synthesis of Multi-Principal Component Alloys via Oxide Reduction

Lehigh University, Bethlehem PA

Investigators

Abstract

Non-technical Summary Multi-principal component alloys (MPCAs) are a radical new class of alloys that consists of approximately equal mixtures of four or more metallic elements. These materials can possess novel and exciting combinations of physical properties. In particular, a sub-group of these alloys, so-called refractory MPCAs exhibit high temperature mechanical properties that surpass those of the alloys currently in use. The application of these new alloys in turbine engines could therefore result in higher efficiency and reduced greenhouse emissions. Current methods used to fabricate refractory MPCAs utilize extremely high temperatures, and can result in compositional variations within the alloy. This program seeks to explore a new process whereby the metallic alloy is produced by reducing a mixture of the metallic oxides. This is achieved by heat-treatment in a hydrogen environment. Not only is this a novel technique, but prior work has shown that it is possible to convert certain oxides that normally would be extremely stable. The research seeks to understand this unexpected aspect of the oxide reduction process. A rationale is suggested whereby the reduction is enhanced by the compositional environment. This model is tested by studying the reduction behavior of a range of MPCAs composed of different elemental combinations. Computer modelling is used to help identify alloy combinations with thermodynamic characteristics that show promise based on the model. These research findings could lead to enhanced processing of tailored MPCA compositions, with potential for significant societal impact in many technological areas, including energy storage and generation, transportation, and medical devices. Outreach activities relating to alloy design and forming focus on encouraging under-represented minorities to pursue opportunities in STEM fields, and to increase awareness of materials engineering amongst middle- and high-school students. Technical Summary The aim of this program is to develop a scientific framework for understanding the complex reduction behavior of a mixture of oxide materials, and hence establish composition-based strategies for the processing of technologically important multi-principal component alloys (MPCAs). This research builds on prior work that demonstrated that MPCAs can be fabricated via the reduction of a compacted metal-oxide mixture. Alloys composed of Co, Cr, Fe, Ni and Mn have been successfully synthesized by this route, using relatively mild heat-treatment conditions for which the reduction of MnO would not be expected. A model is proposed whereby the thermodynamic driving force for reduction is enhanced due to the formation of the alloy solid solution. Electronic structure calculations of the enthalpy of mixing guide the selection of elemental combinations for testing. The reduction behavior of these oxide compositions are studied under controlled conditions of temperature and oxygen partial pressure (pO2). The sequence of reactions and phase evolution is revealed by the characterization of partially reduced samples using scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) techniques, as well as in-situ x-ray diffraction reduction studies using a synchrotron light source. Identification of the nucleation mechanisms of the metallic phases, role of transient phases, and rate-controlling transport mechanisms is of broad fundamental interest, and benefit related research on catalysis and oxygen looping. The enhanced processing of tailored MPCA compositions, both bulk and powder, is expedited by the development of the solid solution model, since it will help identify the heat treatment and compositional parameters that are critical to the process. The research also benefits the research community by generating Reduction-Composition diagrams for select MPCA elemental combinations. Outreach activities relating to alloy design and forming include CHOICES (Charting Horizons and Opportunities In Careers in Engineering and Science), which is dedicated to encouraging middle-school girls to consider careers in science and engineering, and the ASM Teacher and High School Materials Camps, designed to increase awareness of materials engineering amongst high school students. 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.