The Effects of Thermo-mechanical Treatment and Large Current Pulses on the Microstructure and Mechanical Properties of CoCrFeNiX High-entropy Alloys

Description

High entropy alloys (HEA) is a new family of metallic materials developed bymixing five or more elements in roughly equal proportions. A rather highpercentage of these alloys are, surprisingly, in a single phase solid solutionstructure in spite of the numerous known intermetallic phases in the alloysystems of the constituent elements. Different models or controlling parametersbased on thermodynamic, electronic or geometric considerations have beenproposed to predict the composition range for formation of single phasestructure. However, there is not yet a definitive conclusion on this.One of the generally accepted controlling factor is the high mixing entropyarising from the large number of elements occupying the crystal lattice sites in arandom manner. Comparing with the conventional engineering alloys based onone principal element (solvent atoms), the crystal lattice of HEA is highlydistorted and diffusion is found to be sluggish. These give the alloys highmechanical strength at room temperature as well as at high temperatures even ifthe alloy components are not refractory elements. Since HEA has attracted theattention of the materials research community for only slightly more than adecade, a lot of the structure-property correlations are still unknown or not wellunderstood. In order that HEAs can be adopted extensively for manufacturing orstructural uses, knowledge about the effects of thermo-mechanical treatment ontheir microstructures and properties is indispensable.Electropulsing treatment (EPT), or passing electric current of density higherthan 103A/cm2through a metal, is a recently developed process for controllingmicrostructures of engineering alloys, e.g. recrystallization, precipitation, etc.The process is rapid and does not induce large temperature rise. The mechanismof EPT is generally understood in terms of the interaction between the ‘electronwind’ and crystal defects. Since HEA has more distorted crystal lattice thanconventional alloys, stronger electron-lattice interaction is therefore expectedand EPT should be effective for fine-tuning the structures of HEA. The studies ofEPT on HEA are therefore interesting both scientifically and technologically.In this proposed research, the applicant intended to investigate the composition-microstructure-properties correlation of a series of high-entropy alloysconsisting of mainly period 3 transition elements and subject to various thermomechanicaltreatment. Characterization tools for different length-scales will beused to probe the structures from grain-size level to atomic scale short-rangecorrelation. In addition to conventional heat-treatments, attempts to treatdeformed HEA with EPT for recrystallization and/or precipitation strengtheningwill be done to explore the viability of the rapid and energy-efficient EPT processfor HEA. It is expected that when the project is completed, we will have gainedclearer understanding of the microstructure of high-entropy alloys, as well as thecharacteristics of the defects and deformation behaviours in this family of alloys.The expected results will not only facilitate the applications of HEA as structuralmaterials, but will shed light on the deformation mechanisms and phasetransformation behaviours of such distorted and disordered crystal structures.