Alloy Design and Micro/Nano-Scaled Creep Mechanism of Medium-Low Density High Entropy Alloys( II )

High entropy alloys (HEAs) usually possess about five principal elements and demonstrate high entropy of mixing. The effects of high-degree chaos, extensive lattice distortion, and sluggish cooperative diffusion made the HEAs preferentially to form single phase microstructure. As a result, HEAs could exhibit promising mechanical and chemical properties involving of high strength, high thermal stability, and high corrosion resistance. Current typical HEAs are made mainly by transition elements, such as Fe, Co, Ni, Cr, Mn. The density is typically in the range of around 8.1 g/cm3. Density levels in the range from 3.0 to 5.0 g/cm3 are classified as medium-low density (MLD) and those below 3.0 g/cm3 are considered to be low density.In this three-year study, MLD HEAs are intended to be designed and characterized in terms of phase evolution, microstructure, thermal nature, and room temperature, and elevated temperature mechanical properties. The systems start from TiAlNb and TiAlV three-component alloys, followed by adding more elements. The alloy design will be conducted from both experimental casting experience and Calphad approaches. Vacuum induction melting and casting will be used to prepare the alloys. HEAs are typically claimed to exhibit sluggish diffusion, resulting in thermal stability and heat-resistant properties. This study will also explore the thermal diffusion, thermal conductivity, heat capacity, and thermal expansion characteristics for the newly designed medium-low density HEAs. For the application in heat resistant environment, the elevated temperature creep response for the newly designed MLD HEAs will be a focus in this study. To understand the creep resistance contribution from each FCC or BCC phase, the promising HEAs will be indexed by electron backscatter diffraction (EBSD). A new Hysitron nanoindentation system will be adopted to measure the creep response from each phase and each orientation by using Berkovich or micropillar loading. The temperature range will cover from 300 to 800oC. The creep rate and diffusion rate will be extracted to examine the thermomechanical stability and creep resistance. The creep exponent, activation volume, and activation energy will all be extracted to understand the creep mechanisms. The activation volume and activation energy can also help to evaluate the feasibility of sluggish diffusion for multiple-element HEAs. Finite element method (FEM), molecule dynamics (MD), and kinetic Monte Carlo (KMC) simulation and calculation will also be incorporated to support and compare the nano-scaled room and elevated temperature mechanical response, especially the creep resistance. Overall, this joint project will proceed from four aspects, namely, (i) alloys preparation and microstructure control end, (ii) Calphad thermodynamics simulation aspect end, (iii) room and elevated temperature mechanical response end, and (iv) FEM and MD/KMC simulation and calculation end. The three-year workload will be launched from ternary to quaternary and more element HEAs, adjusting their phases, microstructures, thermal and thermomechanical properties. For the last year, these newly designed MLD HEAs will be moved toward industry applications via help from MIRDC and ITRI. Also, an international collaboration team has been established, to work concurrently over the next three years.