Developing new high entropy alloys for high temperature applications using advanced powder metallurgy routes and additive manufacturing

Developing new high entropy alloys for high temperature applications using advanced powder metallurgy routes and additive manufacturing

Author/s: Venkatesh Kumaran Sivagnana Desikan

Director/s: José Manuel Torralba

Defence Date: 13/9/24

Ph.D. Awarding Institution: Carlos III University of Madrid

Abstract

The discovery of new materials is essential due to population growth and the rising quality of life. One significant advancement in this area is high entropy alloys (HEAs), which break away from the traditional alloying method of using a base element and instead combine four or more elements in equiatomic or near-equiatomic proportions. Since their discovery in 2004, HEA research has rapidly expanded, exploring various combinations of 3D transition, refractory, and lightweight metals. HEAs can be customized for specific applications based on the desired properties. Current HEAs show promising performance, often matching or surpassing traditional alloys in strength at cryogenic, room, and high temperatures, among other properties.

Casting and arc melting have been the most common methods for manufacturing HEAs due to their simplicity and accessibility. However, these methods can lead to inhomogeneities, contamination, and large grains, which degrade mechanical properties and necessitate additional heat treatment and mechanical working. Recently, powder metallurgy has emerged as a promising alternative for producing HEAs, offering better control over composition and microstructure, achieving nanocrystalline structures that enhance mechanical properties, and allowing for the creation of complex geometries and near-net shapes. To date, powder metallurgy high-entropy alloys (PMHEAs) have been developed using three classes of powders: fully pre-alloyed gas-atomized powders, pure elemental powders, and fully prealloyed mechanically alloyed powders (also made from elemental powders). Despite their potential, challenges such as cost, availability, and the criticality of pure elemental powders limit HEA development.

This thesis explores a novel approach to developing HEAs through powder metallurgy using blended commercial powders as feedstock. The consolidation techniques employed include field-assisted hot pressing, spark plasma sintering, and laser powder bed fusion for additive manufacturing. The material developed is based on the CoCrFeNi system with additions of Mo, Nb, and Al. Structural, microstructural, and mechanical characterizations were performed on all developed HEAs to identify parameters for achieving a single FCC phase. This method’s success paves the way for the cost-effective, efficient, flexible, and sustainable exploration of HEAs.