Nano/Hetero-structuring and cold consolidation of high-entropy alloy powders using high-pressure torsion

Abstract

Powder metallurgy (PM) is a versatile and cost-effective method for producing alloys compared to casting techniques. This flexible method can engineer the microstructure by combining different powders, including metals, alloys, and ceramics. Recently, PM techniques have been received vast attention from engineering society to produce the newly emerged multi-principal element alloys (MPEAs). A traditional PM route consists of various processes such as powder production, compaction of powders, and sintering at high temperatures. Despite the salient features of the PM, the PM-processed parts usually suffer from porosity, oxidation, and contaminations. Due to these processing-related flaws, the PM products lack tensile elongation, limiting their usage in industrial applications. Exploring an approach based on the PM that can provide good tensile properties is imperative in accelerating industrial applications. The amount of porosity, contamination, and oxidation must be minimized to achieve the desired tensile properties. The present thesis's main idea is to replace the high-temperature sintering with a cold consolidation technique (consolidation at ambient temperature). For cold consolidation, the high-pressure torsion (HPT) process has been used in the present thesis. During the cold consolidation of powders using HPT, the powders are subjected to very high pressures (~4 to 6 GPa). The applied pressure is higher than the pressure used in conventional compaction/sintering processes. Also, removing the high-temperature sintering process eliminates the amount of oxidation and contaminants caused by high-temperature sintering. The consolidation mechanism by the HPT process is as follows: First, the powders are pre-compacted into a disk-shaped sample by a hand-press machine. The disk-shaped sample is subjected to the HPT process, and the powders are deformed and compacted under pressure. Under the application of high pressure, most of the porosities are closed. In the next step, torsion is applied to the sample, generating shear stress inside the sample. This shear stress closes the remaining porosity and results in near full densification. As a severe plastic deformation (SPD) technique, the HPT process produces nano/ultrafine-grained structure beside cold-consolidation. Materials produced through SPD methods usually contain a very high density of dislocations. These structures show excellent strength due to their nano/ultrafine-grained structure and high dislocation density. However, they show a low capability to strain hardening, leading to limited ductility. Subsequent annealing is needed to enhance the strain hardening capacity by recovering the dislocation density. Therefore, annealed specimens provide reasonable ductility, although their strength declines. In the present process, the HPT-consolidated samples are annealed under temperatures much lower than the conventional sintering temperature to obtain the desired mechanical properties. In the initial part of this thesis, the CoCrFeMnNi high-entropy alloy (HEA) powder is processed by cold consolidation using HPT, followed by annealing. Density measurements indicated that the relative density reached 99.62% after cold consolidation. The tensile tests showed a unique combination of 745 MPa yield strength and 58% elongation for the HPT sample annealed at 800 °C for 15 min. These are the best tensile properties reported for this HEA that has never been achieved in the PM so far. The important point of this approach is the controllability of microstructure and mechanical properties with annealing conditions. In the second part, a nanostructured CoCrFeNi medium-entropy alloy (MEA) is produced through cold consolidation and subsequent annealing. The cold-consolidated sample shows outstanding tensile strengths of 2.06 GPa and 2.81 GPa at room and cryogenic temperatures. The enhanced cryogenic tensile properties can be associated with intensive mechanical twin activities. Additionally, engineering the microstructure through subsequent annealing leads to a desirable synergy of tensile strength and ductility, which are highly sought after in structural applications. The present findings pave the way to fabricate the parts with superior tensile properties for cryogenic applications through a PM-based approach. In the third part, the cold consolidation procedure is utilized to fabricate a nanostructured CoCrFeMnNi HEA reinforced with TiC nanoparticles. The microstructural evolutions and hardness changes of the TiC-reinforced HEA composite have been compared with the sole HEA specimen. The TiC-reinforced HEA composite with an uttermost relative density of 99.5% and uniform distribution of TiC nanoparticles exhibit restricted grain growth because of the pinning effect and improved hardness than the sole HEA counterpart. This fabrication procedure can be used to compose different HEA-matrix composites. In the final part, heterogeneous structures with different fractions of CoCrFeMnNi HEA (as the hard phase) and Fe60Co15Ni15Cr10 MEA (as the soft phase) are fabricated through the cold consolidation technique. The generated heterogeneous structures are used as models to investigate the optimal ratio between the hard and soft phases. Also, toughness is introduced as a criterion for evaluating the efficiency of hetero-structuring in this research. As the efficiency of hetero-structuring increases, the tensile toughness becomes higher than the toughness obtained from the rule of mixture. The results indicate that the highest efficiency is obtained when the hard phase is placed in the matrix, and the soft phase is distributed as separated islands into the matrix. Various heterogeneous structures can be produced with this technique, providing an excellent combination of high strength and high ductility. The present fabrication technique as pioneering work on MPEAs would be a breakthrough in the PM area, and it will have far-reaching and significant implications on the development of HEA/MEA-matrix composites and heterogeneous structures with the synergy of strength and ductility. The present work has great potential in hi-tech manufacturing, especially in micro-gears and cryogenic applications.