Tailoring a Refractory High Entropy Alloy by Powder Metallurgy Process Optimization

This paper reports the microstructural evolution and mechanical properties of a low-density Al0.3NbTa0.8Ti1.5V0.2Zr refractory high-entropy alloy (RHEA) prepared by means of a combination of mechanical alloying and spark plasma sintering (SPS). Prior to sintering, the morphology, chemical homogeneity and crystal structures of the powders were thoroughly investigated by varying the milling times to find optimal conditions for densification. The sintered bulk RHEAs were produced with diverse feedstock powder conditions. The microstructural development of the materials was analyzed in terms of phase composition and constitution, chemical homogeneity, and crystallographic properties. Hardness and elastic constants also were measured. The calculation of phase diagrams (CALPHAD) was performed to predict the phase changes in the alloy, and the results were compared with the experiments. Milling time seems to play a significant role in the contamination level of the sintered materials. Even though a protective atmosphere was used in the entire manufacturing process, carbide formation was detected in the sintered bulks as early as after 3 h of powder milling. Oxides were observed after 30 h due to wear of the high-carbon steel milling media and SPS consolidation. Ten hours of milling seems sufficient for achieving an optimal equilibrium between microstructural homogeneity and refinement, high hardness and minimal contamination.</p

Metal Matrix Composites Prepared by Powder Metallurgy Route

Vývoj nových materiálů pro součásti v moderních technologiích vystavené extrémním podmínkám má v současné době rostoucí význam. Děje se tak v důsledku neustále se zvyšujících požadavků průmyslových odvětví na lepší konstrukční vlastnosti nosných materiálů. Ve světle těchto faktů si tato studie klade za cíl posoudit nové složení slitin s vysokou entropií, které se vyznačují vysokým aplikačním potenciálem pro kritické aplikace. Slitiny jsou připravovány práškovou metalurgií, t.j. kombinací mechanického legování a slinování v pevné fázi. Pro účely srovnávaní vlastností jsou vybrané kompozice vyrobeny také tradičními metalurgickými metodami v roztaveném stavu, jako je vakuové indukční tavení a následné lití nebo vakuové obloukové tavení. Prášková metalurgie umožňuje postupný vývoj kompozitů s kovovou matricí (MMC) prostřednictvím přípravy oxidicky zpevněných HEA slitin. To je možné díky inherentním in-situ reakcím během procesu výroby. Když se naopak zvolí výrobní postup z taveniny, připravený kovový materiál vykazuje velké rozdíly v mikrostrukturách a souvisejících vlastnostech, v porovnání se stejným materiálem vyrobeným práškovou cestou (PM). Vyrobené práškové a tavené materiály jsou detailně charakterizovány s ohledem na komplexní vyhodnocení vlivu různých metod zpracování. Práce se zejména orientuje na mikrostrukturní charakteristiky materiálů a jejich mechanické vlastnosti, včetně vlivu tepelného zpracování na fázové transformaci a mikrostrukturní stabilitu připravených materiálů.The development of new materials for applications in extreme environments is currently of a great importance in modern engineering technologies. Hence, the industries' requirement for enhanced structural performance of materials is constantly increasing. In the light of that, this study aims to evaluate promising compositions of high-entropy alloys for critical applications produced by powder metallurgy through a combination of mechanical alloying and solid state sintering. For comparative purposes, the selected compositions were produced by melting routes in liquid state as well, such as vacuum induction melting and subsequent casting or vacuum arc-melting. The powder metallurgy route enables a consequential development of metal matrix composites (MMC) via the manufacturing of oxide dispersed strengthened HEAs. This is possible due to inherent in-situ reactions during the process. In case of melting route fabrication, metallic materials with great differences in structures and related properties are manufactured, compared to those produced by powder routes. The produced MMCs and their melted counterparts are thoroughly studied. A comprehensive evaluation of the influence of the different processing methods, especially on the materials’ microstructural features and their mechanical properties is undertaken, including the effect of heat treatments on the phase transformations and stability of the materials.

Impact of interstitial elements on the stacking fault energy of an equiatomic CoCrNi medium entropy alloy: theory and experiments

We investigated the effects of interstitial N and C on the stacking fault energy (SFE) of an equiatomic CoCrNi medium entropy alloy. Results of computer modeling were compared to tensile deformation and electron microscopy data. Both N and C in solid solution increase the SFE of the face-centered cubic (FCC) alloy matrix at room temperature, with the former having a more significant effect by 240% for 0.5 at % N. Total energy calculations based on density functional theory (DFT) as well as thermodynamic modeling of the Gibbs free energy with the CALPHAD (CALculation of PHAse Diagrams) method reveal a stabilizing effect of N and C interstitials on the FCC lattice with respect to the hexagonal close-packed (HCP) CoCrNi-X (X: N, C) lattice. Scanning transmission electron microscopy (STEM) measurements of the width of dissociated 1/2 dislocations suggest that the SFE of CoCrNi increases from 22 to 42-44 mJ center dot m(-2) after doping the alloy with 0.5 at. % interstitial N. The higher SFE reduces the nucleation rates of twins, leading to an increase in the critical stress required to trigger deformation twinning, an effect which can be used to design load-dependent strain hardening response