Effect of High-Pressure Torsion on Phase Formation and Mechanical Properties of a High-Entropy TiZrHfMoCrCo Alloy.

This investigation delved into the alterations in the mechanical properties of a TiZrHfMoCrCo high-entropy alloy due to phase transformations induced by high-pressure torsion (HPT). The alloy's genesis involved levitation melting within an argon atmosphere, presenting two distinct states for analysis: the initial, post-manufacturing state and the state subsequent to HPT treatment. The original alloy featured a composition comprising a singular A2 phase with a bcc lattice and two Laves phases, C15 and C14. The HPT process triggered significant phase modifications: a retention of one C15 Laves phase and decomposition of the bcc phase into two distinct phases exhibiting different bcc lattice parameters. The HPT-induced effect prominently manifests as strong grain refinement. However, scanning electron microscopy (SEM) observations unveiled persistent inhomogeneities at a micron scale both before and after HPT treatment. Thus, grain refinement occurs separately within each of the bcc and Laves phases, visible in the light, dark, and gray areas in SEM images, while mixing does not occur on the scale of several microns. The examination of Ti, Cr, Co, Zr, Mo, and Hf via X-ray absorption spectroscopy (EXAFS) at specific K-edges and L3-edge revealed that the HPT treatment conserves the local atomic environment of metal atoms, albeit with a slight elevation in static disorder. Assessments through microhardness and three-point bending tests demonstrated the material's inherent hardness and brittleness. The microhardness, standing at a substantial value of 600 HV, displayed negligible augmentation post-HPT. However, the microhardness of individual phases exhibited a notable alteration, nearly doubling in magnitude.

Influence of the Phase Composition of Titanium Alloys on Cell Adhesion and Surface Colonization.

The pivotal role of metal implants within the host's body following reconstructive surgery hinges primarily on the initial phase of the process: the adhesion of host cells to the implant's surface and the subsequent colonization by these cells. Notably, titanium alloys represent a significant class of materials used for crafting metal implants. This study, however, marks the first investigation into how the phase composition of titanium alloys, encompassing the volume fractions of the α, ß, and ω phases, influences cell adhesion to the implant's surface. Moreover, the research delves into the examination of induced hemolysis and cytotoxicity. To manipulate the phase composition of titanium alloys, various parameters were altered, including the chemical composition of titanium alloys with iron and niobium, annealing temperature, and high-pressure torsion parameters. By systematically adjusting these experimental parameters, we were able to discern the distinct impact of phase composition. As a result, the study unveiled that the colonization of the surfaces of the examined Ti-Nb and Ti-Fe alloys by human multipotent mesenchymal stromal cells exhibits an upward trend with the increasing proportion of the ω phase, concurrently accompanied by a decrease in the α and ß phases. These findings signify a new avenue for advancing Ti-based alloys for both permanent implants and temporary fixtures, capitalizing on the ability to regulate the volume fractions of the α, ß, and ω phases. Furthermore, the promising characteristics of the ω phase suggest the potential emergence of a third generation of biocompatible Ti alloys, the ω-based materials, following the first-generation α-Ti alloys and second-generation ß alloys.

Phase Transformations Caused by Heat Treatment and High-Pressure Torsion in TiZrHfMoCrCo Alloy.

In this work the high-entropy alloy studied contained six components, Ti/Zr/Hf/Mo/Cr/Co, and three phases, namely one phase with body-centered cubic lattice (BCC) and two Laves phases C14 and C15. A series of annealings in the temperature range from 600 to 1000 °C demonstrated not only a change in the microstructure of the TiZrHfMoCrCo alloy, but also the modification of phase composition. After annealing at 1000 °C the BCC phase almost fully disappeared. The annealing at 600 and 800 °C leads to the formation of new Laves phases. After high-pressure torsion (HPT) of the as-cast TiZrHfMoCrCo alloy, the grains become very small, the BCC phase prevails, and C14 Laves phase completely disappears. This state is similar to the state after annealing at high effective temperature Teff. The additional annealing at 1000 °C after HPT returns the phase composition back to the state similar to that of the as-cast alloy after annealing at 1000 °C. At 1000 °C the BCC phase completely wets the C15/C15 grain boundaries (GBs). At 600 and 800 °C the GB wetting is incomplete. The big spread of nanohardness and Young's modulus for the BCC phase and (C15 + C14) Laves phases is observed.

Grain Boundary Wetting by a Second Solid Phase in the High Entropy Alloys: A Review.

In this review, the phenomenon of grain boundary (GB) wetting by the second solid phase is analyzed for the high entropy alloys (HEAs). Similar to the GB wetting by the liquid phase, the GB wetting by the second solid phase can be incomplete (partial) or complete. In the former case, the second solid phase forms in the GB of a matrix, the chain of (usually lenticular) precipitates with a certain non-zero contact angle. In the latter case, it forms in the GB continuous layers between matrix grains which completely separate the matrix crystallites. The GB wetting by the second solid phase can be observed in HEAs produced by all solidification-based technologies. The particle chains or continuous layers of a second solid phase form in GBs also without the mediation of a liquid phase, for example by solid-phase sintering or coatings deposition. To describe the GB wetting by the second solid phase, the new GB tie-lines should be considered in the two- or multiphase areas in the multicomponent phase diagrams for HEAs. The GB wetting by the second solid phase can be used to improve the properties of HEAs by applying the so-called grain boundary engineering methods.

Structural and Mechanical Properties of Ti⁻Co Alloys Treated by High Pressure Torsion.

The microstructure and properties of titanium-based alloys can be tailored using severe plastic deformation. The structure and microhardness of Ti⁻4 wt.% Co alloy have been studied after preliminary annealing and following high pressure torsion (HPT). The Ti⁻4 wt.% Co alloy has been annealed at 400, 500, and 600 °C, i.e., below the temperature of eutectoid transformation in the Ti⁻4 wt.% Co system. The amount of Co dissolved in α-Ti increased with increasing annealing temperature. HPT led to the transformation of α-Ti in ω-Ti. After HPT, the amount of ω-phase in the sample annealed at 400 °C was about 80-85%, i.e., higher than in pure titanium (about 40%). However, with increasing temperature of pre-annealing, the portion of ω-phase decreased (60⁻65% at 500 °C and about 5% at 600 °C). The microhardness of all investigated samples increased with increasing temperature of pre-annealing.