The effect of preliminary annealing at 1000°C and subsequent high-pressure torsion (HPT) on the phase composition and mechanical properties of titanium alloys with 2, 4, 6, and 8 wt

The multicomponent equiatomic CrMnFeCoNi alloy was proposed by B. Cantor almost 20 years ago and was the first in the family of the so-called multiprincipal or high-entropy alloys (HEAs). Various mechanical properties of the Cantor alloy and its derivatives, such as corrosion behavior, oxidation resistance, irradiation response, diffusion bonding, and weldability, have been studied these past years. Unfortunately, data on their thermo-physical properties are scarce and the information about infrared emissivity is completely absent. Having reliable infrared emissivity data at working conditions is very important for non-contact temperature measurements and for modeling heat transfer by radiation during manufacturing. In this work, a Cantor alloy, as a typical example of HEAs, was manufactured with levitation melting in vacuum. The alloy contains mainly one phase with face-centered cubic lattice and small amount of oxide precipitates. The angle-dependent spectral directional emissivity was measured between 200 and 700 °C. Reproducible data were obtained upon several thermal cycles. The total directional emissivity is almost constant from 10° to 50°, and it increases up until it reaches a maximum around 70°. Integrating these data, total hemispherical emissivity was determined, and it was observed that this property remains almost constant at 0.28 in a wide temperature range, showing a minor increase with increasing temperature. Spectral directional emissivity measurements allow detecting incipient oxidation processes. These data show the necessity of measuring emissivity at working temperatures to achieve a precise quantification of radiative heat transfer.

The specimens of Ti-2 wt % Fe alloy were annealed at three different temperatures, in the β-Ti, (α-Ti + β-Ti) and (α-Ti + TiFe) fields of the Ti-Fe phase diagram, then water quenched and subjected to high-pressure torsion (HPT). The X-ray diffraction analysis showed that the main phase in all annealed samples was the α phase (more than 90%), while the main phase after HPT was the ω phase. The hardness H and Young’s modulus E were determined by nanoindentation at the center, in the middle of the radius, and near the edge of each specimen. It was found that the H and E values were different for specimens annealed at different temperatures and depended on the radial coordinate of the indentation region. The maximum H values were obtained in the middle of the radius of the specimens. The E values of all specimens decreased from the center to the edge, reaching very low values. The paper discusses structure transformations during HPT, the behavior of the radial dependences of H and E , and probable causes of a strong decrease in E values.

Although vanadium is used in many alloys as an alloying element, the binary Ti–V system has been studied very poorly. In particular, the effect of heat treatment in combination with high-pressure torsion (HPT) has not been studied at all for this system. The Ti-2 wt

The specimens of Ti-2 wt % Fe alloy were annealed at three different temperatures, in the beta-Ti, alpha-Ti + beta-Ti and alpha-Ti + TiFe fields of the Ti-Fe phase diagram, then water quenched and subjected to high-pressure torsion (HPT). The X-ray diffraction analysis showed that the main phase in all annealed specimens was the alpha phase (more than 90%), while the main phase after HPT was the omega phase. Hardness H and Young's modulus E were determined by nanoindentation at the center, in the middle of the radius, and near the edge of each specimen. It was found that the H and E values were different for specimens annealed at different temperatures and depended on the radial coordinate of the indentation region. The maximum H values were obtained in the middle of the radius of the specimens. The E values of all specimens decreased from the center to the edge, reaching very low values. The paper discusses structure transformations during HPT, the behavior of the radial dependences of H and E, and probable causes of a strong decrease in E values.

Three titanium alloys with 0.5, 6, and 9 wt.% iron were investigated, and the samples were pre-annealed in three different regions of the Ti–Fe phase diagram, namely β, α+β, and α+FeTi. After annealing, five samples of different phases and structural compositions were studied. They were then subjected to the high-pressure torsion (HPT). The microstructure of the samples before and after HPT treatment was studied using transmission and scanning electron microscopy. The microstructure of the samples obtained during heat treatment before HPT treatment had a fundamental effect on the microstructure after HPT. Grain boundary layers and chains of particles formed during the annealing process made it difficult to mix the material during HPT, which led to the formation of areas with non-uniform mixing of components. Thus, the grain boundary layers of the α-phase formed in the Ti–6wt % Fe alloy after annealing at 670 °C significantly decreased the mixing of the components during HPT. Despite the fact that the microstructure and phase composition of Ti–6wt % Fe alloys pre-annealed in three different regions of the Ti–Fe phase diagram had significant differences, after HPT treatment, the phase compositions of the studied samples were quite similar. Moreover, the measured micro- and nanohardness as well as the Young’s modulus of Ti–6wt % Fe alloy had similar values. It was shown that the microhardness of the studied samples increased with the iron content. The values of nanohardness and Young’s modulus correlated well with the fractions of β- and ω-phases in the studied alloys.

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.

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, β, 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 β 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, β, 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 β alloys.

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.

The paper analyzes the effect of severe plastic deformation by the high-pressure torsion (HPT) on phase transformations, in particular, on the formation of the ω-phase, and on mechanical properties, such as hardness and Young’s modulus, in Ti alloys with 1.5 and 15 wt.% Mo. Both alloys were pre-annealed at 1000 °C for 24 h and quenched. The microstructure of the initial Ti–1.5 wt.% Mo alloy consisted of the α-phase and α’-martensite, and the initial Ti–15 wt.% Mo alloy contained polycrystalline β solid solution. The hardness tests of the samples were carried out under the load of 10 and 200 mN. The annealed alloys were subjected to HPT, and the micro- and nanohardness of both deformed samples increased up to ~1 GPa compared to their initial state. It turned out that the values of hardness (H) and Young’s modulus (E) depend on the applied load on the indenter: the higher the applied load, the lower H and higher E. It was also found that the HPT leads to the 30% increase in E for an alloy with 1.5 wt.% Mo and to the 9% decrease in E for the alloy with 15 wt.% Mo. Such a difference in the behavior of the Young’s modulus is associated with phase transformations caused by the HPT.

Grain boundaries (GBs) can be wetted by a second phase. This phase can be not only liquid (or melted), but it can also be solid. GB wetting can be incomplete (partial) or complete. In the case of incomplete (partial) wetting, the liquid forms in the GB droplets, and the second solid phase forms a chain of (usually lenticular) precipitates. Droplets or precipitates have a non-zero contact angle with the GB. In the case of complete GB wetting, the second phase (liquid or solid) forms in the GB continuous layers between matrix grains. These GB layers completely separate the matrix crystallites from each other. GB wetting by a second solid phase has some important differences from GB wetting by the melt phase. In the latter case, the contact angle always decreases with increasing temperature. If the wetting phase is solid, the contact angle can also increase with increasing temperature. Moreover, the transition from partial to complete wetting can be followed by the opposite transition from complete to partial GB wetting. The GB triple junctions are completely wetted in the broader temperature interval than GBs. Since Phase 2 is also solid, it contains GBs as well. This means that not only can Phase 2 wet the GBs in Phase 1, but the opposite can also occur when Phase 1 can wet the GBs in Phase 2. GB wetting by the second solid phase was observed in the Al-, Mg-, Co-, Ni-, Fe-, Cu-, Zr-, and Ti-based alloys as well as in multicomponent alloys, including high-entropy ones. It can seriously influence various properties of materials.

The study focused on a 21.99 at.%Ti–22.49 at.%Zr–20.35 at.%Hf–17.45 at.%Mo–17.73 at.%Cr). Analytical techniques such as X-ray diffraction, scanning electron microscopy as well as X-ray absorption spectroscopy were employed to investigate the alloy’s structure, phase transformations, and properties. The alloy in the as-cast state contained three phases, namely the body-centred cubic (A2) phase, hexagonal Laves phase (C14), and cubic Laves phase (C15). The alloy has been annealed for a long time at different temperatures. It led to the disappearance of the hexagonal Laves phase, leaving behind two primary phases, namely the cubic Laves phase (C15) and the body-centered cubic phase (A2). At 1200 °C, the A2 phase almost disappeared, resulting in a practically single-phase sample. After a high-pressure torsion (HPT) treatment, the hexagonal Laves phase disappeared entirely, while the A2 and C15 phases remained. The grain size of the A2 and C15 phases was refined after HPT and grains were elongated, and their configuration resembled a layered structure. The high hardness of the A2 and C15 + C14 phases accounted for this behavior. The lattice parameters in the A2 and C15 phases after HPT treatment approached those observed after prolonged annealing at 1000 °C, indicating that the composition of these phases after short-term high-pressure torsion at ambient temperature is equivalent to the composition of these phases after long tempering at 1000 °C. The rate of diffusion-like mass transfer during severe plastic deformation was estimated to be many orders of magnitude higher than that for conventional bulk diffusion at the HPT treatment temperature and similar to that at elevated temperatures above 1000 °C. X-ray absorption spectroscopy results obtained at K-edges of Ti, Cr, Zr, and Mo as well as at the L3-edge of Hf indicated that the local environment around metal atoms before HPT was similar to that after HPT. However, the static disorder increased after HPT, which could be attributed to an increased specific amount of metal atoms in the disordered grain boundary layers after HPT-driven grain refinement.

The high pressure torsion (HPT) of various binary Ti alloys with beta-stabilizers (Fe, Co, Ni, Mo, Nb) was studied. Before HPT, the samples were annealed and contained (i) pure beta-phase, (ii) alpha+beta mixture with different portion of phases, (iii) alpha(') or alpha'' martensites, (iv) the mixture of alpha-Ti and respective intermetallic phase. The microstructure of Ti alloys before and after HPT was studied by scanning and transmission electron microscopy (also high resolution one), X-rays diffraction (including the high-temperature in situ one), differential scanning calorimetry, atomic probe tomography, synchrotron irradiation. During HPT the (usual) strong grain refinement took place. Also, it was observed that HPT can lead to various phase transitions in the Ti alloys. In particular, the metastable high-pressure omega phase and alpha' martensite can form. The composition of phases (similar to their grain size) reached the steady-state value after about 1.5 plunger revolutions. In some cases, the equifinality of the HPT-driven phase transitions was observed. Equifinality means that the composition and portion of phases after HPT do not depend on the composition and portion of phases before HPT. The HPT-driven phase transitions can be martensitic (i.e. without or almost without mass transfer) or diffusional (i.e. with mass transfer). In case of martensitic beta-to-omega or alpha-to-omega phase transitions, the certain orientation relations between beta and omega or alpha and omega phases were observed. The thermal stability of the 1/2 -phase obtained by HPT has been studied by the in-situ X-rays diffraction at high temperatures. The omega-phase in the HPT-treated Ti alloys with beta-stabilizers can remain in the samples up to 500-600 degrees C. It is much higher than in pure titanium (similar to 180 degrees C). Thus, the HPT-driven phase transitions open the new way for tailoring of grain size and phase composition of Ti-based alloys. In turn, it gives the new instrument in hands of engineers to improvement of the technologically important properties of Ti-based alloys. It is especially important for the medical application like the teeth or bone prosthesis.

The structure of Ti–2 wt % Cr, Ti–4 wt % Cr, and Ti–5.5 wt % Cr alloys, annealed under conditions corresponding to the two-phase region (α + β) of the Ti–Cr phase diagram, is studied using scanning electron microscopy, X-ray diffraction analysis, and microindentation. The work studies the formation and growth of (αTi) phase layers at the grain boundaries (βTi)/(βTi). According to the results of X-ray diffraction analysis, all samples contain both (αTi) and (βTi) phases after annealing. For each alloy, the temperatures are determined at which continuous (αTi) phase layers are formed at the grain boundaries. The thickness and hardness of these layers are measured. The higher the chromium concentration, the harder are both the (αTi) and (βTi) phases. The (αTi)-phase hardness in the Ti–5.5 wt % Cr alloy is independent of the annealing temperature, but the (βTi)-phase hardness increases with decreasing annealing temperature.

High-entropy alloys (HEAs) are called also alloys without a main component or multiprincipal alloys. They consist of five, six or more components in more or less equal proportions and possess unique properties. Several dozens of thousands of publications have already been devoted to bulk HEAs, while HEA coatings are just beginning to develop. More than half of the works on the deposition of HEA coatings are devoted to laser cladding. In the laser cladding process, a mixture of powders on a substrate is melted in a focused laser beam, which sequentially scans the substrate. In the heated zone, the powder mixture melts. At the end of the crystallization process, a solidified polycrystal and a small amount of residual melt are found in the heated zone. It is possible that the grain boundaries (GBs) in the solidified polycrystal are incompletely or fully wetted by this liquid phase. In this way, the GB wetting with a melt determines the morphology and microstructure of HEAs coatings. This review analyzes GB wetting in single-phase HEAs, as well as in HEAs containing two or more phases. We analyze how the HEAs’ composition, laser scanning speed, laser beam power, external magnetic field or ultrasonic impact affect the microstructure and GB wetting. It is also shown how the microstructure and GB wetting change over the thickness of the rather thick as well as multilayer coatings deposited using a laser cladding.

This paper discusses the features of ω-phase formation and its thermal stability depending on the phase composition, alloying element and the grain size of the initial microstructure of Ti–Nb and Ti–Mo alloys subjected to high-pressure torsion (HPT) deformation. In the case of two-phase Ti–3wt.% Nb and Ti–20wt.% Nb alloys with different volume fractions of α- and β-phases, a complete β→ω phase transformation and partial α→ω transformation were found. The dependence of the α→ω transformation on the concentration of the alloying element was determined: the greater content of Nb in the α-phase, the lower the amount of ω-phase that was formed from it. In the case of single-phase Ti–Mo alloys, it was found that the amount of ω-phase formed from the coarse-grained β-phase of the Ti–18wt.% Mo alloy was less than the amount of the ω-phase formed from the fine α′-martensite of the Ti–2wt.% Mo alloy. This was despite the fact that the ω-phase is easier to form from the β-phase than from the α- or α′-phase. It is possible that the grain size of the microstructure also affected the phase transformation, namely, the fine martensitic plates more easily gain deformation and overcome the critical shear stresses necessary for the phase transformation. It was also found that the thermal stability of the ω-phase in the Ti–Nb and Ti–Mo alloys increased with the increasing concentration of Nb or Mo.

The multicomponent alloys with nearly equal concentration of components, also known as high entropy alloys (HEAs), were first proposed 22 years ago. The HEAs quickly became very important in materials science due to their unique properties. Nowadays, the HEAs are frequently used in energy conversion and storage applications. HEAs can consist of five, six or more components. Plasma cladding permits coating of the large surfaces of cheap substrates with (often expensive) HEAs and to enlarge, in such a way, their application area. The large-area coatings deposited by plasma cladding possess multiple advantages such as low thermal distortion, very high energy density, as well as low dilution of the substrate material. Plasma cladding ensures good metallurgical bonding between coating and substrate. The costs of operation and equipment are also very attractive. During plasma cladding, the mixed powders are blown by carrier gas into a plasma torch or are positioned on a substrate. This powder mixture is then melted in or under the plasma torch. The plasma torch, in turn, sequentially scans the substrate. After finalizing the crystallization process, the solid polycrystal appears which contains few residual melts. This remaining melt can completely or incompletely wet the grain boundaries (GBs) in solid phase of the polycrystal. These completely or incompletely wetted GBs can strongly influence the microstructure of HEA coatings and their morphology. In this review we analyze the GB wetting HEAs containing one phase in HEAs with two, three and more phases, as well as in HEAs reinforced with particles of carbides, nitrides, borides, or oxides. We also analyze the microstructure of the rather thick coatings after plasma cladding after additional laser remelting and observe how GB wetting changes over their thickness.

The effect of surface roughness of cold-wrought Ti-6Al-4V (VT6) alloy on the adhesion of mouse MC3T3-pre-osteoblasts cells has been studied. The array of linear grooves has been produced on the substrate surface with the aid of abrasive SiC papers with grit sizes of 220, 400, 600, 800, 1000, 1200, 2000, and 4000 (number of grains per cm^2), as well as different diamond pastes with grain sizes 6, 3, and 1 μm. The grain size of this array of abrasive papers and diamond pastes monotonously decreases from 68 μm (for 220 grit paper) to 1 μm for the finest diamond paste. The adhesion of the mouse MC3T3-pre-osteoblast cells to the samples with different roughness has been measured by fluorescence microscopy. The size and morphology of adhered cells were measured by the light microscopy. The relative fraction of adhered cells behaves non-monotonously with the abrasive grain size. It is about 0.35 for grain size of 1 μm, increases up to 0.95 for grain size of 7 μm, and then slowly decreases down to 0.55 for abrasive grain size of 68 μm. The surface roughness parameters of Ti–6Al–4V (VT6) alloy correlate with the depth and width of grooves produced on the initially flat surface of the alloy. The surface ground with a grinding media corresponding to the grain size of about 10 μm exhibited highest cell adhesion. This grain size corresponding to maximum cells adhesion is comparable with the dimension of MC3T3-pre-osteoblasts cells (~ 14 μm). Thus, the variation of surface roughness opens the way to control and tailor the fraction of adhered cells, depending on the demand of medical techniques. Graphical abstract

In this paper, we measure the nanohardness (H) and Young’s modulus (E) of three alloys: Ti–2.5 wt % Ni, Ti–2 wt % Cr, and Ti–2.2 wt % Fe preliminarily annealed in the two-phase region of the phase diagram (αTi + intermetallic compound) and then subjected to high-pressure torsion. The titanium alloy with the nickel addition showed the highest H and E values, they vary uniformly from the center to the edge of the sample, and the alloy after high-pressure torsion contains two phases: α and ω. The nanohardness of the alloy Ti–2.5 wt % Ni along the sample radius over the surface changes insignificantly: from minimal 4.8 to maximal 5.2 GPa, as does Young’s modulus (from 121 to 155 GPa). The maxima of the H and E values fall in the middle of the sample radius. The alloy Ti–2.2 wt % Fe behaves differently: the presence of four phases α, β, ω, and TiFe leads to a strong scatter in the measured H and E values: from 4.4 to 2.0 GPa and from 131 to 12 GPa, respectively. Processing the P–h diagrams allows the nanohardness of the material to be related to its creep behaviour.