Β-phase Region Research Articles

Effect of rare earth yttrium and the deformation process on the thermal deformation behavior and microstructure of pure titanium for cathode rolls

This study explores the challenges associated with microstructural refinement and homogenization control for large titanium cathode rolls utilized in electrolytic copper foil production. An innovative deformation control strategy was developed by incorporating yttrium microalloying within the β→α dual-phase region to refine the hot-deformed microstructure of pure titanium. Gleeble thermal simulation tests were performed to assess the impact of rare earth Y, deformation temperatures (700–900 °C), and interpass cooling rates (5 °C/s, 25 °C/s, and 50 °C/s) on the hot deformation behavior and microstructural evolution of pure titanium. The results reveal that Y2O3 particles generated by the addition of rare earth Y enhance dynamic recrystallization through the particle-stimulated nucleation (PSN) mechanism, effectively pinning grain boundaries and impeding recrystallized grain growth. Increased interpass cooling rates lead to finer grains and higher stored energy. The mechanism for grain refinement during dual-phase deformation is described as follows: β-phase nonrecrystallization temperature deformation → rapid cooling control → α-phase recrystallization deformation control, energy storage, strain-induced dynamic phase transformation (SIDT, α→β), and dynamic recrystallization (DRX). Under experimental conditions of β-phase region deformation at 980 °C → inter-pass cooling rate of 50 °C/s → α-phase region deformation at 750 °C, the addition of rare earth yttrium achieved the most effective grain refinement, reducing the average grain size to 2.56 μm.

Mechanical Behavior of Additive Manufacturing (AM) and Wrought Ti6Al4V with a Martensitic Microstructure

Processing and microstructure are fundamental in shaping material behavior and failure characteristics. Additively manufactured materials, due to the rapid heating and solidification process, exhibit unique microstructures compared to their as-cast counterparts, resulting in distinct material properties. In this work, the response of the titanium alloy Ti6Al4V has been investigated for different processing conditions through quasi-static testing. AM Ti6Al4V was fabricated by employing Selective Laser Sintering (SLS) and Selective Laser Melting (SLM) techniques. Both materials present a similar microstructure consisting of an acicular martensitic α′-phase. Commercial Ti6Al4V-grade 5 (supplied as bars) was also examined after heat treatment to achieve a microstructure akin to the AM material. The heat treatment involved rapid heating above the β-phase region and water quenching to obtain a full martensite microstructure. A similar constitutive behavior and tensile–compressive asymmetry in strength were noted for the investigated materials. However, AM alloys exhibited a significantly higher deformation at failure, reaching nearly 40%, compared to only 6.1% for the wrought martensitic material, which can be attributed to the dissimilar distribution of both α′ laths and prior-β grain boundaries in the investigated materials. The results indicate that AM can be implemented for the fabrication of martensitic microstructures with mechanical properties superior to those obtained with conventional water-quenching.

Hot pack rolling the Ti2AlNb alloy prepared by SPS: Testing of the high-temperature properties of the alloy directly without the prior heat treatment

Ti2AlNb alloy holds large research significance as a lightweight material for high-temperature structural applications in the next generation of aero-engines. In this work, pre-alloyed Ti2AlNb powders were spark plasma sintered to fabricate the raw compact, which was then hot rolled at different phase fields to fabricate sheets. The high-temperature properties of the sheets were tested directly without the prior heat treatment. Results show that the Ti2AlNb sheets have good high-temperature strengths even without the heat treatment prior to testing. For the sheet rolled at the single B2 phase region, the maximum tensile strength was up to 931.4 MPa at 650 °C. The sheet mainly shows intergranular fracture mode due to the weak bonding at grain boundaries. The sheets rolled at the O + B2 phase region have low tensile strengths at different testing temperatures. For the sheet rolled at α2+O + B2 phase region, the tensile strength gains its maximum strength of 956.6 MPa at 650 °C, and the sheet shows transcrystalline fracture mode. The fracture mechanisms of the sheets rolled at different phase regions are analyzed in detail and proved by experiment. This work shows the potential of fabricating Ti2AlNb sheet by using the spark plasma sintered compact.

Effect of compositional minor adjustment on localized lamellar structure coarsening of TiAl alloys with different solidification modes

Possible lamellar coarsening in the as-cast microstructure of TiAl alloys significantly affects the subsequent processing of ingot metallurgy, and a systematic study of it is necessary. In this paper, two TiAl alloys with different solidification modes (TNM-45Al alloy and TNM-0.3Si-0.7 C alloy) were obtained by compositional minor adjustment basing on TNM alloy. The effect of different adjustment routes on the localized lamellar structure coarseing behaviour and the resulting differences in the uniformity of the nanohardness distribution were investigated by comparing with TNM alloy. It is found that increasing the Al content of the TNM alloy while keeping the solidification mode as β-solidification unchanged leads to severe localised lamellar coarsening in the S-segregation zone of TNM-45Al alloy, including continuous coarsening (CC) and discontinuous coarsening (DC). In contrast, the solidification mode of the TNM-0.3Si-0.7 C alloy with the addition of the strong α stabilizer elements Si and C is transformed into hypoperitectic solidification with severe peritectic segregation, but the lamellar structure exhibits surprising full-domain stability. The results show that the thermodynamic tendency caused by high Al content and the absence of net-like β-segregation leads to severe localised lamellar coarsening in TNM-45Al alloy. CC can be accomplished through the migration of interface defects. DC is formed by the nucleation and expansion of γ-phase Shockley incomplete dislocations, and the interfaces consist of the α2 /γ interface and the first discovered γ /γ twin interface. The lamellar coarsening of TNM-0.3Si-0.7 C alloy can be effectively suppressed by the solid solution of Si/C atoms and the near net-like β-segregation in the primary β-phase region. As a result, the distribution uniformity of the nanohardness of the lamellar structure in TNM-0.3Si-0.7 C alloy is excellent, with the variance decreasing from ±0.24 of TNM alloy to ±0.20, while it increases to ±0.37 for the TNM-45Al alloy.

Study of grain growth kinetics of Zr-2.5%Nb alloy quenched from (α+β) and β-phase region

The Zr-2.5 %Nb is used as the Pressure Tube (PT) material in Pressurized Heavy Water Reactors (PHWRs) in both cold worked and stress relieved as well as heat treated condition. For the development of heat-treated pressure tube, process parameters need to be tuned to achieve optimum microstructure and texture. In microstructural aspect, prior β grain size is a very important factor in influencing the property of the material. Hence, the prior β grain growth kinetics of the Zr-2.5 %Nb alloy has been investigated by subjecting the material to different heat treatments. Different heat treatment temperatures have been chosen from (α+β) and β phase region. The β transus temperature was measured through dilatometry. Specimens were soaked for different time and the grain size were measured using optical metallography. The evolution of grain size with soaking temperature and time were analyzed to study the grain growth kinetics.

In-situ investigation of tensile anisotropy mechanism in an advanced Ti2AlNb-based alloy associated with CRSS ratio and damage model

Combined with in-situ tensile test and electron backscatter diffraction technology, the critical resolved shear stress (CRSS) ratio of O phase and the damage model of grain boundaries were proposed to reveal the tensile anisotropy mechanism of Ti–22Al–25Nb alloy isothermally forged in B2 phase region. In-situ observation suggests that the single slip mode of lamellar O phase is the major deformation behavior, and its preferential slip dominates the yield of the material. Based on the types, number, Schmid factor (SF) and orientation of slip activation, the CRSS ratio of basal <a>, prism <a>, first-order and second-order pyramidal <c+a> slips for O phase is estimated to be 1.19: 1: 1.37: 1.39. Meanwhile, basal and prism <a> slips mainly appear the radial and axial directions (RD and AD) samples with [012] and [100] texture, while the [001], [010] and [212] texture of OD (45° to RD) sample shows an increase in pyramidal <c+a> slips. Thus, the slip activation of O phase depends on the grain orientation and CRSS, resulting in significant strength anisotropy. Furthermore, the microcracks and secondary cracks at grain boundaries are significantly affected by the slip accumulation of lamellar O phase at the O/O and O/B2 interfaces. Considering the stress state of the grain boundary, a damage model about crack nucleation and propagation is proposed to explain the ductility anisotropy. From this, the RD sample has a higher crack nucleation and propagation resistance than the AD and OD samples due to the flattened morphology of B2 grains.

Research on tensile anisotropy of Ti-22Al-25Nb alloy isothermally forged in B2 phase region related with texture and variant selection

Research on tensile anisotropy of Ti-22Al-25Nb alloy isothermally forged in B2 phase region related with texture and variant selection

Enhanced mechanical properties of a near-α titanium alloy by tailoring the silicide precipitation behavior through severe plastic deformation

The microstructure evolution, tensile properties, and strengthening mechanisms of a near-α high-temperature titanium alloy with high Zr and Si content during isothermal multi-dimensional forging (IMDF) at the β-phase region were systematically investigated. The results show that the alloys exhibit a basket-weave microstructure after IMDF, with the silicides diffusely distributed in the matrix. As the forging temperature decreased, the intragranular primary α laths became more refined and the volume fraction of silicides increased, while the average size of silicides gradually decreased. The dynamic spheroidization of α laths occurs preferentially at the prior β grain boundary and extends into the grains as the deformation temperature decreases. The IMDF-1100 alloy possesses excellent high-temperature tensile properties and moderate room-temperature tensile properties, with ultimate tensile strengths (UTS) of 738.4 MPa and 1124.8 MPa at 650 °C and room temperature, respectively, which makes the alloy have tremendous potential for aerospace applications. The strengthening of the alloy at room temperature is primarily attributed to microstructure refinement and the thermal mismatch between silicide and matrix. However, owing to the weakening of grain boundaries at 650 °C, grain refinement leads to a decrease in tensile strengths. When tensile at 650 °C, the fragmentation and decohesion of large-size silicides tend to cause premature fracture of the alloy, and the microstructure with small-size silicide distribution presents better ductility.

Effects of aluminum and oxygen additions on quenched-in compositional fluctuations, dynamic atomic shuffling, and their resultant diffusionless isothermal [formula omitted] transformation in ternary Ti–V-based alloys with bcc structure

Diffusionless isothermal ω transformation is a recently discovered ω transformation that occurs isothermally during aging near room temperature in Ti alloys with a bcc (β-phase) structure. Unlike conventional displacive phase transitions, the diffusionless isothermal ω transformation occurs in locally unstable nanoscale β-phase regions formed by quenched-in statistical compositional fluctuations. In the present study, the effects of Al and oxygen additions in ternary β-phase Ti–V-based alloys on the diffusionless isothermal ω transformation and its attempt process, called dynamic atomic shuffling, were studied. Elastic constant and internal friction measurements for Ti–16.6V–2.1Al and Ti–17.5V–2.5O (at.%) alloy single crystals and analysis of quenched-in compositional fluctuations using the thermodynamic theory of fluctuations indicated that the oxygen addition increases the β-phase stability, and it selectively increases the activation energy of dynamic atomic shuffling in locally unstable Ti-rich regions, which is caused by the strong attractive interaction between Ti and oxygen elements. As a result, oxygen addition suppresses the diffusionless isothermal ω transformation that occurs during aging at 298 K. In contrast, low β-phase stability, which promotes ω-phase nucleation, is retained by Al addition. Furthermore, dynamic atomic shuffling with a low activation energy is retained even though the average activation energy is enhanced by the Al addition, originating from the relatively weak atomic interaction between Ti and Al. Therefore, the diffusionless isothermal ω transformation cannot be suppressed by Al addition.

A novel type of core-shell structured secondary phase particles in Ge-addition modified Zircaloy-4 alloy subjected to β-phase region solution treatment

Secondary phase particles (SPPs) in a Ge-addition modified Zircaloy-4 alloy subjected to β-phase region solution treatment was investigated in detail by transmission electron microscopy to acquire a better understanding on the microstructure of this new zirconium alloy. Result revealed that a novel type of SPPs with core-shell structure were frequently observed in this alloy, in which the “core” and “shell” structures were separately identified as primitive tetragoanl Zr3Ge phase and Zr(Fe,Cr)2 Laves phase with a hexagonal close-packed (HCP) structure. The orientation relationship between the Zr3Ge core, Zr(Fe,Cr)2 shell structures and the surrounding matrix were investigated and can be approximately described as [115]Zr3Ge||[112¯6¯]Zr(Fe,Cr)2||[112¯3]α−Zr. It was strongly suggested that the gradient of Ge concentration occurred during the subsequent β-phase region annealing affected the phase stability of Zr(Fe,Cr,Ge)2 nanophase in the as-cast Ge-addition modified Zr-4 alloy, thereby causing the formation of Zr3Ge/core-Zr(Fe,Cr)2/shell microstructures. The knowledge attained from this study, shedding new lights on the formation mechanism of these novel core-shell SPPs, would also greatly benefit the understanding on the formation of similar composite precipitates in other alloys.

Effect of Mn on microstructure and tensile properties of as-cast Al0.5CoFeNiC0.1 high-entropy alloy

The effect of Mn on the microstructure and tensile properties of Al0.5CoFeNiC0.1 HEA (high-entropy alloy) has been investigated using electron microscopy, atom probe and mechanical testing. The results show that addition of Mn refines dendrite and grain and reduces volume fraction of B2 phase and interdendritic regions. Mn is demonstrated to be not the former of κ′ phase. Therefore, the addition of Mn reduces the size and volume fraction of the κ′ phase, leading to a reduction in the yield strength of the HEA whereas an enhancement in ductility. 7.7 at.% Mn doping makes the HEA (Mn0.3) have a good balance between yield strength (1 GPa) and ductility (23% elongation). Adding Mn does not change deformation mechanism of the HEAs.

Effects of rolling deformation on microstructure orientations and tensile properties of TiB/(TA15-Si) composites

To explore the orientation evolution and interfacial characteristics under the β-phase region, rolling deformation with different reduction ratios of 40– 90% was carried out by hot-pressing sintering and multi-pass hot rolling on a series of TiB whiskers reinforced Ti-6.5Al-2Zr-1Mo-1 V-0.3Si (TiB/(TA15-Si)). As rolling deformation increased, the matrix exhibited slight refining and distortion within lamella α + β, and a stronger T-type α texture was gradually formed. A portion of Burgers Orientation Relation (BOR) between parent β and α phases gradually disappeared due to a decrease in rolling temperature and post deformation after phase transformation. Meanwhile, TiB rotated and presented an intenser orientation of {100} 〈010〉. Silicides heterogeneously precipitated along the α/β phase boundaries both near and away from TiB. Silicide/matrix remained fine interfaces owing to resolution and reprecipitation. Finally, the ultimate strengths at room temperature were monotonously improved with the increase of deformation due to strong work hardening. And an anisotropy of tensile properties was found under large deformation. The composite tensioned along the transverse direction (TD) can exhibit higher strength and lower ductility due to mutual competition of TD strengthening by T-type texture and RD strengthening by TiB distribution, especially at 600 °C. The fracture paths of as-rolled composites were discussed in detail. • Matrix gradually formed an intenser T-type α texture mainly inherited from β by BOR. • TiB can form an obvious orientation of {100}<010 > but have a minor proportion of OR. • Most silicide/matrix interfaces possessed orientation relations in the β-rolled composites. • The ultrahigh ultimate strength of 1505 MPa was obtained in the 90%-rolled composite. • Composites along TD exhibited higher strength and lower ductility especially at 600 °C.

Study on the evolution of phase relations in the Ti–Al–Nb/Cr systems at 0–50 at.% Al region

β-Solidifying γ-TiAl based alloys have gained scientific attention and showed potential applications as structure materials. When adding the β-stabilizing elements (e.g., Nb and Cr), ordered B2 and ωo phases are often formed during the production process of the TiAl based alloys. The information about the evolution of these phases as the temperature changes is necessary and important for design and preparation of desired TiAl based alloys with excellent properties. In this work, phase relationships and evolvement of Ti–Al–Nb/Cr systems have been investigated in the service temperature ranging from 950 to 700 °C. In the Ti–Al–Nb ternary system, B2 phase will directly transform into ωo phase at the temperature range of 913.6–921.1 °C, which results in an increase of the microhardness (i.e. 460 HV to 530 HV). At 700 °C, ωo phase has the composition of 15.8–20.4 at.% Nb, 29.8–33.3 at.% Al, and 46.3–54.4 at.% Ti. Equilibrium composition of ωo is defined as Ti3Al2Nb. The ternary compound O_Ti2AlNb has been found to be formed via β(Ti)+Ti3Al + Nb2Al→O at 981.5 ± 6.5 °C. As for the Ti–Al–Cr ternary system, when temperature decreases from 950 to 900 °C, the center section of the β(Ti) phase region shrinks and separates, resulting in the formation of independent B2 phase region. The island-like B2 region can remain stable at 700 °C. According to the evolution of phase relationship, one solid transition reaction Ti3Al + C14→B2+C15 at 900–800 °C has been deduced to occur in the Ti–Al–Cr ternary system. Present work provides an important guide for fabricating Ti–Al–Nb/Cr alloys with different ratio and distribution of plastic/brittle phases.

Temperature influenced microstructure evolution and strengthening of 4.5 vol% TiBw/TA15 composites with columnar-reinforced structure fabricated by pre-sintering and hot extrusion

In this paper, 4.5 vol% TiBw/TAl5 composite with columnar network reinforced structure was prepared by pre-sintering and subsequent hot extrusion process. Pre-sintering was conducted at 1100 °C for 0.5 h, and billets were extruded with an extrusion ratio of 10: 1. The extrusion speed was 10 mm/s, and water cooling was adopted. An extrusion temperature range of 925–1100 °C (which covers the phase transition) was used to study the influence of the extrusion temperature on the microstructure evolution and its correlation with strengthening. The results showed that the extrusion temperature played a decisive role in the evolution of the matrix morphology. When the extrusion temperature increased from the (α + β) dual-phase region to the β single-phase region, the microstructure of the composite matrix was mainly composed of elongated α phase and recrystallized α phase. It gradually transformed into a lamellar structure from a high-temperature β phase. TiB whiskers continued to grow and coarsen. The corresponding texture also changed from 〈10−10〉 α//ED component in the dual-phase region to 〈0001〉α//ED and 〈10−11〉 α//ED mixed components, which were related to the high-temperature 〈101〉β//ED component in the β-phase region. Further performance analysis showed that the matrix strength was positively correlated with the content of the lamellar α phase, which increased with the extrusion temperature; however, the strengthening of TiB whiskers was affected by its growth and fragility and displayed a weakening trend. As a result, although the strength of the composite was always better than that of the matrix, its overall strength decreased upon increasing the extrusion temperature, showing the best performance when extruded at 925 °C. The stress of strengthening effect could be accurately estimated by the shear lag model.

Composition-dependent interdiffusivity matrices of ordered bcc_B2 phase in ternary Ni–Al–Ru system at 1273∼1473 K

Abstract In this paper, 5 Ni–Al–Ru ternary and 2 NiAl/RuAl binary equilibrium alloys were prepared, and their lattice parameters were evaluated by using the X-ray diffraction technique. The relation between the lattice parameters and Ru content satisfies Vegard’s law, confirming that a continuous single bcc_B2 phase exists between NiAl and RuAl in the ternary Ni–Al–Ru system. After that, three diffusion couples located in the bcc_B2 phase region were prepared at 1273, 1373 and 1473 K, respectively. With the presently measured concentration profiles, the composition-dependent interdiffusivity matrices along the entire diffusion paths were efficiently determined by using the High-throughput Determination of Interdiffusion Coefficients (HitDIC) software. The obtained interdiffusivities show a good agreement with the limited experimental data in the literature. Furthermore, the present results show that the addition of Ru to bcc_B2 NiAl alloys can significantly inhibit the interdiffusion rate of Ni, indicating that the bcc_B2 Ru-containing NiAl bond coats can serve as a potential diffusion barrier for Ni-based superalloy substrate.

Precipitation Behavior of O Phase during Continuous Cooling of Ti-22Al-25Nb Alloy

The microstructure evolution and formation mechanism of the O phase in a Ti-22Al-25Nb (at.%) orthorhombic alloy resulting from different cooling rates were investigated. The results show that the morphology of the precipitated O phase is significantly affected by the cooling rate. As the cooling rate decreases, the floccular O, composed of many fine acicular O phases, gradually grows into the lamellar O phase. When the alloy is cooled from the B2 phase region, the grain boundary O (OGB) preferentially nucleates at the triple junctions and grain boundaries and forms the flat and the zig-zag OGB according to different cooling rates. The OGB consists of separated, flat OGB parts and unconnected, zig-zag OGB composed of multiple short, separated, flat OGB at a higher cooling rate. The zig-zag OGB presents a connected state due to the sufficient diffusion time at a lower cooling rate. When the alloy is cooled from the (B2 + α2) phase region, the increase of the phase boundary provides favorable conditions for the nucleation of the O phase due to the presence of α2 particles. The precipitated rim O phase appears on the periphery of the α2 particles at lower cooling rates. The analysis indicates that the Widmanstätten intragranular O (OWI) precipitated directly from the B2 phase maintains the plane relation with the parent B2 phase, and the Widmanstätten grain boundary O (OWGB) holds the specific orientation relationship with one of the two adjacent B2 grains. The OGB keeps the specific orientation relationship with one of the B2 grains as much as possible. When it cannot maintain the specific orientation relationship with one of the B2 grains, the OGB maintains a near-orientation relationship with B2 grains on both sides to reduce the nucleation activation energy. Moreover, there can be more than one nucleation site for the O phase on a single B2 grain boundary to form the OGB. The rim O phase formed through a decomposition reaction of α2→α2 (Nb-lean) + O (Nb-rich) is controlled by a diffusional mechanism and maintains a specific orientation relationship, i.e., {001} O//{0001} α2 and &lt;110&gt;O//&lt;112¯0&gt; α2, with the parent α2 particles.

Enhancing high temperature mechanical properties via modulating B2 phase with Al contents in FeCrNiAlx(x = 0.63,0.71,0.77) high entropy alloys

High entropy alloy has attracted extensive attention due to the increased requirement for future high temperature structural superalloys. Quaternary Al-Cr-Fe-Ni alloys have been developed in recent years and have the potential to be employed as superalloy materials. How to improve the mechanical properties with strength and ductility balancing at room temperature and further improve the high temperature performance is an urgent problem to be solved. Thus, phase regulations were achieved by designing quaternary Al-Cr-Fe-Ni high-entropy alloys with different Al content to achieve excellent room temperature and high temperature mechanical properties to lay the foundation for its application. Here, a Co-free FeCrNiAl0.77 alloy (Al0.77 alloy) with low density and low cost was proposed and successfully manufactured by arc-melting. With the variation of Al element, the alloys were identified to be eutectic-like spinodal decomposition microstructures with dual BCC phase, consisting of the ordered B2 dendritic regions (enriched in Ni and Al), and disordered BCC inter-dendritic regions (enriched in Fe and Cr elements). The addition of Al can effectively decrease the density and slightly reduce the hardness. The Al0.77 alloy has a minimum density with 6.89 g/cm3 and a high hardness above 470 kgf/mm2. The light-weight alloys exhibit promising comprehensive mechanical properties by virtue to balance the strength and ductility, and have a compressive strength of above 3000 MPa and a compressive strain more than 44%. Furthermore, the Al0.77 alloy still remains high yield strength of 219 MPa at 900 ℃ with the volume fraction of B2 phase of 50.92% via the modulation of B2 phase. The quantitative strengthening mechanisms of Al0.77 alloy were presented, and the competitive strength increments of about 1148 MPa could be mainly attributed to the ordered and coherent microstructures and the nanosized precipitates embedded in the B2 phase regions. The proposed Al0.77 alloy with well balancing of specific strength and strain shows a great potential in the application in aerospace fields.

On phase stability of Mo-Nb-Ta-W refractory high entropy alloys

Refractory high entropy alloys (RHEAs) emerge as promising candidate materials for ultrahigh-temperature applications. Accurate phase stability is the prerequisite for design of high-performance RHEAs, but still missing in the literature due to the big challenge in experiments. In this paper, the reliable 3rd-generation Gibbs energy expressions for pure Mo, Nb, Ta and W were firstly evaluated by integrating the physical based Segmented Regression model with thermal vacancy description from 0 K to high temperatures. The thermodynamic database of the Mo-Nb-Ta-W quaternary system was then developed by combining the established 3rd-generation Gibbs energies with the phase equilibrium and thermodynamic data by using the CALPHAD (CALculation of PHAse Diagram) approach. The calculated phase equilibria, thermodynamic properties as well as the A2/B2 ordering transition behaviors show good agreement with those from experimental determination and theoretical calculation. The phase constitutes and elemental distributions in an equiatomic MoNbTaW as-cast alloy were experimentally investigated and compared with the non-equilibrium solidification simulations to further verify the database reliability. The high-throughput mapping of phase stability within the composition-temperature space in various Mo-Nb-Ta-W alloys was then constructed by applying thermodynamic calculations. The alloying effects on the formation of B2 phase were systematically analyzed. The Mo-Nb-Ta-W alloys have a wide temperature region with a single A2 phase, while the A2 + B2 phase regions exist at very low temperatures. It is highly anticipated that the present accurate 3rd-generation thermodynamic database provides an important basis for efficient design and development of novel Mo-Nb-Ta-W RHEAs.

Influence of cooling rate on ω phase precipitation and deformation mechanism of a novel metastable β titanium alloy

This work investigated the effect of cooling rate (water quenching and air cooling) on the precipitation of ω phase after solution treatment in β-phase region, and its effect on the mechanical properties in a novel metastable β titanium alloy (Ti–5Mo–3Cr–Fe–3Zr). The initial microstructures, phase composition and deformation-induced microstructures have been investigated using SEM, EBSD, XRD, and TEM. The phase composition of water-quenched alloy and air-cooled alloy are β, α", and ω phase. The size and volume fraction of ω phase of air-cooled alloy are larger than that of water-quenched alloy, resulting in an increase in tensile strength and a decrease in ductility. Deformation mechanisms of Ti–5Mo–3Cr–Fe–3Zr alloy with different cooling rate change from stress-induced ω phase transformation and dislocation slip to only dislocation slip. The stress-induced ω lamellas parallel to [1-11]β direction along the [0001]ω1 direction, which is formed by {112}β<111>β slip. Dislocations can cut through the encountered ω phase to form ω-free deformation bands, which accounts for the ductility.

Effect of pre-strain temperature on the subsequent dynamic recrystallization of parent β phase and transformed α-variant selection in a novel near α titanium alloy

The microstructure and micro-orientation evolution of a novel high-temperature titanium alloy were characterized during a two-step deformation process. In the first step, the specimens were deformed at temperatures of 800, 850, 900 and 950 °C, and subsequently deformed in the same β-phase region of 1050 °C. The specimens pre-strained at 800 °C and 950 °C were fully recrystallized during the β-phase deformation process. By contrast, the specimens pre-strained at 850 and 900 °C were not fully recrystallized during the β-phase deformation process. This indicates that the extent of dynamic recrystallization did not increased monotonically with the pre-strain temperature. The extent of dynamic recrystallization was related to the stored energy accumulated in the α and β-phase during the pre-strain deformation. The change in stored energy was based on the difference in microstructure morphology produced by different pre-deformation temperatures,and stored energy was quantitatively estimated based on the local misorientation distribution. Meanwhile, the generation of dynamically recrystallized grains was beneficial to decreasing the generation possibility of the macro-zone. This decrease was attributed to the refinement of β-grains and randomly distributed orientation, which was also related to the decrease in the typically distributed “two parallel wings” orientation between the α and β phase induced by uncrystallized deformation. Additionally, the two-step deformation process with the pre-strain at the (α + β) middle regime was an effective strategy to promote the dynamic recrystallization and decrease the generation probability of the macro-zone.