Β-phase Field Research Articles

Texture of Hot-Compressed Metastable β-Titanium Alloy Ti5321 Studied by Neutron Diffraction.

The textures of the β- and α-phases of the metastable β-titanium alloy Ti5321 after hot deformation were investigated by neutron diffraction. A hot-rolled bar was solutionized in the β-phase field and then hot compressed above and below the β-transus temperature. The initial texture after full recrystallization and grain growth in the β-phase field exhibits a weak cube component {001}<100> and minor {112}<110> and {111}<110> components. After hot compression, a <100> fiber texture is observed, increasing in intensity with compression temperature. Below the β-transus temperature, dynamic recrystallization of the β-phase and dynamic spheroidization of the α-phase interact strongly. The texture of the α-phase is a <11-20> fiber texture, increasing in intensity with decreasing compression temperature. The mechanisms of texture formation during hot compression are discussed.

Microstructure evolution and enhanced mechanical properties of as-rolled TiB/(TA15-Si) composite via heat treatment

To improve the mechanical properties of the as-rolled composite under large rolling deformation, the effect of solid solution and aging on the microstructural characteristics and mechanical properties of the as-rolled TiB/(TA15-Si) composite was detailedly investigated. The solid solution temperatures of 1050 °C in the β-phase field and different aging temperatures of 550–750 °C were chosen to regulate the evolution of matrix and silicides. After solid solution treatment, the matrix transformed from elongated and kinkled lamella into acicular martensite microstructure. And the silicides completely redissolved into matrix and a uniform diffusion of Si atom was presented. Meanwhile, the aging temperature can obviously affect the evolution of martensite and silicides. As aging temperature increased, the martensite gradually decomposed into acicular α+β lamella (at 650–700 °C) and abnormal growth of α phase appeared (at 750 °C), which dominated the decreasing trend of microhardness and high-temperature strength. Besides, the silicides gradually precipitated along the prior β phase and TiB whiskers (at 600 °C) as well as α/β phase boundaries (at 650 °C). Eventually, 600 °C and 700 °C strengths of the as-rolled composite can be obviously improved by 39.7 % and 84.6 % after solid solution and aging processes. And the composite aged at 600 °C can obtain the highest 600 °C strength of 992 MPa, which is mainly attributed to martensite, precipitation of silicides and weak α2 phase.

Hot deformation behavior of a Fe3Al–Ta alloy in the B2-order regime

In the present work, the hot deformation behavior and the corresponding microstructure evolution of an Fe–25Al-1.5Ta (at.%) alloy in the B2-phase field were investigated. Uniaxial compression tests were carried out in a strain rate range from 0.0013 s−1 to 1 s−1 and in a temperature range from 800 °C to 850 °C, where an ordered B2–FeAl matrix phase along with a C14 - (Fe, Al)2Ta Laves phase was confirmed by X-ray diffraction. A dynamic material model was applied to predict the safe and damaging processing windows. The underlying flow softening mechanisms were characterized using scanning electron microscopy and electron-backscatter diffraction. The flow stress-strain curves mostly showed a broad maximum followed by a slight decrease in stress until a steady stress was reached. The optimum processing window for the studied alloy was located at 850 °C/0.0013 s−1, where the efficiency of power dissipation (η) and strain rate sensitivity (m) reached 50% and 0.25, respectively. The processing map also showed a domain of flow instability, resulting from cracking, in the range of lower temperatures and higher strain rates (800 °C/1 s−1). The microstructural analyses confirmed a combination of dynamic recovery (DRV) and dynamic recrystallization (DRX) over the entire range of deformation conditions tested. The current study reveals a well-suited parameter range to achieve a high degree of hot deformability in Fe–Al alloys at significantly lower temperatures than those typically used. This may contribute to optimizing the thermomechanical processing of Fe–Al alloys and reducing energy consumption in industrial forging operations.

Exceptional strength-plasticity synergy in β-Ti alloy via HPT and short-period annealing

A new metastable β-type Ti-3.5Al-5Mo-4 V (Ti-B20) alloy with a high strength has been recently developed. However, the plasticity of the alloy after solution or aging treatment is poor due to its large initial β grain size, and conventional deformation-induced grain refinement is quite limited. In the present work, a high-pressure torsion (HPT) method was used to manufacture a Ti-B20 alloy with large deformed grains. The Ti-B20 alloy subjected to HPT was annealed in a β-phase field (850 ℃) for different periods. The deformation mechanism during HPT processing and the recrystallization behavior of the deformed alloy were systematically revealed. The results show that the deformation mode during HPT processing is mainly dislocation slip and shear band splitting of the original grains and martensitic phase (α″). During recrystallization annealing, the shear bands and high-density dislocations generated by the HPT deformation of the Ti-B20 alloy significantly promote nucleation and growth of recrystallized grains, which shortens the time to complete recrystallization. After recrystallization annealing, exceptional strength-ductility synergy with a high tensile strength of 941.9 MPa and a high plasticity of 39.7% is achieved in the fine-grained (14.68 µm) alloy, which is the highest elongation reported in the Ti-B20 alloy at present. The fine grains with a low dislocation density provide more space for dislocation slip and accumulation while maintaining high strength. This work provides a new possibility for preparing fine-grained β-titanium alloys with synergistic strength and plasticity.

Grain-boundary migration induced abnormally increased flow stress under large strain in a high-temperature compressed Ti-22Al-25Nb alloy

• Impeded grain boundary (GB) slip results in visibly increased flow stress. • The GB slip mechanism was impeded by zigzag GBs. • The zigzag GB result from the GB migration. • Energy difference between <001> and <111> fiber textures leads GB migration. • DDRX is a concomitant phenomenon with increased flow stress. The flow stress of Ti-22Al-25Nb alloy abnormally increased during uniaxial compression at large strains in the B2 phase field. This phenomenon is dependent on the influence of oriented grain boundary (GB) migration on GB sliding. In the early stages of deformation, straight and smooth GBs gradually evolved facilitating GB sliding, and the <001>/<111> double fiber texture has formed. There is a stored energy difference between <001> and <111> grains due to their different Taylor factors. Under low strain rates, the energy difference between the two fibers will drive GB to migrate, making the GBs zigzag and impeding GB sliding. Thus, greater stress is required to achieve further deformation, namely, the flow stress increased. The dislocation density in the shear band near the GB rapidly increased. Although the current alloy has a high stacking fault energy (SFE), the high dislocation density is sufficient to cause discontinuous dynamic recrystallization (DDRX).

Microstructure evolution and mechanical properties of a high-strength Ti2AlNb alloy processed by large-strain two-pass hot extrusion

The microstructure evolution and tensile behavior of Ti2AlNb alloy (Ti-22Al-23Nb-1Mo-1Zr) processed by two-pass hot extrusion and two-step heat treatment were investigated. After extruding deformation in the B2 phase field, the B2 grains were refined from about 1500–50 µm through dynamic recrystallization. The microstructures subjected to solution treatment in the B2 + α2 phase field and aging treatment in the B2 + O phase field include B2 matrix, lamellar O, rim O and lamellarα2/O phases. The microstructure and tensile properties are sensitive to aging temperature. As the increase of the aging temperature, the content of the B2 phase and the size of the lamellar O phase increase, resulting in low strength and high ductility. The alloy aged at 800 °C has a good match of strength and ductility. It has an elongation of 6.3% with a tensile strength as high as 1225 MPa due to the precipitation strengthening of numerous lamellar O phase and fine grain strengthening. The tensile fracture mode predominantly shows quasi-cleavage characteristics with typical cleavage facets and massive dimples, which is consistent with the good balance of tensile strength and ductility. An in-depth understanding of the relationship between microstructure and tensile properties in this study can be helpful for the engineering application of the Ti2AlNb alloy.

Omega phase transformation – morphologies and mechanisms

Abstract Various morphologies of the ω phase are observed in the α and the β phases of group-IV elements and their alloys where formation of this phase is induced by various means, namely, quenching from the β-phase field, isothermal annealing, under high static as well as dynamic pressure, and irradiation. The presence of various morphologies, such as ellipsoidal, cuboidal, granular or plate shapes, under different conditions is demonstrated. It is shown that various factors, including shuffling of atoms, shearing of atomic planes, long-range diffusion of solute atoms, change in volume during transformation, etc., influence the formation of different morphologies of the ω phase. The underlying mechanisms in each case are discussed. Ordered ω phases, which exist as equilibrium phases in various alloy systems, are shown as a case of superimposition of the concentration wave on the displacement wave.

Hot pack rolling of spark-plasma-sintered pre-alloyed powders to fabricate Ti2AlNb sheets

In this work, spark-plasma-sintered Ti-20.3Al-24.7Nb (at.-%) compact was pack rolled in different phase fields to fabricate Ti2AlNb sheets with high strength and ductility without post-heat treatment. The nano O phases in the B2 matrix contributed to the high tensile strength and ductility in a single B2 phase field. In the O + B2 phase field, the O phases remarkably increased in size and resulted in high tensile strength of 1407.1 MPa. Maintaining the O phases to nano sizes and cooperation with micro-sized α 2 phases resulted in extremely high elongation of 13.91% in the α 2+O + B2 phase field. The , , , and strengthening effects of the α 2 and O phases under different conditions were analysed in detail. Highlights Spark-plasma-sintered pre-alloyed powders are used as the raw compact before hot pack rolled Ti2AlNb sheet. Ti2AlNb sheets with good mechanical properties are obtained without any post-heat treatment. This ‘SPS pre-alloyed powder + hot pack rolling’ method has great potential in breaking the limit in sheet properties. The strengthening effects of different factors are analysed.

Effect of temperature and strain rate on the deformation behavior of Ti5321 during hot-compression

The effect of deformation temperature and strain rate (collectively described by the Zener-Hollomon parameter Z) on the deformation mechanism and texture formation of the metastable β-titanium alloy Ti5321 across the β-transus temperature during hot-compression was investigated by electron backscatter diffraction. In the β-phase field, it is found that the deformation behavior and texture formation varies depending on Z. With decreasing Z dynamic recovery and dynamic recrystallization become more and more important. The activation energy for steady state deformation is 240 kJ/mol and 370 kJ/mol in the β- and (α + β)-phase field, respectively. The texture developed is a<100> <111> double-fiber with<100>dominating at all deformation conditions. The<111> fiber gets more prominent with increasing Z suggesting that it is mainly related to deformation. Flow softening behavior of Ti5321 is associated with dynamic globularization of the α-phase and promotion of β-grain formation by continuous dynamic recrystallization.

Effects of Cr alloying on the evolution of solidification microstructure and phase transformations of high-Nb containing γ-TiAl based alloys

Evolution of the as-solidified microstructures of Ti–45Al–8Nb, Ti–45Al–8Nb–2Cr-0.2B, Ti–45Al–8Nb–4Cr-0.2B alloys, and phase stability during high-temperature exposure (1000°, 1100°,1200 °C) have been analyzed in the present report. Macroscopic examinations of the as-cast pancakes indicate that Nb, Cr, and B alloying can be effectively utilized as potential grain/colony size refiners for Ti–45Al base TiAl alloys. However, the as-cast microstructure varies considerably from a nearly fully lamellar (α2+γ) structure for 0 at.% Cr to a dominant coarse-γ laths+β(B2) structure for 4 at.% Cr addition to the baseline Ti–45Al–8Nb alloy. Examination of characteristics of prior-β dendritic morphology and associated crystallographic considerations and surface relief effects establish that all the alloys studied here solidify through the β-phase field. No peritectic α formation could be established. Formation of γ within β phase and lamellar microstructures are attributed to the reactions β→β+γ, and α→α2+γ, respectively in the as-cast condition. It has been established that strong Cr partitioning in the β-phase significantly slows down the kinetics of β-phase decomposition into γ, and σ. This causes departure from the evolution of phases as predicted by thermodynamic calculations. The solidification microstructure resulting from the solid-state phase transformations are further discussed in terms of the micro-segregation and crystallographic characteristics of the parent and the product phases.

The effect of thermomechanical treatments on the properties of powder metallurgy Ti–5Fe alloy

In this study, the low-cost Ti–5Fe alloy was fabricated by a combination of sintering plus thermomechanical treatment (TMT). The primary aim is to improve the tensile properties of the recently widely studied powder metallurgy Ti-xFe alloys made by the press and sinter process. The effect of hot forging or extrusion and subsequent annealing treatment on the microstructure and mechanical properties of the Ti–5Fe alloy was therefore studied. TMT from temperatures in the β-phase field severely changes the microstructure of the sintered alloy. The substantial deformation of the β-grains and the formation of fine α-phase have a significant effect on the mechanical properties. The yield and ultimate tensile strengths of the TMT alloys reach high values, from 1050 MPa to over 1200 MPa. The subsequent heat treatment results in the modification of the α-phase morphology and the partial loss of the crystallographic texture, leading to the enhancement of the elongation to values above 20%, despite of the high oxygen content of the alloy.

Hot deformation behavior and flow stress modeling of Ti–6Al–4V alloy produced via electron beam melting additive manufacturing technology in single β-phase field

The hot working behaviour of additively manufactured Ti–6Al–4V pre-forms by Electron Beam Melting (EBM) has been studied at temperatures of 1000–1200 °C and strain rates of 0.001–1 s−1. As a reference, a wrought Ti–6Al–4V alloy was also analyzed as same as the EBM one. In order to investigate the hot working behaviour of these samples, all the data evaluations were carried out step by step, and the stepwise procedure was discussed. No localized strain as a consequence of shear band formation was found in the samples after the hot compression. The flow stress curves of all the samples showed peak stress at low strains, followed by a regime of flow softening with a near-steady-state flow at large strains. Interestingly, it is found that the initial microstructure and porosity content as well as the chemistry of material (e.g. oxygen content) as being possible contributors to the lower level of flow stress that could be beneficial from the industrial point of view. The flow softening mechanism(s) were discussed in detail using the microstructure of the specimens before and after the hot deformation. Dynamic Recrystalization (DRX) could also explain the gentle oscillation in the appearance of the flow softening curves of the EBM samples. Moreover, the hot working analysis indicated that the activation energy for hot deformation of as-built EBM Ti–6Al–4V alloy was calculated as ~193.25 kJ/mol, which was much lower than the wrought alloy (229.34 kJ/mol). These findings can shed lights on a new integration of metal Additive Manufacturing (AM) and thermomechanical processing. It is very interesting to highlight that through this new integration, it would be possible to reduce the forging steps and save more energy and materials with respect to the conventional routes.

Hot Deformation Behavior and Processing Maps of Zr92Ti8 Alloy

The hot deformation behavior of Zr92Ti8 alloy in the β-phase field was investigated by compression tests at deformation temperatures from 800 to 1050 °C and strain rates from 10−3 to 100 s−1. The true stress–strain curves exhibited a flow softening phenomenon caused by adiabatic heating and dynamic recrystallization (DRX). A hyperbolic-sine Arrhenius-type equation was used to describe the effect of deformation temperature and strain rate on the flow stress. The activation energy for deformation at different true strains was determined to be in the range of 195.0 to 252.2 kJ/mol, and the flow curves were predicted by the strain-modified Arrhenius constitutive equation. The processing maps exhibited two optimal processing domains: one in the temperature range of 825 to 875 °C and the strain rate range of 10−3 to 3.3 × 10−3 s−1 and the other in the temperature range of 1022 to 1050 °C and the strain rate of 10−3 to 3.3 × 10−3 s−1. The observation of microstructure reveled that DRX was difficult to occur during hot deformation of the Zr92Ti8 alloy.

Flow modelling of Ti6Al4V under large strains

This work uses flow stresses obtained experimentally at different strain rates and temperatures to validate flow modelling results. Flow curves of Ti6Al4V are measured via torsion experiments with a Gleeble® 3800 up to effective strains of 8. A physically based model that describes the evolutions of microstructure and the flow stress in the β-phase field was developed. A model of continuous dynamic recrystallization (CDRX) based on the work of Gourdet and Montheillet [1] for aluminium alloys is combined in this work with elements taken from Kocks and Mecking [2]. The model consists of a detailed description of the microstructure, based on different dislocation density populations and grain boundaries. All these internal variables evolve according to a production and a recovery term correlated mathematically with the temperature and the strain rate. The modelled output variables besides the flow stress are the total, the interior and the wall dislocation densities as well as the subgrain and grain sizes developed by continuous dynamic recrystallization. The model describes the softening occurring during large strain deformations, which is partly produced by the formation of new high angle grain boundaries (HAGB). The fraction of HAGB was used to determine the recrystallization grade, validated with microstructural characterization.

Phase transformation and evolution of dislocation structure in the β phase of Ti-17 alloy during hot deformation

The hot deformation behavior and microstructural evolution of Ti–5Al–2Sn–2Zr–4Mo–4Cr (Ti-17) alloy in the β-phase field were examined. Hot compression tests were conducted at temperatures ranging from 700 and 1100 °C and strain rates ranging from 10-3 to 1 s-1. The obtained true stress–true strain curves exhibited steady-state flow behavior at temperatures above the β-transus temperature (Tβ ~ 890 °C). The microstructural observations suggested that the dynamic recovery process represents a dominant mechanism for the β-phase of the alloy. In contrast, the flow softening was identified at 700 °C at a strain rate of 10–3 s–1. The formation of fine acicular α microstructures was noticed in such a deformation condition, indicating that dynamic phase transformation can occur in the metastable β-phase during hot deformation. Such a dynamic phase transformation also occurred preferentially at the initial β-grain boundaries as well as sub-boundaries that created within the βmatrix via dynamic recovery. The β-processing conditions were also examined by constructing processing maps based on the dynamic materials model (DMM).

Diffusional transformation in Ti6Al4V alloy during isothermal compression

Abstract Thermodynamic calculation of the two-phase Ti alloy was completed using CompuTherm Pandat™ and Ti data base, followed by isothermal compression of Ti6Al4V (Grade 5), with an initial colony lamellar structure that was performed in the (α+β) and β-phase field. Microstructural evolution and phase transformation were investigated using X-ray diffraction, scanning and transmission electron microscopy. The presence of the Ti3Al or α2 (hcp), the phase stability and transition temperatures were predicted by the Gibbs free energy−temperature and phase fraction−temperature diagrams. The isothermal compression in the (α+β)-phase field is characterized by reorientation and localized kinking of α/β lamellae, and cracking at α/β interphase regions. While in the α→β-phase transformation area, deformation in β-phase and at α/β interphase boundaries, extensive transformation of α into β-phase, martensitic transformation and spheroidization of α-laths mainly characterize this isothermal compression. A complete transformation of α into β single phase occurs in the β-phase field. Ti3Al or α2 (hcp), β (bcc) and α (hcp)-phase, and additional hcp α' and orthorhombic α” phases in a deformed Ti6Al4V are revealed. The flow stress level, the dynamic recovery and dynamic globularization are affected by deformation temperature.

Effect of Co on the phase stability of CrMnFeCoxNi high entropy alloys following long-duration exposures at intermediate temperatures

The effect of Co on the phase stability of the CrMnFeCoxNi family of alloys, where the atomic ratio x = 0, 0.5, 1.5, has been experimentally established following 1000 h heat treatments at 900 and 700 °C and up to 5000 h at 500 °C. All the alloys were single phase fcc in the homogenised condition, except for CrMnFeNi which also contained bcc precipitates that remained present following exposures at 900 °C and 700 °C. The exposures at 900 °C and 700 °C also resulted in the formation of σ phase precipitates in the CrMnFeNi and CrMnFeCo0.5Ni alloys but not in the CrMnFeCo1.5Ni alloy. These data, in conjunction with results previously published in the literature, conclusively establish that Co stabilises the fcc matrix at elevated temperatures. However, at 500 °C, further bulk decomposition in the CrMnFeNi alloy was observed, consisting of a fine-scale intergrowth of a NiMn L10 phase and CrFe σ phase. Grain boundary precipitates were also observed following exposure at 500 °C in the CrMnFeCo0.5Ni and CrMnFeCo1.5Ni alloys. Four different phases were observed on the grain boundaries of the CrMnFeCo0.5Ni alloy (Cr carbide, σ, FeCo B2 and NiMn L10), whilst only two phases were found on the grain boundaries of the CrMnFeCo1.5Ni alloy (Cr carbide and NiMn L10). The experimental observations facilitated an assessment of the fidelity of current thermodynamic predictions of phase equilibria. All the phases predicted were observed experimentally and the σ stability fitted the experimental observations well. However, at lower temperatures, thermodynamic predictions were less consistent with experimental observations, underpredicting the extent of the B2 phase field and failing to predict the formation of the L10 phase.

Mechanical behaviour of conventional, Pt- and Pt/Ir-modified NiAl diffusion coatings after thermocyclic exposure at 1100°C

ABSTRACTThe influence of Pt- and Pt/Ir-modifications on the mechanical properties of NiAl coatings after thermocyclic exposure at 1100°C was investigated. Four-point-bending tests with in-situ acoustic emission measurement were used to determine the fracture strain. After 100 h of exposure a steep increase in fracture strain was observed for the NiAl coating. In contrast, the increase was found to be moderate for modified coatings. For all investigated coatings the fracture strain was correlated with the mean Al-concentration in the diffusion zone, and a moderate increase in the β-phase field with decreasing Al-concentration was found. In the two-phase (β/γ') stability region fracture strain increased steeply. Indentation hardness instead showed a steep decrease within the single β-phase field, but remained almost constant in the two-phase region. Reduced elastic modulus followed a V-curve behaviour, in which a decrease with ongoing Al-depletion in the β-stability region was followed by an increase in the two-phase region.

Influence of heat treatment conditions on the mechanical properties of Ti–6Al–4V alloy

ABSTRACTIn the current work, several heat treatments were carried out below and above the beta-transition temperature of the Ti–6Al–4V alloy followed by aging at 550 °C for 6 hours. The resultant microstructures and their effects on the mechanical properties of Ti–6Al–4V alloy were investigated. The results showed that solution treatment of Ti–6Al–4V samples followed by water quenching from β and α/β fields raised the alloy hardness from 380 to 575 and 656 HV, respectively, while no remarkable changes were observed after aging. The hot tensile strength of the as-forged sample increased from 671 to 756 MPa after water quenching from the ß- or α/ß- field, while the air cooling from β-phase field decreased the tensile strength to 644 MPa. The fracture mode of the tensile samples was more ductile in case of the solution-treated samples compared to the as-forged samples. A subsurface layer was formed due to the diffusion of oxygen into the surface at high temperatures. This layer which is known as ‘oxygen diffusion layer’ masked the differences of wear behaviour of the specimens.

On the Hydrogen Embrittlement of Commercially Pure Alpha Titanium: An Example from the Petrochemical Industry

Grade 2 of commercially pure Ti consisting of α-phase has many applications in the petrochemical industry such as floaters of gauges used to indicate liquid levels in tanks and reaction vessels. A floater fabricated by welding of 3.5-mm-thick sheet of grade 2 Ti into a thick-walled cylinder to indicate the level of a liquid mixture of isobutane, neobutane and neopentane in a petrochemical plant has lost its structural integrity by puncturing, cracking and blistering particularly at the section in contact with the liquid. The damage has been most severe in the base metal adjacent to the weld. Detailed microstructural characterization of the damaged floater and unwelded section of the same material has been carried out using scanning electron microscopy combined with energy dispersive spectroscopy, x-ray diffraction and transmission electron microscopy, and the results have been complemented by stress analysis and microhardness measurements. It is shown that the mechanical strength of the floater has been degraded by a combination of excessive absorption of hydrogen during welding and rapid cooling from the β-phase field aided by the stresses generated by the liquid pressure. Absorption of hydrogen and rapid cooling are found to alter the desirable morphology of equiaxed grains of α-phase into a multi-phase structure with fine platelet-type morphology. The base metal adjacent to the weld is found to contain the brittle δ-phase of titanium hydride in a low-ductility matrix of α-Ti with some β-Ti. However, β-Ti is found to be the predominant constituent of the weld.