Α-phase Precipitation Research Articles

Influence of the process parameters on the microstructure and the machinability of additively manufactured Ti-5553 titanium alloy

Additive Manufacturing (AM) technologies, particularly laser powder bed fusion (LPBF), are revolutionising the production of complex geometries and lightweight structures. Furthermore, LPBF allows to tailor the microstructure and resulting properties of metallic materials. This study focuses on titanium alloys, crucial for high-performance applications like aircraft components and medical implants. Although AM enables near-net-shape fabrication, many titanium parts still require machining to meet surface and dimensional standards. Titanium’s challenging machinability is well-documented for cast and wrought alloys, but only less is known about additively manufactured variants. In this work, the machinability of an additively manufactured Ti-5Al-5V-5Mo-3Cr alloy (Ti-5553) is investigated, focusing on chip formation, cutting forces, and tool wear across different LPBF process parameters. Four LPBF parameter sets were validated, and results were compared to conventional wrought sample. The findings reveal significant variations in machinability linked to LPBF parameters. Specifically, the highest tool loads and wear were observed for samples produced with the highest energy density of EV = 37.0 J/mm3, likely due to α-phase precipitation. In contrast, samples with lower energy densities (<29.1 J/mm3) exhibited up to 100% longer tool life. Concluding, this study highlights how the machinability of Ti-based components can be significantly influenced by the LPBF processing parameters.

Manipulation of the α-phase precipitation behavior through dual-aging treatment and its correlation with mechanical properties in a metastable β titanium alloy

The precipitation behavior of α-phase and its crucial impact on the mechanical properties have been investigated in a metastable β-type Ti-4Al-6V-5Mo-3Cr-1Zr (wt%, Ti-46531) alloy. Through dedicated design of dual-step aging conditions, the nucleation mechanism, number density and morphology of the secondary α-phase precipitates can be modulated. After the first-step aging treatment at 300 °C for 5−100 h, isothermal ω-phase (ωiso) particles precipitate and are uniformly dispersed in the β-phase matrix, with their number density gradually increasing with aging time. The subsequent second-step aging treatment at 600 °C for 2 h leads to the rapid dissolution of ωiso and the precipitation of α-phase through the classical nucleation process with a steady number density. Lowering the second-step aging temperature to 520 °C promotes the heterogeneous nucleation of the α-phase with the assistance of uniformly dispersed ωiso particles. This leads to a significant increase in the number density of α-phase precipitates. Consequently, the maximum yield strength increases from 1296 MPa for samples aged at 600 °C to 1623 MPa for samples aged at 520 °C. The precipitation of α-phase serves a dual role in enhancing the yield strength, both by providing precipitation strengthening and by increasing dislocation density. The contribution to yield strength enhancement from the former effect is considerably more substantial than the latter. This work underscores the crucial role of α-phase number density in determining the strength of metastable β titanium alloys, offering new insights into the tailoring of microstructures and mechanical properties through dual-aging treatments.

Effects of Ni addition on abnormal grain growth in superelastic CuAlMnCoNi alloy

Large-size grain is one of the effective parameters to determine the superelasticity of Cu-based alloys. In this study, the effects of Ni addition on abnormal grain growth (AGG) and microstructure evolution of CuAlMnCoNi alloys are systematically investigated. The activation energy and driving force of grain boundary (GB) migration during AGG are evaluated. The results reveal that Ni significantly promotes AGG. Average grain size reaches 6.0 cm after three cyclic heat treatments (CHTs) in a CuAlMnCoNi alloy containing 2 at.% Ni. A maximum superelastic strain of 11.8 % can be obtained at 15 % applied strain. The addition of 2 at.% Ni increases the solvus temperature of the α-phase to about 818 °C and raises α-phase volume fraction at phase equilibrium. The precipitation of α-phase with high volume fraction causes high number density of dislocations. The misorientation angle of subgrain is increased by the addition of Ni, contributing to the AGG in CuAlMnCoNi alloys. The addition of Ni reduces the activation energy of GB migration and increases the GB migration driving force.

Relationship between β→α dynamic transformation and dynamic recrystallization under thermomechanical coupling in Ti-5Al-5Mo-5V-1Cr-1Fe alloy

The thermal simulation compression tests of near-β titanium alloy Ti-5Al-5Mo-5V-1Cr-1Fe were performed within the range of deformation temperatures of 710–860 °C and strain rates of 0.001–1 s–1. Based on electron backscatter diffraction (EBSD) characterization and analysis technology, the interaction between dynamic phase transformation (DPT) of β-to-α and dynamic recrystallization (DRX) under thermal-mechanical coupling is deeply and systematically explored. We clarify the effects of temperatures and strain rates on the orientation relationship during dynamic precipitation of α-phase within β grain interiors possessing special orientation. The results show that the intragranular α-phase precipitated on the subgrain boundaries near {001}β with a high degree of dynamic recovery (DRV) deviates more from Burgers Orientation Relationship (BOR) than the α-phase that precipitates near {111}β as temperature increases. The proportion of α-phase precipitated by strain-induced on β recrystallized equiaxed grains for the deviating angle from BOR (θBOR) in the range of 20°–30° increases to 40% with the increase of strain rates below 800 °C. In addition, the α-phase is dynamically precipitated on the grain boundaries with {110}β orientation, which undergoes continuous dynamic recrystallization (CDRX), exhibiting epitaxial recrystallization, namely {110}β//{0001}α. Furthermore, the morphology of grain boundaries α phase (αGBs) precipitated by strain-induced phase transformation (SIPT) on specific types of β grain boundaries (βGBs), as well as the crystallographic orientation relationship and variant selection effect between adjacent α-variant within β grains are elucidated. The orientation relationships between α variants in {111}β grain are related with each other by 50°–60°/<–12–10> and 60°–70°/<–48–43> rotation. The “necklace” αGBs of recrystallization exhibit mainly the rotation of 50°–70° around the 〈2–310〉 zone axis, while the adjacent α-variant in the grain interiors is mainly 60° or 90°/<12−30>. In summary, the study has contributed to a deeper understanding of the deformation behavior and DPT laws of β-to-αp in near-β titanium alloy, which lays a foundation for the optimization of the hot deformation process and mechanical properties.

The effect of preliminary treatment with subsequent aging on structural-phase state and mechanical properties of β titanium alloy

The effect of aging at 723 K on the structural-phase state and mechanical properties of VT35 titanium alloy after β quenching and various treatments using severe plastic deformation has been studied. It is shown that annealing at 1073 K followed by quenching and aging leading to the precipitation of large α-phase lamellae with volume fraction of 20 % doesn’t have a strengthening effect on the alloy. After processing by radial shear rolling and subsequent aging fine α-phase precipitations in the form of plates and nanometer-sized particles with a volume fraction of 24 % are observed in β-phase grains. However, large areas of β-phase without α-phase precipitates are observed. The strength properties of the alloy after the treatment increase by 40 – 45 %. The processing by the method of multiple pressing leads to the formation of a mixed α / β UFG structure with a grain / subgrain size of 0.11 µm. Subsequent aging leads to the decomposition of the residual β-phase and to a homogeneous distribution of α and β phases in the alloy with volume fraction of α-phase 51 %. The formation of such an UFG structural-phase state leads to an increase in the strength properties of the alloy by a factor of almost two compared to the initial state.

Damping Properties of Selective Laser-Melted Medium Manganese Mn-xCu Alloy.

In this work, selective laser melting (SLM) technology was applied to directly realize the in situ synthesis of medium manganese Mn-xCu (x = 30-40 wt.%) alloys based on the blended elemental powders. The effects of heat treatment on the microstructural evolution and damping properties of the SLMed Mn-xCu alloys were investigated. The metastable miscibility gap was studied by thermodynamic modeling and microhardness measurement. The results showed that γ-(Mn, Cu) phase with dendritic arm spacing (DAS) of 0.9-1.2 μm was the main constituent phase in the as-SLMed alloys, which was one to two orders of magnitude finer than those of the as-cast samples. Aging at 400-480°C for the Mn-30%Cu or 430°C for Mn-40%Cu alloys can induce spinodal decomposition, martensitic transformation, and α-phase precipitation, whose direct evidence was provided for the first time by transmission electron microscopy and 3D atom probe tomography in the work. The miscibility gap obtained from thermodynamics calculation was basically consistent with the microhardness results for the SLMed Mn-xCu alloys. Solution and aging (SA) treatment can improve the microstructure, tensile and damping properties of the SLMed Mn-xCu alloys more obviously than aging treatment. A 2.3-2.8 and 4.3-4.5 times increase was produced in damping capacity in the aged SLMed and SLMed+SAed Mn-xCu samples, respectively.

The Effect of ω- and α-Phase Precipitation on the β-Phase Lattice Parameters During 400°C aging in Ti-11Cr(at.%)

The Effect of ω- and α-Phase Precipitation on the β-Phase Lattice Parameters During 400°C aging in Ti-11Cr(at.%)

Grain boundary α-phase precipitation and coarsening: Comparing laser powder bed fusion with as-cast Ti-6Al-4V

Grain boundary α-phase (GB-α) precipitation and coarsening behavior in Ti-6Al-4V (Ti-64) processed by laser powder bed fusion (LPBF) were investigated and compared with those in as-cast Ti-64. GB-α in LPBF processed Ti-64 tended to precipitate at the triple junctions (TJs) of the β-phase grain boundaries during the subsequent annealing process. Kinetic analysis on GB-α showed that the coarsening mechanisms varied in LPBF processed Ti-64 at different annealing temperatures (bulk diffusion at 750 °C and interface reaction at both 850 and 950 °C), while were consistent in as-cast Ti-64 (interface reaction at all the three temperatures). The inconsistent coarsening mechanisms were attributed to the GB-α curvature difference caused by the different α lamellae nucleation mechanisms. These findings could help rationalize the GB-α morphology evolution and benefit the further mechanical property manipulation in LPBF processed titanium alloys.

Oxygen Induced Phase Transformation in TC21 Alloy with a Lamellar Microstructure

The main objective of the present study was to understand the oxygen ingress in titanium alloys at high temperatures. Investigations reveal that the oxygen diffusion layer (ODL) caused by oxygen ingress significantly affects the mechanical properties of titanium alloys. In the present study, the high-temperature oxygen ingress behavior of TC21 alloy with a lamellar microstructure was investigated. Microstructural characterizations were analyzed through optical microscopy (OM), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). Obtained results demonstrate that oxygen-induced phase transformation not only enhances the precipitation of secondary α-phase (αs) and forms more primary α phase (αp), but also promotes the recrystallization of the ODL. It was found that as the temperature of oxygen uptake increases, the thickness of the ODL initially increases and then decreases. The maximum depth of the ODL was obtained for the oxygen uptake temperature of 960 °C. In addition, a gradient microstructure (αp + β + βtrans)/(αp + βtrans)/(αp + β) was observed in the experiment. Meanwhile, it was also found that the hardness and dislocation density in the ODL is higher than that that of the matrix.

Effect of Strain Rate on the Microstructure and β-Texture Evolutions in β-Processed Forging of a Near-β Titanium Alloy

The effect of strain rate on the β texture evolution during two-step hot forging of Ti-6246 alloy was investigated. The two-step forging consisted of 15% or 50% prior-β forging at 980°C and subsequent 60% or 25% forging at 870°C in the (α + β) dual-phase region. The total compression ratio was 75%, and the investigated strain rates were 0.01 and 1.0 s−1. The β forging texture showed typical {001} and {111} body-centered cubic textures. With increasing compression ratio in the (α + β) region and at a strain rate of 0.01 s−1, the amount of precipitated α phase increased. Dynamic recrystallization was rarely observed after forging in the (α + β) region at a strain rate of 0.01 s−1. Large amounts of α precipitates lowered the {001} β texture intensity through slip transmission between the α and β phases under the Burgers orientation relationship. However, in specimens forged at a strain rate of 1.0 s−1, as the compression ratio in the β single-phase region increased, the growth of dynamic-recrystallized β grains was promoted at the prior-β grain boundaries, where α-phase precipitation was not substantial. These effects resulted in a higher {001} texture intensity of the β phase in specimens forged at 1.0 s−1 compared with that of the β phase in specimens forged at 0.01 s−1.

Evolution of the Microstructure and Mechanical Properties of the Ultrafine-Grained VT8M-1 during Isothermal Die Forging and Thermal Treatment

The work addresses the microstructural evolution and mechanical properties of the ultrafine-grained (UFG) VT8M-1 subjected to isothermal die forging (IDF) and subsequent thermal treatment. An UFG microstructure with a mean size of secondary grains of about 0.3 μm was processed by a rotary swaging (RS) at Т=780°С. The ultimate tensile strength (UTS) of the alloy increased by 23% as compared to an initial state due to the formation of an UFG microstructure. It has been shown that isothermal die forging of the UFG alloy at Т=780°С leads to the growth of secondary phase grains by 0.7 μm. Subsequent heat treatment of the forged billets leads to hardening of 11%, which can be attributed both to the formation of additional interphase α/β boundaries at the precipitation of a tertiary α-phase and silicide dispersion.

Grain Boundary α-Phase Precipitation and Coarsening: Comparing Selective Laser Melted and Conventional Manufactured Ti6Al4V

Investigation into the precipitation and coarsening behavior of grain boundary α-phase (GB-α) in Ti6Al4V (Ti-64) manufactured by selective laser melting (SLM) were carried out and compared with that in as-cast Ti64 in this study. It was found that in SLM Ti64, the GB-α tends to precipitate first at the triple junctions (TJs) of the β-phase grain boundaries during the further annealing heat treatments in the α+β phase regime. With 750 ℃ annealings, the coarsening process of GB-α was controlled by the bulk diffusion mechanism with a small coarsening coefficient, different from as-cast Ti64. The interface reaction which has a large coarsening coefficient was dominant for the GB-α coarsening at 850℃ and 950 ℃ annealings. Such a difference in coarsening behavior at different annealing temperatures in SLM Ti64 was found to be related to the different curvatures of GB-α caused by the different nucleation mechanisms of α lamellae.

Heat Treatment of Low Cost Beta Titanium Alloy Produced by Investment Casting

Beta (β) Ti-alloys are promising class of Ti-alloys for high performance structural applications, thanks to their high strength to weight ratios and outstanding corrosion resistance. In the current work, Ti-4.5Fe-7Cr low cost beta Ti-alloy was produced by investment casting. DSC test was carried out to determine the transformation temperatures. The influence of solution treatment and aging temperatures on microstructure and mechanical properties were studied. Six heat treatment cycles were carried out, two solution treatment cycles in β- and α+β-range at 900 °C and 750 °C, respectively, and four precipitation hardening cycles, i.e. α+β-aging at 450 °C and 650 °C after solution treating at both 900 °C and 750 °C. Microstructure examination, XRD analysis and compression test had been conducted to investigate the influence of the different heat treatment regimes on the proposed alloy. Solution treatment at 900 °C results in coarser β-grain size than at 750 °C; hence lower strength. Strengthening by α-phase precipitation and grain refinement was achieved during aging. Aging at 650 °C results in higher strength than at 450 °C although coarser grain size, this is attributed to stronger effect of precipitation hardening of α-phase at 650 °C as a result of its higher amount than at 450 °C. The designed alloy shows high strength and excellent malleability, i.e. the alloy reveals superior plastic deformability, where specimens did not reveal any fracture; and the compression tests were interrupted at a compressive strain of 33%; therefore, this alloy may find applications as a healthcare, automotive or aerospace material.

Ti-15Mo alloy prepared by cryogenic milling and spark plasma sintering

In this study, Ti-15Mo alloy powder was prepared by gas atomization and subsequent cryogenic milling in order to achieve ultra-fine grained microstructure. Both milled and non-milled powders were compacted by spark plasma sintering (SPS) at temperature of 800 °C for different sintering times up to 6 minutes. Sintering temperature and time affect porosity, microstructure and phase composition of the alloy. Milled powder can be sintered at comparatively lower temperature to achieve fully dense material. Sintering below β-transus temperature results in α+β-structure. Furthermore, amount of α-phase is higher in the material sintered from the milled powder due to increased oxygen content and also due to refined microstructure which facilitates α-phase precipitation. Mechanical properties are also affected by formation of ω-phase during uncontrolled cooling in the SPS machine.

Effect of the High-Pressure Torsion (HPT) and Subsequent Isothermal Annealing on the Phase Transformation in Biomedical Ti15Mo Alloy

Ti15Mo metastable beta Ti alloy was solution treated and subsequently deformed by high-pressure torsion (HPT). HPT-deformed and benchmark non-deformed solution-treated materials were annealed at 400 °C and 500 °C in order to investigate the effect of UFG microstructure on the α-phase precipitation. Phase evolution was examined using laboratory X-ray diffraction (XRD) and by high-energy synchrotron X-ray diffraction (HEXRD), which provided more accurate measurements. Microstructure was observed by scanning electron microscopy (SEM) and microhardness was measured for all conditions. HPT deformation was found to significantly enhance the α phase precipitation due the introduction of lattice defects such as dislocations or grain boundaries, which act as preferential nucleation sites. Moreover, in HPT-deformed material, α precipitates are small and equiaxed, contrary to the α lamellae in the non-deformed material. ω phase formation is suppressed due to massive α precipitation and consequent element partitioning. Despite that, HPT-deformed material after ageing exhibits the high microhardness exceeding 450 HV.

Stiffness and hardness gradients obtained by laser surface melting of an aged β-Ti alloy

Components with variable stiffness have been attracting interest because of their potential for use in biomedical implants. Here, a β-titanium alloy aged under different conditions was subjected to laser surface melting (LSM). The high cooling rate typical of LSM avoided α-phase precipitation in the fusion zone (FZ) and heat-affected zone (HAZ) as the α precipitates were solid-solutioned into the β phase, producing a surface layer with full β phase over the aged substrate and reduced stiffness and hardness. The decrease in stiffness occurred independently of the aging condition used while the decrease in hardness was greater for the sample aged at the lower temperature as this had finer precipitates. A route involving aging at this lower temperature and LSM is proposed for use when a graded material combining a less rigid surface with the original core stiffness and strength is required, as in orthopedic implants.

α-Phase Precipitation Behavior in β-Type Ti–15V–3Cr–3Sn–3Al Alloys Aged by Different Aging Processes, Investigated Using Transmission Electron Microscopy

α-Phase Precipitation Behavior in β-Type Ti–15V–3Cr–3Sn–3Al Alloys Aged by Different Aging Processes, Investigated Using Transmission Electron Microscopy

The microstructure and mechanical hardness of cast Ti-30Nb-5Sn after solution treatment

This work reports the investigation on the development of microstructure and the resulting mechanical hardness of a metastable β Ti-30Nb-5Sn alloy as a result of solution treatment. The solution treatment was conducted at 1000°C for 5 h followed by water quenching. The microstructure was studied by a field emission-scanning electron microscope (FE-SEM) which equipped with EDX analysis for identifying the elemental composition. The phase change occurred in the alloy was analyzed by XRD while the mechanical hardness was measured by Vickers’s hardness technique. The results showed that the solution treatment induced the nucleation of intragranular α-phase precipitation in the alloy which formerly composed of only single β-phase with dendritic structure in the as-cast condition. The α-phase exhibited a micro size plate-like morphology distributed randomly in the metallic grain and along grain boundaries of the solution-treated alloy. The α phase precipitate contained a significantly lower Nb concentration (25.07 wt%) than the surrounding matrix (33.78 wt.%). The presence of α-phase precipitate enhanced the mechanical hardness from 274.3 to 335.8 HV. The results implied that the microstructure and hardness of the Ti-30Nb-5Sn were significantly altered by the solution treatment.

Multi-Scale Microstructure Regulation Technology of Near-α High Temperature Titanium Alloy

The served harsh environment of advanced aircraft engine puts forward higher requirements for high temperature titanium alloy performance. The optimized heat treatment technology provides effective theoretical basis for improving the microstructure and properties of high temperature titanium alloys. In this paper, we study the influence of different heat treatment systems on microstructure and mechanical properties of high temperature alloy with equiaxed structure, in order to obtain the corresponding relationship between the process and the microstructure performance of the alloy and the optimal heat treatment process. Analysis the effect of solution treatment on the primary α phase quantitatively by optical microscope and Image-Pro-Plus 6.0 software based on the forged high temperature titanium alloy in α+β phase region. Observe the precipitation of α2 phase and silicide by TEM, optimize the aging process according to hardness test. The results show that the content of primary a phase decrease from 63.3% at 920°C to 15.3% at 990°C with the increase of solution temperature. When the temperature rises to 980~990°C, the structure changes from equiaxed structure to α+β duplex microstructure. And change into lamellar structure when the solution temperature raise to 1010°C. The secondary α phase precipitates more fully when the aging temperature increases. And with increasing aging time, the trend of α2 phase growth become more significantly. The optimum heat treatment system obtained in this experimental is 990°C/1h/AC+700°C/5h/AC, and the α phase is about 15.3%. Hence, the excellent microstructure and properties match has been obtained.

Inhomogeneous Precipitation of the α-Phase in Ti15Mo Alloy Deformed by ECAP

A metastable β-titanium alloy, Ti15Mo was subjected to equal channel angular pressing (ECAP). The resulting microstructure of the material is inhomogeneous consisting of micrometer size β-grains with deformed bands containing ultra-fine β-grains. The ECAP-deformed sample was subjected to thermal treatment in order to elucidate the difference in morphology of the α-phase precipitation in deformed and non-deformed materials. The α-phase formation is accelerated in areas with higher concentration of lattice defects. The detail investigation by transmission electron microscopy revealed equixed α-phase formation in deformed bands. In the transition area between deformed band and β-matrix grain boundary α-phase forms while in the interior of the β-grains the α-phase precipitation is suppressed.