Metastable Β-titanium Alloy Research Articles

Homogeneous α-precipitation and enhanced plasticity in Ti–6Cr–5Mo–5V–4Al high-strength metastable titanium alloy with heterogenous β-structure

A long-standing conflict concerning metastable β-titanium (Ti) alloys is that high strength can be achieved through precipitation strengthening of α-phase while plasticity is largely compromised and even lost completely. Such strength-plasticity trade-off severely limits their structural applications. Here, an attempt to address the issue has been made in Ti–6Cr–5Mo–5V–4Al (Ti6554) high-strength metastable β-Ti alloy. Heterogeneous β-structure is architected by partial static recrystallization (SRX) through properly manipulating solution treatment in the hot-rolled bars. The resulting microstructure is composed of the recrystallized ‘new’ equiaxed β-grains (βe) and the remnant elongated β-fibers (βf). This β-morphology dramatically affects the subsequent αs-precipitation behavior during the aging process. αs-precipitation is rather uniform throughout βf-grains because the pre-existing dislocations left by hot rolling provide copious nucleation sites. However, αGB-films are preferentially formed along βe-grain boundaries, which leads to a boundary-affect zone (BAZ) and thereby renders αs-precipitation in a size gradient from β-grain boundaries to grain interiors. As a result, an ultra-high strength of ∼1700 MPa is achieved whilst the ductility can be up to be 5% in the β-hetero aged samples, which is superior to the β-homo aged siblings that have the same strength level but fully succumb to macroscopic brittleness. The enhanced plasticity in the β-hetero aged samples originates from much better deformation compatibility conferred by the aged βf-grains. However, the soft feature of BAZ around βe-grain boundaries in the β-homo aged samples causes strain localization, which eventually induces intergranular brittle fracture. These findings provide an effective strategy for enhancing plasticity of high-strength metastable β-Ti alloys by structural design, where homogeneous αs-precipitates can be easily constructed by combining conventional thermomechanical processing and heat treatment.

Grain refinement effect of ZrB2 in laser additive manufactured metastable β-titanium alloy with enhanced mechanical properties

Significant grain refinement of β grains in metastable β TB2 titanium alloy (Ti–8Cr–3Al–5Mo–5V) was achieved through the minor addition of zirconium diboride (ZrB2) during laser melting deposition (LMD). The size of β grains decreased from ∼150 μm in the as-deposited TB2 alloy to ∼70 μm in TB2+0.5ZrB2 (TB2 with 0.5 wt% ZrB2 addition) alloy. This study demonstrates that TB2+0.5ZrB2 exhibits good combination of ultimate tensile strength (UTS ≈ 896 MPa) and ductility (EL ≈ 15%) in comparison to the as-deposited TB2 (UTS ≈ 811 MPa and EL ≈ 18%). At similar strength (UTS≈ 900 MPa), TB2+0.5ZrB2 shows superior ductility than TB2+0.4Zr+0.1B and TB2+0.1B because of the formation of fine TiB particles (length of ∼0.6 μm) by uniformly introducing boron (B) into TB2 and lower diffusion rate of B. The excellent mechanical properties of TB2+0.5ZrB2 are attributed to the synergistic effect of strengthening from refined β grains and fine-scale TiB precipitation. The mechanism associated with refinement of β grains is elucidated and discussed in the context of relevant phase diagrams.

Microstructure characterization and tensile properties of Ti–15Mo alloy formed by directed energy deposition

Laser additive manufacturing of titanium (Ti) alloys results in a microstructure that differs from traditional manufacturing, affecting the subsequent alloy's performance. This paper details the correlation between the complex thermal cycles during directed energy deposition (DED) and the microstructure formation of a typical metastable β-titanium alloy Ti–15Mo, and elucidates its strengthening and toughening mechanisms. Relative to the forging standard, the yield and tensile strengths of the DED Ti–15Mo alloy increased by 51.4% and 28.7%, respectively, while retaining its elongation. Combined with experimental observation and finite element (FE) simulation, it was proven that the complex thermal cycles generated in the layer-by-layer stacking process of DED resulted in the presence of sub-structure in the Ti–15Mo alloy, precipitating the metastable ω phase, thereby improving its strength. Furthermore, the formation of {332}<113> twins in the tensile process was verified by SEM and EBSD, and the excellent elongation of the Ti–15Mo alloy was confirmed. This work is expected to lay the foundation for manufacturing excellent performing DED metastable β-titanium (Ti) alloys.

Effect of Solution Temperature on Tension-Compression Asymmetry in Metastable β-Titanium Alloys

Three titanium alloys, Ti-10V-1Fe-3Al, Ti-10V-2Fe-3Al and Ti-10V-2Cr-3Al, were heated by solution treatment at in 700 °C (α + β phase region), 800 °C (near β phase region), 900 °C and 1000 °C (single β phase region). The effects of solution temperature on the microstructure and mechanical properties of the alloys were studied, and the mechanical asymmetry of tension and compression of three titanium alloys was analyzed; the results show that the microstructure of the three alloys changes regularly with the increase of solution temperature. Different solution temperatures have a significant effect on the compressive and tensile properties of the three alloys. During compression deformation, the stress-induced martensite transformation occurs in samples with solution at 800 °C and above; however, there is no phase transformation during the process of tensile tests. The asymmetry of yield strength, work hardening rate and final strength of the three alloys are obvious during compression deformation and tensile deformation. The difference in the number of twins between uniaxial tension and uniaxial compression, the presence or absence of stress-induced martensitic transformation, and the CRSS asymmetry of cone &lt;c + a&gt; slip may be the reasons for the asymmetry of mechanical properties.

Dependence of strength and ductility on secondary α phase in a novel metastable-β titanium alloy

As a novel metastable β titanium alloy, Ti-6Mo-5V-3Al-2Fe, it is essential to establish the correlation among its microstructure, strength and ductility. In this paper, transmission electron microscope (TEM) and scanning electron microscope (SEM) were used to observe the secondary α phase (αs) of the aged alloy. The strength and ductility of the alloy were characterized by a room temperature tensile test. The dependence of strength and ductility on the αs of the alloy was investigated. Results indicate that the minimum width and spacing of αs within β grains can be obtained after being aged at 470°C for 4h. As the increase of aging temperature and time, the average width and average spacing of αs increase. A relational equation among precipitation strengthening and spacing length of αs is established to analyze the strengthening mechanism. It is demonstrated that the average spacing length of αs plays a predominant role in the strengthening of the alloy. The aged alloy exhibits poor ductility due to the formation of continuous grain boundaries α phase (GB α). After being aged at 620°C for 4h, Widmansta¨tten α phase nucleates along β grain boundaries and grows toward β matrix (WGB α). The combination of αs spacing widening and WGB α formation obviously improves the ductility of the alloy.

Towards β-fleck defect free additively manufactured titanium alloys by promoting the columnar to equiaxed transition and grain refinement

Metastable β-Titanium alloys containing a high content of segregated β-stabilising elements such as Cr make segregation defects (known as β-flecks or β-freckles) difficult to avoid during solidification processing. This work investigates the formation of β-flecks in the high strength Ti-3Al-8V-6Cr-4Mo-4Zr (Beta-C/ASTM Grade 19) produced by Wire Arc Additive Manufacturing and explores whether the introduction of grain refiner (La2O3) to the alloy can prevent the segregation defects from forming. Promoting the columnar to equiaxed transition and refining the grain size with La2O3 nucleant particles was found to significantly reduce the presence of β-flecks by 99.9% compared to the un-refined (standard) alloy. Analytical and numerical modelling is used to provide insights into how grain refinement can achieve this significant defect reduction.

In Situ CT Tensile Testing of an Additively Manufactured and Heat-Treated Metastable ß-Titanium Alloy (Ti-5Al-5Mo-5V-3Cr)

Additive manufacturing has been considered a suitable process for developing high-performance parts of medical or aerospace industries. The electron beam powder bed fusion process, EB-PBF, is a powder bed fusion process carried out in a vacuum, in which the parts are melted using a highly focused electron beam. The material class of metastable β-titanium alloys, and especially Ti-5Al-5Mo-5V-3Cr, show great potential for use as small and highly complex load-bearing parts. Specimens were additively manufactured with optimised process parameters and different heat treatments used in order to create tailored mechanical properties. These heat-treated specimens were analysed with regard to their microstructure (SEM) and their mechanical strength (tensile testing). Furthermore, in situ tensile tests, using a Deben CT5000 and a YXLON ff35 industrial µ-CT, were performed and failure-critical defects were detected, analysed and monitored. Experimental results indicate that, if EB-PBF-manufactured Ti-5553 is heat-treated differently, a variety of mechanical properties are possible. Regarding their fracture mechanisms, failure-critical defects can be detected at different stages of the tensile test and defect growth behaviour can be analysed.

The effect of quench rate on the β-α″ martensitic transformation in Ti–Nb alloys

While Ti–Nb binary alloys have been extensively studied over the years, there are discrepancies in the literature relating to the β-α″ martensitic transformation that indicate an incomplete understanding. In particular, there are inconsistencies regarding the phase constitution of as-quenched material and the ability for α″ phase to be thermally-induced. To resolve these issues, the β-α″ phase transformation is studied in a Ti–24Nb (at.%) alloy subjected to two different water quench conditions. It is demonstrated that only the fastest quench rates (≳500 °C/s) result in the formation of α″ phase, and these materials exhibit a reversible thermally-induced β-α″ transformation at low temperatures. In contrast, slightly slower water quench procedures lead to a β+ω microstructure, and these samples do not exhibit a thermally-induced β-α″ transformation, even when cooled to −168 °C. Despite this, room temperature mechanical testing results indicate that α″ can be induced by stresses as low as 200 MPa. To explain these results, it is proposed that quenching stresses play a role in the competition between α″ and athermal ω phases at rapid quench rates. The present results are then reconciled with the body of literature and a broader understanding of phase stability in metastable β-titanium alloys.

A novel deformation mechanism in Ti-V binary metastable β-Ti alloys: Deformation kinking promoted by dislocation accumulation

Unlike stress-induced martensitic transformation, deformation twinning as well as dislocation gliding, deformation kinking is an uncommon deformation mechanism in metastable β-titanium (Ti) alloys. In this study, the unique deformation mechanism was reported in Ti-V binary β-Ti alloys for the first time. It was found that a large number of kink bands were distributed in β-grains after cold forging at a strain rate of ~103 s−1. Microstructural characterization manifests that the appearance of kink bands is frequently accompanied by slip bands, whilst dense dislocations are accumulated around the kink bands on a microscopic scale. Such local plastic deformation is quite strong so that the pre-existing athermal ω-precipitates in the initial microstructure is completely destroyed within the kink bands and instead fresh deformation-induced single variant of ω-precipitates is produced. This deformation localization mainly originates from the impediment of dislocation activity caused by both pre-existing athermal ω-precipitates and the intrinsic slip retardation in BCC crystal structures. The deformation kinking is thus suggested to follow a dislocation-based formation mechanism, whilst the resulting Taylor axis of lattice rotation was deduced to be<011>β on basis of extensive crystallographic analysis. These findings enrich fundamental understanding on deformation kinking, and provide helpful information for utilizing the unique deformation mechanism to tune mechanical properties of β-Ti alloys.

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.

New insights into ω-embrittlement in high misfit metastable β-titanium alloys: Mechanically-driven ω-mediated amorphization

ω-embrittlement is ubiquitous in metastable β-titanium (Ti) alloys, while the fundamental understanding on the damage-fracture mechanism hitherto remains elusive. In this study, we systematically investigate ω-embrittlement of high misfit Ti-10Cr (wt.%) alloys by coupling experiments and first-principles calculation. It is found that brittle cleavage-like fracture prevails in tensile samples, irrespective of the quenching or subsequent aging states. Microscopically, cracks nucleation and propagation proceed along slip bands, inside which ω-lattices are first disordered and then the localized (β + ω)-amorphous-like structures are developed in the shape of white patches. The underlying mechanism of mechanically-driven localized amorphization is that due to the remarkable covalent character of atomic bonding of ω-precipitates caused by composition partitioning of the Cr element, ω-precipitates impart extremely high energy barrier opposed to dislocation gliding and render dislocations pile-up ahead of ω-precipitates, thus leading to their lattice disordering. It is unveiled that the hydrostatic pressure, serving as the driving force for dislocations pile-up, plays a critical role in this unusual cleavage-like fracture of Ti-10Cr alloys caused by mechanically-driven ω-mediated localized amorphization. Accompanied by the transition from the co-operation of deformation twining and ordinary dislocation slip in the quenched Ti-10Cr alloys to the exclusive ordinary dislocation slip in the long-time aged Ti-10Cr samples, it is unexpected that the resulting tensile fracture strength monotonically decreases to a stress level of ~ 100 MPa. These findings provide new insights into the damage and fracture behavior of high misfit β-titanium alloys, such as Ti-Cr alloys.

Plastic instability in Ti–6Cr–5Mo–5V–4Al metastable β-Ti alloy containing the β-spinodal decomposition structures

Metastable β-titanium (Ti) alloys have been extensively used in aerospace industry owing to the outstanding mechanical properties. Temperature arising sometimes becomes unavoidable in service and thus it is necessary to evaluate plastic behavior of the alloys at elevated temperatures prior to applications. Here, plastic stability of metastable β-titanium alloys containing the β-spinodal decomposition structures at different temperatures is investigated for the first time using Ti–6Cr–5Mo–5V–4Al alloy as prototypical material. It is found that a serrated plastic flow with little frequency accompanied by overall strain softening appears in the tensile stress-strain curves at 298 K, while the serrated plastic flow occurs much more frequently and overall strain hardening capability is recovered appropriately when the tensile temperature is increased to 573 K. Microstructural characterizations reveal that ordinary dislocation slip dominates the alloy plasticity at all the testing temperatures. Dislocations strongly interact with the β-striations and dislocation debris are left around the striation interfaces. The resulting local stress accumulation destroys the β-striations in the form of shear bands so that sudden release of an avalanche of dislocations in the soft channels leads to stress drop. Subsequent dynamic reconstruction of the β-striations with the assistance of thermal activation especially at elevated temperature hardens the prior soft channels, which results in the following stress rise. The striking difference on deformation characteristics at 298 K and 573 K is that more homogeneous plasticity conferred by dense deformation bands at the latter, which is the direct reason on high serrated frequency and overall strain hardening. These findings offer valuable insights into plastic behavior of metastable β-Ti alloys, which will have important implications for the applications.

Achieving high strength-ductility synergy in a hierarchical structured metastable β-titanium alloy using through-transus forging

High-strength titanium (Ti) alloys are progressively demanded for structural applications in weight-critical aerospace industry. Improving mechanical properties of the alloys are thus exploited in response to the requirement. In this study, a strategy of through-transus thermomechanical forging was applied to the Ti–10 V–2Fe–3Al metastable β-Ti alloy, and the resulting microstructures and mechanical properties were systematically investigated in comparison to the conventional (α+β) dual-phase field forging. It is found that the through-transus thermomechanical forging significantly enhances strength, ductility and strain hardening capacity, while it is the inferior combinations of mechanical properties in the alloys subjected to the (α+β) dual-phase field forging. Microstructural characterization reveals that the former renders the hierarchical microstructure composed of intergranular αp-necklaces, and intragranular αp-rods as well as αs-lamellae in the β-matrix. The superior combinations of mechanical properties are conferred by good strain compatibility of these microstructural constituents. By contrast, the latter renders the network microstructure comprised of intergranular globular αp-phase and continuous grain boundary (GB) αGB-films as well as the intragranular αs-lamellae. Local deformation-damage is more preferred to initiate in the intergranular network structure. These findings provide fundamental understanding on the correlation between mechanical properties and microstructures, facilitating manufacturing high-performance β-Ti alloys at an industry scale using commercially available thermomechanical processing.

Deformation twinning-induced single-variant ω-plates in metastable β-Ti alloys containing athermal ω-precipitates

The ω-phase in metastable β-titanium (Ti) alloys attracts great research interests owing to the metastability and the involved microstructural complexity upon straining, while the fundamental understanding on its formation mechanism is still insufficient. In this study, deformation-induced ω-plates have been systematically investigated using Ti-10 wt% Cr metastable β-Ti alloy containing athermal ω-precipitates. It is found that single-variant ω-plates form either along {332}β twin interfaces or in the twin interiors, which closely depends on twin morphologies. For the acicular β-twin, the ω-plate appears in the twin interiors accompanied by high density of parallel straight dislocations, while for the lenticular twin, the ω-plate attaches to the twin interface without the surrounding dense parallel straight dislocations. These unique formation characteristics result from local stress field induced by the twinning itself and the passive transformation of athermal ω-precipitates. It is further revealed that the passive transformation of athermal ω-precipitates dominates the formation of ω-plates inside the acicular twins, while it is the locally preferential twin thickening that promotes the ω-plates to form along the lenticular twin interfaces. These findings provide new insight into deformation-induced ω-phase transformation and the intricate deformation microstructures in metastable β-Ti alloys.

Eliminating segregation defects during additive manufacturing of high strength β-titanium alloys

Segregation defects, known as β-flecks, have long detracted from the widespread use of metastable β-Titanium alloys produced via solidification routes. These defects form when segregated β-stabilising solute such as chromium create localised phase instabilities which is detrimental to the alloy’s aging response and mechanical properties. This work explores whether Additive Manufacturing is a viable process for preventing the formation of these defects in the Beta-C alloy (Ti-3Al-8V-6Cr-4Mo-4Zr). Although it is observed that β-flecks initially form during deposition of the top layer, each new subsequent layer and thermal cycle promotes solid-state diffusion of alloy elements that effectively heals the segregation defect. Additive Manufacturing is thus a promising pathway for producing high quality metastable β-titanium alloys free of this defect.

Effects of SMAT at cryogenic and room temperatures on the kink band and martensite formations with associated fatigue resistance in a β-metastable titanium alloy

Surface Mechanical Attrition Treatment (SMAT) has been carried out at cryogenic and room temperatures on a 5553 β-metastable Ti alloy to describe its effects on the modification of the microstructure and fatigue properties. The cryogenic temperature promoted the martensitic transformation and the width of the kink bands produced by SMAT. The kink bands formation was promoted by the high strain rates of the SMAT process. The samples SMATed at cryogenic temperature showed an increase in fatigue resistance, by about 8 % , compared to samples processed by SMAT at room temperature. This improvement was possibly due to a combination of several factors: compressive residual stresses, lower roughness, deeper formation of martensite and larger kink bands. On the contrary, the SMAT at room temperature did not provide any improvement in terms of fatigue compared to the polished condition. The large grain size provoked the appearance of subsurface crystallographic defects on which primary cracks initiated. This greatly limited the effectiveness of the reinforced surface layer produced by SMAT. • Kink bands generated by the high strain rates induced by SMAT. • A lower kink band density in the first μm of SMATed surfaces. • Deeper martensitic formation and lower roughness for cryogenic SMAT. • Increased fatigue resistance for SMAT at cryogenic temperature.

Comparative Study of the Effect of Isothermal Exposure on Metastable β-Titanium Alloy Mechanical Properties

A comparative study of chemical composition, structure, and mechanical properties (Rockwell hardness, HRC) is conducted for pseudo-β-titanium alloys for structural purposes (VT47, VT35, Timet LCB) after isothermal ageing over wide ranges of temperature and exposure times. Diagrams are plotted providing summarized hardness values for VT47 and Timet LCB alloys. Dependencies are revealed between hardness of these alloys and their alloying level with β-stabilizing elements as well as features of phase and structural transformations. “Incubation” periods are established for an increase in hardness in the temperature range studied and comparative evaluation of these periods is provided.

Effect of mechanical milling on the harmonic structure development during spark plasma sintering of Ti-5Al-2Sn-4Zr-4Mo-2Cr-1Fe β-metastable titanium alloy (β-Cez alloy)

The present study focuses on the formation of harmonic microstructures in a metastable β titanium alloy, the β-Cez alloy (Ti-5Al-2Sn-4Zr-4Mo-2Cr-1Fe – Tβ = 895 °C). Previous studies emphasized mainly fabrication and improvements in the mechanical properties of alloys prepared by powder metallurgy and showing a harmonic structure. In this study, the harmonic structure was obtained after mechanical milling of the initial powder, followed by a Spark Plasma Sintering (SPS) process. During the process, phase transformations occur that are dependent on the initial state of the powder. Moreover, their kinetics depend on the thermo-mechanical history of the powder. In order to analyze the microstructure formation, the behavior of milled and non-milled powders was studied during a heat treatment similar to the one applied during the SPS process. In-situ high-energy X-ray diffraction was used to characterize the evolution of phases during the thermal treatment. Additional in situ electrical resistivity measurements were carried out on sintered compact specimens. Characterizations evidenced that the initial powder is in a β metastable state. After mechanical milling, stress/strain induced α” martensite was observed inside the powder’s β grains. The stepwise microstructural characterization revealed the influence of the initial state of the mechanically milled powder on the formation of a harmonic α arrangement in the β matrix consisting of nodular α grains in the powder shell and α lamellae in the powder core. The stress/strain induced martensite formed during the milling associated with the heavier deformation at the powder surface areas contributes highly to the formation of an arrangement of nodular α grains by a recovery/recrystallization phenomenon of β and α”/α phases during the heating.

Hydrogen concentration dependence of phase transformation and microstructure modification in metastable titanium alloy β-21S

The present study aims at revealing the relationships between hydrogen concentration and phase structure, as well as microstructure modification in the β-metastable β-21S titanium alloy. The β-bcc phase can accommodate a large number of interstitial atoms, and hydrogenation by means of molecular hydrogen gas was employed in the present work. The phase structure as well as the microstructure of this alloy was found to be strongly dependent on hydrogen concentration. At lower hydrogen concentration (H/M ≤ 0.300), the microstructure consisting of the single β-phase revealed that the interstitially dissolved hydrogen atoms expanded the bcc lattice and inhibited the decomposition of the β phase upon cooling. The introduction of hydrogen beyond H/M = 0.300 was found to generate a large amount of internal stresses in the microstructure inducing the formation of metastable phases α'' in the form of lamellae and ω in the form of nanoparticles. The generation of the nanosized ω-phase was presumed to relax the strain caused by the volume expansion (2.28%) from the hydrogen-containing β phase to the α'' martensite.

In-situ high energy X-ray diffraction study of the elastic response of a metastable β-titanium alloy

In the present work, the elastic behavior of a metastable β-Ti alloy, Timetal-18, is studied in the β-quenched condition using in-situ far-field (ff) and near-field (nf) high energy synchrotron X-ray diffraction microscopy (HEDM) techniques and full-field crystal elasticity simulation. HEDM enables assessment of the 3D microstructure as well as the complete (6 components) elastic strain tensor and crystallographic orientation of each grain in the gauge volume of a polycrystalline aggregate. Using this elastic strain and orientation data from ~100 grains, a straightforward strategy to obtain the single crystal elastic constants (SEC) is demonstrated. The elastic moduli thus obtained are (in GPa): C11 = 118 ± 5, C12 = 79 ± 3, C44 = 39 ± 4, with a Zener anisotropy ratio (A) of 2.0 ± 0.4. The nf-HEDM 3D microstructure was used to instantiate a virtual microstructure, as input into the parallelized code, Micromechanical Analysis of Stress-strain Inhomogeneities with fast Fourier transform (MASSIF), in order to simulate the grain level constitutive response. The predicted evolutions of the elastic strain tensor components are shown to be in good agreement with the measured values. However, grain-by-grain comparisons emphasize the strong effect of elastic anisotropy, suggesting it may be even higher than A = 2. The accuracy of the approach and implications for modeling the grain-level constitutive response in the plastic regime are discussed.