Phase Titanium Alloy Research Articles

Phase transformation in titanium alloys: A review

Due to a series of exceptional properties, titanium and titanium alloys have received extensive attention in recent years. Different from other alloy systems, there are two allotropes and a sequence of metastable phases in titanium alloys. By summarizing the recent investigations, the phase transformation processes corresponding to the common phases and also some less reported phases are reviewed. For the phase transformation only involving α and β phases, it can be divided into β→α transformation and a reverse transformation. The former one has been demonstrated from the orientation relationship between α and β phases and the regulation of α morphology. For the latter transformation, the role of the stress has been discussed. In terms of the metastable phases, the mechanisms of phase formation and their effects on microstructure and mechanical properties have been discussed. Finally, some suggestions about the development of titanium alloys have been proposed.

Crystal plasticity-phase–field based analyses of interfacial microstructural evolution during dynamic recrystallization in a dual phase titanium alloy

In this study, an integrated crystal plasticity finite element–phase–field (CPFE–PF) model is developed to examine dynamic recrystallization (DRX) in a dual phase Ti alloy. The CP framework is coupled with PF by updating the free energy density with energy contributions due to plasticity. The evolution of grain boundaries through evolving non-conserved order parameters in the PF model is tracked using the Allen–Cahn equation. Nucleation is allowed to occur if the dislocation density exceeds a critical value. DRX is studied in various Ti morphologies such as an α−Ti single crystal containing a stiff elastoplastic particle, α-Ti bicrystals with low and high misorientation between grains, an α−β bicrystal and a globular α−β Ti structure with β phase at α−α interfaces. For an α−Ti bicrystal, a high misorientation facilitates the onset of DRX at the α−α interface at a significantly lower strain than the bicrystal with low misorientation. In an α−β bicrystal, DRX is only observed at the α−β interface. For the globular morphology, nucleation is observed at both α−β interfaces and inside α grains, which is consistent with previous experimental observations for a similar morphology. Nucleation inside α grains is explained by the correlation between SSD density and misorientation indicators such as KAM and GROD at the nucleus site. To correlate slip activity with nucleation propensity immediately prior to different nucleation events, the dislocation density, shear rate and Schmid factors on different slip systems are evaluated at nucleation sites.

Microstructural characterization of gadolinium-rich phases in titanium alloys

This study explores the microstructural characteristics of gadolinium (Gd)-rich phases in titanium (Ti) alloys through comprehensive electron microscopy analysis. The Ti alloys were produced using plasma arc melting and subsequently hot-forged. Elaborate material characterization, including scanning electron microscopy, electron backscatter diffraction, and energy dispersive spectroscopy, revealed the formation of round or angular Gd oxides and elongated Gd-rich grains within the alloy. High-magnification transmission electron microscopy and X-ray diffraction confirmed the presence of the FCC-type γ-Gd phase, influenced by the oxygen intake during casting, coexisting with Gd2O3 due to their similar crystal structures. The study also observed internal twins in the Gd grains, potentially delaying the transformation to the stable α-Gd phase. The significant mechanical property differences between the Gd-rich phases and the Ti matrix caused defects at phase interfaces during hot processing, weakening the Gd phase. This work enhances the understanding of Gd phase formation and its implications on the mechanical properties of Ti–Gd alloys.

Phase-Field Simulations of α Colony Formation Associated with Pre-existing α Phase in Titanium Alloys

Phase-Field Simulations of α Colony Formation Associated with Pre-existing α Phase in Titanium Alloys

Investigation of the Chemical Composition, Microstructure, Density, Microhardness, and Elastic Modulus of the New β Ti-50Nb-xMo Alloys for Biomedical Applications.

β-type titanium alloys with a body-centered cubic structure are highly useful in orthopedics due to their low elastic modulus, lower than other commonly used alloys such as stainless steel and Co-Cr alloys. The formation of the β phase in titanium alloys is achieved through β-stabilizing elements such as Nb, Mo, and Ta. To produce new β alloys with a low modulus of elasticity, this work aimed to produce our alloy system for biomedical applications (Ti-50Nb-Mo). The alloys were produced by arc-melting and have the following compositions Ti-50Nb-xMo (x = 0, 3, 5, 7, and 12 wt% Mo). The alloys were characterized by density, X-ray diffraction, scanning electron microscopy, microhardness, and elastic modulus. It is worth highlighting that this new set of alloys of the Ti-50Nb-Mo system produced in this study is unprecedented; due to this, there needs to be a report in the literature on the production and structural characterization, hardness, and elastic modulus analyses. The microstructure of the alloys has an exclusively β phase (with bcc crystalline structure). The results show that adding molybdenum considerably increased the microhardness and decreased the elastic modulus, with values around 80 GPa, below the metallic materials used commercially for this type of application. From the produced alloys, Ti-50Nb-12Mo is highlighted due to its lower elastic modulus.

The microstructure and impact toughness of vacuum electron beam welded joints of a highly alloyed dual phase titanium alloy

Vacuum electron beam welding of 50 mm thick TC18 titanium alloy plates were carried out at different processing parameters, and full penetration welded joints were obtained. The welded joint presents three typical zones, namely, fusion zone, heat affected zone and base metal zone. It is found that processing parameters have no obvious influence on the width of heat affected zone. By using EBSD technology, the changes of phase composition, grain size and grain boundary density from the fusion zone to heat affected zone and then to base metal zone were clearly observed and quantitatively analyzed. The fusion zone shows coarse equiaxed/columnar β grains with an average diameter/width of more than 100 μm. The volume fraction of α phase in the heat affected zone decreases with the decrease of the distance to the fusion zone until it is completely dissolved into β phase. The formation mechanism of microstructure in different zones and its influence on impact toughness were discussed. The β grain size and undissolved/precipitated fine α grain on β grain boundary are two main factors affecting impact toughness.

Effects of lamellar α orientation on the mechanical behavior of Ti–6Al–4V alloy manufactured by electron beam directed energy deposition

Crystallographic texture of alloy can trigger anisotropic properties, which have been studied extensively. However, the effect of spatial orientation of grains on the mechanical properties, especially for additive manufactured metals with lamellar morphology has remained elusive. Herein, this work theoretically calculated the spatial and crystallographic orientations of α lath from the <100>β//Z-axis (build direction) fiber texture, and investigate the relationship between the orientations of α lath and the deformation behaviors and mechanical properties of the Ti–6Al–4V alloy built via electron beam directed energy deposition (EB DED). Deformation features display the prismatic slip system of α lath that exists a smaller angle to the broad face of α lath showing a lower effective critical resolved shear stress (CRSS). This is because this prismatic slip system has a greater slip range when compares with other slip systems. Such deformation features, combined with the spatial orientation distribution characteristic of α lath that is induced by the <100>β//Z-axis fiber texture, lead to strongly tensile anisotropy, specifically, the samples oriented 45° and 67.5° angles to z-axis present greater strengths but lower ductility, whereas the sample in the z-axis direction generally displays a minimum strength but a superb ductility. This finding is vital for providing an important theoretical basis for optimizing the consistency of mechanical properties of the additive manufactured α+β dual phase titanium alloy.

Substructure evolution in two phases based constitutive model for hot deformation of TC18 in α + β phase region

The α + β dual phase titanium alloys are key structural materials in aviation and aerospace industries, and the complicated flow behavior of these titanium alloys during hot deformation requires to establish a constitutive model incorporating physical mechanism for optimizing processing parameters and designing forming tools. This work aims to establish a constitutive model incorporating physical mechanism for hot deformation of TC18 in α + β phase region. Firstly, the flow behavior and microstructure evolution for hot deformation of TC18 in α + β phase region are characterized. The TC18 shows significant strain hardening rate and negative strain hardening exponent around and after peak flow stress, respectively. After peak flow stress, Dynamic Recovery (DRV) mechanism dominates the evolution of α and β phases according to the results of substructure evolution. Then, the internal state variables method is applied to establish a constitutive model incorporating physical mechanism for hot deformation of dual phase titanium alloys. The variation of dislocation density during the hot deformation of titanium alloys is modeled by considering the accumulation of dislocation due to the impediment to dislocation movement by substructure obstacles and the annihilation of dislocation due to the dynamic restoration effect. The interaction between dislocations, the subgrain boundaries and the grain/phase boundaries obstruct the dislocation movement in the α phase, and the first two obstructs the dislocation movement in the β phase during the hot deformation of TC18. The dislocation annihilation process in the α and β phases during the hot deformation of TC18 is dominated by DRV. Finally, the substructure evolution in the two phases based constitutive model for hot deformation of TC18 in α + β phase region is presented. This model is well applied to predict the flow stress and quantitively analyze the role of DRV effect in the evolution of α and β phases during the hot deformation of TC18.

Crack initiation mechanism of a silicide-containing high-strength Ti–5Al-7.5V alloy under low-cycle fatigue loading

In this work, the dual phase titanium alloy Ti–5Al-7.5V was subjected to low-cycle fatigue at room temperature to reveal the deformation mechanisms of fatigue and crack initiation. Transmission electron microscope analysis showed that planar dislocation slip, most of them localized at the basal and prismatic planes of α phase, is the primary deformation mode of low-cycle fatigue. Multiple slips operate concurrently in the high Schmid factor planes within a single grain. Moreover, a limited number of observed microcracks were formed along the basal slip bands. Electron back-scattered diffraction analysis evidenced that numerous microcracks were formed along basal planes. And, the microcracks were confined in primary α grains without propagation to surrounding transformed β matrix, indicating that the alloy exhibits a high tolerance to microcrack propagation. The α grain aggregate oriented for basal slip is a fatigue-critical microstructure configuration, which provides a high Schmid factor path by linking adjacent grains and potentially lead to internal crack initiation with the formation of a field of facets in the cycling process. While the silicides were observed in contact with the microcracks and slip bands, no evidence of crack initiation from silicides was detected. Given that the microcracks were formed along the pre-existing slip bands, it remains an open question whether the silicides can act as the dislocation sources for planar slips and facilitate crack nucleation from these silicides.

Tuning temperature coefficient of elastic modulus by heat treatment in a compositionally-modulated Ti-Nb-based alloy

Temperature coefficient of elastic modulus can be positive for metastable β phase in titanium alloys whereas the precipitation of α phase from the β matrix phase results in a negative coefficient. This abrupt transition inspires us to tune the coefficient from positive, via zero of Elinvar effect, to negative by aging treatment in a Ti-Nb-based alloy. Here reports that its coefficients are continuously tunable by a compositionally-modulated β+α" dual-phase microstructure induced by spinodal decomposition. As the aging time increases at 723 K, the coefficient keeps positive almost unchanged initially, then drops quickly toward zero, and finally decreases slowly of being negative. A mixture rule modeling suggests that the coefficients of both phases decrease gradually from positive of being metastable to negative of being stable. These findings provide a new strategy to achieve Elinvar effect by optimizing the compositionally-modulated microstructure via heat treatments.

Copper segregation-mediated formation of nanotwins and 9R phase in titanium alloys produced by laser powder bed fusion

Both nanotwins and 9R phase are rarely form in titanium alloys due to their high stacking fault energy. In this work, we report for first time the formation of nanotwins and 9R phase in ultrafine-grained Ti-5wt.%Cu alloys with an average lath width of 223 nm, which were produced by laser powder bed fusion. Transmission electron microscopy showed that the 9R phase originated from lath boundaries with Cu segregation and terminated at the other boundary of the lath. Besides, deformation resulted in curvature of the lath boundaries and an increase in lath width. This promotes the transition of the central 9R phase to nanotwins, implying that the stabilization of the 9R phase is highly correlated to the width of the laths. This work provides a new route for in situ production of nanotwinned Ti by the laser melting-induced segregation of Cu.

Influence of Nonmetallic Interstitials on the Phase Transformation between FCC and HCP Titanium: A Density Functional Theory Study

In addition to the common stable and metastable phases in titanium alloys, the face-centered cubic phase was recently observed under various conditions; however, its formation remains largely unclarified. In this work, the effect of nonmetallic interstitial atoms O, N, C and B on the formation of the face-centered cubic phase of titanium was investigated with the density functional theory. The results indicate that the occupancy of O, N, C and B on the octahedral interstitial sites reduces the energy gap between the hexagonal-close-packed (HCP) and face-centered cubic (FCC) phases, thus assisting the formation of FCC-Ti under elevated temperature or plastic deformation. Such a gap further decreases with the increase in the interstitial content, which is consistent with the experimental observation of FCC-Ti under high interstitial content. The relative stability of the interstitial-containing HCP-Ti and FCC-Ti was studied against the physical and chemical origins, e.g., the lattice distortion and the electronic bonding. Interstitial O, N, C and B also reduce the stacking fault energy, thus further benefiting the formation of FCC-Ti.

Microstructure Evolution and Hot Deformation Behavior of Two Phase Titanium Alloy in α/β Region

Microstructure Evolution and Hot Deformation Behavior of Two Phase Titanium Alloy in α/β Region

Quantitative phase analysis of Ti-3Al-5Mo-4.5 V dual phase titanium alloy by XRD whole pattern fitting method

The reliable and convenient quantitative phase analysis (QPA) of titanium alloys is a meaningful work. In this study, the relative phase fraction of the bulk aviation serviced Ti-3Al-5Mo-4.5 V alloy was investigated by various methods, including Optical Microscope (OM), Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM), Electron Back-Scattered Diffraction (EBSD) and X-ray diffraction (XRD). OM, SEM and TEM can scarcely do phase quantification. From the convincing results of EBSD test, the specimen was primarily constituted by fine, uniformly distributed and equiaxed α grains embedded in β matrix and α/β phase respectively accounted for 74.14% and 25.86%. The obstacles of Peaks overlapping and PO (Preferred Orientation) to XRD methods were greatly dealt with employing XRD whole pattern fitting method according to the 2 θ diffraction pattern by careful scanning. Through initial fitting without PO correction, the calculated α/β phase fractions were 62.25% and 37.75%, respectively. The main PO could be inferred along the directions of (002) α , (101) α and (110) β . When modifying PO of these peaks stepwise utilizing March-Dollase function, the difference of the calculated profile to the observed one was gradually reduced to a low level and the calculated α/β phase components were respectively 73.04% and 26.96%. With the optimization of adding (200) β for PO correction or utilizing spherical harmonics function, the fitting quality was further improved and the quantitative results tended to stability. It was found that PO of α phase was suitable to be modified by eighth order spherical harmonics function because it has multiple diffraction peaks with complicated PO, while β phase adopting March-Dollase function. The final α/β phase contents determined were 73.65% and 26.35% respectively, which was greatly consistent with EBSD results. • The discussion of PO correction in the practical Rietveld refinement process. • A combination of March-Dollase and spherical harmonics function for PO correction. • The relative phase composition of Ti-3Al-5Mo-4.5 V alloy was determined.

A multiscale constitutive model for metal forming of dual phase titanium alloys by incorporating inherent deformation and failure mechanisms

Ductile metals undergo a considerable amount of plastic deformation before failure. Void nucleation, growth and coalescence is the mechanism of failure in such metals. α–β titanium alloys are ductile in nature and are widely used for their unique set of properties such as specific strength, fracture toughness, corrosion resistance and resistance to fatigue failures. Voids in these alloys have been reported to nucleate on the phase boundaries between α and β phase. Based on the findings of crystal plasticity finite element method investigations of the void growth at the interface of α and β phases, a void nucleation, growth, and coalescence model has been formulated. An existing single-phase crystal plasticity theory is extended to incorporate underlying physical mechanisms of deformation and failure in dual phase titanium alloys. Effects of various factors [stress triaxiality, Lode parameter, deformation state (equivalent stress), and phase boundary inclination] on void nucleation, growth and coalescence are used to formulate a phenomenological constitutive model while their interaction with a conventional crystal plasticity theory is established. An extensive parametric assessment of the model is carried out to quantify and understand the effects of the material parameters on the overall material response. Performance of the proposed model is then assessed and verified by comparing the results of the proposed model with the RVE study results. Application of the constitutive model for utilisation in the design and optimisation of the forming process of α–β titanium alloy components is also demonstrated using experimental data.

The effect of TiC on microstructure and mechanical properties of Ti-5553 beta phase titanium alloy

The presence of TiC in Ti alloys is ubiquitous, so, for the interest of exploring its effects, carbon is added in Ti-5553 (beta phase alloy). After fabrication, sintering and multi heat treatments are done to get desired microstructure (TiC in beta, alpha and alpha-matrix). Quasi-static compression, three-point bend test, nano-indentation, and abrasion tests are conducted to study the microstructural response. Different analyzing tools such as SEM, EDS, EBSD, and HRTEM are used to examine microstructural changes before and after fracture. Results showed that TiC crystals are hard and brittle, and favorite sites for crack nucleation and propagation. After comparing both the curves and their corresponding micrographs, it is concluded that the propagation speed of the crack is much faster in TiC as compared to the beta and alpha phases. Furthermore, an increase in the value of strain upon reaching a crack at the TiC/Tiᵦ, and TiC/Tiα interface justifies that these phases offer comparatively more toughness and ductility. In addition, the presence of these crystals in the beta phase initiate fracture almost within half of the elastic limit of Ti-5553 beta phase alloy. Conclusively, TiC has an adverse effect on titanium's mechanical properties if shape and size are not controlled.

Laser micro in situ alloying of γ-Ti46.8Al1Cr0.2Si with Manganese

Titanium alloys have two primary phases (α + β) that are attributed to the α-stabiliser (aluminium) and β-stabiliser (niobium). The latter stabilisers shift α-phase to higher temperature while the former stabilisers shift the β-phase to lower temperatures. To extend their scope of application, (α + β)-phase titanium alloys can be micro-alloyed with β-eutetic elements (Mn, Cr, Si, Co etc) to form intermetallic compounds which are typically referred to as titanium aluminides (Ti-Al). Several intermetallic alloys can be formed in the Ti-Al phase diagram but only (α2)-Ti3Al and (γ)-TiAl are stable and of relevance to structural engineering since they can be used in industries spanning across energy, aerospace and tissue-engineering. In this study, a castable γ-Ti46.8Al1Cr0.2Si pre-alloyed powder, was micro-alloyed with Mn in order to examine if it can improve its tensile properties. When 1–2 at. % Mn is added to Ti-Al alloys, it is able to increase ductility, reduce oxidation resistance and shift the (α2 + γ)/ γ to the titanium side and decrease the Al content in the γ-phase. To effectively ascertain these effects Mn feed rates were varied from 0.2 to 1.0 rpm during the micro-alloying of γ-Ti46.8Al1Cr0.2Si through in-situ laser deposition to form γ-Ti46.8Al1Cr0.2Si-(xMn). The produced clads were characterised for chemistry and microstructure using scanning electron microscope. Results concluded that Al content decrease with an increase in Mn from 56 at. % to 49 at. %. Mn of 10.16 g/min had similar properties to the non-Mn alloyed sample. Moreover, it was shown in this work that laser processing decreased the tensile and yield strength of the alloy when compared to the as-cast in previous studies, and that 2.69 g/min and 10.16 g/min are lower and upper Mn values when ductility of the master alloy is to be improved.

A novel cyclic thermal treatment for enhanced globularisation kinetics in Ti-6Al-4V alloy: Experimental, constitutive and FE based analyses

Secondary hot-working on dual phase titanium alloys are essential for microstructural modification to tailor mechanical properties, which is typically challenging due to a narrow available processing window, especially during industrial scale manufacturing. Poor workability, strain induced porosity and adiabatic temperature rise in α + β phase region (i.e., sub-transus) are some of the main challenges faced. Cyclic thermal treatment (CTT) is an emerging technology showing potentials for microstructure modification (i.e., globularisation) in Ti‐6Al‐4V with significantly reduced mechanical work in the α + β region. This study summarises the results of CTT investigations conducted on a wrought Ti‐6Al‐4V alloy subjected to various thermo-mechanical conditions to develop different initial microstructures. Samples with uniform strain distributions were extracted from pre-forged samples and subsequently subjected to CTT using both conventional electric furnace (i.e., for slow heating and cooling rates), and induction heating (i.e., for faster heating and cooling rates). CTT of the samples forged at sub-transus temperature in conventional furnace led to maximum (i.e. ~100%) globularisation and significant coarsening of α grains, resulting in an equiaxed bimodal microstructure. On the other hand, CTT with induction heating method has resulted in a maximum of 80% globularisation fraction in samples forged to 60% reduction, and ~35% globularisation fraction in those forged to 20% reduction. The globularisation mechanisms during CTT of the sub-transus forged samples was dominated by the boundary splitting and thermal grooving. A Johnson-Mehl-Avarmi-Kolmogorov (JMAK) based model has been developed to predict the evolution of globularisation and grain growth during CTT. The developed JMAK model was then successfully incorporated into DEFORM® software as post-processing user subroutines. The predicted microstructure evolution by Finite Element (FE) simulations shown a good convergence towards the experimentally measured data following CTT.

The microstructure evolution history and the underlying mechanism of an α/β dual phase titanium alloy during elastic-plastic deformation process

In this paper, in situ scanning electron microscope tensile test was carried out on the Ti-4.5Mo-5.1Al-1.8Zr-1.1Sn-2.5Cr-2.9Zn alloy, and the mechanism of grain rotation, shape change and micro-strain redistribution in macroscopic elastic deformation (stage I), macroscopic elastic-plastic transition (stage II) and macroscopic plastic deformation (stage III) stages were quantitatively investigated by electron backscatter diffraction and digital image correlation. In stage I, it was found that 10.1% of the viewing zone underwent grain rotation or shape change, and the micro-strain was mainly distributed in primary α phase (αp) and their intersection. The result of the separately designed elastic loading-unloading tensile test showed that the rotated αp no longer returned to the primary orientation. In stage II, the tensile plastic deformation regions reached 52.9%, and grains in βT regions continued to rotate and began to change shape. In stage III, 84.6% of the viewing zone got into the tensile plastic deformation state. Combined with geometrically necessary dislocation density distribution, it was found that some of the regions in low strain state (0 ≤ ε<1%) actually underwent complex tensile plastic deformation. In addition, the maximum micro-strain of the viewing zone was transferred to βT regions.

A polygonal FEM and continuum damage mechanics based framework for stochastic simulation of fatigue life scatter in duplex microstructure titanium alloys

The scatter in the fatigue behavior of duplex microstructure titanium alloys occurs due to microstructural randomness. The presence of two phases in titanium alloy microstructure also contributes to the scatter in fatigue life data. In the literature, several mathematical frameworks have been developed to consider the effect of microstructural randomness on the prediction of fatigue scatter. However, no numerical study has been found that considers the effect of presence of two phases on the fatigue life scatter. Thus, in the present work, a mathematical framework based on continuum damage mechanics (CDM) and polygonal finite element method (PFEM) is developed to predict the fatigue life scatter in α+β titanium alloys. In this framework, PFEM is applied to perform the numerical simulations and CDM is used to explain the fatigue behavior of material. In PFEM, the Voronoi tessellation approach is used to generate the idealized microstructure models. The Waschpress shape functions are adopted to compute the shape functions over the arbitrary convex polygonal elements. In CDM, a fatigue damage model is adopted to consider the crack initiation and propagation under the cyclic loading conditions. The advantage of combined CDM-PFEM framework is that the numerical simulation can be performed at relatively coarser mesh which decreases the computational cost and increases the efficiency of the computational model.While predicting the fatigue life, it has been observed that micro cracks may initiate and propagate at several locations inside the microstructure. Thus, to track the nucleation and growth of micro cracks effectively, a machine learning based technique ‘cluster analysis’ is employed in the developed framework. The “cluster analysis” identifies the location and dimensions of the micro and dominant crack patterns in the domain. Furthermore, for an idealized representation of duplex microstructures, both phases of titanium alloy (i.e., primary α and β) are modeled using random distribution function. The effect of different sources of randomness in microstructures i.e., topology of microstructure, grain size, volume fraction of alpha phase, inhomogeneity in elastic modulus and internal voids, on the fatigue life is investigated. Several simulations are performed on the numerically generated microstructures at various loads. The statistical analysis of the predicted fatigue lives data is performed through normal and Weibull distribution fits. The simulation results highlight that the combined CDM-PFEM based framework is simple and an effective tool to predict the fatigue life scatter and can effectively analyze the mean fatigue life in titanium alloys under cyclic loading conditions.