Beta Transus Temperature Research Articles

Annealing Heat Treatment for Homogenizing the Microstructure and Mechanical Properties of Electron-Beam-Welded Thick Plate of Ti-6Al-4V Alloy

In the application of Ti-6Al-4V to aerospace structural components, when welding thick plates similar of the thickness of the components, microstructure and hardness gradients emerge between the base material (BM) and the joint. This leads to the issue of significant stress concentration in the BM under tensile stress. To address this problem through post-welding heat treatment, this study conducted heat treatments at temperatures both below (mill annealing, MA) and above the beta-transus temperature (beta annealing, BA) on electron-beam weldments of 18 mm thickness Ti-6Al-4V plates. Subsequently, microstructures and hardness were analyzed at different depths from the upper surface and areas (fusion zone (FZ), heat-affected zone (HAZ), and BM), and tensile properties were measured at various depths. The results indicated that α′ observed in FZ and HAZ was resolved through both MA and BA. Particularly after BA, the microstructural gradient that persisted even after MA completely disappeared, resulting in the homogenization of widmanstätten α + β. Consequently, after BA, the hardness gradient in each zone also disappeared, and the tensile strength was higher than in just-welded and MA heat-treated plates.

On the transformation temperatures of Ti-6Al-4V: Effect of oxygen pick-up during Laser Powder Bed Fusion

Ti-6Al-4V (Ti64) parts manufactured by Laser Powder Bed Fusion (L-PBF) exhibit a fine martensitic microstructure and significant residual stresses that are detrimental for their mechanical performance. Consequently, post-processing heat treatments are necessary to relieve these internal stresses and achieve a microstructure adequate for the mechanical requirements. Given the variations in powder compositions and processing parameters, there is a large scattering of physical properties of both as-built and heat-treated parts. In this context, this study quantifies the link between interstitial contents (specifically oxygen and nitrogen) and the transition temperatures of as-built parts, using as-atomized pre-alloyed powders for reference. The martensite decomposition temperature and beta transus temperature are assessed using high-temperature X-Ray diffraction and differential scanning calorimetry for both the powder and as-built parts. It was found that the martensite that composes the as-built microstructure can be decomposed into a stable α+β structure at temperatures exceeding 490 °C. Oxygen and nitrogen pick-up occurs during the L-PBF process, leading to an elevated beta transus temperature in as-built parts relative to the original powder. This insight provides a guide for optimizing post-processing heat treatments for Ti64 as-built components.

Ti-Cu積層造形合金における熱処理による析出形態の制御と機構解明

Intermetallic compounds (IMCs), known for their hardness, play an important role in the precipitation strengthening of alloys. However, precipitation in excessive amounts or as large particles can result in embrittlement of the material. Thus, controlling the distribution of IMCs among the matrix is crucial for the design of strong and ductile alloys. In this research, a supersaturated Ti-Cu alloy formed by Laser Powder Bed Fusion (L-PBF) was heat-treated at different phase regions to produce lamellar, network, and dispersed distributions of Ti2Cu IMC precipitates. To comprehensively understand the formation mechanisms of each IMC distribution structure, in-situ microstructure observation was performed during the heat-treatments. The lamellar IMC structure obtained from heat treatment above the beta-transus temperature, was found to derive from the rejection of Cu atoms towards the boundaries of lamellar alpha grains formed during cooling. The network IMC structure obtained from heat-treatment in the α+β phase region involved the formation of a Cu-rich β-Ti layer between α-Ti grains during heating, and subsequent precipitation of Ti2Cu into an α-encapsulating network upon cooling. Finally, the dispersed structure of IMCs was observed to result from regular nucleation and growth of Ti2Cu during heat treatment in the α+IMC region.

Machine learning-based beta transus temperature prediction for titanium alloys

Beta transus temperature (βtr) is one of the most crucial features of titanium alloys. It is typically used as the index while designing the heat treatment process for titanium alloys. The βtr is also a significant parameter to optimize the processing technology of titanium alloys. Four machine learning algorithms and one empirical formula is developed in this study to estimate the βtr of titanium alloys: Artificial Neural Networks (ANN), Gauss Processing Regression (GPR), Super Vector Machine (SVM), and Ensemble Regression Trees (ERT). According to the correlation coefficient (R), Mean Absolute Error (MAE) and Root Mean Square Error (RMSE) to verify the accuracy of models, and the experimentally measured phase transition temperature of Ti600 alloy was also used to verify the generalization ability of the model. Choosing the best model to analyze the sensitivity of the elements and determine how each component affects the βtr. The result demonstrated that the ANN model has the highest prediction accuracy among the five models, and different model structures have different effects on predicting new data. The ANN model with 10 neurons has the highest prediction accuracy, while the ANN model with 8 neurons has the strongest generalization ability. The results of the sensitivity analysis proved that all the alloy compositions used as input parameters were valid parameters.

Processing and Characterization of a New Quaternary Alloy Ti10Mo8Nb6Zr for Potential Biomedical Applications

The study of new metallic biomaterials for application in bone tissue repair has improved due to the increase in life expectancy and the aging of the world population. Titanium alloys are one of the main groups of biomaterials for these applications, and beta-type titanium alloys are more suitable for long-term bone implants. The objective of this work was to process and characterize a new Ti10Mo8Nb6Zr beta alloy. Alloy processing involves arc melting, heat treatment, and cold forging. The characterization techniques used in this study were X-ray fluorescence spectroscopy, X-ray diffraction, differential scanning calorimetry, optical microscopy, microhardness measurements, and pulse excitation technique. In vitro studies using adipose-derived stem cells (ADSC) were performed to evaluate the cytotoxicity and cell viability after 1, 4, and 7 days. The results showed that the main phase during the processing route was the beta phase. At the end of processing, the alloy showed beta phase, equiaxed grains with an average size of 228.7 µm, and low Young's modulus (83 GPa). In vitro studies revealed non-cytotoxicity and superior cell viability compared to CP Ti. The addition of zirconium led to a decrease in the beta-transus temperature and Young's modulus and improved the biocompatibility of the alloy. Therefore, the Ti10Mo8Nb6Zr alloy is a promising candidate for application in the biomedical field.

Application of machine learning on tool path optimisation and cooling lubricant in induction heating-assisted single point incremental sheet forming of Ti-6Al-4V sheets

Induction heating-assisted single point incremental sheet forming was established for Ti-6Al-4V thin sheets at closed and above beta-transus temperature (980 °C). In order to eliminate geometric inaccuracy and adherence of lubricant on the surface caused by elevated temperature, a cooling lubricant system was designed for the forming tool to decrease the thermal expansion and friction. A radial basis function (RBF)-based tool path optimisation was developed to study the measured geometric accuracy, temperature, and forming force. By adjusting cooling lubricant control and integrating the RBF framework, the first optimised tool path was used to collect the results and to validate with the finite element (FE) model and theoretical geometric profiles. The output data were further studied by RBF and generate a second optimised tool path. The measured geometric coordinates revealed that the error percentage has been reduced to less than 5%. Further, the microstructure evolution analysed by scanning electron microscopy (SEM) indicated noticeable oxidation and alpha-layer for temperature around 1040 °C and the phenomenon was removed at temperature closed to 950 °C. The surface roughness and energy-dispersive X-ray analysis (EDX) revealed the optimised tool path distributed significant improvement in surface quality. The cooling lubricant system indicated optimal performance with RBF optimised tool path to support constant temperature and reduce friction and lubricant adherence on the surface.

A microabrasion wear study of nitrided α-Ti and β-TiNb PVD metallic thin films, pre-deposited onto titanium alloy substrates

In this paper, we study the structure, mechanical properties and abrasion resistance of two triode-plasma nitrided (TPN) titanium alloys, pre-coated with metallic α-Ti or β-TiNb layers deposited by physical vapour deposition (PVD). The chosen alloys were Ti-6Al-4V and Ti-4Al-10V-22Mo, which are predominantly alpha- and beta-phased, respectively – each presenting different challenges, in terms of their nitriding response. Although duplex nitriding + PVD coating is now quite widely employed on steels (and increasingly on titanium alloys) to improve load-bearing capacity and wear resistance, the use of PVD coatings prior to diffusion treatment to control surface roughness – and, in the case of Ti-alloys particularly, to optimise their ‘challenging’ diffusion kinetics has received little or no attention so far. The TPN process enables diffusion treatments to be carried out at 700 °C – which is lower than the beta-transus temperature of Ti (and of most Ti alloys). Optical microscopy, scanning electron microscopy, atomic force microscopy, X-ray diffraction, nanoindentation, and surface profilometry were used to characterize the nitrided and coated/nitrided samples. It was found that the nitrogen concentration and surface hardness for all samples was significantly increased, and that the surface roughness values significantly increased for uncoated Ti alloy substrates after TPN treatment – due to intense, grain orientation-dependent plasma etching effects. However, our novel approach of PVD pre-deposition of a nanostructured metallic coating prior to nitriding reduces surface roughening effects quite dramatically. The micro-abrasion resistance of Ti- and TiNb-coated Ti-6Al-4V substrates was significantly enhanced after nitriding treatment; on the other hand, similar improvements were not found for the metastable-beta Ti-4Al-10V-22Mo substrates – due to the different nitriding behaviour of the pre-deposited PVD surface layers. The “sputter removal” rates for the metallic coatings were high, but all coatings survived in part until the end of the nitriding process.

Extremely high beta-transus temperature of graphene oxide reinforced high-temperature titanium alloy matrix composite

The beta-transus temperatures (Tβ) of graphene oxide (GO) reinforced high-temperature titanium alloy matrix composites were studied by metallography method. With the increase of GO content, Tβ increases linearly when the content of GO is 0–0.17 wt%, and then stays flat when the content is 0.17–0.23 wt%. The Tβ of the composite with only 0.17 wt% GO is up to 1132.5 ± 2.5 °C, which is 80 °C higher than the matrix alloy without GO. After heat treatment, most GO flakes were located at the interface between primary α phase and transformed β phase. The main reason for Tβ increase is that GO hindered atomic diffusion and α/β interface migration.

Mechanical (compressive) form of driving force triggers the phase transformation from β to ω & α’’ phases in metastable β phase-field Ti-5553 alloy

Most of the structural alloys’ applications are under static, dynamic, and cyclic forms of loading. Ti-5553 alloy in the beta phase field is being investigated to confirm the mechanism of deformation and phase transformation upon quasi-static and dynamic compression. The Ti-5553 alloy was heat-treated at 900 °C (almost 50 °C above beta transus temperature) for one hour of soaking time followed by air quenching to achieve a fully β phase field. After that, Dynamic compression (DC) by Split Hopkinson Pressure Bar (SHPB) and Quasi-static compression (QSC) were performed at a strain rate of ̴103/s and 10−3/s, respectively. Recovered specimens were thoroughly examined by using different tools, such as an Optical microscope (OM), Scanning electron microscope (SEM), High-resolution transmission electron microscope (HRTEM), and Electron backscatter diffraction (EBSD) to get the reliable data for justification of logical conclusions. It is found that the dominating mode of deformation was dislocation slip along with twinning ({332} <113>) to some extent in both of QSC and DC, but sliding & spalling of the grain boundary is observed more in the former. Stress-induced phase transformation, i.e., β to α’’ and β to ω, took place in the grains saturated with dislocation slips, where the former transformation occurred simultaneously with {332} <113> twinning, while β to ω transformation was completed when a set of two adjacent (110)ᵦ planes covered ±1/6th of the total separation distance between two (next to each other) (111)ᵦ planes, by equal but opposite shear in (111)ᵦ direction, and it caused 3% shrinkage of two closed packed (110)ᵦ planes after transformation.

Preliminary study of microstructure, mechanical properties and corrosion resistance of antibacterial Ti-15Zr-xCu alloy for dental application

Ti-15Zr-xCu (3 ≤ x ≤ 7, wt.%) novel antibacterial and antibiofilm alloys with competitive mechanical properties, biological responses and corrosion resistance were designed and fabricated. Annealing heat treatment on Ti-15Zr-7Cu (TZC-7A), after holding for 2 h at slightly above their beta transus temperature (BTT) ensured their tensile strength (UTS), yield strength (YS) and hardness (HRV) were improved by 31.2%, 20% and 12.3% respectively compared to the control without Cu, Ti-15Zr (T-15ZA). Although the 3 wt.% Cu alloy displayed the highest elongation (26%), the TZC-7A alloy also possessed a good ductility. Presence of evenly dispersed Ti2Cu and Zr2Cu Cu-rich intermetallic phases formed as interwoven and alternating lamellae within the α + β matrix as a result of Cu addition, as revealed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). These greatly contributed to their strengthening and bactericidal properties. Over 98% antibacterial effect against E. coli and S. aureus have been imparted, coupled with excellent biofilm inhibition. Potentiodynamic polarization curves showed that the TZC-7A alloy possessed higher corrosion resistance than commercially pure titanium, cp-Ti; contact angle test revealed enhanced hydrophilicity; while confocal laser scanning microscopy (CLSM) and cell counting kit (CCK-8) assays also displayed drastically lowered bacterial adhesion rate with comparatively no cytotoxicity. Cell attachment on all alloys was similar but the best spread was obtained on TZC-7A after 24 h. The developed alloy has good potential as an antibacterial implant material with combination of optimized properties.

Influence of post deposition annealing on microstructure and properties of laser additively manufactured titanium copper alloys

While copper is a potent strengthener in titanium alloys, its use in commercial alloys has been severely restricted due to the strong tendency for segregation during solidification, leading to heterogeneous microstructures and what has often been referred to as the “beta-fleck” problem. This problem can be largely obviated by using additive manufacturing (AM) for processing Ti-Cu alloys. This study focuses on AM of a binary Ti-4Cu and a ternary Ti-4Cu-4Al alloy using the laser engineered net shaping (LENS) process. The influence of post-deposition annealing treatments and the subsequent cooling rate on the microstructure and tensile properties of these alloys has been investigated in detail. The phase fraction of the eutectoid alpha + Ti2Cu product is dependent on the cooling rate from above the beta transus temperature. Additionally, the Ti2Cu phase exhibited a far-from equilibrium composition in case of the water-quenched Ti-4Cu-4Al alloy. Both the yield stress (∼550−650 MPa) as well as the ductility (∼15–18 %) were also higher in case of the ternary alloy. The high strengths exhibited by the water-quenched samples of both alloys, while maintaining appreciable tensile ductility, could be attributed to clustering of Cu within the α laths, revealed by atom probe tomography.

Nano-scale characterisation of tri-modal microstructures in TIMETAL® 575

TIMETAL® 575, developed by Titanium Metals Corporation (TIMET), is a high strength forgeable α+β titanium alloy with comparable density, beta transus temperature and processing characteristics to Ti-6Al-4V but with enhanced static and fatigue strength primarily aimed at aero-engine disc or blade applications. Recent research on this alloy has focussed on microstructure evolution as a means to optimise mechanical behaviour and it has been concluded that a solution heat treatment followed by an ageing step yields a resulting “tri-modal” microstructure, consisting of equiaxed primary α and bi-lamellar transformation product containing nano-scale “tertiary alpha” laths, which appear to provide an excellent balance of strength and ductility. The key objective of the work presented here is to characterise this complex nanoscale microstructure in detail at various stages of alloy processing. For that purpose various advanced and recently developed transmission electron microscopy (TEM) techniques have been used. These include alpha and beta phase mapping Precession Electron Diffraction (PED), overall microstructure imaging with conventional BF and DF TEM, distinction of fine phase detail with high angle annular dark field (HAADF) scanning TEM (STEM), and correlation of the nanostructure to the elemental distribution using scanned Electron Energy Loss Spectroscopy (EELS).

Superplastic Behaviour of Ti54M and Ti64

Even though TIMETAL-54M (Ti-5Al-4V-0.6Mo-0.4Fe or Ti54M) has been commercially available for over 10 years, further study of its superplastic properties is still required in order to assess its applicability within the aerospace industry as a potential replacement for other commercial titanium alloys such as Ti-6Al-4V (Ti64). Ti54M is expected to obtain superplastic characteristics at a lower temperature than Ti64 due to its lower beta-transus temperature. The superplastic forming (SPF) capability of alloys that can be formed at lower temperatures has always attracted the interest of industry as it reduces the grain growth and alpha-case formation, leading to longer life for costly high temperature resistant forming tools. In this work, the SPF characteristics of both Ti54M and Ti64 have been examined by conducting tensile tests according to the ASTM E2448 standard within a range of temperatures and strain values at a fixed strain rate of 1 × 10-4/S. A high strain rate sensitivity and uniform deformation at high strains are key indicators in selecting the optimum superplastic temperature. This was observed at 815˚C and 925˚C for Ti54M and Ti64 respectively. The tensile samples were water quenched to freeze their respective microstructure evolution following superplastic deformation and SEM images were captured for grain size and volume fraction of alpha-phase analyses. A slightly higher alpha-grain growth rate was observed during superplastic deformation of Ti64. The initial fine-grain microstructure of Ti54M (~1.6 micron) resulted in a final microstructure with an average grain size of ~3.4 micron and optimum the alpha/beta ratio. Both the fine-grained microstructure and increased amount of beta-volume fraction promotes the superplastic behaviour of Ti54M by grain boundary sliding (GBS). Thus superplastic properties were observed for Ti54M at a lower temperature (~100˚C) than for Ti64.

Microstructural Modeling of the \u03b1\u2009+\u2009\u03b2 Phase in Ti-6Al-4V: A Diffusion-Based Approach

Complex heat treatment operations and advanced manufacturing processes such as laser or electron-beam welding will see the metallic workpiece experience a considerable range of temperatures and heating/cooling rates. These intrinsic conditions will have a significant bearing upon the microstructure of the material, and in turn upon the thermo-mechanical properties. In this work, a diffusion-based approach to model the growth and shrinkage of precipitates in the alpha + beta field of the common titanium alloy Ti-6Al-4V is established. Further, the numerical model is extended using a JMA-type approach to explore the dependency of the beta-transus temperature on extremely high heating rates, whereby dissolution alone is too slow to accurately describe the alpha to beta-phase transformation. Experimental heat treatments at rates of 5, 50, and 500 °C/s were performed, and metallographic analysis of the samples was used to validate the numerical modeling framework predictions for lamellar shrinkage, while data from the literature has been used to evaluate the numerical modeling framework for the growth of equiaxed microstructures. The agreement between measurements and numerical predictions was found to be good.

The Effect of Heat Treatment and the Volume Fraction of the Alpha Phase on the Workability of Ti-5Al-5Mo-5V-3Cr Alloy

Ti-5Al-5Mo-5V-3Cr (Ti-5553) is a metastable beta titanium alloy with high hardenability, good ductility, and high strength. Different microstructures with different volume fractions and distribution of α phase can be produced in this alloy by suitable heat treatment. In this study, the workability of this alloy has been determined as a function of the volume fraction of α phase in the microstructure. Workability is an important parameter indicating the ability of the material to be deformed without the occurrence of fracture. Specimens with three different volume fractions of alpha phase were produced using suitable heat treatments. Two different volume fractions of α (approx 3 and 23%) were obtained by air and furnace cooling, respectively, after solutionizing at 877 °C, i.e., 25 °C above the beta transus temperature. The third specimen with approximately 18% volume fraction of α was obtained by solutionizing at 800 °C for 4 h, followed by water quenching. Workability was investigated by conducting upsetting tests on cylindrical specimens with varying height-to-diameter ratios. Workability limit diagrams have been obtained as a function of axial and hoop strains at fracture. The workability for the furnace cooled specimens was found to be very poor compared to the other two specimens. The results were evaluated using four popular ductile fracture criteria, and the values of critical damage constants at fracture initiation have been calculated using the experimental upsetting test data. The theoretical fracture strains were estimated using the critical constants obtained for all the four ductile fracture criteria and were compared with experimental test data. The Cockroft and Latham, Brozzo et al., and Oh et al. showed close agreement with the experimental values and therefore can be used as ductile fracture criteria for this alloy.

Enhancement of fatigue resistance using the accelerated diffusion/sintering phenomenon near beta transus temperature in Ti-6Al-4V powder metallurgy alloy

Rapid sintering at the beta-transus temperature (1010 °C) of a titanium alloy (Ti-6Al-4V), by exploiting the accelerated self-diffusion, is shown here to yield a high density (~98%), and a refined microstructure, relative to that obtained at a higher sintering temperature in powder-metallurgical processing. While saving energy and cost, this approach is shown to produce a nearly equivalent tensile stress-strain behavior, and, strikingly, a significant improvement in overall SN (stress-life) fatigue resistance, relative to that of the alloys sintered at a relatively higher temperature. This new approach may be broadly applicable to all titanium alloys exhibiting alpha-to-beta phase transformation.

Microstructure Evolution of Ti-5Al-5V-5Mo-3Cr after Hot Deformation at Large and Moderate Strains

This work deals with the analysis and modelling of the microstructural evolution of the metastable titanium alloy Ti-5Al-5V-5Mo-3Cr during hot deformation up to moderate and large strains. Experimental flow curves and deformed samples are obtained by hot compression and hot torsion tests using a Gleeble ® 3800 device. The samples are deformed above and below the beta transus temperature and in a wide range of strain rates. Microstructures are characterized after deformation and in-situ water quenching using light optical and scanning electron microscopy and electron back scattered diffraction (EBSD). Dynamic recovery of the beta phase is found to be the main deformation mechanism up to moderated strains. By increasing the strain, continuous dynamic recrystallization (cDRX) is confirmed by the progressive conversion of low angle boundaries into high-angle boundaries. Alpha phase plays a secondary role in the deformation of the material by pinning the movement of beta high angle grain boundaries (HAGB). The evolution of the microstructure is modelled using dislocation density as internal variable in the single β field.

Determination of Beta-Transus Temperature of Two-Phase Titanium Alloys Using Differential Scanning Calorimetry

The new technique has been developed for measuring the β-transus temperature of (α+β)-titanium alloys with any initial microstructure by means of the DSC method. The technique is based on a linear correlation between extremums of temperature on the DSC time-derivative curve, the Mo-equivalent of an alloy and the equilibrium β-transus temperature. The developed technique enables the determination of the β-transus temperature with the use of a single specimen and it is characterized by high accuracy comparable with that of structural analysis techniques.

The effect of heat treatment on the tensile strength and ductility of pure titanium grade 2

The commercially pure titanium (CP) and titanium alloy are widely used in medical, aerospace and automotive industries due to their attractive mechanical properties especially because of its strength-to-weight ratio that is the highest among all known metal. A thorough understanding of the mechanical properties of titanium is very important so that an optimize machining parameters can be selected. Pure titanium has a higher level of corrosion resistance compared to titanium alloy due to the presence of an oxide layer on its surface. In this paper, heat treatment especially the annealing process was used prior to machining in order to study on the variation of its tensile strength and ductility at two different annealing conditions. The three conditions that were used in this experiment are untreated (T1), heat-treated at 700°C for 1 hour (T2) and 900°C for 1 hour (T3). In this work, a higher level of oxidation was observed for T3 compared to T2 due to the exposure to a temperature that is above the beta transus temperature that triggered the formation of acicular martensite. It was also observed that there was a decrease in the tensile strength when treated at T2 and a recovery of the strength was noticed at T3. The ductility properties were more significant at T2.

A Review of Metastable Beta Titanium Alloys

In this article, we provide a broad and extensive review of beta titanium alloys. Beta titanium alloys are an important class of alloys that have found use in demanding applications such as aircraft structures and engines, and orthopedic and orthodontic implants. Their high strength, good corrosion resistance, excellent biocompatibility, and ease of fabrication provide significant advantages compared to other high performance alloys. The body-centered cubic (bcc) β-phase is metastable at temperatures below the beta transus temperature, providing these alloys with a wide range of microstructures and mechanical properties through processing and heat treatment. One attribute important for biomedical applications is the ability to adjust the modulus of elasticity through alloying and altering phase volume fractions. Furthermore, since these alloys are metastable, they experience stress-induced transformations in response to deformation. The attributes of these alloys make them the subject of many recent studies. In addition, researchers are pursuing development of new metastable and near-beta Ti alloys for advanced applications. In this article, we review several important topics of these alloys including phase stability, development history, thermo-mechanical processing and heat treatment, and stress-induced transformations. In addition, we address recent developments in new alloys, phase stability, superelasticity, and additive manufacturing.