Beta Transus Research Articles

Microstructure evolution during hot deformation of Ti-6Al-4V double cone specimens

Double cone specimens of Ti-6Al-4V alloy with a bimodal starting structure have been heat treated and then compressed at temperatures near to the beta transus (Tβ + 70 K) with strain rates between 0.1 and 5 s-1. The deformed specimens were immediately quenched to conserve the phase distribution. Alpha grains maintain their morphology during deformation in the alpha + beta region up to 0.2 of strain. Dynamic recrystallization of the alpha grains is observed using EBSD technique at strains of 1 for all the strain rates. Dynamic recrystallization of beta phase takes place in samples deformed above the beta transus temperature. There, the beta grains’ shape does not change up to 0.3 of strain. The grade of recrystallization depends on the previous heat treatment, and the recrystallized grain size on the strain rate and the temperature of deformation.

Effects on the Surface Texture, Superplastic Forming, and Fatigue Performance of Titanium 6AL-4V Friction Stir Welds

The speed and feed effects of the friction stir welding (FSW) process on the surface texture along the top of a butt welded nugget were studied. The tests were conducted using fine grain (0.8-2 μm) titanium alloy 6Al-4V with a nominal thickness of 2.5 mm. It was shown that the pin tool marks along the top surface of the weld can be highly detrimental to both the superplastic forming (SPF) characteristics and the fatigue performance of welded panels. Removing the marks by machining the top surface after FSW was found to eliminate the predominant tearing of the weld during SPF and most of the fatigue life of across the weld was also restored. Through additional development of the FSW process parameters, the butt welded nugget was made to have equivalent SPF characteristics as the parent sheet material. By using a water-cooled pin tool and other cooling techniques, it is believed that the weld zone can be kept below the beta transus temperature during FSW, which enables the formation of a grain structure that is uniquely conducive to superplastic behavior, when compared to conventional fusion welding processes.

Investigation of microstructure, surface and subsurface characteristics in titanium alloy friction stir welds of varied thicknesses

Friction stir welding of titanium alloy (Ti–6Al–4V) was demonstrated on 3, 6, 9 and 12 mm thickness square groove butt joints. Complete microstructural and microhardness evaluations were conducted in addition to surface and subsurface examinations for each case. The 3 mm welds exhibited an extremely fine grained microstructure with evidence of processing temperatures below the beta transus temperature of the alloy. The 6, 9 and 12 mm samples possessed larger grains formed by a slower cooling rate from above the beta transus temperatures. The thick section weld exhibited a nearly uniform microhardness, while the thinner welds showed a slight, 6%, increase in hardness compared with the parent material.

Ti-Fe-Cr-Al 4 元系合金のミクロ組織と機械的性質に及ぼす鉄添加量と冷却速度の影響

Titanium and its alloys first found applications in the fields of aerospace and chemical engineering. Recently, titanium alloys have been used in consumer products due to their high specific strengths and corrosion resistances. They have also been employed in medical applications because of their excellent biocompatibility. Unfortunately, the principal alloying elements of titanium alloys are very expensive due to their low crustal abundances; this is especially true of the beta-stabilizing elements (e.g., V, Mo, Nb, and Ta). Although titanium is considered to be a ubiquitous element since it has the tenth highest Clarke number of all the elements, it is actually classified as a rare metal because its current refinement process is more environmentally damaging than the processes used to refine iron or aluminum. By contrast, iron and chromium are very attractive as beta stabilizers for titanium alloys since iron is highly ubiquitous and chromium is relatively inexpensive and abundant, while aluminum is very attractive as an alpha stabilizer. It is thus very important to investigate the properties of Ti-Fe-Cr-Al system alloys. In this study, the effects of iron content and the cooling rate after solution treatment on the tensile properties of Ti-3.2 to 5.1 mass%Fe-7.1 mass%Cr-3.0 mass%Al alloys were investigated. This investigation was done by measuring their electrical resistivities and Vickers hardnesses, by performing X-ray diffraction and optical microstructure investigations, and by conducting tensile testing. It was found that the beta phase could only be identified in solution-treated and quenched (STQed) alloys. For alloys in the STQed state, the resistivity ratio decreased with an increase in iron content, whereas the Vickers hardness exhibited the opposite trend. From the results of isochronal heat treatment, the beta transus was estimated to be 1093 K in 3.2 and 3.5Fe alloys and 1073 K in 3.9Fe to 5.1Fe alloys. Alloys in the STQed state have almost the same tensile strength, elongation, and area reduction. The tensile strength, elongation, and area reduction in all the alloys cooled in air were almost the same as those of the STQed alloys. Employing furnace cooling increased the tensile strength of the alloys, but reduced their elongation and area reduction. For the air-cooled alloys, the tensile properties of the Ti-3.2 to 5.1 mass%Fe-7.1Cr-3.0Al alloys were almost the same as those of the STQed alloys.

Nature of growth of dual titanium boride layers with nanostructured titanium boride whiskers on the surface of titanium

The nature of growth of hard surface layers consisting of a titanium diboride (TiB 2 ) layer and a titanium boride (TiB) whisker layer on titanium, and the kinetics have been critically examined. High density of nanostructured TiB whiskers have been found to form in the sub-layer in most conditions. A very surprising and unusual result is the kinetics of growth of TiB whiskers at temperatures within a few degrees of the beta transus – the whiskers grew denser and to depths larger than that at any other temperature investigated.

Possibilities and limits in thermohydrogen processing of beta titanium alloy Timetal®10-2-3

Abstract The microstructure of the metastable -titanium alloy Timetal®10-2-3 was changed systematically by means of thermohydrogen processing, in order to improve the mechanical properties. The thermodynamic and kinetic data-base was determined experimentally and the change in the beta transus temperature was determined quantitatively. The results were used to design a 4-step heat treatment process involving hydrogen during the first two steps. A comparison of the fatigue properties of the standard heat-treated and the thermohydrogen-processed alloy revealed that, despite the increase of the monotonic strength of the thermohydrogen-treated alloy, the fatigue limit is slightly reduced, while the fatigue crack growth threshold remains almost unaffected. From this correlation a strategy can be deduced for a further improvement of the thermohydrogen-processing procedure, in order to tailor the fatigue properties according to the requirements of the respective application.

Improvement of pseudoelasticity and ductility of Beta III titanium alloy--application to orthodontic wires

The pseudoelasticity of metastable Beta III titanium alloy (TMAtrade mark) used for orthodontic applications is obtained by cold wiredrawing. This wire has higher rigidity than cold-drawn NiTi (Nitinoltrade mark, superelastic NiTi SE) and lower recoverable deformation. The low ductility value of Beta III is due to the deformation imposed by wiredrawing. The aim of this research was to improve the behaviour of this alloy by modifying the microstructural parameters to decrease the rigidity and increase the recoverable deformation and ductility of the alloy. The effects of second phase precipitate, grain size, and deformation on the wire mechanical properties were also examined. The isothermal precipitation of alpha (alpha) or omega (omega(isoth)) phases precludes the expression of the pseudoelastic effect. The presence of an omega(isoth) phase considerably increases fracture strength, whereas the alpha phase strongly decreases the ductility and adversely affects the strain recovery (epsilon(r)). To control the grain size, the growth of the recrystallized grains was studied by considering several parameters, which are known to have an influence on grain size, including the cold rolled strain, the temperature, the time of annealing, and the initial grain size. A structure with coarse grains, quenched from a temperature higher than the beta transus (T(beta)), associated with a plastic pre-deformation, contributed to an improved pseudoelastic behaviour, due to the presence of a reversible martensite phase (alpha'') induced by the pre-deformation.

Effect of boron on the beta transus of Ti–6Al–4V alloy

Abstract The influence of boron on the β transus of Ti–6Al–4V is established in a powder metallurgy Ti–6Al–4V–1.7B alloy using microstructural analysis, electrical resistivity, and dilatometry. An increase in the β transus by ∼60 °C was observed, which is attributed to the equilibrium as well as to supersaturated B in the solid solution.

Influence of the microstructure on the pseudoelasticity of a metastable beta titanium alloy

The pseudoelasticity of metastable Beta III titanium alloy, used for orthodontic applications, is obtained by cold wiredrawing. The role of the microstructure and the conditions of deformation on the stability of the β phase are studied in order to increase the strain recovery and to decrease the apparent modulus. The effects of a precipitate second phase, the grain size and the predeformation will be successively examined. The isothermal precipitation of α or ω phase does not improve the pseudoelastic effect. However, a structure with coarse grains, quenched from a temperature higher than the beta transus, associated with a uniaxial predeformation, increases the pseudoelasticity. This behavior is due to the variant reorientation of the martensite induced by the predeformation.

Flow Stress Behavior and Deformation Characteristics of Ti-22Al-25Nb Alloys at Elevated Temperature

The characteristics of deformation of an orthorhombic phase based alloy, Ti-22Al-25Nb (at%), have been studied by hot compression tests in the temperature range of 940-1150°C with the strain rates of 0.01s-1 and 0.1s-1. A flow curves typically controlled by dynamic recovery were observed in the temperature range of 1090-1150 °C for the strain rate of 0.01s-1 , while discontinuous yield phenomena was found for the strain rate of 0.1s-1. The dynamic recovery can be identified by the microstructure characteristics of the deformation specimens. At 1060°C, the temperature of beta transus, the flow curves and microstructure exhibited the same deformation charateristics as that above the beta transus. At the subtransus temperature, a long period of flow softening followed by the steady-state flow can be observed. During the hot deformation, the hard phases α2 and O elongated, subboundary produced followed by cusp formation in the elongated α2 and O phases, the equiaxed morphology phases occurred by B2 phase penetrating along the subboundaries. The softening mechanism was discussed.

An investigation of a new near-beta forging process for titanium alloys and its application in aviation components

In this paper, the authors present a new near-beta forging process, in which materials are heated at about 15 °C below the beta transus, to improve the combined properties of titanium alloys. Materials processed by the near-beta forging process, followed by rapid water-cooling, then high temperature toughening and low temperature strengthening heat treatments, produce a new kind of microstructure for titanium alloys. This new tri-modal microstructure consists of about 15% equiaxed alpha, 50–60% lamellar alpha and transformed beta matrix. Materials with tri-modal microstructure show a high low cycle fatigue property, high creep-fatigue interaction life, high fracture toughness and a high service temperature without decreasing ductility and thermal stability. The experimental fundamentals of the new process and the strengthening and toughening mechanism have been discussed. A critical issue in the practical application of the near-beta forging process is the control of temperature. A new metallographic inspection method was proposed to solve this problem. This new near-beta forging process has been reliably applied to produce several aeroengine compressor disks, rotators, and other airplane components.

Modelling beta transus temperature of titanium alloys using artificial neural network

An artificial neural network (ANN) model is developed to simulate the non-linear relationship between the beta transus (βtr) temperature of titanium alloys and the alloy chemistry. The input parameters to the model consist of the concentration of nine elements, i.e. Al, Cr, Fe, Mo, Sn, Si, V, Zr and O, whereas the model output is the βtr temperature. Good performance of the ANN model was achieved. The interactions between the alloying elements were estimated based on the obtained ANN model. The results showed good agreement with experimental data. The influence of the database scale on ANN model performance was also discussed. Estimation of βtr temperature through thermodynamic calculation was carried out as a comparison.

Microstructural evolution in laser-deposited multilayer Ti-6Al-4V builds: Part II. Thermal modeling

The thermal history developed in laser metal deposition (LMD) processes has been shown to be quite complex and results in the evolution of an equally complex microstructure. A companion article (Part I. Microstructural Characterization) discussed the LMD of Ti-6Al-4V, where the resultant microstructure consists of a periodic, scale-graded layer of basketweave Widmanstatten alpha and a banding that consists of colony Widmanstatten alpha. In order to understand the microstructural evolution in Ti-6Al-4V, a numerical thermal model based on the implicit finite-difference technique was developed to model LMD processes. The effect of different laser-scan velocities on the characteristics of the thermal history was investigated using an eight-layer single-line build. As the laser-scan speed decreases and the position within a layer increases, the peak temperature increases. The heating rate and the peak thermal gradient within a deposited layer were shown to follow the same trend as the peak temperature after two layers were deposited on top of the substrate. In general, the laser-scan speed or z-position within a layer did not have a significant effect on the cooling rate. The cooling rate in a newly deposited layer decreases as the number of layer additions increases. Given the predicted temperature vs time profile from the thermal model, the evolution of phase transformations occurring in the deposit is mapped as each layer is deposited. As a result of the thermal cycling imposed by the periodic deposition of material, a characteristic layer, consisting of two regions heated above and below the beta transus, forms in layer n due to the deposition of layer n+1.

Influence of microstructural changes on corrosion behaviour of thermally aged Ti‐6Al‐7Nb alloy

AbstractSolution treatment and ageing (STA) is an effective strengthening method for α + β titanium alloys. This paper reports the effect of solution treatment and aging on the corrosion behaviour of Ti‐6Al‐7Nb alloy in a simulated body fluid (Ringer's solution). Ti‐6Al‐7Nb alloy is hot rolled in the α + β field and subjected to solution treatment above and below its beta transus temperature (1283 K). The solution treated specimens are water quenched (WQ), air‐cooled (AC), and furnace cooled (FC) at three different rates, and subsequently aged at 823 K for 4 h. Microstructural changes were examined using optical microscopy and phases developed were analyzed using XRD. The influence of microstructure on the corrosion performance of the alloys are discussed in detail based on the Open Circuit Potential (OCP), passive current density and area of repassivation loop values obtained from the cyclic polarization study in Ringer's solution. The passive current density was low (0.5 μA/cm2) for the specimen with duplex microstructure obtained for specimen solution treated at 1223 K, air‐cooled, and aged, in comparison with that for as‐rolled specimen (1.5 μA/cm2). The corrosion aspects resulting from various heat treatments are discussed in detail.

β型チタン合金における内部起点型疲労破壊とそれに関連する挙動(J10-1 チタン系材料の力学・強度特性,J10 チタン系材料の力学・強度特性と応用技術)

Rotaing bending and axial fatigue tests have been performed using solution treated and aged materials in two beta titanium alloys, Ti-15Mo-5Zr-3Al and Ti-22V-4Al alloy. Subsurface crack initiation was found to be a microstrructure and a stress ratio related phenomenon. In the former alloy, cracks were initiated internally only in a material that was solution treated above the beta transus and at a certain stress ratio in relatively short life regime. The effects of grain size and mean stress were not characterized entirely by the Hall-Petch relationship and the Goodman relationship, respectively, which was attributed to different crack initiation mechanisms. In the latter alloy, subsurface crack initiation was also seen in the long life regime around or above 10^7 cycles, resulting in a step-wise S-N curve. A smooth facet invariably existed at the crack initiation site. The occurrence of this mode of failure is discussed based on microstructural and mechanical considerations.

Effect of texture and slip mode on the anisotropy of plastic flow and flow softening during hot working of Ti-6Al-4V

The effect of crystallographic texture and slip mode on the plastic flow of Ti-6Al-4V with either a colony- or globular-alpha microstructure was determined by conducting isothermal, constant-strainrate, hot-compression tests on specimens cut at various orientations (rolling direction (RD), transverse direction (TD), 45 deg, and normal) from hot-rolled plate. Testing was performed using a fixed strain rate (0.1 s−1) and various temperatures below the beta transus. The flow curves from all of the experiments exhibited a peak flow stress followed by a large and a small amount of flow softening for the colony and globular microstructures, respectively. Although the flow softening response did not depend noticeably on test direction for a given microstructure and test temperature, the peak flow stress and development of sample ovality did. This orientation dependence was interpreted using both lower-bound (isostress-type) and upper-bound (isostrain, Taylor/Bishop-Hill) models to deduce the operative slip systems in the alpha phase. These analyses suggested that prism 〈a〉 and basal 〈a〉 slip are considerably easier than pyramidal 〈c+a〉 or 〈a〉 slip at hot-working temperatures. A comparison of the flow curves for the colony and globular alpha microstructures suggested that slip transfer across alpha/beta interfaces and loss of Hall-Petch boundary strengthening can account for a substantial portion of the flow softening observed during hot working.

Plastic flow and microstructure evolution during thermomechanical processing of laser-deposited Ti-6Al-4V preforms

Plastic flow behavior and microstructure evolution during hot working and heat treatment of Ti-6Al-4V synthesized via a laser-deposition, Laser Engineered Net Shaping (LENSℳ), process were established. To this end, isothermal, hot compression tests were conducted on samples in either a deposited + stress relieved condition or a deposited + hot isostatically pressed (hipped) condition. The starting microstructures consisted of columnar grains with fine or coarse Widmanstatten (basketweave) alpha platelets. At subtransus temperatures, the flow curves of both microstructural conditions exhibited a peak stress at low strains followed by extensive flow softening; these curves were almost identical to previous measurements on ingot-metallurgy (IM) Ti-6Al-4V with similar transformed microstructures. In addition, the kinetics of globularization of the alpha phase during subtransus deformation or subsequent static heat treatment were found to be the same as for IM Ti-6Al-4V with comparable alpha-platelet thicknesses. During supertransus heat treatment, moderately fine beta-grain microstructures were developed in samples that had been predeformed below the beta transus. Such a heat treatment for samples previously deformed above the transus gave rise to a nonuniform distribution of coarse beta grains, an effect attributed to critical grain growth.

Superplastic roll forming of Ti alloys

The high cost of aerospace alloys and their components makes them prime candidates for net-shape manufacturing techniques. Conventional processes for manufacturing disk components include hammer, hot die, and isothermal forging. This paper will examine the potential of a revolutionary approach for the manufacture of aircraft engine disks, superplastic roll forming. The process of superplastic roll forming, developed at the Institute for Metals Superplasticity Problems, Ufa, employs pairs of small opposed rollers to shape a cylindrical workpiece into a complex axisymmetric shape by simultaneously adjusting the roll gap and by moving the rolls radially outward on the workpiece while it is rotated about its axis of symmetry. Both the workpiece and the rolls are maintained at temperatures close to the beta transus. This paper will describe metallurgical evaluations of superplastically roll formed disks of alloy VT25. The evaluations of the disks included microstructure, crystallographic texture, heat treatment response, tensile strength, stress rupture resistance, and ultrasonic characteristics. The disk microstructures were found to be uniform and without any strongly textured colonies. Mechanical properties of the roll formed VT 25 were compared with those of Ti-6242S, IMI834, and conventionally forged VT25. The RF VT25 disk was found to possess low ultrasonic noise and high inspectability, which provided an increase in signal to noise for synthetic flaws.

Effect of initial microstructure on plastic flow and dynamic globularization during hot working of Ti-6Al-4V

Plastic flow behavior and globularization kinetics during subtransus hot working were determined for Ti-6Al-4V with three different transformed beta microstructures. These conditions consisted of fine lamellar colonies, a mixture of coarse colonies and acicular alpha, and acicular alpha. Isothermal hot compression tests were performed on cylindrical samples at subtransus temperatures and strain rates typical of ingot breakdown (i.e., T∼815 °C to 955 °C, \(\bar \varepsilon \)∼0.1 s−1). For all three material conditions, true stress-true strain curves exhibited a peak stress followed by noticeable flow softening; the values of peak stress and flow softening rate showed little dependence on starting microstructure. On the other hand, the kinetics of dynamic globularization varied noticeably with microstructure. By and large, the globularization rate under a given set of deformation conditions was most rapid for the fine acicular microstructure and least rapid for the mixed coarse-colony/acicular structure. At temperatures close to the beta transus, however, the difference in globularization rates for the three microstructures was less, an effect attributed to the rapid (continuous) coarsening of the laths in the acicular microstructure during preheating prior to hot working. The absence of a correlation between the globularization kinetics and the observed flow softening at low strains suggested platelet/lath bending and kinking as the primary deformation mechanism that controls the shape of the flow curves.

Fatigue life prediction techniques for notch geometries in titanium alloys

This paper focuses on the α+β titanium alloy, Ti–6Al–4V, in a range of microstructural conditions from fine grained alpha and retained beta (annealed) through bimodal morphologies consisting of primary alpha and transformed beta to coarse fully transformed structures produced by heat treatment above the beta transus. Material was taken from several processing routes including cross rolled plate, hot rolled round bar stock and small disc forgings. The strain control LCF response measured from plain testpieces is related to data obtained on a variety of notch geometries with K t factors in the range 1.4 to 4.7. A correlation technique based on the parameter K tΔ σ for notches and Δ σ for plain specimens with Δ σ the stress range defined at the ‘stabilised’ or ‘saturated’ condition is used to relate the data sets. Notch behaviour that is not consistent with this correlation is related to the microstructural condition, the cyclic stress–strain behaviour of that condition and the presence of `fatigue sensitive' features.