Two-phase Titanium Alloy Research Articles

Low-frequency internal friction of α+β titanium alloy SP-700

Abstract Internal friction of a commercial α+β two-phase titanium alloy SP-700 has been measured at frequencies around 1 Hz in the temperature range from 90 to 400 K to characterize the damping capacity of the material. Two components of internal friction have been observed for specimens quenched from temperatures around the α/β transus (1173 K): a well-defined peak located at 120 K and a broader one around 200 K. The magnitude of the former peak is increased by hydrogen charging, and the activation energy is close to that of the diffusion of hydrogen in β-titanium. The peak is thus attributed to stress-induced redistribution of hydrogen atoms in β phase. The component at 200 K appears when the alloy has been quenched from temperatures around the transus but does not when cooled slowly. This component is due either to defects in α″ martensite produced by quenching or to interfaces between α″ and the β phases. The damping capacity can be as high as 10−2 at low temperatures but is much lower at room temperature.

Macroscopic and microscopic subdivison of a cold–rolled aluminium single crystal of cubic orientation

An aluminium single crystal of cube orientation has been rolled to 15, 30 and 50% reductions under controlled homogeneous rolling conditions. The deformation structure of the rolled specimens was investigated by both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) over several scales of magnification. The local crystallographic orientations have been measured by an automatic electron back scattering patterns (EBSP) technique and a semiautomatic TEM method. Orientation image maps based on the local orientation data have been used to reveal the evolution of the deformation structure during rolling. It is observed that by an opposite rotation around transverse direction (TD) the crystal was subdivided into four macroscopic bands, termed matrix bands in the present paper, which are parallel to the rolling plane. Between the four bands there are three transition bands in which the orientation changes continuously from that of a matrix band to that of the adjoining one. A model based on the idea of location–dependent shear strain caused by geometric and friction effects together with a plasticity analysis has been used to explain the macroscopic subdivision of the crystal. In addition to the macroscopic subdivision, a microscopic subdivision by the formation of cell–blocks within the matrix bands and a cell structure within transition bands has also been observed. A difference related to shear amplitude difference between the active slip systems changing continuously across the crystal has been observed. Both the macroscopic orientation of the dislocation boundaries and the misorientation angles and axes across dislocation boundaries are analysed and it is found that Frank9s formula is a useful tool in analysing the dislocation boundaries formed during deformation.

Influence of processing on microstructure and mechanical properties of ( α+β) titanium alloys

The present paper tries to summarize the relationship between processing, microstructure, and mechanical properties of two-phase ( α+β) titanium alloys. Although for most structural applications of titanium alloys a variety of important mechanical properties (yield stress, ductility, HCF, LCF, da/dN of micro- and macrocracks, K IC, and creep) have to be optimized or balanced, and although both processing as well as microstructure contain many variables, it can be shown that from the numerous correlation possibilities only a few underlying basic principles are really important. One of them is the relationship between cooling rate, colony size, and slip length leading directly to the advantages of a bi-modal (duplex) type of microstructure usable for most applications and involving a reproducible and insensitive processing route.

The effect of microstructure on the mechanical properties of two-phase titanium alloys

This article presents the results of investigations of microstructure and mechanical properties of two-phase α+β titanium alloys with different volume fraction of the β-phase. Microstructure of the specimens was examined using an optical microscope. Fracture surfaces were observed by SEM technique. The influence of the microstructure and phase composition on the mechanical properties of the alloys was studied. Static tensile tests, hardness tests and fatigue investigations were performed. It was noticed that the volume fraction and chemical composition of the β-phase has a significant effect on mechanical properties and cracking process during fatigue.

Effect of forging conditions and annealing temperature on fatigue strength of two-phase titanium alloys

This paper reports on the results of studies on the influence of deformation degree and temperature in the die forging process and annealing temperature on fatigue strength of forgings made of two-phase, martensitic titanium alloys (Ti–6Al–4V and Ti–6Al–2Mo–2Cr). Dilatometric and metallographic studies have been carried out along with fatigue tests. The influence of phase composition and microstructure obtained after cooling at different rates on fatigue strength has been determined.

The influence of simple and complex loading on structure changes in two-phase titanium alloy

Titanium alloys are widely used in modern engineering. The ensuring of property of articles and designs during their service is one of the most important problems. It is known that mechanical properties of metals and alloys are determined by their microstructure, formed in a process of thermomechanical treatment according to a history of strain and conditions of a thermal effect. The processing of the required properties of an alloy is an important and difficult task. Its decision assumes comprehensive study of the alloy strain history and determination of an effect of a complex loading that takes place almost in any real technological process. However, as a rule, investigations devoted to this question consider only results of uniaxial loading. This paper presents the results of experimental studies of the influence of a deformation path on changes in globular and lamellar structures of the two-phase titanium alloy Ti-6.5Al-3.5Mo-1.6Zr-0.27Si.

Phase transformation and heat treatment of titanium (? + ?) alloys

The variety of phase and structural transformations occurring in two-phase (α + β) titanium alloys in heat and deformation treatments yield articles with different combinations of mechanical properties. However, specific heat treatment regimes are often prescribed on the basis of empirical regularities based on practical experience rather than on data of a comprehensive analysis of phase and structural transformations. As a result, to apply the prescribed regimes of treatment to new alloys a great number of tests have to be conducted, which is laborious and material-consuming. This work is an attempt to substantiate scientifically the principles for prescribing heat treatment regimes proceeding from the regularities of the processes of decomposition of metastable phases in high-alloyed titanium alloys.

Analysis of volume effects of phase transformation in titanium alloys

Some results of studies have been presented, concerning possibility of computer technology application in the prediction of transformation temperatures, lattice parameters and volume effects of transformations in two-phase titanium alloy heated at various rates. To do so, a methodology has been introduced, based upon a mathematical description of a physical model of a diffusional phase transformation. To estimate the changes of lattice parameters Vegard's rule have been applied that allows the phase lattice parameters to be determined out of its chemical composition. Furthermore, it has been assumed that the external volume effects of transformations are the resultant of changes of the alloy phase composition, the chemical composition of the component phases and their specific volummes. The calculated results have been experimentally verified in high temperature x-ray diffraction studies, dilatometric tests and microanalytical observations. Good agreement of the calculations with the experimental data has been proved.

Fatigue strength and cyclic crack resistance of titanium alloy VT3-1 in different structural states. Communication 2. Procedure for considering the effect of structure on fatigue limit

On the basis of experimental results presented in communication 1 for a study of the fatigue strength of two-phase titanium alloy VT3-1 in different structural states it is shown that the fatigue limit for the alloy increases with a reduction in the average size of a structural element critical for cyclic strength. A numerical model is developed by means of linear fracture mechanics approaches which makes it possible from available data about the size of structural parameters for the alloy to predict the fatigue limit of smooth specimens.

Calculation of Superplastic Temperatures in Two-Phase Titanium Alloys

Calculation of Superplastic Temperatures in Two-Phase Titanium Alloys

Microstructural interactions during flow of two-phase titanium alloys

To describe the stress-strain behaviour of two-phase alloys in many cases a very simple law, the so-called rule of mixture, has been employed: σ = Σσ iƒ i and ε = Σε iƒ i where i is a phase index and ƒ the volume fraction. The paper gives a short review of applications and adaptation of this rule to experimental observations on titanium alloys. The problem with this approach and its modifications is that it does not consider adequately the great variety of possible microstructural interaction mechanisms, which depend on factors such as phase morphology, crystallographic relationship between different phases, work hardening characteristics etc. The importance of these aspects for multiphase behaviour will be demonstrated by two examples, namely: strength and flow in a system with two ductile components; and flow and work hardening of a ductile matrix with hard lamellar inclusions. This view shows that for an adequate description of a given material an individual adaptation to the particular ruling mechanisms, which vary from case to case, is necessary.

Structure and mechancal properties of titanium alloy VT35 in the cast condition

In a large ingot (1 ton) of alloy VT35 the distribution of chemical elements both over the cross section and over the height is uniform, the macrostructure of the ingot is similar to the structure of other two-phase titanium alloys, and the microstructure is almost the same in different parts of an ingot. The mechanical properties in different parts of an ingot are the same, i.e., the strength is low and the ductility is significant. All of this points to the high promising cast properties of alloy VT35 and also that there should not be any complicated technical problems connected with chemical inhomogeneity.

Overview of titanium dioxide industry

In this paper, a set of physically based constitutive model coupling microstructure evolution was developed for unified prediction of flow stress and globularization evolution during hot working of two-phase titanium alloys with initial lamellar microstructure. The dislocation density variation, dynamic globularization and effect of Hall–Petch strengthening were considered in the microstructure model. The dynamic globularization was modeled by two parts: critical strain for initiation of globularization and globularization rate, both of which are function of processing conditions (temperature and strain rate); In the modeling of Hall–Petch strengthening, the dependence of Hall–Petch coefficient on processing conditions were considered and the loss of Hall–Petch strengthening with deformation process was molded. The microstructure model was implemented to a physically based constitutive model, composed of a thermally activated stress and an athermal stress, to realize the unified prediction of flow stress and globularization evolution. The material parameters were determined by calibration using the experimental flow stress and globularization fraction in isothermal compression tests through the genetic algorithm based optimization method. Based on the set of constitutive models, flow stress and globularization evolution of Ti–6Al–4V and TA15 alloys at different temperatures and strain rates were predicted. Good agreements between the experimental and computed results were obtained.

New constitutive equation for two-phase superplastic titanium alloys

In two-phase α + β titanium alloys, the phase ration and consequently the phase sizes vary with temperature. The flow stress of this type of alloy is not only affected by the thermal contribution of temperature but also by changes in the phase ratio and sizes. This prompted us to develop a constitutive equation in a superplastic titanium alloy (Ti6.3Al2.7Mo1.7Zr), incorporating volume fraction and phase sizes. The phase ratio was altered at constant temperature by means of charging with hydrogen. The constants of the constitutive equation were evaluated. Flow stresses were then predicted at the optimum superplastic temperature of 1073 K and strain rates of 1.9 × 10 −3 s −1 and 7.1 × 10 −4 s −1 for various β volume fractions. The predicted flow stresses were found to correlate fairly well with the experimentally determined values.

Correlation of grain-structure parameters with the properties of thermally strengthened titanium alloys VT6 and VT23

1. The original grain size of two-phase titanium alloys VT6 and VT23 after high-speed heating, quenching, and aging has almost no effect on the ultimate breaking strength of these alloys, but it changes their ductility characteristics to a significant degree. 2. The dependence of mechanical properties for thermally strengthened alloys VT6 and VT23 on original β-grain size may be described by empirical equations which are the well-known Petch-Hall expression only under conditions of constancy for values of ductility characteristics (δ and ϕ).

Effect of high-temperature deformation on the texture of a two-phase titanium alloy

Changes in the texture of both the α- and β-phases of the two-phase alloy Ti-6AI-4V have been determined in order to clarify the mechanisms of high-temperature deformation. A strain of ∼ 1.5 was applied to the alloy at 928 °C, at strain rates representative of superplastic and nonsuperplastic conditions. The α-phase texture changed very little with strain rate whereas that for the β-phase was much more sensitive. The β-phase texture was weakened at superplastic strain rates but developed a fibre texture at non-superplastic rates. It is postulated that under superplastic conditions the alloy deforms predominantly by grain boundary sliding of the soft β-phase grains, with the hard primary α-phase grains remaining in their original orientations and the measured loss in texture intensity being attributed to the loss in texture of the secondary α-phase only. Under non-superplastic conditions there is a greater contribution from plastic deformation in the β-phase which, in turn, can enhance the secondary α-phase texture.

Structure ortimization of two-phase titanium alloy by way of hydrostatic extrusion

Abstract Two-phase titanium alloy was deformed up to 50% by hydrostatic pressure. Deformation combined with various schemes of heat treatment sufficiently changes morphology and internal structure of all constitutent alloy phases. Structural changes cause improvement of mechanical properties.

Isothermal particle growth in two-phase titanium alloys

Isothermal particle growth studies were carried out on several Ti−Mn and T−V alloys consisting of varying amounts of α and β phases at 973 K. It was found that the particle growth kinetics of the α and the β phases could be represented by simple equations in terms of time and volume percents of the phases. The growth process is presented as a two-way diffusion process where the solvent and the solute atoms move in opposite directions resulting in conversion of α to β and β to α leading to the growth of the particles. The Lifshitz-Wagner-Slyozov theory, which considers only solute diffusion for growth of particles, is slightly modified to incorporate the diffusion of both solute and solvent into the growth equation. The observation of the growth kinetics indicated that, under identical conditions, the growth of α particles in a β matrix was faster than the growth of β particles in an α matrix. Furthermore, for identical conditions, the growth kinetics of Ti−Mn alloys are faster than those of the Ti−V alloys. While the faster kinetics are consistent with higher bulk interdiffusivities of the Ti−Mn system, the magnitude of the difference is much smaller than that expected from the bulk interdiffusivity considerations. In addition, it was found that the growth exponent,n, values are in between the theoretically predictedn values for grain boundary diffusion control and bulk diffusion control. Based on these observations, it is suggested that a mixed mode diffusion mechanism consisting of bulk diffusion as well as grain boundary diffusion is controlling the growth process at 973 K. Details of the investigation and various growth models for α-β titanium alloys are presented.

Effects of volume fraction and grain size on creep characteristics of .ALPHA./.BETA. titanium alloys.

Effects of volume fraction of α phase and grain size on the creep properties of a series of two-phase titanium alloys were examined. The chemical compositions of α and β phases of the alloys were designed to be constant.The alloys were quenched after solution-treatments for 1, 100, and 1 000 h at 1 173 K, followed by aging for 4 h at 773 K. The volume fraction of α phase (VFAP) was varied from 17 to 73%. Creep rupture test was carried out at 392 MPa and 773 K in air.The creep rupture life was the longest at 50 % of VFAP, and the shortest at the minimum VFAP when the specimens were solution-treated for 1 and 100 h. It increased with VFAP when solution-treated for 1 000 h. The steady state creep rates decreased with the increase in VFAP. This might be due to the higher creep resistance of the α phase. Irrespective of the wide range of the average grain sizes, the steady state creep rates become insensitive of grain size as VFAP decreased. This would be attributed to grain boundaries of fine α precipitates generated in the prior β phase. The nucleation of voids occurred in most cases at the α/β and α/α interfaces, and within α phase. The void growth rate decreased with the volume-fraction-ratio of β and α phases and increased with the average transverse grain size of α phase.

On the improvement in fatigue resistance of Ti-6Al-4V castings

The fatigue resistance of two-phase titanium alloys like Ti -6AI-4V is known to critically depend upon microstructurat parameters such as Widmanst~tten colony sizes, their relative orientation and the size of individual a/~ platelets [t ]. This improvement in fatigue fracture resistance with fine microstructure is established in the forged products [2]. Recently widespread interest has been evinced in the use of net-shaped titanium alloy castings for aerospace and other applications. The castings under normal freezing conditions have a coarse microstructure with targe a/~ p/atelets which can lead to poor fatigue properties. One of the viable routes to refine this structure without any mechanical working is by faster cooling [2-4]. However, the use of ceramic moulds impose an upper limit on the achievable cooling rates. Hydrogen dissolved in titanium brings down the /3-transus temperature. This beneficial effect has been used to improve the superplastic formability of titanium alloys [5], In this paper, we report on the use of h}~drogen to ref'me the as-cast microstructure and thereby improve the fatigue life of near net-shaped castings made of these alloys. Ti-5.9A1-3.8V pancakes were melted and cast by a consumable arc melting process, The alloy was melted four times to obtain homogeneity. Oxygen content in the alloy was 80 to 100 ppm. These alloys were made to simulate the cast T i -6AI-4V alloy. However, on account of solidification in contact with the water-cooled copper mould, the alloy displayed a much finer mdcrostructure than if it were to be cast in investment ceramic moulds, The hydrogen treatment described betow was carried out to examine if this microstructure could be refined further. Cut pieces of the pancake were hydrogenated in a tubular furnace fitted with on inconel tube at 850°C for 24 h by maintaining a steady flow of hydrogen over the hot samples. At the end of the run, the samples were cooled to room temperature in a fast flowing argon atmosphere, It was noted that coolhag in the hydrogen gas itself led to shattering of the samples possibly due to the formation of hydride while cooling. The samples were then degassed at 650 ° C at a 10 -4 torr vacuum level for 24h and then quenched by a jet of helium gas. The weight gain and subsequent loss of hydrogen was 0.6wt%. After the hydrogen treatment, the hydrogen content of the samples was measured to be about 20 ppm. Some of cast samples were directly subjected to the vacuum