Phase Transformations In Titanium Alloys Research Articles

Paths of phase transformations in titanium alloys based on four-dimensional structural description

Paths of phase transformations in titanium alloys based on four-dimensional structural description

Influence of stress-induced α’’ phase transformation on mechanical properties in metastable β type Ti-5Al-2.5Cr-5Mo-1Sn alloy

The stress-induced α’’ phase transformation in titanium alloys has received extensive attention due to its significant effect on mechanical properties. In this work, by adjusting the heat treatment process, the stress-induced α’’ phase transformation is activated in Ti-5Al-2.5Cr-5Mo-1Sn. The quasi-static tensile mechanical properties of the alloy under air-cooled and as-quenched states have been compared. Further, the morphology and orientation of the stress-induced α’’ phase and its interaction with dislocations are investigated by X-ray diffraction (XRD), optical microscope (OM), electron backscattered diffraction (EBSD) and transmission electron microscope (TEM) experiments. The value of average misorientation at the stress-induced α’’ bands region is larger than that in the β matrix, which indicates that the degree of plastic deformation in the stress-induced phase transformation region is higher. Therefore, the stress-induced α’’ phase exhibits a strong strain partitioning effect during the stretching process and improves the ductility of the material. The β/α’’ interface has a weak effect to hinder the dislocations, which results in low tensile strength of the alloy. This work can provide insight into the influence of stress-induced α’’ phase transformation on the mechanical properties of transformation induced plasticity (TRIP) type titanium alloys. • The Ti-5Al-2.5Cr-5Mo-1Sn alloy is elaborately regulated to produce TRIP effect. • Average misorientation of α’’ band region is generally larger than that of β matrix. • α’’ phase exhibits a strong strain partitioning effect to increase alloy ductility. • The β/α’’ interface is semi-coherent and has a weak effect to hinder dislocations.

Improvement of mechanical properties and microstructural refining of cast titanium alloys by coupling of electropulsing and temporary alloying element hydrogen

The electric pulse treatment and hydrogen as temporary alloying element have been widely used to improve the microstructure and mechanical properties of titanium alloys, respectively. In this study, the microstructure and mechanical properties of as-cast Ti–6Al–4V alloy are improved by electric pulse treatment combined with temporary alloying element hydrogen instead of traditional thermo-mechanical treatment and heat treatment. In comparison to the traditional thermo-mechanical treatment, the microstructure of the as-cast Ti–6Al–4V alloy after solution-aging treatment in pulsed thermal hydrogen treatment is homogeneous and finer, which is due to the lamellar β phase decomposition after thermal hydrogen treatment, accordingly, the ultimate tensile strength and total elongation of the alloy are effectively improved after pulsed thermal hydrogen treatment. It’s the acceleration of atomic diffusion and phase transformation in titanium alloys by hydrogen as temporary alloying element and electric pulse treatment that leads to the decomposition of the lamellar β phase, thereby refining the microstructure and enhancing mechanical properties. Finally, a novel method for improving the microstructure and mechanical properties of titanium alloys using hydrogen as temporary alloying element and electric pulse treatment is presented.

Four-dimensional structural description of phase transformations in titanium alloys

Titanium alloys display formation of $$\beta$$ , $$\alpha$$ , and $$\omega$$ phases under different processing conditions. Understanding structural transformations involving these phases on a unified basis and identifying a possible path of transformation seem to be interesting from the point of view of studies on phase transformations. An alternative and unified description pertaining to these is being presented here. This is accomplished by making an appeal to four dimensional structural description. It will be shown that such a model requires an angular distortion ( $$\theta$$ ) around the threefold axis. Such a distortion helps one define a scalar order parameter ( $$\eta$$ ) with values lying between 0 and 1. The two terminal values recover all the symmetrical characteristics of $$\beta$$ phase corresponding to $$\eta =0$$ and that of $$\omega$$ phase pertaining to $$\eta =1$$ . $$\alpha$$ phase formation in this model takes place for $$\theta = \cos ^{-1} (\frac{1}{4})$$ and $$\eta =\frac{13}{16}$$ . It is important to point out here that all structures related to choice of $$\eta$$ always preserve a minimal threefold symmetry. This led us to conclude that formation of trigonal phases may be taking place during the paths of transformation. In addition to all these advantages, the model also provides a general possibility of understanding commensurate and incommensurate modulations along threefold axis.

Strengthening Induced by Instantaneous Phase Transformation in Titanium Alloy

High-strength titanium alloys are important for aircraft and space vehicles. However, it is hard to reach 1300 MPa with adequate ductility for the strength of most widely used Ti-6Al-4V alloy via conventional deformation and heat treatment. A new strategy based on electropulsing was used to break the limitation. The strong electron flow induced rapid temperature rise and accelerated local element diffusion, which not only caused efficient grain refinement and phase transformation, but also improved the stability of parts of β phases and dislocation density, thus, the subsequent aging effect was greatly enhanced. Electropulsing as a novel method and idea could be considered to improve and control the mechanical properties of Ti-6Al-4V or other alloys with diffusionless transformation.

First-principles investigations of [formula omitted] variant selection during athermal [formula omitted] transformation of binary Ti-xMo alloy

Variant selection during solid–solid phase transformation in titanium alloys affects greatly the microstructure and mechanical properties of the alloys. Theoretical investigations of the variant selection were generally performed by using phase field simulation that considers solely the elastic coherency strain energy. In the present work, we develop a model to determine the variant selection directly from first-principles calculations. The source of the variant selection is considered to be the varying free energy gains (including both bulk and interface contributions) induced by the transformation from the parent phase to different variants of the product phase. This model is applied to investigate the effects of shear stress and alloy composition on ω variant selection during athermal β→ω transformation in binary Ti-xMo (x≤25 at.%) alloys. The random distribution of the atoms in the alloy is described by using virtual crystal approximation (VCA). We show that the tendency of variant selection becomes stronger with increasing shear stress. With increasing Mo concentration, the favorable ω variant transfers from one to another, except for the ω phase with very small particle size (e.g., radius R≤1 nm) where one variant is always selected. The critical Mo concentration for the transfer of the favorable ω variant approaches to 5 at.% with increasing size of the ω phase particles and remains almost unchanged against the shear stress. At the critical Mo concentration, the considered variants have equal free energy gains and there is no variant selection. This finding opens the possibility of controlling the variant selection and the associated microstructure by changing the composition of the alloys.

Variant selection in laser melting deposited α + β titanium alloy

Variant selection in laser melting deposited (LMD) α + β titanium alloy (TC21) was investigated using linear thermal expansion and EBSD test. At micro level, volume change of lattice is anisotropic during solid phase transformation in titanium alloy. If several variants are more abundant than others, the anisotropic volume changes could be observed at macro level. In linear thermal expansion test, the growth directions of as-deposited and α + β treated LMD TC21 alloy expanded during α→β phase transformation while at the transverse direction, a contraction was observed. The β treated alloy showed isotropic expansion in the same temperature region. Anisotropic volume changes of as-deposited and α + β treated LMD TC21 alloy confirmed the variant selection happened in deposition process. During depositions, the anisotropic internal thermal stress generated in the as-solidified metal before solid phase transformations started. The existed thermal stress made the lattice spacing a little distorted. The anisotropic distortion of lattice spacing led to the variant selection. The theoretical analysis was supported by EBSD results.

Simulation of coupled temperature, microstructure and internal stresses evolutions during quenching of a β-metastable titanium alloy

The evolution of the internal stresses and strains during quenching of Ti17 alloy has been simulated numerically by taking into account the coupled thermal, mechanical and microstructural evolution phenomena. Emphasis was put on the influence of the β→α+β phase transformation on the internal stresses evolutions during quenching, which is rarely discussed in the case of titanium alloys as compared to steel [Denis et al., J. Mat. Eng. Perf., 2002;11(1):92]. The simulation of these coupled phenomena was built on a thorough knowledge of internal stresses development during quenching of different metallic alloys, the solid phase transformations in titanium alloys, existing models of transformations kinetics and characterizations of the mechanical behavior.The flow stress was calculated by using an isotropic thermo–visco–elasto-plastic law depending on temperature and microstructure. The parameters were determined experimentally as a function of temperature and the microstructural state: either only β or α+β. In the latter case, the amount of the α phase, as well as its morphology, was accounted for. As for the prediction of phase transformation kinetics, the model developed in [Teixeira et al., Mat. Sci. Eng. A, 2007; 448:135] was used. It is based on a JMAK rule and an additivity hypothesis and the parameters were deduced from isothermal kinetics determination. The effects of stress on the phase transformation (transformation plasticity) were examined as well as the small volume change due to the transformation. The coupled calculation of the thermal, microstructural and mechanical evolutions was set up in the finite element code ZeBuLoN. Cylindrical geometry was considered with a diameter sufficiently large to obtain significant thermal and microstructural gradients. The calculation results show that taking into account the relatively slow kinetics of the β→α+β phase transformation in Ti17 has a significant effect on the level of the residual stresses and strains.

钛合金β→α相变的变体选择

β→α转变是钛合金中最基本和最重要的一种相变，两相之间存在严格的Burgers取向关系，即(0001)α//{110}β和20>α//β，这种关系决定了在一个β晶粒内可能会出现12个α相变体。在各种类型的钛合金中，均会出现在一个β晶粒内只有少数几个取向的α相能够存在的现象，即发生变体选择，其对强度、塑性、疲劳等力学性能的负面影响逐步被关注，抑制其发生将是进一步提高钛合金性能的潜在方法之一。本文结合作者的研究工作，介绍了钛合金中β→α转变时发生变体选择的本质、成因以及对合金力学性能的影响等方面的研究现状，并分析了该研究方向目前存在的问题和今后的研究重点。 β→α transformation is the most essential phase transition in titanium alloys. The α phase formed in the β matrix is oriented according to the so-called Burgers orientation relationship, viz. (0001)α//{110}β and 20>α//β. Because of the symmetry of the two crystals, 12 α va-riants can form in a β grain theoretically. Variant selection can take place in almost all kinds of ti-tanium alloys and decline the strength, plasticity and fatigue resistance, which have been attended by the researchers gradually. To control the occurrence of variant selection will be one of the po-tential methods to improve the properties of titanium alloys. In this paper, by integrating our re-search work, the progress in research of variant selection of β→α phase transformation in titanium alloys is briefly reviewed from aspects of the substance and the reason of variant selection, and its influence on the mechanical properties. The problems at present study and the research directions in the future are also analyzed.

Time resolved X-ray diffraction observations of phase transformations in transient arc welds

In situ X-ray diffraction methods have been developed at Lawrence Livermore National Laboratory for direct observation of microstructural evolution under quasi-steady state and transient welding conditions. Using intense highly collimated synchrotron radiation, the crystal structures in the weld heat affected and fusion zones are probed in real time to monitor solidification and solid state phase transformations during welding. Here the authors review recent work on the development and use of the time resolved X-ray diffraction (TRXRD) technique during transient welding and illustrate its unique capabilities to: directly observe the solidification mode; discover, in real time, the definitive sequence of phase transformations that lead to the final microstructure; and provide quantitative kinetic data of phase transformations through synthesis of TRXRD data with the temperature history obtained through heat transfer modelling. The TRXRD technique has been used to investigate welding induced phase transformations in titanium alloys, low alloy steels, and stainless steel alloys. The results show some of the first real time observations of the weld solidification mode and the evolution of equilibrium and non-equilibrium phases during rapid heating and cooling. When combined with numerical modelling, quantitative phase transformation kinetic data are obtained, allowing for its use in a wide variety of isothermal and non-isothermal processing. The potential for future applications of this and similar techniques is also addressed.

In situ observation of texture evolution during α → β and β → α phase transformations in titanium alloys investigated by neutron diffraction

Texture changes during recrystallization and the α–β–α phase transformation in two titanium alloys were investigated in situ by time-of-flight neutron diffraction by heating in a vacuum furnace to 950 °C. In commercially pure titanium, a strong texture memory effect is observed. This effect is a direct consequence of an orientation-selective α → β transformation, favoring new orientations produced during nucleation and grain growth. The β–α transformation favors β orientations with minimal misorientations, resulting in a strong final α texture that emphasizes the grain growth component. In Ti–6Al–4V, the texture memory effect is less pronounced. The high-temperature β texture is obtained by growth of pre-existing β nuclei. In a similar way, during cooling, the growth of α domains is controlled by high-temperature α orientations inherited from the β grains with Burgers orientation relation.

Studies of Phase Transformations in Ti-Al-X Alloys

Phase transformation in titanium alloys on high temperature exposure has been studied. The central theme of review includes the study of basic development in phase transformation, microstructural features, acceptability of crystalline modification, precipitation and intermetallics formation. Effects of alloying elements to the standard systems have been studied. Lamellar microstructure in titanium aluminide systems forms various typical fronts of microstructural changes, where equiax, duplex, ultra fine grained combinations in lamellar or other phases produce versatile implications to the scenario. Ti-6Al-4V systems have been referred to as the 'work horse' of component manufacturing in industry and airways. Twinning, an important deformation mechanism, arises from stress fields associated to misfitting interfacial dislocation or shear reshuffle. Mostly lamellar topography of these systems shows a better creep life after reduction of dislocation climb process by segregated solutes at ledges of lamella. Industrial, biomedical, airways, aerospace and electrical sensors industry are main component manufacturer from these different standard alloy systems.

Modelling, Simulations and Monitoring of Lamella Structure Formation in Titanium Alloys Controlled by Diffusion Redistribution

A mathematical model and computer programs are developed for simulation and monitoring of the processes of nucleation and growth of the α phase Widmanstatten plates during the course of the β ⇒ α phase transformation in titanium alloys. Based on integration of thermodynamic modelling and finite element simulations, the α phase appearance at the grain boundary of β phase is described by a numerical procedure for random nucleation as a function of the alloy composition and the temperature. The rate at which an interface moves depends both on the intrinsic mobility and on the rate at which diffusion can remove the excess of β stabilising atoms ahead of the interface. This process is controlled by diffusion redistribution of the alloying elements between the α and β phases that are different for different titanium alloys. The finite element method was used for solving the diffusion equation on the domain occupied by β phase. The models and program packages developed are used to simulate the morphology of the microstructure evolution (Fig. 1) in different titanium alloys during variety of heat treatment processings and can be used in the real processing route.

Electropulsing-induced Phase Transformation in Titanium Alloy

Electropulsing-induced Phase Transformation in Titanium Alloy

Correlation Between Diagrams of Isothermal and Anisothermal Transformations and Phase Composition Diagram of Hardened Titanium Alloys

A brief review of works devoted to phase transformations in titanium alloys and published by the authors during a long period and of recent publications on the topic is presented. When plotting phase diagrams of hardened titanium alloys (metastable diagrams) the authors used as a basis their relationship to the equilibrium diagrams and the transformations of the β-phase of various compositions during hardening from the β-range temperatures. After studying the structure of hardened alloys they investigated their transformations during aging, isothermal treatment, and continuous cooling from the β-range temperatures. Using the interrelation between the phase diagrams of hardened alloys and the phase transformations of β-phase under various kinds of heat treatment the authors developed theoretical diagrams of isothermal and anisothermal transformations, which were later confirmed by experimental diagrams for pilot and commercial alloys. The suggested classifications of diagrams of isothermal and anisothermal transformations are applicable to titanium alloys of various structural classes with allowance for the coefficient Kβ of stabilization of the β-phase.

In situ high temperature microscopy study of the surface oxidation and phase transformations in titanium alloys

Two popular commercial titanium alloys, Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo-0.08Si, were used for in situ high temperature microscopy study. The experiments were performed on an optical microscope equipped with high temperature stage using both normal and florescence lights. Two kinds of experiments were performed, at continuous heating/cooling with different rates and in isothermal conditions at different temperatures. The changes taking place on the sample surface during the experiments were monitored. The morphology of the alpha ==> beta ==> alpha phase transformation was recorded at different heat treatment conditions using the effect of thermal etching. An effect of sample surface oxidation and deoxidation was observed during continuous heating. The appearance and disappearance of ordered titanium oxides Ti3O and Ti2O are discussed based on the phase equilibrium diagram. The kinetics of the surface oxidation was monitored in both isothermal and continuous cooling conditions.

Synchrotron X-ray diffraction study of the phase transformations in titanium alloys

High-resolution synchrotron X-ray diffraction was used to study the phase transformations in titanium alloys. Three titanium alloys were investigated: Ti–6Al–4V, Ti–6Al–2Sn–4Zr–2Mo–0.08Si and β21s. Both room and high temperature measurements were performed. The room temperature experiments were performed to study the structure of the alloys after different heat treatments, namely as received (AR), furnace cooling (FC), water quenching (WQ) and water quenching followed by ageing. The α, α′, α″ and β phases were observed in different combinations depending on the heat treatment conditions and the alloy studied. A multicomponent hexagonal close packed ( hcp) α phase, with different c and the same a lattice parameters, was detected in Ti–6Al–4V after FC. High temperature synchrotron X-ray diffraction was used for ‘in situ’ study of the transformations on the sample surface at elevated temperatures. The results were used to trace the kinetics of surface oxidation and the concurrent phase transformations taking place under different conditions. The influence of the temperature and oxygen content on the lattice parameters of the α phase was derived and new data obtained on the coefficients of thermal expansion in the different directions of the hcp α phase, for Ti–6Al–4V and Ti–6Al–2Sn–4Zr–2Mo–0.08Si.

Dislocation-hydride interactions at low plastic strain in titanium

The electric pulse treatment and hydrogen as temporary alloying element have been widely used to improve the microstructure and mechanical properties of titanium alloys, respectively. In this study, the microstructure and mechanical properties of as-cast Ti–6Al–4V alloy are improved by electric pulse treatment combined with temporary alloying element hydrogen instead of traditional thermo-mechanical treatment and heat treatment. In comparison to the traditional thermo-mechanical treatment, the microstructure of the as-cast Ti–6Al–4V alloy after solution-aging treatment in pulsed thermal hydrogen treatment is homogeneous and finer, which is due to the lamellar β phase decomposition after thermal hydrogen treatment, accordingly, the ultimate tensile strength and total elongation of the alloy are effectively improved after pulsed thermal hydrogen treatment. It’s the acceleration of atomic diffusion and phase transformation in titanium alloys by hydrogen as temporary alloying element and electric pulse treatment that leads to the decomposition of the lamellar β phase, thereby refining the microstructure and enhancing mechanical properties. Finally, a novel method for improving the microstructure and mechanical properties of titanium alloys using hydrogen as temporary alloying element and electric pulse treatment is presented.

Structural and phase transformations in titanium alloys

Structural and phase transformations in titanium alloys

Study of the α → β phase transformation of a Ti-6Al-4V sheet by means of texture change

Although a large number of studies on the {alpha}/{beta} phase transformation in titanium alloys have been reported, only a few works deal with the texture change observed during the {alpha}{yields}{beta} transformation during continuous heating. This is due to the difficulty to obtain information about the metallurgical state of the high temperature {beta} phase, which is not stable at room temperature. In pure titanium, the hcp {alpha} phase generally transforms into the bcc {beta} phase according to the Burgers relation with sometimes orientation variant selections. In the case of Ti-6Al-4V alloy, the transformation of the {alpha} phase is performed in the presence of a low volume fraction of {beta} phase which can influence the transformation mechanism. In the present paper, the {alpha}{yields}{beta} texture change of Ti-6Al-4V material is characterized using adapted methods. The resulting textures allows us to discuss the influence of the residual {beta} phase on the transition texture.