Flow Characteristics Analysis of TC18 Titanium Alloy during Hot Deformation Based on Phase Transformation

An accurate flow stress model was established by considering the parameters of strain rate,strain and temperature as well asβ→a+βphase transformation in order to develop the plastic forming theory of TC18 titanium alloy.Firstly,the phase transition kinetics of TC18 titanium alloy during isothermal and continuous cooling at 1073 and 1273 K was studied by thermodynamic calculation,meanwhile,the relationship of volume fraction of phase transition with temperature and time was obtained.Constitutive models were calculated by investigating flow behaviors under hot compression tests with the strain rates of 0.001-1s^(-1) and temperatures of 973-1223 K in the singleβand a+βregions in TC18 titanium alloy,respectively.By combining the phase transformation dynamic kinetics with constitutive models,an accurate flow stress model was established,providing theoretical basis and data support for the hot forging of TC18 titanium alloy.

Multi-scale analyses of phase transformation mechanisms and hardness in linear friction welded Ti17(a+β)/Ti17(β)dissimilar titanium alloy joint

The Ti17(a+β)-Ti17(β)dual alloy-dual property blisk produced using Linear Friction Welding(LFW)is considered as high-performance component in advanced aeroengine.However,up to now,microstructure evolution and relationship between microstructure and micro mechanical properties of LFWed Ti17(a+β)/Ti17(β)dissimilar joint have not been thoroughly revealed.In this work,complex analyses of the phase transformation mechanisms of the joint are conducted,and phase transformations in individual zones are correlated to their microhardness and nanohardness.Results reveal that a dissolution occurs under high temperatures encountered during LFW,which reduces microhardness of the joint to that of Ti17(a+β)and Ti17(β).In ThermoMechanically Affected Zone of Ti17(a+β)(TMAZ-(a+β))side joint,a large number of nanocrystalline a phases form with different orientations.This microstructure strengthens significantly by fine grains which balances partial softening effect of a dissolution,and increases nanohardness of a phase and microhardness of TMAZ-(a+β).Superlattice metastableβphase precipitates from metastableβin Weld Zone(WZ)during quick cooling following welding,because of short-range diffusion migration of solute atoms,especiallyβstabilizing elements Mo and Cr.The precipitation of the superlattice metastableβphase results in precipitation strengthening,which in turn increases nanohardness of metastableβand microhardness in WZ.

ω-Assisted refinement of α phase and its effect on the tensile properties of a near β titanium alloy

In this work, the phase transformation sequence during the continuous heating process(3℃/min) was investigated in a near β titanium alloy. The results show that the staring formation of ω phase is about267℃, and the ending precipitation temperature about 386℃ during the heating process. When the heating temperature is greater than 485℃, there are no ω phase detected within the β matrix. Combined with the microstructural characterization, it is found that ω phase facilitates the nucleation of αphase nearby the ω/β interface and has a great effect on the refinement for α phase. As compared with the specimens directly aged, the specimens with ω-assisted refinement of α phase possess high tensile strength, but there is no yield stage detected on their stress-strain curve. Combined with the analyses of the fracture morphology, the specimens with ω-assisted refinement of α phase present a brittle fracture.This is mainly ascribed to its relatively lager width of grain boundaries and the absence of widmanst?ttenα precipitates.

Research on the Characteristics of Hot Deformation in BT20 Titanium alloy and Its Optimum Spinning Temperature Range

Hot compression tests were conducted on a Gleeble-1500 simulator to investigate the hot deformation behavior of BT20 Ti alloy （Ti-6Al-2Zr-IMo-1V） in the temperature range from 550 to 1000℃ at constant strain rate in the range of 0.01-1s^-1, and then the optimum spinning temperature range was determined. Moreover, tube spinning experiments were executed to verify the reasonability of the optimum temperature range. The results show that the flow stress declines gradually with increasing deformation temperature and decreasing strain rate. In α＋β phase region the dynamic recrystallization is the main softening mechanism and in β phase region the hot deformation softening is controlled by dynamic recovery. In α＋β phase region with reducing strain rate dynamic recrystallization is fully developed. The optimum temperature of hot spinning is 850-900℃ and that of warm spinning is 600-650℃. Meanwhile, at the temperature above 600℃ tubular workpieces of BT20 Ti alloy have been spun without surface cracks and microstructure inhomogeneity, which proves that the optimum spinning temperature range obtained through hot compression experiments is reasonable.

Phase transformation kinetics in metastable titanium alloys

We investigated the phase transformation sequence,kinetics,and microstructural evolutions during heating,isothermal treatment,and continuous cooling of the metastable Ti–B19 alloy.On the basis of these results,we summarized the phase transformation characteristics of this kind of alloys,especially the metastable b-Ti alloy.We also analyzed and discussed the possibility of describing the size,morphology,and distribution of precipitates as well as the possibility of and complications in establishing relationships among the chemical composition,microstructure,processing parameters,and properties of the metastable Ti–B19 alloy.

Effects of Deformation and Phase Transformation Microstructures on Springback Behavior and Biocompatibility in β-Type Ti-15Mo Alloy

This study examined the mechanical properties, springback behavior from three-point bending loading–unloading tests and biocompatibility from human osteoblast cell adhesion and proliferation experiments in Ti-15Mo alloy with different microstructures. The springback ratio increased after the appearance of deformation microstructures including {332} < 113 > twins and dislocations, due to the increased bending strength and unchanged Young’s modulus. By contrast, the change in springback ratio was dependent on the competing effect of the simultaneous increase in bending strength and Young’s modulus after phase transformation, namely, the isothermal ω-phase formation. Good cell adhesion and proliferation were observed on the alloy surface, and they were not significantly affected by the deformation twins, dislocations and isothermal ω-phase.The diversity of deformation and phase transformation microstructures made it possible to control the springback behavior effectively while keeping the biocompatibility of the alloy as an implant rod used for spinal fixation devices.

Microstructure Evolution and Phase Transformation of Ti–6.5Al–2Zr–Mo–V Alloy During Thermohydrogen Treatment

Thermohydrogen treatment （THT） is an effective way to refine microstructure and improve the mechanical properties of the titanium alloys. In the current work, as-cast Ti-6.5Al-2Zr-Mo-V alloy was hydrogenated with different hydrogen contents and processed solution aging. Accordingly, the microstructure evolution and phase transformation were analyzed. Results show that during solution aging, eutectoid decomposition occurs and the product is a mixture of coarse primaryα, fine eutectoid product and undecomposed βH. The size of primary α is closely dependent on the hydrogen content, and large primary α can be obtained at medium hydrogen content. Further, the influence of hydrogen content on the growth of primary α phase was revealed. The primary α is much fine, and the eutectoid product is relatively homo- geneous with 0.984 wt% H. After THT, the ultimate strength is beyond 1,100 MPa that has increased by 23.15% compared with that in as-cast state.

DEFORMATION-INDUCED α2→γ TRANSFORMATION IN A HOT DEFORMED Ti-45A1-10Nb ALLOY

High-resolution transmission electron microscope (HRTEM) was employed to investigate the deformation-induced α2→γ phase transformation phenomenon in a hot deformed Ti-45Al-10Nb alloy. Such a tronsformation can be nucleated either at α2/γ interfaces or at stacking faults on the basal planes of the α2 phase. The growth of deformation-induced γplate is accomplished by the motion of α/6<100> Shockley partials on alternate basal planes (0001)α2, and the α/6<100> Shockley partials move in coordination rather than sweep on (0001)α2 plane one by one. It appears that no atom transportation is involved in this stress-induced α2→γ transfromation.

Morphology characteristics of α precipitates related to the crystal defects and the strain accommodation of variant selection in a metastableβtitanium alloy

Profound and comprehensive investigations on the morphology characteristics ofαprecipitates are essential for the microstructural control of metastableβtitanium alloys.At the very beginning of aging treatment,intragranularαprecipitates with a dot-like morphology begin to generate nearby the dislocations,then those dot-likeαprecipitates with the same crystallographic orientation tend to connect with each other to develop a lath-like morphology.With the progress of aging treatment,the orientated lath-likeαprecipitates gradually combine with each other to form the V-shaped clusters or the triangular ones.The dislocations of{110}_(β)<111>βedge type are evidenced within theβgrains,and it is found that variant selection ofαprecipitates induced by the transformation strain and the interplay betweenαvariants and the dislocations are confirmed as the key factors for the formation of the V-shaped or triangular clusters.The results of this work could provide underlying knowledge on the morphology characteristics of intraguranularαprecipitates related to the crystal defects and the strain accommodation ofαvariants in metastableβtitanium alloys.

On the nature of a peculiar initial yield behavior in metastable β titanium alloy Ti-5Al-5Mo-5V-3Cr-0.5Fe with different initial microstructures

The compressive yielding phenomenon of titanium alloys is not as focused and sufficiently ascertain as the tensile yielding phenomenon.In the present work,the peculiar compressive yielding behavior and the different dynamic responses of three different initial microstructures(singleβ,clavateβand lamellarβ)were investigated in an attractive metastableβtitanium alloy Ti-5553 using electron microscopes/crystallographic calculation/crystal plastic finite element simulation.Results reveal that the distinct compressive yielding behavior,steep peaks of sudden drop in the initial stage(very small true strain 0.03)of stress loading have appeared in the compression stress-strain curves except for the lamellarβinitial microstructure.Dislocation slip is the essential mechanism of the initial yielding behavior.Interlaced multiple-slip bands formed in the singleβinitial microstructure during the warm deformation process.A small quantity of single slip bands was observed in the deformed clavateβinitial microstructure.The abundant varied nano/ultrafineβsprecipitates were nucleated dynamically and dispersedly in all the three deformed initial microstructures.The multiple-slip bands formation and substantial nanoscaleβsresult in the highest peak of flow stress for singleβinitial microstructure.The compressive slip bands are formed early in the elastic–plastic deformation stage.As the increasing strain,the sample showed a significant compressive bulge,or eventually forming a strong adiabatic shear band or crack.These results are expected to provide a reference for the study of deformation behavior and mechanical properties of metastableβtitanium alloys.

Formation of adiabatic shearing band for high-strength Ti-5553 alloy:A dramatic thermoplastic microstructural evolution

By using split Hopkinson pressure bar, optical microscopy and electronic microscopy, we investigate the influence of initial microstructures on the adiabatic shear behavior of high-strength Ti-5Al-5V-5Mo-3Cr(Ti-5553) alloy with lamellar microstructure and bimodal microstructure. Lamellar alloy tends to form adiabatic shearing band(ASB) at low compression strain, while bimodal alloy is considerably ASBresistant. Comparing with the initial microstructure of Ti-5553 alloy, we find that the microstructure of the ASB changes dramatically. Adiabatic shear of lamellar Ti-5553 alloy not only results in the formation of recrystallized β nano-grains within the ASB, but also leads to the chemical redistribution of the alloying elements such as Al, V, Cr and Mo. As a result, the alloying elements distribute evenly in the ASB.In contrast, the dramatic adiabatic shear of bimodal alloy might give rise to the complete lamination of the globular primary a grain and the equiaxial prior β grain, which is accompanied by the dynamic recrystallization of a lamellae and β lamellae. As a result, ASB of bimodal alloy is composed of a/β nanomultilayers. Chemical redistribution does not occur in ASB of bimodal alloy. Bimodal Ti-5553 alloy should be a promising candidate for high performance armors with high mass efficiency due to the processes high dynamic flow stress and excellent ASB-resistance.

Phase/grain boundary assisted-3D static globularization mechanism of TC17 alloy based on the microstructure reconstruction and in-situ TEM observation

Lamellar globularization in the dual-phase titanium alloy is the key to improving plasticity and strength.However,the mechanism has not been fully elucidated so far.In this work,the role of phase/grain bound-ary in the static globularization of TC17 alloy was systematically studied by setting differentαphase con-tent before annealing through low-and high-temperature deformation.Isothermal compression causes the parallel distribution and fragmentation of 3Dαplates and few globularαparticles are formed at a strain rate of 1 s^(-1).Post-deformation annealing promotes the static globularization ofαphase while it is affected by initialαphase content.After 730°C deformation,the development ofα/αinterface by absorbing dislocations promotes the formation of globularαgrains based on the nucleation of sepa-ratedαparticles and pre-recoveryαsubgrain during subsequent annealing.Theα/α/βandα/β/βtriple junctions formed due to highαcontent with about 36%volume fraction are favorable for the further nucleation and growth of globularαgrains by reducing interface energy,forming a 3D irregularαplate.Then nucleation and growth of theβphase dominate the microstructure evolution during subsequent an-nealing,resulting in the local dissolution of the plate and formation ofαrods.After 850°C deformation,theαphase tends to nucleate at theβ/β/βtriple junctions and grow into a lamellar shape along the high energyβ/βgrain boundary due to lowαcontent with about 7%volume fraction.Theαnucleation that maintains the Burgers orientation relationship(BOR)with the surroundingβphase grows along the habit plane and thickens slowly,resulting in the formation of a precipitatedαplate with a flat surface and the suppression of static globularization.The comprehensive investigation of lamellar globularization provides guidance for optimizing the 3D microstructure and properties of dual-phase titanium alloy.

Two-dimensional cellular automaton model for simulating structural evolution of binary alloys during solidification

Two-dimensional cellular automaton(CA)simulations of phase transformations of binary alloys during solidification were reported.The modelling incorporates local concentration and heat changes into a nucleation or growth function,which is utilized by the automaton in a probabilistic fashion.These simulations may provide an efficient method of discovering how the physical processes involved in solidification processes dynamically progress and how they interact with each other during solidification.The simulated results show that the final morphology during solidification is related with the cooling conditions.The established model can be used to evaluate the phase transformation of binary alloys during solidification.

面心立方结构钛的研究进展

面心立方结构钛是一种亚稳相,最早发现于超薄钛膜。除此之外,钛及钛合金在经历大塑性变形之后,也常常会发生hcp→fcc钛及钛合金的相变。近年来,大量研究报道了钛及钛合金经冷轧、压缩、拉伸和球磨等加工后观察到面心立方结构钛。为此,对近几十年来面心立方结构钛的制备方法、相变机理及其变形行为的研究进展进行了详细介绍。现有研究已证实,面心立方结构钛不仅能够协调材料的变形,而且有利于材料强度的提高。

Introducingωand O'nanodomains in Ti-6Al-4V:The mechanism of acceleratingα→βtransformation kinetics via electropulsing

Conventional kinetics theory for diffusion-controlled phase transformation shows that the reverse transition should lag behind the temperature rise through rapid heating,i.e.,overheating is required.In this work,we found that theβ-transus temperature decreased by∼50℃ during studying theα→βtransformation in Ti-6Al-4V alloy via electropulsing treatment(EPT).The calculation suggests that the acceleration of transformation kinetics cannot be fully explained by Joule heat and athermal effects of the electromigration effect and electron wind theory.The microstructural evolution during EPT was systematically investigated utilizing scanning electron microscope(SEM),electron backscattered diffraction(EBSD),X-ray diffraction(XRD),transmission Kikuchi diffraction(TKD),and transmission electron micro-scope(TEM).Microscopic analysis shows that the nano-sizedωand O'phases formed in theβphase,which causes large numbers of lattice distortion regions.The defects are conducive to accelerating the bulk diffusion of alloying elements inβ.Moreover,the nanodomains limited the growth of martensite,therefore nanocrystalline martensite formed after quenching.These findings develop the understanding of the destructive effect of current on metallic crystal,which will help to guide microstructural regulation in titanium and other alloys.

Phase field simulation of the stress-inducedαmicrostructure in Ti–6Al–4 V alloy and its CPFEM properties evaluation

Variant selection under specific applied stresses during precipitation of a plates from prior-βmatrix in Ti-6 Al-4 V was investigated by 3 D phase field simulations.The model incorporates the Burgers transformation path fromβto a phase,with consideration of interfacial energy anisotropy,externally applied stresses and elastic interactions among a variants andβmatrix.The Gibbs free energy and atomic mobility data are taken from available thermodynamic and kinetic databases.It was found that external stresses have a profound influence on variant selection,and the selection has a sensitive dependence,as evidenced by both interaction energy calculations and phase field simulations.Compared with normal stresses,shear stresses applied in certain directions were found more effective in accelerating the transformation,with a stronger preference to fewer variants.The volume fractions of various a variants and the final microstructure were determined by both the external stress and the elastic interaction among different variants.The a clusters formed by variants with Type2 misorientation([11-20]/60°)relation were found more favored than those with Type4([-1055-3]/63.26°)under certain applied tensile stress such as along<111>β.The mechanical properties of different microstructures from our phase field simulation under different conditions were calculated for different loading conditions,utilizing crystal plastic finite element simulation.The mechanical behavior of the various microstructures from phase field simulation can be evaluated well before the alloys are fabricated,and therefore it is possible to select microstructure for optimizing the mechanical properties of the alloy through thermomechanical processing based on the two types of simulations.

Microstructure evolution and deformation mechanism ofα+βdual-phase Ti-xNb-yTa-2Zr alloys with high performance

Biomedicalβ-phase Ti-Nb-Ta-Zr alloys usually exhibit low elastic modulus with inadequate strength.In the present work,a series of newly developed dual-phase Ti-xNb-yTa-2Zr(wt.%)alloys with high performance were investigated in which the stability ofβ-phase was reduced under the guidelines of ab initio calculations and d-electronic theory.The effects of Nb and Ta contents on the microstructure,compressive and tensile properties were investigated.Results demonstrate that the designed Ti-xNb-yTa-2Zr alloys exhibit typical characteristics ofα+βdual-phase microstructure.The microstructure of the alloys is more sensitive to Nb rather than Ta.The as-cast alloys exhibit needle-likeα′martensite at a lower Nb content of 3 wt.%and lamellarα′martensite at an Nb content of 5 wt.%.Among the alloys,the Ti-3Nb-13Ta-2Zr alloy shows the highest compressive strength(2270±10 MPa)and compressive strain(74.3%±0.4%).This superior performance is due to the combination ofα+βdual-phase microstructure and stressinducedα"martensite.Besides,lattice distortion caused by Ta element also contributes to the compressive properties.Nb and Ta contents of the alloys strongly affect Young's modulus and tensile properties after rolling.The as-rolled Ti-3Nb-13Ta-2Zr alloy exhibits much lower modulus due to lower Nb content as well as moreα"martensite andβphase with a good combination of low modulus and high strength among all the designed alloys.Atom probe tomography analysis reveals the element partitioning between theαandβphases in which Ta concentration is higher than Nb in theαphase.Also,the concentration of Ta is lower than that of Nb in theβphase,indicating that theβ-stability of Nb is higher than that of Ta.This work proposes modernα+βdual-phase Ti-xNb-yTa-2Zr alloys as a new concept to design novel biomedical Ti alloys with high performance.

Ti-Nb合金中β/α″相界面的研究

本文用高分辨透射电子显微镜研究了β型Ti-Nb基合金中变形带内部的β/α″相界面,还用马氏体相变拓扑模型(TM)估算了α″相惯习面附近的弹性应力场。结果表明,α″相惯习面附近的弹性应力太小,以至于不会引起应力诱发β→ω相变。此外,{332}<113>_(β)孪晶是在应力诱发的α″相内部产生的,并且孪晶和{110}_(β)/{00-1}_(α")相界面有关。

钛合金热氢处理技术及其应用前景

钛合金热氢处理技术是利用氢致塑性、氢致相变以及钛合金中氢的可逆合金化作用以实现钛氢系统最佳组织结构、改善加工性能的一种新体系、新方法和新手段 ,利用该技术不仅可以改善钛合金的加工性能 ,而且可以提高钛制件的使用性能 ,降低钛产品的制造成本 ,提高钛合金的加工效率。综述了钛合金中氢对改善压力加工、扩散加工、机械加工和铸造钛合金变质加工的组织、力学性能和加工性能的作用 ,简要分析了其改性机理 ,展望了钛合金热氢处理技术的应用前景。