置氢Ti-6Al-4V合金组织与室温变形行为的相关性

应用压缩实验研究置氢Ti-6Al-4V合金的室温力学行为,采用OM、SEM分析了氢对钛合金组织的影响和断口形貌特征,探讨了置氢钛合金组织和室温变形行为之间的相关性.结果表明:氢的固溶强化作用使置氢Ti-6Al-4V合金硬化效应增加,但适量的氢可以显著降低其压缩屈服强度和弹性模量,且断裂时发生的变形量增加,此时合金组织为α+β的双态组织,当合金中产生粗大的β晶粒时,断裂时发生的变形量显著降低;氢的加入促进了合金中斜方马氏体α″的生成;置氢Ti-6Al-4V合金的室温压缩断口为延性沿晶断裂或脆性沿晶断裂和解理型穿晶断裂两种断裂方式的混合断口.     

Effect of Cooling Rate on Microstructure Evolution and Plastic Flow of Ti-6Al-4V

The effect of cooling rate following subtransus solution treatment on microstructure evolution and plastic flow of Ti-6Al-4V was established. For this purpose, three types of tests were performed using samples of Ti-6Al-4V with an initial structure of equiaxed α in a matrix of β and a cooling rate of 11, 42, or 180 K/min: (i) Static cooling following solution treatment without or with a prestrain, (ii) constant-strain-rate hot compression during concurrent cooling, and (iii) static cooling to a specified temperature followed by constant-strain-rate isothermal hot compression. The volume fraction of equiaxed α developed during cooling was strongly dependent on cooling rate, but pre- or concurrent deformation resulted in relatively-small changes in this quantity. In addition, the cooling rate through its effect on the growth kinetics of equiaxed α had a noticeable effect on plastic-flow behavior under both isothermal and non-isothermal conditions. In both instances, the retention of the high-temperature microstructure (characterized by a low fraction of equiaxed α) during rapid cooling gave rise to lower flow stresses than samples with equilibrium equiaxed phase fractions. By contrast, when secondary α was formed during cooling, higher flow stresses were generated due to a Hall-Petch-like effect. The results were interpreted using models for the diffusional growth of equiaxed α, the onset of nucleation of secondary α, and predictions of the plastic-flow response of equiaxed two-phase microstructures based on a self-consistent approach. Unlike previous findings which indicated a large increase in the rate of dissolution of equiaxed α due to concurrent deformation/pipe diffusion during heating transients, the present work did not reveal a corresponding enhancement of growth during cooling.

     

Fracture characteristics,microstructure, and tissue reaction of Ti-5Al-2.5Fe for orthopedic surgery

The microstructure of Ti-5Al-2.5Fe, which is expected to be used widely as an implant material not only for artificial hip joints but also for instrumentations of scoliosis surgery, was variously changed by heat treatments. The effect of the microstructure on mechanical properties, fracture toughness, and rotating-bending fatigue strength in the air and simulated body environment, that is, Ringer’s solution, was then investigated. Furthermore, the effect of the living body environment on mechanical properties and fracture toughness in Ti-5Al-2.5Fe were investigated on the specimens implanted into rabbit for about 11 months. The data of Ti-5Al-2.5Fe were compared with those of Ti-6Al-4V ELI, which has been used as an implant material mainly for artificial hip joints, and SUS 316L, which has been used as an implant material for many parts, including the instrumentation of scoliosis surgery. The equiaxedα structure, which is formed by annealing at a temperature belowβ transus, gives the best balance of strength and ductility in Ti-5Al-2.5Fe. The coarse Widmanstättenα structure, which is formed by solutionizing overβ transus followed by air cooling and aging, gives the greatest fracture toughness in Ti-5Al-2.5Fe. This trend is similar to that reported in Ti-6Al-4V ELI. The rotating-bending fatigue strength is the greatest in the equiaxedα structure, which is formed by solutionizing belowβ transus followed by air cooling and aging in Ti-5Al-2.5Fe. Ti-5Al-2.5Fe exhibits much greater rotating-bending fatigue strength compared with SUS 316L, and equivalent rotating-bending fatigue strength to that of Ti-6Al-4V ELI in both the air and simulated body environments. The rotating-bending fatigue strength of SUS 316L is degraded in the simulated body environment. The corrosion fatigue, therefore, occurs in SUS 316L in the simulated body environment. Fatigue strength of Ti-5Al-2.5Fe in the simulated body environment is degraded by lowering oxygen content in the simulated body environment because the formability of oxide on the specimen surface is considered to be lowered comparing with that in air. The mechanical property and fracture toughness of Ti-5Al-2.5Fe and Ti-6Al-4V ELI are not changed in the living body environment. The hard-surface corrosion layer is, however, formed on the surface of SUS 316L in the living body environment. The C1 peak is detected from the hard-surface corrosion layer by energy-dispersive X-ray (EDX) analysis. These facts suggests a possibility for corrosion fatigue to occur in the living body environment when SUS 316L is used. The fibrous connective tissue and new bone formation are formed beside all metals. There is, however, no big difference between tissue morphology around each implant material.     

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 (LENS?), 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 Widmanstätten (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.     

Microstructure and Texture Evolution during Friction Stir Processing of Fully Lamellar Ti-6Al-4V

Friction stir processing (FSP) was used to locally refine a thin surface layer of the coarse, fully lamellar microstructure of investment-cast Ti-6Al-4V. Depending on the peak temperature reached in the stir zone during processing relative to the β transus, three distinct classes of microstructures were observed. Using accepted wrought product terminology, they are equiaxed, bimodal, and lamellar, except for the case of FSP, the length scale of each was smaller by at least an order of magnitude compared to typical wrought material. The evolution of an initially strain-free fully lamellar microstructure to each of these three refined conditions was characterized with scanning electron microscopy, transmission electron microscopy and electron backscatter diffraction. The fundamental mechanisms underlying grain refinement during FSP, including both the morphological changes and the formation of high-angle grain boundaries, were discussed.     

Friction Stir Processing of Investment-Cast Ti-6Al-4V: Microstructure and Properties

Investment-cast titanium components are becoming increasingly common in the aerospace industry due to the ability to produce large, complex, one-piece components that were previously fabricated by mechanically fastening multiple pieces together. The fabricated components are labor-intensive and the fastener holes are stress concentrators and prime sites for fatigue crack initiation. The castings are typically hot-isostatically-pressed (HIP) to close internal porosity, but have a coarse, fully lamellar structure that has low resistance to fatigue crack initiation. The as-cast + HIP material exhibited 1- to 1.5-mm prior β grains containing a fully lamellar α + β microstructure consistent with slow cooling from above the β transus. Friction stir processing (FSP) was used to locally modify the microstructure on the surface of an investment-cast Ti-6Al-4V plate. Friction stir processing converted the as-cast microstructure to fine (1- to 2-μm) equiaxed α grains. Using micropillars created with a dual-beam focused ion beam device, it was found that the fine-grained equiaxed structure has about a 12 pct higher compressive yield stress. In wrought products, higher strength conditions are more resistant to fatigue crack initiation, while the coarse lamellar microstructure in the base material has better fatigue crack growth resistance. In combination, these two microstructures can increase the fatigue life of titanium alloy castings by increasing the number of cycles prior to crack initiation while retaining the same low-crack growth rates of the colony microstructure in the remainder of the component. In the current study, high-cycle fatigue testing of investment-cast Ti-6Al-4V was performed on four-point bend specimens. Early results show that FSP can increase fatigue strength dramatically. This article is based on a presentation given in the symposium entitled “Materials Behavior: Far from Equilibrium” as part of the Golden Jubilee Celebration of Bhabha Atomic Research Centre, which occurred December 15–16, 2006 in Mumbai, India.     

The effect of heat treatment on the fatigue strength of microknurled Ti-6Al-4V

Microknurling, a high pressure surface indentation technique, was devised as an alternative to traditional heat-bonded porous coatings found on many orthopedic implants designed for fixation by tissue ingrowth. Heat-bonded porous coating can cause at the surface of an implant stress concentrations that reduce fatigue strength. However, microknurling may reduce stress intensification without eliminating it. Thus the purpose of this work was to explore surface thermal/mechanical processing of Ti-6Al-4V to improve the fatigue strength of microknurled specimens via the production of a Ti-6Al-4V dual microstructure. The latter consists of a surface layer of equiaxed grains known to be effective against crack initiation and a bulk microstructure of lamellar grains that possesses optimum fatigue crack propagation resistance. Rotating-bending fatigue tests showed that such a microstructure had some benefits, but this was offset by the reduction in compressive strains imparted to the surface by the heat treatments needed to obtain this microstructure.     

The influence of α/β interface phase on tensile properties of Ti-6Al-4V

The effects of α/β interface phase on room temperature tensile properties of Ti-6Al-4V having an equiaxed primary α microstructure have been studied systematically. Due to the conditions under which it grows, manipulation of the interface phase width also results in alteration of the volume fraction of primary α in the alloy. Tensile yield strength and elongation were correlated to interface phase width and volume fraction primary α. The relative individual influence of each of these microstructural features on properties is not unambiguously clear, but evidence indicates that yield strength increases with increasing interface phase width when the interface phase exceeds about 250 nm, and elongation decreases with increasing interface phase width up to about 250 nm, and is unaffected at widths above 250 nm. It is speculated that the interface phase raises yield strength and lowers elongation by acting as a barrier to slip.     

The effect of matrix strength on void nucleation and growth in a widmanstätten alpha-beta titanium alloy,CORONA-5

This paper presents the results of a study of the effect of matrix yield strength, at a constant Widmanstätten α microstructure, on the nucleation and growth of voids in an α-β titanium alloy, CORONA-5, Ti-5Al-4.5Mo-l.5Cr. Four microstructures with a retained β matrix and involving coarse or fine Widmanstätten α particles in coarse or fine β grains were used in different heat treatment conditions resulting in yield strengths from 765 to 1018 MPa. Void nucleation occurred atα/α boundaries, grain boundary α/matrix interfaces, α twin/matrix interfaces, and α twin/untwin boundaries. The void nucleation strain varied from 0 to 0.26 and was a function of both microstructure and yield strength. Void growth rates increased with yield strength for all microstructures except for those containing coarse α which decreased with yield strength. Intense shear was observed in colonies of fine α structures and was considered to be the cause of the observed rapid void growth rates in the fine β+ fine α microstructures. Surface cracking occurred in several aged conditions during straining. This cracking was attributed to strain concentration in α.     

Improvement in mechanical properties of dental cast Ti-6Al-7Nb by thermochemical processing

Hydrogenation and dehydrogenation, that is, thermochemical processing (THP) and its variation with a post-heat treatment (THPH), are investigated in order to improve the balance of strength, elongation, and fatigue strength of cast Ti-6Al-7Nb and Ti-6Al-4V for dental applications. Microstructures of both cast alloys change from coarse Widmanst?tten α structure to super fine α structure with an average diameter of 3 μm by conducting THP or THPH. Tensile strength and fatigue limit of cast Ti-6Al-7Nb and Ti-6Al-4V increase by around 10 and 40 pct, respectively, as compared with those of both as-cast alloys. The balance of strength and ductility of cast Ti-6Al-7Nb is improved by conducting THPH as compared with the case where THP is conducted. This improvement is due to the plastic deformability of unstable β phase because the lattice constant of β phase in each alloy conducted with THPH is much greater than that of each as-cast alloy.     

Ti-44Al-5V-1Cr-0.3Ni-0.1Hf-0.15Gd合金的微观组织和拉伸性能

针对变形量达86%的锻造Ti-44Al-5V-1Cr-0.3Ni-0.1Hf-0.15Gd(原子分数,%)合金,对其进行热处理获得近层片组织,研究变形合金及其近层片组织的微观组织特征,并进行了室温、700℃和800℃拉伸实验.组织观察发现,近层片组织由层片团、分布于层片团界的β相以及弥散分布于基体的椭圆状Gd析出物组成.层片团的平均尺寸为40μm,层片组织、β相和Gd析出物的体积分数分别为93.73%、5.25%和1.02%.拉伸结果显示,室温下合金试样的平均抗拉强度为865MPa,平均延伸率可达4.17%,700℃时其平均抗拉强度和延伸率分别为643MPa和22%.Ti-44Al-5V-1Cr-0.3Ni-0.1Hf-0.15Gd合金不仅具有与高β相TiAl合金相当的塑性变形能力,且室温塑性也得到显著提高,这主要归因于β相体积分数的下降和Gd化合物对微观组织室温塑性的贡献.     

Effect of isothermal forging on microstructure and fatigue behavior of blended elemental Ti-6Al-4V powder compacts

The effect of isothermal hot forging (IHF) on microstructure, pore closure, and tensile and fatigue properties of Ti-6A1-4V blended elemental cold pressed and sintered powder compacts was investigated. Two types of sponge fines were used: (a) high chloride produced by the Hunter sodium reduction process (HP) and (b) low chloride produced by the electrolytic process (EP). The as-sintered HP compacts were 99 pct dense while the EP compacts were only 92 pct dense. All sintered preforms were isothermally hot forged below the beta transus temperature and reached almost full density. The microstructure of the HP forged compacts consisted of fine equiaxed alpha, while the EP forged compacts exhibited a coarse lenticular alpha structure after 30 pct reduction and a partially recrystallized structure after 68 pct reduction. It was found that EP compacts forged to a 30 pct reduction exhibited a low fatigue limit of 172 MPa (25 ksi), since the lenticular alpha morphology and the residual porosity resulted in premature fatigue crack initiation. On the other hand, a higher fatigue strength of 485 MPa (70 ksi) was obtained for EP compacts forged to a 78 pct reduction due to the mixed equiaxed/lenticular alpha morphology as well as removal of stress concentration features such as interparticle pore interfaces.     

Microstructure and Mechanical Properties of Wire and Arc Additive Manufactured Ti-6Al-4V

Wire and arc additive manufacturing (WAAM) is a novel manufacturing technique in which large metal components can be fabricated layer by layer. In this study, the macrostructure, microstructure, and mechanical properties of a Ti-6Al-4V alloy after WAAM deposition have been investigated. The macrostructure of the arc-deposited Ti-6Al-4V was characterized by epitaxial growth of large columnar prior-β grains up through the deposited layers, while the microstructure consisted of fine Widmanstätten α in the upper deposited layers and a banded coarsened Widmanstätten lamella α in the lower layers. This structure developed due to the repeated rapid heating and cooling thermal cycling that occurs during the WAAM process. The average yield and ultimate tensile strengths of the as-deposited material were found to be slightly lower than those for a forged Ti-6Al-4V bar (MIL-T 9047); however, the ductility was similar and, importantly, the mean fatigue life was significantly higher. A small number of WAAM specimens exhibited early fatigue failure, which can be attributed to the rare occurrence of gas pores formed during deposition.     

The effect of microstructure and deformation behavior on the hot ductility of Ti-6Al-2Nb-1Ta-0.8Mo

Hot ductility of the alloy Ti-6Al-2Nb-lTa-0.8Mo has been correlated with microstructure and fracture behavior. Low hot ductility was found to be associated with strain localization within the grain boundary alpha phase, producing void formation along the prior-beta grain boundaries and inter-granular fracture. Microstructural features that appear to be critical to the strain localization process are beta grain shape and alpha phase morphology. For the case of Widmanstätten + grain boundary alpha phase morphologies, equiaxed prior-beta grains formed by annealing above the beta transus are required to produce significant strain localization. For the beta processed structure with elongated beta grains due to working above the beta transus temperature, the orientation of the grain boundary alpha phase limits strain localization due to low resolved shear stress. The martensitic Widmanst?ten alpha prime structure formed by quenching from above the beta transus temperature rapidly forms grain boundary alpha upon reheating to temperatures high in the alpha + beta phase field. This results in strain localization in the grain boundary regions in an apparently similar manner to that observed in the Widmanstätten + grain boundary alpha phase morphologies with equiaxed prior-beta grains.     

热处理制度对Ti-55531合金组织和力学性能的影响

通过采用不同的热处理制度研究了时效温度和β退火温度对Ti-55531合金显微组织和力学性能的影响。结果表明:Ti-55531合金固溶加时效处理后可获得初生α相呈长条或等轴状的组织,β基体上大量析出的次生α相使其获得较高的强度,且强度随时效温度升高而显著降低,延伸率变化不明显,断面收缩率在620℃以上随着时效温度升高有所增加,但该组织状态断裂韧度偏低;β退火后可获得均匀的片状组织,具有较高的断裂韧性,抗拉强度在600~650℃之间随退火温度升高呈线性关系降低,可根据需要很方便地调整强度级别,塑性随退火温度升高变化不太明显。     

热处理对Ti-5331合金板材组织及力学性能的影响

研究了核反应堆壳体用Ti-5331合金热轧板材在不同退火温度下的显微组织与力学性能。结果表明:Ti-5331合金板材在相变点以下随着退火温度的升高,初生α相含量逐渐减少,β转变相含量明显增加。当退火温度为700℃时,开始发生静态再结晶,800℃时为等轴组织,900℃时为双态组织,950℃时为网篮组织。随着退火温度的升高,合金板材的抗拉强度先下降后上升,屈服强度呈下降趋势,屈强比逐渐减小;当退火温度在相变点以下时,板材冲击韧性随退火温度升高呈上升趋势,当超过相变点后冲击韧性急剧下降;退火温度对塑性影响较小。经900℃×1 h/AC退火处理的Ti-5331合金板材有着较好的综合性能,抗拉强度为920 MPa,延伸率为15%,V型缺口冲击韧性达到93 J/cm^2。     

Microstructural Evolution in Ti-5111 Friction Stir Welds

The microstructural evolution occurring during friction stir welding of a near-α titanium alloy, Ti-5111, has been examined by backscattered electron imaging and electron backscatter diffraction. The unaffected baseplate (BP) microstructure consists of millimeter-scale prior β grains containing ~100 μm large colonies of aligned α laths, related to each other by a strain-accommodating Burgers orientation relationship. The α laths are separated by fine, 100 to 150-nm-thick, interlath β ribs. A heat-affected zone (HAZ) is observed ~1.5 to 2.5 mm from the tool surface, characterized by a thickening of the β ribs and the formation of secondary α platelets within them closer to the tool. There is a narrow thermomechanically affected zone (TMAZ), comprised of outer and inner regions, observed ~1.0 to 1.5 mm from the tool surface. Deformation is first observed in a ~200-μm-wide outer TMAZ, where the microstructure is refined through an increase in fine secondary (α laths) α laths and the lattice orientations rotate to align the close-packed \(\langle{11\bar{2}0}\rangle\) directions with the shear direction (SD). Continued deformation closer to the tool produces periodic shear bands within a ~300-μm-wide inner TMAZ, resulting in alternating regions of material that are deformed below and above the β transus. Material in the stir zone (SZ) within ~1 mm of the tool surface consists of fine (~10 to 20-μm diameter) equiaxed prior β grains that are delineated by ~500-nm-thick α and contain 150-500-nm thick α laths. The texture exhibits both \(D_1({\bar{1}\bar{1}{2}})[111]\) bcc and \({P_1({1}\bar{1}00)}[11\bar{2}0]\) hcp shear texture components, indicating that this material exceeded the β transus during welding.     

注射成形Ti-6Al-4V合金的显微组织和力学性能

采用金属注射成形方法制备Ti-6Al-4V合金坯体,然后利用溶剂脱脂和热脱脂工艺脱除坯中粘结剂,研究了合金在真空烧结和热等静压烧结条件下的显微组织和力学性能.结果表明:真空烧结Ti-6Al-4V合金具有典型的魏氏体组织,其初始的β晶粒粗大,β晶粒内为次生片状α和薄β相片,空隙较多,合金的强度和塑性较低;合金经热等静压处理后,组织明显细化且均匀,空隙很少或几乎没有,从而强度和塑性都有所提高.     

A Self-Consistent Approach for Modeling the Flow Behavior of the Alpha and Beta Phases in Ti-6Al-4V

The flow behavior of the α and β phases in Ti-6Al-4V was interpreted in the context of a self-consistent modeling formalism. For this purpose, high-temperature compression tests were conducted at various temperatures for a single-phase α alloy (Ti-7Al-1.5V), a variety of near-β alloys, and the two-phase alloy Ti-6Al-4V, each with an equiaxed microstructure. The flow behavior of the α phase in Ti-6Al-4V was deduced from the experimental results of the single-phase α alloy. The flow behavior of the β phase, which was predicted by using the self-consistent approach and the measured flow behaviors of Ti-6Al-4V and Ti-7Al-1.5V, showed good agreement with direct measurements of the various near-β alloys. From these results, it was shown that the strength of the α phase is approximately three times higher than that of the β phase at temperatures between 1088 K and 1223 K (815 °C and 950 °C). It was also concluded that the relative strain rates in the two phases varies significantly with temperature. The usefulness of the approach was confirmed by comparing the predicted and measured flow stresses for other Ti-6Al-4V and near-α alloys.