Hydrogen-induced slow crack growth in Ti-6Al-6V-2Sn

The effect of hydrogen and temperature on threshold stress intensity and crack growth kinetics was studied in Ti-6Al-6V-2Sn containing 38 ppm hydrogen. A slight decrease in threshold values occurred as temperature decreased from 300 K while they increased significantly above 300 K. For a given test temperature, crack growth rates exhibited an exponential dependence on stress intensity over a major portion of growth. At 300 K the rates reached a maximum. Slow crack growth occurred predominately by cleavage ofα grains which has been associated with hydride formation. The stress intensity required for hydride formation at a crack tip can be determined from hydrogen concentration and solubility considerations under stress. As these values differed from observed thresholds, a strong influence of microstructure was suggested and subsequently revealed by crack front examination. Quantification of this effect with a modified Dugdale-Barenblatt model relates the effective stress intensity at the crack tip to the applied stress intensity. Microstructure was also found to exert a strong influence on slow crack growth behavior when examined in terms of the effective stress intensity,K _eff. From Arrhenius plots of crack growth rates for variousK _eff, activation energies of 27.0 to 32.8 kJ/mol were obtained and related to the diffusion of hydrogen through theβ phase. The increase in crack growth rates with increasing temperatures up to 300 K is attributed to the temperature dependence of hydrogen diffusion. The decrease in crack growth rates above 300 K is related to a hydride nucleation problem.     

Silicide Formation in Ti-3Al-8V-6Cr-4Zr-4Mo

TEM, including convergent beam electron diffraction (CBED), SEM, and EDX techniques, were used to] characterize the silicide (Ti, Zr)₅ Si₃ which forms in Beta-C (Ti-3Al-8V-6Cr-4Zr-4Mo) and other similar titanium alloys. Space group of this silicide is determined as P6/mmm. In addition, solvus of this silicide was determined and a complete phase field diagram was constructed for Beta-C alloy. It was found that morphology and distribution of the silicide strongly depends on thermal and thermomechanical treatments. Continuous grain boundary silicide was found to be detrimental for tensil ductility which results in intergranular fracture. Thermal and thermomechanical treatments were identified which can reduce or eliminate continuous grain boundary silicide precipitation.     

High-temperature mechanical behavior of Ti-6Al-4V alloy and TiC p /Ti-6Al-4V composite

Mechanical behaviors at 538 °C, including tensile and creep properties, were investigated for both the Ti-6Al-4V alloy and the Ti-6Al-4V composite reinforced with 10 wt pct TiC particulates fabricated by cold and hot isostatic pressing (CHIP). It was shown that the yield strength (YS) and ultimate tensile strength (UTS) of the composite were greater than those of the matrix alloy at the strain rates ranging from approximately 10⁻⁵ to 10⁻³ s⁻¹. However, the elongation of the composite material was substantially lower than that of the matrix alloy. The creep resistance of the composite was superior to that of the matrix alloy. The data of minimum creep strain rate vs applied stress for the composite can be fit to a power-law equation, and the stress exponent values of 5 and 8 were obtained for applied stress ranges of 103 to 232 MPa and 232 to 379 MPa, respectively. The damage mechanisms were different for the matrix alloy and the composite, as demonstrated by the scanning electron microscopy (SEM) observation of fracture surfaces and the optical microscopy examination of the regions adjacent to the fracture surface. The tensile-tested matrix alloy showed dimpled fracture, while the creep-tested matrix alloy exhibited preferentially interlath and intercolony cracking. The failure of the tensile-tested and creep-tested composite material was controlled by the cleavage failure of the particulates, which was followed by the ductile fracture of the matrix.     

Elevated-Temperature Mechanical Behavior of As-Cast and Wrought Ti-6Al-4V-1B

This work studied the effect of processing on the elevated-temperature [728 K (455 °C)] fatigue deformation behavior of Ti-6Al-4V-1B for maximum applied stresses between 300 to 700 MPa (R = 0.1, 5 Hz). The alloy was evaluated in the as-cast form as well as in three wrought forms: cast-and-extruded, powder metallurgy (PM) rolled, and PM extruded. Processing caused significant differences in the microstructure, which in turn impacted the fatigue properties. The PM-extruded material exhibited a fine equiaxed α + β microstructure and the greatest fatigue resistance among all the studied materials. The β-phase field extrusion followed by cooling resulted in a strong α-phase texture in which the basal plane was predominately oriented perpendicular to the extrusion axis. The TiB whiskers were also aligned in the extrusion direction. The α-phase texture in the extrusions resulted in tensile-strength anisotropy. The tensile strength in the transverse orientation was lower than that in the longitudinal orientation, but the strength in the transverse orientation remained greater than that for the as-cast Ti-6Al-4V. The ratcheting behavior during fatigue is also discussed.     

添加Nd元素对Ti-6Al-4V-2Cr合金组织细化的影响

采用冷等静压–真空烧结法制备Ti-6Al-4V-2Cr-1Nd合金,然后进行固溶及时效热处理,通过实验与最小错配度理论计算,研究Nd元素对该合金组织细化的影响,并分析细化机理。结果表明,添加1%(质量分数)的稀土元素Nd后,析出相Nd2O3能有效促进晶粒细化。二维错配度的计算结果证明析出相Nd2O3是有效的形核剂,可促进非均匀形核,增加形核率,从而使晶粒细化。通过对合金试样薄区进行高分辨率观察,发现另一种絮状的、非常细小的、弥散分布的Nd2Ti4O11相,由于其界面错配度较低,也可作为非均匀形核的核心,促进形核,起到细化晶粒的作用。     

Fabrication and properties of heavy section extrusions of Ti-6Al-6V-2Sn and Ti-8Mo-8V-2Fe-3AI alloys

This study is concerned with the fabrication in heavy section of the titanium alloys Ti-8Mo-8V-2Fe-3Al (Ti-8823) and Ti-6Al-6V-2Sn (Ti-662). The technique utilized to achieve 84 pct reduction during the extrusion of heavy section cylindrical hollows is given. The response to subsequent aging of both alloys is described in terms of the effect on important mechanical properties such as yield and ultimate tensile strength, ductility and fracture toughness. For the Ti-8823 alloy extruded in heavy section, it is shown that the optimum heat treatment consists of aging directly after hot working rather than the more common solution treatment and age cycle. With the former heat treatment, uniform through the thickness mechanical properties are obtained. Uniform mechanical properties are also obtained through the section of the Ti-662 extrusion with a solutionizing and overaging heat treatment.     

In situ scanning auger analysis of hydrogen-induced fracture in Ti-6Al-6V-2Sn

Metallurgical and Materials Transactions A -     

Design for Isothermal Forging of Ti-46.5Al-2.5V-1.0Cr-0.3Ni Alloy

 Abstract: The three-dimensional finite element code was utilized to investigate the isothermal forging of Ti-46.5Al-2.5V-1.0Cr-0.3Ni (at.%). The partition of height reduction, effective strain and damage during the forging were analyzed. And the experiment was employed to verify the simulation results. The results show that severe plastic deformation can improve the homogeneity of the deformed microstructure and the higher accumulative height reduction can be achieved through two-step forging. With the isothermal forging of 60%+62.5% (total 85%), the entire forged pancake can acquire refined microstructure and expand the uniform flow zone to almost the entire volume without generating strain-induced defects or cracking.     

Fatigue crack growth behavior of Ti-6Al-6V-2Sn in methanol and methanol-water solutions

Differences in the corrosion fatigue crack growth behavior of anα–β titanium alloy in chloride-containing aqueous and methanol environments are reported, and discussed in relation to differences in repassivation behavior for the two types of environments. Experiments have been conducted with various solution mixtures of water (a passive film-forming environment) and methanol (a nonfilm-forming environment) to define the role of repassivation in controlling fracture modes and crack growth rates at different frequencies. The critical event in determining whether the repassivation process can suppress environmental fatigue fracture is the interaction between the rate of exposure of fresh metal surfaces at the crack tip and the rate at which they can be repassivated. The out-come of this mechano-chemical interaction is shown to be dependent on the frequency and stress intensity(ΔK) level as well as the chemistry of the environment. As a result, differences in repassivation behavior for methanol-water solutions can be correlated with major differences in fatigue crack growth rates and fracture modes at low ΔK levels, whereas repassivation differences have little effect at high ΔK levels. Based on these low-ΔK corrosion fatigue characteristics, methanol solutions are concluded to be far more detrimental to titanium alloys than aqueous solutions.     

Creep deformation and damage in a continuous fiber-reinforced Ti-6Al-4V composite

Mechanisms of longitudinal creep deformation and damage were studied in an eight-ply unidirectional-reinforced SCS-6/Ti-6Al-4V composite. The composite was creep tested in air under constant tensile load at temperatures from 427 °C to 650 °C and stresses from 621 to 1380 MPa.In situ acoustic emission (AE) monitoring and post-test metallographic evaluation were used to study fiber fracture and damage during creep. At low creep stresses, creep rates continuously decreased to near-zero values. This was attributed to a mechanism of matrix relaxation and the time-dependent redistribution of load from the ductile matrix to the elastic fibers. At higher stresses, progressive fiber overload occurred during creep loading. In this case, the composite exhibited a stage of decreasing creep rate (due primarily to matrix relaxation), followed by a secondary stage of nearly constant creep rate due to fiber fracture. The results indicate that interfacial oxidation damage and inefficient load transfer at elevated temperatures significantly decreased the capability of broken fibers to carry load. As a result, additional time-dependent stress redistribution occurred in the composite, which was responsible for the secondary creep stage.     

Diffusion Bonding of Ti-6Al-4V Sheet with Ti-6Al-4V Foam for Biomedical Implant Applications

Advanced metallic bone implants are designed to have a porous surface to improve osseointegration and reduce risks of loosening. An alternative approach to existing surface treatments to create a porous surface is to bond separately produced metallic foams onto the implant. To assess the feasibility of this approach, a Ti-6Al-4V foam was diffusion bonded onto bulk Ti-6Al-4V in an argon atmosphere at temperatures between 1173 K and 1223 K (900 °C and 950 °C) for times between 45 and 75 minutes. These specimens were tested in tension to determine bond quality: failures occurred in the foam, indicating a strong diffusion-bonded interface. The quality of the bond was confirmed by metallographic studies, indicating that this approach, which can also be applied to creating of sandwich with porous cores, is successful.     

Synthesis and material characterization of beryllium/Ti-6Al-4V composites

The synthesis of Be/Ti−6Al−4V alloy composites from mixed powders of the elemental constituents is described and bulk mechanical properties of the composites are compared to theoretical predictions. A synergistic strengthening effect is predicted based on interface constraints imposed on ductile beryllium fibers during plastic deformation. Composites fabricated with three separate beryllium compositions,i.e., 40, 50 and 60 vol. pct, had measured yield strengths between 240 MPa and 454 MPa and tensile moduli between 155 and 175 GPa. The ultimate fracture strengths for these compositions is between 825 MPa and 950 MPa and was found to be limited by the fracture strength of the Be/Ti-alloy interface. Both authors were formerly with KBI, Reading, PA.     

Ti-46.5Al-2.5V-1.0Cr-0.3Ni合金高温变形动态再结晶行为

采用热模拟实验研究了变形参数(应变量、变形温度、应变速率)对Ti-46.5Al-2.5V-1.0Cr-0.3Ni合金微观变形组织的影响.研究结果表明,在应变量逐渐增加的热压缩变形过程中,一定变形量后开始出现层片弯曲、折曲或破碎,随着应变量的增加,动态再结晶形成的等轴晶粒增多,残余层片团减少.变形温度对钛铝合金的变形组织有着显著影响,提高热变形温度,有利于合金中的动态再结晶以及变形组织的均匀化,降低残余层片团的体积分数.较低的应变速率能促进变形时的动态再结晶,有利于提高变形组织的均匀性.还对γ-TiAl合金动态再结晶新晶粒形成过程进行了讨论.     

Processing-microstructure relationships for Ti-6Al-2Sn-4Zr-2Mo-0.1Si

The detailed relationships between thermal-mechanical processing parameters and resulting microstructures for Ti-6Al-2Sn-4Zr-2Mo-0.1 Si (Ti-6242) have been established through compression testing and heat treatment. Beginning with either an equiaxed alpha or Widmanstätten alpha preform microstructure, isothermal compression tests were run at strain rates typical of isothermal forging (10^?3 to 10^?1 s^?1) and conventional forging (1 to 100 s^?1). Metallographic investigation of these test specimens in as-deformed and heat treated conditions was used to characterize deformation-induced microstructures and transformations. For the equiaxed alpha microstructure, it was shown that deformation, as well as post-deformation heat treatment, were more effective in promoting microstructures close to the expected equilibrium ones than heat treatment alone, a finding similar to that for other alloy systems. For the metastable Widmanstätten alpha microstructure, the deformation and heat treatment parameters that promote the development of an equilibrium, equiaxed alpha microstructure have been determined. For this microstructure, two separate temperature-strain rate regimes have been identified, and the resulting microstructures correlated with the measured flow stress behavior. For the low temperature regime, deformation is highly nonuniform, and the microstructural features are shown to be similar to those in pearlitic steels and other lamellar alloys. In the higher temperature regime, on the other hand, deformation is much more uniform. The results presented can be applied to select hot forging parameters for the control of final microstructure and properties in Ti-6242 and similarα/β titanium alloys.     

Cavitation-Induced erosion of Ti-6Al-4V

Cavitation-induced erosion has been examined in Ti-6A1-4V in the mill annealed, solution-treat and aged, and beta annealed conditions. Weight loss data show only small differences between heat treatments with the solution-treat and aged microstructure exhibiting the lowest weight loss rates. Sequential micrographs of the same specimen area as a function of erosion time show that initial fracture occurs along the α-β interfaces and along crystallographic slip bands in the α-phase. The early stages of erosion are also dependent on the orientation of the Widmanstatten colonies in the beta annealed condition. These observations strongly suggest that fatigue fracture is important, at least in the early stages of the cavitation erosion process. Depression of the softer α- phase also occurs at short exposure times, and this facilitates fracture and removal of the exposed material;i.e., β-phase or tempered martensite. Examination of the eroded surfaces in the later stages where considerable material has been removed shows little evidence of the underlying microstructure, despite the distinct differences in the micro-structures of the samples tested. Formaly Undergraduate Students at Michigan Technological University     

Transverse creep of SiC/Ti-6Al-4V fiber-reinforced metal matrix composites

The transverse creep response of an 8-ply SiC (SCS-6)/Ti-6Al-4V composite was measured at 482 °C from 69 to 276 MPa. Creep samples with fibers exposed at the edges as well as specimens with fully embedded fibers were tested under stepped loading conditions with increasing load. The response of each sample geometry was compared with creep data from the unreinforced matrix (‘neat’ material). The samples with exposed fiber ends exhibited minimum creep rates that were higher than those of the neat material at all stresses, and the stress exponent was slightly large than the neat material. The embedded fiber samples possessed minimum creep rates that were smaller than the neat material at low stresses (<115 MPa), but became equivalent to the exposed fiber data at the highest stress (276 MPa). The apparent stress exponent for the embedded fiber composite was significantly larger than the neat material. The exposed fiber test data were well represented by a standard Crossman analysis, where the fibers were considered to have completely debonded. A stress singularity in the interfacial region at the sample edge is responsible for this behavior. The Crossman model was modified to incorporate the effect of a finite interface strength (120 MPa), and this was used to describe the response of the samples with embedded fibers. A reasonable fit to this representation was obtained. However, the measured minimum creep rate at the lowest stress was significantly lower than that predicted by the Crossman analysis for fully bonded fibers. This article is based on a presentation made in the symposium “Fatigue and Creep of Composite Materials” presented at the TMS Fall Meeting in Indianapolis, Indiana, September 14–18, 1997, under the auspices of the TMS/ASM Composite Materials Committee.     

Fretting wear behaviour of plasma nitrided Ti-6Al-4V fretted against unnitrided Ti-6Al-4V and alumina counterbodies

Ti-6Al-4V samples were plasma nitrided at 520°C in two environments (nitrogen and a mixture of nitrogen and hydrogen in the ratio of 3:1) for two different time periods (4 h and 18 h). Fretting wear tests were conducted on unnitrided and nitrided samples for 50,000 cycles using two counterbody materials (unnitrided Ti-6Al-4V and alumina). Gross slip prevailed at a normal load of 4.9 N while mixed stick-slip prevailed at 9.8 N. Tangential force coefficient values of plasma nitrided samples were lower than those of unnitrided samples. The tangential force coefficient nearly stabilised after thousand cycles in case of samples tested against Ti-6Al-4V counterbody. On the other hand, it showed a continuously increasing trend in case of specimens tested against alumina counterbody. The samples nitrided for 4 h exhibited higher hardness and lower tangential force coefficient compared to the specimens nitrided for 18 h. The samples nitrided in nitrogen-hydrogen mixture environment exhibited higher hardness and lower tangential force coefficient compared to the specimens nitrided in nitrogen. The samples plasma nitrided in nitrogen-hydrogen mixture for 4 h exhibited the highest hardness and the lowest tangential force coefficient. The wear volume of the plasma nitrided samples was lower than that of the unnitrided samples. Owing to tribochemical reactions, the wear volume of unnitrided and nitrided samples fretted against alumina ball was higher than that of the samples fretted against Ti-6Al-4V. A consistent trend was not observed regarding which nitriding condition would result in lower wear volume at different loads.     

Low temperature creep of Ti-6 Al-4 V

Creep tests were conducted at 295 K on Ti-6 Al-4 V in the solution treated and aged (4 h at 815 K) condition, and in the as-welded condition. Some aged specimens were tested after pre-straining. Creep stresses ranged from 40 to 90 pct of the aged material yield strength. Results showed that creep was of the primary or transient kind in all cases, and was much greater in welded than in aged material. In general, pre-strains reduced creep, although a strain larger than 10^-3 was needed to do this at the highest creep stress. Activation areas A* were between 10 and 20 b², and thus were similar to tensile results on titanium and its alloys. The microstructural rationale applied to Ti-5 Al-2.5 Sn in earlier work, based on the character of dislocation sources, proved successful in understanding the effects of prestrain in this work. Formerly with Sandia Laboratories, Livermore, Calif.