Materials Science Forum Vols. 654-656

Abstract: In order to develop a perspective of the new metastable β titanium alloy——TB-13, the influence of ageing treatments on the microstructure and properties has been studied. The results show that TB-13 alloy displays high age strengthening effect and fine precipitation. During the one-step aging process, the higher the aging temperature, the coarser the α plate thickness. As a result, TB-13 alloy shows the lower the strength and the higher the ductility. The best combination of strength and ductility is acquired in the TB-13 alloy after solution treatment at 1133K following aging at 813K for 16h. The ultimate strength is near 1400MPa and the specific elongation is 7.0% by ageing. At last, the details of the transformation mechanism and microstructural evolution accompanying ω-assisted α nucleation during continuous heating are discussed.

Abstract: Within the past decade, it has been shown that twinning in α, β, and α + β titanium alloys can occur at speeds much lower than the speed of sound by many orders of magnitude. This is related to the twinning deformation mechanisms controlled by the diffusion of oxygen as compared to simply a shear process. Very recent developments, such as strain-rate effects on twinning, support a recent hypothesis that the twinning in these materials is controlled by a slow diffusion process, resulting in time-dependent twinning. These recent developments, along with the ramifications of the findings will be outlined in this article.

Abstract: The deformation microstructure of a metastable beta-Ti alloy with chemical composition of Ti-23Nb-0.7Ta-2Zr-1.2O at.% after cold swaging has been studied by electron backscattering diffraction and high-resolution transmission electron microscope. The results showed that the deformation structure of the cold-swaged alloy appears to be similar to the swirled structure that commonly found in bcc metals heavily deformed either by wire-drawing or by rotary swaging process, and <110> fiber texture is a typical texture component of bcc metals. HRTEM results further showed <111> dislocations in the deformed alloy. Additionally, {112}<111> mechanical twinning and stress-induced omega transition were further revealed.

Abstract: The elastic response of a metastable β titanium alloy, Ti-25Nb-3Zr-3Mo-2Sn, is characterised. The effects of processing and heat treatment on phase composition and mechanical behaviour are reported. The alloy exhibits pseudoelastic behaviour which is related to the reversible formation of the stress induced α" phase. The solution treated condition displays the greatest pseudoelasticity and the hot rolled condition the least. Tensile deformation reduces the modulus of the alloy. This is due to increases in the relative proportions of the low modulus α" phase.

Abstract: Titanium alloy sheets have excellent specific strength and corrosion resistance as well as good performance at high temperature. Recently, titanium alloys are widely employed not only aerospace parts but also prosthetics and motorcycle. However, titanium and its alloys are difficult-to-form materials due to limited slip system and plastic anisotropy. Titanium alloy sheets were usually formed by slow forming or hot forming with heating die and specimen. In the sheet metal forming area, FE simulation technique to optimize forming process is widely used. To achieve high accuracy FE simulation results, identification of material properties and deformation characteristics such ad yield behaviors are very important. In this study, the yield locus of Ti-6Al-4V sheet was obtained at warm temperature. The experimental results are compared with the theoretical predictions. Also, the Forming Limit Curves (FLC) was achieved at warm temperature.

Abstract: The hot deformation behaviour of the alpha / beta Ti-6Al-4V alloy was investigated at various temperatures and strain rates by means of compression and torsion tests. As expected, the peak flow stress increased with increasing strain rate and decreased as the initial sample temperature was increased. The different flow behaviours observed are discussed in terms of the volume fraction of each phase. The dual phase Ti-6Al-4V alloy was assumed to be a composite material containing a soft phase and a hard phase. By taking into consideration the phase-to-phase interaction and volume fraction change with temperature, constitutive models are proposed to simulate the deformation behaviours. By application of the rule of mixtures the modelled yield stresses showed good agreement with the experiment results.

Abstract: The microstructural evolution of TIG welded joint in Ti-6.5Al-2Zr-1Mo-1V alloy treated by thermo hydrogen processing was investigated by optical microscopy, transmission electron microscopy and scanning electronic microscopy and the mechanical properties of welded joints were tested and analysed. The results showed that δ-hydride formed after hydrogenising at 700°C for 3 hours, and it is a face-centered cubic crystal. And these hydrides brought a strong fuzzy effect on microstructure. A lot of rhombic martensite phase α" generated in the area of welded joint. Metastable β(H) phase and α" phase were gradually decomposed to α and δ phases during aging. Hydrides were dissolved, the hydrogen was removed from the alloy and the recrystallization occurred in the process of annealing in vacuum. After hydrogen treatment, the refinement effect of grains in the fusion area was very obviously, that is, coarse columnar grains changed to finer equiaxed grains. Non-equilibrium phase reduced in grains. After hydrogen treatment (hydrogenization at 700°C for 3 hours, eutectoid at 300°C for 8 hours and annealing at 750°C for 8 hours in vacuum), the tensile strength of welded samples increased by 25.7%, and the elongation increased by 37.6% compared with welded samples without hydrogen treatment. After hydrogen treatment, the surface morphology of tensile fracture was relative flatness.

Abstract: -annealed Ti-6Al-4V ELI alloy has a special chemical composition and manufacturing process, intended to optimise its fatigue and fracture properties. This alloy is used in primary fatigue-critical structures of advanced military aircraft, but little has been generally published about its properties, particularly the fatigue and crack growth behaviour under service loading. A test programme has been set up to assess the fatigue durability and damage tolerance properties of the alloy, in the context of the two most relevant analysis methods. The first is strain – life analysis, used to estimate safe lives (durability). The second is fatigue crack growth analysis, used to assess the damage tolerance behaviour. This paper presents preliminary results of the test programme.

Abstract: The fusion zone of an electron beam welded Ti-6Al-4V alloy presents a ' martensitic structure which leads to a change of mechanical properties. Starting from two manufacturing processing routes for the alloy (1) a  processing followed by the weld (the reference microstructure), (2) an  processing followed by welding and a post weld heat treatment (PWHT), the microstructure can be adjusted to avoid local difference of strength, fatigue properties and impact toughness. This results from the optimisation of the process and of the PWHT. The present work investigates the mechanical behaviour and the damage mechanism of both base metal and fusion zone in regards to the microstructure and to the heat treatment parameters.

Abstract: A three 3D transient finite element model has been developed to predict the temperature distribution in Ti6Al4V alloy plate workpiece. It is found that the temperature profile is strongly dependent on the parameters of the laser beam and material properties. Also the thermal model results were compared with results produced by experimental work and these show close agreement.