Materials Science Forum Vols. 539-543

Abstract: New affordable 2024 series aerospace aluminum alloy has been developed. Fracture toughness has been demonstrated increase in inverse proportion to the root of the distance between constituents, Cu2FeAl7, formed during ingot solidification. Higher fracture toughness material is obtained by means of combination of reduction of Fe content and wider spacing between the constituents. The fractured surface of those materials has been confirmed to show larger dimples due to the wider constituents. An outcome is the fracture toughness increases 20% through broadening the space from 75 to 140μm. Fatigue crack growth (FCG) has been governed by the morphology of dispersoids such as Cu2MnAl20, Cr2Mg3Al18 and ZrAl3, formed in homogenization process during heat treatment of ingot. In a low ΔK region, the FCG rate is slower when Cu2MnAl20 becomes larger. It is reconfirmed that the FCG rate is still faster for small dispersoids, Cr2Mg3Al18 or ZrAl3 bearing materials than Cu2MnAl20 bearing one through bridging effect of dispersoids. In a high ΔK region, on the other hands, the FCG rate becomes slower by broadening the spacing of the constituents. A new 2024 series alloy (2x24) with high fracture toughness and excellent FCG resistance has been developed on the basis of process- microstructure-structure methods.

Abstract: Thermal desorption spectroscopy (TDS) was applied to measure the hydrogen in titanium (Ti). Because fracture by hydrogen embrittlement for medical/dental devices of Ti and Ti alloys was reported, dependence of surface oxidation film on hydrogen absorption and desorption behaviors of cp-Ti was carried out. To form the surface oxide film (rutile), the Ti wire samples were annealed in an ambient air at 800oC for 2 hours. Half of the specimens were immersed in a mixed solution of NaF and H3PO4 (APF). The part of the specimens were removed the surface films by abrasion after the heat and the immersion in the APF solution. TDS analysis was applied to detect released ions/molecules during heating from room temperature to 1200oC in a high vacuum vessel for the TiH2 powders, the Ti samples with and without the above treatments. High ion intensity was detected for hydrogen (m/z = 2), and ion intensities of the other ions/molecules were almost the background level in measurements. The concentration of hydrogen in the samples related with heat and chemical process was determined quantitatively, and was not significantly increased by the annealing and the immersion in APF solution if the sample had the tough surface film. However, the sample, which had been removed the surface films before the immersion in APF, showed typical chemical degradation after the immersion in APF for 7 days, that is, high ion intensities of hydrogen, fluoride, water and hydroxyl ions were detected. It was concluded that the control of the surface oxide film on the medical/dental Ti devices is important in determining its longevity.

Abstract: Two-Dimensional finite element analysis was carried out to optimize the equal channel angular pressing process (ECAP) for strain homogeneity under frictionless and frictional conditions. The effect of outside corner angle (Ψ), inner radius (r) and shear friction (m) on the strain homogeneity was investigated. The strain homogeneity can be increased by correcting the outside corner to eliminate the corner gap between the sample and the die at the expense of average strain. Small inside radius and outside corner radius would provide large deformations without much loss in the strain homogeneity under frictionless conditions. The work piece deformation is by bending if the inner radius exceeds a critical value. No improvement in strain homogeneity was observed under frictional conditions.

Abstract: The aim of this work is to study the flow instabilities occurring during hot forging of titanium alloy blades. In this view, the viscoplastic deformation behaviour of Ti-6Al-4V alloy is investigated by means of torsion tests under isothermal hot working conditions at temperatures ranging from 800 to 1020 °C and strain rates of 0.01, 0.1 and 1s−1. The thermomechanical processing is performed up to a true strain of 10. The flow stress data are analysed in terms of strain rate and temperature sensitivities. A constitutive equation that relates not only the dependence of the flow stress on strain, strain rate and temperature, but also for the fraction of each phase α and β is proposed. Two mechanical models are compared : the uniform strain rate model (Taylor) and the uniform plastic energy model (IsoW). The usual strain rate sensitivity and activation energy values of Ti-6Al-4V alloy are obtained by fitting the experimental data. Furthermore, specific values of strain rate sensitivities and activation energies are calculated for the α and β phases providing thus a constitutive law based on the physics of the α / β phase diagram. The flow stress is then related to strain by an empirical equation taking into account the flow softening observed after a true strain of 0.5 and the steady state flow reached after a true strain of 4. Comparison of the calculated and measured flow stresses shows that the constitutive equation predicts the experimental results with a reasonable accuracy. The above constitutive equation is then used for simulating forging processes by the finite element method. The calculations exhibit the localisation of deformation produced by shearing effects in the form of the classical X shape.

Abstract: In this study, gas nitriding was processed for various sizes of Ti specimens which were produced by metal injection molding (MIM) process, for the sake of enabling the high functionality at low processing cost. It was shown by the nitriding treatment that the hardness of the surface increased extremely and the microstructure changed to the TiN and acicular α-Ti phase. With respect to the effect of the nitriding on the size of parts, micro dumbbell specimen had around five times higher content of nitrogen than the block ones. It was suggested that the size of μ-MIM products is so small that the surface treatment can contribute significantly to improve the properties, and then gas nitriding process may be one of effective surface treatment methods for high functionality of μ-MIM Ti products.

Abstract: This work reports the influence of oxidation on the superplasticity of commercially pure titanium at high temperatures. Uniaxial tensile tests were conducted at temperatures in the range 600-800°C with an initial strain rate of 10s-1 to 10s-3. This study shows that oxidization at the surface of the alloy causes oxide film on the surface of commercially pure titanium alloy, and the thickness of oxide film increase with increasing exposure time and temperature. XRD analysis shows that the oxide film consists of TiO2. Because this oxide film is very brittle, it can induce clefts and degrade the ductility of the titanium at high temperatures. The mechanism of the initial clefts was investigated and a model for the cleft initiation and propagation during high temperature tensile test was proposed.

Abstract: The growth mechanism of bainitic α plates in Ti-4.5Fe-6.8Mo-1.5Al, a metastable β Ti alloy, has been investigated by optical microscopy, electron probe microanalysis (EPMA) and dilatometry. The observations are compared with the transformation characteristics of primary α plates, which form at relatively high temperatures. The primary α plates form predominantly on β grain boundaries, whereas the bainitic α plates nucleate both at grain boundaries and intragranularly. It is shown that the morphological transition with decreasing temperature is associated with a change in transformation mechanism. The EPMA results show that the primary α plates are formed by a partitioning transformation. In contrast, the growth of the bainitic α plates is partitionless, followed by a post-transformation redistribution of Fe. This mechanism is similar to bainite in steel. The Fe diffusion from the supersaturated bainitic α plates to the β matrix causes the observed dilatation signal. The results of dilatometry in conjunction with optical microscopy indicate that a low misfit between the lattice structures exists, which is favourable for a partitionless transformation to occur at a low undercooling below T0.

Abstract: The microstructure and mechanical properties including room temperature fracture toughness Kq, tensile strengthσb and elongationδ at 1250°C of the Nb based alloy directionally solidified in an electron beam floating zone melting (EBFZM) furnace have been evaluated. The microstructure is primarily composed of Nb solid solution (Nbss), α-(Nb)5Si3 and (Nb)3Si phases. After directional solidification with the moving rate of electron beam gun R being respectively 2.4, 4.8 and 7.2 mm/min, the primary Nbss dendrites, Nbss + (Nb)5Si3/(Nb)3Si eutectic colonies (lamellar or rod-like) and divorced Nb silicide plates align along the longitudinal axes of the specimens. When R = 2.4 mm/min, the best directional microstructure is obtained. Directional solidification has significantly improved theσb at 1250°C and Kq. The maximumσb occurs for the specimens with R = 2.4 mm/min and is about 85.0 MPa, meanwhile, the Kq is about 19.4 MPam1/2.