Materials Science Forum Vols. 546-549

Abstract: The femtosecond laser ablation has been investigated on second generation single crystal superalloy CMSX-4 using a commercial titanium:sapphire laser system (λ = 780nm, τ = 120 fs). The ablation thresholds of bulk alloy CMSX-4 have been determined as a function of the pulse number (1, 10, 100, 1000) in air, argon gas and vacuum. The results indicate that the multiple-pulse threshold of this material decreased with increasing the pulse number in all the cases. For the same pulse number, the threshold changed in the descending order: air, vacuum and argon gas. The incubation coefficient in air has been determined: ξ = 0.86 ± 0.03. The preliminary results showed the better quality of femtosecond laser machining of CMSX-4 in vacuum and argon gas than in air.

Abstract: The sheet metal of a new Ni-based superalloy has been prepared by Electron Beam Physical Vapor Deposition (EB-PVD) technology. The phases, the microstructures and mechanical properties of this alloy before and after heat treatment have been analyzed by X-ray diffractometer, transmission electron microscope, optical microscope, scanning electrical microscope and tensile equipment. Results showed that the size of γ' particles increases gradually and the morphologies of γ' particles changed from spherical shape into cubical shape when temperature increased from low to high. Compared with as-deposited alloy, mechanical properties of heat-treated alloy were improved obviously. It is feasible that superalloy of better properties can be prepared by EB-PVD technology.

Abstract: The strength of nickel-based superalloys usually consists of solid solution strengthening from the gamma matrix and precipitation hardening due to the gamma' and/or gamma" precipitates. In the present work, a model was developed to calculate the high temperature strength of nickel-based superalloys, where the temperature dependence of each strengthening contribution was accounted for separately. The high temperature strength of these alloys is not only a function of microstructural changes in the material, but the result of a competition between two deformation modes, i.e. the normal low to mid temperature tensile deformation and deformation via a creep mode. Extensive validation had been carried out during the model development. Good agreement between calculated and experimental results has been achieved for a wide range of nickel-based superalloys, including solid solution alloys and precipitation-hardened alloys with different type/amount of precipitates. This model has been applied to two newly developed superalloys and is proved to be able to make predictions to within useful accuracy.

Abstract: The optimisation of spray forming IN718 alloy rings for aeroengine applications was investigated using both modelling and experimental approaches. A multiphysics numerical model has been developed and implemented to assist in the optimisation of the spray forming process. IN718 alloy ring preforms were spray formed at University of Oxford (UK) and The University of Bremen (Germany). A variety of on-line monitoring facilities were integrated onto spray forming units to (1) investigate the dynamics of alloy melt atomisation and droplet deposition at a sprayed surface; and (2) acquire ring preform thermal history and various thermal boundary conditions for the numerical model. Modelling and experiments were performed iteratively to investigate the effects of key spray forming parameters including gas metal flow ratio, atomiser scan, substrate heating schemes on the resulting ring preform shape, internal heat flow and solidification. It was found that preform top surface temperature and alloy liquid fraction inside the preform during spray forming were critical factors in governing the formation of macro/microporosity and the grain size of as-sprayed preforms. In the optimised conditions, IN718 alloy ring preforms were characterised by a microporosity of less than 1.5% and randomly oriented equaxied grains of 20-50 μm.

Abstract: A calculation of the interface energy for the Ni-Al binary alloy, including the inter-phase boundary (IPB) energy and the anti-phase boundary (APB) energy, has been performed using the Cluster Variation Method (CVM) with the tetrahedron approximation within the temperature range of 600°C~1300°C. The calculated IPB energies range between 8 and 13 mJ/m2, while the APB energies range between 24 and 46 mJ/m2. Additionally, the dependence of the average composition and the order parameter on distance with the compositionally diffuse interfacial regions has been computed. The calculation also shows the width of the diffuse IPB increases with the temperature linearly.

Abstract: The density of Ni-Co-Al ternary alloys with a relative fixed mole ratio of Ni/Co(xNi:xCo≈86:14) and aluminum concentration change from 0 to 10% mass fraction in liquid states were measured by a modified pycnometric method(MPM). It was found that the density of the liquid Ni-Co-Al alloy decreases with increasing temperature, Al concentration and the ratio of Al concentration to Ni concentration in the alloy. The temperature coefficient of density changes with increasing the Al concentration or the ratio of Al concentration to Ni concentration in the ternary alloys. The measured densities by MPM were comparable with that by the modified sessile drop method, the maximum difference of density between them was about ±0.56%. The molar volume of liquid Ni-Co-Al alloy increases with increasing temperature and changed with increasing Al concentration. And the partial molar volume of aluminum in Ni-Co-Al alloy was calculated approximately. The molar volume of liquid Ni-Co-Al ternary alloy determined in the present work shows a negative deviation from the ideal linear molar volume.

Abstract: By means of pre-compressive stress treated, the cubic γ΄ phase in alloy is transformed into the P-type structure along the direction parallel to the applied stress axis. The influence of the P-type structure on the creep lifetimes of alloy has been investigated by means of the tensile creep testing and microstructure observation. Results show that, compared with the A structure alloy, the P-type γ′ rafted alloy displays a shorter creep lifetimes under the experimental conditions. The microstructure evolution of the P-type structure alloy occurs during tensile creep, in which the p-type γ′ rafted phase is transformed into the N-type structure. The microstructure evolution alloy reduces the creep resistance of the alloy, this is one of the main reasons for reducing the creep resistance of the one.

Abstract: Martensitic β→α′(α″) transformation, β→ω transformation and eutectoid decomposition in a series of Ti-base alloys with d transition metals of Groups I, IV-VIII have been investigated using the techniques of X-ray diffraction, optical and transmission electron microscopy. Phase and structural information is given on the non-equilibrium and metastable modifications occurring in these alloys after quenching from high-temperature β-field and aging. The conditions of the orthorhombic α″-phase, ω-phase and metastable β-phase formation in binary titanium–base alloys with d-metals of V-VIII groups were investigated. It was established that the position of the alloying metal in the Periodic Table defines the presence or absence of the α″-phase in the alloy after quenching and the minimum concentration of the alloying metal necessary for formation of the α″-phase, ω-phase and metastable β-phase.

Abstract: Beta-21S titanium alloy is ranked among the most important advanced materials for a variety of technological applications, due to its combination of a high strength/weight ratio, good corrosion behavior and oxidation resistance. However, in many of these technological applications, this alloy is exposed to environments which can act as sources of hydrogen, and consequently, severe problems may arise. The objective of this paper is to investigate the influence of high fugacity hydrogen on Beta-21S alloy in as-received (mill-annealed and hot-rolled) condition. Hydrogen effects on the microstructure are studied using X-ray diffraction and electron microscopy, while the absorption and desorption characteristics are determined respectively by means of a hydrogen determinator and thermal desorption spectroscopy. Preliminary results at room temperature revealed hydrogen-induced straining and expansion of the lattice parameters. However, neither second phases formation (hydrides), nor hydrogen-induced cracking, were observed after hydrogenation. The main characteristics of hydrogen absorption/desorption behavior, as well as hydrogen-induced microstructural changes in both microstructures are discussed in detail.

Abstract: Superplastic blow forming with diffusion bonded sheet is an effective forming technology for the production of multi-cell structures which should have light weight and high stiffness for aerospace purpose. In the current study, finite element analysis on superplastic blow forming process has been carried out in order to improve the forming process when manufacturing axi-symmetric multi-cell structures using diffusion bonded Ti-6Al-4V multi-sheets. The simulation focused on the reduction of forming time and obtaining finally required shape throughout investigating the deformation mode of sheet according to the forming conditions, which are diffusion bonding pattern and die geometry. To reduce forming time, a preforming die was required, and to obtain the final shape the bonding pattern should be also modified within allowable geometrical margin, so that the sheet is easy to deform. Moreover, an intermediate simulation result, which was forming pressure profile, was employed in real forming test to check if the prediction was reasonably on progress. In the future, a study on the thickness ratio between each sheet should be followed to obtain optimum process parameters.