Materials Science Forum Vol. 944

Abstract: the effects of solid solution temperature on microstructure and properties in extruded and forged FGH95 superalloys are studied in this work. The superalloys with different micrtstructures are prepared through solid solution heat treatment at different temperature, and the microstructures were analyzed by SEM and EBSD. The results show that following with the increase of solid solution temperature, the grain size increase and the amount of primary γ′-phase located at the grain boundary decrease. When the solid solution temperature is higher than the solvus, the primary γ′-phase disappear completely, and as a result, the grain size grow significantly. The orientation of the FGH95 superalloys would not be influenced by the solid solution temperature, but the amount of twins increases following with the increase of solid solution temperature. The tensile and creep property is also studied in this work, the results show that the sub-solid solution heat treatment is suitable for Extruded and Forged FGH95 superalloys.

Abstract: Intermediate temperature creep properties are considered a key indicator of single crystal superalloys used for turbine blades of aircraft engines. The interrupted and ruptured creep tests were carried out in a second generation single crystal superalloy under the conditions of 760°C/785MPa. The creep rupture life as well as minimum creep rate were also in the same level of those in CMSX-4 and PWA1484. The microstructural evolution at different creep stages were analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed the γ’ phases kept the cuboid morphology mostly until the creep rupture, and super lattice stacking faults (SSFs) extended along [-1 1 0] and [-1-1 0] orientations within the γ’ precipitate were the typical dislocation configuration.

Abstract: With the development of aircraft engine, higher requirement was put forward on turbine disk materials. In the present work, new thoughts on improving high temperature properties of superalloys have been proposed and a newly developed candidate turbine disk material for 850°C-900°C application with a composition of Ni-Co-Cr-W superalloy has been investigated. The results show that W is beneficial for mechanical properties. Microstructural characteristics and hot deformation of this new alloy were studied by optical microscope (OM), field emission scanning electric microscope (FESEM) and energy dispersive X-ray spectrometer (EDX) and differential scanning calorimetry (DSC). The results show that the main precipitates in the as-cast condition are γ’ phase, primary MC carbides and eutectic phase. The incipient melting temperature, γ’ solvus and MC solvus are 1312°C, 1220°C and 1356°C respectively. Cracks are observed in the tested samples after hot deformed at 1160°C to 1220°C with 30% strain. They initiated at the surface of the samples and propagated along the grain boundaries and also initiated at the interface of carbides and matrix.

Abstract: A study was carried out to determine the deform behavior in a polycrystalline Ni-based superalloy based on micropillar compression tests. Three different heat treatments of this alloy were evaluated by systematically controlling the cooling rate from the supersolvus solutioning step, in order to examine the effect of γ' microstructure on the CRSS (Critical Resolved Shear Stress). It is shown that the γ' precipitates have the marked effect on the deform behavior of micropillar, as the size of the secondary γ' in the general microstructure decreased, the CRSS were increased; SEM and TEM examination show that compressive deformation behavior of [110] oriented micropillars is dominated by dislocation plasticity. Cross slip is occurring in different slip planes, and exhibits away slip lines as well as stacking faults across γ/γ' microstructure. The γ' and carbides in this alloy effectively impeded the motion of dislocations and inhibit the strain burst behavior, caused by the running out of dislocations to the surface of micropillars,which leads to the sustained strain hardening the plastic deformation stage of micropillars.

Abstract: In this work, double annealing processes of Ti-5Al-5Mo-5V-1Cr-1Fe titanium alloy were systemically investigated, and the microstructure and property of different processes were compared. The results show that, when the first stage annealing is air cooling, the microstructure with continuous and straight grain boundary is obtained, and corresponding mechanical property is not very good; When the first stage annealing is furnace cooling, the influence of first stage annealing temperature on microstructure and property is obvious. Excellent comprehensive property can be obtained when β grain boundary and a phase are discontinuous and tortuous. Further, based on this research, a new double annealing process is developed, which can obtain excellent ductility when large size forging is fabricated above the phase transformation point and has small deformation.

Abstract: In this paper, the flow stresses, the constitutive equation, processing map and the critical conditions of dynamic recrystallization (DRX) of the hot forged QCr0.8 alloy are studied by hot compressive test in the 750-900°C temperature and 0.01-10s^-1 strain rate ranges using Gleeble-1500D thermo-mechanical simulator. The compression reduction of thermal compression deformation is 50%. The results show that the thermal deformation temperatures and strain rates have a significant effect on the high temperature deformation behavior of the alloy. The higher the temperature, the smaller the strain rate and the easier the DRX of the alloy is found.The peak stresses of the alloy decreases with the increase of temperature and increases with the increase of the strain rates.The flow stresses during hot deformation can be described by a hyperbolic sine function. The activation energy Q of the thermal compression deformation is determined to be 370.8KJ/mol. The constitutive equation and processing map of the alloy are established. Critical strains of DRX ε_c are studied by the inflection point characteristic of the lnθ-ε curve of the alloy and the corresponding minimum value of the ∂θ (∂θ)/∂ε-ε curve.

Abstract: Uniform direct chill (UDC) casting is coupled annular electromagnetic stirring and intercooling, having been utilized for the preparation of large-sized aluminum alloy billet. In this paper, the UDC casting was applied to 2A14 aluminum alloy billets with a diameter of 584 mm. Hot compression tests, cogging and ring rolling procedures were carried out for the billets, respectively. The results show that during the deformation temperature of 420 °C and the strain rate of 0.01 s⁻¹ to 10 s⁻¹, the flow stresses of different positions are higher and more stable in the UDC casting billet than in the normal direct chill (NDC) casting billet. The dislocation glide is the dominant deformation mechanism of 2A14 aluminum alloy. Meanwhile, the UDC casting significantly improves the mechanical properties of the rolled rings in tangential and axial directions compared with the NDC casting.

Abstract: The transient electromagnetic phenomena and solidification of Al-Cu alloy under a typical pulsed magnetic field (PMF) are numerically studied by a two-dimensional (2D) axisymmetric model. The results show that the magnetic flux density, eddy current density, Lorentz force and Joule heat all inherit the instantaneous and intermittent feature of the PMF, and their amplitudes and phases decrease with the increasing distance to the side surface of the ingot. The Lorentz force appears alternatively as pressure force and pull force mainly in the radial direction. Forced convection is induced in the liquid metal, and the flow field is composed of a clockwise vortex and a counter-clockwise vortex in the meridian plane of the ingot. The melt velocity is accompanied with a dramatic periodic oscillation. The temperature field in the ingot with the PMF tends uniform due to the mixing effect of the melt flow. However, the convection is damped soon after the solidification starts due to the increasing penetration resistance, and the temperature field gradually approximates that in the case without the PMF.

Abstract: The directional solidification process of Sn–10 wt% Bi alloy with low melting point was observed by synchrotron X-ray imaging technology. The mold temperature was controlled, and the dynamic images of a series of alloy solidification behavior were obtained. The results show that columnar crystal grows in dendrite morphology. It is also found that dendrite morphology changes at different mold temperature. With the decrease of the mold temperature, the dendrite morphology begins to change from irregular to regular, and finally, the primary dendrites and the secondary dendrites are perpendicular to each other.