Materials Science Forum Vol. 850

Abstract: The effects of melt flow on dendrite growth during solidification are studied by the quantitative phase field model coupling the Navier-Stokes equations. Through analyzing the relationship between flow velocity and dendrite growth rate in simulations, a flow Péclet number involving with characteristic flow velocity, characteristic length of the zone affected by flow and thermal (solute) diffusion coefficient, is suggested for dendrite growth under convections. The growth rate increment due to flow follows a power-law relationship with the Péclet number. As the Péclet number is much higher than one, the influence of convection on dendrite growth is apparent, whereas as it is below one, the flow effects can be neglected.

Abstract: Nuclear grade 316LN austenitic stainless steel (ASS) with an exceptional combination of mechanical properties and corrosion resistance was used to produce AP1000 primary coolant pipe. In order to evaluate the microstructure evolution of the pipe during its forging process, the material database of the 316LN ASS is established with high integrity and reliability. In this paper, the thermal physical parameters, flow stress-strain data and the recrystallization kinetic equations of the 316LN steel are coupled, and the material database is systematically established. Most important, the reliability of the database is verified by an experiment.

Abstract: The structural, electronic, phonon and thermodynamic properties of rocksalt (RS) structure LiF are studied using a plane-wave pseudopotential method within the local density approximation (LDA). The values of lattice constants, elastic constants, and bulk modulus and its pressure derivatives are in well agreement with the available experimental data and other theoretical results. The LiF crystal exhibits a wide band gap of about 8.727 eV. The linear response method is applied to determine the phonon dispersion, phonon density of states and Born effective charge. The phonon frequencies at the Γ, X, L points are analyzed using group theory. We also calculate the thermodynamic functions such as free energy, enthalpy, entropy, specific heat using the phonon density of states. We compare the present calculation results satisfactorily to experimental and previous theoretical results.

Abstract: Phase stability and elastic properties of seven one dimensional long period structures (1D-LPSs) of Al₃Ti under high pressure have been systematically investigated by first-principles calculations. The enthalpy differences indicate that Al₃Ti will undergo a phase transition from 1D-LPSs to L1₂ structure at high pressure. With increase of antiphase boundary period parameter M’, the enthalpy initially decreases and then increases, and the enthalpy for D0₂₃ is the smallest. Oppositely, the phase transition pressure firstly increases and then decreases, and the maximum is for D0₂₃. The elastic constants and elastic moduli B, G and E increase monotonically with increase of pressure, and the corresponding second-order polynomial fits are also obtained. Interestingly, the pressure dependence of Poisson’s ratio show similar tendency with that of B/G ratio. Both the B/G ratios and the Cauchy pressures reveal that these 1D-LPSs exhibit brittleness at high pressure.

Abstract: Chemisorbed atomic oxygen inducing Co segregation in CoNi (111) alloy is studied using periodic self-consistent density functional theory (DFT) calculations. In particular, the coverage dependence and possible adsorption-induced segregation phenomena are addressed by investigating segregation energies (the difference in calculated total free energy between surface sites and bulk-like sites) of isolated Co in CoNi (111) alloy. In agreement with previous experimental and theoretical investigations, segregation of Co is found to be oxygen-coverage dependent. While for ‘clean’ CoNi (111), Co prefers to be in the bulk. In the presence of more than 2/9 ML of oxygen, Co segregates to the surface. The analysis of oxygen adsorption trends and surface electronic structures explains the change in the local atomic arrangement which is expected to occur on the surface of alloys under reaction conditions. Our predictions for the high oxygen coverage cases are particularly relevant in underlining the importance of segregation phenomena to the hydrogen evolution performance of CoNi alloy hydrogen evolution electrode.

Abstract: TRIP steels are prone to crack during forming process, mainly because of the transformation of retained Austenite to Martensite, causing discordance between external force and deformation. The mechanical behavior of TRIP steels has conventional elastic and plastic deformation, as well as time-dependent viscous properties. So it is necessary to consider the effect of time to their deformation behavior during constitutive model building. The rheological model used in TRIP steels is not only to reveal the soft and hard phase properties accurately and reasonably by combination of different components but also to indicate the TRIP effect during deformation process by change and combination of coefficients of components. In this paper, rheological constitutive models for TRIP600 are adopted and discussed based on rheological theory, and finally the uniform constitutive equations under creep condition and stress relaxation are built.

Abstract: In this paper, the uniaxial compression of Mg, Ti, Zr and Co single crystals along the direction is performed by molecular dynamics (MD) to investigate the elastic-to-plastic transition in these hexagonal close-packed (hcp) metals. Two deformation twinning modes are observed in these simulations, including the twinning in Ti, Zr and Co and the [0001] twinning in Mg. The underlying atomistic mechanisms of these twinning modes are analyzed in detail.

Abstract: As a new method, liquid-metal cooling (LMC) process is used in manufacturing industrial gas turbines (IGT) blades. Numerical simulation is an effective way to investigate the grain’s growth and morphology, and optimize the process. In this paper, mathematical models for heat dynamic radiation and convection boundary of LMC process is established to simulate the temperature fields. Cellular Automaton (CA) method and KGT growth model are used to describe the nucleation and growth. Simulation results and experimental results are compared. The mushy zone and microstructure evolution are studied in detail. This study indicates that simulation and experimental results agree very well with each other. The withdrawal rate has an important influence on the shape of mushy zone and growth rate of the grain directly. A concave mushy zone is formed and the grain tends to convergent under an excessive high of withdrawal rate. But, the mushy zone has a convex shape and the grain is divergent under a smaller withdrawal rate. A variation withdrawal rate (from 2mm/min to 9mm/min) is found to obtain smooth mushy zone, which improves the parallelism of grain and produces high quality IGT blades.

Abstract: In this paper, based on mass conservation, momentum conservation and turbulence k-ε model, the incompressible flow fluid dynamics model in sand discharging process is established. Computational Fluid Dynamics (CFD) technology is adopted to solve the flow field in the bend pipe and the erosion laws of the pipeline wall are predicted coupling material erosion model. Four erosion models of carbon steel material are adopted respectively to calculate the erosion laws and comparison is conducted. Further parametric study is carried out and different influence factors (such as pipeline structure, flow of fluid, et al.) to the erosion laws of the blooie line are discussed. The results indicate that the distribution characteristics of the erosion damage and the law of the erosion rate are changed with impact parameters in the bend pipe. There are obvious distribution differences of the erosion rate within a certain angle range obtained from different erosion models.

Abstract: In this study, the effects of residual stresses on the ductile crack growth resistance was investigated using single-edge-notched bending (SENB) and single-edge-notched tension (SENT) specimens as well as axisymmetric model. Weld residual stresses were introduced by the so-called eigenstrain method. The crack tip opening displacement (CTOD) and constraint parameter R were calculated for different specimens and residual stresses. Results show that the residual stresses slightly reduced the ductile crack growth resistance. However, crack tip constraint R elevated with the increase of residual stress.