This study presents a novel approach for enhancing combustion resistance of a nickel-based superalloy by the application of platinum coating. The combustion behaviors of Pt-coated samples were ...studied using the promoted-ignition combustion standard test at 7–25 MPa (≥ 99.5 % oxygen). In contrast to uncoated samples, the Pt-coated samples exhibit notable improvements in resistance to burning, as demonstrated by reduced combustion length and rate. The underlying mechanism can be ascribed to two factors. One notable effect of the Pt coating is the influence on diffusion and distribution of combustion elements. In particular, it impedes the oxidation and combustion of Al with the highest combustion heat. The second one is the comparatively lower heat released by the Pt-coated samples throughout the combustion process, as well as the substantial capacity to reduce the temperature at which combustion ceases. The aforementioned discoveries offer new perspectives on enhancing the compatibility of metal materials with oxygen.
•Pt-coating notably improve in resistance to burning of superalloy.•Pt-coating decrease the temperature when combustion terminates.•Pt-coating alters the diffusion and distribution patterns of combustion elements.
The coarse primary carbides in high-carbon alloy steels can significantly impair their overall performance. Reducing and refining the primary carbides efficiently are always of great significance in ...material processing. Here, an innovative approach ― the selective microzone remelting (SMR) method, is proposed to dramatically reduce and refine the coarse primary carbides. The method is achieved by utilizing an electric current and is facilitated with the substantial difference in electrical resistivity between carbides and the matrix phase. When a sample contains massive coarse primary carbides is subjected to an electric current, the high electrical resistivity of carbides leads to the selective remelting of local carbides before the sample reaches solidus temperature during heating while during cooling the big difference in electrical resistivity suppresses the carbides formation but promotes the austenite nucleation and growth. This mechanism has been validated through the successful treatment of M50 bearing steel, in which the large-sized blocky primary carbides were eliminated completely, leaving only small-sized eutectics with extremely thin lamellas. Besides, the microporosity in the as-cast sample has also been reduced significantly via SMR treatment. These studies demonstrated that the proposed SMR treatment can be a promising method to reduce and refine coarse precipitates formed during solidification in various alloys.
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•Selectively remelting of local carbides is realized via electrical heating.•The different electrical resistivity between carbides and matrix is utilized.•The proposed method can efficiently refine the primary carbides in steels.•The method is applicable for other alloys with coarse precipitates.
•Increasing ratio of DS/DL reduces segregation and primary carbide precipitation.•Fraction of residual liquid to precipitate carbide is reduced by enhancing DS/DL.•Driving force for phase transition ...of carbide is also reduced by increasing DS/DL.
This study provides a potentially viable approach to manipulate the precipitation of primary carbides in molten steel by modifying the diffusion coefficient of carbon. The solidification process of a Fe-C alloy is simulated using the multi-phase-field method, and we focus on the impact of diffusion coefficient of carbon on the solute segregation and cementite precipitation. Two benefits have been revealed as the ratio of the diffusivities of carbon in solid to that in liquid is increased. A potential advantage is the reduction in the volume fraction of the residual liquid enriched with carbon during the late solidification. Furthermore, the magnitude of the chemical driving force for the phase transition of cementite precipitation will be lower. The combined influence of both factors results in an exponential decrease in the volume fraction of cementite formed at the end of solidification.
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The cooling process of liquid Fe-C and Fe-Cr-C systems from 2100 K to 1100 K was studied by ab initio molecular dynamics simulations. The evolution of local atomic structure was ...statistically analyzed by the pair correlation function, structure factor, coordination number, bond angle distribution, and the Honeycutt-Anderson (HA) index as well as the Voronoi index. We found that C atoms tend to be surrounded by metal atoms and the strong chemical bonds of Fe-C and Cr-C make the C-centered clusters more stable than Fe-centered clusters. The two alloy systems are dominated by icosahedral order in the liquid state but show different evolutionary trends of atomic structures during the cooling process. The Fe-C system undergoes a bcc-like crystalline phase transition at 1500 K, while the Fe-Cr-C system can be supercooled to an amorphous state with decreasing disclination density in the polytetrahedral packing. The analyses of clusters indicated that doping Cr can introduce new local topological and chemical orderings and thus renders a better glass-forming ability. Furthermore, to understand the mechanism of carbides precipitation from undercooled liquid of high-carbon chromium bearing steel, we simulated the local structures of several typical Fe and Cr carbides. We identified a close connection in the short-range order between the carbides and metallic liquids, which provides a theoretical basis for the formation of primary carbides.
We consider the effect of a mutual misorientation between two interacting equiaxed dendrites in polycrystalline materials on the scaling law of growth from undercooled melt. This effect is ...investigated by three-dimensional quantitative phase-field simulations of Al-Cu alloy solidification in thin samples. It has been found that the equiaxed dendritic growth kinetics changes due to the reduced solute interaction when considering mutual grain misorientation, and both the characteristic growth rate and primary dendritic arm length can be approximately correlated linearly to the misorientation of two dendrites. The scaling law, which originally uses the solute composition and the distance between two nuclei as physical parameters to describe the dynamics of primary dendritic arms, is then adapted with consideration of the misorientation. Compared with the previous scaling law that only concerns face-to-face growth of two dendrites, the predictions of the newly proposed scaling law taking the grain orientation in account are in better agreement with the experimental data.
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•Microsegregation during solidification is studied by 2D and 3D PF simulation.•Grain refining, cooling rate and back diffusion slightly affect microsegregation.•A new microsegregation ...model is derived to predict accurate solute redistribution.
Microsegregation formed during solidification is of great importance to material properties. The conventional Lever rule and Scheil equation are widely used to predict solute segregation. However, these models always fail to predict the exact solute concentration at a high solid fraction because of theoretical assumptions. Here, the dynamics of microsegregation during polycrystalline solidification of refined Al-Cu alloy is studied via two- and three-dimensional quantitative phase-field simulations. Simulations with different grain refinement level, cooling rate, and solid diffusion coefficient demonstrate that solute segregation at the end of solidification (i.e. when the solid fraction is close to unit) is not strongly correlated to the grain morphology and back diffusion. These independences are in accordance with the Scheil equation which only relates to the solid fraction, but the model predicts a much higher liquid concentration than simulations. Accordingly, based on the quantitative phase-field simulations, a new analytical microsegregation model is derived. Unlike the Scheil equation or the Lever rule that respectively overestimates or underestimates the liquid concentration, the present model predicts the liquid concentration in a pretty good agreement with phase-field simulations, particularly at the late solidification stage.
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•Solidification experiments of Al-Cu alloys are observed in situ and real-time.•Large-scale 2D PF simulations of solidification experiments are realized.•Effects of cooling rate and ...liquid flow on multi-grain growth are discussed.
The equiaxed dendritic growth of Al-Cu alloys in nearly isothermal temperature field under continuous cooling condition is studied using in situ and real-time observation of experiments by synchrotron X-ray radiography and large-scale quantitative two-dimensional (2D) phase-field (PF) simulations. It is revealed that the equiaxed dendritic morphology and the secondary dendritic arm spacing (SDAS) in the 2D PF simulations are in a reasonable agreement with the experimental data. Increasing the cooling rates results in a smaller SDAS, as predicted by the analytical Kattamis-Flemings model. The transformation kinetics of solid fraction can be described by the Johnson-Mehl-Avrami-Kologoromov (JMAK) theory, but quantitative differences between the experiments and 2D PF simulations are significant. The maximum solute concentration Cmax in liquid is approximately equal to the equilibrium concentration, which depends on the undercooling rather than the cooling rate. But the minimum solute concentration Cmin in solid decreases with the cooling rate, thus leading to a larger segregation ratio SR = Cmax/Cmin. Moreover, the liquid gravity-driven natural convection is considered in simulations. The liquid flow slightly increases the SDAS but has no apparent effect on solid fraction, and the segregation ratio is slightly reduced by the liquid convection, which could be attributed to the almost same Cmax and enlarged Cmin.
