Defect and Diffusion Forum Vols. 297-301

Abstract: Steel produced in Electric Arc Furnaces (EAF) contain a high amount of copper that causes a detrimental surface cracking phenomenon called hot shortness. Studies have found that nickel can alleviate hot shortness by increasing copper solubility in the Fe phase, decreasing oxidation rate and promoting occlusion [1-3]. Occlusion is a phenomenon whereby the copper-rich phase becomes incorporated into iron oxides. Nickel promotes occlusion by causing an uneven interface and increasing the number of internal oxides. The uneven interface is likely a result of the two concentration fields resulting from ternary diffusion of nickel, copper and iron in the Fe phase. This work is aimed at explaining why nickel causes wavy oxide/liquid-Cu and liquid-Cu/Fe interfaces. Constitutional super-saturation criterion [4] was applied to explain uneven interfaces caused by nickel. A model simulating diffusion behaviors of copper and nickel in Fe was developed by coupling Comsol Multiphysics® and Matlab®. Interface concentrations of copper and nickel and perturbation criterion values were calculated as a function of time. Modeling results show that (i) the nickel interface concentration first increases to a peak value then decreases slowly during oxidation process as a result of the change in oxidation rates, and (ii) the alloys with higher nickel contents have more potential for interface breakdown and this occurs within the initial linear oxidation regime.

Abstract: In order to investigate the crystallization behavior of the Co67Fe4Cr7Si8B14 amorphous metallic alloy, ribbons of this alloy were prepared by planar flow melt spinning process (PFMS). Differential scanning calorimetery (DSC) and differential thermal analyzer (DTA) were used to analyze the thermal properties and crystallization behavior of the samples at three heating rates of 10, 20 and 30º C/min. The experimental data were fitted to the Avrami model to determine the crystallization behavior. The results showed that the crystallization exotherm became wider and shifted toward a higher temperature range as the heating rate increased. The Avrami analysis also showed that n is about 1, which is related to the same transformation mechanism at different heating rates. The Kissinger method was used to determine the activation energy for the first crystallization peaks. The measured value is approximately 332.67 kJ/g.

Abstract: NiTi alloys containing more than 55%wt nickel undergo precipitation of Ni4Ti3, Ni3Ti2, and Ni3Ti phases during various heat treatments which could have a great effect on the chemical composition of the matrix and behavior of alloy. In this investigation, a NiTi alloy with Ti-57.5%wt nickel content, produced by vacuum induction melting in a graphite crucible, were subjected to the homogenization heat treatments in 1100oC and for various time periods (0.5, 1, 2, and 4 hours). The subsequent cooling was conducted in different cooling media (furnace and air) in order to examine the effect of cooling rate. Microstructural investigations show Ni4Ti3 particles with bimodal size distribution in furnace cooling. Differential scanning calorimetry demonstrates the correlation between homogenization time and transformation temperatures of the alloy.

Abstract: The present paper deals with different effects of homogenization time and cooling environment on Ni-42.5wt%Ti-7.5wt%Cu alloy. The alloy was prepared by vacuum arc melting. Afterwards, three homogenization times (half, one and two hour) and three cooling environments (water, air and furnace) at 1373 K were selected. Optical and Scanning Electron Microscopic methods, EDX, DSC and hardness tests have been used to evaluate the microstructure, transformation temperatures and hardness. Results indicate that specimens that were cooled in air are super-saturated. Also, the microstructure from furnace cooling has many disparities with the other cooling environments’ microstructure and two types of precipitates exhibit in the matrix, but in other cooling environments, only one phase can be seen. Particles of the Ti2(Ni,Cu) phase are distributed in the matrix in all of the microstructures irrespective of cooling rate. Observations show that increasing the time of homogenization results in finer precipitates and uniform distribution in the matrix. In addition, alteration of cooling rate and time of homogenization affect the martensitic transformation temperatures. On the other hand, the hardness varies slightly for different homogenization times but declines extremely with decreasing cooling rate. Moreover homogenization time and the cooling environment affect the transformation temperatures on furnace cooled samples.

Abstract: Processes to transform natural gas into hydrogen or synthesis of gas (H2+CO) have been extensively studied in recent decades. H2 can be used in fuel cells as a power source and syngas may be converted into hydrocarbons via the Fischer-Tropsch synthesis. An attractive alternative process for syngas production is the partial oxidation of methane (POM). The objective of this study is to find effective conditions in using of nanofine particles for producing high activity and novel nanocatalysts to syngas, especially selectivity green fuel H2 gas as a power source. Therefore, we could produce 98.6% methane conversion, and 97.1% H2 selectivity with the help of unique stable and new nanofine materials: Eco-friendly x%Ni / SiO2, Co /Ce/ZrO2, Ru / Ce / ZrO2, and highly active Co-Ru and Ni-Ru bimetallic over Ce-ZrO2 nanosized mixed oxide support catalysts. Recently, we prepared new and interesting nanostructures by Cu and Ni sputtering on the nanosized Ce-ZrO2 solid solution support in the first time. Several surfactants, dispersants and mixture of alcohol solvents were applied in synthesis of nanostructures and nanofluids as remarkable templates.

Abstract: Recrystallization processing of cold-rolled copper after isothermal annealing was investigated using high-resolution electron backscattered diffraction pattern analysis. The fiber texture is obtained by cold rolling with the rolled direction oriented along {111}, and the transverse and nominal directions have a random orientation. An isothermal recrystallization process at 150°C was investigated. Initially, rotations of the orientations occur from {111} to {100} and then small misfit angle boundaries decreased. Accompanying this change, the fiber-shaped grains change to a round shape grain and their sizes decrease. Considering these tendencies, we determined that rotation at subgrain boundaries is activated by isothermal annealing and subboundaries grow the boundary misfit angle >15 º. With further annealing, those grains surrounded by grain boundaries greater than 15º expanded. The rolling, transverse, and nominal orientations rotate {100}. Subsequently, a twin boundary appeared, and the fraction of twin boundaries increased.

Abstract: This paper presents the mechanical performance of the annealed NiTi Shape Memory Alloy (SMA) coating deposited onto 316L stainless steel substrate. The as-deposited SMA coating, Ni55.9 Ti44.1, showed an amorphous behaviour. The crystalline NiTi (SMA) coating was produced by annealing the as-deposited NiTi with a thickness about 2.0 µm, at above its crystallisation temperature in a vacuum ambient. The annealed NiTi coatings were characterised to determine the effect of the annealing parameters on their mechanical behaviour. The NiTi phases and structures were determined by x-ray diffraction (XRD) and scanning electron microscopy (SEM) whereas the mechanical properties were measured using the Rockwell C adhesion test. Three main phases; NiTi B2 parent phase, Ni3Ti and TiO2 were found in the annealed samples and the intensities of each phase were dependent on the annealing temperature and annealing time. Each phase significantly affected the mechanical behaviour of the coatings. Higher intensities of Ni3Ti and TiO2 phases were believed to contribute to the low adhesion of the annealed NiTi coatings due to their brittle properties. The annealing parameters; 600 °C for durations of 30 min was considered as the optimum parameter, yielding no fine cracks at the Rockwell C indentation interface compared to other samples at high magnification under the SEM. Adding a hard top layer of TiN would potentially provide a hard coating with an interlayer capable of absorbing impact which would be very suitable for ball joints used in hip replacement therapy.

Abstract: In order to improve the magnetic properties of electrical steels, it may be desirable to increase the Si and/or Al content of the steel. A possible and alternative route to realize this is through the application of an Al-Si-rich coating on the steel substrate using a hot dipping process, followed by a diffusion annealing treatment. Previously, a series of compositions were used for dipping, namely: pure Al, Al + 10wt% Si (hypo-eutectic composition) and Al + 25wt% Si (hypereutectic composition). After these dipping experiments, and the subsequent evaluation of the coating and its formed intermetallic phases, the use of a hypo-eutectic Al-Si-bath was recommended for further investigation, because of certain advantages: i.e. hypo-eutectic concentrations allow lower dipping temperatures and reduce the formation of ordered Fe-Si-structures that cause brittleness in the coating and substrate. The present work reports on the results obtained on materials that were hot dipped in a hypo-eutectic Al-Si bath. An Al + 1wt%Si bath was used to coat electrical steel substrates with different silicon contents with dipping times, varying between 0 to 20 seconds, after a preheating of the samples to a temperature of 700°C. A thorough characterization of the formed intermetallics was made by Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and X-Ray Diffraction (XRD). Three different compounds were identified as Fe2Al5, FeAl3 and a nearly pure Al phase.

Abstract: A phase-field simulation is performed to examine the effect of elastic inhomogeneity between the  and ’ phases on coarsening of the ’ phase in Ni-based superalloys. In the calculation of elastic strain energy, the mechanical equilibrium equation in elastically inhomogeneous system is solved by an iterative-perturbation scheme. On the basis of the elastic constants of a practical Ni-based superalloy, a series of simulations is performed in which both elastic anisotropy and shear modulus are varied independently. The variation of elastic anisotropy gives significant effect on both morphology and size distribution function of the ’ particles, whereas the variation of shear modulus gives little effect on them. Furthermore, it is found that the coarsening rate constant of the cubic growth raw changes and increases with increasing the standard deviation of the ’ size distribution.

Abstract: Interdiffusion coefficients of Al replacing elements in Ni-Al-X (X=Ti, V and Nb) were estimated by a series of experiments using diffusion couples of Al rich pseudo-binary systems at three different temperatures of 1423, 1473 and 1523K. In order to obtain interdiffusion coefficients of the pseudo-binary systems, the experimental data was analyzed by the Sauer and Freise method, and also impurity diffusion coefficients of Ti, V and Nb in Ni3Al were estimated by applying the Darken-Manning equation. The magnitude of interdiffusion coefficient decreased in order of V, Ti and Nb at all three temperatures. Impurity diffusion coefficients were described by the expressions: , , . The activation enthalpies obtained from the experimental data confirmed the retardation of Ti, V and Nb diffusion in Ni3Al by the anti-site diffusion mechanism. These results are consistent with our previous work on diffusion of Re and Ru in Ni3Al .