Papers by Keyword: Grain Boundary Diffusion

Abstract: A critical issue when joining aluminium alloys to dissimilar metals such as magnesium, titanium, and steel is to control the formation of brittle intermetallic compounds (IMCs) that occur due to the reaction at the joint interface. It is demonstrated that once the IMC exceeds a critical thickness, failure of welds always occurs in a brittle manner, with cracks propagating through the IMC. One approach to minimize IMC thickness is to use a solid state joining process, such as friction stir or ultrasonic welding. However, even using these processes, an IMC that exceeds the critical thickness can either form during welding or post-weld heat treatment. In this paper, a number of approaches are discussed to control IMC formation in welds between aluminium alloys and magnesium alloy. Modelling predictions indicate that interfacial reaction rates and grain size of the IMC phases are two critical factors to control layer growth. Experimental results demonstrate that the grain size of IMC layers changes as the layers grow. These modelling predictions and experimental results offer new design strategies to optimize dissimilar metal welding involving aluminium.

Abstract: In order to investigate the creep behavior and understand its deformation mechanism of automotive exhaust pipe materials, this study conducted creep experiments of 409L and 436 ferritic stainless steels at both 600°C and 750°C under different stress levels. After creep tests, crept specimens were analyzed on the microstructure and fracture surfaces by the use of scanning electron microscopy. From creep data, two important material coefficients, namely, M of Monkman-Grant relationship and K of Coble creep equation are calculated for predicting the material creep life. Results show the creep resistance of 436 stainless steel is better than that of 409L stainless steel, because the 436 stainless steel has more Ni, Cr, and Mo contents. As the creep mechanism, all tests show grain boundary diffusion or Coble creep is the dominate deformation mechanism, except at higher temperature 750 °C and higher stress levels.

Abstract: The temperature dependence of the bulk diffusion coefficient of Fe in Cu is determined by EDX in the temperature range from 923 to 1273 K, , m²/s. These results are different from that obtained earlier by radiotracer technique: activation energy is less by 30 kJ/mol and pre-exponential factor is 50 times smaller. Deviations from ideality of investigated solutions do not explain the differences; consequently, the thermodynamical factor would not responsible for such an effect. Fast grain boundary diffusion of Fe in Cu was not observed in the temperature range from 823 to 1073 K.

Abstract: Effect of microalloying of Al by Ce on Cu grain boundary diffusion (GBD) in Al and other results of GBD study in metals with different purity were analyzed. It is shown that in dependence on the chemical composition of the matrix the triple product of GB diffusion can be higher or less than that for pure metals. The main concept used for description of the results is local ordering of the atoms (including atomic complex formation and appearance of «poisonous» places around atoms of the alloying element). The increasing of atom mobility at GB is discussed also in terms of solidus temperature changes.

Abstract: Grain boundary (GB) diffusion of Ag in Cu-based alloys with Fe (0, 0.14, 0.29, 0.43, 0.55 and 0.99 at. % Fe) was investigated. The experiments were performed at 600, 650 and 700 °C, which corresponded to B-regime. It was proved by the presence of diffusion wedges. The triple product of silver GB diffusion was obtained by measuring of angle at the top of isoconcentration profiles which was detected by electrochemical etching. It was shown that iron decreased the triple product at higher temperatures and didnt affect significantly at lower temperatures. SEM investigation showed the presence of small (10-100 nm) precipitates on the copper grain boundaries. Analysis of particles composition by Auger electron spectroscopy indicated higher concentration of iron comparing with the alloy composition.

Abstract: A method of grain boundary diffusion characterization has been suggested based on combined analysis of radiotracer serial-sectioning technique and Mössbauer spectroscopy data and a specified model of grain boundary diffusion. Its application for determination of parameters of Co grain boundary diffusion in W is demonstrated.

Abstract: The problem of whisker formation in tin (Sn) wiring in small electronic devices has become an important issue with the requirement of lead-free wiring, because doping of Pb to reduce whisker formation cannot be applied. It is therefore urged to better understand stress migration in tin, which is suspected to play a key role in whisker growth. We aim to study grain boundary diffusion in tin by atomistic simulation. After constructing an efficient interatomic potential suitable for diffusion of atoms using the genetic algorithm (GA), we perform molecular dynamics (MD) simulation of grain boundary diffusion in Sn under stress. We find that the magnitude of stress effect on diffusion depends on the boundary structure. Moreover, we examine the effect of impurities on vacancy migration by ab initio calculation to find atom doping that has potential to suppress diffusion.

Abstract: Crack-healing effectiveness was investigated on 5 vol% nano-Ni dispersed Al₂O₃ hybrid materials. Influence of the Y or Si doping or SiC co-dispersion was also studied on the crack healing behavior. Cracks were introduced by a Vickers indentation to be a crack length of approximately 60 μm. Cracks of nano-Ni/Al₂O₃ were completely disappeared, for example, by oxidation at 1200°C for 6 h in air, Y/Si doped one and SiC co-dispersed one have similar performance of crack disappearance. Bending strength of crack-disappeared samples showed about 550 MPa and was comparable or improved with that of as-sintered one. Mechanism of crack healing was considered as filling up of cracks by NiAl₂O₄ oxidation product which is developed by outward diffusion of cations at grain boundary of Al₂O₃ matrix. Nano-Ni/Al₂O₃ with Y or Si doping or SiC co-dispersion are realized to have crack-healing effectiveness with improved high-temperature oxidation resistance.

Abstract: It was shown more recently in our Laboratory [1,2,3] that having a substrate/diffusant/thin-film/cap-layer structure (the thin film was typically several 10 nm thick, with the same order of magnitude of grain size; the refractory metal cap layer was used just to avoid the oxidation), first the diffusant atoms migrated very fast across the thin film and segregated at the film/cap-layer interface. The accumulated atoms at the film/cap layer interface form a secondary diffusion reservoir and atoms diffuse back to the layer. Later on, the thin film was gradually filled up with the diffusing atoms and composition depth profiles, determined by Secondary Neutral Mass Spectroscopy (SNMS), showed a maximum at the cap layer-thin film interface. The accumulated atoms at this interface formed a secondary diffusion reservoir and atoms diffused back to the layer. These observations can be interpreted supposing a bimodal grain boundary structure with different (fast and low) diffusivities. The observed grain boundary diffusion phenomena can be classified as C-type diffusion. The appearance of the peak observed at the cap layer interface can be used as a tool to determine the grain boundary diffusivity along the fast boundaries. Because the fast boundaries were saturated in the first stage of the process, this back-diffusion took place along the low-diffusivity boundaries only. Thus the SNMS depth-profiling is a good method to determine grain boundary diffusivities in a bimodal structure. In addition, from the overall impurity content inside the film the segregation can also be estimated, if the bulk solubility is low and the GB density is known. Numerical simulations of C-type GB diffusion in thin films with a bimodal structure confirmed that the interpretation of the result depicted above is reasonable [4]. In order to estimate roughly the GB diffusion data we determined the fast diffusivity using the first appearance method. The lower diffusivity was determined from the time evolution of the broadening of the diffusant/thin film interface. In addition both (slow and fast) diffusivities were also estimated from fitting numerical solutions obtained in [4] too.

Abstract: Recent models of grain-boundary diffusion are briefly reviewed. Models of diffusion along equilibrium boundaries of recrystallization origin in coarse-grained materials and along non-equilibrium boundaries in nanocrystalline materials obtained by gas condensation and compacting or by severe plastic deformation are considered separately.