E. Rabkin

Thermodynamic aspects of the grain boundary segregation in Cu(Bi) alloys

Abstract The grain boundary segregation of Bi in dilute polycrystalline Cu–Bi alloys was systematically studied as a function of temperature and composition. The temperature dependencies of the Gibbsian excess of Bi at the grain boundaries exhibited discontinuous changes at the temperatures close to, but different from the bulk solidus temperatures. The observed segregational phase transition was interpreted in terms of prewetting model.

Laser produced functionally graded tungsten carbide coatings on M2 high-speed tool steel

Abstract The objective of the investigation was to produce functionally graded, carbide alloyed multilayer coatings on M2 high-speed steel by laser alloying with direct injection of WC powder into the melt pool. Single layer coatings with a wide alloying range corresponding to 12–58 wt.% W and 1.3–4.3 wt.% C, respectively, were produced by varying laser beam power and beam traverse velocity. Depending on the alloying degree, four different types of structures were observed in laser alloyed coatings; they were characterized by scanning electron microscopy and X-ray microanalysis. Multiple laser alloying with beam power decreasing at each successive stage was used for producing a triple-layer coating with tungsten content increasing from layer to layer and reaching 75 wt.% in the upper layer. The observed hardness was in the 1100–1200 HV range for single layer coatings with 40–50% W and as high as 1600 HV in the upper layer of a triple coating with 75% W. The coating with 58 wt.% W showed wear resistance five times as high as compared with the unalloyed laser-melted M2 steel.

Nanohardness of copper in the vicinity of grain boundaries

The nanohardness in the vicinity of grain boundaries in high purity Cu was investigated. It was found that the nanohardness increases while approaching the grain boundary, the characteristic distance at which the grain boundary influences the nanohardness being in the range of few micrometers.

Penetration of Tin and zinc along tilt grain boundaries 43° [100] in Fe-5 at.% Si alloy : premelting phase transition ?

Abstract Tin and zinc penetration along the tilt grain boundary 43° [100] in b.c.c. Fe-5 at.% Si alloy is studied in the temperature range from 652 to 975°C. Wetting transition of grain boundary by the tin-rich melt at Tw = 810 ± 5°C is observed. About Tw there is a thin wetting film at grain boundary. With zinc penetration along the grain boundary a wetting film has been observed at all temperatures studied. Behind that film there is a region with an unusually high diffusivity of zinc, and below that region there is a region of “ordinary” grain boundary diffusivity. Such a phenomenon may be explained in terms of the phase transition “grain boundary-thin wetting film on the boundary”, which is commonly known as a premelting phase transition. A model is proposed which explains the form of the temperature dependence of the concentration cBt, at which such transition occurs, and, in particular, the influence of the “paramagnet-ferromagnet” transition in the bulk on the premelting transition. The influence of the temperature dependence of the volume solubility limit, c0, on the cBt(T) dependence is also discussed. In critical region below Curie point Tc critical exponents d of magnetic part of activation free energy of bulk and grain boundary diffusion are calculated. Critical index d for grain boundary diffusion by premelting layer, as well as activation energy in paramagnetic region, lies in the interval between bulk values of d and estimation of d for truly two-dimensional grain boundary diffusion.

Pressure influence on the grain boundary wetting phase transition in FeSi alloys

Abstract The influence of hydrostatic pressure up to 1.4 GPa at 905°C on the wetting of grain boundaries in Fe-6 at.% Si and Fe-12 at.% Si bicrystals by a Zn-rich melt has been studied. The dihedral angle θ at the intersection of the grain boundary with the solid/liquid interface has been measured by light microscopy. The transition from complete (θ = 0) to partial (θ > 0) wetting of the grain boundary (dewetting phase transition) was found to occur as the pressure increased. The dewetting transition pressure is higher for special boundaries than for the general boundary studied and decreases with increase in the Si content in the alloy. The pressure effect on the solidus concentration of Zn in FeSi alloys has been determined. This enabled us to construct the surfaces of grain boundary wetting/dewetting phase transitions in three-dimensional phase diagrams in Zn concentration-temperature-pressure and Zn concentration-Si concentration-pressure coordinates. A thermodynamic analysis of the wetting phenomena in binary and ternary systems is given, taking into account the effect of pressure, chemical interactions and structural misfit on the energy of interfaces.

On the kinetics of grain growth inhibited by vacancy generation

Department of Mechanical and Materials Engineering, The University of Western Australia, Nedlands, Western Australia 6907, Australia Institut für Metallkunde und Metallphysik, RWTH Aachen, Kopernikusstrasse 14, D-52074 Aachen, Germany Department of Materials Engineering, TECHNION-Israel Institute of Technology, 32000 Haifa, Israel Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Moscow District 142432, Russia

Beyond the fisher model of grain boundary diffusion : Effect of structural inhomogeneity in the bulk

Abstract An extension of the Fisher model of grain boundary diffusion is suggested, in which the diffusion along the short-circuit paths in the bulk of the crystalline grains (dislocations, subgrain boundaries, interphase boundaries in the lamellar structures) is taken into account. In the framework of the suggested model the bulk is treated as a stochastic mixture of defect-free crystalline regions and regions of bad material inside the short-circuit paths. The Harrison classification of the diffusion regimes is extended to the new D -regime, in which the kinetics of the penetration of the diffusing element along the grain boundaries is dominated by diffusion along these short-circuit paths. Three different kinetic modes during the grain boundary diffusion in the D -regime are uncovered: for the short annealing times the penetration kinetics follows the Whipple law, but with the bulk diffusion coefficient substituted by g 2 D d , for the intermediate annealing times the penetration distance along the grain boundary is a weak function of time and for the long times the Whipple law is valid again, but with the bulk diffusion coefficient substituted by gD d , where g and D d are the volume fraction of the material inside the short-circuit paths and the diffusion coefficient along them, respectively. The applications of the suggested model to the analysis of experimental data are discussed.

The Solidus Line of the Cu-Bi Phase Diagram

The solid solubility of Bi in Cu single crystals has been experimentally determined. It is shown that the solidus line is a retrograde curve without a monotectic transition. The solid and liquid phases are successfully described with simple thermodynamic models. The experimentally measured maximum solubility of 0.0207 at. % Bi at 975 °C correlates well with that from the model (0.0193 at. % Bi at 968 °C). A linear temperature dependence of the interchange energies is suggested, and the values of the optimized coefficients are in accordance with those estimated from the thermal expansion coefficients. The calculated thermodynamic functions are in good agreement with the assessed experimental data.

Role of the solid/liquid interface faceting in rapid penetration of a liquid phase along grain boundaries

A model is developed for describing rapid penetration of a liquid phase along a grain boundary. It is based on the assumption of a highly faceted solid/liquid interface. Experiments showing the faceting of the solid/liquid interface in grain boundary penetration experiments are presented. The basic hypothesis of the model are an undersaturated solid and a positive spreading coefficient of the liquid phase along the grain boundary. The model explains the apparent concave shape of the tip of the groove and the reason why penetration also occurs if the liquid phase is pre-saturated with the material of the solid. Moreover it predicts a power law with an exponent close to unity for the time dependence of the depth of penetration of the liquid layer along the grain boundary.

Grain growth in thin metallic films

Abstract Grain growth in thin films deposited on a substrate was studied theoretically. The thrust of the model proposed is the effect of vacancy generation accompanying grain growth on the rate of the process. In addition, the magnitude of a tensile stress developing in the film was considered. It was shown that due to the contribution of vacancies to the free energy of the system, discernible grain growth is preceded by an “incubation” period, during which the grain structure can be considered as stable, as the rate of growth is relatively small over this incubation time. During this time, the vacancy concentration remains nearly constant, staying at a level much higher than the thermal equilibrium concentration. Based on numerical analysis, a simple expression for the incubation time in terms of the vacancy sink spacing, temperature and grain boundary characteristics was derived. With this formula, the stability of the grain structure of a thin film can be assessed for given conditions.