Defect and Diffusion Forum Vols. 323-325

Abstract: New processes and materials are vital nowadays, since specific features of materials are essential to accomplish the more demanding technological and environmental requirements. Al/Ni system has stimulated a large number of phase equilibrium, thermodynamic, diffusion and related studies. Its relevance relays on first principle features as well as technological importance. Transient Liquid Phase Bonding (TLPB) process is presented in this work due to the advantages of this method above others. Low temperatures of processing are needed, to obtain stable intermetallic phases (IPs) suitable for high temperatures of service. The aim of this research is to investigate the impact of temperature on the IPs growth and formation mechanisms, completing previous works on microstructure and kinetics characterization of IPs in Al/Ni system at 720°C. The results of SEM-EDS/WDS analyses of the cross-sections of the bonds showed the presence of Al₃Ni and Al₃Ni₂ at the beginning of the interaction between Al and Ni, which are consistent with the phase equilibrium diagram. The Al₃Ni₂ layer growth showed linear correlation with time at 800°C, whereas at 900 and 1000°C it suggested a diffusive growth. Inert markers experiments allowed distinguishing individual growth. Vickers micro-hardness was determined obtaining values for Ni and Al₃Ni₂ of about 100 and 800 HV/ 0.025, respectively.

Abstract: Recent scientific work has opened new fields of application to mechanical treatments such as shot blasting or peening. Indeed, it has been shown that this treatment, performed before a nitriding treatment on the surface of ferrous alloy, lowers processing temperatures and significantly increases the diffusion kinetics. We undertook to test this combination of mechanical pretreatment and thermochemical treatment on stainless steels and nickel-based alloys. The mechanical treatments were done by surface attrition peening. The pretreated samples were then nitrided at low temperature using remote plasma. In this paper, the results obtained after nitriding treatments on samples treated by attrition peening are compared to those nitrided only. The use of X-ray diffraction, microhardness measurement, observations by optical and scanning electron microscopy, texture analysis by EBSD (Electron BackScatered Diffraction) and measurement of nitrogen concentration profiles by SIMS (Secondary Ion Mass Spectrometry) allows quantifying the effects of the combined treatments.

Abstract: Austenitic Fe-Mn-C steels are Ultra High Strength Steels which may be used for the production of deep drawn automotive parts containing extremely high residual stress and strain levels. In consequence, hydrogen absorption occurring during the corrosion process in aqueous environments may enhance the sensitivity of these steels to different kinds of hydrogen-induced damage, in particular Stress Corrosion Cracking (SCC). In order to predict and prevent SCC, it is important to study the behaviour of hydrogen in these austenitic steels exposed to aqueous environments and in particular the dependence on the alloy chemistry and microstructure. SIMS profiles of deuterium introduced by cathodic charging in selected specimens were used to characterize the diffusion of hydrogen in these steels. This allowed to be studied the role of chemical composition and microstructure on the kinetics of H absorption at room temperature. The competition between bulk matrix diffusion and short-circuit diffusion phenomena along grain boundaries was investigated. The results show a strong dependence of H diffusion and distribution on the alloy chemistry and grain size.

Abstract: The electrochemical permeation technique was used to evaluate the effect of the microstructure on hydrogen diffusivity and hydrogen trapping at room temperature in martensitic steels. A detailed study of the electrochemical permeation technique was first performed in order to identify the boundary conditions of a permeation test in the selected experimental set-up. The validity of the apparent diffusion coefficient derived from this test is also discussed. A 34CrMo4 quenched steel has been selected and designed at three tempering temperatures (200°C, 540°C and 680°C) in order to obtain three different microstructures. According to permeation measurements, H diffusion strongly depends on the microstructure. The material tempered at 540°C exhibits the smallest diffusion coefficient and the largest fraction of reversible traps at room temperature.

Abstract: Diffusion couple technique is used to study interdiffusion in Nb-Mo, Nb-Ti and Nb-Zr systems. Interdiffusion coefficients at different temperatures and compositions are determined using the relation developed by Wagner. The change in activation energy for interdiffusion with composition is determined. Further, impurity diffusion coefficient of the species are determined and compared with the available data in literature.

Abstract: Nb is one of the common refractory elements added in Ni, Co and Fe based superalloys. This lead to the formation of brittle topological close packed (tcp) μ phase, which is deleterious to the structure. It mainly grows by interdiffusion and in the present article, the interdiffusion process in different Nb-X (X=Ni, Co, Fe) systems is discussed. The activation energy for interdiffusion is lower in the Co-Nb system (173 kJ/mol) than Fe-Nb system (233 kJ/mol), which is again lower than the value found in the Ni-Nb system (319.7 kJ/mol). The mole fraction of Nb in this phase is less than Fe or Co at stoichiometric compositions in the Nb-Fe (that is Fe₇Nb₆) and Nb-Co (that is Co₇Nb₆) systems. On the other hand, the mole fraction of Nb is higher than Ni in the same phase (Ni₆Nb₇) in Ni-Nb system. However, in all the phases, Nb has lower diffusion rate. Possible diffusion mechanism in this phase is discussed with respect to the crystal structure.

Abstract: The method of perturbed angular correlation (PAC) was used to determine lattice locations of ¹¹¹In impurity probe atoms present in extreme dilution in the intermetallic compound FeGa₃. In slightly Ga-poor samples, probes were found to strongly prefer one of two inequivalent Ga-sites. In slightly Ga-rich samples at room temperature, 293 K, the PAC spectrum exhibited an unperturbed quadrupole interaction signal that is consistent with indium probes dissolved in small liquid pools of the excess Ga. A myriad of such pools are probably located along grain boundaries in the sample. Cooling from 293 K down to 12 K, the site fraction of indium in liquid decreased, being offset by the increase in a signal attributed to indium solutes in precipitates with other impurities at the sides of the Ga pools. However, these changes were completely reversible upon heating, and no crystallization of the liquid gallium pools was observed down to 12 K. This is attributed to the extraordinarily small volumes for the pools, which, while not measured directly, are orders of magnitude smaller than cubic microns. The measured temperature dependence of the site fraction of indium in the liquid was used to extend the metastable solubility curve for indium in liquid gallium down to a temperature of 150 K, much lower than the eutectic temperature of Ga-In at 288.5 K.

Abstract: Diffusion couples of the type WC-Co/VC-Co were employed to observe the diffusion behaviour of the grain-growth inhibitor (GGI) VC within nanoand ultrafine-grained WC-10wt.% Co samples. Since diffusion of the GGI during heating occurs already in the solid-state regime, interrupted sintering experiments were performed up to the liquid phase formation temperatures. By the use of light-optical and scanning electron microscopy the impact of GGI concentration on the microstructure as a function of depth from the interface was investigated. Electron Probe Microanalysis (EPMA) was used in order to quantify the amount of vanadium diffused into WC-Co as a function of distance. In first approximation, an activation energy of 3.45 eV (332,6 kJ/mol) for vanadium transport was determined.

Abstract: A strong gravitational field causes the changes in composition and structure through sedimentation or displacement of atoms in multi-component condensed matter. We have developed a high-temperature ultracentrifuge to generate a strong acceleration field of even over 1 million (1x10⁶) G, and, for the first time succeeded in realizing the sedimentation of the constitutive solute atoms and aeven isotope atoms in solids or liquids. The changes in composition and crystalline state of various alloys, polymers, and other substances have been investigated. Recently, we started the experiments on compounds and semiconductors with the aims of new materials synthesis and semiconductor control. The chemical reaction, crystal structure change have been found for metallic compound (Bi₃Pb₇), or covalent compounds (Y₁Ba₂Cu₃O_7-x, TiO₂, etc.). The impurity control was also examined in semiconductor. In this article, the recent progress and the future prospects for materials processing are described.

Abstract: The theoretical model describing the sedimentation process, i.e. diffusion induced by the gravity (centrifugal) field in multicomponent system is presented. This procedure starts from the combined mass conservation law, equation of motion, volume continuity equation and Nernst-Planck flux formulae. Two dimensional calculations of sedimentation process in InPb system are presented. Example applications of this theory applied to binary system is evidence of sedimentation in condensed matter via diffusion mechanisms. Keywords: sedimentation, interdiffusion, gravity, pressure, thermodynamics, volume continuity.