Sergiy V. Divinski

Grain boundaries in ultrafine grained materials processed by severe plastic deformation and related phenomena

Grain boundaries in ultrafine grained (UFG) materials processed by severe plastic deformation (SPD) are often called “non-equilibrium” grain boundaries. Such boundaries are characterized by excess grain boundary energy, presence of long range elastic stresses and enhanced free volumes. These features and related phenomena (diffusion, segregation, etc.) have been the object of intense studies and the obtained results provide convincing evidence of the importance of a non-equilibrium state of high angle grain boundaries for UFG materials with unusual properties. The aims of the present paper are first to give a short overview of this research field and then to consider tangled, yet unclear issues and outline the ways of oncoming studies. A special emphasis is given on the specific structure of grain boundaries in ultrafine grained materials processed by SPD, on grain boundary segregation, on interfacial mixing linked to heterophase boundaries and on grain boundary diffusion. The connection between these unique features and the mechanical properties or the thermal stability of the ultrafine grained alloys is also discussed.

Thermodynamics, diffusion and the Kirkendall effect in solids

Thermodynamics, phases and phase diagrams.- Structure of Materials.- Ficks laws of diffusion.- Development of interdiffusion zone in different systems.- Atomic mechanism of diffusion.- Interdiffusion and the Kirkendall effect in binary systems.- Growth of phases with narrow homogeneity range and line compounds by interdiffusion.- Microstructural evolution of the diffusion zone.- Interdiffusion in multicomponent systems.- Short circuit diffusion.- Reactive phase formation in thin film systems.

Percolating network of ultrafast transport channels in severely deformed nanocrystalline metals

Severe plastic deformation is nowadays used to produce sizable amounts of bulk nanocrystalline materials, which render them suitable for innovative applications ranging from biomedical implants to off-shore or aerospace structures, owing to favorable combinations of high mechanical strength and enhanced ductility they offer. Enhanced atom diffusion along internal interfaces is largely responsible for the resulting property combinations. Severe plastic deformation processing of metals is demonstrated to create bulk nanostructured materials with a hierarchy of internal interfaces. On top of that, specific diffusion channels providing pathways for ultrafast transport of atoms have been identified. The defects that represent the constituents of the fast diffusion network were visualized by means of the focused ion beam technique. Nonequilibrium grain boundaries, nonequilibrium triple junctions, and microvoids/microcracks compose the percolating network of ultrafast diffusion channels, which represent an impor...

Diffusion in Ultrafine Grained Materials

The paper provides an overview of recent results of the radiotracer investigations of short-circuit diffusion in ultra fine grained (UFG) materials produced by severe plastic deformation (SPD). Different material classes (copper of different purity levels and Cu alloys) are considered. The study is focused on the existence of non-equilibrium grain boundaries after SPD. Although a dominant contribution of common high-angle grain boundaries with very similar diffusivities as those in the corresponding coarse-grained material is established, much faster diffusion rates are also observed experimentally. The nature and kinetic properties of these “high mobility” paths in different materials are investigated and critically discussed.

Diffusion of Ag and Co in ultrafine-grained α-Ti deformed by equal channel angular pressing

Grain boundary diffusion of Co and Ag was investigated in coarse-grained and ultrafine-grained (UFG) α-Ti. Ultrafine grained Ti was produced by equal channel angular pressing (ECAP) and diffusion measurements were performed in a temperature interval where no significant grain growth occurred. Grain boundary diffusion of Co was found to be 1-2 orders of magnitude slower in UFG Ti, despite the attendant activation enthalpy being similar to that in coarse-grained α-Ti. By contrast, grain boundary diffusion of Ag occurred at a significantly higher rate in the ECAP-modified UFG Ti. This behaviour is associated with the specific diffusion mechanisms of Ag and Co: while Ag diffuses preferentially via substitutional sites, diffusion of Co is dominated by interstitial jumps. The existence of so-called non-equilibrium interfaces in UFG Ti, providing an increased density of traps for the interstitially diffusing Co and simultaneously enhancing substitutionally diffusion of Ag, is confirmed by the diffusion measurements.

Grain Boundary Diffusion and Linear and Non-Linear Segregation of Ag in Cu

Ag grain boundary (GB) diffusion was measured in the Cu-0.2at%Ag alloy in a wide temperature range from 473 to 970 K. The direct measurements of Ag GB diffusivity Dalloygb under conditions of the Harrison C regime revealed that Dalloygb is almost identical to Dpuregb determined earlier for Ag diffusion in high-purity Cu (Divinski, Lohmann, and Herzig, 2001). The penetration profiles determined in the Harrison B regime showed a complex, multi-stage shape. This diffusion behavior can be rationalized assuming that besides GBs significantly covered by segregated Ag atoms, some fraction of GBs remains almost free from Ag atoms in the studied temperature interval. The total amount of “pure” GBs drastically decreases with decreasing temperature. This hypothesis was proven by measurements of Ag GB diffusion in Cu near Σ5 bicrystals, which allowed us to analyze in detail the non-linear segregation of Ag in Cu GBs.

Self- and Interdiffusion in Ternary Cu-Fe-Ni Alloys

Diffusion of Cu, Fe, and Ni radiotracers has been measured in Cu–Fe–Ni alloys of different compositions at 1271 K. The measured penetration profiles reveal grain boundary-induced part along with the volume diffusion one. Correction on grain boundary diffusion was taken into account when determining the volume diffusivities of the components. When the Cu content in the alloys increases, the diffusivities increase by order of magnitude. This behaviour correlates well with decreasing of the melting temperature of corresponding alloys, as the Cu content increases. Modelling of interdiffusion in the Cu–Fe–Ni system based on Danielewski-Holly model of interdiffusion is presented. In this model (extended Darken method for multi-component systems) a postulate that the total mass flow is a sum of the diffusion and the drift flows was applied for the description of interdiffusion in the closed system. Nernst-Planck’s flux formula assuming a chemical potential gradient as a driving force for the mass transport was used for computing the diffusion flux in non-ideal multi-component systems. In computations of the diffusion profiles the measured tracer diffusion coefficients of Cu, Fe and Ni as well as the literature data on thermodynamic activities for the Cu–Fe–Ni system were used. The calculated interdiffusion concentration profiles (diffusion paths) reveal satisfactory agreement with the experimental results. Introduction Advanced materials are very often multi-component and have complex structure (gradient materials, coatings, etc.) and from thermodynamic point of view are non-ideal. Interdiffusion and interfacial reactions play important role during processing of many functional materials and limit their long term exploitation. Understanding of these processes has fundamental practical importance. There are different approaches to reach this goal. These are Onsager phenomenology [1] and Darken method [2], they differ in form of the constitutive flux formula. Up to now they were not unified and the fundamental question is whether the computed diffusivities or interdiffusion coefficients represent real material constants [3]. Situation is even more difficult because the proof of the uniqueness of the inverse Darken problem does not exist [4]. We selected relatively simple Cu-Fe-Ni system to study the interdiffusion in non-ideal alloys showing limited solubility. This system has advantage because its thermodynamic and kinetic data are fairly well known. This paper has two goals: first to measure the tracer diffusivities of all components as a function of the ternary alloy composition, second the theoretical examination of the extended Darken method (Danielewski-Holly model) for a ternary system. Experimental measurements Sample preparation and radiotracer measurements Copper, iron and nickel (99.99 pct purity) were used as starting materials. The ternary alloys (Table 1) were melted in induction furnace in argon atmosphere. Such obtained ingots were homogenized at 1273 K for 250 hour in argon ( 2 6 10 O p atm − < ). Subsequently, the alloys of different composition were reannealed at the temperature 50 C below their melting temperature for 4 hours for recrystalization. The grain size of polycrystalline alloys was measured to vary from 30 (Cu-richest alloy) to about 300 μm. Alloys in the form of the cylindrical ingots of 8 mm in diameter were cut into slices of 3 mm thick by spark erosion. One face of the specimen was polished to optical quality by standard metallographical procedure. The samples were encapsulated into Cu–Fe–Ni containers (made of nearly the same alloy which was under study), wrapped in Ta foil, and sealed in silica tubes under purified Ar atmosphere. The use of the containers guarantees that the chemical composition of the specimen was not changed during thermal treatment. The samples were subsequently annealed at 1373 K for 24 h in order to remove the mechanical stresses, which could be built up during cutting and polishing procedures and which could affect the diffusion behaviour. After the pre-annealing the blanc surface was slightly chemically polished with Syton colloidal silica slurry to remove the effects of thermal etching. Table 1. Alloy compositions in wt. % Alloy Fe Ni Cu #1 12 68 20 #2 25 50 25 #3 10 45 45 #4 28 37 35 #5 45 40 15 #6 6 24 70 #7 75 20 5 #8 10 80 10 Penetration profile measurements The radiotracer Cu (half-life 12.7 hours) was produced by neutron irradiation of a cupper chip at the research reactor in Geesthacht, Germany. The nuclear reaction was used. Its initial specific activity was about 500 MBq/mg. Due to short life-time of the isotope it was delivered to the lab in Munster in few hours after irradiation. The activated chip was dissolved in nitric acid and then diluted with double-distilled water. 63 64 29 29 Cu(n, ) Cu γ The radiotracer Fe (half-life 45 days) was also produced by neutron irradiation of iron powder at the research reactor in Geesthacht according to the nuclear reaction . The activated isotope material was dissolved in 30% HCl and then further diluted with double-distilled water. 58 59 26 26 Fe(n, ) Fe γ The radiotracer Ni (half-life 100 years) was purchased in form of a HCl solution and dissolved with double-distilled water. Each radiotracer (Cu, Fe, or Ni) was dropped as a dilute acid solution on the polished face of the samples. The samples were again encapsulated into containers, which were then wrapped into Ta foil, to avoid any undesirable contamination. The specimens were sealed in silica tubes under purified Ar atmosphere and subjected to the diffusion anneals at T = 1271 K. The temperatures were measured and controlled with Ni–NiCr thermocouples with an accuracy of about ±1 K. After the diffusion anneal the samples were reduced in diameter (at least 1 to 2 mm) by grinding on a lathe to remove the effect of lateral and surface diffusion. The penetration profiles were determined by the precision grinding sectioning technique using special abrasive mylar foils. The radioactivity of each section was determined with high efficiency using a Packard TRI CARB 2500 TR liquid scintillation counter. The penetration profiles present the plots of the normalized activity of each sections (the activity divided by the weight of the section) against the penetration depth x. The latter was calculated from the weight loss of the sample with known geometry after each sectioning step. The uncertainties of individual points on the penetration profiles, stemming from the counting procedure and the errors of the depth determination, were estimated to be typically less than 10 %. 0 5 10 15 20 25 Ni Fe Cu42.8Fe10.8Ni46.4

Grain Boundary Radiotracer Diffusion of Ni in Ultra-Fine Grained Cu and Cu - 1wt.% Pb Alloy Produced by Equal Channel Angular Pressing

The radiotracer technique was applied for measuring grain boundary diffusion of Ni in ultrafine grained (UFG) copper materials with different nominal purities and in a Cu—1wt.%Pb alloy. The UFG specimens were prepared by equal channel angular pressing at room temperature. The stability of the microstructure was studied by focused ion beam imaging. Grain boundary diffusion of the 63Ni radioisotope was investigated in the temperature interval from 293 to 490K under the formal Harrison type C kinetic conditions. Two distinct short-circuit diffusion paths were observed. The first (relatively slow) path in the UFG materials corresponds unambiguously to relaxed high-angle grain boundaries with diffusivities which are quite similar to those in the respective coarse-grained reference materials. The second path is characterized by significantly higher diffusivities. The experimental data are discussed to elucidate the contribution of nonequilibrium grain boundaries in the deformed materials. Alternative contributions of other shortcircuit diffusion paths cannot be ruled out, particularly for the Cu-Pd alloy.

Silicon diffusion in molybdenum disilicide: correlation effects

Correlation factors for silicon diffusion by a vacancy mechanism in the silicon sublattice of the tetragonal MoSi2 structure have been calculated by combining an analytical and a Monte Carlo approach. The ratio of the silicon diffusivity perpendicular to the tetragonal axis to that parallel to the tetragonal axis is also deduced. An effect of forward correlation of tracer atom jumps in the silicon sublattice with the corresponding partial correlation factor of 1.5 appears at small frequencies of silicon atom jumps along the tetragonal axis with respect to the jump frequencies in the silicon layer perpendicular to the tetragonal axis of the MoSi2 structure. The anisotropy of silicon diffusion in MoSi2 measured by Salamon et al. is explained in terms of correlation effects of silicon diffusion on its own sublattice.

Diffusion of Solute Elements in Fe3Al

Diffusion of Co, Mn, Ni, and Cr was studied in the intermetallic phase Fe3Al. The radiotracer method in combination with serial-sectioning has been used for diffusion profile determination. Wide temperature ranges were covered including the A2 and B2 phase fields of Fe3Al. The Arrhenius plots show only a small influence of the A2 B2 phase transition on the diffusivity. The results are discussed together with previous results from our laboratory on selfdiffusion in the Fe-Al system, which concerns self-diffusion of iron and the Al substitutes Zn and In. Solutes, which substitute primarily on the Fe sublattice of the B2 structure (Ni, Co) are slower diffusers than solutes, which substitute on the Al sublattice (Zn, In, Cr, Mn).