Papers by Author: Sergiy V. Divinski

Abstract: Systematic radiotracer diffusion studies on metals present in severely deformed, ultra-fine grained (UFG) states have revealed the existence of ultra-fast transport paths, which include the so-called “non-equilibrium” grain boundaries and other defects including excess free volume. Under certain experimental conditions percolating porosity is produced even in a ductile metal like pure copper. This result indicates the importance of the cavitation phenomena in severe plastic deformation under those conditions. It is well known that micro-cracking can take place in metals rather early, if the local maximum shear stress equals or exceeds the shear yield stress of the material. However, the growth and propagation of these cracks will be postponed till very late in the deformation process because of the intrinsic ductility of metals, the effect of the superimposed hydrostatic component of the stress system and/ or concurrent dynamic recovery/ recrystallization, when the latter two are present (which is likely to be the case, if the severe plastic deformation operation is successful). That is, the stage in which crack growth and propagation is present represents a material state in which the scope for further deformation is exhausted and fracture processes have taken over. Using these and similar ideas, the load required for equal channel angular pressing, the change in the slope of the Hall-Petch plot with decreasing grain size and the theoretical limit for the smallest grain size attainable in a metal subjected to a severe plastic deformation (SPD) process are predicted and checked against experimental results.

Abstract: A model that considers diffusion in nanocrystalline materials undergoing recrystallization was developed. Application of this model enabled us deriving 63Ni radiotracer diffusion coefficients along the grain boundaries in ultrafine grain copper produced by equal channel angular pressing from the experimentally measured radiotracer penetration profiles.

Abstract: Recent results on diffusion in nanostructured materials are reviewed. The analysis highlights the importance of the proper account for a hierarchic microstructure which is often formed in nanostructured materials. The diffusion kinetics is such a material requires a special consideration in dependence on the temperature, diffusion time and the segregation level of the solute. Pressure-less sintering results in clustering of nanograins with significantly enhanced diffusivity of the inter-agglomerate boundaries. Severe plastic deformation produces a broad spectrum of high-angle grain boundaries (GBs) with different kinetic properties. The majority of the high-angle GBs reveals diffusivities very similar to those of general high-angle GBs in their well-annealed coarse-grained counterparts. Nevertheless, considerably faster short-circuit diffusion paths are detected, too. The origin, geometric arrangement, structural and kinetic properties of these high-diffusivity paths are comprehensively investigated and discussed.

Abstract: A nanostructured surface layer with a gradient microstructure was produced on a Cu plate by means of the surface mechanical attrition treatment (SMAT). Diffusion of Ni in the nanostructured layer was investigated by the radiotracer technique at temperatures from 383 to 438 K. The measured diffusion profiles consist of two distinct sections with different slopes, the steep one corresponding to the top surface layer with the grain size of 10 to 25 nm and the shallow one corresponding to a subsurface layer with a grain size of 25 to 100 nm. The effective diffusivities derived from both sections are more than 2 orders of magnitudes higher than the grain boundary diffusivities in coarse-grained Cu. The significantly accelerated diffusion rates are expected to be associated with the “non-equilibrium” states of interfaces in the nanostructured surface layer induced by SMAT. The difference between the diffusivities in the top and sub- surface layer might result from the fact that most interfaces developed from twin boundaries in the former while produced by dislocation activities in the latter.

Abstract: Diffusion of both titanium and nickel was measured in the near stoichiometric Ni-49.4at.%Ti alloy with the B2 ordered structure. The radiotracer technique and the 44Ti and 63Ni isotopes were applied in the temperature interval from 900 to 1300 K. The penetration profiles were determined by precision parallel grinding or by ion beam sputtering at larger and smaller penetration depths, respectively. Titanium and nickel diffusivities were found to follow linear Arrhenius dependencies with the pre-exponential factors of 2.710-7 and 4.710-9 m2/s and the activation enthalpies of 205 and 143 kJ/mol, respectively. A vacancy mediated diffusion mechanism is suggested to provide diffusion of both nickel and titanium in the compound NiTi.

Abstract: Starting from some fundamentals of solid-state diffusion, we remind the reader to the major techniques for lattice diffusion measurements. Self-diffusion is the most basic diffusion phenomenon in any solid. The paper covers main features of self-diffusion in pure fcc and bcc metals and some important facts about diffusion of substitutional solutes in metals. Binary intermetallics are compounds of two metals or of a metal and a semimetal. Their structures are different from those of the constituents. Some intermetallics are interesting functional materials others have attracted attention as high-temperature structural materials. The paper reviews some results mainly from our laboratory on diffusion in binary intermetallics from the systems Cu-Zn, Ni-Al, Fe-Al, Ni-Ge, Ni-Ga, Fe-Si, Ti-Al, Ni-Mn, Mo-Si and Co-Nb, which have been published in detail elsewhere. Some results for the ternary system Ni-Fe-Al are also mentioned.

Abstract: Grain boundary (GB) diffusion of 59Fe in high purity polycrystalline copper was measured using the radiotracer technique and precision parallel sectioning in an extended temperature interval. The results of diffusion measurements below 900 K are consistent with Harrison's C kinetics and yield the GB diffusion coefficient of Fe in Cu with the pre-exponential factor of 5.610-6 m2/s and the activation enthalpy of 121 kJ/mol. Unexpectedly strong GB segregation of Fe in Cu hindered reliable determination of the Fe diffusivity in the B kinetics. Additionally, unconventional penetration profiles were measured for GB diffusion of 59Fe in Cu at high temperatures. Molecular dynamics simulation with the literature Finnis-Sinclair type interatomic potentials was performed to shed light into the observed features. A strong effect of the Fe coverage on GB structure and kinetics in pure Cu is predicted at increased temperatures above 900 K.

Abstract: Kinetic characteristics of internal interfaces in ultrafine grained (UFG) materials represent an aspect that is central to their potential applications. Interface diffusion is strongly affected by ultrafine crystallinity and incorporated defects. In the present study the radiotracer technique was applied for measuring grain boundary (GB) diffusion of Ni in UFG Cu-1wt.%Pb alloy. The UFG specimens were prepared by equal channel angular pressing at room temperature. GB diffusion was investigated at room temperature in Harrison’s C kinetic regime under conditions in which Ni diffusion - even along general high-angle GBs in pure Cu - would hardly be measurable by the mechanical sectioning (as the corresponding penetration depths would be less than 100 nanometers). The present study highlights the existence of ultra-fast diffusion paths in severely deformed material. An atypical time dependence of the room-temperature diffusion data indicates a quite involved nature of these “fast” diffusion paths in the Cu-1wt.%Pb alloy.

Abstract: Classically a master curve as Dorn's equation is applied for elucidating stationary creep behaviour within high temperature range (T > 0.6 Tm). As the diffusion of both 63Ni and 44Ti have been measured in an equiatomic NiTi, an effective choice of creep-relevant diffusion coefficient D may be possible. Moreover, creep measurements in the same temperature range performed can be found in the literature. The correlation does not permit to establish precisely what D coefficient to integrate in the Dorn's equation.

Abstract: 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.