Grain Boundary Diffusion and Mechanisms of Creep of Nanostructured Metals

Comparative investigations of diffusion in coarse-grained (d = 20 μm), nanocrystalline (d = 0.04 μm) and nanostructured nickel (d = 0.3 μm) have been carried out in a temperature interval of 0.2–0.3 melting temperature. The reasons for difference of parameters of copper grain-boundary diffusion in the above materials are discussed. The effect exerted by grain boundary state and grain boundary diffusion fluxes of impurity on creep mechanisms of nanostructured nickel and copper in the temperature interval of 373–473 K have been studied. Significant change in the apparent creep activation energy under copper grain boundary diffusion fluxes is described as a consequence of different contribution of grain boundary sliding to overall deformation.     

Characteristic features of diffusion-controlled processes in ordinary and ultrafine-grained polycrystaline metals

An analysis of manifestation of the effect of acceleration of creep in polycrystalline metals and alloys in the presence of grain-boundary diffusion fluxes of an impurity from an external medium (coating) is performed. The influence of the softened (due to impurity diffusion fluxes) layer thickness on the creep character in molybdenum is discussed for the case of its diffusion contact with nickel. An analysis of the effects of the regime of impurity grainboundary diffusion on the value of strain-rate sensitivity and on the behavior of grain-boundary migration as a function of external stress is made. Using submicrocrystalline nickel as an example, the influence of grainboundary state on properties of metallic materials in submicrocrystalline state is investigated. Direct measurements, theoretical calculations and indirect estimations of the differences in grain-boundary diffusion coefficients in coarse- and ultrafine-grained states are performed. The role of dispersion hardening in stabilizing the submicrocrystalline structure and improving the creep resistance is addressed, including the case where the grain-boundary fluxes of impurity atoms are forced from an external medium.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 49–64, August, 2004.     

On physical nature of creep acceleration effect in submicrocrystalline materials

An analysis of special features of manifestation of the creep acceleration effect in polycrystalline metals and alloys in the presence of grain-boundary diffusion fluxes of an impurity from an external medium is made. It is noted that in certain cases where the diffusion properties of sub-boundary regions differ greatly from those within the boundary and in the bulk of the grain (e.g., in materials with submicrocrystalline structure (grain size 0.1–0.5 µm) and/or in the case of diffusion along the grain boundary of a strongly segregating impurity), conventional theoretical methods of calculation of the temperature-temporal range of the occurrence of the above effect have to be corrected. Based on the analysis of the experimental data, feasible solutions to the problem are discussed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 29–32, October 2004.     

Changes in the diffusion properties of nonequilibrium grain boundaries upon recrystallization and superplastic deformation of submicrocrystalline metals and alloys

The effect of an increase in the coefficient of the grain-boundary diffusion upon recrystallization and superplastic deformation of submicrocrystalline (SMC) materials prepared by severe plastic deformation has been studied. It is shown that the coefficient of the grain-boundary diffusion of the SMC materials is dependent on the intensity of the lattice dislocation flow whose value is proportional to the rate of the grain boundary migration upon annealing of SMC metals or the rate of the intragrain deformation under conditions of superplastic deformation of SMC alloys. It is found that, at a high rate of grain boundary migrations and high rates of superplastic deformation, the intensity of the lattice dislocation flow bombarding grain boundaries of SMC materials is higher than the intensity of their diffusion accommodation, which leads to an increase in the coefficient of the grain-boundary diffusion and a decrease in the activation energy. The results of the numerical calculations agree well with the experimental data.     

Effects of grain boundary diffusion of copper on nickel creep

We have investigated the effects of surface copper diffusion on the creep of large-grain nickel (average grain size about 20 m) and submicron crystalline nickel (grain size about 0.3 m). For both structural states of nickel we find an acceleration of creep over its value in vacuum, and an increase in plasticity during creep when copper has diffused deeply into the nickel from the surface. The temperature range over which these effects are observed in the submicron nickel is nearly 300° lower than that for the large-grain samples. This result is probably due to a significant increase in the grain-boundary and bulk diffusion coefficients of copper in submicron nickel when compared with large-grain nickel.Siberian Branch. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 83–86, December, 1994.     

High-temperature creep of nickel under conditions of grain-boundary diffusion of impurities from the surface

Comparative investigations of the effect of diffusion streams of copper atoms (a weakly segregating impurity) and silver atoms (a strongly segregating impurity) from the surface in the high-temperature plastic deformation of nickel have been carried out. It has been established that in the high-temperature plastic deformation of nickel, when there are diffusion streams of copper and silver from the surface, there is a reduction in the creep resistance of nickel due to an increase in the contribution of grain-boundary slip to the overall deformation. Two stages, characterized by different values of the rate sensitivity factor m, are observed on the curve of the stress against the rate of deformation over a certain temperature range for each impurity. In the region of deformation rates of sec⁻¹, m≊0.2, and for sec⁻¹, m becomes less than 0.05. Institute of Physics of the Strength and Study of Materials. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 119–125, July, 1997.     

Variation of apparent creep activation energy in plastic deformation of molybdenum in diffusion contact with nickel

The variation of the apparent creep activation energy as a function of the state of grain boundaries is investigated in the deformation of molybdenum in the presence of diffusion fluxes of nickel at the grain boundaries. It is shown that this energy varies in the same way as in the plastic deformation of classical superplastic materials.Physics Institute of Strength and Materials Science, Siberian Branch, Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 110–113, May, 1993.     

Grain boundary deformation in Cd-Zn and Zn-Al alloys

Activation energies for creep have been measured on fine-grained and coarse-grained specimens of pure cadmium, zinc, and Cd-Zn and Zn-Al alloys in the temperature range (0·4–0·8)T _m. It is found that in the case of fine-grained specimens the activation energies for creep are equal to the activation energies for grain boundary diffusion in cadmium and zinc, and in the case of coarse-grained specimens — to that of volume self-diffusion.     

Structure and temperature dependence of the magnetization of the DyFe<Subscript>11</Subscript>Ti nanocrystalline compound

The coercive force, the temperature dependence of the magnetization, and the structure of a DyFeTi alloy based on the DyFe₁₁Ti compound with an excess content of α-Fe in the initial coarse-grained, nanocrystalline, and submicrocrystalline states are investigated experimentally. It is found that, in the submicrocrystalline sample, the coercive force is three times stronger and the temperature of the first spin-reorientation transition is 20 K higher than those in the coarse-grained sample. In the nanocrystalline sample, the coercive force is five times stronger than that in the coarse-grained sample, the first spin-reorientation transition is not revealed, and the transition at the Curie temperature is smeared. It is demonstrated that the changes observed in the magnetic properties are unrelated to the phase transformations but stem from the small size of crystal grains and high imperfection of the structure. The thermal instability of the DyFe₁₁Ti compound is observed in submicrocrystalline and nanocrystalline states.     

Effect of the grain structure on the evolution of martensitic transformations in a nanostructured titanium nickelide

Experimental data are presented on the formation of a nanostructure in coarse-grained and submicrocrystalline titanium nickelides under severe plastic deformation in the premartensitic state followed by annealing. The effect of the grain size in nano- and submicrocrystalline-structured materials on the evolution of martensitic transformations is considered.     

Grain-size-dependent thermal conductivity of nanocrystalline materials

In order to study the influence of grain size and lattice strain on the thermal conductivity of nanocrystalline (NC) materials, both experimental and theoretical studies were carried out on NC copper. The NC copper samples were prepared by hot isostatic pressing of nano-sized powder particles with mean grain size of 30 nm. The thermal behaviors of the samples were measured to be 175.63–233.37 W (m K)^?1 by using a laser method at 300 K, which is 45.6 and 60.6 % of the coarse-grained copper, respectively. The average grain size lies in the range of 56–187 nm, and the lattice strain is in the range of ?0.21 to ?0.45 % (in the direction of 111) and ?0.09 to 0.92 % (in the direction of 200). In addition, a modified Kapitza resistance model was developed to study the thermal transport in NC copper. The theoretical calculations based on the presented theoretical model were in good agreement with our experimental results, and it demonstrated that the thermal conductivity of NC materials show obvious size effect. It is also evident that the decrease in the thermal conductivity of NC material can be mainly attributed to the nano-size effect rather than the lattice strain effect.     

The influence of crystalline grain size on the thermal stability of the Er<Subscript>0.45</Subscript>Ho<Subscript>0.55</Subscript>Fe<Subscript>2</Subscript> compound

The phase composition and the temperature dependence of the magnetization of the Er_0.45Ho_0.55Fe₂ compound in coarse-grained, microcrystalline, and submicrocrystalline states are investigated experimentally. It is found that, upon heating under vacuum, the Er_0.45Ho_0.55Fe₂ microcrystalline powder with a crystalline grain size of ∼1 μm undergoes decomposition into pure iron and rare-earth (erbium and holmium) oxides and nitrides at a temperature of 500 K. The changes observed in the phase composition of the microcrystalline powder due to annealing are confirmed by x-ray diffraction analysis. Heating of the Er_0.45Ho_0.55Fe₂ submicrocrystalline sample leads to a partial change in the phase composition. The phase composition of a large crystal (∼1 mm in size) remains unchanged upon heating to 1080 K. It is shown that the thermal stability of the Er_0.45Ho_0.55Fe₂ compound depends on the crystalline grain size. __________ Translated from Fizika Tverdogo Tela, Vol. 44, No. 6, 2002, pp. 1060–1063. Original Russian Text Copyright ? 2002 by Mulyukov, Sharipov, Korznikova.     

Study on the effect of temperature rise on grain refining during fabrication of nanocrystalline copper under explosive loading

Nanocrystalline (NC) copper was fabricated by severe plastic deformation of coarse-grained copper at a high strain rate under explosive loading. The feasibility of grain refinement under different explosive loading and the influence of overall temperature rise on grain refinement under impact compression were studied in this paper. The calculation model for the macroscopic temperature rise was established according to the adiabatic shock compression theory. The calculation model for coarse-grained copper was established by the Voronoi method and the microscopic temperature rise resulted from severe plastic deformation of grains was calculated by ANSYS/ls-dyna finite element software. The results show that it is feasible to fabricate NC copper by explosively dynamic deformation of coarse-grained copper and the average grain size of the NC copper can be controlled between 200～400 nm. The whole temperature rise would increase with the increasing explosive thickness. Ammonium nitrate fuel oil explosive was adopted and five different thicknesses of the explosive, which are 20 mm, 25 mm, 30 mm, 35 mm, 45 mm, respectively, with the same diameter using 20 mm to the fly plate were adopted. The maximum macro and micro temperature rise is up to 532.4 K, 143.4 K, respectively, which has no great effect on grain refinement due to the whole temperature rise that is lower than grain growth temperature according to the high pressure melting theory.     

Microstructure dependence of steady state creep in Cd–Sn eutectic alloy

The steady state creep of Sn–33 wt.% Cd alloy was studied under various constant stresses ranging from 25.56 to 30.85 MPa in the temperature range from 353 to 433 K. The stress exponent n was found to change from 6.25 to 4.55 in the above temperature range. The energy activating the steady state creep amounted to 59.3kJ/mol in the temperature range from 353 K to 393 K and to 37 kJ/mol in the temperature range from 413 K to 433 K characterizing the grain boundary diffusion in Cd and in Sn, respectively. Microstructure analysis confirmed that the above mentioned mechanisms took place during steady state creep.     

Curie Transition of NC Nickel by Mechanical Spectroscopy and Magnetization Study

Mechanical spectroscopy measurement is performed to study the internal friction of nanocrystalline （NC） nickel with an average grain size of 23 nm from room temperature to 610 K. An internal friction peak is observed at about 550 K, which corresponds to the Curie transition process of the NC nickel according to the result of magnetization test. Moreover, the fact that the explained by an analytical model Curie temperature of NC nickel is based on the weakening of cohesive lower than that of coarse-grained nickel is energy.     

Field Electron And X-Ray Emission Spectroscopy of Submicroscopic Nickel

Samples of submicrocrystalline nickel (d = 100 nm) prepared under the action of strong plastic strains are investigated by the methods of field electron and x-ray emission spectroscopy and transmission submicroscopy. Qualitative and quantitative differences of autoelectron distributions over total energies for a submicrocrystalline metal from those for a coarse-grained metal are detected.     

Low-temperature plasticity in nanocrystalline titanium and copper

The stress-strain compressive curves, temperature dependences of the yield stress, and small-inelastic-strain rate spectra of coarse-grained and ultrafine-grained (produced by equal-channel angular pressing) titanium and copper are compared in the temperature range 4.2–300 K. As the temperature decreases, copper undergoes mainly strain hardening and titanium undergoes thermal hardening. The temperature dependences of the yield stress of titanium and copper have specific features which correlate with the behavior of their small-inelastic-strain rate spectra. Under the same loading conditions, the rate of microplastic deformation of ultrafine-grained titanium is lower than that of coarse-grained titanium and the rate peaks shift toward high temperatures. The deformation activation volumes of titanium samples differing in terms of their grain size are (10–35)b ³, where b is the Burgers vector magnitude. The dependences of the yield stress on the grain size at various temperatures are satisfactorily described by the Hall-Petch relation.     

Assessment of surface relief and short cracks under cyclic creep in a type 316LN austenitic stainless steel

Formation of surface relief and short cracks under cyclic creep (stress-controlled fatigue) in type 316LN stainless steel was studied at temperatures ranging from ambient to 923 K using scanning electron microscopy technique. The surface topography and crack distribution behaviour under cyclic creep were found to be strong functions of testing temperature due to the difference in strain accumulation. At 823 K, surface relief mainly consisted of fine slip markings due to negligible accumulation of strain as a consequence of dynamic strain ageing (DSA) which led to an increase in the cyclic life. Persistent slip markings (PSM) with distinct extrusions containing minute cracks were seen to prevail in the temperature range 873–923 K, indicating a higher slip activity causing higher strain accumulation in the absence of DSA. Besides, a large number of secondary cracks (both transgranular and intergranular) which were partially accentuated by severe oxidation, were observed. Extensive cavitation-induced grain boundary cracking took place at 923 K, which coalesced with PSM-induced transgranular cracks resulting in failure dominated by creep that in turn led to a drastic reduction in cyclic life. Investigations on the influence of stress rate were also carried out which underlined the presence of DSA at 823 K. At 923 K, lowering the stress rate caused further strengthening of the contribution from creep damage marked by a shift in the damage mechanism from cyclic slip to diffusion.     

Effect of the microstructural porosity parameters on the fracture and deformation of copper during creep at 773 K

The parameters of intergranular fracture of copper during creep under tension at T = 773 K and σ = 12.5 MPa are determined, and the contribution of grain-boundary porosity to the increase in the creep rate at stage III is estimated. The increase in the creep rate is shown to occur due to the pore-induced decrease in the grain boundary area, an increase in the mobile-dislocation density, and the deformation of the material because of the formation of pores and cracks.     

The Structural Features and Mechanical Properties of Submicrocrystalline Nickel Produced by Severe Plastic Deformation

Transmission electron microscopy is used to study the grain-subgrain structure of submicrocrystalline nickel produced by severe plastic deformation. The parameters of the grain-subgrain structure elements and the relative fractions of high- and low-angle grain boundaries in the microstructure subjected to severe plastic deformation are determined. The effect of the true grain size found by direct measurements on the mechanical properties of submicrocrystalline nickel is investigated.