Papers by Author: Boris B. Straumal

Abstract: Severe plastic deformation (SPD) can lead to the phase transformations in the materials. Even the SPD-treatment at ambient temperature T_SPD = 300 K is frequently equivalent to the heat treatment at a certain elevated temperature (effective temperature) T_eff > 300 K. However, if the real annealing at effective temperature leads to the grain growth, SPD leads to strong grain refinement. SPD also accelerates the mass transfer in the materials. In this review the methods of determination for effective temperature after high-pressure torsion of metallic alloys are discussed as well as SPD-driven acceleration of diffusion.

Abstract: The thin layers of a second phase (also called complexions) in grain boundaries (GB) and triple junctions (TJs) are more and more frequently observed in polycrystals. The prewetting (or premelting) phase transitions were the first phenomena proposed to explain their existence. The deficit of the wetting phase in case of complete wetting can also lead to the formation of thin GB and TJ phases. However, only the phenomenon of pseudopartial (or pseudoincomplete, or constrained complete) wetting permitted to explain, how the thin GB film can exist in the equilibrium with GB lenses of a second phase with non-zero contact angle.

Abstract: Deformation-enhanced diffusion in single-crystalline Ni-based superalloy specimens have been investigated under the conditions of hard cyclic viscoplastic tension-compression deformation. The chemical composition of phases before and after cyclic deformation was investigated by filed-emission scanning electron microscopy. At low strain amplitude values (0-0.05%; 0-0.2%; 0-0.5%) the material shows upscaled viscoelastic behavior and microstructural stability. At the increase of strain amplitude in the γ+γ’-phase (0-1%), the Ni, Re and Co content decreases, whereas Al and Mo content increases significantly. On the contrary, in the single γ’-phase area, the Ni and Co content was increased, which was accompanied by a decrease of Nb, Cr, Ta and Al content. The length of dendrite arms was significantly decreased as compared to primary dendrite arms and γ+γ’-rafts were formed parallel to the stress axis direction. As a result of the deformation-enhanced, diffusion the necking of dendrites accompanied with longitudinal cracking by the dendrite axis and cross-sectional radial cracking by interdendritic region of single crystalline specimen occurs.

Abstract: In this work ultrafine-grained (UFG) structure was successfully produced in the commercial Al 6061 and Al-30%Zn alloys using new modifications of two severe plastic deformation (SPD) techniques, namely equal channel angular pressing (ECAP) with parallel channels (PC) and high pressure torsion (HPT) with enhanced load. Variation of SPD processing regimes made it possible not only to perform strong grain refinement but also to modify the phase composition through the formation of grain boundary (GB) segregations and precipitations. This enabled to achieve a unique combination of high strength and ductility in the Al 6061 alloy and demonstrate super-ductility in the Al-30%Zn alloy, when elongation to failure exceeded 150% at room temperature.

Abstract: The wetting behavior of grain boundaries is affected by temperature, pressure and misorientation of grain boundaries. However, the influence of quenching baths on liquid state grain boundary wetting is rarely reported. In this work, this effect in the Sn−25 at% In alloy was investigated. The Sn−In alloy was prepared by smelting of In and Sn at 300°C for 6 hr in Ar atmosphere. Samples were annealed at temperature between 130 and 165°C and quenched in two kinds of baths: −10°C salt-saturated water and liquid nitrogen. The results from X-ray diffraction show a difference in preferential orientations between samples quenched in these two baths. Metallographic analysis reveals that the percentage of grain boundaries fully wetted in samples quenched in −10°C salt-saturated water is lower than that quenched in liquid nitrogen. It is pointed out that a proper quenching bath is necessary for preserving the initial microstructure of grain boundary wetting.

Abstract: In studying grain boundary segregation in Cu–Bi alloys by means of Auger electron spectroscopy samples must be broken in-situ. Consequently, the distribution of segregants on both sides of fracture path must be considered quantitatively. This question can be addressed by studying the influence of an adsorbed oxygen layer on the intensity of Auger peaks. This oxygen layer forms on the fracture surface when it is intentionally exposed to air. In this work, the values of Bi coverage have been measured both on the as-fractured Cu fracture surface and on the fracture exposed to air. The coverage values evaluated from the model of a crosstie-like pattern agree better with each other than those from the model of a continuous layer. Our study reveals that the Cu-Bi bond is weaker than the Bi-Bi bond.

Abstract: The temperature dependence of the energy of various facets of twin GBs has been measured. For the investigation of GB faceting the Al bicrystals of 99.999% wt. purity were grown by the modified Bridgman technique. One grain in these bicrystals is semi-surrounded by another one. Bicrystals were coated with a layer of Sn–Al alloy and annealed at various temperatures. Contact angles at the junction of a GB and two solid/liquid interfaces have been measured. The ratios of GB energy to solid/liquid interface energy have been calculated. Using these data, the Wulff-Herring plots and GB phase diagrams were constructed. Three different crystallographic facets were observed for the coincidence GB. Two of them are stable at all studied temperatures, the third one becomes metastable below ~ 800K. In GBs with θ = 3° only one facet (symmetric twin GB) is stable.

Abstract: The grain boundary (GB) wetting was investigated in the Sn – 25 at.% In alloy. It was found that the portion of GBs wetted by the melt depends on the annealing temperature. No GB completely wetted by melt was observed at 140°C, while all GBs were fully wetted after annealing at 180°C. Between 140°C and 180°C the portion of wetted GBs increases with increasing temperature. The tie-lines of GB wetting phase transition were constructed in the Sn–In bulk phase diagram.

Abstract: The composition and microstructure of historic tongues and shallots from reed pipes of various Baroque organs have been studied. They contain Cu–Zn solid solution (α-brass with 23-29 wt. % Zn) and lead particles. Grain size in brass scatters from 10 to 200 μm. Around 50% of all GBs in brass are Σ=3 twin GBs. The high-indexed coincidence site lattice facets were observed in twin GBs. The increase of number of various facets roughly correlates with decreasing grain size. It may indicate the variation in annealing temperature used by organbuilders in Baroque Era. New brass with 25 wt. % Zn and 2 wt. % Pb has been prepared for reconstruction of historic tongues and shallots by restoration of reed pipes in Baroque organs. The morphology of lead inclusions and twin GBs has been investigated in temperature interval from 400 to 700°C and compared with that of historic alloys. The annealing temperature has been estimated.

Abstract: The effect of pressure on the kinetics of grain boundary (GB) segregation in the Cu–50 at. ppm Bi alloy has been studied. It was found by means of Auger electron spectroscopy that at a temperature of 1173 K the segregation level decreases from 2 to 1.5 monolayer as the pressure increases from 0.01 to 1.2 GPa. This behavior was explained by considering the physical parameters controlling kinetics and thermodynamics of GB segregation. A simplified model based on dislocation pipe diffusion, proposed previously and discussed in more details in this work, was used to calculate the non-equilibrium GB segregation during cooling under high pressure. The pressure effect on bulk diffusion is responsible for the suppression of GB segregation, while the pressure effect on the phase stability in Cu–Bi alloys provides a negligible contribution on GB segregation in the pressure range studied.