Papers by Author: Boris B. Straumal

Abstract: The composition and microstructure of historic tongues and shallots from reed pipes of various Baroque organs has been studied. They contain Cu-Zn solid solution (α-brass with 23-29 wt. % Zn) and lead particles. Lead is mainly present as spherical bulk or lens-like grain boundary (GB) inclusions. However, in two samples Pb wets the brass GBs. In this case Pb forms the branched root-like structures. 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. The annealing temperature has been estimated using the faceting phase diagram for twin GBs in Cu.

Abstract: The kinetics and morphology of the grain boundary grooving of Zn bicrystals with 16° <1010> tilt GB by Sn(Zn) melt has been studied at 325°C in equilibrium conditions in vacuum. It is shown that grooving process is interface controlled at least on the first stage. Groove walls mobility is evaluated. Changing of GB grove shape from “faceted walls” corner for annealing time < 78 h to concave “Mullins type” groove for annealing time > 78 h at the same experimental temperature was observed for the first time.

Abstract: The migration of individual special [ ] 0 1 10 tilt grain boundary (GB) with Σ =15 and misorientation angle 29° in Zn bicrystal have been investigated. The stationary shape of migrating GB has been studied and the migration rate has been measured by optical microscopy in situ between 558 and 683 K using polarized light. In certain experimental runs the migrating GB is faceted and moves thermally activated. Its kinetics follows the Arrhenius type dependence despite the fact that shape of moving GB depends on temperature. After detachment from impurity cloud the [ ] 0 1 10 tilt GB migrates activationless in the temperature interval 618÷683 K. The detachment temperature is 618 K. The non-physically high value of the apparent migration activation enthalpy can appear due to the change of GB shape from faceted to smooth and back.

Abstract: Microstructure and hardness of ternary Al–Zn–Mg alloys were studied both in as cast state and after high pressure torsion (HPT) with 5 torsions (shear strain about 6). The size of (Al) grains and of reinforcing second phase precipitates decreases drastically after HPT reaching nanometer range. During HPT, the Zn- and Mg-rich supersaturated (Al) solid solution decomposes and reaches the equilibrium state corresponding to the room temperature. In the as cast state the hardness of the supersaturated solid solutions increases with increasing Zn and Mg content due to the solid-solution hardening. However, after HPT the work hardening and Hall-Petch hardening due to the decreasing grain size competes with softening due to the decomposition of a supersaturated solid solution. In the net effect, the severe plastic deformation results in softening of ternary Al–Zn– Mg alloys.

Abstract: Structure and phase composition of binary Al–Zn, Al–Mg and ternary Al–Zn–Mg alloys were studied before and after high pressure torsion (HPT) with shear strain 300. The size of (Al) grains and crystals of reinforcing second phases decreases drastically after HPT reaching nanometer range. As a result of HPT, the Zn-rich (Al) supersaturated solid solution decomposes completely and reaches the equilibrium state corresponding to room temperature. The decomposition is less pronounced for Al–Mg and Al–Zn–Mg alloys. We conclude that the severe plastic deformation of supersaturated solid solutions can be considered as a balance between deformation-induced disordering and deformation-accelerated diffusion towards the equilibrium state.

Abstract: Diffusional growth of the grain boundary (GB) groove permits one to measure the ratio between GB energy sGB and surface energy ssur. The faceting of twin tilt grain boundaries in Cu has been studied using the GB thermal groove method. No rough facet edges were observed. It means that melting temperature is lower than the roughening temperature for the observed facets in Cu. The influence of orientation and misorientation deviation Dq = ½q – qS½ from coincidence misorientation qS has been studied. By increase of Dq the energy of (100)CSL facet increases. The convenient method for construction 3D three-dimensional Wulff diagrams was found. The 3- dimensional Wulff diagrams were constructed using this method and measured sGB / ssur values.