O. A. Kogtenkova

Grain boundary phase observed in Al-5 at.% Zn alloy by using HREM

The nature and behaviour of grain boundary (GB) phases is very important since they can control strength, plasticity, resistivity, grain growth, corrosion resistance, etc, especially in nanocrystalline materials. For nanocrystalline Al-based light alloys, extremely high plasticity has been observed in restricted temperature and concentration intervals close to the solidus line. This phenomenon is not fully understood. It can be explained by formation of GB phases not included in the bulk phase diagram. Therefore, the structure of GB phases, as well as thermodynamic conditions for their existence, has to be carefully studied. In this work the structure and composition of GBs and GB triple junctions in Al–5 at.% Zn polycrystals annealed in the temperature region above and below the bulk solidus line were studied by high-resolution electron microscopy and analytical transmission electron microscopy. Evidence has been obtained that a thin layer of a liquid-like phase exists in GBs and GB triple junctions slightly below the bulk solidus line.

Phase transitions induced by severe plastic deformation: steady-state and equifinality

Abstract During severe plastic deformation (SPD), a steady-state is usually reached after a certain value of strain (i. e. number of passes during equal-channel pressing or number of rotations during high pressure torsion). The structure and properties of a material in a steady state (including composition of phases) do not depend on those in the starting state before SPD. In other words they are equifinal, and the production of lattice defects is in dynamic equilibrium with defect elimination. Moreover, the SPD-treatment at ambient temperature TSPD = 300 K is frequently equivalent to the heat treatment at a certain elevated temperature Teff > 300 K. For example, the composition of phases in Cu–Ni, Co–Cu and Nd–Fe–B-based alloys after high pressure torsion corresponds to the states at 200, 890 and 1 170 °C, respectively, and is rather insensitive to the high pressure torsion rate (between 0.2 and 2 rpm) and pressure (between 3 and 8 GPa).

Review: grain boundary faceting–roughening phenomena

Similar to free surfaces, the grain boundaries (GBs) in metals, semiconductors and insulators can contain flat (faceted) and curved (rough) portions. In the majority of cases, facets are parallel to the most densely packed planes of coincidence sites lattice formed by two lattices of abutting grains. Facets disappear with the increasing temperature (faceting–roughening transition) and the increasing angular distance from coincidence misorientation. The temperature of GB faceting–roughening transition TR decreases with the increasing inverse density of coincidence sites Σ. In case of fixed Σ, TR decreases with the decreasing density of coincidence sites in the GB plane. The intersection line (ridge) between facets or between facets and curved (rough) portions of surfaces can be of first order (two different tangents in the contact point) or of second order (common tangent, continuous transitions). The rough (curved) portions of GB can also form the first-order rough-to-rough ridges (with two tangents). GB facets control the transition from normal to abnormal grain growth and strongly influence the GB migration, diffusion, wetting, fracture and electrical conductivity.

Heat effect of grain boundary wetting in Al-Mg alloys

Grain boundary wetting transitions were previously observed in the Al–Mg system. The melting of as-cast Al–5 wt% Mg and Al–10 wt% Mg alloys was studied by the differential scanning calorimetry. The asymmetric shape of the melting curve permitted the observation of the thermal effect of grain boundary wetting. The difference in the shape of the melting curve for the two studied alloys is explained by the different temperature dependence of the fraction of completely wetted grain boundaries.

Formation of nanostructure during high-pressure torsion of Al-Zn, Al-Mg and Al-Zn-Mg alloys

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.

Structural changes in aluminum alloys upon severe plastic deformation

The structure and phase composition of Al-Zn, Al-Mg, and Al-Mg-Zn alloys were studied before and after severe plastic deformation of these alloys. The deformation was performed by high pressure torsion with true strain of ∼6. It was established that, as a result of severe plastic deformation, the grains of Al and Zn and also of the β and τ phases revealed in the structure decrease significantly in size and reach nanometer scales. A supersaturated solid solution of Zn in Al decomposes completely in this case and achieves the equilibrium state corresponding to room temperature. The decomposition is less pronounced for the magnesium-containing alloys. Based on the obtained experimental data, a conclusion is drawn concerning the possible mechanisms of this process. Microhardness measurements revealed softening of the alloys as a result of the deformation, which is due to the decomposition of the supersaturated solid solution.

Effect of the Wetting of Grain Boundaries on the Formation of a Solid Solution in the Al-Zn System

Phase transitions in the bulk and at grain boundaries in the (Al-20 wt % Zn) alloy have been studied by means of differential scanning calorimetry and transmission electron microscopy. Polycrystals with a high specific area of grain boundaries have been obtained using severe plastic deformation (high-pressure torsion). It has been shown that the Zn-based solid phase completely wets the grain boundaries in aluminum at a temperature of 200°C. The position of the grain boundary solvus line (solubility line), which is above the bulk solvus by 40–45 K, has been determined.

Faceting of σ3 grain boundaries in Al

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.

Reversible “Wetting” of grain boundaries by the second solid phase in the Cu-In system

The reversible wetting of grain boundaries by the second solid phase in the copper-indium system has been observed. With an increase in the temperature, the contact angle θ between the (Cu)/(Cu) grain boundary in a Cu-based solid solution based and particles of the δ-phase (Cu70In30) decreases gradually. Above TW = 370°C, the first (Cu)/(Cu) grain boundaries completely “wetted” by the δ phase appear in Cu-In polycrystals. In other words, the δ phase forms continuous layers along grain boundaries and θ = 0. At 440°C, the fraction of completely wetted grain boundaries reaches a maximum (93%), whereas the average contact angle reaches a minimum (θ = 2°). With a further increase in the temperature, the fraction of completely wetted grain boundaries decreases and vanishes again at TDW = 520°C. This phenomenon can be explained by an anomalous shape of the solubility limit curve of indium in a solid solution (Cu).

Pseudopartial Grain Boundary Wetting: Key to the Thin Intergranular Layers

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.