A. S. Gornakova

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.

Effective Temperature of High Pressure Torsion in Zr-Nb Alloys

Abstract Severe plastic deformation by the high pressure torsion (HPT) leads to the phase transitions and strong grain refinement. The starting α Zr-phase in Zr alloyed by 2.5 and 8 mass% Nb transforms into β + ω mixture. This β + ω phase mixture can be found in the equilibrium phase diagram at higher (effective) temperature (Teff = 620°C for Zr-2.5 mass% Nb and (Teff = 550°C for Zr-8 mass% Nb). The published papers on phase transitions during HPT are analysed and the values of effective temperature are estimated. Contrary to the increasing temperature, the increasing pressure slows down the diffusion and grain boundary migration. Therefore, the forced atomic movement during HPT produces the states equivalent to higher temperature, but not to the higher pressure.

Reversible transformation of a grain-boundary facet into a rough-to-rough ridge in zinc

The transition of two first-order facet-to-rough ridges into a single first-order rough-to-rough ridge theoretically predicted for free surfaces has been observed in a slowly migrating grain boundary (GB). The reversible faceting-roughening transition of the tilt GB in a Zn bicrystal has been studied in situ. Above T R+ = 673 K, the slowly migrating GB semi-circle was discontinuously curved, e.g. two rounded (rough) GB portions intersected with a slope discontinuity. Above T R+, a single first-order facet-to-rough GB ridge has been observed. Below T R− = 668 K, a facet appeared and intersected with rounded (rough) GB portions with a slope discontinuity. Below T R−, two first-order facet-to-rough GB ridges existed. The GB facet and tangents to the rounded (rough) GB portions were parallel to the closely packed planes of the constrained coincidence sites lattice. The steady state length of the facet increased with decreasing temperature. A hysteresis of about 5 K between roughening temperatures measured on ...

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.

Formation regularities of grain-boundary interlayers of the α-Ti phase in binary titanium alloys

The microstructure of polycrystalline alloys of titanium with chromium (2, 4, and 5.5 wt %), cobalt (2 and 4 wt %), and copper (2 and 3 wt %) is investigated. Series of prolonged isothermal annealing of these materials are performed in a temperature range from 600 to 850°C (in vacuum). Annealing temperatures fall in two-phase regions α(Ti,Me) + β(Ti,Me) of phase diagrams Ti–Cr, Ti–Co, and Ti–Cu. Temperature dependences of the fraction of grain boundaries β(Ti,Me)/β(Ti,Me) completely “wetted” by interlayers of the second solid phase α(Ti,Me) and average contact angle are measured. The results of microstructural investigations showed that the type and concentration of the second component in the alloy strongly affect the formation of equilibrium grain-boundary interlayers. A nonmonotonic temperature dependence of the fraction of grain boundaries completely wetted by interlayers of the second solid phase in the absence of ferromagnet–paramagnet phase transformations in the volume is revealed for the first time.

Second-order faceting-roughening of the tilt grain boundary in zinc

Abstract The faceting of the almost stationary tilt grain boundary with a misorientation angle of 84° in the Zn bicrystal has been investigated. The shape of the very slow migrating grain boundary has been studied in situ between 350 and 400 °C using polarized light. Two intersecting facets lie in the closely-packed planes of the constrained coincidence sites lattice with coincidence parameter = 15. In the as-grown sample flat grain boundary facets form the sharp first-order ridge with the break of the first derivative y/x of the grain boundary shape similar to those observed in our previous works on GB faceting in Zn. However, above 350 °C the sharp first-order ridge is substituted by the smooth grain boundary portion without a ∂y/∂x derivative break similar to that observed in twin grain boundaries in Mo. The transformation of the as-grown first-order grain boundary ridge into continuous one has been observed for the first time. The critical parameter α has been calculated using the grain boundary shape in the transition region. α increases from α = 1.7 ± 0.07 for 350 °C to α = 1.9 ± 0.07 for 385 °C approaching the α = 2 value predicted in the mean-field Andreev model for the continuous surface roughening.

Grain Boundary Faceting-Roughening in Zn

The change in the grain-boundary (GB) shape with an increase in temperature near the GB faceting-roughening phase transition has been studied. The GB facet length decreases with an increase in temperature to complete facet disappearance. The facet orientation is determined by the constrained coincidence site lattice (CCSL). Facets are located along close-packed CCSL planes. Above the roughening temperature TR, the tangents to the faceted and rough GB portions at the point of emergence of the first-order ridge are located along the close-packed CCSL planes (as facets below TR). The faceting-roughening phase transition is reversible. The presence of temperature hysteresis is indicative of first-order phase transition.

Growth of (αTi) grain boundary layers in Ti–Co alloys

The influence of temperature on the formation of the (αTi) grain-boundary interlayer in alloys Ti–2 wt % Co and Ti–4 wt % Co in the two-phase region (αTi) + (βTi) of the Ti–Co phase diagram is studied in a temperature range of 690–810°C. The growth kinetics of the thickness (Δ) of the grain-boundary interlayer of the (αTi) phase in the Ti–2 wt % Co alloy is investigated at 750°C. Δ depends on the annealing time as ~ t1/3. The analysis of the results of experimental observations makes it possible to assume that an increase in Δ is the manifestation of coalescence of (αTi), which is controlled by the bulk diffusion.

Crystallochemiluminescence of Solutions

It is shown that the chemiluminescence intensity from luminol solutions reaches a maximum when the latter are crystallized. This phenomenon is explained by the complex dynamics of the phase transition, chemical reactions, and degradation of electronic excitation energy. Luminescence of new type, called crystallochemiluminescence, is revealed.