Grain Boundary Grooving as an Indicator of Grain Boundary Phase Transformations

The atomic force microscopy (AFM) was used to study the grain boundary (GB) groove profiles far away from the melting temperature T _m. It is shown that AFM allows one to measure the temperature dependence of the GB energy in a rather broad temperature interval (from 0.85 T _m to T _m). The GB energy and GB segregation of Bi were measured at 1123 K in the interval of the Bi bulk concentration x ^v _Bi from 5 to 140 ppm Bi. The transition from monolayer to multilayer adsorption is observed for the 19a GB at 1123 K and x ^v _Bi = 60 at. ppm Bi. At the same point (1123 K and x ^v _Bi = 60 at. ppm Bi) a discontinuity of the first derivative of the GB energy is observed. These features were explained using the model of GB prewetting phase transformation developed previously.     

Characterization and Oxidation of Magnetron Sputtered Fe-Al Intermetallic Alloys

Fe-Al intermetallic alloys have been considered as protective materials against corrosion. We have studied the properties of such intermetallic coatings Fe _x Al_1?x (x ≤ 0.5) on the aluminum-rich side and their oxidation behaviour. The samples were prepared using rf-magnetron sputtering. The composition and layer thickness were determined by Rutherford Backscattering Spectrometry (RBS) and high-resolution scanning electron microscopy (HRSEM). Conversion Electron Mössbauer Spectroscopy (CEMS) and X-Ray Diffraction (XRD) were applied for phase analysis. We report here on the oxidation of such coatings.     

Phase Transformations in Nd–Fe–B-Based Alloys under High Pressure Torsion at Different Temperatures

In this work, we studied the behavior of the Nd–Dy–Fe–Co–Cu–B alloy for permanent magnets under high pressure torsion (HPT). In the initial state of the studied alloy, it mainly contained the crystalline phase τ₁ (Nd, Dy)₂(Fe, Co, Cu) ₁₄B. After HPT at room temperature (T_HPT = 30°C), a mixture of an amorphous phase with nanocrystalline inclusions of the τ₁ phase is observed in the alloy. In the equilibrium phase diagram, this state is equivalent to a mixture of the τ₁ phase with the melt at the temperature T_eff= ∼1100°C. The thus determined T_eff value is called the effective temperature. When the T_HPT temperature of the HPT treatment increases to 300 and 400°C, the amorphous phase disappears, and the Fe₂B and γ-Fe phases appear instead. In the equilibrium phase diagram, this state is equivalent to a mixture of phases τ₁+ Fe₂B + γ-Fe, which is observed in the temperature range from ∼950 to ∼1050°C. We explain this phenomenon by the fact that with an increase in the HPT temperature T_HPT, the rate of formation of defects during deformation remains constant, but the rate of their thermal relaxation (annihilation) increases. This is equivalent to decrease in the effective temperature T_eff in the equilibrium phase diagram. The previously predicted decrease in T_eff with an increase in T_HPT is observed for the first time.

     

Formation of the ω Phase in the Titanium—Iron System under Shear Deformation

JETP Letters - The effect of the phase composition on the α/β-Ti(Fe)→ω-Ti(Fe) transformation in the Ti-4 wt % Fe alloy under shear strain with high-pressure torsion (HPT) has...     

Phase Transformations in Copper—Tin Solid Solutions at High-Pressure Torsion

JETP Letters - Phase transformations of some Hume-Rothery phases (electron compounds) to others in a copper-tin system subjected to high-pressure torsion were detected in our previous work [B.B....     

Grain boundary layers in nanocrystalline ferromagnetic zinc oxide

The complete solubility of an impurity in a polycrystal increases with decreasing grain size, because the impurity dissolves not only in the crystallite bulk but also on the grain boundaries. This effect is especially strong when the adsorption layers (or the grain boundary phases) are multilayer. For example, the Mn solubility in the nanocrystalline films (where the size of grains is ∼20 nm) is more than three times greater than that in the ZnO single crystals. The thin nanocrystalline Mn-doped ZnO films in the Mn concentration range 0.1–47 at % have been obtained from organic precursors (butanoates) by the “liquid ceramic” method. They have ferromagnetic properties, because the specific area of the grain boundaries in them is greater than the critical value [B.B. Straumal et al., Phys. Rev. B 79, 205206 (2009)]. The high-resolution electron transmission microscopy studies show that the ZnO nanocrystalline grains with the wurtzite lattice are separated by amorphous layers whose thickness increases with the Mn concentration. The morphology of these layers differs greatly from the structure of the amorphous prewetting films on the grain boundaries in the ZnO:Bi₂O₃ system.     

Grain Boundary Phase Transitions and their Influence on Properties of Polycrystals

Grain boundary (GB) phase transitions can change drastically the properties of polycrystals. The GB wetting phase transition can occur in the two-phase area of the bulk phase diagram where the liquid (L) and solid (S) phases are in equlibrium. Above the temperature of the GB wetting phase transition a GB cannot exist in equlibrium contact with the liquid phase. The experimental data on GB wetting phase transitions in numerous systems are analysed. The GB wetting tie-line can continue in the one-phase area of the bulk phase diagram as a GB solidus line. This line represents the GB premelting or prewetting phase transitions. The GB properties change drastically when GB solidus line is crossed by a change in the temperature or concentration. The experimental data on GB segregation, energy, mobility and diffusivity obtained in various systems both in polycrystals and bicrystals are analysed. In case if two solid phases are in equilibrium, the GB “solid state wetting” can occur. In this case the layer of the solid phase 2 has to substitute GBs in the solid phase 1. Such GB phase transition occurs if the energy of two interphase boundaries is lower than the GB energy in the phase 1.     

Ferromagnetism of zinc oxide nanograined films

The reasons for the appearance of ferromagnetic properties of zinc oxide have been reviewed. It has been shown that ferromagnetism appears only in polycrystals at a quite high density of grain boundaries. The critical size of grains is about 20 nm for pure ZnO and more than 40 μm for iron-doped zinc oxide. The solubility of manganese and cobalt in zinc oxide increases significantly with a decrease in the size of grains. The dependences of the saturation magnetization on the concentrations of cobalt, manganese, and ion are nonmonotonic. Even if the size of grains is below the critical value, the ferromagnetic properties of zinc oxide depend significantly on the texture of films and the structure of amorphous intercrystallite layers.     

“Wetting” Phase Transitions by the Second Solid Phase for Linear Defects (Grain Boundary Triple Junctions)

JETP Letters - In this work, the wetting phase transition of grain boundaries (GBs) and their triple junctions (GB TJs) by the second solid phase in the magnesium-based alloy EZ33A is studied. The...