A. B. Straumal

Observation of Pseudopartial Grain Boundary Wetting in the NdFeB-Based Alloy

The NdFeB-based alloys were invented in 1980s and remain the best-known hard magnetic alloys. In order to reach the optimum magnetic properties, the grains of hard magnetic Nd2Fe14B phase have to be isolated from one another by the (possibly thin) layers of a non-ferromagnetic Nd-rich phase. In this work, we observe that the few-nanometer-thin layers of the Nd-rich phase appear between Nd2Fe14B grains due to the pseudopartial grain boundary (GB) wetting. Namely, some Nd2Fe14B/Nd2Fe14B GBs are not completely wetted by the Nd-rich melt and have the high contact angle with the liquid phase and, nevertheless, contain the 2-4-nm-thin uniform Nd-rich layer.

Pseudopartial Wetting of Grain Boundaries in Severely Deformed Al-Zn Alloys

After severe plastic deformation by the high-pressure torsion, Al-Zn alloys have three various classes of Al/Al grain boundaries (GBs) wetted with a second zinc-rich phase. Completely wetted Al/Al GBs are coated with the layer of a zinc-rich phase more than 30 nm thick. Partially (incompletely) wetted Al/Al GBs contact particles of the zinc-rich phase with a contact angle >60°, but contain no measurable zinc concentration. Pseudopartially wetted Al/Al GBs also contact Zn particles with a contact angle >60°. However, they have a thin interlayer of the zinc-rich phase with a uniform thickness of 2–4 nm, the presence of which explains the unusually high ductility of Al-Zn alloys after high-pressure torsion.

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).

First observation of a wetting phase transition in low-angle grain boundaries

The wetting phase transition at low-angle intercrystallite grain boundaries has been experimentally observed. In contrast to the high-angle grain boundaries with the misorientation angels θ > 15°, the low-angle grain boundaries (θ < 15°) are not continuous two-dimensional defects, but constitute a discrete wall (network) of lattice dislocations (edge and/or helical). The theory predicts that, depending on θ, either a continuous layer of the liquid phase or a wall (network) of microscopic liquid tubes on wetted dislocation nuclei is formed at completely wetted low-angle grain boundaries. It has been shown that the continuous liquid layers at low-angle grain boundaries in the Cu-Ag alloys appear at the temperature TwminL = 970°C, which is 180°C higher than the onset temperature Twmin = 790°C and 50°C lower than the finish temperature of the wetting phase transition at high-angle grain boundaries, Twmax = 1020°C.

Solid-phase wetting at grain boundaries in the Zr-Nb system

The microstructure of Zr-Nb polycrystalline alloys with niobium concentrations of 1, 2.5, 4, and 8 wt % is investigated in the temperature interval of 620–840°C. It is revealed that the second solid phase β-Nb forms either a chain of separate lens-like precipitates or continuous homogeneous layers at grain boundaries in zirconium, depending on the annealing temperature and the energy of the Zr/Zr grain boundary. It is shown that the greater the quantity of the second solid phase, the lower is the temperature of the termination of grain-boundary wetting. A model is constructed that explains the dependence of the temperature of grain boundary wetting on the amount of wetting phase. It is found that the complete wetting of all grain boundaries in zirconium by the second solid phase does not occur in Zr-Nb alloys.

Wetting of grain boundaries in hard-magnetic Nd-Fe-B alloys

Samples of Nd-Fe-B alloys, which have been the main hard-magnetic materials with the highest magnetic energy since the end of the 1980s, are investigated. Magnets based on them are obtained by liquid-phase sintering or spin coating. In this article, wetting of the Nd2Fe14B grains by the neodymium-enriched liquid phase is investigated. The microstructure of the Fe-12.3 at %Nd-7.6 at %B alloy quenched after annealing at T = 700−1100°C in the presence of a neodymium-enriched melt is studied. The acquired data indicate that the transition from incomplete to complete wetting of grain boundaries occurs as the temperature increases. The results are compared with the reference data for alloys of the Nd-Fe-B system obtained by liquid-phase sintering. The relation between the wetting phase transition of grain boundaries and magnetic properties is also discussed.