GRAIN BOUNDARY WETTING PHASE TRANSITIONS IN THE Al-BASED ALLOYS

Svetlana Protasova ¹ Boris Straumal ^1,3 Olga Kogtenkova ¹ Tomasz Czeppe ² Pawel Zięba ²

¹Institute of Solid State Physics, Institute of Solid State Physics RAS, Chernogolovka
²Polish Academy of Sciences, Institute of Metallurgy and Materials Science, Cracow
³Institut für Nanotechnologie, Karlsruher Institut für Technologie, Eggenstein-Leopoldshafen

The nature and behavior 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, composition and thermal evolution of binary Al–Zn alloys with different grain size were studied by high-resolution microscopy, analytical transmission electron microscopy and method of differential scanning calorimetry. 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. The formation of liquid-like GB layers close to the solidus temperature explains the phenomenon of high strain-rate superplasticity observed in several nanostructured Al ternary alloys and nanostructured Al metal-matrix composites, containing Zn and Mg, in a very narrow temperature interval under the bulk solidus lines. This phenomenon remained long time unexplained. This work was partly financed by the RFBR (grant 11-08-90439) and Grant of President of Russia for support of young scientists (MK- 3748.2011.8)