Abstract
High-pressure torsion has been used to obtain the ultra-fine grained (UFG) state with a high specific area of grain boundaries (GBs) in Al-Zn, Al-Mg, Cu-Ag, Cu-Co, and Cu-Ni solid solutions with face-centered cubic (fcc) lattices. The UFG samples were heated in a differential scanning calorimeter (DSC). Small endothermic peaks in the DSC curves were observed in the one-phase solid-solution area of the respective phase diagrams, i.e., far away from the bulk solidus and solvus lines. A possible explanation of these endothermic peaks is based on the hypothesis of phase transformations between GB complexions. This hypothesis has been supported by observations with transmission electron microscopy and electron backscattering diffraction. The new lines of GB phase transformations have been constructed in the Al-Zn, Al-Mg, Cu-Ag, Cu-Co, and Cu-Ni bulk phase diagrams.
Highlights
Phase transformations can take place in the volume of a material and on free surfaces or in the interphase boundaries (IBs) and grain boundaries (GBs)
It is clear that the new GB lines in the alloys contained the supersaturated (Al)-zero contact angle between (Zn), Al-Mg, contained supersaturated (Cu)-Co, and Cu-Ni phase diagrams proposed in this work are still very hypothetic, and the physical reasons for the small endothermic peaks observed in differential scanning calorimeter (DSC) curves (Figure 3) should be studied in more details
Small endothermic peaks were observed in ultra-fine grained (UFG) Al-Zn, Al-Mg, Cu-Ag, and Cu-Co solid solutions during their heating in the differential scanning calorimeter (DSC)
Summary
Phase transformations can take place in the volume of a material (as 3D transformations) and on free surfaces or in the interphase boundaries (IBs) and grain boundaries (GBs). In the case of premelting or prewetting, the GBs can contain a few-nanometers-thin layer of a second phase which can be stable in GB but cannot be thermodynamically stable in bulk [18,46,47,48] Such intergranular films (or IGFs) are not easy to observe, mainly because they are very thin (a few nanometers). The goal of this work is to study GB phase transformations and GB complexions (i) in-situ; (ii) in poorly quenchable metallic alloys; (iii) deep in the solid-solutions area (i.e., removed from the solidus or solvus lines); (iv) by comparing the samples with low and high specific area of GBs; and (v) by the use of thermal effect of such transformations and abnormal grain growth. The choice of alloys (Al-Zn, Al-Mg, Cu-Co, Cu-Ag, and Cu-Ni) is primarily related to the wide technological application of these materials, such as aircraft construction, mechanical engineering, construction, as well as for solid oxide fuel cells interconnect material
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