Nanocrystallization by Heating of Amorphous Zr3Al Made by Repeated Cold Rolling

Full text: Nanocrystalline materials (i.e. materials with a grain size < 100 nm) are of strong scientific interest because of the possible changes of the materials properties. Kilmametov et al. showed that the ion irradiation resistance of the intermetallic compound NiTi can be enhanced by nanostructuring the material by severe plastic deformation (SPD). Zr3Al is an interesting intermetallic compound in this context because it was proposed as a nuclear structural material, but showed the drawback of amorphization upon ion irradiation. Nanostructuring could be a way to bypass this disadvantage. Repeated cold rolling and intermediate folding (RCR) is a promising SPD method because it was shown that the grain sizes that can be achieved are often smaller than those achieved by any other SPD method. Because of its iterative character, in principal unlimited amounts of strain can be induced by this method rendering the samples nanocrystalline or amorphous. In the present work, coarse crystalline Zr3Al was deformed by RCR. Samples deformed by up to 80 folding cycles (true strain ε80 = 6 600 %) were studied by Xray diffraction (XRD) and transmission electron microscopy (TEM). After 80 foldings, a large volume fraction of the material was amorphous. The mostly amorphous samples were heated to temperatures up to 973 K in a differential scanning calorimeter. Using different heating rates, the activation enthalpy of the recrystallization process and the crystallization enthalpy were determined. TEM and XRD measurements of the heat-treated samples showed that the initial nanocrystallization does not result in the equilibrium Zr3Al structure. A metastable crystal structure forms, which slowly transforms into the equilibrium structure at higher temperatures. (author)