Formation of Fe-Cu and Fe-Bi nonequilibrium solid solutions upon large plastic deformation and subsequent heating

Nanocrystalline structure and nonequilibrium nonuniform solid solutions were observed to form in Fe-Cu and Fe-Bi alloys upon large plastic deformation. The deformed specimens were studied by transmission electron microscopy, X-ray diffraction, and Mossbauer spectroscopy. Fine-grained structure is formed in the initially coarse-grained Fe-Cu alloys due to large deformation by shear under pressure. In the Fe-20 at. % Cu alloy, two maxima (15 nm and 40 nm) in the grain-size distribution were found in the alloy after deformation to e = 5-6, and one maximum at 10 nm was found after deformation to e = 7.2. Deformation to e = 6-7, occurring virtually without change in average grain size, may be considered as a cold constrained superplastic deformation. The model of grain rolling was proposed for the description of the mechanism of this superplastic deformation. According to the X-ray diffraction and Mossbauer data, the diffusive redistribution of atoms and the formation of the nonequilibrium Fe-Cu and Fe-Bi solid solutions occur mainly at deformations ranging from e = 5-6 to e = 7. The rapid diffusion upon cold plastic deformation is caused by high internal stresses both at grain boundaries and inside fine grains. A nonequilibrium solid solution Fe-20 at. % Cu is also formed upon heating of specimens with bimodal size distributions of nanograins caused by deformation to e = 5-6.