It has been shown through a variety of methods that a crystal can be destabilized and removed from equilibrium under conditions of external forcing to form non-equilibrium phases, such as for example, highly-defective nanocrystalline and amorphous or glass-like structures. For such materials the phase stability and the path of phase transformations are often very different from those which are valid for bulk phases under equilibrium conditions. Effects such as shifts of phase boundaries in phase diagrams, extension of solubility limits and even lowering of the melting point have been reported recently for nanostructured systerns. In comparison with conventional materials these forced alloys also exhibit unusual mechanical properties, for example, many recent experimental results demonstrated increases in hardness and yield stress by a factor 4 to 5 and a slight decrease in Young's modulus. A variety of mechanisms can explain this behaviour, including the difficulty of generating and propagating dislocations through the nanoscale grains and the large volume fraction of grain boundaries. Therefore, alternative deformation modes must be relevant for nanocrystalline materials, such as grain boundary sliding and grain rotation, together with partial dislocation activity resulting in stacking fault formation and twinning. In this contribution we report on a unique phase transformation and novel deformation mechanism for bulk samples of a pearlitic carbon steel subjected to severe plastic deformation via novel processing routes, specifically high-pressure torsion, which enables grain refinement to nanometre dimensions at room temperature under a pressure of 7 GPa.
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