STRAIN INDUCED CEMENTITE DISSOLUTION IN PEARLITIC STEELS AS A CLASSICAL EXAMPLE OF MECHANICAL ALLOYING

摘要

Severe plastic deformation (SPD) is widely used now to produce a nanocrystalline structure in metals. Being applied to alloys it allows not only to refine the microstructure, but also lead to dissolution of second phases. An example of such effect of SPD is complete dissolution of cementite in eutectoid steel. In the present paper the mechanism of the strain induced cementite dissolution was analysed in terms of a model where a plastic phase (ferrite) flows under high pressure and high external stress around hard precipitates like a viscous fluid. The friction at the precipitate/matrix interface leads to two effects. One is a high strain energy accumulated in the carbides, which may strongly contribute to their thermodynamic instability, and the second is their wear due to the flow of the ferrite. The dissolution of carbon with the ferrite phase can be considered as a driven transformation, where two driving forces for mass transport flow are competing. One is the mechanically induced drag of carbon atoms, which depends mainly on the deformation rate. The second is diffusion of carbon induced by thermodynamic energy gradients, which are connected with high strains in the precipitates.