Paradox of strength and ductility in metals processed by severe plastic deformation

It is well known that plastic deformation induced by conventional forming methods such as rolling, drawing or extrusion can significantly increase the strength of metals However, this increase is usually accompanied by a loss of ductility. For example, Fig. 1 shows that with increasing plastic deformation, the yield strength of Cu and Al monotonically increases while their elongation to failure (ductility) decreases. The same trend is also true for other metals and alloys. Here we report an extraordinary combination of high strength and high ductility produced in metals subject to severe plastic deformation (SPD). We believe that this unusual mechanical behavior is caused by the unique nanostructures generated by SPD processing. The combination of ultrafine grain size and high-density dislocations appears to enable deformation by new mechanisms. This work demonstrates the possibility of tailoring the microstructures of metals and alloys by SPD to obtain both high strength and high ductility. Materials with such desirable mechanical properties are very attractive for advanced structural applications.