Nanostructured Metals with Multiscale Microstructures

The successful synthesis of bulk nanostructured metals (BNMs, grains sizes in the 100’s of nm) for engineering applications is of technological and scientific significance. From a technological point of view, BNMs can be engineered for ultra-high strengths, for example. From a scientific point of view, BNMs allow systematic investigation of physical and mechanical behavior, and sometimes yield novel deformation mechanisms.

In the last few decades, severe plastic deformation (SPD) techniques have been broadly applied to prepare a variety of BNMs, that not only exhibit notable combinations of strength and ductility, but also reveal non-equilibrium deformation mechanisms, such as twinning in the case of fcc crystals. Results from various groups around the world reveal considerable improvements in the physical performance of a variety of SPD processed metals and alloys, using cryomilling (milling in liquid nitrogen) and high pressure torsion, for example. While increases in strength of several 100% are commonly documented, ductility, however, appears to scale inversely with strength in these materials; this behavior has been attributed to limited dislocation activity at these length scales. This challenge has been addressed via the introduction of additional size scales that facilitate plasticity during deformation. The concept of a bimodal microstructure has recently been extended into the realm of metal composites with tri-modal microstructural characteristics, to attain ultra-high strength values. In this lecture, published data of cryomilled BNMs are reviewed and discussed with particular emphasis on the following topics: recent findings in the area of cryomilled materials; primary consolidation and secondary processing methods; microstructural evolution from nanostructured powders to bulk materials during consolidation; and mechanical behavior of BNMs. The deformation behavior and the underlying mechanisms of cryomilled materials are discussed in an effort to shed light into the fundamental behavior of BNMs.