Hall–Petch k dependencies in nanopolycrystals

Measurements at nanopolycrystal grain sizes of the Hall–Petch plastic flow stress dependence on the reciprocal square root of polycrystal grain diameter occasionally fall short of expectations. The present report reviews experimental and theoretical aspects of such behaviours for a number of metal systems. Comparison with the results on copper had led to prediction on a dislocation pile-up mechanics basis of an even stronger grain size dependence that is now shown to carry over to explanation of similar results reported recently for cold-drawn eutectoid steel wires. In the current review, emphasis is given on additional important results achieved on the production of nanoscale zinc and tungsten carbide materials by Professor Jay Narayan and colleagues at North Carolina State University.