ABSTRACT
Metallurgists have known for many years that grain refinement often leads to an improvement in the properties of metals and alloys. For example, reducing grain size lowers the ductile-to-brittle transition temperature in steel. Of interest in this chapter is the influence of grain size reduction on the mechanical behavior of materials. The well known empirical Hall-Petch equation [1, 2] relates yield stress u y to average grain size d
where u0 is a friction stress and k is a constant. A similar relationship exists between hardness and grain size. Nanocrystalline metals represent the ultimate in grain refinement. If equation (13.1) were valid down to grain sizes in the nanometer range with the same value of k found at conventional grain sizes, remarkable increases in strength would be realized. Reducing d from 10 J.Lm to 10 nm would increase the strength by a factor of about 30. Although very significant improvement is seen, for all nanocrystalline metals measured to date, the amount has fallen short of this rosy prediction. Reasons for failure to live up to the promise of equation (13.1) will be discussed later. In the part of this chapter that treats nanocrystalline metals, we first examine the deformation mechanisms that are expected to be important in the nanometer grain size range, then survey the experimental data available on mechanical properties, and evaluate the results. Discussion of the outstanding questions and challenges will appear at the end of the chapter.
