The strengths of metals are sensitive to microstructure. Most hardening mechanisms involve making dislocation motion more difficult. These mechanisms include decreased grain size, strain-hardening, solid-solution hardening, and dispersion of fine particles. With finer grain sizes there are more grain boundaries to impede dislocation motion. Metals strain-harden because deformation increases the number of dislocations and each interferes with the movement of others. In solid solutions, solute atoms disrupt the periodicity of the lattice. Fine dispersions of hard particles create obstacles to dislocation motion. Martensite formation and strain aging in steels are sometimes considered separate mechanisms, but both are related to the effects of interstitial solutes on dislocations.
Other factors affecting strength are constraints from neighboring grains, preferred orientations, and crystal structure. This chapter will review all of these mechanisms.
Deformation of polycrystals
In polycrystalline materials, each grain is surrounded by others and must deform in such a way that its change of shape is compatible with its neighbors. Slip on a single system within a grain will not satisfy the need for compatibility. Early attempts to calculate the stress–strain behavior of polycrystals by averaging the stresses to cause single slip in each grain did not meet with success. G. I. Taylor achieved much better agreement by assuming that each grain of a polycrystal undergoes the same shape change (set of strains) as the whole polycrystal.