Chapter 7: Dislocations and Strengthening Mechanisms

Dislocation density quantifies these defects, ranging from 10³ mm⁻² in pristine single crystals to 10¹⁰ mm⁻² in severely deformed metals. Slip occurs when dislocations move along specific crystallographic planes under applied shear stress, with strain fields surrounding individual dislocations enabling interactions that may cause repulsion, attraction, or mutual annihilation. The concept of slip systems specifies combinations of slip planes and directions active in different crystal structures. FCC and BCC metals possess numerous slip systems, conferring high ductility, whereas HCP metals exhibit limited slip systems and consequently reduced ductility and increased brittleness. The resolved shear stress equation and critical resolved shear stress threshold define when slip initiates. Single crystals display characteristic slip lines during deformation, while polycrystalline materials accommodate strain through grain elongation while preserving boundary integrity. Mechanical twinning provides an alternative deformation mechanism in materials with restricted slip systems by creating mirror symmetry across twin boundaries. Strengthening mechanisms operate on the principle that restricting dislocation motion increases material strength. Grain size reduction, quantified by the Hall–Petch equation relating yield strength inversely to the square root of grain diameter, simultaneously enhances both strength and toughness. Solid-solution strengthening occurs when impurity atoms create lattice distortions that immobilize dislocations. Strain hardening, or cold working, increases dislocation density during plastic deformation, elevating yield and tensile strength while reducing ductility, measurable through percent cold work calculations. The chapter concludes by addressing thermal processes that modify these effects: recovery reduces stored strain energy through dislocation rearrangement without eliminating defects, recrystallization generates new strain-free grains at temperatures typically between 0.3–0.5 of absolute melting temperature, and grain growth causes larger grains to consume smaller ones. These processes demonstrate how controlling dislocation behavior through mechanical and thermal treatments optimizes the balance between strength, ductility, and toughness in engineering applications.