Back to basics: dislocations, the engine of plastic deformation

The word "defect" can sound inherently negative. Yet it is precisely dislocations — imperfections in the crystal lattice of metals — that make plastic deformation possible: without them, drawing a wire or bending a sheet of metal would require practically unsustainable forces.

After exploring the origins of wire drawing and the metallurgical fundamentals of plastic deformation, the "Back to basics" column arrives at its third installment. In this issue, Ing. Sergio Rusconi delves into the heart of physical metallurgy, examining deformation mechanisms, the enabling role of dislocations, and their different types.

Three key points for understanding dislocations

How do dislocations move within the crystal lattice? How does their density change as deformation proceeds? And how do these phenomena affect the plastic deformation of the material?

There are three key aspects to consider:

• the dynamics of dislocation slip, which explains how dislocations move along the lattice's slip systems, making plastic deformation possible at stresses far lower than those theoretically required in a perfect crystal;
• the interaction with microstructural barriers such as grain boundaries, alloying atoms and precipitates, responsible for the main strengthening mechanisms (Hall-Petch for grain boundaries, solid-solution strengthening for alloying atoms, precipitation hardening for precipitates);
• the multiplication and accumulation of dislocations, including through mechanisms such as the Frank-Read source, phenomena closely tied to strain hardening and the subsequent recovery and recrystallization processes.

The "defect" we started from thus becomes a fundamental feature of the microstructure — one whose behavior can be controlled, and even exploited, through processing and heat treatments.

We thank Eng. Sergio Rusconi for his valuable contribution to this project. Follow the series to stay updated on upcoming installments.