Metallurgical fundamentals of plastic deformation processes

A thorough understanding of metallic materials, their structures, and the transformations that characterize them, represents the indispensable foundation for analyzing and optimizing plastic deformation processes, of which wire drawing is a prime example. Metals are crystalline solids distinguished by a precise microstructural order that, taken as a whole, determines the macroscopic properties of the material. It is therefore essential to understand the correlations between metallurgical aspects, mechanical properties, and in-service behavior, enabling a well-informed study and optimal planning of manufacturing processes and operating conditions.

The metallic bond

Metallic materials, at the atomic level, are characterized by a particular type of bond called the metallic bond, which can be defined as an electrostatic interaction in which valence electrons detach from their parent nuclei to form a delocalized "electron cloud" surrounding the cations (positively charged ions), which occupy fixed equilibrium positions in the lattice. This electron mobility not only explains the high thermal and electrical conductivity of metals, but also underlies their ductility (the ability of a material to undergo plastic deformation under tensile loading before fracture) and malleability (the ability of a material to be reduced to thin sheets, typically through the application of a compressive force). The electron cloud acts as a "glue" holding the system together; the bond energy derives from the electromagnetic interaction between the negative charges of the valence electrons and the positive charges of the metal cations. This very bond allows atomic planes to slide relative to one another without the brittle fracture typical of many materials characterized by ionic or covalent bonds, such as numerous ceramic materials.

Crystal structures: unit cells

Atoms aggregate according to repetitive geometric patterns called unit cells, which, when repeated in space, form the crystal lattice. The most common unit cells in metallic materials are:

• Body-Centered Cubic (BCC), characterized by one atom at the center of the cube and eight atoms at the vertices. In this structure, atoms can be thought of as rigid spheres in contact along the body diagonals of the cube.
• Face-Centered Cubic (FCC), characterized by one atom on each of the six faces of the cube and eight atoms positioned one at each vertex. In this structure, contact between atoms occurs along the face diagonals.
• Hexagonal Close-Packed (HCP), in which the unit cell is bounded by two faces, each with six atoms at the corners and one atom at the center, and an intermediate plane — offset relative to the basal planes — that accommodates three atoms.

Allotropy and polymorphism: the case of iron

Iron is the prime material in metallurgy because, when combined with carbon and other elements, it forms the broad family of steels. It is characterized by polymorphic structures, meaning it exists in different elementary crystal structures, known as allotropic forms, depending on the temperature at which it exists. Broadly speaking, at room temperature, iron adopts the BCC structure, known as the alpha phase (α); above 912 °C, it transforms to FCC (gamma phase γ); between 1,394 °C and 1,538 °C – the melting point of pure iron – it reverts to BCC (the delta phase δ), which is structurally identical to alpha but named differently to reflect the different temperature ranges. The temperatures at which the transition from one allotropic form to another occurs are called critical points and are usually denoted by the letter "A" followed by a progressive number (e.g., A1, A2, etc.).

Copper, aluminum, nickel, silver, and gold are examples of metallic materials that adopt FCC crystal structures. Among materials with hexagonal close-packed structures, we find titanium in its alpha phase (whereas in the beta phase it has a BCC lattice), zinc, and magnesium.

Deformability and slip systems

The structure of the unit cell greatly influences the deformability of the material. To understand how, it is useful to draw a distinction between two types of deformation, elastic and plastic. Plastic deformation — the permanent type that is the primary focus of this discussion — occurs through the motion of dislocations, which are lattice defects present in the crystal structure, without which the material would not be able to deform. Elastic deformation, by contrast, is temporary: it results from a slight displacement of atoms from their equilibrium positions and is fully recovered once the applied force ceases. The ease of this dislocation motion depends on the atomic density of the crystal planes: the closer the atoms and the greater the interplanar spacing, the lower the energy required for slip. For this reason, the FCC lattice — with its densely packed planes — is the most readily deformable, and materials possessing it exhibit the greatest ductility. Materials with a BCC lattice are harder to deform and therefore classified as less ductile, while HCP lattice materials are the least deformable of the three, as their particular geometry offers very few slip systems for dislocation movement.

Solidification and grain formation

The grain structure of a metal originates during the cooling and solidification process from a molten mass. The transition from the liquid to the solid state occurs through a mechanism known as nucleation and growth. During nucleation, crystalline embryos appear in certain regions of the liquid, capable of attracting further atoms from the liquid phase and growing until the phase transformation is complete. Initially, small solid nuclei form (nucleation stage), which then grow by capturing atoms from the surrounding liquid until it is completely consumed (growth stage). At this point, the various crystalline grains that have formed meet one another, giving rise to grain boundaries (boundary zones between the different crystals making up the metal's structure).

Initially, the system is entirely in the liquid state (molten metal). The atoms, although in close proximity, do not adopt a well-defined lattice arrangement but are free to move randomly. This occurs because the temperature is above the melting/solidification point, so the kinetic energy of the atoms is sufficient to prevent the consolidation of the solid bonds that would allow crystal lattice formation.

As the liquid metal is allowed to cool, atomic motion progressively decreases in both intensity and frequency. Consequently, the probability of some atoms finding themselves at the correct configuration and interatomic spacing characteristic of the solid crystal lattice increases steadily. When the temperature falls below the melting/solidification point, the first solid nuclei are observed to form spontaneously at numerous random points in the molten pool. This nucleation mode is referred to as homogeneous nucleation.

This form of nucleation is considered purely theoretical in industrial practice. In the vast majority of practical cases, the liquid-to-solid phase transition proceeds via heterogeneous nucleation, which involves the formation of solid aggregates starting from well-defined nucleation sites, such as the internal walls of the ingot mold and/or solid impurities always present in the molten metal.

A crucial aspect is that solidification never occurs exactly at the melting point, as in industrial practice it generally requires a certain degree of undercooling: the liquid must fall below its nominal solidification temperature before solidification. Undercooling, by reducing the kinetic energy in the system, decreases atomic mobility and promotes the onset of solidification.

If the extent of undercooling is small, a limited number of nuclei form, resulting in the growth of only a few coarse grains. If undercooling is more pronounced, a greater number of solid nuclei form, producing many small grains.

The degree of undercooling in practice governs the final grain size: rapid cooling produces many nuclei and consequently a fine-grained structure, preferred for its superior mechanical properties. Conversely, slow cooling leads to coarse grains or branched dendritic structures, often associated with chemical segregation and porosity phenomena that can compromise subsequent processability of the material.

Grain size and mechanical properties

Crystal grain size is inversely correlated with the mechanical strength and hardness of the material, since grain boundaries act as effective barriers to dislocation motion; this obstacle to dislocation propagation limits plastic deformation and translates, on a macroscopic scale, into an increase in mechanical properties.

In a fine-grained material, the total grain boundary area per unit volume is greater, creating a larger number of obstacles that impede the slip of crystallographic planes. This relationship is quantified, for example, by the Hall–Petch equation, which states that yield strength increases as the mean grain diameter decreases: during wire drawing, a fine-grained material will therefore require higher drawing forces to activate plastic deformation compared to a coarse-grained material.

Work hardening and recrystallization

As mentioned above, the microscopic mechanism that allows a material to be plastically deformed — as occurs in wire drawing — is the motion of dislocations that slip along preferential planes under the action of a shear stress generated by an external force. Wire drawing is typically a cold plastic deformation process, meaning it is carried out below the recrystallization temperature of the material being deformed.

Under these conditions, dislocations not only move but multiply exponentially, mutually obstructing one another and causing a phenomenon in the metal known as work hardening, or strain hardening.

Work hardening produces changes in the mechanical and physical properties of the material. Among the most significant: an increase in hardness, yield strength, and ultimate tensile strength at the expense of residual ductility, which decreases; a reduction in electrical conductivity; and an increase in magnetic permeability.

Macroscopically, the crystal grain is elongated and flattened in the drawing direction, creating a finer structure with improved mechanical properties. Excessive accumulation of elastic energy and lattice distortion leads to the development of residual stresses — internal stresses that persist in the drawn wire.

These can cause distortion or fracture if not properly managed through stress-relief annealing. When work hardening has consumed the plastic reserve of the metal, an intermediate recrystallization anneal becomes necessary.

This heat treatment activates the phenomena of recovery and nucleation of new crystals, restoring the original ductility and allowing further passes through the drawing line or additional processing, without the risk of breakage.

Phase diagrams

Given the importance of thermomechanical processes, managing them correctly requires consulting phase diagrams — thermodynamic maps showing the phases present in equilibrium as a function of temperature and composition.

A key example is the Iron-Carbon (Fe-C) diagram, the fundamental reference for the study of steels, which allows identification of the main critical points and phase transformations occurring as a function of temperature and composition.

Iron-Carbon (Fe-C) phase diagram. Image credit: AG Caesar, CC BY-SA 4.0, via Wikimedia Commons

The main reasons why these diagrams are indispensable are detailed below:

• Prediction and control of microstructure: the development of a microstructure in an alloy is directly linked to the characteristics of its phase diagram.
• Design of heat treatments: knowledge of phase diagrams is crucial for designing and controlling industrial heat treatment procedures.
• Quantitative determination of phases: thanks to phase diagrams, it is possible to precisely determine the chemical composition of individual phases at a given temperature.
• Guidance for industrial processes: they provide valuable information for correctly managing technological processes such as melting, casting, and crystallization.
• Study of the influence of alloying elements: phase diagrams allow visualization of how the addition of chemical elements modifies the stability fields of phases.
• Reference for non-equilibrium structures: they are indispensable for understanding the development and preservation of martensite or bainite.
• Mapping solubility limits: phase diagrams show the solubility limits of a solute in a solid solvent as a function of temperature.

In conclusion, it is important to remember that the microstructure of a material — as characterized by the unit cell structure, grain size, and dislocation density — is not merely a passive parameter. It is the driving force behind every plastic deformation operation, setting both its technological limits and its quality potential. For this reason, any technician working in the wire drawing sector must count the metallurgical foundations covered in this article among their core competencies.

by Eng. Sergio Rusconi