Metallic Materials

Overview of Metallic Materials

Metallic materials are solids whose structure and properties are dominated by metallic bonding and a crystal lattice of metal atoms. In technology and everyday life, they are used because they combine:

• High electrical and thermal conductivity
• Strength and toughness
• Formability (they can be rolled, drawn, forged)
• Often good recyclability

This chapter focuses on how metallic materials are structured, how that structure can be modified, which main classes of metallic materials exist, and how these aspects determine their technical use. General aspects of materials and polymers are treated in other chapters.

Atomic and Microstructural Features of Metals

Crystal Structure of Metals

Metals are mostly crystalline: atoms are arranged in a regularly repeating pattern (crystal lattice). Common lattice types in elemental metals include:

• Body-centered cubic (bcc), e.g. $\text{α-Fe}$ (room-temperature iron), chromium
• Face-centered cubic (fcc), e.g. aluminum, copper, nickel, $\text{γ-Fe}$ (high-temperature iron)
• Hexagonal close-packed (hcp), e.g. magnesium, titanium, zinc

These structures differ in:

• Packing density (how tightly atoms are packed)
• Available slip systems (directions and planes along which atoms can slide)

Slip systems strongly influence:

• Ductility (how far a metal can deform plastically before fracture)
• Ease of forming operations (rolling, deep drawing, wire drawing)

Fcc metals typically have many slip systems and are therefore very ductile. Bcc and hcp metals can be less ductile, especially at low temperatures.

Grains and Grain Boundaries

Real metallic materials consist of many small crystals (grains). Important features:

• Grain size: Fine grains vs. coarse grains
• Grain boundaries: Regions between grains with slightly misoriented crystal lattices

Grain size has a strong impact on mechanical properties:

• Finer grains → higher strength and hardness, often better toughness
• Coarser grains → lower strength but sometimes improved formability at high temperature

Controlling grain size is a key lever in metal processing and heat treatment.

Defects and Their Role

Crystals are not perfect; defects strongly influence behavior:

• Point defects: Vacancies (missing atoms), interstitials (extra atoms)
• Line defects (dislocations): Lines where the regular atomic order is disrupted
• Planar defects: Grain boundaries, stacking faults

Dislocations are especially important because plastic deformation occurs when dislocations move. The ease of their movement determines strength and ductility:

• Easy dislocation motion → soft, ductile metal
• Hindered dislocation motion → stronger, harder, but often less ductile

Many strengthening mechanisms in metallic materials are based on hindering dislocation motion.

Strengthening Mechanisms in Metallic Materials

In practice, metals must reach specific combinations of strength, ductility, and toughness. Several microstructural strategies are used to adjust these properties.

Solid Solution Strengthening

When foreign atoms (alloying elements) dissolve in a metal’s crystal lattice, they distort it slightly. This distortion hinders dislocation motion.

Two types:

• Substitutional solid solution: Foreign atom replaces a host atom (e.g. Cu in Ni, Cr in Fe)
• Interstitial solid solution: Small atoms occupy interstitial spaces between host atoms (e.g. C in Fe)

Effects:

• Increased strength and hardness
• Often reduced ductility
• Modified physical properties (corrosion resistance, electrical conductivity, etc.)

Solid solution strengthening is fundamental in most alloys.

Work Hardening (Strain Hardening)

When a metal is plastically deformed (e.g. cold rolling, bending, drawing), the dislocation density increases:

• Dislocations interact and hinder each other
• Further deformation becomes more difficult

Consequences:

• Strength and hardness increase
• Ductility and formability decrease

Work hardening is exploited in:

• Cold-rolled steel sheets
• Drawn copper wires
• Many forming processes where final strength is achieved by deformation

It can be reversed or reduced by appropriate heat treatment (annealing).

Grain Refinement

Decreasing grain size (e.g. by controlled solidification, deformation plus recrystallization) strengthens metals:

• More grain boundaries → more obstacles to dislocation motion

Empirically, yield strength increases with decreasing grain size (Hall–Petch relationship, conceptually). Grain refinement is particularly important in steels and light alloys (Al, Mg).

Precipitation (Age) Hardening

Some alloys (e.g. certain Al-, Cu-, Ni-alloys, and some steels) can be hardened by forming fine second-phase particles (precipitates):

Basic steps:

1. Solution treatment: Alloy heated until a homogeneous solid solution forms
2. Quenching: Rapid cooling to retain a supersaturated solid solution
3. Aging: Holding at a moderate temperature so that fine precipitates form

These fine particles:

• Pin dislocations and hinder their motion
• Significantly increase strength and hardness

Practical examples:

• High-strength aluminum alloys in aircraft structures
• Age-hardenable stainless steels
• Certain copper-based alloys (e.g. Cu–Be)

Dispersion and Second-Phase Strengthening

In many alloys, phases with different composition and structure form (second phases). If they are finely distributed, they strengthen the metal:

• Hard carbides in tool steels
• Oxide particles in oxide-dispersion-strengthened alloys

These particles obstruct dislocation motion, similar to precipitates, but are often more stable at high temperatures.

Types of Metallic Materials

Metallic materials can be classified by composition and structure into several main groups.

Pure Metals vs. Alloys

• Pure metals: Nearly single-element materials (e.g. pure Cu, Al, Fe)
• Often high ductility and good conductivity
• Usually lower strength compared to alloys
• Alloys: Intentional mixtures of a base metal with one or more alloying elements (e.g. steel, brass, bronze, aluminum alloys)
• Targeted tailoring of strength, corrosion resistance, melting behavior, etc.
• Most technical metallic materials are alloys

Ferrous Metals (Iron-Based)

Ferrous metals are based on iron and are typically magnetic (in many compositions). They dominate structural applications.

Steels

Steels are Fe–C alloys with a carbon content typically below about 2%. They may also contain other alloying elements (Cr, Ni, Mo, Mn, etc.).

Subgroups (by microstructure and properties):

• Unalloyed (carbon) steels: Mainly Fe and C
• Low-carbon steels (soft, ductile, good weldability)
• Higher-carbon steels (stronger, harder, can be hardened by heat treatment)
• Alloy steels: Additional alloying elements improve hardenability, toughness, or corrosion resistance
• Stainless steels (corrosion-resistant steels):
• Characterized by high Cr content (typically ≥ 10.5%)
• Form a stable, protective oxide layer
• Used e.g. in chemical apparatus, cutlery, building facades

Steels are extremely versatile because:

• Composition and microstructure can be varied widely
• Many heat treatments are possible (see below)

Cast Irons

Cast irons have a higher carbon content than steels (typically > 2%). They are:

• Easier to cast into complex shapes
• More brittle compared to most steels
• Often good in compression and with good damping properties

Types (by form of carbon):

• Gray cast iron (graphite flakes)
• Ductile/nodular cast iron (graphite nodules)
• White cast iron (carbon in cementite rather than graphite)

Cast irons are widely used in engine blocks, machine frames, pipes, and heavy components.

Non-Ferrous Metals and Alloys

Non-ferrous metals are all metallic materials that do not have iron as their main component.

Light Metals

Light metals have low density and are important when weight must be minimized.

• Aluminum and aluminum alloys
• Low density, good corrosion resistance
• Many alloys are precipitation-hardenable
• Widely used in transport (aircraft, vehicles) and construction
• Magnesium and magnesium alloys
• Even lower density than aluminum
• Good specific strength (strength per unit weight)
• Limitations: corrosion sensitivity, limited room-temperature ductility in many alloys
• Titanium and titanium alloys
• Intermediate density between aluminum and steel
• Very high specific strength, excellent corrosion resistance
• Used in aerospace, medical implants, and chemical industry

Copper and Copper Alloys

Copper is notable for its:

• High electrical and thermal conductivity
• Good corrosion resistance

Common copper alloys:

• Brass: Cu–Zn alloys
• Bronze: Traditionally Cu–Sn alloys; in practice, “bronze” can mean various Cu-based alloys with other elements

Applications:

• Electrical engineering (wires, busbars)
• Plumbing and heating
• Bearings and sliding components (certain bronzes)

Other Non-Ferrous Metals

Some other important metallic materials (selection):

• Nickel and nickel alloys:
• Good corrosion resistance, often good high-temperature properties
• Basis of many high-temperature alloys (“superalloys”) used in gas turbines
• Zinc and its alloys:
• Often used as a coating metal for corrosion protection of steel (galvanizing)
• Cast zinc alloys used for die castings
• Precious metals:
• Gold, silver, platinum, etc.
• High chemical stability, often good electrical properties
• Used in jewelry, electronics, chemical catalysis

Heat Treatment of Metallic Materials

Heat treatments adjust microstructure and thus properties without changing the composition.

Annealing

General goals of annealing:

• Reduce internal stresses
• Restore ductility after work hardening
• Adjust grain size and microstructure

Typical steps:

• Heating to a specific temperature
• Holding (soaking) to allow microstructural changes
• Controlled cooling

Examples:

• Recrystallization annealing of cold-worked steel sheets
• Annealing of copper wires to increase flexibility

Hardening and Tempering of Steels

In many steels, a combination of hardening and tempering is used:

1. Austenitizing and hardening:
• Heating steel to a high temperature to form austenite (fcc iron with dissolved C)
• Rapid cooling (quenching) to produce a hard, supersaturated phase (martensite)
2. Tempering:
• Reheating to a moderate temperature
• Allowing partial decomposition and relaxation of stresses

Result:

• High strength and hardness
• Improved toughness compared to untempered martensite

This treatment underpins many structural and tool steels used in machinery, automotive components, and tools.

Precipitation Hardening

As described above, in precipitation-hardenable alloys an age-hardening treatment:

• Optimizes size and distribution of precipitates
• Achieves a compromise between strength and toughness

Precise control of time and temperature is essential for obtaining desired properties.

Corrosion Behavior and Protection

Types of Corrosion in Metallic Materials

Corrosion is the deterioration of metals by chemical or electrochemical reaction with their environment. Important forms include:

• Uniform (general) corrosion: Relatively even material loss over large areas
• Localized corrosion: Pitting, crevice corrosion, intergranular corrosion
• Galvanic corrosion: Accelerated corrosion of a metal when coupled to a more noble metal in an electrolyte

Susceptibility depends on:

• Alloy composition and microstructure
• Presence of stable, protective oxide films (e.g. on Al, Cr-containing steels, Ti)
• Environmental factors (pH, temperature, oxygen content, salts)

Protective Measures

To extend the lifetime of metallic materials:

• Alloying for corrosion resistance
• Addition of Cr, Ni, Mo in steels
• Use of inherently resistant alloys (e.g. Ti alloys, certain Ni alloys)
• Coatings
• Metallic coatings (galvanizing with Zn, plating with Ni or Cr)
• Organic coatings (paints, polymer coatings)
• Inorganic coatings (anodizing of Al)
• Cathodic protection
• Sacrificial anodes (more active metal corrodes instead of the protected structure)
• Impressed current systems
• Design and maintenance
• Avoid crevices and water accumulation
• Provide drainage and ventilation
• Regular inspection and repair of coatings

Mechanical and Physical Properties Relevant to Applications

Mechanical Properties

For engineering use, several mechanical properties of metallic materials are important:

• Strength (yield strength, tensile strength): resistance to permanent deformation and fracture
• Ductility (elongation, reduction in area): ability to deform plastically without breaking
• Toughness: ability to absorb energy before fracture, especially under impact or at low temperatures
• Hardness: resistance to local plastic deformation (e.g. indentation, wear)
• Fatigue resistance: behavior under cyclic loading

These properties depend strongly on:

• Chemical composition (alloy design)
• Microstructure (grain size, phases, precipitates, dislocation density)
• Processing history (deformation, heat treatment)

Physical Properties

Other crucial properties for selection and design:

• Density: mass per unit volume – important for lightweight design
• Elastic modulus: stiffness of the material
• Thermal conductivity and expansion: relevant to heat transfer and dimensional stability with temperature changes
• Electrical conductivity: key in electrical and electronic applications
• Melting range: relevant for casting, welding, and service temperature limits

Metallic materials can therefore be tuned to very different requirement sets, from highly conductive but soft copper wires to extremely strong but relatively brittle high-alloy tool steels.

Processing and Applications of Metallic Materials

Common Processing Routes

Metallic materials are produced and shaped through a variety of processes, often in sequence:

• Casting:
• Melting and pouring into molds
• Suitable for complex shapes and large components (engine blocks, pump housings)
• Forming (shaping without melting):
• Rolling, forging, extrusion, drawing, deep drawing
• Exploits metals’ ability to undergo plastic deformation
• Machining:
• Turning, milling, drilling, grinding
• Produces precise shapes and surfaces
• Joining:
• Welding, brazing, soldering, mechanical fastening (screws, rivets)
• Choice depends on material, joint design, and service conditions

Each metallic material family responds differently to these processes; for example, some alloys are easily welded, others require special precautions or are unsuitable.

Selected Application Fields

Because of their wide-ranging properties, metallic materials are ubiquitous:

• Construction and infrastructure:
• Structural steels in buildings, bridges, pipelines
• Reinforcing steel in concrete
• Transportation:
• Steels and aluminum alloys in vehicles, ships, railways
• Titanium alloys and high-strength Al alloys in aerospace
• Energy and power engineering:
• Turbine blades (Ni-based superalloys)
• Boilers and pipelines (high-temperature steels, stainless steels)
• Electrical conductors (Cu, Al)
• Mechanical engineering and tools:
• Machine components (shafts, gears, bearings)
• Cutting and forming tools (tool steels, hard metals with metallic binders)
• Electronics and communication:
• Conductors and contacts (Cu, Au, Ag)
• Structural components for housings and heat sinks (Al alloys)

Selecting a metallic material always involves balancing performance, cost, processability, availability, and environmental aspects such as recyclability and corrosion resistance.

Environmental and Sustainability Aspects

Metallic materials play a significant role in resource and energy use:

• Production of many metals, especially primary steel and aluminum, is energy-intensive and can emit significant greenhouse gases.
• Recycling is often highly efficient:
• Metals can be melted and reused with little loss of quality
• Scrap collection and separation are important
• Durability and corrosion resistance influence lifetime and replacement frequency of components.

Designing metallic materials and structures with long service life, low maintenance, and good recyclability is an important goal in modern materials engineering and environmental chemistry.