Properties of Metals

Learning Objectives

• Define engineering stress and engineering strain.
• Identify the critical features of a uniaxial stress-strain curve.
• Recognize the crucial role metals play in modern society as indispensable materials for technological advancement, infrastructure, transportation, energy production, and everyday life.

What are Metals?

• Metals are substances with high electrical and thermal conductivity.
• They are remarkable and indispensable, a testament to human ingenuity and technological progress.
• Metals are formed naturally below the surface of the Earth.
• Metals have a distinct appearance, characterized by a shiny and reflective surface.
• Metals are inorganic, meaning they are made of substances that were never alive.

Metals in the Periodic Table

• Around 2/3 of the elements in the periodic table are metals.
• Metals are located on the left-hand side and in the middle of the periodic table.
• The elements in group 1 and 2 are metals, as well as the transition metals in the middle.

Metal Examples

• Copper
• Iron
• Zinc
• Sodium
• Potassium
• Gold

Stress

• Stress refers to the internal resistance of a material when subjected to external forces.

Different Types of Stress

• Residual Stress
• Structural Stress
• Pressure Stress
• Flow Stress
• Thermal Stress
• Fatigue Stress

Residual Stress

• Residual stresses are internal stresses that remain in a material after manufacturing processes like welding, casting, machining, or cold working, even when no external loads are applied.
• These stresses can be tensile (pulling) or compressive (pushing).
• Residual stresses can significantly affect a component's performance, potentially leading to premature failure or distortion.
• Understanding and controlling residual stresses is crucial for ensuring the reliability and longevity of metal structures.

Structural Stress

• Structural stress refers to the stresses that develop within a material when it is subjected to external loads during its intended use.
• These stresses are directly related to the applied forces and the geometry of the structure.
• Engineers analyze structural stresses to ensure that components can withstand the anticipated loads without failure, contributing to safety and performance.

Pressure Stress

• Pressure stress arises when a metal component is subjected to internal or external pressure from fluids or gases.
• Examples include pressure vessels, pipelines, and hydraulic systems.
• The magnitude of pressure stress depends on the pressure, the geometry of the container, and the material's properties.
• Proper design and material selection are essential to prevent failure due to excessive pressure stress.

Flow Stress

• Flow stress is the stress required to plastically deform a metal during manufacturing processes like rolling, extrusion, or forging.
• It represents the material's resistance to plastic flow at a specific temperature and strain rate.
• Understanding flow stress is critical for optimizing manufacturing processes and predicting the forces required to shape metal components.

Thermal Stress

• Thermal stresses occur due to temperature variations, which cause metals to expand or contract.
• If the metal is constrained, these expansions or contractions generate internal stresses.
• Rapid or uneven temperature changes can lead to significant thermal stresses, potentially causing warping, cracking, or fatigue failure.
• Thermal expansion coefficients and temperature gradients are key factors.

Fatigue Stress

• Fatigue stress is the stress experienced by a material under cyclic loading, leading to progressive and localized structural damage and eventual failure.
• This failure, known as fatigue failure, can occur even when the maximum stress is significantly less than the material's static tensile or yield strength.

Types of Applied Stress

• Applied stress refers to any stress imposed on a material by external forces.
• It's typically categorized by the nature of the force causing the stress.
• The types of applied stress align with the basic types of stress:
  • Tensile Stress
  • Compressive Stress
  • Shear Stress

Tensile Stress

• Tensile Stress is that type of stress in which the two sections of material on either side of a stress plane tend to pull apart or elongate.
• When you apply a pulling force, you're inducing tensile stress, which tries to elongate the material.
• In engineering terms, it's the force applied per unit area, pulling the material apart.
• Tensile stress is seen in cables in bridges, ropes, or anything that's designed to be pulled.

Compressive Stress

• Compressive Stress is the reverse of tensile stress; adjacent parts of the material tend to press against each other through a typical stress plane.
• Think of squeezing a sponge or the weight of a building pushing down on its foundation.
• This type of stress is common in columns, pillars, and any structure designed to support a load pushing down on it.
• It's the force per unit area pushing the material together.

Shear Stress

• Shear Stress exists when two parts of a material tend to slide across each other in any typical plane of shear upon application of force parallel to that plane.
• Instead of pulling or pushing, shear stress occurs when forces cause layers of the material to slide past each other.
• Imagine cutting paper with scissors or the stress on a bolt being cut by a force parallel to its surface.
• Shear stress is the force applied parallel to the surface of the material.
• This type of stress is critical in understanding how materials behave under forces that try to slide them.

Strain

• Strain in metals is the measure of deformation resulting from applied stress, representing the change in a material's dimensions relative to its original size.
• It's a dimensionless quantity, often expressed as a ratio or percentage, indicating how much a metal stretches, compresses, or shears under load.
• Strain is intrinsically linked to stress; applied stress induces strain, and their relationship is characterized by the material's stress-strain curve.
• We distinguish between tensile, compressive, and shear strain, corresponding to stretching, compressing, and sliding deformations, respectively.
• Furthermore, strain can be elastic, where the material returns to its original shape upon stress removal, or plastic, where permanent deformation occurs.
• Understanding strain is vital for engineers to ensure structural integrity, predict material behavior under load, and select appropriate materials for diverse applications, preventing excessive deformation and potential failures.

Elastic Strain

• Elastic strain is a temporary deformation.
• When a material experiences elastic strain, it changes shape under stress, but it returns to its original shape once the stress is removed.
• Think of stretching a rubber band slightly; it returns to its original length when you release it.

Plastic Deformation

• Plastic deformation is a permanent deformation.
• When a material undergoes plastic deformation, it changes shape under stress, and it does not return to its original shape when the stress is removed.
• Think of bending a paperclip; it stays bent.

Young's Modulus

• Young's modulus, also known as the modulus of elasticity, is a measure of a solid material's stiffness or resistance to elastic deformation under tensile or compressive stress.
• It essentially describes how much a material will deform when a force is applied to it.
• Young's modulus is defined as the ratio of tensile stress to tensile strain.
• $E = stress / strain$

Sample Problem

A steel rod with an initial length of 2 meters and a cross-sectional area of 0.005 $m^2$ is subjected to a tensile force of 10,000 N. If the Young’s modulus of steel is $200 \times 10^9$ Pa, determine:

The stress in the rod.

$σ = F/A$
$σ = 10,000 N / 0.005 m^2$
$σ = 2,000,000 Pa$ or $2 \times 10^6 N/m^2$

The strain in the rod.

$ϵ = σ / E$
$ϵ = (2 \times 10^6) / (200 \times 10^9)$
$ϵ = 0.00001$

The elongation of the rod.

$ΔL = ϵ \times L$
$ΔL = 0.00001 \times 2$
$ΔL = 0.00002 m$ or 0.02 mm

Mechanical Properties of Metals

• Mechanical properties of metal indicate its inherent behavior under external force, including its ability to resist failure.

Important Mechanical Properties

• Elasticity
• Plasticity
• Ductility
• Brittleness
• Hardness
• Toughness
• Resilience
• Stiffness
• Creep

Importance of Mechanical Properties

• Engineers and manufacturers use mechanical features to help them select the best metal for a given task.

Mechanical Properties (Detailed)

Elasticity

• The metal's ability to regain shape and size after load removal.
• Metals have an elastic limit, where force cannot leave deformation.
• Applying beyond this limit results in deformation.

Plasticity

• The metal's ability to form and shape without fracture when subjected to external forces, a property crucial in extruding operations.
• Most metals exhibit good plasticity, making it essential for various applications.

Ductility

• The ability of metals to be drawn into wires or elongated before rupture, influenced by tenacity and hardness.
• It is higher in cold conditions, with common metals having decreasing ductility.

Brittleness

• Refers to a metal's sudden fracture without noticeable deformation, with less ductile metals being brittle, such as cast iron.

Hardness

• The metal's ability to resist abrasion, indentation, and scratch from harder materials like diamonds, quartz, and corundum, crucial for cutting tool materials and metallic components.

Toughness

• The maximum energy a material can maintain before fracture, measured by stress up to the fracture point, and decreases with temperature, crucial for selecting materials.

Resilience

• The metal's ability to store energy and resist shock and impact loads, measured by energy storage per unit volume.
• Higher resistance material is used for springs.

Stiffness

• Stiffness, or metal rigidity, prevents deformation or deflection under load.
• Cast iron is preferred for machine beds and frames due to its greater rigidity and accuracy.

Creep

• The continuous and slow deformation of metal under steady load, especially at higher temperatures, which is crucial for designing IC engine and turbine blades.

Electrical Properties of Metals

• Metals are excellent conductors of electricity due to the presence of free electrons in their atomic structure, resulting in low resistivity and high conductivity.
• Electrical Property: The characteristic of metal that enables the flow of electric current.

Electrical Conduction

• Permittivity
• Electrical Conductivity
• Electrical Resistivity
• Capacitance
• Ferroelectricity
• Dielectric Strength
• Dielectric Constant

Electrical Conduction

• The movement of electrically charged particles through a material, resulting in an electric current.
• Ohm’s Law states the relationship between electric current and potential difference.
• The current that flows through most conductors is directly proportional to the voltage applied to it.

Electrical Conductivity

• A measure of the ability of the material to conduct an electrical current.

Electrical Resistivity

• A measure of a material's property to oppose the flow of electric current.

Capacitance

• It is the ability of an object to store electric charge.

Permittivity

• A property of a material that measures the opposition it offers against an electric field.

Dielectric Constant

• A measure of the amount of electric potential energy, in the form of induced polarization that is stored in a given volume of material under the action of an electric field.

Dielectric Strength

• A measure of how much electrical stress an insulating material can withstand before breaking down.

Ferroelectricity

• A property of certain materials that exhibit spontaneous polarization.

Thermal Properties of Metals

• Thermal properties are characteristics that describe how a metal responds to heat.
• Thermal properties are associated with a material-dependent response when heat is supplied to a solid body, a liquid, or a gas.

Thermal Properties

• Thermal Conductivity
• Heat Capacity
• Thermal Expansion
• Thermal Stress

Thermal Conductivity

• Thermal conductivity (often denoted by k, λ, or κ) refers to a material’s intrinsic ability to transfer or conduct heat.

Heat Capacity

• Heat capacity is the heat required to increase an object's temperature by one degree.

• The heat Capacity formula is mathematically given as:

Specific Heat Capacity

• The specific heat capacity of a substance is the amount of energy required to raise the temperature of 1 kg of the substance by 1°C.

Molar Heat Capacity

• Molar heat capacity is defined as the amount of heat required to raise 1 mole of a substance by 1 degree Kelvin.

Thermal Expansion

• Thermal Expansion, the general increase in the volume of a material as its temperature is increased.

Thermal Stress

• Thermal stress is the internal stress experienced by matter during temperature fluctuations.

Magnetic Properties

• Magnetic properties of metal refer to the metal and alloys such as iron and steel and associated alloying elements such as cobalt and nickel.
• Metal and alloys can be classified as either hard or soft.
  • Soft magnetic materials can be easily magnetized or demagnetized and they retain no magnetism when the magnetizing force is remove.
  • Hard magnetic cannot be easily magnetize and use to make permanent magnet.

Magnetic properties

• Magnetism
• Magnetic Dipoles
• Magnetic Field Strength
• Magnetic Flux Density
• Permeability
• Diamagnetism
• Paramagnetism
• Ferromagnetism
• Antiferromagnetism
• Ferrimagnetism

Magnetism

• Magnetism occurs through a magnetic field which allows object to attract or repel each other.

Magnetic Dipoles

• A magnetic field is created by two poles similar on how an electric dipole is created by two electric charges.

Magnetic Field Strength (H)

• Measure of how strong a magnetic field is in a given area.

Magnetic Flux Density (B)

• Magnitude of the internal field strength with the substance that is subjected to an H field.

Permeability

• Is the property of a specific medium through which the H field passes and in which B is measured.

Diamagnetism

• Property of material that are repelled by a magnetic field.

Paramagnetism

• Form of magnetism whereby some material are weakly attracted by an externally applied magnetic field and for external induced magnetic fields in the direction of the applied magnetic field.

Ferromagnetism

• Certain metallic materials possess a permanent dipole moment in the absence of an external field and manifest very large and permanent magnetizations.

Antiferromagnetism

• Magnetic moment coupling between adjacent atoms or ions also occurs in the materials other than those that are ferromagnetic.

Ferrimagnetism

• Some ceramics also exhibits a permanent magnetization.

Optical Properties

• This properties describes how metals interact with light

Opacity

• Most visible light is either reflected or absorbed and converted to heat, making metals generally opaque to visible light.

High Reflectivity

• Metals effectively reflect a significant amount of incoming light, making it highly reflective. This is a key reason why they appear shiny.

Colors of Metals

• The color of a metal is determined by the specific wavelengths of light it reflects.

Drude Model

• Proposed by Paul Drude in 1900
• It depicts the electrons in metal as a “sea” of free electrons

Chemical Properties of Metals

• Chemical properties are material characteristics that relate to the structure of a material and its formation from chemical elements.
• Describes how a substance reacts with other substances.
• It's observed only when the substance undergoes a chemical change, meaning its chemical composition changes.

Five Key Chemical Properties of Metals

• pH
• Hygroscopy
• Surface Tension
• Reactivity
• Corrosion Resistance

pH

• pH was introduced in 1909 by Danish chemist Soren Peter Lauritz Sorensen.
• is a measure of hydrogen ion concentration, a quantitative measure of the acidity or alkalinity of a solution.
• the pH scale usually ranges from 0 to 14.
  • ACIDIC- solutions with pH less than 7.
  • BASIC/ALKALINE- solution with pH level greater than 7.

Hygroscopy

• The ability of a substance to attract and hold water molecules from the environment through absorption or adsorption, causing physical changes like increased volume or stickiness.
• Deliquescence: A specific form of hygroscopy where the substance absorbs so much moisture that it dissolves in the absorbed water, forming a liquid solution. This is often seen with salts like calcium chloride.

Surface Tension

• A property of a liquid's surface allowing it to resist external force; caused by molecular cohesion.
• This property is caused by the cohesion of molecules and is responsible for many of the behaviors of liquids.

Reactivity

• The rate at which a chemical substance undergoes a chemical reaction.
• In pure compounds, reactivity is regulated by physical properties.

Corrosion Resistance

• Refers to a material's resistance to chemical degradation (like rusting).

Examples of Corrosion-Resistant Metals

1. Stainless steel
2. Aluminum
3. Copper and Copper Alloys (Brass, Bronze)
4. Nickel Alloys
5. Titanium Alloys