MATS105 Introduction to Engineering Materials Semester 2

Material Classification

Atomic Level Classification

Engineering Materials Classification

• Metals: Steels, Cast irons, Al-alloys, Cu-alloys, Zn-alloys, Ti-alloys

• Ceramics: Aluminas, Silicon carbides, Silicon nitrides, Zirconias

• Polymers: PE, PP, PET, PC, PS, PEEK, PA (nylons), Polyesters, Phenolics, Epoxies

• Glasses: Soda glass, Borosilicate glass, Silica glass, Glass-ceramics

• Elastomers: Isoprene, Neoprene, Butyl rubber, Natural rubber, Silicones, EVA

• Composites: Sandwiches, Hybrids, Segmented structures, Lattices and foams

Taxonomy of Materials

Kingdom

Family

Class

Sub-class Member

Attributes

• Density

• Mechanical props.

• Thermal props.

• Electrical props.

• Optical props.

• Corrosion props.

• Documentation

• specific

• general

Material records

Factors Affecting Material Property

Temperature

Atomic

Processing

Micro-structure

Topics Covered in the Module

Crystal Structures, defects, microstructures, and material types

Mechanical Properties and Deformation

Different material types (Metals, Ceramics, Polymers & Composites)

Modifications of alloys using processing

Sustainable materials science

Natural materials

Module Objectives / Learning Outcome

Relationships between materials properties, structure/microstructure, processing

Modification of mechanical properties of alloys using processing methods

Appreciate materials failure processes

Materials applications in various engineering disciplines

Appreciate of how materials are selected and specified in industry

Practical Skills

• Hands-on mechanical testing of materials (Optical microscopy, Tensile testing, hardness measurements & impact testing).

• Using GRANTA Edupack for materials and process selection.

Bonding Types & Classification of Materials

Crystal Structures

Simple cubic

Face-centered cubic

Body-centered cubic

Simple tetragonal

Body-centered tetragonal

Hexagonal

Simple orthorhombic

Body-centered orthorhombic

Base-centered orthorhombic

Face-centered orthorhombic

Rhombohedral

Simple monoclinic

Base-centered monoclinic

Triclinic

Point Defects in Metals

Vacancies: vacant atomic sites in a structure.

Self-Interstitials: "extra" atoms positioned between atomic sites.

Impurities in Metals (alloying)

Two outcomes if impurity (B) added to host (A):

• Solid solution of B in A (i.e., random distribution of point defects)

• Solid solution of B in A plus particles of a new phase (usually for a larger amount of B)

OR

• Substitutional solid solution (e.g., Cu in Ni)

• Interstitial solid solution (e.g., C in Fe)

Second phase particle

• different composition

• often different structure.

Dislocations

Very important during deformation of crystalline materials

Deformation is associated with the movement (“slip”) of dislocations

The harder it is for the dislocations to move, the stronger the metal

Extra half-plane of atoms “Edge dislocation”

Plastic Deformation

Stresses greater than the yield stress cause plastic deformation (as well as a small amount of elastic deformation).

Plastic deformation strengthens and hardens (hence “work hardening”).

Any elastic strain is recovered on unloading (or failure), whereas the plastic strain is permanent.

Plastic yielding and “work-hardening” starts at the yield stress

Necking starts at the UTS (tensile strength)

Note unloading line is not vertical – it recovers the elastic strain.

Stress vs. Strain

Stress \sigma = F/A。

Strain ε = δL/L

Strengthening Mechanisms

Solid-solution strengthening: Different alloy atoms on the metal lattice cause local lattice distortion/strain, hindering dislocation movement and increasing strength.

Precipitation strengthening: Alloy atoms react to form small intermetallic compound precipitates, which hinder dislocation slip during deformation, increasing yield stress and strength; requires careful alloying and heat-treatment.

True Stress and True Strain

True stress ($\sigmat$) and true strain ($\epsilont$) represent the real stress and strain on the sample, considering instantaneous changes in cross-section area and length during deformation.

Engineering stress ($\sigma_0$) and engineering strain ($\epsilon$) are calculated from the original starting dimensions.

Effect of Temperature on Working

Cold-working: Occurs at T < 0.3Tmelt, increases strength and hardness but decreases ductility.

Hot-working: Occurs at T > 0.5Tmelt, structure recrystallizes simultaneously with deformation, maintaining softness and ductility.

Recrystallization: Heating cold-worked metals to ~0.4Tm-0.5Tm restores ductility by forming new, undeformed grains.

Hardness

Definition: Resistance of a material to indentation or scratching.

Engineering: Ability to resist indentation by an applied load

Minerology: Ability to resist cutting or scratching

Strength of a material increases in proportion to hardness.

Not a fundamental property because it measures a combination of properties (elastic, plastic, compressive).

Toughness

Area under stress-strain curve = energy per unit volume (Jm-3) required to deform the material

Tough materials are STRONG and DUCTILE

Impact Fracture

Definition: Property of resisting fracture or distortion; measured by impact test for high strain-rate deformation/fracture.

Toughness: Energy (J) absorbed in fracturing a metal sample in impact at high strain rates.

Ductile vs Brittle Fracture

• Most metals are relatively ductile

Creep

Definition: Time-dependent (permanent) deformation of metals under constant load (stress) below yield stress at elevated temperature (above ~0.4Tm).

Limits lifetime of components at high temperatures.

Fatigue Failure

Occurs due to dynamic and fluctuating cyclic stresses well below the ultimate tensile strength.

~90% of metal component failures are due to fatigue.

Final failure often occurs suddenly and without warning

Allotropy of Iron

Iron exists in different crystal structures depending on temperature:

• Delta ($\delta$) ferrite: Body-centered cubic (bcc) at high temperatures.

• Austenite ($\gamma$): Face-centered cubic (fcc) at intermediate temperatures, more close-packed.

• Alpha ($\alpha$) ferrite: Body-centered cubic (bcc) at lower temperatures.

Types of Steel

Steels contain only up to ~2 weight% C

Carbon Steels

• C, plus < 5 wt% non-C alloying elements

Stainless Steels

• >12% Cr

• produces a passive oxide film that protects Fe against corrosion.

Phase Diagrams

Nickel atom

Copper atom

Phase A

Phase B

• Describe the equilibrium state when combining two elements.

Solubility Limit

Maximum concentration for which only a single phase solution exists.

Components and Phases

Components: The elements or compounds which are present in the alloy (e.g., Al and Cu).

Phases: The physically and chemically distinct material regions that form (e.g., α and β).

Binary Phase Diagram

Indicate phases as a function of T, C, and P.

binary systems: just 2 components.

independent variables: T and C (P = 1 atm is almost always used).

• Isomorphous : complete solubility of one component in another; α phase field extends from 0 to 100 wt% Ni.

Rules phase diagrams.

Rule 1: If we know T and Co, then we know:

• which phase(s) is (are) present.

Rule 2: If we know T and C0, then we can determine:

• the composition of each phase.

Ferrous Alloys

i.e. Steels and cast irons

Fe-Fe3C Phase Diagram

Concrete Making + Testing Lab

CMT1 + CMT2 (over 2 successive weeks): Concrete Making + Testing

When: Fridays (check lab schedule)

Where: Basement in Brodie Tower