Materials Science – Comprehensive Notes
Mechanical Properties, Testing, Microstructure & Phase Diagrams

1. Mechanical Properties & Stress-Strain Behavior

Engineering Stress-Strain Curve (Tensile Test)

• Elastic region: Obeys Hooke’s law → Stress ∝ Strain (E = modulus of elasticity)
• Yield Strength (σ_y): Stress at which plastic deformation begins (0.2% offset)
• Ultimate Tensile Strength (UTS): Maximum stress the material can withstand
• Fracture Strength: Stress at breaking point
• % Elongation: Measure of ductility
• % Reduction in Area: Another ductility indicator

Ductile vs Brittle Materials

Property	Ductile (e.g., Mild steel, Cu)	Brittle (e.g., Glass, Ceramics, CI)
% Elongation	> 5–10%	< 5%
Necking	Significant	Almost none
Fracture	Cup-and-cone (45°)	Cleavage (flat, shiny)
Warning before failure	Yes	No

Important Mechanical Properties

• Strength: Ability to resist stress without failure
• Toughness: Energy absorbed before fracture (area under stress-strain curve)
• Hardness: Resistance to indentation/plastic deformation
• Stiffness: Modulus of elasticity (E)
• Resilience: Energy absorbed in elastic region

Failure Modes

• Fracture: Ductile fracture (void coalescence), Brittle fracture (cleavage)
• Fatigue: Failure under cyclic loading (even below yield strength) → Beach marks, striations
• Creep: Time-dependent plastic deformation at high temperature & constant load (important in turbines)

2. Mechanical Testing Methods

Test	Property Measured	Common Standards/Methods
Tensile Test	YS, UTS, %Elongation, E	ASTM E8
Hardness	Resistance to indentation	Brinell, Rockwell, Vickers, Microhardness
Impact Test	Toughness (energy absorbed)	Charpy & Izod (DBTT detection)
Fatigue Test	S-N curve, Endurance limit	Rotating beam test
Creep Test	Creep rate, Rupture time	Constant load at high temp
Nondestructive Testing (NDT)	Detect internal defects without damage	Ultrasonic, Radiography, Magnetic Particle, Dye Penetrant, Eddy Current

3. Microstructural Examination

Optical Microscope Principle

• Magnification up to 1500×
• Resolution ~0.2 μm
• Works on reflected light (metallography)

Sample Preparation Steps

1. Sectioning
2. Mounting (bakelite/hot mounting)
3. Grinding (SiC papers: 180 → 1200 grit)
4. Polishing (Alumina/Diamond paste → mirror finish)
5. Etching (Nital 2% for steel, Picral for cast iron, etc.)

Grain Size Determination

• ASTM Grain size number n → N = 2^n-1 (grains per sq. inch at 100×)
• Methods: Comparison, Planimetric (Jeffries), Intercept method

Microstructures of Common Materials

Material	Microstructure	Key Features
Mild Steel (Low C)	Ferrite (α) + Pearlite	Soft, ductile, light + dark regions
Medium Carbon Steel	More Pearlite	Higher strength
High Carbon Steel	Pearlite + Cementite network	Hard but brittle
Gray Cast Iron	Ferrite/Pearlite + Graphite flakes	Excellent machinability, damping
White Cast Iron	Cementite + Pearlite	Very hard, brittle
Ductile Iron	Ferrite + Nodular graphite	Good ductility
Brass (Cu-Zn)	α-solid solution (FCC)	Golden color, highly ductile
Bronze (Cu-Sn)	α + δ eutectoid	Good corrosion resistance

4. Phase Diagrams & Equilibrium Diagrams

Gibbs Phase Rule

P + F = C + 2 (for condensed systems → P + F = C + 1)

Types of Binary Phase Diagrams

1. Complete Solid Solution (Isomorphous): e.g., Cu-Ni → Single phase α throughout
2. Eutectic System: Limited solubility → Liquid → α + β at eutectic point (lowest melting)
3. Peritectic System: Solid + Liquid → New solid phase
4. Eutectoid: Solid γ → α + β (e.g., 0.8% C in steel)
5. Peritectoid: Solid phases react to form new solid

Iron-Carbon Equilibrium Diagram (Most Important!)

• Phases:
  • Ferrite (α): BCC, soft, ductile, max 0.02% C at RT
  • Austenite (γ): FCC, non-magnetic, dissolves up to 2.1% C
  • Cementite (Fe₃C): Hard, brittle
  • Pearlite: Lamellar mixture of Ferrite + Cementite (0.8% C)
  • Ledebrite: Austenite + Cementite (at high temp)
• Critical Temperatures:
  • A₁ = 727°C (Eutectoid)
  • A₃ = Upper critical (varies with C%)
  • A_cm = For hypereutectoid steels
• Key Points:
  • 0.02% C → Pure ferrite
  • 0.8% C → 100% Pearlite (max strength & toughness balance)
  • 4.3% C → Cast iron region

Summary of Steel Types (Fe-C Diagram)

Type	Carbon %	Microstructure (slow cooling)	Properties
Hypoeutectoid	< 0.8%	Ferrite + Pearlite	Ductile, weldable
Eutectoid	0.8%	100% Pearlite	Best strength-ductility
Hypereutectoid	> 0.8–2.1%	Pearlite + Cementite network	Very hard, wear resistant

This HTML note is exam-ready! Save as .html and revise anytime.
Best of luck for your Materials Science exam!