Chapter 1 Notes: Introduction to Materials Science & Engineering

What is Materials Science and Engineering?

  • Materials science
    • Investigates the relationships between the structures and properties of materials.
    • Involves designing and developing new materials.
  • Materials engineering
    • Focuses on creating products from existing materials.
    • Involves developing materials processing techniques.

Why Are Materials Important?

  • Materials drive advancements in our society, marking different ages:
    • Stone Age
    • Bronze Age
    • Iron Age
  • Current Material Age:
    • Silicon (Electronic Materials) Age?
    • Nanomaterials Age?
    • Polymer Age?

Why is it Important for Engineers to Understand Materials?

  • Products, devices, and components engineers design are made of materials.
  • Engineers need to:
    • Select appropriate materials and processing techniques for specific applications.
    • Have knowledge of material properties.
    • Understand the structure-property relationships.

Relationships Among Processing, Structure, & Properties

  • Processing (e.g., cooling rate of steel from high temperature) affects the structure (microstructure).

  • Structure, in turn, affects properties like hardness.

    Example: Hardness vs. Cooling Rate The hardness of a material varies based on the cooling rate during processing which alters the material's microstructure.

Types of Materials

  • Metals:
    • Strong, ductile
    • High thermal & electrical conductivities
    • Opaque, reflective
  • Polymers/plastics:
    • Compounds of non-metallic elements
    • Soft, ductile, low strengths, low densities
    • Low thermal & electrical conductivities
    • Opaque, translucent, or transparent
  • Ceramics:
    • Compounds of metallic & non-metallic elements (oxides, carbides, nitrides, sulfides)
    • Hard, Brittle
    • Low thermal & electrical conductivities
    • Opaque, translucent, or transparent

Materials Selection

Engineers often solve materials selection problems using the following procedure:

  1. Determine Required Properties
    • To provide the required set of properties.
    • To produce components having desired shapes and sizes.
    • Example techniques: casting, mechanical forming, welding, heat treating.
    • Properties: mechanical, electrical, thermal, magnetic, optical, deteriorative.
  2. From List of Properties Identify Candidate Material(s)
  3. Best Candidate Material Specify Processing technique(s)

Material Property Types

Properties of materials fall into six categories:

  • Mechanical
  • Electrical
  • Thermal
  • Magnetic
  • Optical
  • Deteriorative

Mechanical Properties

Affect of carbon content on the hardness of a common steel: Increasing carbon content increases hardness of steel.

Electrical Properties

Factors that affect electrical resistivity for copper:

  • Increasing impurity content (e.g., Ni) increases resistivity.
  • Deformation increases resistivity.
  • Increasing temperature increases resistivity.

Thermal Properties

  • Thermal Conductivity – measure of a material’s ability to conduct heat

  • Increasing impurity content (e.g., Zn in Cu) decreases thermal conductivity.

  • Highly porous materials are poor conductors of heat

    Example: Ceramic Fibers: significant void space leads to low thermal conductivity and makes them suitable in applications like space shuttle thermal insulators.

Magnetic Properties

  • Magnetic Permeability vs. Composition: Adding 3 atomic % Si makes Fe a better recording medium!

    Example: Magnetic Storage: Recording medium is magnetized by recording write head.

Optical Properties

  • The light transmittance of some materials depends on their structural characteristics:
    • Aluminum oxide single crystal (high degree of perfection)—is optically transparent
    • Aluminum oxide polycrystalline material (having many small grains)—is optically translucent
    • Aluminum oxide polycrystalline material having some porosity—is optically opaque

Deteriorative Properties

  • Small cracks formed in a steel bar that was simultaneously stressed and immersed in seawater indicate stress-corrosion cracking.

  • For stress-corrosion cracking, the rate of crack growth is diminished by heat treating.

    For Aluminum alloy 7178 that is stressed while immersed in a saturated aqueous NaCl solution, crack growth rate is reduced by heat treating (160°C for 1 h prior to testing).

Example of Materials Selection: Artificial Hip Replacement

  • Hip joint problems can be painful and disabling
    • Joint deterioration (loss of cartilage) as one ages
    • Joint fracture

Materials: Artificial Hip Replacement (cont.)

  • Damaged and diseased hip joints can be replaced with artificial ones
  • Materials requirements for artificial joints
    • Biocompatible – minimum rejection by surrounding body tissues
    • Chemically inert to body fluids
    • Mechanical strength to support forces generated
    • Good lubricity and high wear resistance between articulating surfaces

Components of an artificial hip:

  • Femoral stem — inserted into the top of the hip bone (femur)
  • Head (Ball) — affixed to femoral stem
  • Shell — attached to pelvis
  • Liner — into which head fits

Materials Used in Artificial Hip Replacement

  • Femoral stem — titanium or CoCrMo alloy
  • Head (Ball) — CoCrMo alloy or Al2O3 (ceramic)
  • Shell — titanium alloy
  • Liner — polyethylene (polymer) or Al2O3 (ceramic)

Summary

  • Appropriate materials and processing decisions require engineers to understand materials and their properties.
  • Materials' properties depend on their structures; structures are determined by how materials are processed
  • In terms of chemistry, the three classifications of materials are metals, ceramics, and polymers
  • Most properties of materials fall into the following six categories: mechanical, electrical, thermal, magnetic, optical, and deteriorative.
  • An important role of engineers is that of materials selection.