Summary
Highlights
Engineering materials are categorized into metals, polymers, ceramics, and composites. Understanding metals, their properties, and effective use is crucial. This video will explore metals, their microstructure, alloying, and heat treatment to improve properties. Common engineering metals include iron (for steel), aluminum (high strength-to-weight, low melting point), titanium (excellent strength-to-weight, high melting point but expensive), magnesium, copper, and nickel.
The effectiveness of metals comes from their atomic structure. Pure metals have closely packed atoms in a regular grid, making them crystalline materials with a crystal lattice. Unlike amorphous materials like glass, metals have repeating unit cells. Common metallic unit cell structures include Face-Centred Cubic (FCC) for copper, Body-Centred Cubic (BCC) for iron, and Hexagonal Close-Packed (HCP) for titanium. FCC and HCP have a 74% packing factor, while BCC is 68%. This close packing contributes to metals' high densities.
Real crystal lattices contain defects. Point defects include vacancy defects (missing atom), interstitial defects (extra atom in gap, either self or impurity), and substitutional defects (impurity atoms replacing lattice atoms). Linear defects are dislocations, where atoms are offset. Edge dislocations involve an extra half plane of atoms. Screw dislocations involve a spiral shift in atomic alignment. Dislocations move through the lattice by bond breaking and re-forming, causing irreversible plastic deformation in metals, unlike elastic deformation which is reversible stretching of bonds.
Plastic deformation is the motion of dislocations. High dislocation density increases strength because dislocations tangle, preventing movement. Dislocations move easiest along close-packed atomic planes. Even pure metals don't maintain a perfect crystalline structure. As molten metal cools, atoms form lattices in multiple locations with different orientations, creating grains. These polycrystalline materials have grain boundaries, which impede dislocation motion, making them stronger than single crystals. Smaller grain sizes lead to stronger materials (Hall-Petch equation).
Grain size can be controlled to strengthen metals through grain boundary strengthening. Adding inoculants to molten metal promotes more crystal nucleation, leading to smaller grains. Rapid cooling also results in finer grain structures. Another strengthening technique is work hardening (plastic deformation like cold rolling), which increases dislocations and material strength, but reduces ductility.
Mixing metals with other elements creates alloys, improving base metal properties. Alloys are classified as ferrous (iron-based) or non-ferrous. Brass, a non-ferrous alloy of copper and zinc, is valued for appearance and machinability. Aluminum alloys are crucial for their high strength-to-weight ratio and are classified for casting or working. Steel, an iron-carbon alloy, is arguably the most important engineering alloy.
Pure iron is too soft, but carbon (and other elements) transform it into steel. Low-carbon (mild) steel (up to 0.25% carbon) is ductile and low-cost. Medium-carbon (0.25-0.6% carbon) and high-carbon (0.6-2% carbon) steels are stronger and benefit from heat treatment. Cast iron (2-4% carbon) has good fluidity for casting but is brittle. Additional elements like chromium (in stainless steel) provide specific properties, e.g., corrosion resistance.
Alloys are formed by melting metals. They can be substitutional or interstitial (like steel, where smaller carbon atoms fit into iron lattice). Alloying elements distort the crystal lattice, impeding dislocation motion and strengthening the material (solid solution strengthening). If alloying elements exceed solubility, they form distinct phases within the microstructure. These phase boundaries also impede dislocations, increasing strength. Precipitation hardening uses heat treatment to create uniformly dispersed particles of a second phase to strengthen the material.
Pure iron undergoes phase transformations with temperature changes: BCC ferrite below 912°C, FCC austenite between 912°C and 1394°C, then back to BCC. These are iron 'allotropes'. The iron-carbon phase diagram shows how phases change with carbon content and temperature. Ferrite (BCC) holds little interstitial carbon; excess carbon forms cementite (a hard, brittle compound of Fe3C) alongside ferrite. Austenite (FCC) can hold more carbon. The combination of ferrite and cementite makes steel much stronger than pure iron.