Beyond mechanical behavior, design must weigh density, melting point, specific heat, thermal & electrical conductivity, thermal expansion, and corrosion resistance.

Density

• Density = mass per unit volume; weight saving is critical for aircraft/aerospace.
• Specific strength = strength-to-weight ratio. Titanium has the highest among metals; fibers beat most metals.
• High-speed equipment (printing, textiles) favors low-density metals (e.g. magnesium) to cut inertial forces.

Melting Point

• Depends on the energy needed to separate atoms.
• A pure metal has a definite melting point; an alloy melts over a range.

Specific Heat

• Energy to raise unit mass by one degree.
• Temperature rise in forming/machining depends on the work done and the specific heat.
• Excessive temperature rise hurts surface finish & dimensional accuracy, increases tool/die wear, and can cause adverse metallurgical changes.

Thermal Conductivity

• Heat from plastic deformation/friction must be conducted away fast enough to avoid a severe temperature rise.
• Titanium machines poorly largely because of its very low thermal conductivity.

Thermal Expansion

• The coefficient of thermal expansion is roughly inversely proportional to melting point; alloying has only a minor effect.
• Shrink fits exploit it: heat a part with a hole, slip it over a cool shaft; on cooling it shrinks into an integral assembly.

Electrical Properties

• Resistivity = inverse of conductivity; high-resistivity materials are dielectrics / insulators.
• Superconductors: ~zero resistivity below a critical temperature (near 0 K; record ≈150 K = −123°C). Promise for high-power magnets, power lines, electronics.
• Semiconductors (Si, Ge, GaAs): conductivity is highly sensitive to temperature and dopant type/level (e.g. P, B in Si).
• Piezoelectric effect: reversible coupling of elastic strain and electric field (quartz, some ceramics) — used in transducers, strain gages, sonar, microphones.

Corrosion Resistance

• Corrosion = deterioration of metals/ceramics; in plastics the analog is degradation.
• Important in chemical, food, and petroleum industries; corrosion lowers strength and structural integrity (U.S. cost ≈ $200 billion/yr).
• Steels and cast irons resist poorly → need coatings/surface treatments (Ch33).
• Stainless steels form a protective chromium-oxide film (passivation).