Overview of Metals

Combined Yield Strength

$${\sigma }_y={\sigma }_{\text{PN}}+k\sqrt{c}+\frac{k_y}{\sqrt{d}}+\alpha Gb\sqrt{\rho }$$

• ${\sigma }_{\text{PN}}$: the “starting point” for the material (Peierls-Nabarro Stress)
• $k\sqrt{c}$: Solid Solution
• $\frac{k_y}{\sqrt{d}}$: Grain Boundaries
• $\alpha Gb\sqrt{\rho }$: Strain Hardening

An application in the production of 7XXX Al-alloy:

Main strengthening methods of alloys

Unique Properties of Metals

• many methods of part shaping (wrought, casting, PM, joining, etc.)
• Many methods of strengthening (as well as a large variety of mechanical properties)
• Excellent compromise between high-strength and high-toughness
• Sensitive to cooling/heating rates
• Can combine structural and functional performance
• Can undergo significant plastic deformation

Inconveniences:

• Affected by environmental and on-service degradation and embrittlement
• Very sensitive to strain rate
• Sensitive to transition temperature

Properties of “The Big 6” Metals

Property	Iron (Fe)	Aluminium (Al)	Copper (Cu)
Available?	Yes (Very Abundant)	Yes (Most Abundant Metal)	Yes (Moderate)
Cheap? (£ / tonne 1997)	Yes (~£100 - £150)	Medium (~£1,000)	No (~£1,500 - £1,800)
M.P. (°C)	1538	660	1085
Density ($kg{m}^{-3}$)	7870	2700	8960
Allotropy	Yes (BCC $\alpha$, FCC $\gamma$)	No (Always FCC)	No (Always FCC)
Dissolves things?	Yes (Carbon, Nitrogen)	Yes (Copper, Magnesium, Silicon)	Yes (Zinc, Tin, Nickel)
T-Variable solubilty	Yes (Crucial for Steel hardening)	Yes (Crucial for Age hardening)	Yes (Variable)
Compounds	Carbides ($F{e}_{3}C$), Oxides	Alumina ($A{l}_2{O}_3$), Intermetallics	Oxides ($CuO$), Sulfides
Stability	Poor (Rusts/Oxidizes rapidly)	Good (Protective Oxide Layer)	Good (Noble / Forms Patina)
Hazardous?	No (Essential nutrient)	No (Dust can be explosive)	No (Salts can be toxic)

Property	Nickel (Ni)	Titanium (Ti)	Magnesium (Mg)
Available?	Medium (Sulfide ores)	High (Abundant but hard to extract)	High (Sea water / Dolomite)
Cheap? (£ / tonne 1997)	No (~£4,000)	Very Expensive (~£8,000+)	Medium (~£1,500)
M.P. (°C)	1455	1668	650
Density ($kg{m}^{-3}$)	8908 (Heavy)	4506 (Light-ish)	1738 (Very Light)
Allotropy	No (FCC)	Yes (HCP $\alpha$ $\to$ BCC $\beta$)	No (HCP)
Dissolves things?	Yes (Cr, Fe, Co - Superalloys)	Yes (Al, V, O)	Yes (Al, Zn, Mn)
T-Variable solubility	Yes (Precipitation hardening)	Yes (Phase transformation)	Yes (Age hardening)
Compounds	Intermetallics (${\gamma }^{\prime }$ $N{i}_{3}Al$)	Oxides ($Ti{O}_2$), Carbides	Oxides ($MgO$), Hydrides
Stability	Excellent (High Temp & Corrosion)	Excellent (Passivates instantly)	Poor (Galvanic corrosion / Flammable)
Hazardous?	Yes (Allergen / Carcinogen dust)	No (Biocompatible)	Yes (Dust/Chips are explosive)

Recycling Rate of Elements

Materials Substitution Table

Material	Consumption	Value	Import content	Energy	Resource life	Security of supply	Ease of use	Recycle potential
Steel	1	0	4	2	4	4	4	3
Aluminium	2	3	3	0	3	3	4	4
Copper	2	2	3	2	3	2	5	4
Lead	3	3	3	3	2	3	5	5
Zinc	3	3	2	1	2	3	4	3
Tin	4	4	2	3	1	1	-	2
Nickel	4	4	1	3	4	4	-	1
Magnesium	4	4	2	0	5	5	3	3
Mercury	5	5	0	2	1	0	-	4
Glass	2	2	5	3	5	5	2	3
Brick	0	2	5	5	5	5	2	0
Concrete	0	1	4	4	5	5	3	0
Timber	1	1	2	5	4	3	4	2
Plastic	2	1	2	1	3	4	4	1

Info

The above table shows ratings for several categories for certain materials. The ratings are in the range of 0 (meaning very bad) to 5 (meaning excellent).

Properties Involved In Selection of Materials

• Chemical composition
• Contamination of contents by corrosion products
• Corrosion characteristics in:
  • Atmosphere
  • Water
  • Soil
  • Chemicals
  • Gases
  • Molten metals
• Creep characteristics @ temperature range
• Crystal structure
• Damping Coefficient
• Density
• Effect of cold working
• Effect of high temperature on corrosion resistance
• Effect on strength after exposure to hydrogen and high temperatures
• Electrical conductivity
• Electrical resistivity
• Fire resistance
• Hardenability
• Maximum temperature not affecting strength
• Melting point
• Corrosion factor
• Susceptibility to corrosion:
  • General
  • Hydrogen damage
  • Pitting
  • Galvanic
  • Corrosion fatigue
  • Fretting
  • Stress corrosion cracking
  • Corrosion/erosion
  • Cavitation damage
  • Intergranular
  • Selective attack
  • High temperature
• Thermal coefficient of expansion
• Thermal conductivity
• Wearing quality:
  • Inherent
  • Via heat treatment
  • Via plating