Common Alloy Families: Steel, Aluminum, Copper, and Titanium

## Steel: The Universal Structural Material Steel is an alloy of iron and carbon, with carbon content between 0.008 and 2.14 wt%. Additional elements give rise to the hundreds of grades available today. **Carbon steels** rely primarily on carbon for their properties. Low-carbon steels (under 0.3% C), such as AISI 1018, are easily welded, formed, and machined but have modest strength. Medium-carbon steels (0.3–0.6% C) offer higher strength after heat treatment. High-carbon steels (0.6–1.0% C) are used for springs and cutting edges. **Alloy steels** add chromium, molybdenum, nickel, manganese, or other elements to improve hardenability, toughness, or specific property profiles. AISI 4140 (Cr-Mo steel) achieves tensile strengths of 655–1080 MPa depending on heat treatment and is one of the most widely used engineering steels. **Stainless steels** contain at least 10.5% chromium, which forms a self-healing passive oxide layer. The three main families are austenitic (304, 316), ferritic (430, 444), and martensitic (410, 440C), each with distinct strengths and trade-offs. **Tool steels** are formulated for cutting, forming, and stamping operations, prioritizing hardness, wear resistance, and hot hardness. Grades like D2, H13, and M2 are common. ## Aluminum Alloys: Lightweight and Versatile Aluminum alloys are classified by a four-digit designation where the first digit indicates the primary alloying element: | Series | Primary Alloying Element | Representative Grade | Typical UTS | |--------|-------------------------|---------------------|-------------| | 1xxx | None (pure Al) | 1100 | 110 MPa | | 2xxx | Copper | 2024-T4 | 470 MPa | | 3xxx | Manganese | 3003-H14 | 150 MPa | | 5xxx | Magnesium | 5052-H32 | 228 MPa | | 6xxx | Mg + Si | 6061-T6 | 310 MPa | | 7xxx | Zinc | 7075-T6 | 572 MPa | 6061-T6 is the most commonly specified structural aluminum alloy, offering a good balance of strength (310 MPa tensile), corrosion resistance, weldability, and machinability at moderate cost. 7075-T6 is favored for aerospace structural applications where maximum strength-to-weight ratio is critical, though it has lower corrosion resistance and is more difficult to weld. The temper designation (T4, T6, H32, etc.) describes the thermal or mechanical processing state and has as much influence on properties as the alloy composition itself. ## Copper Alloys: Conductivity and Corrosion Resistance Copper alloys encompass brasses, bronzes, cupronickel, and several specialty families: **Brasses** are Cu-Zn alloys. C26000 (70% Cu, 30% Zn), known as cartridge brass, combines excellent cold formability with moderate strength and is used for ammunition cases, heat exchanger tubes, and hardware. C36000 (61.5% Cu, 35.5% Zn, 3% Pb) is free-cutting brass, the most machinable copper alloy, widely used for fittings, valves, and precision parts. **Bronzes** traditionally referred to Cu-Sn alloys but now includes Cu-Al (aluminum bronzes), Cu-Si (silicon bronzes), and Cu-Be (beryllium bronzes). Aluminum bronze C95400 (81% Cu, 4% Fe, 4% Ni, 11% Al) offers tensile strengths above 620 MPa combined with excellent seawater corrosion resistance, making it suitable for marine propellers, pump impellers, and valve bodies. **Cupronickel** alloys (70-30 and 90-10) resist biofouling and seawater corrosion better than brasses or bronzes. They are used for condenser tubing in power plants and desalination equipment. **Electrical applications** use the high-conductivity copper alloys: C11000 electrolytic tough pitch (ETP) copper at 101% IACS (International Annealed Copper Standard) for electrical wire, and beryllium copper C17200 for springs and connectors where conductivity must be combined with high spring force. ## Titanium Alloys: Strength Where Weight Matters Titanium alloys are categorized by their phase composition at room temperature: **Alpha alloys** (e.g., Ti-5Al-2.5Sn) are weldable, have good cryogenic toughness, and are used in airframe and cryogenic applications. They cannot be strengthened by heat treatment beyond the as-annealed condition. **Alpha-beta alloys** contain both alpha and beta phases. Ti-6Al-4V (Grade 5) is the workhorse of the titanium industry, accounting for more than half of all titanium consumption. In the annealed condition it delivers 930 MPa tensile strength; solution treated and aged, it can reach 1170 MPa. It is used for airframe structures, jet engine fan blades, medical implants, and high-performance bicycle frames. **Beta alloys** (e.g., Ti-3Al-8V-6Cr-4Mo-4Zr, or Beta-C) can be cold worked and are age-hardenable to very high strengths (above 1200 MPa), but they are denser than alpha-beta alloys. They find application in high-strength fasteners and springs. The density of titanium alloys (4.4–4.7 g/cm³) is roughly 60% that of steel but they can match or exceed steel's strength, giving a specific strength (strength per unit density) that exceeds most steels and aluminum alloys at the extreme. The main barrier to wider use is cost: titanium ore extraction, the Kroll process to produce metal, and the high reactivity that demands inert-atmosphere or vacuum processing all contribute to prices roughly 5–10 times those of comparable steel grades.