Metals and Alloys: Properties, Types, and Applications
T10.1 Metals and Alloys
General Properties of Mechanical Properties
Physical: Metallic materials have a high melting point (Tm), are heat and electrical conductors, and have a medium thermal expansion coefficient.
Mechanical: Metallic materials are ductile (less than polymers), have a high tensile strength, and high toughness.
Chemical: Metallic materials are highly reactive (oxidation).
Steels
Steels are alloys of iron and carbon. They contain other elements like silicon, manganese, sulfur, etc. The alloying elements are either intentionally added or retained during the refining process.
Low Carbon Steels (C < 0.25%)
Moderately priced, low-carbon steels are soft and have low resistance and high ductility.
- Can be hardened by cold working but not by heat treatment.
- Have good weldability and high formability.
- The microstructure is ferrite + perlite.
Typical applications of low-carbon steels: car body components, beams, pipelines.
Medium Carbon Steels (0.25% < C < 0.6%)
Are stronger than low-carbon steels but less ductile. The mechanical properties depend on carbon content.
Alloying elements such as Cr, Ni, and Mo can improve heat treatment capacity.
Heat treatment is needed to harden materials with this carbon percentage.
The microstructure will be tempered martensite.
Typical applications of this steel are train wheels, train rails, and gears.
High Carbon Steels (0.6% < C < 1.4%)
This steel has high strength, high hardness, and low ductility. It has high wear resistance, and alloying elements such as V, W, and Mo (formation of very hard carbides) can be added.
The typical applications are cutting tools, punches, and knives.
Tool Steel
They are basically high-carbon alloys. Tool steel has properties such as good toughness, good wear resistance, very good machinability, and resistance to softening on heating. The applications are for cutting, pressing, extruding, and coining. It is also used in the injection molding process.
Stainless Steel
If it has >4% Cr addition, it will have good rust resistance.
- Austenitic stainless steel has high corrosion resistance but is expensive. It is used in the food industry.
- Ferritic stainless steel is used in exhaust pipes and valves.
- Martensitic stainless steel is used in surgical instruments.
Alloy Steel
The alloying elements are added to steels to achieve one or more of the following properties:
- Increase resistance to corrosion.
- Improve machinability.
- Increase resistance to abrasion.
- Enhance high-temperature properties.
If alloying elements are less than 8%, it is classified as low-alloy steel.
If alloying elements are more than 8%, it is classified as high-alloy steel.
Manganese (Mn)
The properties depend on the % of Mn you have:
- 0.05-0.85% -> Improves strength and ductility.
- 1.5-2% -> Further increases the strength in heat-treated conditions.
- 2-10% -> Along with carbon, is responsible for brittleness in steel.
- 11-16% -> Along with 1-1.5% carbon, the alloy produced is hard and wear-resistant.
Nickel (Ni) – Steel
A small quantity of nickel with steel will improve the toughness and impact resistance.
- 1.5%-6% -> Increases the elastic limit, hardness, and tensile strength of steel.
- 8-22% -> Improves corrosion resistance and provides additional strength and hardness.
Chromium (Cr) – Steel
It provides hardness and increased elastic limit and tensile strength without affecting the ductility.
Addition of chromium to plain carbon steel improves the hardenability, strength, and wear resistance.
Nickel and chromium steel are also called stainless steel; it has increased strength, surface hardness, and resistance to corrosion.
Martensitic Stainless Steel
11.5-18% Cr + 1.2% C + 1-1.2% nickel
These are hardenable and have moderate corrosion resistance. It is used in turbine springs and turbine blades.
Ferritic Stainless Steel
10-27% Cr + 0.2% carbon
These are resistant to corrosion and have great strength. It is used for household and automobile industries.
Austenitic Stainless Steel
16-26% Cr + 35% nickel
They have high corrosion resistance, won’t be hardenable by heat treatment, and are nonmagnetic. It is used in aircraft and the food processing industry.
High-Speed Steel
It contains a large amount of tungsten, chromium, vanadium, and cobalt. It withstands high temperatures without losing hardness. It is used in drills and milling cutters.
Tungsten High-Speed Steel
Tungsten is commonly used as an alloying element in high-alloy steels. It varies from 1%-20% in the form of ferrite and also carbide. It is used in high-speed machining operations.
Effect of Alloying Elements in Steel
Less than 5% -> Improve strength or hardenability.
Up to 20% -> Gives corrosion resistance or stability at high or low temperatures.
Manganese
It’s used to improve hot ductility. At low temperatures, it acts as an austenite stabilizer, and at high temperatures, it will stabilize ferrite.
Manganese increases the solubility of nitrogen and is used to obtain high nitrogen contents in duplex and austenitic stainless steel.
Silicon
Increases resistance to oxidation, both at high temperatures.
Chromium
It gives corrosion resistance, increases the resistance to oxidation at high temperatures, and promotes a ferritic microstructure.
Molybdenum
Increases the hardness penetration of steel, slows the critical quenching speed, and increases high-temperature tensile strength.
Vanadium
It helps control grain growth during heat treatment and increases the toughness and strength of the steel.
Titanium
When used together with boron, it increases the effectiveness of the boron in the hardenability of steel.
Aluminum
Improves oxidation resistance. In precipitation hardening steels, aluminum is used to form the intermetallic compounds that increase the strength in the aged condition.
Silicon
Increases resistance to oxidation, promotes a ferritic microstructure, and increases the strength.
Copper
It enhances corrosion resistance, decreases work hardening, improves machinability, and improves formability.
Tungsten
It is added to improve pitting corrosion resistance.
T10.2 – Non-Ferrous Metals: Metals and Alloys
Non-ferrous materials: All the metallic elements other than iron are referred to as non-ferrous materials. These materials are:
- Lighter in weight
- Higher electrical and thermal conductivity
- Better resistance to corrosion
- Ease of fabrication
- Color
Aluminum Alloys
- Low density and high corrosion resistance
- Electrical conductivity
- Ease of fabrication
- Easily recycled
The application of this type of aluminum is for aircrafts, outer foil, aircraft racks, or aircraft couplings.
Titanium Alloys
- Low density and excellent corrosion resistance
- High resistance and high ductility
- Ease of fabrication and machining
- High reactivity
- α stabilizing elements: Al, O, N, C.
- β stabilizing elements: Mo, V, Nb, Ta
The applications of titanium alloys are turbojet engines, biomedical implants, and heat exchangers.
Copper
- Pure copper is reddish in color
- It’s highly malleable, ductile, and is a good conductor of heat and electricity.
Used in wire and some industrial applications like heat exchangers, bearings.
Brass
- Brass is an alloy of copper and zinc containing at least 50% copper.
- It is bright yellow to golden in color
- It is soft and ductile and stronger than copper
- Good casting properties and resistant to corrosion
It is used to make bearings, pumps.
Bronzes
Higher strength than copper and is non-corrosive and wear-resistant.
Silver (Ag)
Silver is a very soft, white, and lustrous transition metal. It has high electrical and thermal conductivity and good reflectivity of any other metal.
Silver has utilities such as photography, dentistry, high-capacity zinc long-life batteries, and in electrical or electronic items such as printed circuits and computer keyboards.
Gold
It is a bright, yellow, and soft metal. Gold is highly malleable and ductile, has good reflectance, and a high electrical and thermal conductivity.
Gold is used in electronic devices, particularly in printed circuit boards, connectors, and keyboard contractors. It is also used as a reflector of infrared radiation in radiant heating and drying devices.
Platinum
Platinum is a soft, lustrous, and silver metal. It is highly dense, malleable, and ductile, has high corrosion resistance, a high boiling point, and is a noble metal because of its high stability.
There are different platinum alloys.
Refractory Metals
Are a group of metallic elements that are highly resistant to heat and wear. Tungsten, molybdenum, niobium, tantalum, and rhenium are refractory materials. Those materials have high densities and hardness at room temperature.
Metals considered to be refractory typically meet these two thresholds:
- Melting point above 2200º
- Creep resistance above 1500º
Niobium
Is the least dense of the refractory metals and is the only one that can realistically be annealed. It can be found in aerospace and nuclear reactors.
Molybdenum
Is mainly used over tungsten when cost is a factor. It is cheaper than tungsten but still has comparable properties. It is commonly used as a strengthening alloy in steel.
Tantalum
Has superior corrosion resistance. It is found most often in the medical field as a following element in surgical tooling. Can be found in the capacitors of cell phones.
Rhenium
It is commonly used as an alloy in other refractory metal alloys, adding ductility and stile strength.