Metals make up a significant portion of the world's crust. However they are not distributed evenly around the world, nor are they often found in pure deposits. Rather, metals are typically found combined with other elements in localized areas. Rock that contains metals that can be economically extracted is called ore.

Ore deposits are not evenly distributed around the world - they are often localized in certain areas. Moreover, the ore may be mined from one area, and then transported to another area where the metals are extracted.

Design Context

Designers should understand that the process for extracting metals from ore is complex, energy intensive,  and has many environmental, economic, and social implications. (See Topic 2) Choosing materials carefully, and ensuring that they can be easily isolated for disposal or reuse/recycling is essential.

Metals are crystalline structures made up of grains. The size of the grains has an impact on the physical and mechanical properties of a metal.

Grain size is influenced by the type of heat treatment a metal receives while it is cooling. Manufacturers will apply certain heat treatment to achieve desirable mechanical properties. Essentially, rapid cooling forms small grains, while slow cooling forms large grains.

Design Context

Designers should understand that the physical and mechanical properties of a metal are influenced by the manufacturing process; Hence, precisely specifying the metal to be used, or any material for that matter, can ensure the best match with performance needs.

Alloying

Alloys are the mixture of one metal with another metal or non-metal.  This combination creates a material that has unique properties than if the metal was just used alone. 

In general, alloying increases hardness and strength and reduces malleability and ductility.

Pure iron, for example, is quite soft. By allowing it with carbon, carbon steel can be produced which has much greater strength, and thus is more useful for construction and engineering applications. 

Design Context

Stainless steel actually has Chromium in it. This alloy produces a durable protective layer that prevents surface corrosion. Sanitary surfaces, such as fridges, kitchen knives and cutlery are common examples of stainless steel. 

Work hardening and tempering

Work hardening is the process of increasing the hardness of a metal through applying a process when the metal is cold. The most common work hardening method is the application of force to compress the grains through cold rolling. 

Tempering is the application of heat after work hardening. This reduces some of the hardness and stiffness and increase the toughness and ductility of the metal.

Watch the video at 1m:50s point to see a demonstration of the impact that heat treatment has on the strength of steel.

Design Context

Cutting edges of blades, such as knives and razors, are common examples of tempering. The sharpness of the blade is preserved for longer.

Super Alloys are metal alloys that exhibit high degrees of mechanical strength, resistance to corrosion, and surface stability.

In particular, they are defined by a high resistance to creep and high oxidation and corrosion resistance.

• Creep is the slow expansion or deformation of a material over time. Creep can be exaggerated by high temperatures and prolonged forces. 

• Oxidation is the reaction of a metal with oxygen. The most familiar of this type of reaction is rust (oxygen reacting with iron in steel). However, metals may also corrode in the presence of high temperatures and acids or alkalis.

Design Context

Super alloys are a good example of design requirements driving the development of a material. Turbine engines used in jet aircraft need to operate under high temperatures for long periods of time. For obvious safety reasons they must have an extremely low failure rate.  The use of super alloys in the engines allows for the design to meet these  performance expectations.

Recovery and disposal of metals and metallic alloys

Metals and metal alloys are easily recyclable. Compared to the extraction of raw materials, recycling metals uses less energy, produces less waste, and uses materials that have already been extracted (thereby reducing the demand to extract more natural resources).

Metals can be indefinitely recycled, unlike plastics which can degrade in quality and properties through recycling. 

Design Perspective

Design for Disassembly (DfD) allows for a products to be easily and efficiently disassembled without the need for specialized tools, machines, or processes. Since many consumer products such as phones, computers, and even cars, use large quantities of valuable metals, effective DfD can ensure that the environmental impact of the design is reduced.