Mechanical Characters of Metals

Stress-strain curve of a metal obtained from a tensile test

From its stress-strain curve we can tell a lot about mechanical properties of a metal. Such stress-strain curve is produced routinely in a tensile testing done in many labs around the world. The curve behaves linearly initially and the slope of this linear portion is equal to its Young's modulus. A rigidity of a metal can be attributed to its large Young's modulus. Aluminum is often the most optimum metal as it offers sufficient rigidity, cheap cost, and light weight.

Deformation in the linear regime is called elastic deformation since the metal would return to its original shape when the loading is removed. This elastic deformation originates from the distance change between two adjacent atoms in a crystalline metal. It is possible for a metal to be not crystalline; we say it is a polycrystalline since it is composed of many small crystals. The region between two adjoining small crystals in a polycrystalline metal is called grain boundary and can disrupt the elastic deformation process, so that the elastic deformation terminates prematurely.

In any case, elastic deformation terminates when dislocations in the metal start moving. Dislocations arise from imperfections that emerge when the metal is processed and fabricated. It is impossible to have a 100% pure elemental metal; it can easily incorporate impurity atoms - such as oxygen, nitrogen, and hydrogen - and vacancies are always present. Vacancies are atomic-size bubbles and when they bunch together they will create a dislocation. Larger or smaller impurity atoms than the host elemental metal can also create dislocations.

It takes energy to move these dislocations since they have to break chemical bonds. When they move, they have to push aside nearby atoms. This creates a friction, and thus require a minimum energy before they can move. The metal is said to develop plastic deformation when these dislocations move. The threshold energy density corresponds to the upper yield strength.

Since these dislocations are distributed randomly and are usually bunched up near grain boundaries, the stress will fluctuate rather wildly as the dislocations move. In the stress-strain curve, this region is called discontinuous yielding. A metal that does not have many dislocations will have a smooth curve around the yield strength. After the dislocations can move more easily, we get a uniform elongation in the plastic deformation region and the elongation continues until the metal breaks.

When the loading is removed, the metal no longer returns to its original shape once plastic deformation occurs. The dislocations, in other words, do not remember their original positions. The energy spent for the loading is now stored in the metal.

The maximum loading that the metal can withstand before it breaks is called tensile strength. The breaking strength is called fracture strength. The strain value at fracture strength is called fracture strain. A metal that is ductile will have a large fracture strain, i.e., large elongation.

This mechanical behavior of a metal is analogous to how a person might respond to a life stress. When the person is subjected to a small stress, he will be able to bounce back to his normal, unstressful state. He can alter his behavior and even character, however, when the stress exceeds some value. There is a stress limit as well a person can take. Characters that are used to describe mechanical behavior of a metal are similar to people's characters: strength, hardness, rigidity, toughness.

Strength has to do with how much stress a metal can take and is proportional to its tensile strength. Hardness has to do with how much stress a metal can take before it produces plastic deformation and it is thus proportional to its yield strength. Rigidity is proportional to its Young's modulus. Toughness has to do with how much energy a metal can absorb and is proportional to the area under the stress-strain curve. A person is said to be tough if he can take physical (and sometimes mental) abuses. We can learn a lot about people by studying mechanical characters of metals.