Mumbai University > Mechanical Engineering > Sem 4 > Material Technology

Marks: 5M

Year: Dec 2014

In materials science, creep (sometimes called cold flow) is the tendency of a solid material to move slowly or deform permanently under the influence of mechanical stresses. It can occur as a result of long-term exposure to high levels of stress that are still below the yield strength of the material. Creep is more severe in materials that are subjected to heat for long periods and generally increases as they near their melting point.

According the above concepts, in selecting a metal able to withstand dislocation creep, the following criteria should be followed

• Choosing a material with a high melting point
• Producing a large grain size, reducing the volume fraction of grain boundaries. In this case, a long diffusion path is required for grain-boundary diffusion. Further, a large grain size reduces the contribution of grain-boundary sliding (submicron grain size should be avoided in this sense).
• Inducing by proper alloying and heat treatment the precipitation of particles on grain boundaries, thus introducing a further obstacle to grain-boundary sliding. A continuous chain of precipitates should nevertheless be avoided, since it constitutes an easy path for cracks

According to this scheme, the most widely used methods to improve tensile strength at room temperature very often do not result in increased creep strength. For example

• The reduction of the grain size strengthens the material at low temperature, but could be dangerous in creep.
• Cold working increases tensile strength at low temperature; yet, exposure to sufficiently high temperature produces recovery or even recrystallization, with resulting softening. Thus, cold working is effective in enhancing creep strength only at moderately low temperatures.
• The precipitation of secondary-phase particles enhances both tensile strength and creep response. Fine particles can obstruct dislocation mobility, thus reducing creep. Yet, prolonged exposure to high temperature causes the coarsening of the precipitates, with progressive loss of the hardening effect

Burgers vector

The Burgers vector associated with a dislocation is a measure of the lattice distortion caused by the presence of the line defect. The diagram shows the convention for measuring the Burgers vector. A circuit is made around a dislocation line in a clockwise direction (top picture) with each step of the circuit connecting lattice sites that are fully coordinated. This circuit is then transferred to a perfect lattice of the same type. Because of the absence of a dislocation within this circuit, it fails to close on itself, and the vector linking the end of the circuit to the starting point is the Burgers vector, b = QM.