(a) Creep

The tensile properties of most engineering materials at room temperature are practically independent of time. For example, during a tensile test, whether the test is completed in two minutes or two hours makes little difference to the results. At room temperature, the anelastic behaviour of materials—where irreversible structural changes occur—has little practical significance.

However, at elevated temperatures, material behaviour changes significantly. The strength of materials becomes strongly dependent on time and strain rate (rate of deformation). Under such conditions, many materials exhibit behaviour similar to viscoelastic materials, where the response transitions from elastic to viscous behaviour with time.

When a material is subjected to a constant tensile load at elevated temperature, it undergoes time-dependent deformation. This phenomenon is known as creep.

Creep is defined as the slow and progressive deformation of a material with time under constant stress, particularly at elevated temperatures.

Nature of Creep Deformation

• The simplest form of creep deformation is viscous flow.
• Once creep begins, deformation continues progressively.
• With increasing strain, necking and reduction in cross-sectional area occur.
• As the effective load-bearing area reduces, the rate of deformation increases, ultimately leading to rupture.

Materials Exhibiting Creep

Creep is observed in:

• Metals
• Ionic and covalent crystals
• Amorphous materials such as glasses and polymers

General behaviour:

• Metals exhibit creep primarily at high temperatures.
• Plastics, rubbers, and other amorphous materials are highly temperature-sensitive and may creep even at relatively low temperatures.

Conditions Where Creep Becomes Important

Creep is significant in the following applications:

• Soft metals used near room temperature
  • Example: lead pipes and white-metal bearings
• Steam and chemical plants operating at 450–550°C
• Gas turbines operating at very high temperatures
• Rockets, missiles, and supersonic jets
• Nuclear reactor systems

(b) Metallurgical Processing Techniques to Enhance Creep Resistance of Metal Alloys

To improve creep resistance, metallurgical techniques aim to reduce time-dependent deformation at high temperatures. Three important techniques are discussed below.

1. Solid Solution Strengthening

• Alloying elements are dissolved in the base metal to form a solid solution.
• These solute atoms cause lattice distortion, which impedes dislocation movement.
• Reduced dislocation mobility slows down creep deformation.
• Commonly used in high-temperature alloys such as nickel-based and iron-based alloys.

2. Precipitation (or Dispersion) Strengthening

• Fine, stable precipitates are uniformly distributed within the matrix.
• These particles act as barriers to dislocation motion, especially at elevated temperatures.
• Effective only when precipitates remain stable and resist coarsening at high temperature.
• Widely used in superalloys for turbine blades and aerospace components.

3. Grain Size and Grain Boundary Control

• Coarse-grained or single-crystal structures are preferred for creep resistance.
• Grain boundaries are weak points where creep deformation and diffusion occur.
• Increasing grain size reduces grain boundary area, thereby reducing creep rate.
• Directionally solidified and single-crystal alloys are commonly used in gas turbines.