As a general rule, creep starts occurring when the homologous temperature is greater than 0.4.  Most metals do not suffer from creep at room temperature, as they have much higher melting points than solder.  However, creep is still a major concern in when designing metallic components that have to function at high temperatures.

*The solder used in the practical is a eutectic mixture of lead and tin (61.5% Sn 38.5% Pb)

An example of one such engineering challenge is in the design of turbine blades for use in jet engines.  The blades in these engines can be exposed to temperatures of up to 850°C.  It is critical that the blades can withstand these temperatures without deforming.

Nickel-based Superalloys - an example of a creep-resistant material

The materials used to make jet engine turbine blades are nickel-based superalloys.  These not only have a high melting point (lowering the homologous temperature) but have also undergone special alloying procedures to make them resistant to creep at homologous temperatures in excess of 0.7.

Resistance to Dislocation Creep

The microstructure of a superalloy has 2 distinct phases: g and g¢.  g forms the matrix in which g¢ precipitates.  Both phases, g and g’ are cubic and have similar lattice parameters, so the phase boundaries between the phases are coherent but strained.  A lot of energy is required for the dislocation to permeate the strain field and cut through the precipitate.

Smaller, more finely spaced precipitates give the greatest resistance to creep.  A finely spaced array of precipitates is produced by solution treatment and ageing of the alloy.

There are 3 main stages in the heat treatment process:

1. Solution heat treatment: the alloy is heated to a very high temperature at which it exists as a single phase γ.
2. Quenching: the alloy is cooled very rapidly to ‘quench in’ the one phase microstructure.  This forms a super-saturated solid solution.
3. Ageing: the alloy is heated to a temperature below the solvus temperature. The second phase γ' gradually precipitates out.  The longer the alloy is held at the ageing temperature, the larger the precipitates grow.  For optimal strength, a fine dispersion of particles is necessary, although these particles must be sufficiently large to impede the motion of dislocations.

Resistance to Diffusional Creep

The rate of diffusional creep in a metal or alloy can be controlled by altering the grain size.  Grain boundaries provide pathways for diffusion, so the rate of diffusion creep can be reduced by increasing the grain size, as this reduces the number of grain boundaries.

In a directionally solidified structure, the grain boundaries are all aligned, meaning that diffusional distances for diffusion creep are very large, leading to high creep resistance in the direction parallel to the grain boundaries.  In the turbine blades, the axis of greatest stress is in the radial direction, along the length of the blade, so the grains are grown with their boundaries parallel to this axis to maximise the blade’s creep resistance.

A single crystal structure shows the highest creep resistance as there are no grain boundaries.

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