The analysis of any creep data is made by assuming the microstructure to be constant. Some of the microstructural features that could change during the course of a test are phase composition, precipitate size and distribution, and grain size. Thus to estimate the different creep parameters and to deter­mine the mechanism of creep, it is necessary to keep the microstructure constant. To this end, materials are usually heat treated at temperatures higher than the test temperature. Even though thermal stabilization estab­lishes a constant microstructure during the course of a test, stress-assisted processes altering the microstructure cannot be ruled out. Non-equilibrium structures, namely nanocrystalline materials undergo stress-assisted micro­structural changes that prevent the attainment of a constant creep micro — structure.14 Creep tests on such materials should be carried out at stresses lower than the critical stress at which microstructural changes could be initiated.15

The most important microstructural parameter that plays a major role in controlling the creep properties of a material is the grain size. The depen­dence of the strength of a material on its grain size can be understood through the Hall-Petch relationship which states that materials with finer grain sizes possess greater strength than materials with larger grain size3:

[3.15]

where ay is the yield strength, a0 the friction hardening (stress felt by dislo­cations while moving through the lattice), d the grain size and ky is known as the Petch-unpinning coefficient that frees the dislocations locked by inter­stitial solute atoms. This is true at low temperatures where grain boundary sliding is not dominant.

On the contrary, under creep conditions the reverse is true. Materials with finer grain size creep faster than coarse grained materials at higher tem­peratures and at lower stresses. There are certain creep mechanisms that operate faster in finer grained materials in comparison to coarse grained materials. Hence, it is necessary to have knowledge of the grain size of a material. The dependence of the steady-state creep rate on the grain size is understood through the following equation:

[3.16 ]

where K2 is a constant, d is the grain size and p is the grain size exponent. As is clear from this equation at a given stress and temperature, finer grain sized materials are expected to creep faster than coarser grained materials. However for dislocation-based mechanisms which are not grain size depen­dent, the strain rate of deformation would be the same for both fine grained and coarse grained materials.