Through their classic experiments on high purity aluminum (99.95%), Harper and Dorn22 came across a rate controlling mechanism that was seemingly independent of the grain size but still displayed characteristics

generally associated with Newtonian viscous creep. The creep experiments carried out at 0.99Tm provided stress exponent and activation energy values (n = 1 and Q = Ql) considered unique to N-H creep. However the grain size-independent behavior of the material combined with experimental strain rates around 1400 times larger than theoretical N-H creep predictions were suggestive of a new mechanism of creep. When the results obtained by Harper and Dorn were compared with theoretical N-H creep predictions, a large discrepancy was noted. In addition, by using markers Harper and Dorn22 found that the strains in the center of the grain are equal to the macroscopic strains noted in the creep experiments. The steady-state strain rate of deformation of this creep mechanism, now known as Harper-Dorn (H-D) creep mechanism, is given by

Studies over the years, on a host of other materials have led to a belief that

H-D creep is seen only in large grained materials (studies carried out by

Harper and Dorn were on Al with a grain size of 3.3 mm) and at very low

stresses and high temperatures. The primary characteristics of high temper­ature H-D creep are summarized below:5

• The stress exponent is equal to one.

• The creep rate is independent of grain size and similar creep rates are observed both in polycrystals and single crystals.

• The activation energy for creep is equal to that for lattice diffusion.

• The creep curves show a distinct primary stage which is followed by a steady-state stage.

• There is a random and reasonably uniform distribution of dislocations in specimens crept to the steady state.

• The dislocation density is low, of the order of 5 x 107 m-2, and is indepen­dent of stress.

• Very similar results are obtained in pure metals and solid solution alloys revealing that solute concentration has no effect on the creep behavior at these conditions.

While the initial studies were confined to very high temperatures (>0.95 TM), recent studies show that H-D creep can be rate controlling at inter­mediate temperatures as well. Creep studies in alpha titanium,23 beta cobalt,24 alpha iron25 and alpha zirconium26 have shown the presence of a H-D regime at homologous temperatures of around 0.35 to 0.6 for applied stresses around 9 x 10-5G (G is shear modulus) and grain sizes of around 500 pm.

Several models were proposed to understand the mechanism of H-D creep. The models of high temperature H-D creep were discussed in detail by Langdon and Yavari.27 Barrett et a/.28 proposed a model based on the creep strain resulting from dislocation glide with dislocation multiplication through climb. Murty29 suggested that H-D creep in Al-Mg solid solution arises from a modified viscous creep glide process (described later) with stress-independent dislocation density. More recently Kumar et a/.30 summarized the experimental results obtained on ceramic single crystals. Purity of crystals and a low initial dislocation density were cited as necessary conditions to unequivocally estab­lish the presence of H-D creep as a viable mechanism of deformation. A review of the viscous creep with n = 1 was recently made by Lingamurty et a/.31