The roles of grain-boundary sliding (GBS) and of other creep mechanisms in creep and fine-grain superplasticity are presented in relation to a model based on the division of grains into their central “cores“ and peripheral “mantles”; GBS and its accommodation is limited to the latter, which changes with the mode of accommodation,viz by fold formation, dislocation motion in the mantle or pure diffusion. This description is used to adapt from the literature or develop equations for creep rate based on dislocation or vacancy mechanisms, which are then combined to give plots of the various regimes of superplastic (or creep) behavior, all of which involve GBS in a quantitatively defined manner. The predictions of these equations are compared with a number of results in the literature and with those of a lead-thallium alloy of grain sizes intermediate between superplastic behavior and normal creep. Some preliminary comparisons of measured GBS are also made with the predictions of the model. Agreement is good.

Grain-boundary sliding and its accommodation during creep and superplasticity

Over the past 15 years important advances have been made in the experimental study of the microstructural changes occurring during the non-linear steady-state creep of single phase crystalline matter at elevated temperatures. Curiously, although the results of these painstaking studies have gone a long way toward elucidating the mechanism of this phenomenon, they have been largely ignored in favour of some simple dislocation mechanisms that are not only inconsistent with these observations, but are also unable to describe correctly the known phenomenology. This review concentrates primarily on the recent experiments on microstructural alterations occurring during creep; however, it also surveys the many mechanistic models that attempt to describe this phenomenon, and finds them all deficient.

Steady-state creep of single-phase crystalline matter at high temperature

On the creep of crystals

Five-power-law creep in single phase metals and alloys

An examination of the breakdown in creep by viscous glide in solid solution alloys at high stress levels

The dislocation structure of alpha iron formed in high-temperature creep mas investigated in the transmission electron microscope and its quantitative characteristics—dislocation density, subgrain diameter and subgrain misorientation—were measured in the primary as well as in the steady-state creep. The steady state substructure characteristics are correlated with the steady state values of the applied, effective and internal stress and the development of the substructure in the primary creep is discussed. The results of the structure observation correspond well to the macroscopic creep data (Pahutora, Orlova, Kuchařova, Cadek, to be published).

Dislocation structure and applied, effective and internal stress in high-temperature creep of alpha iron

Creep in copper of 99·99% purity in the temperature interval 550-1025°K was investigated by the isothermal tests technique. Two dislocation mechanisms operating in parallel were detected. In the region of lower creep rates-Region 1-the apparent activation energy of creep depends linearly on stress: [image omitted] where Q01 = 47·4 ± 2·8 kcal mol-1 and B1 = 3·2 ± 0·5 kcal mol-1 Kg-1 mm2. The value of Q01 is close to the activation enthalpy of lattice self-diffusion in copper. Consequently, creep is controlled by lattice self-diffusion. The stress exponent n ≡ n1 = δ In ∊/δ In σ is a function of both stress and temperature. Generally, n1 decreases, reaches a minimum and increases again with the increasing stress. The lowest value of n1 ≃ 5·4. Non-conservative motion of jogs on screw dislocations dependent on lattice self-diffusion was suggested to be a rate-controlling process in Region 1.In the region of higher creep rates-Region 2-the apparent activation energy Qc2, is considerably higher than the activat...

M. Pahutová

Papers

Dislocation structure and applied, effective and internal stress in high-temperature creep of alpha iron

High temperature creep in copper

Dynamics of dislocations in high temperature creep of an Fe-3.5% Si alloy

High temperature creep of alpha iron

Some stress change experiments on creep in α zirconium