The activity of non-basal slip systems and dynamic recovery at room temperature in fine-grained AZ31B magnesium alloys

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

The alloy development, microstructure, properties and uses of cast magnesium alloys for elevated temperature applications are reviewed. The alloying principles and strengthening mechanisms of magnesium are discussed to identify the potential alloy systems for elevated temperature applications in automotive and aerospace industries. It is concluded that the Mg-Zr family of sand cast alloys exhibit adequate mechanical properties at both ambient and elevated temperatures for aerospace applications, and Ca-modified sand cast AS41 alloy might provide a cost-effective alternative for the Zr-containing alloys. For diecasting applications, no current alloy systems meets the requirements of good high temperature properties, acceptable castability and low cost for critical automotive components, future development is especially needed in this area. Development of dispersion strengthened magnesium alloys and improvement of current Mg-Al-RE and Mg-Al-Si systems are the potential routes to expand diecast magnesium alloys to elevated temperature applications.

Cast magnesium alloys for elevated temperature applications

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

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).

Josef Čadek

Papers

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

High temperature creep mechanisms in magnesium

Effective stress in steady state creep in an Al-5.5 at.% Mg solid solution

Interpretation of high temperature creep in alpha-zirconium in terms of effective stress and dislocation dynamics☆

Dislocation structure in the high temperature creep of metals and solid solution alloys: a review