No abstract

Creep properties of specimens taken from the core of AZ91D magnesium alloy ingots (9 pct Al, 1 pct Zn) were examined in the temperature range of 120 ЊC to 180 ЊC and stress range of 40 to 115 MPa. Solution-treated and aged creep specimens were also tested. The creep rates observed were about three orders of magnitude lower than those of pure magnesium, and elongations to fracture were seen to be at least twice those of pure magnesium. A minimum creep rate was reached after approximately 2/3 of the creep life of the specimens (except for the aged specimens, in which the minimum creep rate appears at the beginning of the test). A qualitative model for the creep process is proposed on the basis of the creep tests and optical, scanning electron, and transmission electron microscopy. This model proposes that dislocation motion on additional slip systems is the primary creep mechanism and that cracking acts as a stress relief mechanism. Structural instability dictates the amount of hardening and, hence, creep resistance.

The available studies of the creep behaviour of AZ91 alloy produced by die casting, ingot casting, or thixoforming are reviewed in the present paper. Differences in microstructure and creep response are analysed and discussed with a view to providing a unitary, comprehensive description of the creep behaviour of this material. In particular, the minimum creep rate dependence on applied stress is described by means of a modified form of power law, which takes into consideration the effects of grain size and of intragranular precipitates interacting with dislocations. Analysis of the data obtained from the die cast alloy indicates that, in this material, creep is controlled by climb. Based on this observation and on theoretical considerations, as well as on the microstructural similarities, it is concluded that creep is controlled by climb also in thixoformed and ingot alloys. The effect of grain size is then quantified, and the strengthening effect of intragranular precipitates is described by introducing the threshold stress concept.MST/4714

No abstract

No abstract