Sei Miura

Mechanisms of Plastic Deformation in Pure Mg and AZ31 Mg Alloy Polycrystals

Analyses based on the Hall-Petch relation and tensile loading-unloading tests were made to clarify the deformation mechanisms in pure Mg and AZ31 alloy polycrystals. The Hall-Petch slope and the frictional stress of both specimens decrease with increasing temperature, especially at higher than 423K. The fraction of the activated non-basal slip system is about 40% of all at 293K in pure Mg. The elastic recovery strain ratio εer/εt of pure Mg and AZ31 alloys decreases with an increase of the plastic strain εp at the range of 293 to 523K, becoming almost constant at the strain εp exceeding 1.5%. In consequence of decreasing with an increase of temperature, the ratio εer/εt at 523K is below half of that for 293K. The present results suggest that the non-basal slips activated in the plastic deformation at 293K, which cause to interfere with the back motion of piled-up dislocations on basal slip planes. Introduction Mg alloys are expected to be quite a super-light material in the next generation applications, due to the excellent specific strength and good recycling ability among the conventional metals. It is also well known that the grain size refinement leads frequently to remarkable improvement of their strength [1]. In contrast, they possess low creep resistance and poor cold formability, in which the basal slip system contributes mainly to the deformation. In order to overcome this serious problem, it is important to understand their deformation behavior in detail. Trojanová et al. [2] pointed out that the activity of non-basal slip systems is required for the deformation of microcrystalline Mg and the glide of dislocations in the pyramidal slip system is very probably the main thermally activated process. Kobayashi et al. [3] reported that the non-basal segment of dislocations was found to consist of 40% of the total density in deformed AZ31B alloys by electron microscopy. Therefore, it is clear that the non-basal slip systems such as prismatic and pyramidal slips as well as twinning must be activated to deform Mg alloy polycrystals [4, 5], nevertheless their evaluation based on the dislocation theory remains disputable. The present investigation addressed to make clear the relation between the flow stress and the grain size for pure Mg and AZ31 alloy polycrystals at the wide range of temperatures. Furthermore, the elastic recovery strain ratio was estimated by using the tensile loading-unloading loops registered during the plastic deformation. From the obtained results, the deformation mechanisms in pure Mg and AZ31 alloys were examined in terms of crystal plasticity. Experimental Procedure Rolled sheets of pure Mg with 99.95mass% and commercial AZ31 alloys made by Osaka Fuji Industry were employed in the present study. They were prepared in the form of rectangles with the dimensions of 1×5×50mm in gauge length. Five kinds of average grain sizes ranging from 43 to 172μm for pure Mg and from 16 to 35μm for AZ31 alloys were selected for mechanical testing. The process was contolled by the strain-annealing method at 573 and 673K. Subsequently, the specimens were electrolytically and chemically polished in order to be able to reveal grain boundaries using Materials Science Forum Vols. 488-489 (2005) pp 555-558 Online: 2005-07-15

Etch Pit Observation of Dislocations in Early Deformation of Cu-Al Alloy and Pure Mg Polycrystals

In order to make clear the micro-yielding mechanisms of polycrystalline metals including twins, the movement of dislocations in the surface grains of Cu-6.8at%Al alloy and pure Mg polycrystals during the early stages of deformation was directly observed by using etch pit technique. The fresh dislocations multiply from the Frank-Read sources within the grains, and pile up against the twin and grain boundaries of two kinds of specimens. The pile-up dislocations on the primary and/or secondary slip planes are also confirmed in Cu-6.8at%Al alloys. Especially during the compressive loading for pure Mg, the occurrence of deformation twins is remarkable with an increase of strain rate, whereas the distribution of fresh dislocations tends to decrease in the surface grains. The present results suggest that the effect of twin boundaries on micro-yielding is almost equivalent for that of grain boundaries, which act as barriers to moving dislocations even in the pre-yield deformation.

Sub-Grain Size and Hall-Petch Relation in Pure Copper Single Crystals

The effect of sub-grain on the yield stress of pure copper single crystals with the [253] orientation was investigated by using the etch pit technique. The single crystal plates were successfully prepared from the seed crystals, which were produced at the melting temperature of 1473 K by the Bridgeman method. The present investigation confirmed the Hall-Petch relation concerning the effect of sub-grain boundaries on the macroscopic yielding of pure copper. The result derived from the extrapolation of the relationship of critical resolved shear stress (CRSS) and the initial dislocation density and sub-grain size is in good agreement with the evaluation in high purity copper single crystals of low dislocation density.