Thermally activated processes in microcrystalline Mg
Introduction
Materials with small grains exhibit interesting mechanical properties for the technological use. However, the very high strength at room temperature may very often decrease rapidly with increasing temperature. The reason for a decrease of the mechanical properties at higher temperatures has not been explained up to now. Growth of the small grains with increasing temperature can cause the drop in the strength. In our previous paper we studied the thermal stability of the microstructure of microcrystalline Mg using internal damping method [1]. We estimated very stable grain structure; no grain growth was observed even at higher temperatures up to 550°C. The occurrence of diffusion processes could also lead to a rapid decrease of the yield stress and it may be apparent in the thermally activated analysis.
The aim of the present work was to study deformation characteristics of microcrystalline Mg and to try to find an explanation for a rapid decrease of the flow stresses at elevated temperatures and to determine possible thermally activated processes.
Section snippets
Experimental procedure
The microscaled Mg powder having particle diameter of about 40 μm was prepared by gas atomisation of a Mg melt with Ar containing 1% oxygen for powder passivation. The powder was subsequently pre-compressed followed by hot extrusion at 150 MPa at temperature 400°C. The original more or less equiaxial grains changed into elliptical grains with the long axis parallel to the extrusion direction. The grain size was in the cross section about 3μm and in the extrusion direction tens μm.
Cylindrical
Experimental results and discussion
The temperature dependencies of the yield stress of pure Mg and two composites containing 1 and 3 vol.% of alumina nanoparticles (for details see [1]) are given in Fig. 1. The yield stress decreases rapidly with increasing temperature in the whole temperature range studied. Lukác̆ [3] has reported that the critical resolved shear stress of pure Mg as well as Mg-Cd alloys decreases with increasing temperature between 77 and 230 K. The critical resolved shear stress of these materials is not
Conclusions
The strong temperature dependence of the yield stress indicates the occurrence of thermally activated processes during plastic deformation of microcrystalline Mg. The activity of non-basal slip systems is required for the grain accommodation in polycrystalline material. The glide of dislocations in pyramidal slip systems is very probably the main thermally activated process. The non-basal dislocations intersect the basal dislocations that are forest dislocations for the moving dislocation in
Acknowledgements
The authors acknowledge support from the Grant Agency of the Czech Academy of Sciences under Grant A2112901. Z.T. is also most grateful to Deutsche Forschung Gemeinschaft for its financial support during her stay in Clausthal.
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