كليدواژه :
Dislocations , Mg alloys , Nano , mechanics
چكيده فارسي :
Mg and its alloys stand for the lightest structural metals and present high specific-strength, excellent bio-compatibility and reasonable cost. However, there are some limitations as well. One main limitation for the extensive use of Mg and Mg alloys in engineering application is the reduced ductility and formability at room temperature, whose origin can be traced to the hcp lattice structure. There is huge difference between critical resolved shear stress (CRSS) of basal slip and other modes of deformation, especially at low temperatures, which leads to plastic anisotropy. Alloying with different elements is a natural strategy to improve the strength of different slip systems and modify the plastic anisotropy. However, the accurate determination of the effect of solute atoms and precipitates on the CRSS for each mode of deformation is very expensive because it requires the manufacturing of single crystals with different compositions. This problem may be overcome by the combination of micropillar compression tests and high-throughput processing techniques based on the diffusion couples. This methodology requires a detailed analysis of the effect of micropillar dimensions on the flow strength and can be easily extended to high temperatures. In this paper, after explaining these issues with more details, the mico-pillar compression technique would be used to determine CRSS of different slip systems in an Mg-4Zn alloy. In addition, interaction of dislocations with precipitates would be studied in details at high temperatures. Using this methodology, the effect of MgZn2 precipitates on basal and pyramidal slip was determined. It was found that the mechanical properties were independent of the micropillar size when the cross-section was 3 ×3 µm2. Transmission electron microscopy showed that deformation involved a mixture of dislocation bowing around the precipitates and precipitate shearing. The initial yield strength was compatible with the predictions of the Orowan model for dislocation bowing around the precipitates. Nevertheless, precipitate shearing was dominant afterwards, leading to the formation of slip bands in which the rod precipitates were transformed into globular particles, limiting the strain hardening. The importance of precipitate shearing increased with temperature and was responsible for the reduction in the mechanical properties of the alloy from 23 °C to 100 °C. Also, the obtained experimental results were compared with predictions from atomistic and/or continuum models and showed the effect of alloying on the strength and plastic anisotropy. It was found that the dislocations initially overcame the precipitate by the formation of an Orowan loop that penetrated in the precipitate. The precipitate was finally sheared after several Orowan loops were piled-up. The number of loops necessary to shear the precipitate decreased as precipitate cross-section decreased and the temperature increased but was independent of the precipitate spacing.
