Resumen de High-throughput screening of strength and creep properties in mg-zn alloys
Magnesium has attracted significant attention due to its lowest density among the structural metallic alloys and its potential to solve severe global environment issues, not only in transport, but also in electronics and medical applications. However, some drawbacks limit its widespread application, like its low ductility and formability, resulting from its high mechanical anisotropy and the large difference in critical resolved shear stress (CRSS) between the soft (basal slip and extension twinning) and the hard (prismatic and pyramidal slip) deformation modes. Some of these limitations might be overcome by chemical alloying and suitable thermal treatments that have the potential to balance the CRSS between the soft and hard deformation modes. However, conventional alloy design is costly and time consuming, because it relies on processing a large number of alloys and testing them mechanically using conventional methods. In addition, it is often difficult to extract the effect of alloying elements on individual deformation modes using these conventional methods, because the mechanical behaviour is strongly influenced by other factors like texture or grain size. In this work, a novel high-throughput methodology is proposed, based on the combination of diffusion couple and advanced nanomechanical testing methods, to directly measure the effect of alloying on the CRSS of individual deformation modes in Mg alloys.
The methodology was tested on Mg-Zn alloys. Preliminary studies using nanoindentation revealed that this technique can provide qualitative trends on the strengthening effect of Zn and the rate controlling deformation mechanisms as a function of Zn content and temperature. However, the technique was unable to provide quantitative estimates of the CRSS of individual deformation modes. On the contrary, micropillar compression tests in selected orientations were much more suitable to isolate individual deformation modes. This way, a complete assessment of the effect of Zn content and temperature on the CRSS for basal slip, extension twinning, and prismatic slip was performed in Mg-Zn alloys, up to Zn contents of 2 at.%. This range covered both diluted and supersaturated Mg-Zn alloys. The study was performed in three different metallurgical conditions to assess the effect of Zn distribution on the strength of the deformation modes: as-quenched, for which the Zn solute atoms remain homogenously dispersed in solid solution; room-aged, for which the Zn atoms tend to form small Zn rich clusters; and peak-aged, for which the Zn atoms form rod-shape β1′ (MgZn2) precipitates.
It was found that, in the solid solution regime, the Zn atoms induce basal slip and extension twinning strengthening and softening of prismatic slip. The strengthening effect for basal slip was moderate, but much larger than what can be expected for diluted alloys, and this was related to short range order effects. The strengthening for twinning was significantly larger than for basal slip, and the mechanism responsible was found to be preferential Zn segregation to the twin boundaries. Finally, the softening of prismatic slip was found to contribute to the activation of cross-slip between basal and prismatic planes.
With respect to aged conditions, it was found that the strengthening of basal slip and extension twinning provided by the β1′ (MgZn2) precipitates is much larger than that found for the solid solution condition with the same Zn content. The precipitates were shearable by the basal dislocations and order strengthening was the mechanism responsible for the hardening. It was found that the β1′ (MgZn2) precipitates induce a strong pinning effect on twin boundary migration, but the strengthening of extension twinning was slightly lower than for basal slip.
The results demonstrate that Zn alloying in both the solid solution and aged conditions, contribute to a reduction in the plastic anisotropy of Mg. Overall, the methodology was successful not only for screening purposes, but also to answer fundamental questions on the mechanisms of interaction of the solute atoms with the different deformation modes. Therefore, the novel high throughput experimental methodology can be readily applied to other Mg alloys.
