The important benefits of ultrafine-grained (UFG) alloys for various applications stem from their enhanced superplastic properties. However, decreasing the temperature of superplasticity and providing superplastic forming at lower temperatures and higher strain rates is still a priority. Here, we disclose, for the first time, the mechanism by which grain boundary sliding and rotation are enhanced, when UFG materials have grain boundary segregation of specific alloying elements. Such an approach enables achieving superplasticity in commercial Al alloys at ultralow homologous temperatures below 0.5 (i.e. below 200°C), which is important for developing new efficient technologies for manufacturing complex-shaped metallic parts with enhanced service properties.
IMPACT STATEMENTFor the first time, ultralow-temperature superplasticity is found in commercial 7xxx Al alloy. This discovery enables the development of new technologies for the superplastic forming of complexshaped products with enhanced service properties.
Low temperature tensile and impression creep tests were carried out on an ultrafine-grained 7xxx series Al–4.8Zn–1.2 Mg–0.14Zr (wt%) alloy, which can be deformed for maximum elongation of about 200% at 150 °C. The characteristics of the deformation process, such as the strain rate sensitivity (SRS) and activation energy (Q) were determined by considering also the effect of threshold stress. Relatively high SRS of $$\sim$$
∼
0.35 and low activation energy of $$\sim$$
∼
92 kJ/mole were obtained, confirming the super ductility of the investigated ultrafine-grained alloy in the low temperature region between 140 and 160 °C.
Graphical abstract
Al–Zn alloys having different Zn contents of 2, 5, 10 and 30 wt% were processed by high-pressure torsion (HPT) to produce ultrafine-grained (UFG) materials. Microstructural features of these UFG Al–Zn alloys were investigated using depth-sensing indentations, focused ion beam (FIB), scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). Emphasis was placed on the microstructure evolution of the alloys with different Zn-concentration which demonstrated substantially different mechanical behavior, exhibiting superductility with increasing Zn content. It was shown that in every case, HPT resulted decomposition in the microstructure, but there is a significant difference between the microstructures of alloys with low and high Zn content. Based on the microstructural observations, a scenario is proposed about that how the decomposed microstructure developed during HPT process in low- and high Zn-containing Al–Zn alloys, influencing their mechanical behavior.
Graphical abstract
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