Ultralow-temperature superplasticity and its novel mechanism in ultrafine-grained Al alloys

Abstract

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.

Advances in Superplasticity from a Laboratory Curiosity to the Development of a Superplastic Forming Industry

“…This low temperature superplasticity was first reported almost twenty years ago in materials processed by ECAP, as, for example, in a tensile elongation of ~800% in an Mg-9% Al alloy at a temperature of 423 K [49] and an elongation of ~460% in an AZ31 Mg alloy also at 423 K [50]. More recently, there have been similar reports of exceptional low temperature superplatsicity after processing by HPT, as in an Al-Zn-Mg-Zr alloy with an elongation of >500% at 443 K [51]. In practice, however, the development of high strain rate superplasticity generally has more relevance than low temperature superplasticity to the development of an industrial superplastic forming capability.…”

Nanostructural design of superstrong metallic materials by severe plastic deformation processing

“…The coming years may witness the study of strengthening mechanisms and their control using SPD techniques to become a relevant trend in the development of metallic materials with not only very high strength but also ductility and other enhanced mechanical properties. [26] .…”

“…Recent model calculations in Ref. [26] show [Figure 6] that the formation of segregations of impurities or alloying elements at grain boundaries may significantly inhibit the dislocation nucleation at grain boundaries, thereby contributing to the additional hardening of UFG materials. Simultaneously, computer simulations [27,28] and experimental studies [11,12,14,[17][18][19] provide convincing evidence of the formation of grain boundary segregations during the formation of UFG structures in metallic materials using SPD techniques.…”

“…(C) Dislocation loop expansion realized via its bow-out, thereby increasing the applied load if segregation-induced pinning is strong. (D) If pinning is weak, loop expansion is realized via the unpinning and lateral motion of points A and B. Reproduced with permission, Copyright 2019, Elsevier[26] .…”

A Short Review on Superplasticity of Aluminum Alloys

“…The kinetics of GBS is greatly influenced by grain boundary structure and state [33]. The grain boundary structure approach has been explored in ultra-fined grained Al-Zn-Mg alloys [33,34]. In those alloys, Mg and Zn atoms segregate at the grain boundary and form a grain boundary structure, which responds readily to GBS [33,34].…”