Structure and mechanical behavior of ultrafine-grained aluminum-iron alloy stabilized by nanoscaled intermetallic particles

Abstract

Ultrafine-grained aluminum alloys offer interesting multifunctional properties with a combination of high strength, low electrical resistivity, and low density. However, due to thermally induced grain coarsening, they typically suffer from an intrinsic poor thermal stability. To overcome this drawback, an Al-2%Fe alloy has been selected because of the low solubility of Fe in Al and their highly positive enthalpy of mixing leading to the formation of stable intermetallic particles. The two-phase alloy has been processed by severe plastic deformation to achieve simultaneously submicrometer Al grains and a uniform distribution of nanoscaled intermetallic particles. The influence of the level of deformation on the microstructure has been investigated thanks to transmission electron microscopy and atom probe tomography and it is shown that for the highest strain a partial dissolution of the metastable Al6Fe particle occurred leading to the formation of a Fe super saturated solid solution. The thermal stability, and especially the precipitation of particles from the ultrafine-grained solid solution and the way they pin grain boundaries has been investigated both from static annealing and in-situ transmission electron microscopy experiments. The correlation between microstructural features and microhardness has been established to identify the various strengthening contributions. Finally, it is 2 shown that ultrafine grained high purity Al with less than 0.01 at. % Fe in solid solution could preserve a grain size only 300nm after 1h at 250°C.

Understanding the role of Ca segregation on thermal stability, electrical resistivity and mechanical strength of nanostructured aluminum

“…In the Al-Ca composite before SPD, there are two phases: the fcc Al matrix and elongated submicrometer fcc Ca filaments [37]. These two phases are ductile and easily co-deform during the HPT process, which is the main difference with as-cast structures containing brittle intermetallic particles [19,[28][29][30]. During the early stage of deformation (one revolution by HPT), only a smooth hardness gradient appears (Fig.…”

“…However, the influence of segregations, both on electrical resistivity and mechanical strength of nanostructured Al alloys is not yet completely understood. It has already been demonstrated that grain refinement and intermetallic fragmentation can be successfully achieved by SPD processing of as-cast Al alloys containing immiscible elements (Rare Earth [28], Fe [19,29] or Ca [30]). However, starting from as-cast structures with intermetallic compounds requires huge plastic strains, and only Zener pinning can be achieved.…”

Strain Induced Segregations in Severely Deformed Materials

“…There are situations where SPD leads to mechanical mixing and the formation of super saturated solid solutions in multi-phase metallic alloys, 57,58) but there are also situations where super saturated solid solutions decompose during SPD. 41,42,59,60) In the latter case, the initial material is single phase and the super saturated solid solution is usually obtained by quenching from high temperature where it is stable.…”

“…Indeed, in such materials the driving force for grain growth is intrinsically large, thus if enough mobility is provided, grains will coarsen and the mechanical strength decreases. However, an enhanced thermal stability could be achieved if GBs are pinned by particles (so-called Zener pinning) 58) or segregations. 25) Besides, equilibrium GB segregations are driven by a minimization of the GB energy, thus they could help also stabilizing UFG or nanocrystalline structures by reducing the driving force for grain growth.…”

“…For example, the tensile strength of SPD processed Al alloys might be over one GPa. 58) Beyond problems connected to the up-scaling of processes, one of the major limitations for applications is the low ductility that usually results from the very limited strain hardening capability of UFG alloys achieved by SPD. Therefore, various strategies have been proposed to tackle this issue.…”