An Overview of High-energy Ball Milled Nanocrystalline Aluminum Alloys

Aluminum (Al) alloys, due to their lightweight and excellent physical properties are commonly used in aerospace, marine and automotive applications [1–5]. Various types of wrought Al alloys, with designations ranging from the so-called 1xxx to 8xxx series (and named according to the predominant alloying additions) have been developed in the past century [2]. Alloy properties depend upon chemical composition and thermomechanical processing employed, both of which influence the microstructure and the properties to a large extent. Among these alloys, the 7xxx series alloys (based on the Al-Zn-Mg system) have traditionally been the most commonly employed “high strength” alloys. The tensile strength of these alloys is as high as ~ 600 MPa, with modest ductility [2].

Alloying of elemental blends achieved through high-energy ball milling (HEBM) is referred to as mechanical alloying (MA), which is a solid-state powder processing technique involving the repeated deformation, fracture and welding of powder particles [1–4]. This technique was originally developed to produce oxide-dispersion strengthened (ODS) nickel and iron-base superalloys for aerospace applications [5]. Later, MA has been substantiated to be capable of synthesizing a variety of equilibrium and non-equilibrium phases, including nanocrystalline and amorphous materials. Recently MA has been demonstrated to be a most versatile and economical process for synthesis of nanocrystalline materials, due to its simplicity, low cost, and ability to produce large amount of material [1–4, 6]. Historically, from the point of Al based alloys, MA was used to produce dispersion hardened Al alloys [7–9]. Commercial production of Al alloys by ball milling was first reported by INCO alloys in year 1989 [10].

High-energy ball milling (HEBM) of Al and Al based alloys results in the nanocrystalline alloys in the form of powder, which subsequently needs to be consolidated for engineering applications, as well as for the investigation of associated properties. Consolidation of HEBM nanocrystalline alloy powders is not a trivial task. High mechanical strength (Chap. 4) and poor thermal stability (Chap. 5) of nanocrystalline materials make consolidation into fully dense bulk products challenging. Various conventional and non-conventional consolidation techniques used for consolidation of ball milled Al powders are summarised in Table 3.1 and described briefly in this chapter.

Nanocrystalline metallic materials [1–3] produced by the high-energy ball milling (HEBM) [4] are reported to be much stronger and apparently less ductile than conventional coarse grained materials. This difference in the properties is attributed to the unique grain structure, defects, defect activity and the arrangement of such features in nanocrystalline materials. For example, a paucity of dislocations in nanocrystalline materials is well documented. Dislocation pile-up in deformed specimen has not been reported so far and any dislocation activity is primarily believed to originate and terminate at grain boundaries. Due to the fine grain size, grain boundary sliding and/or Coble creep can dominate deformation, which may cause softening. Various aspects of mechanical properties of nanocrystalline materials produced via several processing routes are discussed in the literature. This chapter is focused on mechanical properties of nanocrystalline Al alloys as prepared by HEBM.

High-energy ball milling (HEBM) is a non-equilibrium processing techniques that imparts extended solid solubilities of alloying elements and grain refinement to the nanoscale level, this results in a thermodynamically metastable system. Such a metastable system can be prone to grain growth and decomposition of the solid solution, with elevated temperature exposure, or in the timed dependent the pursuit of thermodynamically stable phases [1, 2]. Therefore, an understanding the thermal stability of ball milled Al alloy and developing technologies to prevent grain growth and retain the alloying elements in solid solution upon high temperature exposure are of great interest—as any in-service applications require predictability of properties for the duration of a component lifetime.

Aluminum exhibits excellent corrosion resistance in the pH range of 4–8 due to the presence of a thin “passive film” upon the metal surface [1–9]. Breakdown of the passive film caused by several metallurgical (defects, impurities, secondary phases etc.) and environmental (halide ions, high or low pH, temperature) factors leads to the significant localized corrosion [9–17]. The corrosion of Al is escalated by the presence of intermetallics, which are formed due to the alloying additions and impurities as most of the elements possess limited solid solubility in Al and therefore intermetallics form during production, processing, and service conditions [18–22].

High-energy ball milled Al alloys have demonstrated extraordinary properties due to grain refinement to the nanoscale, and increased solid solubility of alloying elements. For example, the compressive yield strength of 1200 and 1105 MPa was observed in Al-Fe [1] Al-Cr alloys [2], both prepared via HEBM followed by consolidation. The specific compressive yield strength of an Al-20Cr alloy was compared with that of the commercial alloys and high-energy ball milled alloy surpassed the specific yield strength of the commercial alloys indicating a possibility of a development of a new class of light metals with high strength (Fig. 7.1). Moreover, corrosion performance of the nanocrystalline alloys was shown to improve significantly. For example, Gupta et al. [2, 3] reported a significant improvement in corrosion performance of both nanocrystalline alloys. In contrary to commercial alloys, where corrosion performance diminished with strength, high-energy ball milled alloys exhibited simultaneous improvement in corrosion resistance and strength due to the extended solid solubility of suitable solute and grain refinement to nanoscale. In summary, the high-energy ball milled Al alloys exhibited revolutionary properties and showed promise to develop future lightweight materials exhibiting ultra-high strength and improved durability. In spite of the attractive properties, the research on these alloys is in an early stage and further research is required to gain fundamental insight and ensure the engineering applications.