High-strength Al-based alloys from amorphous precursors
Methods to strengthen aluminum alloys have been employed since the discovery of the age-hardening phenomenon in 1901. The upper strength limit of bulk Al alloys is ~0.7 GPa by conventional precipitation strengthening and increases to >1 GPa through grain refinement and amorphization. Unfortunately, the non-equilibrium state and the limited thermal stability of nanostructured and amorphous Al alloys limit their use at high temperatures. In order to overcome these limitations, we have developed a microstructural strategy for the production of high-strength Al-based alloys with good thermal stability. Our processing strategy consists of three steps: gas atomization, ball milling and hot pressing. Gas atomization is used to produce the amorphous particulate precursor with composition Al84Ni7Gd6Co3 (Figure 1a). The rapidly cooled small gas atomized particles are amorphous, whereas the slowly cooled large particles display minor amounts of crystalline phases. The powders are then treated by ball milling, which has the purpose to designedly vary the microstructure and the crystallization behavior of the amorphous precursor (Figures 1b). Finally, the powders are consolidated into highly dense bulk samples by hot pressing at a relatively high temperature, where the combined devitrification and consolidation of the amorphous particulate precursor take place (Figure 1c).
Figure 1. Processing of the Al84Ni7Gd6Co3 alloys: Gas atomization (a(1–4)), ball milling (b(1–4)) and hot pressing (c(1–4)). Schematic processing methods (a1, b1 and c1) and corresponding SEM backscattered electron (BSE) imaging (a2, b2 and c2), XRD (a3, b3 and c3) and differential scanning calorimetry (a4, b4 and c4) results of obtained microstructures.
The microstructure of the bulk samples clearly resembles the composite structure of the parent milled powder (compare Figures 1b2 and 1c2) and consists of a bimodal-like microstructure with coarse and fine precipitates regions. At the nanoscale, the material exhibits hybrid structures composed of nanostructured fcc-Al and intermetallic compounds. Such a hybrid microstructure leads to high strength at both room and high temperatures along with large Young’s modulus, which adds a new and promising region to the Ashby map of specific yield strength versus the specific Young’s modulus (Figure 2).
The basic principles for achieving such high strength are based on the composite structure and the effect of the mutual confinement between the nanosized phases. Confinement can effectively prevent the nanocrystalline fcc-Al and intermetallics from premature brittle fracture, thereby providing the possibility to deform plastically and to exhibit intrinsic strength rather than the flaw-controlled strength. The microstructural strategy can, in principle, be applicable to other materials and may thus provide a potential approach to developing high-performance hybrid materials.
Z. Wang, R.T. Qu, S. Scudino, B.A. Sun, K.G. Prashanth, D.V. Louzguine-Luzgin, M.W Chen, Z.F. Zhang, J. Eckert: “Hybrid nanostructured Aluminum alloy with super-high strength”, NPG Asia Materials (2015) 7, e229; doi:10.1038/am.2015.129.