High-strength and ductile ultrafine-grained Al–Y–Ni–Co alloy for high-temperature applications

Highlights

• Ultrafine-grained Al alloy synthesized from Al-based metallic glass precursors.

• Al alloy with high strength and ∼5% tensile ductility at room temperature.

• Tensile strength at elevated temperature superior to most Al alloys.

• High phase stability and sluggish coarsening of Al₁₉(Ni, Co)₅Y₃ phases.

Abstract

Ultrafine-grained Al_90.4Y_4.4Ni_4.3Co_0.9 was synthesized by powder metallurgy from partially-amorphous powder precursors. A good combination of mechanical properties was obtained by optimizing size and density of second phases. The resultant alloy shows high specific strength and specific modulus as well as a tensile ductility of ∼5% at room temperature. Particularly, this alloy exhibits good mechanical stability at elevated temperature, which is superior to most high-temperature aluminum alloys. These results provide evidence for the effectiveness of Al-based metallic glasses as precursors for the development of high-temperature aluminum alloys.

Introduction

Aluminum alloys are among the most widely used structural materials owing to their low density, good mechanical properties, good workability, superior corrosion resistance as well as excellent recyclability [1]. One limitation of most Al alloys lies in the low service temperature, which is typically limited to 473 K since the precipitates responsible for strengthening coarsen rapidly at elevated temperature [[2], [3], [4]]. Alloying elements that exhibit both limited solid solubility and low diffusivity in Al have been used to develop high strength and thermally-stable Al alloys [5]. These include rapid solidified Al–Fe alloys with ternary and often quaternary additions of elements such as Mo, V, Ni, Co, Zr, Ce [5,6] and systems forming Al₃TM (where TM is a transition metal) coherent L1₂ precipitates from a supersaturated solid solution containing at least one of the elements Sc, Zr, Ti, Hf, Er and Yb [[7], [8], [9]]. Based on the diffusion-controlled coarsening theory, the second phase resulting from the addition of these elements would be resistant to Ostwald ripening.

Ultrahigh strength and appreciable compressive ductility in Al-rich Al-RE-TM (RE is a rare earth metal) metallic glasses or derived nanostructured alloys have been extensively reported [[10], [11], [12]]. However, limited ductility under tension remains a constant issue for these alloys, significantly restricting their applications. The microstructure of devitrified Al-RE-TM alloys usually consist of nano-sized α-Al and Al-based binary and/or ternary intermetallic compounds [13,14]. Nanocrystalline Al shows low ductility because of the lack of dislocation multiplication and accumulation, as grain-boundary-controlled dislocation activity rapidly gives rise to a dynamic equilibrium, thereby strain hardening is absent leading to plastic instability [15]. A compromise can be achieved at submicron grain size, as ultrafine-grained metals following the Hall-Petch relationship present a better balance between strength and ductility [16]. Yttrium locates at the same main group as Sc in the periodic table and the two elements show similar chemical properties. One can expect that an Al alloy containing Y and/or other low diffusivity elements would resist to particle coarsening, and yielding higher recrystallization temperatures. Indeed, a former study has demonstrated that Al–Y–Ni–Co alloys have potential for high temperature applications [17].

In this study, bulk ultrafine-grained Al_90.4Y_4.4Ni_4.3Co_0.9 was synthesized by powder metallurgy (PM) from partially-amorphous powder precursors. The alloy shows good mechanical properties at both ambient and elevated temperature. In particular, the tensile strength at elevated temperature is superior to most high temperature aluminum alloys, while the room temperature Young’s modulus, yield strength and elongation reach 90 GPa, 500 MPa and 5%, respectively. The mechanisms regarding strength and thermal stability of this alloy are discussed.