Microstructure and hardness evolution of Al8Zn7ni3mg alloy after casting at very different cooling rates

Abstract

In this study, we combined both a high strength Al-8%Zn-3%Mg aluminum matrix and a reinforcing contribution of Al3Ni intermetallics in Al8Zn7Ni3Mg hypereutectic alloy with a tuned microstructure via a variation of cooling rates from 0.1 K/s to 2.3 × 105 K/s. Using the Thermo-Calc software, we analyzed the effect of nickel content on the phase equilibria during solidification and found out that 7%Ni provides a formation of equal fractions of primary (6.5 vol.%) and eutectic (6.3 vol.%) crystals of the Al3Ni phase. Using microstructural analysis, a refinement of intermetallics with an increase in cooling rate was observed. It is remarkable that the structure after solidification at ~103 K/s across 1 mm flake casting consists of a quasi-eutectic with 1.5 μm Al3Ni fibers, while an increase in the cooling rate to ~105 K/s after melt spinning leads to the formation of 50 nm equiaxed Al3Ni particles. Under these conditions, the alloy showed an aging response at 200 °C, resulting in hardness of 200 HV and 220 HV, respectively. After 470 °C annealing, the fibers in the 1 mm sample evolved to needles. However, in melt-spun ribbons, the particles were kept globular and small-sized. Overall, the results may greatly contribute to the development of new eutectic type composites for rapid solidification methods.