Effects of Zr Additions on Structure and Microhardness Evolution of Eutectic Al-6Ni Alloy

Dilute Al-Zr alloys form upon aging, Al₃Zr nano-precipitates which resist coarsening up to 475 °C. Binary eutectic Al-Ni alloys exhibit Al₃Ni microfibers formed during solidification which also have good thermal stability. Here, we combine both strengthening approaches and study the effects of Zr micro-additions to an eutectic Al-6Ni (wt%) alloy on macro-, microstructure and hardness evolution upon long-term aging at 350 and 375 °C. The Al-6Ni-0.2Zr and Al-6Ni-0.4Zr (wt%) alloys exhibit a dendritic microstructure with primary Al₃Zr micro-precipitates. The Al-6Ni alloy maintains a constant room-temperature hardness after aging at 350 and 375 °C for 432 and 96 h, respectively, followed by a slow softening due to coarsening of the Al₃Ni microfibers. The Al-6Ni-0.4Zr alloy shows a strong precipitation-hardening response from Al₃Zr nano-precipitates achieving peak hardness after 48 and 24 h upon aging at 350 and 375 °C, respectively, and maintaining peak hardness for 768 and 264 h after aging at these temperatures. However, no precipitation hardening is occurring in Al-6Ni-0.2Zr, due to the lower Zr amount present in the matrix after solidification.