Development of ultra-fine lead-free duplex brass by promoting homogeneous nucleation

Abstract

Despite increasing restrictions on lead usage, improving the machinability of lead-free duplex brass remains challenging due to high ductility. Herein, we propose a novel microstructural control strategy based on homogeneous nucleation, achieved by tailoring cooling rate during β → α transformation. We investigated the effects of Zn content and cooling rate on microstructure and mechanical properties of binary Cu-Zn alloys. Specifically, hot-extruded Cu60Zn40 and Cu58Zn42 (wt.%) alloys were compared, and two Cu58Zn42 specimens with different cooling rates were fabricated by varying sample dimensions. Cooling behavior depending on geometry and position was quantitatively analyzed using a MATLAB-based heat transfer model, and effects on microstructure and mechanical properties were systematically evaluated. Our results show that Cu58Zn42 exhibited higher β′ phase fraction and finer α-Widmanstätten structure due to enhanced undercooling. Furthermore, rapid cooling of Cu58Zn42 at 10.0 K/s promoted homogeneous nucleation within prior β grains, forming ultrafine duplex structure without additional alloying. This fast-cooled Cu58Zn42 exhibited significantly improved hardness and yield strength, and reduced Rm/Rp0.2 ratio indicative of enhanced machinability. These findings demonstrate that optimizing cooling rate alone enables the formation of ultrafine duplex microstructures with improved strength and machinability, offering an eco-friendly, scalable route for high-performance lead-free brass and potentially other dual-phase alloys undergoing coherent solid-state transformations.

Keywords

• Duplex brass alloy
• Microstructural evolution
• Ultra-fine microstructure
• Cooling rate
• Homogeneous nucleation
• Mechanical property