Influence of solution heat treatment on microstructure and hardness of as-cast biodegradable Zn–Mn alloys

Zn–(< 1 wt%)Mn alloy is a promising base alloy for developing biodegradable Zn–Mn-based alloys with high strength and high ductility. In this paper, the influence of solution heat treatment on microstructure and hardness of as-cast Zn–0.34Mn and Zn–0.76Mn alloys has been investigated. The temperature of 380 °C is a good choice for the solution heat treatment. ζ-MnZn13 phase is completely dissolved in Zn matrix after 20 h for Zn–0.34Mn alloy and after 30 h for Zn–0.76Mn alloy. Unexpectedly, temporary precipitation of ζ-MnZn13 is observed within grains of Zn–0.76Mn alloy at 5 h, which reaches the peak at 10 h and then decreases until it finally disappears. During the solution heat treatment, hardness of the Zn–Mn alloys decreases with time. Zn–0.76Mn alloy is always about 4–8 HV harder than Zn–0.34Mn alloy. The temporarily formed ζ-MnZn13 particles are rod-like or lath shaped. The orientation relationship between a representative lath-shaped ζ-MnZn13 precipitate and Zn matrix can be described as [101]ζ~//[2-1-10]m and (-131)ζ ~ 5.4° from (0002)m, which is firstly reported as far as our knowledge. The ζ-MnZn13 lath has the long axis parallel to [03-32]m and the habit plane parallel to (-111)ζ//(0-113)m. Simulation of lattice matching reveals that every two ζ-MnZn13 lattice sites match with one Zn lattice site along the direction of the long axis, which explains why the precipitate grows along this direction. No serious segregation of Mn is detected within Zn grains in the as-cast Zn–0.76Mn alloy. The reason why ζ-MnZn13 precipitates temporarily form during the solution heat treatment requires further investigation.