溶融Zn–Al–Mg合金めっき皮膜の高温におけるZn₂Mg→Zn₁₁Mg₂相変態に伴う硬化

抄録

The present study was undertaken to investigate changes in the solidification microstructure consisting of Zn(hcp), α-Al(fcc), and Zn₂Mg phases in the hot-dip Zn–6Al–3Mg (mass%) alloy coating during isothermal holding at an elevated temperature of 200°C (below the solidus temperature), in terms of the transformation from Zn₂Mg metastable phase to Zn₁₁Mg₂ stable phase. The macroscopic Vickers hardness increased approximately from 150 HV to 180 HV after holding at 200°C for 36 ks. The hardness change corresponded well with the occurrence of the Zn₂Mg→Zn₁₁Mg₂ phase transformation identified by X-ray diffraction analyses. The Zn₁₁Mg₂ stable phase nucleated at the interface between Zn₂Mg and Zn(hcp) phases, and grew by consuming both phases formed in the initial solidification microstructure. Nanoindentation measurements revealed that the Zn₁₁Mg₂ phase had a lower hardness than the Zn₂Mg phase. Therefore, the significant growth of the Zn₁₁Mg₂ phase, which consumes a large part of the soft Zn(hcp) phase, could contribute to the observed hardening of the hot-dip Zn–Al–Mg alloy coating during isothermal holding.