鉄とアルミニウム含有亜鉛融液との合金層形成反応

概要

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Ti interstitial free steel specimens with grain size of 50 μm in diameter polished with buffs were immersed in molten Zn containing 0.1-1.0 mass% Al for 4 to 3600 s at 733K. Formation and growth kinetics of Fe-Zn and Fe-Al alloy layers was examined metallographically. A fragmentary δ_1(f. δ_1) layer composed of small discrete δ_1 phase particles embedded in a matrix of η phase was formed on the surface of the specimen in molten Zn containing 0.1 mass% Al and its thickness increased with immersion time according to a linear rate law. In molten Zn containing more than 0.3 mass% Al, a very thin layer composed of small Fe_2Al_5 crystals was formed on the specimen surface, and then another type of small Fe_2Al_5 particles in the shape of lens were formed between the Fe_2Al_5 crystal layer and the specimen. The Fe_2Al_5 crystal layer disappeared, as the lens-like particles of Fe_2Al_5 phase developed to a layer, the thickness of which increased with immersion time according to a parabolic rate law that means the layer growth is controlled by diffusion. In molten Zn containing 0.15-0.25 mass% Al, f. δ_1 phase was formed between the crystal or diffusion layer of Fe_2Al_5 phase and the specimen. It appears that the formation of f. δ_1 phase is caused by penetration of the molten Zn through cracks of the Fe_2Al_5 layer.
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