Effect of Nb/W Ratio on Fe2 (Nb, W) Laves Phase Formation in Fe-Cr-Al-Nb-W Alloy

Laves phases may be one of effective reinforcements at higher temperature for creep. resistant ferritic steels. In this study, the effect of Nb/Wratio on crystallization and precipitation behavior of Fe-2 (Nb, W) Laves phase was investigated for Fe-Cr-Al- Nb-W ferritic alloys. Fe-9Cr-6.5Al-10 (Nb, W) (at%) alloys were prepared by a conventional arc. melting technique. In the As. cast condition, the 10 at% Nb alloy was entirely composed of a eutectic phase of ferrite and Fe2Nb. By substituting W for a part of Nb, the microstructure was changed to the combination of the primary ferrite and the eutectic of ferrite and Fe2 (Nb, W). The microstructural change indicates that the composition shifted from eutectic to hypo-eutectic with increasing the W/(Nb+W) ratio in the alloy. For the 10 at% W alloy, there was no Laves phase in the as-cast microstructure. However, a number of Laves phase precipitates were observed inside grains after heat-treatment at 1000 degrees C. The volume fraction of Laves phases decreased with increasing the W/(Nb+ W) ratio in the alloy since the solubility limit of W in the ferritic matrix is higher than that of Nb at 1000 degrees C. Therefore, the volume fraction and morphology are controllable by the Nb/W ratio in the Fe-Cr-Al-Nb-W alloy.