Precipitation of the Icosahedral Quasicrystalline Phase, R-phase and Laves Phase in Ferritic Alloys

Abstract

Precipitation behavior of intermetallic phases in ferrite matrix is investigated by transmission electron microscopy (TEM) in Fe-10Cr1.4W-4.5Co (at%) alloys with and without 0.3 at%Si. It is intended to provide basic information for the alloy design of ferritic heat resistant alloys strengthened by intermetallic compounds. In the alloy containing Si, icosahedral quasicrytalline phase (I-phase) is found to precipitate during aging at 873 K, It is confirmed that selected area diffraction (SAD) patterns of the precipitates exhibit two-, three- and five-fold symmetry and have diffraction spots in the positions related to the golden section. In the Si-free alloy, the R-phase precipitates instead of I-phase at 873 K, and the Laves phase precipitates in both alloys during aging at higher temperature, 973 K. The Laves phase formed at 973 K transforms to the I-phase in the alloy containing Si but to the R-phase in the Si-free alloy during subsequent aging at 873 K. It is found that the structures among icosahedral quasicrystalline and crystalline R- and Laves phases are closely similar by comparing the orientation relationship, precipitate morphology and stability of I-phase with those of R- and Laves phases.