Orientation Relationships of Icosahedral Quasicrystalline Phase and Laves Phase Precipitates in a Ferritic Alloy

Abstract

The precipitation behavior and orientation relationships of icosahedral quasicrystalline phase (I-phase) and Laves phase precipitates in ferrite matrix have been investigated by transmission electron microscopy (TEM) in an Fe-10Cr-1.4W-4.5Co-0.3Si (at%) alloy. It is found that the precipitates of the alloy aged at 873 K are the I-phase but those of the alloy aged at 973 K are the Laves phase. Through a double aging experiment at both temperatures, it is shown that the transformation between the I-phase and the Laves phase occurs. Although a single orientation relationship is established between the I-phase and the ferrite matrix, three different types of the orientation relationships between the Laves phase and the ferrite matrix are obtained by the analysis of SAD patterns. The results can be explained by the coincidence between the five-fold symmetrical plane of the I-phase and the (11� 2) plane of the Laves phase on the phase transformation between them. High Cr ferritic heat resistant steels containing W have been developed for the power generating plants. These steels are composed generally of the tempered martensite matrix and the precipitates of MX carbonitride, M23C6 carbide and intermetallic compounds such as the Laves phase, � phase and z phase, etc. 1) It is suggested that fine precipitation of the Fe2W Laves phase under the service condition improves the creep resistance of the ferritic heat resistant steels. 2) On the other hand, the coarsening of the Laves phase reduces the creep strength and toughness of the steels particularly at ambient temperature. Then it is important to make it clear the precipitation behavior of the Laves phase in order to best utilize the Laves phase. The present authors have investigated the precipitation behavior of the intermetallic phases by transmission electron microscopy (TEM) in Fe-10Cr-1.4W-4.5Co alloys with and without 0.3 at%Si. 3) From a series of SAD patterns taken from the precipitates of the Si-containing alloy aged at 873 K, the two-, three- and five-fold symmetry and diffraction spots at the positions related to the golden section have been found to originate from the icosahedral quasicrystal, hereafter denoted as I-phase. It has been also found that the precipitates of the Si-containing alloy aged at 873 K are the I-phase but those of the Si-free alloy are the crystalline R-phase. However, the precipitates in both the Si-containing and Si- free alloys aged at 973 K have been found to be the Laves phase. Furthermore, it is interesting that the transformation between the Laves phase and the I-phase (or R-phase) occurs between the two aging temperatures. The objective of this work is to investigate the orientation relationships among the I-phase, the Laves phase and the � - Fe matrix which are related to the transformation between the I-phase and the Laves phase precipitates in an Fe-10Cr- 1.4W-4.5Co-0.3Si (at%) alloy. 2. Experimental Procedure