TEM studies of the ThMn12–type and Th2Zn17–type phases in the Nd-Fe-Ti system

Abstract

Rare-earth intermetallic compounds with high Fe concentrations and adopting the ThMn12–type and Th2Zn17-type structures have attracted considerable attention in the field of permanent magnets. A large number of experimental studies has been done on these compounds, which are usually measured on alloys structurally characterized only by powder X-ray diffraction (XRD). However, the materials are often multiphasic and their quenched or annealed microstructures evidence homogenization ranges, second phases (resulting from segregation) and on-going phase transformations that may not be easily detectable by XRD. Additional microstructural studies on these systems are therefore required.Among the Fe-based systems, the ThMn12–type and Th2Zn17–type structures usually require stabilization by a third element, such as Ti. In the present work Nd:11Fe:Ti has been prepared by melting Nd, Fe and Ti in an arc furnace followed by subsequent splat-quenching and/or annealing treatments. The resulting materials have been characterized by XRD, scanning and transmission electron microscopy coupled with energy dispersive spectroscopy.The results have shown that the presence of α-Fe(Ti) could not be avoided during the solidification of the Nd:11Fe:Ti alloy. Furthermore, the microstructure morphologies and elemental analyses showed that at moderate cooling rates a secondary crystallization phase of Th2Zn17–type appeared in the Nd:11Fe:Ti alloy as result of a peritectic reaction. This lower temperature phase was however not detected in the splat-quenched material, where the high cooling rate route suppressed its crystallization. Nevertheless during a subsequent heat-treatment at 800 ºC the following decomposition took place: NdFe11Ti → Nd2(Fe,Ti)17 + α-Fe(Ti) + Fe2Ti. This reaction had already been proposed by Jang and Stadelmaier, who suggested that the NdFe11Ti compound is unstable at temperatures below 1000 °C. The current study shows that this transformation results in the fine lamellar intergrowth of the Th2Zn17-type phase in ThMn12-type grains, displaying random distribution of planar defects (Figure 1). The reciprocal space of the combined parent and intergrown phases has been mapped through a series of 3-D microdiffraction experiments (Figure 2). This allowed to establish that the preferred orientation relation between the two phases is (020)1:12//(003)2:17 and [100]1:12//[110]2:17, with the invariant plane sitting at 1:12//(333)2:17 planes.The work was supported by the Portuguese Science Foundation through the CTM/48617/2002, PEst-OE/CTM-UI0084/2011 and CFMC-PEst-OE/FIS/UI0261/2011 grants.