Date Published: April 9, 2014
Publisher: Public Library of Science
Author(s): Thomas L. Reichmann, Herta S. Effenberger, Herbert Ipser, Andreas Hofmann.
The complete Cd-Pr equilibrium phase diagram was investigated with a combination of powder-XRD, SEM and DTA. All intermetallic compounds within this system, already reported in literature, could be confirmed: CdPr, Cd2Pr, Cd3Pr, Cd45Pr11, Cd58Pr13, Cd6Pr and Cd11Pr. The corresponding phase boundaries were determined at distinct temperatures. The homogeneity range of the high-temperature allotropic modification of Pr could be determined precisely and a limited solubility of 22.1 at.% Cd was derived. Additionally, single-crystal X-ray diffraction was employed to investigate structural details of Cd2Pr; it is isotypic to the AlB2-type structure with a z value of the Cd site of 0.5. DTA results of alloys located in the adjacent two-phase fields of Cd2Pr suggested a phase transformation between 893 and 930°C. For the phase Cd3Pr it was found that the lattice parameter a changes linearly with increasing Cd content, following Vegard’s rule. The corresponding defect mechanism could be evaluated from structural data collected with single-crystal XRD. Introduction of a significant amount of vacancies on the Pr site and the reduction in symmetry of one Cd position (8c to 32f) resulted in a noticeable decrease of all R-values.
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All samples were prepared from pure elements, using Cd shot (99.9999%, AlfaAesar, Johnson Matthey Chemicals, Karlsruhe, Germany) and Pr pieces (99.9%, Smart Elements, Vienna, Austria). The surface-oxide layer of the Pr pieces was removed with a file. The elements were weighed with a semi-micro balance to an accuracy of about ±0.5 mg, resulting in an accuracy of ±0.01 at.% in the nominal composition for a total sample mass of 1 g. To prevent Pr from oxidation, the whole sample preparation was carried out in a glove box under Ar atmosphere (oxygen level: <1 ppm, water level: <1 ppm). The metals were placed in Ta crucibles which were designed in our laboratory and subsequently enclosed by means of arc welding under an Ar atmosphere of 0.25 bar. A total number of 29 samples were annealed and characterized by powder-XRD, SEM and DTA to obtain a complete description of the Cd-Pr phase diagram. Equilibrated samples and as-cast alloys were consulted to define homogeneity ranges, phase equilibria and crystallization behaviour. All relevant samples, examined with isothermal methods, are listed in Table 2. Heat treatments, identified phases and phase compositions are given. The corresponding results are discussed in detail in chapter 1. Samples studied by DTA are listed in Table 3, together with their thermal effects from two heating and cooling cycles, respectively. These thermal effects are discussed properly in chapter 2. On the basis of the combined results a complete version of the Cd-Pr phase diagram was drawn, which is given in Fig. 1. The complete Cd-Pr phase diagram was investigated with a combination of powder-XRD, SEM and DTA; it is presented in Fig. 1. Based on the present results, the homogeneity ranges of the altogether seven intermetallic compounds and of the solid solution of Cd in Pr were derived (Table 4). The solubility ranges fit quite well with values reported previously by Reichmann and Ipser . Additionally, melting or decomposition temperatures of all intermetallic compounds, derived from DTA measurements, are listed. It was found that the intermetallic compounds Cd11Pr, Cd6Pr, Cd45Pr11 and Cd3Pr are formed incongruently whereas Cd58Pr13, Cd2Pr and CdPr exhibit congruent formation behaviour. The invariant reaction temperature for the peritectic formation of Cd11Pr was comparable with values reported by Refs.  and . The solid solubility of Cd in β-Pr could be determined reliably and is given as about 22.1 at.% Cd. The addition of Cd stabilizes the high-temperature modification β-Pr down to 450°C where it decomposes in terms of a eutectoid reaction. Source: http://doi.org/10.1371/journal.pone.0094025