The Ag-Ce phase diagram I: Ag-Ag3Ce partial phase diagram

Journal

of the Less-Common

Metals,

167

92 (1983) 167-175

THE Ag-Ce PHASE DIAGRAM I : Ag-Ag,Ce PARTIAL PHASE DIAGRAM

ISOLDE STAPF and HERMANN Max-Planck-Znstitut (F.R.G.)

JEHN

fiir Metallforschung,

Znstitut fiir Werkstoffwissenschaften,

D-7000 Stuttgart 1

(Received December 6,1982)

Summary As part of a systematic investigation of the silver-rare earth metal systems the Ag-Ce system was re-examined to clarify discrepancies reported in the literature. In the Ag-Ag,Ce partial phase diagram only Ag,Ce (tetragonal) was found in addition to the Ag,Ce phase (hexagonal, Ag,Pu type). The Ag,Ce phase is formed peritectically at 815 “C by the reaction between Ag,Ce and the liquid. A eutectic reaction liquid + Ag + Ag,Ce occurs at 793 “C. The Ag,Ce phase melts congruently at 1030°C. The phase diagram was based on the results of thermal analyses and microstructure examinations of cast and annealed specimens.

1. Introduction The systematic investigation of the systems of silver with the rare earth (RE) metals yttrium [l], holmium [a], erbium [3], gadolinium [4], scandium [5], praseodymium [S], neodymium [S] and samarium [7] has been extended to the Ag-Ce system. Detailed phase diagram studies of this system have been performed by Delfino et al. [S] and recently by Heumann and Preval [9]. The results of the present work based on thermal analysis and microstructure examination of cast and annealed samples are rather different from those obtained in the earlier investigations [S-lo]. The Ag-Ag,Ce partial phase diagram is presented in this paper. The Ag,Ce-Ce partial phase diagram and the intermetallic phases of the whole system will be discussed in a subsequent paper. OElsevier

Sequoia/Printed

in The Netherlands

168

2. Experimental

details

2.1. Alloypreparation Silver (Johnson-Matthey Chemicals Ltd.; purity, 99.993%) and cerium (Koch-Light Laboratories Ltd.; purity, 99.9%) were used as starting materials. The Ag-Ce alloys containing 2-26 at.% Ce were produced by arc melting using a non-consumable tungsten electrode in water-cooled copper crucibles under an argon atmosphere of 400 mbar. A zirconium ingot was melted before preparing the alloy to getter the impurities of the argon atmosphere. Only one remelting procedure was required to ensure the homogeneity of the samples. During the melting process maximum weight losses of 0.15% were observed. The alloy composition was analysed chemically after the arc melting and after the thermal analyses. Silver and cerium were determined by potentiometric titration and complexometric titration respectively. 2.2. Constitution studies Some ofthe cast ingots were used for differential thermal analysis (DTA). A detailed description of the procedure is given elsewhere [7]. A second set of samples was heat treated in sealed argon-filled quartz ampoules at temperatures between 730 and 850 “C for periods of up to 6 months in a resistance-heated furnace and was then quenched in water. The microstructures of cross sections of as-cast samples, samples which had undergone DTA and heat-treated samples were examined. The metallographic preparation is described in detail elsewhere [5]. The phases were easily differentiated by the tarnishing colours formed during exposure of the polished surfaces to air for several hours at room temperature. As observed for other AgRE systems, RE-rich phases are darker than RE-deficient phases. 3. Results Figure 1 shows the Ag-Ag,Ce partial phase diagram constructed on the basis of the microstructural observations and the DTA data obtained in the present investigation. Two intermetallic phases are present: the congruently melting phase Ag,Ce and the Ag,Ce phase formed peritectically from Ag,Ce and the liquid. The melting point of silver is 961.3 “C. The eutectic reaction liquid + Ag solid solution + Ag,Ce appears at 793 “C; the eutectic composition was observed at 10 at.% Ce. The intermetallic compound Ag,Ce is formed peritectically at 815 “C by the reaction liquid + Ag,Ce + Ag,Ce The cerium concentration of the liquid at the peritectic temperature is 12 at.%. The Ag,Ce phase melts congruently at 1030 “C. Figures 2-10 show the microstructure of as-cast and slowly cooled (after DTA runs) Ag-Ce alloys containing 1,7,10,12,14,18,22,24 and 26 at.% Ce. The concentrations of these alloys relative to the phase diagram are shown by

169

x130% .- / (.-+-*--*\,

1000

\

961.3T

*g

IO Cerium

Fig. I. Ag-A&e

20 ,

at.%

partial phase diagram: 0, DTA; 0,

30

annealedsP@cim@ns.

Fig. 2. Ag-lat.%Ce, slow solidification (DTA): primary, silver solid solution eutectic silver (white) plus Ag,Ce (black). (Ma~ification, 160x.)

(white); remainder,

Fig. 3. Ag-?at.%Ce, slow solidification (DTA): primary, silver solid solution eutectic silver (white) plus Ag,Ce (black). (Magnification, 160x.)

(white); remainder,

arrows in Fig. 1. The alloys Ag-lat.%Ce (Fig. 2) and Ag-7at.%Ce (Fig. 3) show primary crystals of silver solid solution and a eutectic consisting of silver and Ag,Ce. The Ag-lOat.%Ce alloy solidifies completely in the eutectic mode (Fig. 4). Figure 5 represents the hypereutectic structure of the Ag-lBat.%Ce all<> formed by primary Ag,Ce dendrites and the Ag-Ag,Ce eutectic. Alloys containing 14 and 18 at.% Ce show the typical microstructure of a peritectic reaction (Figs. 6 and 7). An Ag,Ce phase solidifies first and on further

Fig. 4. Ag-lOat.%Ce, slow solidification (black). (Magnification, 160x.)

(DTA): eutectic

silver solid solution

(white) plus Ag,Ce

Fig. 6. Ag-lPat.%Ce, slow solidification (DTA): primary, Ag,Ce (black); remainder, eutectic silver (white) plus Ag,Ce (black). (Magnification, 160x.)

Fig. 6. Ag-llat.%Ce, slow solidification (DTA): primary, Ag,Ce (black) (grey); remainder, eutectic silver (white) plus Ag,Ce (grey). (Magnification,

surrounded 160x.)

by Ag,Ce

Fig. 7. Ag-lBat.%Ce, slow solidification (DTA): primary, Ag,Ce (black) (grey); remainder, eutectic silver (white) plus Ag,Ce (grey). (Magnification,

surrounded 160x.)

by Ag,Ce

cooling reacts with the liquid phase to form Ag,Ce. This reaction (liquid + Ag,Ce + Ag,Ce) proceeds only incompletely and the remainder of the liquid solidifies eutectically. The distribution of the phases corresponds to the actual compositions of the alloys and thus indicates the existence of the Ag,Ce compound. The microstructure of the slowly cooled Ag-22at.%Ce alloy shows primary Ag,Ce and the Ag-Ag,Ce eutectic; the amount of peritectically formed Ag,Ce is too small to be detected (Fig. 8). The Ag-24at.%Ce alloy (Fig. 9) shows principally the same structure as the Ag-22at.%Ce alloys, except that the amount of primary Ag,Ce phase is greater. Ag,Ce appears black in Fig. 9 because of its higher cerium content. In contrast, in the alloy containing 26 at.% ;ze the “white” primary phase Ag,Ce is surrounded by a cerium-rich black phase (Fig. 10). These results indicate that the congruently melting compound has a composition of 25at.z Ce which suggests that Ag,Ce has only a small homogeneity range.

171

Fig. 8. Ag-22at.%Ce, slow solidification (DTA): primary, Ag,Ce (black); remainder, eutectic silver (white) plus Ag,Ce (black). (Magnification, 160x.) Fig. 9. Ag-%lat.%Ce, slow solidification (DTA): primary Ag,Ce (black); remainder, eutectic silver (white) plus Ag,Ce (black). (Magnification, 160x.)

Fig. 10. Ag-26at.%Ce, fication, 160x.)

cast structure:

primary, Ag,Ce

(white); remainder, Ag,Ce (black). (Magni-

Fig. 11. Ag-lat.%Ce annealed at 730 “C for 4 months in an argon atmosphere and quenched in water: Ag,Ce (black, spheroidal) plus silver solid solution (white, twins). (Magnification, 160x.)

Further data for the construction of the partial phase diagram are obtained from alloys subjected to extended homogenization heat treatments. Figures ll16 show microstructures of Ag-Ce alloys containing 1-18 at.% Ce after annealing at 730,305 and 850 “C for periods of from 7 days to 6 months. Figure 11 shows the etched microstructure of the Ag-lat.%Ce alloy annealed for 4 months at 730°C. The cast structure (Fig. 2) has transformed to a typical annealing structure. The existence of a two-phase structure indicates that the maximum solubility of cerium in silver must be below 1 at.% Ce. Figure 12 shows the slow transformation of the cast structure of the hypereutectic alloy (12 at.% Ce (Fig. 5)) during different homogenization treatments at 730 “C. After 7 days no significant change is observed, at most a slight coarsening of the eutectic structure (Fig. 12(a)). Annealing for up to 2 months results in a strong coarsening of the eutectic structure as well as the

(4

(4 Fig. 12. Ag-lZat.%Ce annealed and quenched in water: (a) annealed at 730 “C for 7 days in an argon atmosphere (Ag,Ce (black); remainder, eutectic silver (white) plus Ag,Ce (black)); (b) annealed at 730 “C for 2 months in an argon atmosphere (Ag,Ce (black); remainder, eutectic silver (white) plus Ag,Ce (black)); (c)annealed at 730 “C for 4 months in an argon atmosphere (Ag,Ce (black) and silver solid solution (white)). (Ma~jfications, 160x.)

Fig. 13. Ag-lZat.%Ce annealed at 606 “C for 6 months in an argon atmosphere and quenched in water: Ag,Ce (black); remainder, eutectic silver (white) plus Ag,Ce (black). (Magnification, 160x.) Fig. 14. Ag-lBat.%Ce annealed at 730 “C for 7 days in an argon atmosphere and quenched in water: Ag,Ce (black) and silver solid solution (white). (Magnification, 160x.)

173

Fig. 15. Ag-18at.%Ce annealed at 806 “C for 7 days in an argon atmosphere and quenched in water: Ag,Ce (black); remainder, eutectic silver (white) plus Ag,Ce (black). (Magnification, 80x.) Fig. 16. Ag-18at.%Ce annealed at 850 “C for 7 days in an argon atmosphere and quenched in water: Ag,Ce (black); remainder, primary Ag,Ce (black, dendrites) and extremely fine eutectic silver (white) plus Ag,Ce (black) (compare Fig. 5). (Magnification, 400x.)

development of dendrites (Fig. 12(b)). Further homogenization for 4 months results in the annealing structure (Fig. 12(c)). The microstructure of Ag-12at.x Ce after heat treatment for 6 months at 805 “C, i.e. above the eutectic temperature, is shown in Fig. 13. The rounded annealing structure of Ag,Ce (black) is surrounded by the cast structure of the Ag-Ag,Ce eutectic. Figures 14-16 show the microstructures of Ag-18at.%Ce after annealing for 7 days at 730 “C, 805 “C and 850 “C respectively. The samples quenched in water show Ag-Ag,Ce after annealing at 750 “C (Fig. 14). The primary Ag,Ce has reacted to become Ag,Ce. The structure of Fig. 14 is similar to that observed after annealing the Ag-12at.%Ce alloy at 730 “C for 4 months (Fig. 12(c)) but has a higher proportion of Ag,Ce (black). Homogenization at 805 “C!results in Ag,Ce plus eutectic (Fig. 15) analogous to the results obtained for the Ag-12at.%Ce alloy (Fig. 13). Figure 16 shows the microstructure after annealing at 850 “C with subsequent quenching in water; coarsened Ag,Ce (black) and fine Ag,Ce dendrites with extremely fine eutectic are observed (similar to the result for slowly cooled Ag-12at.%Ce (Fig. 5)). The same heat treatments were performed on Ag-14at.%Ce alloys and microstructures analogous to those of the Ag18at.%Ce were obtained.

4. Discussion Figure 17 shows the results of the present investigation of the Ag-Ag,Ce system compared with the partial phase diagrams reported in the literature [8,9]. All the investigations show a eutectic and a peritectic reaction and a congruently melting compound. The Ag,Ce compound reported by Heumann and Preval[9] is confirmed to have the highest silver content. The existence of an Ag,Ce phase was ruled out on the basis of the microstructure data for alloys containing 12-22 at.% Ce. In particular, Ag-18at.%Ce shows the same structure

174

in the cast state (Fig. 7) as Ag-14at.%Ce (Fig. 6), apart from the higher proportion of the Ag,Ce phase. In addition, the annealed samples (18 and 12 at.% Ce) show similar microstructures after heat treatment at 730 “C (Figs. 14 and 12(c)) and at 805 “C (Figs. 15 and 13). The Ag,Ce phase reported by Delfino et al. [8] was also not observed by Heumann and Preval [9]. Compounds of the formula Ag,RE are also found in other systems, e.g. Ag,Sc [5,11] and Ag,La

cm

4

IO Cer1um ,

20

30

at.%

Fig. 17. Comparison of Ag-Ag,Ce -, this work.

partial phase diagram with published data: -.

-, ref. 8; ..., ref. 9;

On the basis of the present investigations (DTA and microstructure examination of cast and annealed alloys) the homogeneity range of the Ag,Ce phase (25at.x Ce) must be assumed to be relatively small. A composition Ag,,Ce,, (21.5at.x Ce) [S, 91 which increases to Ag,Ce (25at.x Ce) [9] is reported for this congruently melting phase. The melting point of this phase (maximum of the liquidus line) was determined in the present study to be 1030 “C which is somewhat lower than the values reported by Heumann and PrBval[9] (1053 “C) and Delfino et al. [S] (1040 “C). The Ag,Ce compound has been observed earlier by Vogel and Heumann [lo] and Steeb et al. [13]. The correct composition of this phase is believed to be Ag,Ce. The value of 793 “C obtained for the eutectic temperature in this work is slightly lower than that of 800 “C given by Delfino et al. [S] and Heumann and Preval[9]. The Ag-lOat.%Ce alloy represents exactly the eutectic composition found by DTA and microstructure investigations (Fig. 4). This composition agrees with the results of Heumann and P&al [9] and is slightly below that reported by Delfino et al. [8] (10.8 at.% Ce). The peritectic reaction was found to be at 815 “C and thus again is lower than the values reported in the literature (835 “C [S] and 842 “C [9]).

175

Acknowledgment The authors thank Professor Dr. A. M. Mulokozi, Chemistry Department, University of Dar-es-Salaam, for his interest and helpful discussions during the tenure of his Alexander-von-Humboldt fellowship at the Max-Planck-Institut fur Metallforschung.

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