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Phase diagram studies of the Ti–Bi–Zn system
Journal of Alloys and Compounds 385 (2004) 181–191
Phase diagram studies of the Ti–Bi–Zn system G.P. Vassilev a,∗ , K. Ishida b b
a University of Sofia, Faculty of Chemistry, 1164 Sofia, Bulgaria Department of Materials Science, Graduate School of Engineering, Tohoku University, Aoba-yama 02, Sendai 980-8579, Japan
Received 1 April 2004; received in revised form 3 May 2004; accepted 3 May 2004
Abstract The purpose of the present work is to obtain data about the Ti–Bi–Zn phase diagram. The latter was studied, by means of differential scanning calorimetry (DSC), optical and scanning electron microscopy. Ternary Ti–Bi–Zn alloys were prepared in quartz ampoules by classical metallurgical methods and annealed at 500, 615 and 900 ◦ C. The simultaneous existence of the ternary compounds TiBiZn and ∼Ti4 Bi3 Zn was found for the first time. The homogeneity range of the phase ∼Ti4 Bi3 Zn may include the following compositions: 47–51 at.% Ti; 36–38 at.% Bi; 12–16 at.% Zn. Unidentified crystals (or dendrites) having an approximate composition TiBi2 Zn have been observed. The compounds TiBiZn, ∼Ti4 Bi3 Zn and TiZn3 are stable until around 900 ◦ C. Thermal arrests at 255.4, 259.3, 263.5, 265.9, 266.6 and 267.8 ◦ C were detected. Evidence about the existence of a binary Ti–Bi compound with approximate formula TiX BiY (X ≈ Y) has been found. The isothermal sections of the ternary phase diagram at 500 ◦ C and at 615 ◦ C are constructed. © 2004 Elsevier B.V. All rights reserved. Keywords: Alloys; Scanning electron microscopy; Phase equilibria; Thermal analysis; Electrical conductivity
1. Introduction Although soldering and brazing are general ways to join materials, the method of transient liquid phase (TLP) bonding is more attractive in some cases. The general idea applied by the latter method is to achieve, at relatively low temperatures, the formation of a high-melting intermediate phase serving as adhesive layer between different substances. This approach is especially useful when at least one of the reacting elements has a low melting point. The alloying of high-melting (e.g. Ti) with low-melting elements (Bi, Zn) could also serve to regulate the working temperatures, wetting and mechanical properties of lead-free solders that are currently under development. Thus, the knowledge of the pertinent phase diagrams could be a source of important information. Recently, we revealed, the existence of two ternary compounds in the Ti–Bi–Zn system (TiBiZn and ∼Ti4 Bi3 Zn) [1]. Nevertheless, their simultaneous presence in the phase diagram was questionable. The purpose of the present work is to clarify which ternary compounds form,
∗ Corresponding author. Tel.: +359-81-22217-4010; fax: +359-296-25-438. E-mail address: [email protected] (G.P. Vassilev).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.05.012
and to obtain more details about the disposition of the phase boundaries. Considering the binary end-systems one should be aware that the Bi–Zn system is well studied [2–8], while Ti–Zn [7–23] and the Bi–Ti [7,8,24–29] phase diagrams are known only roughly (Table 1).
2. Experimental Shots of Bi and Zn (3N), small pieces cut from bulk arc-melted Ti and Ti-lamella (0.20 mm thick) have been used. Carefully weighted mixtures have been put into quartz tubes, sealed under vacuum (10−5 to 10−6 Tore) and annealed (Table 2) in order to produce the specimens. Three isoplethic sections (Table 2) of the ternary Ti–Bi–Zn system have been planned for studies: section A – alloys with mole fraction ratio Bi/Zn ≈ 1/1, section B – Bi/Ti ≈ 1/1, and section C – Zn/Ti ≈ 1/1. Preliminary studies have shown that Bi–Zn liquid alloys are rather sensitive to the annealing conditions. Because of strong positive deviations from Raoult’s law, even small inequality of the furnace temperature along the ampoule causes zinc-dew formation at the colder end. Thus, care has been taken in this respect. However, the formation of zinc-dew on the
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Table 1 Description of the solid phases relevant to the Ti–Bi–Zn system [8,12–23] Phase
Approximate concentration interval (at.%)
(Bi) (␣-Ti) (-Ti)a Ti3 Bi Ti2 Bi Ti2 Bie Ti8 Bi9 d TiZn16 b TiZn15 b,d TiZn10 d TiZn8 c,d Ti3 Zn22 c,d TiZn7 c TiZn5 d TiZn3 TiZn2 Ti2 Zn3 d TiZn Ti2 Zn (-Zn) a b c d e f
Pearson symbol
Space group
Prototype
≈100 at.% Bi ≈100 at.% Ti 0–17 at.% Sn ≈25.0 at.% Bi 33.3 at.% Bi
hR2 hP2 CI2 Tetragonalf tP12
52.9 at.% Bi 5.88 at.% Ti 6.2 at.% Ti 9.1 at.% Ti 11.1 at.% Ti ≈11.4 at.% Ti 12.5 at.% Ti 16.7 at.% Ti 25.0 at.% Ti 33.3 at.% Ti 40.0 at.% Ti 50.0 at.% Ti 66.7 at.% Ti ≈100 at.% Zn
tP34 – – – – – – – cP4 hP12 – cP2 tI6 hP2
¯ R3m P63 /mmc ¯ Im3m – P42 /mmc I4/mmm P4/nmm Cmcm – – – P42 /mbc – – ¯ Pm3m P63 /mmc – ¯ Pm3m I4/mmm P63 /mmc
␣As Mg W – Ti2 Bi; Cu2 Sb Eu4 As2 O Ti8 Bi9 TiZn16 FeZn13 ? – – Ti3 Zn22 – – AuCu3 MgZn2 – CsCl CuZr2 Mg
High temperature phase. Probably the same phase (TiZn16 ). Ti3 Zn22 might be identical with TiZn7 or with TiZn8 , or homogeneity range might exist. Phase whose existence or formula should be verified. Oxidized form [26]. Similar to DO19 , but with lower than hexagonal symmetry [24].
upper (colder) end of some tubes could not be prevented sometimes. In such a case the corresponding ampoule has been moved to a more appropriate position. It also happens that dark films form along the walls of the tubes so that the latter become obscure. This is probably due to bismuth propagation and usually such tubes break upon quenching (due to the difference of specific volumes of solid and liquid bismuth).
Another experimental complexity arises from the large negative standard Gibbs energies of formation of titanium oxides [30,31] so that the titanium contained in the alloys, could react with SiO2 (under some conditions). For this reason the application of long annealing times at high temperatures should be avoided. Nevertheless, our experience gives evidence that when such a reaction is observed the result is the formation of reactionary layers along the walls [1].
Table 2 Initial chemical compositions and heat treatment of the Ti–Bi–Zn specimens No.
XTi
XZn
XBi
Heat treatment
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
0.101 0.100 0.196 0.195 0.452 0.427 0.333 0.201 0.201 0.098 0.099 0.225 0.277 0.280 0.422 0.424
0.452 0.451 0.406 0.408 0.284 0.064 0.332 0.597 0.591 0.802 0.800 0.225 0.281 0.280 0.404 0.408
0.448 0.449 0.398 0.397 0.264 0.509 0.335 0.202 0.208 0.100 0.099 0.550 0.442 0.440 0.174 0.176
14 14 14 14 16 13 14 14 14 13 13 25 15 15 14 14
days days days days days days days days days days days days days days days days
at at at at at at at at at at at at at at at at
615 ◦ C, 615 ◦ C, 615 ◦ C, 615 ◦ C, 615 ◦ C, 500 ◦ C, 615 ◦ C, 615 ◦ C, 615 ◦ C, 500 ◦ C, 500 ◦ C, 615 ◦ C, 500 ◦ C, 500 ◦ C, 500 ◦ C, 500 ◦ C,
35 min at 900 ◦ C, 14 days at 500 ◦ C 35 min at 900 ◦ C, 14 days at 615 ◦ C, 7 h at 900 ◦ C 10 min at 900 ◦ C, 14 days at 500 ◦ C 10 min at 900 ◦ C, 14 days at 615 ◦ C, 7 h at 900 ◦ C 24 h at 900 ◦ C, 11 days at 500 ◦ C 23 h at 900 ◦ C, 10 days at 500 ◦ C 24 h at 900 ◦ C, 10 days at 615 ◦ C 35 min at 900 ◦ C, 2 days at 500 ◦ C 35 min at 900 ◦ C, 2 days at 615 ◦ C 40 min at 900 ◦ C, 10 days at 500 ◦ C 40 min at 900 ◦ C, 10 days at 615 ◦ C 7 h at 900 ◦ C 23 h at 900 ◦ C, 10 days at 500 ◦ C 23 h at 900 ◦ C, 10 days at 615 ◦ C 24 h at 900 ◦ C, 11 days at 500 ◦ C 24 h at 900 ◦ C, 11 days at 615 ◦ C
No.: consecutive number of the specimen, Xij : mole fraction of the corresponding element. Section A (XZn /XBi ≈ 1:1) nos. 1–5; section B (XTi /XBi ≈ 1:1) nos. 6–11; section C (XTi /XZn ≈ 1:1) nos. 12–16.
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Thus, the bulk of the alloys is, usually, prevented by contamination. The silicon and oxygen (together with Ti, Bi and Zn) content of the specimens has been determined by electron probe microanalyses (EPMA). The wave disperse system method (WDS) with sequential determination of the element concentrations was used. Alloys where any phase contained more than 5 at.% O or 0.1 at.% Si have been discarded. Optical microscopy observations and thermal analyses were also done. Differential scanning calorimetry (DSC) analyses have been performed using a NETZSCH DSC 404C instrument and later, a Mettler TA3000 System DSC. In the first case an evacuated quartz ampoule containing 252.8 mg powder of alloys no. 6 was prepared and two heating/cooling cycles were performed. The second analysis was done eight months after the first one. During this period the alloys were kept in air. For the experiment, the powder of alloys no. 6 was encapsulated into standard aluminum crucible and heated in pure argon stream.
3. Results and discussion The results obtained with specimens annealed at 500 ◦ C are shown in Table 3. Eight alloys have been successfully studied (nos. 1, 3, 5, 6, 8, 10, 13 and 15, see Table 2). Some of them (those rich in bismuth) corrode in air. An optical micrograph of specimen no. 6 (taken before the specimen
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corroded and converted to powder) is shown in Fig. 1. Four phase fields could be distinguished in the quenched sample: ∼Ti4 Bi3 Zn crystals [1], TiY BiX (with X ≈ Y) (probably related with the compound Ti8 Bi9 [26]) and Bi-rich and Zn-base areas (actually, both of them appertain to the liquid phase where separation occurs due to the Bi–Zn miscibility gap with critical temperature around 600 ◦ C). The degeneration took place within 10–14 days after the specimen has been brought in contact with air. The results obtained by EPMA with specimens annealed at 615 ◦ C (nos. 7, 9, 11, 14, 16) and 900 ◦ C (nos. 2, 4, 12) are shown in Table 4. A micrograph of specimen no. 9, annealed at 615 ◦ C, is shown in Fig. 2. Three phases: TiZn3 , TiBiZn and the former liquid solution could be distinguished. In Fig. 3a and 3b micrographs in characteristic X-rays of specimen no. 4, annealed at 900 ◦ C, are exposed. The white areas correspond to the former liquid phase while the gray needles belong to the ternary TiBiZn compound. The dark hexagonal crystals in the center and below appertain to the TiZn3 phase (Fig. 3a). The dark dendrites either have a composition near TiZn3 or correspond to the formula TiZn6 . The latter composition is intermediate between the probable Ti–Zn compounds (TiZn7 or TiZn5 ) that could form under these conditions or during cooling, from the liquid [19,20]. Crystals of the compound TiBiZn are situated in the center of Fig. 3b. Needles of the same phase are seen as well. The presence of relatively small amounts of TiZn3 and TiBiZn crystals (compared to the amounts of the dendrites
Fig. 1. Optical micrograph of specimen no. 6, annealed at 500 ◦ C (before suffering corrosion). Four phase fields are seen: 1: ternary compound Ti4 Bi3 Zn crystals; 2: Zn-rich areas; 3: TiX BiY compound crystals; 4: Bi-rich areas (a liquid phase miscibility gap is formed at the working temperature).
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Table 3 Results obtained with specimens annealed at 500 ◦ C No.
Phase
Average composition
Notes
1
TiBiZn
XTi = 0.342 ± 0.022 XBi = 0.331 ± 0.012
TiBiZn (4 p.) + liquid The overall composition of the liquid phase has not been measured, because it consists of Bi- and Zn-rich areas
XZn = 0.322 ± 0.022 3
TiBiZn
XTi = 0.334 ± 0.023 XBi = 0.350 ± 0.034
TiBiZn (4 p.) + liquid The overall composition of the liquid phase has not been measured, because it consisted of Bi- and Zn-rich areas
XZn = 0.323 ± 0.022 5
TiBiZn
XTi = 0.343 ± 0.004 XBi = 0.327 ± 0.007 XZn = 0.340 ± 0.008
TiBiZn, TiZn and Ti2 Bi TiBiZn gray crystals (4 p.)
5
Ti2 Bi
XTi = 0.664 ± 0.027 XBi = 0.328 ± 0.003 XZn = 0.030 ± 0.030
Gray crystals (3 p.)
5
TiZn
XTi = 0.520 ± 0.005 XBi = 0.003 ± 0.002 XZn = 0.478 ± 0.001
Dark crystals (2 p.) Small amount of Bi-rich liquid is observed
6
In around 2 weeks the specimen corroded (converted to powder) in air
6
Ti4 Bi3 Zn
XTi = 0.475 ± 0.05 XBi = 0.368 ± 0.02 XZn = 0.157 ± 0.01
Approximate formula Ti9 Bi7 Zn3 (6 p.)
6
Ti9 Bi8
Small dark crystals (3 p.)
6
Liquid
XTi = 0.527 ± 0.005 XBi = 0.472 ± 0.005 XZn = 0.002 ± 0.002 XTi = 0.006 ± 0.004 XBi = 0.911 ± 0.009 XZn = 0.083 ± 0.009
8
TiZn7
XTi = 0.125 XBi = 0.000 XZn = 0.875
TiZn7 , Liq, TBZ
8
TiBiZn
XTi = 0.339 ± 0.004 XBi = 0.324 ± 0.005 XZn = 0.335 ± 0.004
Gray crystals (6 p.)
10
TiZn7
XTi = 0.122 XBi = 0.000 XZn = 0.878
TiZn7 , Liq, TiBiZn
10
TiBiZn
XTi = 0.337 ± 0.003 XBi = 0.323 ± 0.005 XZn = 0.331 ± 0.005
TiZn7 , Liq, TiBiZn
13
TiBiZn
XTi = 0.340 ± 0.002 XBi = 0.324 ± 0.003 XZn = 0.335 ± 0.004
Gray crystals (6 p.)
13
Liq
XTi <0.01 XBi = 0.932 ± 0.014 XZn = 0.068 ± 0.014
6 a.a.
13
Ti4 Bi3 Zn
XTi = 0.508 ± 0.02 XBi = 0.363 ± 0.01 XZn = 0.129 ± 0.01
Dark crystals (5 p.)
13
Liq
XTi <0.01 XBi = 0.915 ± 0.002 XZn = 0.084 ± 0.003
2 a.a
13
Liq
XTi <0.001 XBi = 0.941 ± 0.006 XZn = 0.059 ± 0.006
4 a.a.
1 a.a. + 3 p.
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Table 3 (Continued ) No.
Phase
Average composition
Notes
13
u.s.
XTi = 0.195 XBi = 0.555 XZn = 0.250
Unidentified crystal, probably dendrite
13
u.s.
XTi = 0.254 XBi = 0.473 XZn = 0.273
Unidentified crystal, probably dendrite
15
TiBiZn
XTi = 0.339 ± 0.003 XBi = 0.330 ± 0.001 XZn = 0.331 ± 0.003
Gray crystals (6 p.)
XTi = 0.495 ± 0.004 XBi <0.001 XZn = 0.505 ± 0.004
Gray crystals (6 p.)
15
TiZn
No.: specimen’s number; p.: point, where electron microprobe analyses is done (the digit preceding (p.) shows the number of measurements); a.a.: area analyses (usually applied to the liquid phase); u.s.: unidentified stoichiometry; Liq: liquid phase.
of the same phases) indicates that the working temperature of 900 ◦ C is near the melting temperatures of the corresponding compounds. In Fig. 4 (micrograph of specimen no. 14 annealed at 900 ◦ C) the phase separation in the liquid phase region is observed (due to the miscibility gap in the Bi–Zn system): the dark areas consist of almost pure Zn while the light-gray areas have almost equiatomic ratios of Bi and Zn. The gray areas appertain to the TiBiZn compound. The rough surface areas represent corroded crystals of the Ti4 Bi3 Zn compound (but the exact composition could not be found). Small TiZn dendrites are found in the Zn-rich areas, while dendrites
with the approximate composition TiZn6 form in the Bi–Zn regions. Silicon containing layers form along the internal tube walls after annealing at 900 ◦ C. We also found that the titanium content in these layers is higher than in the adjacent phases, meaning that the partial titanium Gibbs energy has larger negative values. Undoubtedly, the formation of such layers is the result of the reaction between the SiO2 (the material of the tubes) and the titanium dissolved in the specimen. Nevertheless, a detailed study of these formations is out of the scope of this work. We have been satisfied finding out that Si has not penetrated the bulk of the samples.
Fig. 2. Micrograph, in characteristic X-rays (COMP), of specimen no. 9, annealed at 615 ◦ C. Three phases could be distinguished: 1: TiZn3 (dark areas); 2: TiBiZn (gray crystals); 3: liquid phase (white matrix with Zn-rich dendrites).
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Fig. 3. (a) Micrograph, in characteristic X-rays (COMP), of specimen no. 4, annealed at 900 ◦ C. The white areas represent the former liquid phase (Bi-rich) while the gray needles (sections of plate crystals) belong to the ternary TiBiZn compound. The dark hexagonal crystals in the center and below it are of TiZn3 . The dendrites (small dark particles) have compositions either near to TiZn3 or to TiZn6 . (b) Micrograph, in characteristic X-rays (COMP), of specimen no. 4, annealed at 900 ◦ C. The gray crystals in the center and the needles belong to the ternary TiBiZn compound while the white areas represent the former liquid phase (Bi-rich). The dendrites (small dark particles) have compositions either near to TiZn3 or to TiZn6 .
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Table 4 Results obtained with specimens annealed at 615 ◦ C No.
Phases
Measured composition
Notes
2
TiBiZn
XTi = 0.339 ± 0.002 XBi = 0.327 ± 0.003 XZn = 0.333 ± 0.002
TiBiZn crystals (5 p.) + liquid
2
Liq
n.a.
Regions of almost pure Zn and other regions with approximately equal Bi and Zn content
XTi = 0.136 ± 0.005 XBi = 0.002 ± 0.002 XZn = 0.862 ± 0.005
3 p. TiZn7 dendrites in the Bi + Zn regions
XTi = 0.330 ± 0.004 XBi = 0.326 ± 0.003 XZn = 0.343 ± 0.005
TiBiZn needles (7 p.) + liquid
XTi = 0.132 ± 0.006 XBi = 0.003 ± 0.002 XZn = 0.867 ± 0.008
8 p. Dendrites, probably of TiZn7
2
4
TiBiZn
4
4
TiZn3
XTi = 0.255 ± 0.009 XBi = 0.009 ± 0.004 XZn = 0.736 ± 0.005
3 p. Dendrites
4
Liq
XTi = 0.08 ± 0.01 XBi = 0.51 ± 0.06 XZn = 0.42 ± 0.05
2 a.a.
4
Liq
XTi <0.001 XBi = 0.932 ± 0.006 XZn = 0.068 ± 0.05
3 p.
7
TiBiZn
XTi = 0.332 ± 0.002 XBi = 0.327 ± 0.007 XZn = 0.340 ± 0.008
TiBiZn + TiZn Inclusions of liquid phase, containing 88 at.% Bi, 6 at.% Zn, 5 at.% Ti (5 p.) There is a Si-containing layer around the wall
7
TiZn
XTi = 0.515 ± 0.017 XBi = 0.003 ± 0.003 XZn = 0.474 ± 0.019
6 p.
9
TiBiZn
XTi = 0.329 ± 0.009 XBi = 0.335 ± 0.013 XZn = 0.336 ± 0.010
TiBiZn + Liq + TiZn3 6 p. There is a Si-containing layer around the wall
9
TiZn3
XTi = 0.260 ± 0.005 XBi = 0.014 ± 0.001 XZn = 0.7260 ± 0.00
9 p. Dark crystals
11
TiBiZn
XTi = 0.33 ± 0.01 XBi = 0.34 ± 0.02 XZn = 0.34 ± 0.01
TiBiZn + Liq + TiZn3 6 p. There is a Si-containing layer around the wall
11
TiZn3
XTi = 0.266 ± 0.006 XBi = 0.018 ± 0.001 XZn = 0.723 ± 0.006
9 p. Dark crystals
12
TiBiZn
XTi = 0.339 ± 0.003 XBi = 0.333 ± 0.004 XZn = 0.328 ± 0.004
TiBiZn + Liq Gray rectangular crystals (4 p.)
12
Liq
XTi = 0.01 ± 0.01 XBi = 0.95 ± 0.03 XZn = 0.04 ± 0.03
5 a.a.
12
Ti4 Bi3 Zn
XTi = 0.498 ± 0.009 XBi = 0.374 ± 0.004 XZn = 0.127 ± 0.009
5 p. Dark crystals with approximate formula Ti4 Bi3 Zn
14
TiBiZn
XTi = 0.339 ± 0.003 XBi = 0.330 ± 0.001 XZn = 0.331 ± 0.003
TiBiZn + Ti4 Bi3 Zn + Liq TiBiZn gray crystals (6 p.) The composition of the Ti4 Bi3 Zn crystals could not be measured quantitatively because of the rough surface
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Table 4 (Continued ) No.
Phases
Measured composition
Notes
14
Liq
XTi = 0.03 XBi = 0.90 XZn = 0.07
1 p.
16
TiBiZn
XTi = 0.334 ± 0.005 XBi = 0.332 ± 0.003 XZn = 0.334 ± 0.005
TiBiZn + TiZn 6 p. Gray crystals no. 16
16
TiZn
XTi = 0.495 ± 0.004 XBi <0.001 XZn = 0.505 ± 0.004
Gray crystals (6 p.)
No.: specimen’s number; p.: point, where electron microprobe analyses is done (the digit preceding (p.) shows the number of measurements); a.a.: area analyses (usually applied to the liquid phase); u.s.: unidentified stoichiometry; Liq: liquid phase.
In specimen no. 12, annealed at 900 ◦ C, except the recently found TiBiZn [1,32], another phase corresponding approximately to the formula Ti4 Bi3 Zn has been observed (Fig. 5). Its crystals exhibit fissures and are always in contact with the TiBiZn phase. Layers having similar composition have been observed in solid (␣-Ti)/liquid (Bi + Zn) diffusion couples annealed at 700 and 800 ◦ C [1]. Taking into account the present results and the authors’ previous data [1,32], we assume the existence of two stable ternary phases in the Ti–Bi–Zn system (TiBiZn and ∼Ti4 Bi3 Zn). Our observations show that the corrosion resistance of the phase Ti4 Bi3 Zn is intermediate between that of TiBiZn and
the binary TiX BiY . The former does not corrode in air, while the latter converts quickly to powder. The results of the first and the second DSC analyses are displayed in Figs. 6 and 7, respectively. The mass was controlled before and after the heating, but mass loss has not been found. Thus, the hypothesis that the TiX BiY and Ti4 Bi3 Zn absorb air humidity could not be maintained, while the assumption for a reaction with the oxygen is highly probable. Five endothermic thermal peaks (Fig. 7) were observed in the interval 255–268 ◦ C. The two strongest peaks (F and L) are reproducible, especially the second one (at 267.8 ◦ C).
Fig. 4. Micrograph, in back scattered electrons (SEI), of specimen no. 14, annealed at 900 ◦ C. The gray areas (1) appertain to the ternary TiBiZn compound. Separation in the crystallized liquid phase is observed: the dark areas (2) consist of almost pure Zn; the light-gray areas (3) have more or less equiatomic ratios of Bi and Zn. The rough-surface areas (4) represent corroded crystals of Ti4 Bi3 Zn. Small TiZn dendrites are found in the Zn-rich areas, while dendrites with approximate composition TiZn6 form in the Bi–Zn regions.
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Fig. 5. Micrograph, in characteristic X-rays, of specimen no. 12, annealed at 900 ◦ C. The gray crystals appertain to the ternary TiBiZn compound, the dark gray areas (fissures due to corrosion are seen in them) correspond approximately to the formula Ti4 Bi3 Zn (the oxygen content has not been taken into account). The white matrix belongs to the Bi-rich liquid phase.
They correspond to the peaks B and C (Fig. 6), while the hypothetical peak A was not observed. The thermal arrests at around 255 ◦ C are related with the binary eutectic reaction (254.5 ◦ C, 8.1 at.% Zn) in the Bi–Zn system [8]. A ternary
Fig. 6. Thermal curves of the corroded specimen no. 6, obtained by NETZSCH DSC 404C. Heating rate 10 K min−1 . The mass of the specimen is 252.8 mg. Curves 1 and 2 correspond to the heating to 302 ◦ C and cooling to 192 ◦ C branches, respectively, during the first cycle. The endothermic peaks A (≈230 ◦ C), B (≈250 ◦ C) and C (≈260 ◦ C) are observed during the first heating, while the peak C only (curve 3) is registered during the second heating (up to 406 ◦ C). Curve 4, corresponds to the second cooling. The temperature (◦ C) is plotted along the abscissa, and DSC units (mW) – along the ordinate. The endothermic peaks show downward.
eutectic reaction with the following reactants could be anticipated as well: LE ⇔ (Bi) + (η-Zn) + TiX BiY
(1)
Fig. 7. Thermal curves of the corroded specimen no. 6, obtained by Mettler TA3000 System DSC. Heating rate 10 K min−1 . The mass of the specimen is 25.57 mg. Two sequential heatings from 50 to 280 ◦ C have been performed (curves 1 and 2, respectively). The peaks F and L appear in both cycles, while G and H – in the first cycle and J and K – in the second cycle. The temperatures of the thermal arrests F, G, H, J, K and L are as follows: 255.4, 259.3, 263.5, 265.9, 266.6 and 267.8 ◦ C, respectively.
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Fig. 8. Temperature dependence of the dc specific conductivity (ρ, Ohm cm) for specimen no. 6. The inversed temperature is plotted along the abscissa and specific conductivity (䊊)—along the ordinate.
Here, LE is the liquid phase with eutectic composition, (Bi) and (-Zn) – phases on the basis of the pure pertinent components. The rest of the peaks were not identified, including the strong reproducible peak (C in Fig. 6, and L in Fig. 7) at around 268 ◦ C. The peaks G and H (Fig. 7, curve 1) might be related with the peaks J and K (Fig. 7, curve 2), respectively, because they appear as couples, shifted by a few degrees.
Fig. 9. Isothermal section of the Ti–Bi–Zn phase diagram at 500 ◦ C. The experimental data are presented as follows: ( ) nominal alloys compositions; ( ) liquid phase (L); (䊉) TiX BiY (X ≈ Y); (䉫) TiZn; (×) TiZn3 ; 夹 TiZn7 ; (䊏) Ti4 Bi3 Zn; ( ) TiBiZn. The miscibility gap is represented by dots. The thick solid lines represent the phase field boundaries; the thin—the tie-lines. The short dashes stand for the phase field boundaries the most recommendable for further studies.
The electrical conductivity of specimen no. 6 was measured as well. For this purpose a tablet was prepared pressurizing the powder and the four-point method was used. The Arrhenius temperature dependence of the dc specific conductivity (ρ, Ohm cm) is shown in Fig. 8. The specific conductivity increases with increasing temperature. Moreover, Hall voltages are not found at all, suggesting that a part of the material has metallic character (that is confirmed by the DSC studies) while the crystallite boundaries or surfaces are non-metallic. In Figs. 9 and 10 isothermal sections of the Ti–Bi–Zn system at respectively 500 and 615 ◦ C are plotted, based on this work as well as on previous authors’ studies [1,19,20,32]. Pertinent data obtained at 900 ◦ C (specimens 2, 4, 12) are plotted in Fig. 10 too. This is admissible because of the stoichiometric character of the binary and ternary compounds (i.e. vertical phase boundaries). Actually, there are data that the phase Ti4 Bi3 Zn is non-stoichiometric. Its homogeneity range might include the following compositions: 47–51 at.% Ti; 36–38 at.% Bi; 12–16 at.% Zn (Tables 3 and 4; see also [1]). Anyhow, for the sake of simplicity, it has been plotted as stoichiometric compound in Figs. 9 and 10. The melting temperature of the compound TiZn7 is around 600 ◦ C [19,20] and in this study only probable TiZn7 dendrites have been observed in the specimens annealed at 615 and 900 ◦ C. For this reason TiZn7 has not been presented in Fig. 10. Unidentified crystals (probably dendrites) with an approximate composition TiBi2 Zn have been observed in specimen no. 13 (annealed at 500 ◦ C).
Fig. 10. Isothermal section of the Ti–Bi–Zn phase diagram at 615 ◦ C. The experimental data are presented as follows: ( ) nominal alloys compositions (specimens nos. 2, 4 and 12 are annealed at 900 ◦ C); ( ) liquid phase (L); (䉫) TiZn; (×) TiZn3 ; (䊏) Ti4 Bi3 Zn; ( ) TiBiZn. The thick solid lines represent the phase field boundaries; the thin—the tie-lines. The short dashes stand for the phase field boundaries the most recommendable for further studies.
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The Ti-solubility in molten zinc is around 2.0 at.% Ti and around 4.4 at.% Ti at 500 and 615 ◦ C, respectively. The zinc solubility in (␣-Ti) probably does not depend strongly on the temperature and a Zn content of ∼9 at.% is to be expected [8,19,20] in this temperature interval. At the working temperatures (500 and 615 ◦ C), the bismuth solubility in (␣-Ti) and the titanium solubility in molten bismuth are expected to be, respectively, around 1.5 at.% Bi and 1 at.% Ti [8,29]. Concerning the construction of the ternary system at 900 ◦ C, one should be aware that, at this temperature, ternary solid solutions based on (␣-Ti) are expected to be in equilibrium with Ti3 Bi and with TiZn (rather then with Ti2 Zn [33]) in the Ti–Bi and Ti–Zn systems, respectively.
4. Conclusion The phase equilibria in the system Ti–Bi–Zn have been studied at 500, 615 and 900 ◦ C. The data show the existence of two stable ternary compounds – the stoichiometric TiBiZn and the non-stoichiometric ∼Ti4 Bi3 Zn. The homogeneity range of the latter phase may include the following compositions: 47–51 at.% Ti; 36–38 at.% Bi; 12–16 at.% Zn. Thermal arrests at 255.4, 259.3, 263.5, 265.9, 266.6 and 267.8 ◦ C have been found. The first one is related with the binary eutectic reaction or with a ternary one. The melting temperatures of the compounds TiBiZn, ∼Ti4 Bi3 Zn and TiZn3 are above 900 ◦ C. Tentative isothermal sections of the Ti–Bi–Zn system have been constructed at 500 ◦ C and at 615 ◦ C. In the pertinent end-systems, confirmations about the existence of the following binary compounds have been found: TiX BiY (X ≈ Y), Ti3 Bi, Ti2 Bi, TiZn, TiZn3 , TiZn7 . The Zn solubility in the ternary extensions of the Ti–Bi compounds is superior to the Bi solubility in the Ti–Zn compounds. Acknowledgements The authors appreciatively acknowledge a fellowship granted by the Japanese Society for Promotion of Science to one of them (G.P.V.). The latter also thanks Dr. X.J. Liu for helpful discussions and Mr. Takaku (both with Tohoku University, Sendai, Japan) who kindly was of assistance during the work with the scanning electron microscope. The help of Dr. N. Avramova and Prof. B. Arnaudov (both with University of Sofia) with the Mettler DSC and
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specific conductivity experiments, respectively, is gratefully accepted.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
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G.P. Vassilev, Cryst. Res. Technol. (2004), accepted for publication. W. Seith, H. Johnen, J. Wagner, Z. Metallkd. 46 (1965) 773. K. Okamjima, H. Sakao, Trans. Jpn. Inst. Met. 21 (1980) 226. V. Glazov, S. Kim, V. Timoshenko, K.B. Nurov, Zh. Fiz. Khim. (in Russian) 62 (1988) 2510. R. Khairulin, S. Stankus, J. Alloys Compd. 234 (1996) 260. D. Malakhov, CALPHAD 24 (2000) 1. M. Hansen, K. Anderko, Constitution of Binary Alloys, second ed., McGraw-Hill, New York, 1958, p. 1488. T. Massalski, CD ROM: Binary Alloy Phase Diagrams, ASM International, OH, USA, 1996. G. Leone, P. Niessen, H.W. Kerr, Metall. Trans. B 6 (1975) 503. G. Leone, H. Kerr, J. Crystal Growth 32 (1976) 111. E. Gebhardt, Z. Metallkd. 33 (1941) 355. W. Heine, U. Zwicker, Z. Metallkd. 53 (1962) 380. J. Murray, Bull. Alloy Phase Diagr. 5 (1984) 52. X. Chen, W. Jeitschko, M. Danebrock, C. Evers, K. Wagner, J. Sol. St. Chem. 11 (1995) 219. M. Saillard, G. Develey, C. Becle, Acta Crystallogr. B37 (1981) 224. T. Gloriant, G. Reumont, P. Perrot, Z. Metallkd. 88 (1997) 539. P. Villars (Ed.), Pauling File Binaries Edition, Inorganic Materials Database and Design System, CD-ROM, Germany, ISBN 3-00-009043-6. W. Piotrowski, Zeszyty Nauk. Politech. Lodz. Mech. 10 (1963) 33 (in Polish). G.P. Vassilev, X.J. Liu, K. Ishida, J. Alloys Compd. 375 (2004) 162. G.P. Vassilev, Contribution to the zinc-rich side of the Ti–Zn system, Z. Metallkd., accepted for publication. P. Pietrokowsky, Trans. AIME 200 (1954) 219. F. Laves, H. Wallbaum, Naturwissienschaften 27 (1939) 674. K. Schubert, K. Frank, R. Gohle, A. Maldonado, H.G. Meissner, A. Raman, W. Rossteutscher, Naturwissienschaften 50 (1963) 41. H. Nowotny, J. Pesl, Monatsh. Chem. 82 (1951) 344. I. Obinata, Y. Takeuchi, S. Saikawa, Trans. ASM 52 (1960) 1059. C. Richter, W. Jeitschko, J. Solid State Chem. 134 (1997) 26. H. Auer-Welsbach, H. Nowotny, A. Kohl, Monatsh. Chem. 89 (1958) 154. J. Weeks, Trans. ASM 58 (1965) 302. J. Murray, Bull. Alloy Phase Diagr. 5 (1984) 610. R. Swalin, Thermodynamics of Solids, J. Wiley & Sons, New York, London, 1961, p. 316. I. Barin, Thermochemical Data of Pure Substances, VCH Verlags, Weinheim, Part II, 1993, p. 1739. G.P. Vassilev, E.S. Dobrev, J.-C. Tedenac, T. Czeppe, in: Proceedings of the VIIIth Seminar on Diffusion and Thermodynamics of Solids, 4–6 September 2002, Brno, Czech Republic. S. Ono, H. Ohtani, M. Hasebe, in: Proceedings of 9th Symposium on Microjoining and Assembly Technology in Electronics, 6–7 February 2003, Yokohama, Japan.
Phase diagram studies of the Ti–Bi–Zn system G.P. Vassilev a,∗ , K. Ishida b b
a University of Sofia, Faculty of Chemistry, 1164 Sofia, Bulgaria Department of Materials Science, Graduate School of Engineering, Tohoku University, Aoba-yama 02, Sendai 980-8579, Japan
Received 1 April 2004; received in revised form 3 May 2004; accepted 3 May 2004
Abstract The purpose of the present work is to obtain data about the Ti–Bi–Zn phase diagram. The latter was studied, by means of differential scanning calorimetry (DSC), optical and scanning electron microscopy. Ternary Ti–Bi–Zn alloys were prepared in quartz ampoules by classical metallurgical methods and annealed at 500, 615 and 900 ◦ C. The simultaneous existence of the ternary compounds TiBiZn and ∼Ti4 Bi3 Zn was found for the first time. The homogeneity range of the phase ∼Ti4 Bi3 Zn may include the following compositions: 47–51 at.% Ti; 36–38 at.% Bi; 12–16 at.% Zn. Unidentified crystals (or dendrites) having an approximate composition TiBi2 Zn have been observed. The compounds TiBiZn, ∼Ti4 Bi3 Zn and TiZn3 are stable until around 900 ◦ C. Thermal arrests at 255.4, 259.3, 263.5, 265.9, 266.6 and 267.8 ◦ C were detected. Evidence about the existence of a binary Ti–Bi compound with approximate formula TiX BiY (X ≈ Y) has been found. The isothermal sections of the ternary phase diagram at 500 ◦ C and at 615 ◦ C are constructed. © 2004 Elsevier B.V. All rights reserved. Keywords: Alloys; Scanning electron microscopy; Phase equilibria; Thermal analysis; Electrical conductivity
1. Introduction Although soldering and brazing are general ways to join materials, the method of transient liquid phase (TLP) bonding is more attractive in some cases. The general idea applied by the latter method is to achieve, at relatively low temperatures, the formation of a high-melting intermediate phase serving as adhesive layer between different substances. This approach is especially useful when at least one of the reacting elements has a low melting point. The alloying of high-melting (e.g. Ti) with low-melting elements (Bi, Zn) could also serve to regulate the working temperatures, wetting and mechanical properties of lead-free solders that are currently under development. Thus, the knowledge of the pertinent phase diagrams could be a source of important information. Recently, we revealed, the existence of two ternary compounds in the Ti–Bi–Zn system (TiBiZn and ∼Ti4 Bi3 Zn) [1]. Nevertheless, their simultaneous presence in the phase diagram was questionable. The purpose of the present work is to clarify which ternary compounds form,
∗ Corresponding author. Tel.: +359-81-22217-4010; fax: +359-296-25-438. E-mail address: [email protected] (G.P. Vassilev).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.05.012
and to obtain more details about the disposition of the phase boundaries. Considering the binary end-systems one should be aware that the Bi–Zn system is well studied [2–8], while Ti–Zn [7–23] and the Bi–Ti [7,8,24–29] phase diagrams are known only roughly (Table 1).
2. Experimental Shots of Bi and Zn (3N), small pieces cut from bulk arc-melted Ti and Ti-lamella (0.20 mm thick) have been used. Carefully weighted mixtures have been put into quartz tubes, sealed under vacuum (10−5 to 10−6 Tore) and annealed (Table 2) in order to produce the specimens. Three isoplethic sections (Table 2) of the ternary Ti–Bi–Zn system have been planned for studies: section A – alloys with mole fraction ratio Bi/Zn ≈ 1/1, section B – Bi/Ti ≈ 1/1, and section C – Zn/Ti ≈ 1/1. Preliminary studies have shown that Bi–Zn liquid alloys are rather sensitive to the annealing conditions. Because of strong positive deviations from Raoult’s law, even small inequality of the furnace temperature along the ampoule causes zinc-dew formation at the colder end. Thus, care has been taken in this respect. However, the formation of zinc-dew on the
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Table 1 Description of the solid phases relevant to the Ti–Bi–Zn system [8,12–23] Phase
Approximate concentration interval (at.%)
(Bi) (␣-Ti) (-Ti)a Ti3 Bi Ti2 Bi Ti2 Bie Ti8 Bi9 d TiZn16 b TiZn15 b,d TiZn10 d TiZn8 c,d Ti3 Zn22 c,d TiZn7 c TiZn5 d TiZn3 TiZn2 Ti2 Zn3 d TiZn Ti2 Zn (-Zn) a b c d e f
Pearson symbol
Space group
Prototype
≈100 at.% Bi ≈100 at.% Ti 0–17 at.% Sn ≈25.0 at.% Bi 33.3 at.% Bi
hR2 hP2 CI2 Tetragonalf tP12
52.9 at.% Bi 5.88 at.% Ti 6.2 at.% Ti 9.1 at.% Ti 11.1 at.% Ti ≈11.4 at.% Ti 12.5 at.% Ti 16.7 at.% Ti 25.0 at.% Ti 33.3 at.% Ti 40.0 at.% Ti 50.0 at.% Ti 66.7 at.% Ti ≈100 at.% Zn
tP34 – – – – – – – cP4 hP12 – cP2 tI6 hP2
¯ R3m P63 /mmc ¯ Im3m – P42 /mmc I4/mmm P4/nmm Cmcm – – – P42 /mbc – – ¯ Pm3m P63 /mmc – ¯ Pm3m I4/mmm P63 /mmc
␣As Mg W – Ti2 Bi; Cu2 Sb Eu4 As2 O Ti8 Bi9 TiZn16 FeZn13 ? – – Ti3 Zn22 – – AuCu3 MgZn2 – CsCl CuZr2 Mg
High temperature phase. Probably the same phase (TiZn16 ). Ti3 Zn22 might be identical with TiZn7 or with TiZn8 , or homogeneity range might exist. Phase whose existence or formula should be verified. Oxidized form [26]. Similar to DO19 , but with lower than hexagonal symmetry [24].
upper (colder) end of some tubes could not be prevented sometimes. In such a case the corresponding ampoule has been moved to a more appropriate position. It also happens that dark films form along the walls of the tubes so that the latter become obscure. This is probably due to bismuth propagation and usually such tubes break upon quenching (due to the difference of specific volumes of solid and liquid bismuth).
Another experimental complexity arises from the large negative standard Gibbs energies of formation of titanium oxides [30,31] so that the titanium contained in the alloys, could react with SiO2 (under some conditions). For this reason the application of long annealing times at high temperatures should be avoided. Nevertheless, our experience gives evidence that when such a reaction is observed the result is the formation of reactionary layers along the walls [1].
Table 2 Initial chemical compositions and heat treatment of the Ti–Bi–Zn specimens No.
XTi
XZn
XBi
Heat treatment
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
0.101 0.100 0.196 0.195 0.452 0.427 0.333 0.201 0.201 0.098 0.099 0.225 0.277 0.280 0.422 0.424
0.452 0.451 0.406 0.408 0.284 0.064 0.332 0.597 0.591 0.802 0.800 0.225 0.281 0.280 0.404 0.408
0.448 0.449 0.398 0.397 0.264 0.509 0.335 0.202 0.208 0.100 0.099 0.550 0.442 0.440 0.174 0.176
14 14 14 14 16 13 14 14 14 13 13 25 15 15 14 14
days days days days days days days days days days days days days days days days
at at at at at at at at at at at at at at at at
615 ◦ C, 615 ◦ C, 615 ◦ C, 615 ◦ C, 615 ◦ C, 500 ◦ C, 615 ◦ C, 615 ◦ C, 615 ◦ C, 500 ◦ C, 500 ◦ C, 615 ◦ C, 500 ◦ C, 500 ◦ C, 500 ◦ C, 500 ◦ C,
35 min at 900 ◦ C, 14 days at 500 ◦ C 35 min at 900 ◦ C, 14 days at 615 ◦ C, 7 h at 900 ◦ C 10 min at 900 ◦ C, 14 days at 500 ◦ C 10 min at 900 ◦ C, 14 days at 615 ◦ C, 7 h at 900 ◦ C 24 h at 900 ◦ C, 11 days at 500 ◦ C 23 h at 900 ◦ C, 10 days at 500 ◦ C 24 h at 900 ◦ C, 10 days at 615 ◦ C 35 min at 900 ◦ C, 2 days at 500 ◦ C 35 min at 900 ◦ C, 2 days at 615 ◦ C 40 min at 900 ◦ C, 10 days at 500 ◦ C 40 min at 900 ◦ C, 10 days at 615 ◦ C 7 h at 900 ◦ C 23 h at 900 ◦ C, 10 days at 500 ◦ C 23 h at 900 ◦ C, 10 days at 615 ◦ C 24 h at 900 ◦ C, 11 days at 500 ◦ C 24 h at 900 ◦ C, 11 days at 615 ◦ C
No.: consecutive number of the specimen, Xij : mole fraction of the corresponding element. Section A (XZn /XBi ≈ 1:1) nos. 1–5; section B (XTi /XBi ≈ 1:1) nos. 6–11; section C (XTi /XZn ≈ 1:1) nos. 12–16.
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Thus, the bulk of the alloys is, usually, prevented by contamination. The silicon and oxygen (together with Ti, Bi and Zn) content of the specimens has been determined by electron probe microanalyses (EPMA). The wave disperse system method (WDS) with sequential determination of the element concentrations was used. Alloys where any phase contained more than 5 at.% O or 0.1 at.% Si have been discarded. Optical microscopy observations and thermal analyses were also done. Differential scanning calorimetry (DSC) analyses have been performed using a NETZSCH DSC 404C instrument and later, a Mettler TA3000 System DSC. In the first case an evacuated quartz ampoule containing 252.8 mg powder of alloys no. 6 was prepared and two heating/cooling cycles were performed. The second analysis was done eight months after the first one. During this period the alloys were kept in air. For the experiment, the powder of alloys no. 6 was encapsulated into standard aluminum crucible and heated in pure argon stream.
3. Results and discussion The results obtained with specimens annealed at 500 ◦ C are shown in Table 3. Eight alloys have been successfully studied (nos. 1, 3, 5, 6, 8, 10, 13 and 15, see Table 2). Some of them (those rich in bismuth) corrode in air. An optical micrograph of specimen no. 6 (taken before the specimen
183
corroded and converted to powder) is shown in Fig. 1. Four phase fields could be distinguished in the quenched sample: ∼Ti4 Bi3 Zn crystals [1], TiY BiX (with X ≈ Y) (probably related with the compound Ti8 Bi9 [26]) and Bi-rich and Zn-base areas (actually, both of them appertain to the liquid phase where separation occurs due to the Bi–Zn miscibility gap with critical temperature around 600 ◦ C). The degeneration took place within 10–14 days after the specimen has been brought in contact with air. The results obtained by EPMA with specimens annealed at 615 ◦ C (nos. 7, 9, 11, 14, 16) and 900 ◦ C (nos. 2, 4, 12) are shown in Table 4. A micrograph of specimen no. 9, annealed at 615 ◦ C, is shown in Fig. 2. Three phases: TiZn3 , TiBiZn and the former liquid solution could be distinguished. In Fig. 3a and 3b micrographs in characteristic X-rays of specimen no. 4, annealed at 900 ◦ C, are exposed. The white areas correspond to the former liquid phase while the gray needles belong to the ternary TiBiZn compound. The dark hexagonal crystals in the center and below appertain to the TiZn3 phase (Fig. 3a). The dark dendrites either have a composition near TiZn3 or correspond to the formula TiZn6 . The latter composition is intermediate between the probable Ti–Zn compounds (TiZn7 or TiZn5 ) that could form under these conditions or during cooling, from the liquid [19,20]. Crystals of the compound TiBiZn are situated in the center of Fig. 3b. Needles of the same phase are seen as well. The presence of relatively small amounts of TiZn3 and TiBiZn crystals (compared to the amounts of the dendrites
Fig. 1. Optical micrograph of specimen no. 6, annealed at 500 ◦ C (before suffering corrosion). Four phase fields are seen: 1: ternary compound Ti4 Bi3 Zn crystals; 2: Zn-rich areas; 3: TiX BiY compound crystals; 4: Bi-rich areas (a liquid phase miscibility gap is formed at the working temperature).
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Table 3 Results obtained with specimens annealed at 500 ◦ C No.
Phase
Average composition
Notes
1
TiBiZn
XTi = 0.342 ± 0.022 XBi = 0.331 ± 0.012
TiBiZn (4 p.) + liquid The overall composition of the liquid phase has not been measured, because it consists of Bi- and Zn-rich areas
XZn = 0.322 ± 0.022 3
TiBiZn
XTi = 0.334 ± 0.023 XBi = 0.350 ± 0.034
TiBiZn (4 p.) + liquid The overall composition of the liquid phase has not been measured, because it consisted of Bi- and Zn-rich areas
XZn = 0.323 ± 0.022 5
TiBiZn
XTi = 0.343 ± 0.004 XBi = 0.327 ± 0.007 XZn = 0.340 ± 0.008
TiBiZn, TiZn and Ti2 Bi TiBiZn gray crystals (4 p.)
5
Ti2 Bi
XTi = 0.664 ± 0.027 XBi = 0.328 ± 0.003 XZn = 0.030 ± 0.030
Gray crystals (3 p.)
5
TiZn
XTi = 0.520 ± 0.005 XBi = 0.003 ± 0.002 XZn = 0.478 ± 0.001
Dark crystals (2 p.) Small amount of Bi-rich liquid is observed
6
In around 2 weeks the specimen corroded (converted to powder) in air
6
Ti4 Bi3 Zn
XTi = 0.475 ± 0.05 XBi = 0.368 ± 0.02 XZn = 0.157 ± 0.01
Approximate formula Ti9 Bi7 Zn3 (6 p.)
6
Ti9 Bi8
Small dark crystals (3 p.)
6
Liquid
XTi = 0.527 ± 0.005 XBi = 0.472 ± 0.005 XZn = 0.002 ± 0.002 XTi = 0.006 ± 0.004 XBi = 0.911 ± 0.009 XZn = 0.083 ± 0.009
8
TiZn7
XTi = 0.125 XBi = 0.000 XZn = 0.875
TiZn7 , Liq, TBZ
8
TiBiZn
XTi = 0.339 ± 0.004 XBi = 0.324 ± 0.005 XZn = 0.335 ± 0.004
Gray crystals (6 p.)
10
TiZn7
XTi = 0.122 XBi = 0.000 XZn = 0.878
TiZn7 , Liq, TiBiZn
10
TiBiZn
XTi = 0.337 ± 0.003 XBi = 0.323 ± 0.005 XZn = 0.331 ± 0.005
TiZn7 , Liq, TiBiZn
13
TiBiZn
XTi = 0.340 ± 0.002 XBi = 0.324 ± 0.003 XZn = 0.335 ± 0.004
Gray crystals (6 p.)
13
Liq
XTi <0.01 XBi = 0.932 ± 0.014 XZn = 0.068 ± 0.014
6 a.a.
13
Ti4 Bi3 Zn
XTi = 0.508 ± 0.02 XBi = 0.363 ± 0.01 XZn = 0.129 ± 0.01
Dark crystals (5 p.)
13
Liq
XTi <0.01 XBi = 0.915 ± 0.002 XZn = 0.084 ± 0.003
2 a.a
13
Liq
XTi <0.001 XBi = 0.941 ± 0.006 XZn = 0.059 ± 0.006
4 a.a.
1 a.a. + 3 p.
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Table 3 (Continued ) No.
Phase
Average composition
Notes
13
u.s.
XTi = 0.195 XBi = 0.555 XZn = 0.250
Unidentified crystal, probably dendrite
13
u.s.
XTi = 0.254 XBi = 0.473 XZn = 0.273
Unidentified crystal, probably dendrite
15
TiBiZn
XTi = 0.339 ± 0.003 XBi = 0.330 ± 0.001 XZn = 0.331 ± 0.003
Gray crystals (6 p.)
XTi = 0.495 ± 0.004 XBi <0.001 XZn = 0.505 ± 0.004
Gray crystals (6 p.)
15
TiZn
No.: specimen’s number; p.: point, where electron microprobe analyses is done (the digit preceding (p.) shows the number of measurements); a.a.: area analyses (usually applied to the liquid phase); u.s.: unidentified stoichiometry; Liq: liquid phase.
of the same phases) indicates that the working temperature of 900 ◦ C is near the melting temperatures of the corresponding compounds. In Fig. 4 (micrograph of specimen no. 14 annealed at 900 ◦ C) the phase separation in the liquid phase region is observed (due to the miscibility gap in the Bi–Zn system): the dark areas consist of almost pure Zn while the light-gray areas have almost equiatomic ratios of Bi and Zn. The gray areas appertain to the TiBiZn compound. The rough surface areas represent corroded crystals of the Ti4 Bi3 Zn compound (but the exact composition could not be found). Small TiZn dendrites are found in the Zn-rich areas, while dendrites
with the approximate composition TiZn6 form in the Bi–Zn regions. Silicon containing layers form along the internal tube walls after annealing at 900 ◦ C. We also found that the titanium content in these layers is higher than in the adjacent phases, meaning that the partial titanium Gibbs energy has larger negative values. Undoubtedly, the formation of such layers is the result of the reaction between the SiO2 (the material of the tubes) and the titanium dissolved in the specimen. Nevertheless, a detailed study of these formations is out of the scope of this work. We have been satisfied finding out that Si has not penetrated the bulk of the samples.
Fig. 2. Micrograph, in characteristic X-rays (COMP), of specimen no. 9, annealed at 615 ◦ C. Three phases could be distinguished: 1: TiZn3 (dark areas); 2: TiBiZn (gray crystals); 3: liquid phase (white matrix with Zn-rich dendrites).
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Fig. 3. (a) Micrograph, in characteristic X-rays (COMP), of specimen no. 4, annealed at 900 ◦ C. The white areas represent the former liquid phase (Bi-rich) while the gray needles (sections of plate crystals) belong to the ternary TiBiZn compound. The dark hexagonal crystals in the center and below it are of TiZn3 . The dendrites (small dark particles) have compositions either near to TiZn3 or to TiZn6 . (b) Micrograph, in characteristic X-rays (COMP), of specimen no. 4, annealed at 900 ◦ C. The gray crystals in the center and the needles belong to the ternary TiBiZn compound while the white areas represent the former liquid phase (Bi-rich). The dendrites (small dark particles) have compositions either near to TiZn3 or to TiZn6 .
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Table 4 Results obtained with specimens annealed at 615 ◦ C No.
Phases
Measured composition
Notes
2
TiBiZn
XTi = 0.339 ± 0.002 XBi = 0.327 ± 0.003 XZn = 0.333 ± 0.002
TiBiZn crystals (5 p.) + liquid
2
Liq
n.a.
Regions of almost pure Zn and other regions with approximately equal Bi and Zn content
XTi = 0.136 ± 0.005 XBi = 0.002 ± 0.002 XZn = 0.862 ± 0.005
3 p. TiZn7 dendrites in the Bi + Zn regions
XTi = 0.330 ± 0.004 XBi = 0.326 ± 0.003 XZn = 0.343 ± 0.005
TiBiZn needles (7 p.) + liquid
XTi = 0.132 ± 0.006 XBi = 0.003 ± 0.002 XZn = 0.867 ± 0.008
8 p. Dendrites, probably of TiZn7
2
4
TiBiZn
4
4
TiZn3
XTi = 0.255 ± 0.009 XBi = 0.009 ± 0.004 XZn = 0.736 ± 0.005
3 p. Dendrites
4
Liq
XTi = 0.08 ± 0.01 XBi = 0.51 ± 0.06 XZn = 0.42 ± 0.05
2 a.a.
4
Liq
XTi <0.001 XBi = 0.932 ± 0.006 XZn = 0.068 ± 0.05
3 p.
7
TiBiZn
XTi = 0.332 ± 0.002 XBi = 0.327 ± 0.007 XZn = 0.340 ± 0.008
TiBiZn + TiZn Inclusions of liquid phase, containing 88 at.% Bi, 6 at.% Zn, 5 at.% Ti (5 p.) There is a Si-containing layer around the wall
7
TiZn
XTi = 0.515 ± 0.017 XBi = 0.003 ± 0.003 XZn = 0.474 ± 0.019
6 p.
9
TiBiZn
XTi = 0.329 ± 0.009 XBi = 0.335 ± 0.013 XZn = 0.336 ± 0.010
TiBiZn + Liq + TiZn3 6 p. There is a Si-containing layer around the wall
9
TiZn3
XTi = 0.260 ± 0.005 XBi = 0.014 ± 0.001 XZn = 0.7260 ± 0.00
9 p. Dark crystals
11
TiBiZn
XTi = 0.33 ± 0.01 XBi = 0.34 ± 0.02 XZn = 0.34 ± 0.01
TiBiZn + Liq + TiZn3 6 p. There is a Si-containing layer around the wall
11
TiZn3
XTi = 0.266 ± 0.006 XBi = 0.018 ± 0.001 XZn = 0.723 ± 0.006
9 p. Dark crystals
12
TiBiZn
XTi = 0.339 ± 0.003 XBi = 0.333 ± 0.004 XZn = 0.328 ± 0.004
TiBiZn + Liq Gray rectangular crystals (4 p.)
12
Liq
XTi = 0.01 ± 0.01 XBi = 0.95 ± 0.03 XZn = 0.04 ± 0.03
5 a.a.
12
Ti4 Bi3 Zn
XTi = 0.498 ± 0.009 XBi = 0.374 ± 0.004 XZn = 0.127 ± 0.009
5 p. Dark crystals with approximate formula Ti4 Bi3 Zn
14
TiBiZn
XTi = 0.339 ± 0.003 XBi = 0.330 ± 0.001 XZn = 0.331 ± 0.003
TiBiZn + Ti4 Bi3 Zn + Liq TiBiZn gray crystals (6 p.) The composition of the Ti4 Bi3 Zn crystals could not be measured quantitatively because of the rough surface
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Table 4 (Continued ) No.
Phases
Measured composition
Notes
14
Liq
XTi = 0.03 XBi = 0.90 XZn = 0.07
1 p.
16
TiBiZn
XTi = 0.334 ± 0.005 XBi = 0.332 ± 0.003 XZn = 0.334 ± 0.005
TiBiZn + TiZn 6 p. Gray crystals no. 16
16
TiZn
XTi = 0.495 ± 0.004 XBi <0.001 XZn = 0.505 ± 0.004
Gray crystals (6 p.)
No.: specimen’s number; p.: point, where electron microprobe analyses is done (the digit preceding (p.) shows the number of measurements); a.a.: area analyses (usually applied to the liquid phase); u.s.: unidentified stoichiometry; Liq: liquid phase.
In specimen no. 12, annealed at 900 ◦ C, except the recently found TiBiZn [1,32], another phase corresponding approximately to the formula Ti4 Bi3 Zn has been observed (Fig. 5). Its crystals exhibit fissures and are always in contact with the TiBiZn phase. Layers having similar composition have been observed in solid (␣-Ti)/liquid (Bi + Zn) diffusion couples annealed at 700 and 800 ◦ C [1]. Taking into account the present results and the authors’ previous data [1,32], we assume the existence of two stable ternary phases in the Ti–Bi–Zn system (TiBiZn and ∼Ti4 Bi3 Zn). Our observations show that the corrosion resistance of the phase Ti4 Bi3 Zn is intermediate between that of TiBiZn and
the binary TiX BiY . The former does not corrode in air, while the latter converts quickly to powder. The results of the first and the second DSC analyses are displayed in Figs. 6 and 7, respectively. The mass was controlled before and after the heating, but mass loss has not been found. Thus, the hypothesis that the TiX BiY and Ti4 Bi3 Zn absorb air humidity could not be maintained, while the assumption for a reaction with the oxygen is highly probable. Five endothermic thermal peaks (Fig. 7) were observed in the interval 255–268 ◦ C. The two strongest peaks (F and L) are reproducible, especially the second one (at 267.8 ◦ C).
Fig. 4. Micrograph, in back scattered electrons (SEI), of specimen no. 14, annealed at 900 ◦ C. The gray areas (1) appertain to the ternary TiBiZn compound. Separation in the crystallized liquid phase is observed: the dark areas (2) consist of almost pure Zn; the light-gray areas (3) have more or less equiatomic ratios of Bi and Zn. The rough-surface areas (4) represent corroded crystals of Ti4 Bi3 Zn. Small TiZn dendrites are found in the Zn-rich areas, while dendrites with approximate composition TiZn6 form in the Bi–Zn regions.
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Fig. 5. Micrograph, in characteristic X-rays, of specimen no. 12, annealed at 900 ◦ C. The gray crystals appertain to the ternary TiBiZn compound, the dark gray areas (fissures due to corrosion are seen in them) correspond approximately to the formula Ti4 Bi3 Zn (the oxygen content has not been taken into account). The white matrix belongs to the Bi-rich liquid phase.
They correspond to the peaks B and C (Fig. 6), while the hypothetical peak A was not observed. The thermal arrests at around 255 ◦ C are related with the binary eutectic reaction (254.5 ◦ C, 8.1 at.% Zn) in the Bi–Zn system [8]. A ternary
Fig. 6. Thermal curves of the corroded specimen no. 6, obtained by NETZSCH DSC 404C. Heating rate 10 K min−1 . The mass of the specimen is 252.8 mg. Curves 1 and 2 correspond to the heating to 302 ◦ C and cooling to 192 ◦ C branches, respectively, during the first cycle. The endothermic peaks A (≈230 ◦ C), B (≈250 ◦ C) and C (≈260 ◦ C) are observed during the first heating, while the peak C only (curve 3) is registered during the second heating (up to 406 ◦ C). Curve 4, corresponds to the second cooling. The temperature (◦ C) is plotted along the abscissa, and DSC units (mW) – along the ordinate. The endothermic peaks show downward.
eutectic reaction with the following reactants could be anticipated as well: LE ⇔ (Bi) + (η-Zn) + TiX BiY
(1)
Fig. 7. Thermal curves of the corroded specimen no. 6, obtained by Mettler TA3000 System DSC. Heating rate 10 K min−1 . The mass of the specimen is 25.57 mg. Two sequential heatings from 50 to 280 ◦ C have been performed (curves 1 and 2, respectively). The peaks F and L appear in both cycles, while G and H – in the first cycle and J and K – in the second cycle. The temperatures of the thermal arrests F, G, H, J, K and L are as follows: 255.4, 259.3, 263.5, 265.9, 266.6 and 267.8 ◦ C, respectively.
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Fig. 8. Temperature dependence of the dc specific conductivity (ρ, Ohm cm) for specimen no. 6. The inversed temperature is plotted along the abscissa and specific conductivity (䊊)—along the ordinate.
Here, LE is the liquid phase with eutectic composition, (Bi) and (-Zn) – phases on the basis of the pure pertinent components. The rest of the peaks were not identified, including the strong reproducible peak (C in Fig. 6, and L in Fig. 7) at around 268 ◦ C. The peaks G and H (Fig. 7, curve 1) might be related with the peaks J and K (Fig. 7, curve 2), respectively, because they appear as couples, shifted by a few degrees.
Fig. 9. Isothermal section of the Ti–Bi–Zn phase diagram at 500 ◦ C. The experimental data are presented as follows: ( ) nominal alloys compositions; ( ) liquid phase (L); (䊉) TiX BiY (X ≈ Y); (䉫) TiZn; (×) TiZn3 ; 夹 TiZn7 ; (䊏) Ti4 Bi3 Zn; ( ) TiBiZn. The miscibility gap is represented by dots. The thick solid lines represent the phase field boundaries; the thin—the tie-lines. The short dashes stand for the phase field boundaries the most recommendable for further studies.
The electrical conductivity of specimen no. 6 was measured as well. For this purpose a tablet was prepared pressurizing the powder and the four-point method was used. The Arrhenius temperature dependence of the dc specific conductivity (ρ, Ohm cm) is shown in Fig. 8. The specific conductivity increases with increasing temperature. Moreover, Hall voltages are not found at all, suggesting that a part of the material has metallic character (that is confirmed by the DSC studies) while the crystallite boundaries or surfaces are non-metallic. In Figs. 9 and 10 isothermal sections of the Ti–Bi–Zn system at respectively 500 and 615 ◦ C are plotted, based on this work as well as on previous authors’ studies [1,19,20,32]. Pertinent data obtained at 900 ◦ C (specimens 2, 4, 12) are plotted in Fig. 10 too. This is admissible because of the stoichiometric character of the binary and ternary compounds (i.e. vertical phase boundaries). Actually, there are data that the phase Ti4 Bi3 Zn is non-stoichiometric. Its homogeneity range might include the following compositions: 47–51 at.% Ti; 36–38 at.% Bi; 12–16 at.% Zn (Tables 3 and 4; see also [1]). Anyhow, for the sake of simplicity, it has been plotted as stoichiometric compound in Figs. 9 and 10. The melting temperature of the compound TiZn7 is around 600 ◦ C [19,20] and in this study only probable TiZn7 dendrites have been observed in the specimens annealed at 615 and 900 ◦ C. For this reason TiZn7 has not been presented in Fig. 10. Unidentified crystals (probably dendrites) with an approximate composition TiBi2 Zn have been observed in specimen no. 13 (annealed at 500 ◦ C).
Fig. 10. Isothermal section of the Ti–Bi–Zn phase diagram at 615 ◦ C. The experimental data are presented as follows: ( ) nominal alloys compositions (specimens nos. 2, 4 and 12 are annealed at 900 ◦ C); ( ) liquid phase (L); (䉫) TiZn; (×) TiZn3 ; (䊏) Ti4 Bi3 Zn; ( ) TiBiZn. The thick solid lines represent the phase field boundaries; the thin—the tie-lines. The short dashes stand for the phase field boundaries the most recommendable for further studies.
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The Ti-solubility in molten zinc is around 2.0 at.% Ti and around 4.4 at.% Ti at 500 and 615 ◦ C, respectively. The zinc solubility in (␣-Ti) probably does not depend strongly on the temperature and a Zn content of ∼9 at.% is to be expected [8,19,20] in this temperature interval. At the working temperatures (500 and 615 ◦ C), the bismuth solubility in (␣-Ti) and the titanium solubility in molten bismuth are expected to be, respectively, around 1.5 at.% Bi and 1 at.% Ti [8,29]. Concerning the construction of the ternary system at 900 ◦ C, one should be aware that, at this temperature, ternary solid solutions based on (␣-Ti) are expected to be in equilibrium with Ti3 Bi and with TiZn (rather then with Ti2 Zn [33]) in the Ti–Bi and Ti–Zn systems, respectively.
4. Conclusion The phase equilibria in the system Ti–Bi–Zn have been studied at 500, 615 and 900 ◦ C. The data show the existence of two stable ternary compounds – the stoichiometric TiBiZn and the non-stoichiometric ∼Ti4 Bi3 Zn. The homogeneity range of the latter phase may include the following compositions: 47–51 at.% Ti; 36–38 at.% Bi; 12–16 at.% Zn. Thermal arrests at 255.4, 259.3, 263.5, 265.9, 266.6 and 267.8 ◦ C have been found. The first one is related with the binary eutectic reaction or with a ternary one. The melting temperatures of the compounds TiBiZn, ∼Ti4 Bi3 Zn and TiZn3 are above 900 ◦ C. Tentative isothermal sections of the Ti–Bi–Zn system have been constructed at 500 ◦ C and at 615 ◦ C. In the pertinent end-systems, confirmations about the existence of the following binary compounds have been found: TiX BiY (X ≈ Y), Ti3 Bi, Ti2 Bi, TiZn, TiZn3 , TiZn7 . The Zn solubility in the ternary extensions of the Ti–Bi compounds is superior to the Bi solubility in the Ti–Zn compounds. Acknowledgements The authors appreciatively acknowledge a fellowship granted by the Japanese Society for Promotion of Science to one of them (G.P.V.). The latter also thanks Dr. X.J. Liu for helpful discussions and Mr. Takaku (both with Tohoku University, Sendai, Japan) who kindly was of assistance during the work with the scanning electron microscope. The help of Dr. N. Avramova and Prof. B. Arnaudov (both with University of Sofia) with the Mettler DSC and
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specific conductivity experiments, respectively, is gratefully accepted.
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