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Influence of tin on the structure and properties of as-cast Ti-rich Ti–Si alloys
Journal of Alloys and Compounds 350 (2003) 164–173
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Influence of tin on the structure and properties of as-cast Ti-rich Ti–Si alloys M. Bulanova*, L. Tretyachenko, K. Meleshevich, V. Saltykov, V. Vereshchaka, O. Galadzhyj, L. Kulak, S. Firstov I.N. Frantsevich Institute for Problems of Materials Science, Str. Krzhyzhanovsky, 3, Kiev, 03142, Ukraine Received 17 June 2002; received in revised form 5 August 2002; accepted 5 August 2002
Abstract By the methods of DTA, X-ray diffraction, metallography and microprobe analysis, phase equilibria in the Ti-corner (more than 50 at.% Ti) of the Ti–Si–Sn system were studied. The solidus projection and the melting diagram (solidus1liquidus) were constructed. A new ternary compound T of composition Ti 5 Si 1.2 – 1.6 Sn 1.8 – 1.4 was found to form with the crystal structure of W5 Si 3 -type. The ternary eutectic equilibrium L↔kb-Til1kTi 5 Si 3 l1kTi 3 Snl was established to occur at 1460 8C with the composition of the invariant point E at |77Ti–9Si–14Sn. Microhardness measurements were carried out for the primary grains of the alloys with 5 at.% Si. 2002 Elsevier Science B.V. All rights reserved. Keywords: Transition metal alloys; Crystal structure; Phase diagram; Metallography; X-Ray diffraction
1. Introduction Si and Sn are important constituents of multicomponent Ti-alloys. Si is known to increase mechanical characteristics (creep and corrosion resistance, high-temperature strength) of both titanium and Ti–Al alloys. Sn increases both the working temperatures of the alloys, and viscosity of the b-phase [1]. Despite this, multicomponent phase diagrams on the basis of Ti, Si and Sn are mostly unknown. Moreover, phase equilibria in the boundary Ti–Si–Sn ternary were reported only in Ref. [2], however, in a narrow concentration interval. On the basis of the boundary binaries Ti–Si (Fig. 1a) and Ti–Sn (Fig. 1b), the Ti-rich Ti–Si–Sn alloys may be expected to form Ti–Si-based binary eutectic structures of natural composite materials, and a refractory ternary eutectic, as well. The latter can have a positive influence on the mechanical characteristics of multicomponent materials. So a knowledge of the Ti–Si–Sn phase diagram is necessary to understand the character of the interaction of the components in multicomponent systems of practical interest. Thus, the goal of the present paper is to present the
*Corresponding author. E-mail address: [email protected] (M. Bulanova).
results of our investigation of phase equilibria in the Ti-corner of the Ti–Si–Sn system at crystallization. The Ti–Si phase diagram accepted in this work has been assessed by thermodynamic evaluation [3] (Fig. 1a) mainly on experimental data [4]. The homogeneity range of the Ti 5 Si 3 -based phase (Z) and the temperature of the eutectic reaction L↔kb-Til1Ti 5 Si 3 are shown due to our corrections [5]. Polymorphism of the compounds Ti 5 Si 4 and TiSi was not considered in Ref. [3]. These corrections are involved according to the data in Refs. [4,6]. The Ti–Sn system (Fig. 1b) is accepted from Ref. [5].
2. Experimental The purity of the starting materials was Ti 99.85%, Si 99.999% and Sn 99.9995%. The alloys were melted in an arc-furnace with unconsumable tungsten electrode on a water-cooled copper hearth in an Ar atmosphere purified by a Ti-melt. To achieve homogeneity, the buttons were turned over and remelted three times. The alloys for which weight losses were more than 1% were subjected to chemical analysis. The alloy compositions are located along 10, 5 at.% Si (series I, II) and 62.5 at.% Ti (series III). The oxygen content in the samples was determined to be 0.01–0.02%. The alloys were studied in the as-cast state by DTA,
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 02 )00971-4
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165
Fig. 1. The edge binary systems: (a) Ti–Si [3] with corrections [4–6]; (b) Ti–Sn [7].
X-ray diffraction, metallography, EPMA and microhardness measurements. DTA was performed in a VDTA-7-type device with a W/ W-Re thermocouple in helium. The rate of heating / cooling was |30 8C / min. Al 2 O 3 crucibles were used. The liquidus temperatures were taken from the heating curves as the maximum of the thermal effect (end of the melting process). In some cases, the liquidus temperatures were taken on cooling as the beginning of the thermal effect (beginning of crystallization process). The accuracy of the temperature measurements was estimated to be 61%. X-Ray diffraction was performed by the powder method in Debye cameras (d557.3 mm) with Cu Ka-filtered radiation in a URS-2.0 device. The lattice parameters were calculated by a least-squares deviation method. The samples for microstructure examination were polished with a water suspension of Cr 2 O 3 . For etching, a solution of HF / HNO 3 / H 2 O (1:2:3–5) was used. EPMA was carried out by means of Jeol Superprobe733 and Camebax SX-50 devices. A PMT-3 device was used to measure the microhardness of the phases. The loads were chosen in accordance with the grain size.
3. Results and discussion For convenience the following designations are used below: a and b for the a-Ti and b-Ti solid solutions, respectively; a 2 for the Ti 3 Sn-based phase; 2 / 1, 5 / 3 and 6 / 5 for the Ti 2 Sn, Ti 5 Sn 3 and Ti 6 Sn 5 -based phases,
respectively; Z for the Ti 5 Si 3 -based phase; T for the ternary compound. The phase composition of the alloys studied is summarized in Table 1. The solidus projection and the melting diagram (solidus1liquidus) which resulted from this investigation are shown in Fig. 2. The solidus temperatures are shown in Fig. 2a. The most prominent feature of the system is the formation of the ternary compound T (Ti 5 Si 1.2 – 1.6 Sn 1.8 – 1.4 ), found for the first time. This takes part in the equilibria at crystallization. The compound has a rather wide homogeneity range due to mutual substitution of Si and Sn, the Ti concentration being stable at 62.5 at.%. The crystal structure of the compound was shown to be tetragonal, of the W5 Si 3 type. The X-ray pattern of the annealed three-phase sample 62.5Ti–22.5Si–15Sn with indexing of the reflections is shown in Table 2. Other phases taking part in the equilibria during solidification are the phases of the binary systems b, a 2 , 2 / 1, Z, 5 / 3 and 6 / 5. The crystal structure data are shown in Table 3. The boundaries of the homogeneity regions of the phases were established on the basis of EPMA results (Table 4). It should be noted that the EPMA results obtained for the cast alloys were attributed for the compositions representative for the solidus temperatures as it was observed in Ti–Si–Al [5], and Ti–Si–Ge–Al systems [12]. The solubility of Si in Ti 3 Sn was determined to be about 1.5 at.%, |3 at.% in Ti 2 Sn and |0.5 at.% in Ti 6 Sn 5 . The solubility of Si in Ti 5 Sn 3 is |10 at.%, the homogeneity range being wider in the ternary system than in the binary with respect to the titanium content.
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Table 1 Phase composition of as-cast Ti–Si–Sn alloys and solidification temperatures according to the results of this examination Alloy
Phase composition According to microstructures
According to X-ray
According to EPMA
At solidus according to the entity of data
Primary phase
Series I 85Ti–10Si–5Sn 80Ti–10Si–10Sn 75Ti–10Si–15Sn 70Ti–10Si–20Sn 65Ti–10Si–25Sn 60Ti–10Si–30Sn 55Ti–10Si–35Sn 48Ti–10Si–42Sn b
b1eutectic (b1Z) b1eutectic (b1Z) Eutectic (Z1a 2 )1eutectic (b1Z1a 2 ) Z1T1a 2 Z1a 2 12 / 1 Z1T15 / 3 5 / 31? ?
a1Z – a1Z1a 2 a 2 1T12 / 1 a 2 1T12 / 1 a 2 12 / 11T15 / 3 2 / 11kb-Snl a 15 / 31? 6/5
b1mixture (b1Z) b1mixture (b1Z) Z1a 2 1mixture (b1Z) T1mixture (T1a 2 ) T12 / 1 Z1T15 / 3 Z15 / 316 / 5 Z15 / 3
b1Z b1Z b1Z1a 2 Z1a 2 1T T12 / 1 Z1T15 / 3 5 / 31? ?
b b – Z Z Z 5/3 5/3
Series II 85Ti–5Si–10Sn 80Ti–5Si–15Sn 75Ti–5Si–20Sn 70Ti–5Si–25Sn 55Ti–5Si–40Sn
b1eutectic (b1Z) b1eutectic (b1Z) a 2 1eutectic (Z1a 2 ) 2 / 11eutectic (a 2 12 / 1) 6 / 51?
a1Z a1Z1a 2 a 2 1? a 2 12 / 11T 2 / 11kb-Snl15 / 316 / 51?
b1Z b1Z1a 2 Z1a 2 2 / 11T
b1Z b1Z b1Z1a 2 a 2 12 / 11T ?
b b a2 a2 5/3
Series III 62.5Ti–27.5Si–10Sn c 62.5Ti–22.5Si–15Sn c 62.5Ti–17.5Si–20Sn c 62.5Ti–12.5Si–25Sn c 62.5Ti–7.5Si–30Sn c 62.5Ti–2.5Si–35Sn c
Z1T1? Z1T1? Z1T1? Z1T1? T1eutectic (T15 / 3) 5 / 31eutectic (T15 / 3)
Z1T1? Z1T1? Z1T1? T15 / 312 / 11? 5 / 31? 5 / 31?
Z12 / 1 Z12 / 1 Z1T12 / 11a 2 Z1T15 / 312 / 1 T15 / 312 / 1 5/3
Z1T1[a 2 ] d Z1T1[a 2 ] T1[2 / 1]1[a 2 ] T12 / 115 / 3 T12 / 115 / 3 2 / 115 / 31[T]
Z Z Z Z T 5/3
a
Tin is present because of a sequence of crystallization reactions coming at the end to an invariant equilibrium with Sn participance. Composition according to chemical analysis. c Based on the totality of results. The chemical composition of the samples is shifted a little, while chemical analysis did not reveal this. d Traces of phases are shown in brackets. b
The maximum tin solubility in Ti 5 Si 3 is |4 at.%. The homogeneity region is substantially narrower in the ternary system than in the Ti–Si binary system with respect to Ti / Si substitution. A drastic decrease in the Ti solubility is observed when the Sn additions are less than |1 at.%. At higher Sn concentration, the phase contains a stable
quantity of Ti equal to |62.5 at.%. This reflects the complicated character of the component substitutions at the Ti-rich boundary of the homogeneity range of the Z phase. Thus, Sn atoms when added to the binary Ti 5 Si 3 first substitute for Ti atoms. Adding 1 at.% Sn results in a |2.5 at.% decrease in the Ti concentration. When the Sn
Fig. 2. Solidus surface (a) and melting diagram (solidus1liquidus) (b) of the Ti-corner of the Ti–Si–Sn system: f, two-phase sample; d, three-phase sample; , sample with unknown phase composition; ^, EPMA results.
M. Bulanova et al. / Journal of Alloys and Compounds 350 (2003) 164–173 Table 2 X-Ray powder pattern of the sample 62.5Ti–22.5Si–15Sn, annealed at 1300 8C for 30 h N
2
Sin u
I
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
0.0343 0.0445 0.0486 0.0507 0.0562 0.0837 0.0894 0.0923 0.0951 0.0989 0.1042 0.110S 0.1136 0.1170 0.1213 0.1269 0.1315 0.1357 0.1422 0.1477 0.1732 0.1809 0.1848 0.1924 0.2078 0.2287 0.2363 0.2605 0.2723 0.2791 0.2914 0.3003 0.3164 0.3267 0.3515 0.3681 0.3815 0.3978 0.4075 0.4146 0.4290 0.4456 0.4583 0.4720 0.4860 0.4984 0.5167 0.5338 0.5437 0.5544 0.5711 0.5841 0.6011 0.6138 0.6282 0.6448 0.6588 0.6691 0.6924 0.7019
VW VW W M W VW M M W S M M M VS S M S VW VW W W W M W VW VW W M W M W VS M M M M M W VW VW VW M W W M VW VW VW W VW VW VW M W VW M M W VW VW
Table 2. Continued N
Sin 2 u
I
hkl Ti 5 Si 3 (Z)
T
Ti 3 Sn (a 2 )
220 101 200
310
002
400 002 321 330
210 102
200
002 420 202 411
211 300 112
201 102
222 510 202 220
112 4021521
310 221 311 400
332
301 512
222
321 213 402 004 223 420 331
502 214 332 304
4421631 323 602 413 730 6511004 5521712 642 660 732 831 802
424 851 842 912
520 521 333 602 610 4141215
211 202
813 624 833 7141554 415
167
61 62 63 64 65 66 67 68 69 70 71 72 73 74
0.7149 0.7223 0.7460 0.7551 0.7686 0.7849 0.7982 0.8124 0.8294 0.8466 0.8684 0.9004 0.9125 0.9654
M W W W VW M VW VW VW M W W VW W
hkl Ti 5 Si 3 (Z)
T
504
11.2.1
4241621
824 664
Ti 3 Sn (a 2 )
532
116
10.6.2
631 614
12.2.2
Cu Ka radiation. Tetragonal, W5 Si 3 -type, lattice parameters: a5 ˚ VS, very strong; S, strong; M, moderate; 10.3560.01, c55.08460.007 A. W, weak; VW, very weak.
concentration in the compound is |1 at.%, the boundary changes its direction, and additional Sn atoms substitute for Si atoms. This is reflected in the lattice parameters of the phase Z (Table 3). When the Sn content increases, the lattice spacings corresponding to the boundary of the homogeneity range, first decrease (at less than 1 at.% Sn), then increase (at more than 1 at.% Sn). A similar situation was observed for the Ti 5 Si 3 -based phase in the system Ti–Si–Al [5]. The homogeneity region of the ternary compound T is located along 62.5 at.% Ti. In contrast to the Z and T phases, the homogeneity region of the Ti 5 Sn 3 -based phase in the ternary system does not proceed along 62.5 at.% Ti, not arriving at 62.5 at.% Ti in the Ti–Sn binary system. When Si is added to the binary compound Ti 5 Sn 3 the homogeneity range shifts towards higher tin concentration. So the Si atoms substitute for both the Sn and Ti atoms. The question concerning mutual substitution of the components in the phases Ti 5 Sn 3 and Ti 5 Si 3 needs investigation by single-crystal X-ray diffraction. The solidus surface (Fig. 2a) is characterized by the three-phase fields Z1b1a 2 , Z1a 2 1T, a 2 1T12 / 1, T12 / 115 / 3, Z1T15 / 3 and the appropriate two-phase regions. The three-phase regions result from the following fourphase invariant reactions: Z1b1a 2 : L↔Z1b1a 2 (1460 8C, point E, |77Ti–9Si– 14Sn); Z1a 2 1T: L1T↔a 2 1Z (1480 8C, point U 4 ); a 2 1T12 / 1: L12 / 1↔a 2 1T (1560 8C, point U 3 ); T12 / 115 / 3: L1T↔2 / 115 / 3 (between 1520 and 1580 8C, point U 2 ); Z1T15 / 3: L1Z↔T15 / 3 (1620 8C, point U 1 ). These, in turn, are due to the following monovariant incoming and outgoing reactions: L↔b1Z1a 2 : L↔b1Z, L↔b1a 2 , L↔Z1a 2 ;
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M. Bulanova et al. / Journal of Alloys and Compounds 350 (2003) 164–173
Table 3 The crystal structure data for the Ti-rich Ti–Si–Sn phases, measured for as-cast alloys Phase
Crystal structure
˚ Lattice periods (A)
Remarks
Refs.
a, a-Ti
Mg, hP2 –P63 /mmc Mg, hP2 –P63 /mmc Mg, hP2 –P63 /mmc
a52.9503, c54.6836
25 8C
[8]
a52.93960.003 c54.65860.008 a52.94360.001 c54.69060.003 a52.94760.002 c54.70960.005 a52.96260.005 c54.73160.008 a52.98660.004 c54.72060.008 a52.95060.003 c54.69360.005 a55.916(4), c54.764(4) a55.92260.005 c54.74260.003 a55.94360.008 c54.75460.009 a55.89460.006 c54.74560.006 a55.92660.009 c54.73160.007 a55.91860.009 c54.74360.007 a55.90760.009 c54.74660.006 a54.653, c55.700
90Ti–10Si
[2]
90Ti–7Si–3Sn
[2]
85Ti–5Si–10Sn
Th.w.a
80Ti–5Si–15Sn
Th.w.
75Ti–5Si–20Sn
Th.w.
85Ti–10Si–5Sn
Th.w.
75Ti–25Sn, 1030 8C 80Ti–5Si–15Sn
[9] Th.w.
75Ti–5Si–20Sn
Th.w.
70Ti–5Si–25Sn
Th.w.
75Ti–10Si–15Sn
Th.w.
70Ti–10Si–20Sn
Th.w.
65Ti–10Si–25Sn
Th.w.
65.2Ti–34.8Sn
[9]
a54.64260.005 c55.68760.008 a54.65360.004 c55.70660.005 a54.65760.004 c55.70560.006 a57.465, c55.162 a57.429, c55.139 a57.431(6), c55.135(5) a57.45660.002 c55.15760.002 a57.46660.006 c55.1660.01 a57.42460.008 c55.1260.01 a57.7560.01 c55.20860.007 a57.44260.006 c55.11460.007 a57.47160.005 c55.14560.005 a57.47360.009 c55.15660.009 a57.41660.009 c55.12460.009 a57.44860.008 c55.1760.01
70Ti–5Si–25Sn
Th.w.
65Ti–10Si–25Sn
Th.w.
62.5Ti–12.5Si–25Sn
Th.w.
a, a-(Ti,Si) a, a-(Ti,Si,Sn)
a 2 , Ti 3 Sn a2, Ti 3 (Sn,Si)
2 / 1, Ti 2 Sn 2 / 1, Ti 2 (Sn,Si)
Z, Ti 5 Si 3
Z, Ti 5 (Si,Sn) 3
Ni 3 Sn, hP8 –P41 21 2 Ni 3 Sn, hP8 –P41 21 2
Ni 2 In, hP6 –P63 /mmc Ni 2 In, hP6 –P63 /mmc
Mn 5 Si 3 , hP16 –P63 /mcm
Mn 5 Si 3 , hP16 –P63 /mcm
[10] [11] [4] 62.5Ti–37.5Si
[5]
86.5Ti–13.5Si, 1250 8C 90Ti–7Si–3Sn
[5]
85Ti–5Si–10Sn
Th.w.
85Ti–10Si–5Sn
Th.w.
75Ti–10Si–15Sn
Th.w.
62.5Ti–27.5Si–10Sn
Th.w.
62.5Ti–22.5Si–15Sn
Th.w.
62.5Ti–17.5Si–20Sn
Th.w.
[2]
M. Bulanova et al. / Journal of Alloys and Compounds 350 (2003) 164–173
169
Table 3. Continued Phase
Crystal structure
˚ Lattice periods (A)
5 / 3, Ti 5 Sn 3
Mn 5 Si 3 , hP16 –P63 /mcm Mn 5 Si 3 , hP16 –P63 /mcm
a58.049(2), c55.454(2) a58.05160.008 c55.4860.01 a58.03660.005 c55.48960.005 a58.05260.007 c55.47060.007 a58.02960.005 c55.46160.005 a58.04460.004 c55.46460.005 a58.06560.003 c55.48460.004 a510.16260.009 c55.09860.008 a510.23860.007 c55.08160.007 a510.2260.01 c55.05860.009 a510.3560.01 c55.1360.01 a510.2560.01 c55.1060.01 a510.2860.01 c55.1260.01 a510.43260.006 c55.12960.003
5 / 3, Ti 5 (Sn,Si) 3
T
W5 Si 3 , tI32 –I4 /mcm
a
Remarks
Refs. [9,10]
55Ti–5Si–40Sn
Th.w.
60Ti–10Si–30Sn
Th.w.
55Ti–10Si–35Sn
Th.w.
62.5Ti–12.5Si–25Sn
Th.w.
62.5Ti–7.5Si–30Sn
Th.w.
62.5Ti–2.5Si–35Sn
Th.w.
70Ti–5Si–25Sn
Th.w.
70Ti–10Si–20Sn
Th.w.
65Ti–10Si–25Sn
Th.w.
60Ti–10Si–30Sn
Th.w.
62.5Ti–27.5Si–10Sn
Th.w.
62.5Ti–17.5Si–20Sn
Th.w.
62.5Ti–12.5Si–25Sn
Th.w.
Th.w., results of this examination.
L1T↔a 2 1Z: L↔a 2 1T, L1Z↔T, L↔Z1a 2 ; L12 / 1↔a 2 1T: L↔a 2 12 / 1, L↔T12 / 1, L↔a 2 1T; L1T↔2 / 115 / 3: L↔T12 / 1, L↔T15 / 3, L↔2 / 115 / 3; L1Z↔T15 / 3: L1Z↔T, L↔T15 / 3, L↔T15 / 3. The two-phase fields Z1b, T12 / 1, a 2 12 / 1, Z1T and Z15 / 3 of the solidus surface have temperature maxima corresponding to the invariant three-phase equilibria L↔b1Z, L↔T12 / 1, L↔a 2 12 / 1, L1Z↔T, L↔Z15 / 3. The coordinates of the invariant points for the first two equilibria were estimated to be 1470 and 1580 8C, and |79Ti–11.5Si–9.5Sn and |62Ti–10.5Si–27.5Sn, respectively. The maximum of the b1Z solidus is located very close to the Z1b1a 2 three-phase field, almost on its boundary. The melting diagram (solidus1liquidus) is presented in Fig. 2b. The character of the equilibria and the location of invariant points and monovariant curves were determined from microstructure examinations (Fig. 3). It should be noticed that the b-phase was never observed. The X-ray patterns contained the reflections of the a-phase, resulting from the solid-state transformation b↔a. The alloys with 10 at.% Si containing 5 and 10 at.% Sn (Figs. 3a,b) are hypoeutectic with primary crystallization of the b-phase. In the alloy 80Ti–5Si–15Sn (Fig. 3f) the b-phase crystallizes first, as well. Meanwhile the striped
character of the primary grains indicates a transformation in the solid state. According to the EPMA results, light and dark regions of the primary grains correspond to the a 2 and Ti-phases, respectively. So, on cooling, the a 2 -phase precipitates from the b-phase due to the intersection of the (b1Z) /(b1Z1a 2 ) phase boundary. No traces of solid-state transformations of the primary grains were observed for the alloy 75Ti–5Si–20Sn (Fig. 3g). So the alloy is located in the field of primary crystallization of the a 2 -phase. The morphology of the primary grains in the alloy 70Ti–5Si–25Sn (Fig. 3h) is different. This allows to conclude that the 2 / 1-phase gives rise to primary crystallization for this alloy. In the alloy 75Ti–10Si–15Sn (Fig. 3c), no primary grains are found, while two types of eutectics were observed. This shows its location on the monovariant curve L↔Z1a 2 , the liquid-phase composition being moved during solidification towards the ternary eutectic L↔b1Z1a 2 . The alloys (65–60)Ti–10Si–(25–30)Sn (Fig. 3d,e) and 62.5Ti–(27.5–12.5)Si–(10–25)Sn (Fig. 3i–k) are hypereutectic with primary crystallization of the Z-phase. The amount of Z-phase in the alloy 65Ti–10Si–25Sn is extremely low, so the alloy is located very close to the monovariant curve L1Z↔T. In the alloys 62.5Ti–7.5Si–30Sn (Fig. 3l) and 62.5Ti–
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170
Table 4 Compositions of Ti-rich Ti–Si–Sn phases according to the EPMA examination of as-cast alloys Alloy
90Ti–10Si a 90Ti–7Si–3Sn a 90Ti–5Si–5Sn a 85Ti–10Si–5Sn
80Ti–10Si–10Sn
75Ti–10Si–15Sn
70Ti–10Si–20Sn
65Ti–10Si–25Sn
60Ti–10Si–30Sn
55Ti–10Si–35Sn
50Ti–10Si–40Sn
85Ti–5Si–10Sn
80Ti–5Si–15Sn
75Ti–5Si–20Sn
70Ti–5Si–25Sn
62.5Ti–27.5Si–10Sn
Chemical composition (at.%) Ti
Si
Sn
95.4 95.3 92.2 92.1 91.6 92.0 90.9 91.0 79.6 84.1 85.2 78.0 78.0 62.4 62.2 75.0 74.7 77.2 80.3 71.9 62.6 62.6 70.6 62.6 62.5 67.7 68.0 62.0 62.5 62.2 62.3 59.6 59.8 62.0 62.4 55.2 57.6 58.3 62.0 61.9 57.2 56.5 56.7 87.1 87.0 63.9 78.2 77.4 88.1 81.3 80.8 75.2 74.8 64.9 66.9 66.5 66.9 65.3 64.5 64.7
4.5 4.6 3.8 3.8 2.9 2.6 2.7 2.6 16.3 2.0 2.3 13.2 13.5 35.9 34.7 1.5 1.4 14.9 10.5 8.8 20.1 19.4 7.5 17.7 19.7 2.9 2.7 34.1 15.1 34.0 17.2 4.1 4.8 35.6 35.2 0.6 8.7 10.5 35.8 35.4 10.6 8.9 8.6 1.5 1.6 9.3 0.8 0.6 1.3 2.3 3.3 0.0 0.0 29.1 5.3 4.8 5.0 2.7 34.3 34.0
0 0 4.0 4.0 5.5 5.4 6.4 6.4 4.0 13.9 12.5 8.7 8.5 1.7 3.1 23.4 23.9 7.9 9.2 19.2 17.3 18.0 21.9 19.6 17.8 29.4 29.3 3.9 22.4 3.8 20.5 36.2 35.4 2.4 2.4 44.2 33.7 31.2 2.2 2.7 32.3 34.6 34.7 11.4 11.4 8.2 21.0 22.1 10.6 16.4 15.8 24.8 25.2 6.0 27.8 28.6 28.0 32.0 1.1 1.4
Table 4. Continued Alloy
Identification of the measurement
b b b b b b b b Mixture b b Mixture Mixture Z Z a2 a2 Mixture Mixture Mixture T T Mixture T T 2/1 2/1 Z T Z T 5/3 5/3 Z Z 6/5 5/3 5/3 Z Z 5/3 5/3 5/3 b b Mixture Mixture Mixture b Mixture Mixture a2 a2 Mixture Mixture Mixture Mixture 2/1 Z Z
62.5Ti–22.5Si–15Sn
62.5Ti–17.5Si–20Sn
b1Z
b1Z b1Z
62.5Ti–12.5Si–25Sn
62.5Ti–7.5Si–30Sn
b1Z b1Z a 2 1Z
a 2 1T
b1Z b1a 2 b1a 2 b1Z b1Z
a 2 1Z 2 / 11T 2 / 11T 2 / 11T
62.5Ti–2.5Si–35Sn
a
Chemical composition (at.%) Ti
Si
Sn
67.6 67.4 63.1 64.4 65.5 66.9 64.1 63.9 63.6 65.3 65.5 63.4 63.6 64.0 61.8 64.3 63.4 60.8 60.4 64.0 59.8 60.2 60.4
1.9 2.1 35.4 33.9 2.3 2.2 33.2 15.4 19.1 1.4 1.0 12.2 16.0 13.6 2.5 16.4 17.5 3.9 4.0 2.2 2.9 3.1 2.9
30.4 30.6 1.5 1.7 32.2 30.9 2.4 20.6 17.3 33.3 33.5 24.4 20.4 22.4 35.8 19.2 19.1 35.4 35.6 33.8 37.2 36.8 36.6
Identification of the measurement
2/1 2/1 Z Z 2/1 2/1 Z T T 2/1 2/1 Mixture 2 / 11T T Mixture 2 / 11T Mixture 2 / 115 / 3 T T kTi 5 Sn 3 l kTi 5 Sn 3 l Mixture 2 / 115 / 3 5/3 5/3 5/3
Data from Ref. [2]. All the other results are obtained from this study.
2.5Si–35Sn (Fig. 3m), the primary phases are T and 5 / 3, respectively. The complicated character of the solidification of the alloys with 62.5 at.% Ti, located in the field of Z-phase primary crystallization can be seen from the microstructure of the alloy 62.5Ti–27.5Si–10Sn (Fig. 3i,j). Fig. 3j shows enlarged fields between the primary grains. This allows to consider the following route of crystallization: Z (primary)→T (L1Z↔T)→2 / 1 (L1T↔2 / 1)→a 2 (L↔a 2 12 / 1). For the alloys with 5 at.% Si, the microhardness of the primary grains was measured versus the tin concentration in the alloys. The results are shown in Fig. 4. As it is seen, the microhardness of the primary b-grains, transformed on cooling to a (0–10 at.% Sn), increases when the tin concentration increases. The microhardness of the primary b-grains in the alloy with 15 at.% Sn has a substantially lower value. As discussed above, in this alloy (Fig. 3f) in addition to the b→a transformation, a 2 precipitates from the primary b-phase on cooling. This allows to suppose that the microhardness represents the average value between b (transformed into a) and a 2 . This is proved by the lower value of the microhardness for the alloy with 20 at.% Sn, where the primary phase is a 2 and no transformations occur on cooling (Fig. 3g). Thus, the microhardness of kTi 3 Snl is essentially lower than that of the ternary kTi,Si,Snl solid solution, while it is of the same level as for the binary kTi,Sil solid solution.
M. Bulanova et al. / Journal of Alloys and Compounds 350 (2003) 164–173
171
Fig. 3. Microstructures of as-cast Ti–10Si–Sn alloys: (a) 85Ti–10Si–5Sn, 31000, b1eutectic (b1Z); (b) 80Ti–10Si–10Sn, 31000, b1eutectic (b1Z); (c) 75Ti–10Si–15Sn, 31000, eutectic (Z1a 2 )1eutectic (b1Z1a 2 ); (d) 65Ti–10Si–25Sn, 31000, T1Z1a 2 12 / 1; (e) 60Ti–10Si–30Sn, 31000, Z1T15 / 3; (f) 80Ti–5Si–15Sn, 31000, b1eutectic (b1Z); (g) 75Ti–5Si–20Sn, 31000, a 2 1eutectic (a 2 1Z); (h) 70Ti–5Si–25Sn, 34000, 2 / 11eutectic (a 2 12 / 1); (i) 62.5Ti–27.5Si–10Sn, 3600, Z1T12 / 11a 2 ; (j) 62.5Ti–27.5Si–10Sn, 32400, intergrain space, Z1T12 / 11a 2 ; (k) 62.5Ti–12.5Si–25Sn, 31000, Z1T12 / 115 / 3; (l) 62.5Ti–7.5Si–30Sn, 31000, T1eutectic (T15 / 3); (m) 62.5Ti–2.5Si–35Sn, 3600, 5 / 31eutectic (T15 / 3).
172
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Fig. 3. (continued)
4. Conclusions The character of phase equilibria at crystallisation is studied for the Ti-corner of the Ti–Si–Sn system. The
solidus projection, melting diagram (solidus1liquidus) and reaction scheme at crystallization are constructed. The most prominent feature of the system is the existence of the ternary compound T, which has been found for the first time. The crystal structure of the T-compound was shown to be tetragonal, of the W5 Si 3 type. A ternary eutectic equilibrium occurs in the system at a temperature of 1460 8C. The invariant point (E) is located at |77Ti–9Si–14Sn. The microhardness of the primary grains was measured for the alloys with 5 at.% Si. The microhardness of the kTil-grains increases versus tin concentration in the alloys. The microhardness of kTi 3 Snl was shown to be substantially lower than that of kTi,Si,Snl.
Acknowledgements
Fig. 4. Microhardness of the primary grains in the alloys with 5 at.% Si versus tin content in the alloys.
The work was performed within the Partner Project Agreement N P-060 between the Science and Technology Center in Ukraine, the I.N. Frantsevich Institute for
M. Bulanova et al. / Journal of Alloys and Compounds 350 (2003) 164–173
Problems of Materials Science of National Academy of Science and EOARD.
References [1] A.A. Popov, L.I. Anisimova, Metallovedeniye i termicheskaya obrabotka 12 (1985) 45–50, in Russian. [2] M. Bulanova, A. Soroka, P. Zheltov, V. Vereshchaka, K. Meleshevich, Z. Metallkd. 90 (1999) 505–507. [3] H.J. Seifert, H.L. Lukas, G. Petzow, Z. Metallkd. 87 (1996) 2–13. [4] V.N. Svechnikov, Yu.A. Kocherzinsky, L.M. Yupko et al., Dokl. Acad. Nauk. SSSR 193 (1970) 393–396, in Russian. [5] M. Bulanova, L. Tretyachenko, M. Golovkova, Z. Metallkd. 88 (1997) 256–265.
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[6] V.V. Zavodyanny, V.Ya. Markiv, N.M. Belyavina, V.A. Makara, Dop. NAN Ukrainy 10 (1997) 117–121, in Ukrainian. [7] T.B. Massalski (Ed.), Binary Alloy Phase Diagrams, 2nd Edition, ASM International, Metals Park, OH, 1990. [8] H.W. King, Bull. Alloy Phase Diagr. 2 (1981) 401–402. [9] P. Rogl, Titanium: Physico-chemical Properties of its Compounds and Alloys, IAEA, Vienna, 1983. [10] L.I. Mirkin, Handbook on X-ray Structural Analysis of Polycrystals, Fizmatgiz, Moscow, 1961, in Russian. [11] P. Villars, L.D. Calvert, Pearsons Handbook of Crystallographic Data for Intermetallic Phases, ASM, Metals Park, OH, 1985. [12] M. Bulanova, L. Tretyachenko, M. Golovkova, A. Soroka, Z. Metallkd. 89 (1998) 783–789.
L
www.elsevier.com / locate / jallcom
Influence of tin on the structure and properties of as-cast Ti-rich Ti–Si alloys M. Bulanova*, L. Tretyachenko, K. Meleshevich, V. Saltykov, V. Vereshchaka, O. Galadzhyj, L. Kulak, S. Firstov I.N. Frantsevich Institute for Problems of Materials Science, Str. Krzhyzhanovsky, 3, Kiev, 03142, Ukraine Received 17 June 2002; received in revised form 5 August 2002; accepted 5 August 2002
Abstract By the methods of DTA, X-ray diffraction, metallography and microprobe analysis, phase equilibria in the Ti-corner (more than 50 at.% Ti) of the Ti–Si–Sn system were studied. The solidus projection and the melting diagram (solidus1liquidus) were constructed. A new ternary compound T of composition Ti 5 Si 1.2 – 1.6 Sn 1.8 – 1.4 was found to form with the crystal structure of W5 Si 3 -type. The ternary eutectic equilibrium L↔kb-Til1kTi 5 Si 3 l1kTi 3 Snl was established to occur at 1460 8C with the composition of the invariant point E at |77Ti–9Si–14Sn. Microhardness measurements were carried out for the primary grains of the alloys with 5 at.% Si. 2002 Elsevier Science B.V. All rights reserved. Keywords: Transition metal alloys; Crystal structure; Phase diagram; Metallography; X-Ray diffraction
1. Introduction Si and Sn are important constituents of multicomponent Ti-alloys. Si is known to increase mechanical characteristics (creep and corrosion resistance, high-temperature strength) of both titanium and Ti–Al alloys. Sn increases both the working temperatures of the alloys, and viscosity of the b-phase [1]. Despite this, multicomponent phase diagrams on the basis of Ti, Si and Sn are mostly unknown. Moreover, phase equilibria in the boundary Ti–Si–Sn ternary were reported only in Ref. [2], however, in a narrow concentration interval. On the basis of the boundary binaries Ti–Si (Fig. 1a) and Ti–Sn (Fig. 1b), the Ti-rich Ti–Si–Sn alloys may be expected to form Ti–Si-based binary eutectic structures of natural composite materials, and a refractory ternary eutectic, as well. The latter can have a positive influence on the mechanical characteristics of multicomponent materials. So a knowledge of the Ti–Si–Sn phase diagram is necessary to understand the character of the interaction of the components in multicomponent systems of practical interest. Thus, the goal of the present paper is to present the
*Corresponding author. E-mail address: [email protected] (M. Bulanova).
results of our investigation of phase equilibria in the Ti-corner of the Ti–Si–Sn system at crystallization. The Ti–Si phase diagram accepted in this work has been assessed by thermodynamic evaluation [3] (Fig. 1a) mainly on experimental data [4]. The homogeneity range of the Ti 5 Si 3 -based phase (Z) and the temperature of the eutectic reaction L↔kb-Til1Ti 5 Si 3 are shown due to our corrections [5]. Polymorphism of the compounds Ti 5 Si 4 and TiSi was not considered in Ref. [3]. These corrections are involved according to the data in Refs. [4,6]. The Ti–Sn system (Fig. 1b) is accepted from Ref. [5].
2. Experimental The purity of the starting materials was Ti 99.85%, Si 99.999% and Sn 99.9995%. The alloys were melted in an arc-furnace with unconsumable tungsten electrode on a water-cooled copper hearth in an Ar atmosphere purified by a Ti-melt. To achieve homogeneity, the buttons were turned over and remelted three times. The alloys for which weight losses were more than 1% were subjected to chemical analysis. The alloy compositions are located along 10, 5 at.% Si (series I, II) and 62.5 at.% Ti (series III). The oxygen content in the samples was determined to be 0.01–0.02%. The alloys were studied in the as-cast state by DTA,
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 02 )00971-4
M. Bulanova et al. / Journal of Alloys and Compounds 350 (2003) 164–173
165
Fig. 1. The edge binary systems: (a) Ti–Si [3] with corrections [4–6]; (b) Ti–Sn [7].
X-ray diffraction, metallography, EPMA and microhardness measurements. DTA was performed in a VDTA-7-type device with a W/ W-Re thermocouple in helium. The rate of heating / cooling was |30 8C / min. Al 2 O 3 crucibles were used. The liquidus temperatures were taken from the heating curves as the maximum of the thermal effect (end of the melting process). In some cases, the liquidus temperatures were taken on cooling as the beginning of the thermal effect (beginning of crystallization process). The accuracy of the temperature measurements was estimated to be 61%. X-Ray diffraction was performed by the powder method in Debye cameras (d557.3 mm) with Cu Ka-filtered radiation in a URS-2.0 device. The lattice parameters were calculated by a least-squares deviation method. The samples for microstructure examination were polished with a water suspension of Cr 2 O 3 . For etching, a solution of HF / HNO 3 / H 2 O (1:2:3–5) was used. EPMA was carried out by means of Jeol Superprobe733 and Camebax SX-50 devices. A PMT-3 device was used to measure the microhardness of the phases. The loads were chosen in accordance with the grain size.
3. Results and discussion For convenience the following designations are used below: a and b for the a-Ti and b-Ti solid solutions, respectively; a 2 for the Ti 3 Sn-based phase; 2 / 1, 5 / 3 and 6 / 5 for the Ti 2 Sn, Ti 5 Sn 3 and Ti 6 Sn 5 -based phases,
respectively; Z for the Ti 5 Si 3 -based phase; T for the ternary compound. The phase composition of the alloys studied is summarized in Table 1. The solidus projection and the melting diagram (solidus1liquidus) which resulted from this investigation are shown in Fig. 2. The solidus temperatures are shown in Fig. 2a. The most prominent feature of the system is the formation of the ternary compound T (Ti 5 Si 1.2 – 1.6 Sn 1.8 – 1.4 ), found for the first time. This takes part in the equilibria at crystallization. The compound has a rather wide homogeneity range due to mutual substitution of Si and Sn, the Ti concentration being stable at 62.5 at.%. The crystal structure of the compound was shown to be tetragonal, of the W5 Si 3 type. The X-ray pattern of the annealed three-phase sample 62.5Ti–22.5Si–15Sn with indexing of the reflections is shown in Table 2. Other phases taking part in the equilibria during solidification are the phases of the binary systems b, a 2 , 2 / 1, Z, 5 / 3 and 6 / 5. The crystal structure data are shown in Table 3. The boundaries of the homogeneity regions of the phases were established on the basis of EPMA results (Table 4). It should be noted that the EPMA results obtained for the cast alloys were attributed for the compositions representative for the solidus temperatures as it was observed in Ti–Si–Al [5], and Ti–Si–Ge–Al systems [12]. The solubility of Si in Ti 3 Sn was determined to be about 1.5 at.%, |3 at.% in Ti 2 Sn and |0.5 at.% in Ti 6 Sn 5 . The solubility of Si in Ti 5 Sn 3 is |10 at.%, the homogeneity range being wider in the ternary system than in the binary with respect to the titanium content.
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Table 1 Phase composition of as-cast Ti–Si–Sn alloys and solidification temperatures according to the results of this examination Alloy
Phase composition According to microstructures
According to X-ray
According to EPMA
At solidus according to the entity of data
Primary phase
Series I 85Ti–10Si–5Sn 80Ti–10Si–10Sn 75Ti–10Si–15Sn 70Ti–10Si–20Sn 65Ti–10Si–25Sn 60Ti–10Si–30Sn 55Ti–10Si–35Sn 48Ti–10Si–42Sn b
b1eutectic (b1Z) b1eutectic (b1Z) Eutectic (Z1a 2 )1eutectic (b1Z1a 2 ) Z1T1a 2 Z1a 2 12 / 1 Z1T15 / 3 5 / 31? ?
a1Z – a1Z1a 2 a 2 1T12 / 1 a 2 1T12 / 1 a 2 12 / 11T15 / 3 2 / 11kb-Snl a 15 / 31? 6/5
b1mixture (b1Z) b1mixture (b1Z) Z1a 2 1mixture (b1Z) T1mixture (T1a 2 ) T12 / 1 Z1T15 / 3 Z15 / 316 / 5 Z15 / 3
b1Z b1Z b1Z1a 2 Z1a 2 1T T12 / 1 Z1T15 / 3 5 / 31? ?
b b – Z Z Z 5/3 5/3
Series II 85Ti–5Si–10Sn 80Ti–5Si–15Sn 75Ti–5Si–20Sn 70Ti–5Si–25Sn 55Ti–5Si–40Sn
b1eutectic (b1Z) b1eutectic (b1Z) a 2 1eutectic (Z1a 2 ) 2 / 11eutectic (a 2 12 / 1) 6 / 51?
a1Z a1Z1a 2 a 2 1? a 2 12 / 11T 2 / 11kb-Snl15 / 316 / 51?
b1Z b1Z1a 2 Z1a 2 2 / 11T
b1Z b1Z b1Z1a 2 a 2 12 / 11T ?
b b a2 a2 5/3
Series III 62.5Ti–27.5Si–10Sn c 62.5Ti–22.5Si–15Sn c 62.5Ti–17.5Si–20Sn c 62.5Ti–12.5Si–25Sn c 62.5Ti–7.5Si–30Sn c 62.5Ti–2.5Si–35Sn c
Z1T1? Z1T1? Z1T1? Z1T1? T1eutectic (T15 / 3) 5 / 31eutectic (T15 / 3)
Z1T1? Z1T1? Z1T1? T15 / 312 / 11? 5 / 31? 5 / 31?
Z12 / 1 Z12 / 1 Z1T12 / 11a 2 Z1T15 / 312 / 1 T15 / 312 / 1 5/3
Z1T1[a 2 ] d Z1T1[a 2 ] T1[2 / 1]1[a 2 ] T12 / 115 / 3 T12 / 115 / 3 2 / 115 / 31[T]
Z Z Z Z T 5/3
a
Tin is present because of a sequence of crystallization reactions coming at the end to an invariant equilibrium with Sn participance. Composition according to chemical analysis. c Based on the totality of results. The chemical composition of the samples is shifted a little, while chemical analysis did not reveal this. d Traces of phases are shown in brackets. b
The maximum tin solubility in Ti 5 Si 3 is |4 at.%. The homogeneity region is substantially narrower in the ternary system than in the Ti–Si binary system with respect to Ti / Si substitution. A drastic decrease in the Ti solubility is observed when the Sn additions are less than |1 at.%. At higher Sn concentration, the phase contains a stable
quantity of Ti equal to |62.5 at.%. This reflects the complicated character of the component substitutions at the Ti-rich boundary of the homogeneity range of the Z phase. Thus, Sn atoms when added to the binary Ti 5 Si 3 first substitute for Ti atoms. Adding 1 at.% Sn results in a |2.5 at.% decrease in the Ti concentration. When the Sn
Fig. 2. Solidus surface (a) and melting diagram (solidus1liquidus) (b) of the Ti-corner of the Ti–Si–Sn system: f, two-phase sample; d, three-phase sample; , sample with unknown phase composition; ^, EPMA results.
M. Bulanova et al. / Journal of Alloys and Compounds 350 (2003) 164–173 Table 2 X-Ray powder pattern of the sample 62.5Ti–22.5Si–15Sn, annealed at 1300 8C for 30 h N
2
Sin u
I
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
0.0343 0.0445 0.0486 0.0507 0.0562 0.0837 0.0894 0.0923 0.0951 0.0989 0.1042 0.110S 0.1136 0.1170 0.1213 0.1269 0.1315 0.1357 0.1422 0.1477 0.1732 0.1809 0.1848 0.1924 0.2078 0.2287 0.2363 0.2605 0.2723 0.2791 0.2914 0.3003 0.3164 0.3267 0.3515 0.3681 0.3815 0.3978 0.4075 0.4146 0.4290 0.4456 0.4583 0.4720 0.4860 0.4984 0.5167 0.5338 0.5437 0.5544 0.5711 0.5841 0.6011 0.6138 0.6282 0.6448 0.6588 0.6691 0.6924 0.7019
VW VW W M W VW M M W S M M M VS S M S VW VW W W W M W VW VW W M W M W VS M M M M M W VW VW VW M W W M VW VW VW W VW VW VW M W VW M M W VW VW
Table 2. Continued N
Sin 2 u
I
hkl Ti 5 Si 3 (Z)
T
Ti 3 Sn (a 2 )
220 101 200
310
002
400 002 321 330
210 102
200
002 420 202 411
211 300 112
201 102
222 510 202 220
112 4021521
310 221 311 400
332
301 512
222
321 213 402 004 223 420 331
502 214 332 304
4421631 323 602 413 730 6511004 5521712 642 660 732 831 802
424 851 842 912
520 521 333 602 610 4141215
211 202
813 624 833 7141554 415
167
61 62 63 64 65 66 67 68 69 70 71 72 73 74
0.7149 0.7223 0.7460 0.7551 0.7686 0.7849 0.7982 0.8124 0.8294 0.8466 0.8684 0.9004 0.9125 0.9654
M W W W VW M VW VW VW M W W VW W
hkl Ti 5 Si 3 (Z)
T
504
11.2.1
4241621
824 664
Ti 3 Sn (a 2 )
532
116
10.6.2
631 614
12.2.2
Cu Ka radiation. Tetragonal, W5 Si 3 -type, lattice parameters: a5 ˚ VS, very strong; S, strong; M, moderate; 10.3560.01, c55.08460.007 A. W, weak; VW, very weak.
concentration in the compound is |1 at.%, the boundary changes its direction, and additional Sn atoms substitute for Si atoms. This is reflected in the lattice parameters of the phase Z (Table 3). When the Sn content increases, the lattice spacings corresponding to the boundary of the homogeneity range, first decrease (at less than 1 at.% Sn), then increase (at more than 1 at.% Sn). A similar situation was observed for the Ti 5 Si 3 -based phase in the system Ti–Si–Al [5]. The homogeneity region of the ternary compound T is located along 62.5 at.% Ti. In contrast to the Z and T phases, the homogeneity region of the Ti 5 Sn 3 -based phase in the ternary system does not proceed along 62.5 at.% Ti, not arriving at 62.5 at.% Ti in the Ti–Sn binary system. When Si is added to the binary compound Ti 5 Sn 3 the homogeneity range shifts towards higher tin concentration. So the Si atoms substitute for both the Sn and Ti atoms. The question concerning mutual substitution of the components in the phases Ti 5 Sn 3 and Ti 5 Si 3 needs investigation by single-crystal X-ray diffraction. The solidus surface (Fig. 2a) is characterized by the three-phase fields Z1b1a 2 , Z1a 2 1T, a 2 1T12 / 1, T12 / 115 / 3, Z1T15 / 3 and the appropriate two-phase regions. The three-phase regions result from the following fourphase invariant reactions: Z1b1a 2 : L↔Z1b1a 2 (1460 8C, point E, |77Ti–9Si– 14Sn); Z1a 2 1T: L1T↔a 2 1Z (1480 8C, point U 4 ); a 2 1T12 / 1: L12 / 1↔a 2 1T (1560 8C, point U 3 ); T12 / 115 / 3: L1T↔2 / 115 / 3 (between 1520 and 1580 8C, point U 2 ); Z1T15 / 3: L1Z↔T15 / 3 (1620 8C, point U 1 ). These, in turn, are due to the following monovariant incoming and outgoing reactions: L↔b1Z1a 2 : L↔b1Z, L↔b1a 2 , L↔Z1a 2 ;
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Table 3 The crystal structure data for the Ti-rich Ti–Si–Sn phases, measured for as-cast alloys Phase
Crystal structure
˚ Lattice periods (A)
Remarks
Refs.
a, a-Ti
Mg, hP2 –P63 /mmc Mg, hP2 –P63 /mmc Mg, hP2 –P63 /mmc
a52.9503, c54.6836
25 8C
[8]
a52.93960.003 c54.65860.008 a52.94360.001 c54.69060.003 a52.94760.002 c54.70960.005 a52.96260.005 c54.73160.008 a52.98660.004 c54.72060.008 a52.95060.003 c54.69360.005 a55.916(4), c54.764(4) a55.92260.005 c54.74260.003 a55.94360.008 c54.75460.009 a55.89460.006 c54.74560.006 a55.92660.009 c54.73160.007 a55.91860.009 c54.74360.007 a55.90760.009 c54.74660.006 a54.653, c55.700
90Ti–10Si
[2]
90Ti–7Si–3Sn
[2]
85Ti–5Si–10Sn
Th.w.a
80Ti–5Si–15Sn
Th.w.
75Ti–5Si–20Sn
Th.w.
85Ti–10Si–5Sn
Th.w.
75Ti–25Sn, 1030 8C 80Ti–5Si–15Sn
[9] Th.w.
75Ti–5Si–20Sn
Th.w.
70Ti–5Si–25Sn
Th.w.
75Ti–10Si–15Sn
Th.w.
70Ti–10Si–20Sn
Th.w.
65Ti–10Si–25Sn
Th.w.
65.2Ti–34.8Sn
[9]
a54.64260.005 c55.68760.008 a54.65360.004 c55.70660.005 a54.65760.004 c55.70560.006 a57.465, c55.162 a57.429, c55.139 a57.431(6), c55.135(5) a57.45660.002 c55.15760.002 a57.46660.006 c55.1660.01 a57.42460.008 c55.1260.01 a57.7560.01 c55.20860.007 a57.44260.006 c55.11460.007 a57.47160.005 c55.14560.005 a57.47360.009 c55.15660.009 a57.41660.009 c55.12460.009 a57.44860.008 c55.1760.01
70Ti–5Si–25Sn
Th.w.
65Ti–10Si–25Sn
Th.w.
62.5Ti–12.5Si–25Sn
Th.w.
a, a-(Ti,Si) a, a-(Ti,Si,Sn)
a 2 , Ti 3 Sn a2, Ti 3 (Sn,Si)
2 / 1, Ti 2 Sn 2 / 1, Ti 2 (Sn,Si)
Z, Ti 5 Si 3
Z, Ti 5 (Si,Sn) 3
Ni 3 Sn, hP8 –P41 21 2 Ni 3 Sn, hP8 –P41 21 2
Ni 2 In, hP6 –P63 /mmc Ni 2 In, hP6 –P63 /mmc
Mn 5 Si 3 , hP16 –P63 /mcm
Mn 5 Si 3 , hP16 –P63 /mcm
[10] [11] [4] 62.5Ti–37.5Si
[5]
86.5Ti–13.5Si, 1250 8C 90Ti–7Si–3Sn
[5]
85Ti–5Si–10Sn
Th.w.
85Ti–10Si–5Sn
Th.w.
75Ti–10Si–15Sn
Th.w.
62.5Ti–27.5Si–10Sn
Th.w.
62.5Ti–22.5Si–15Sn
Th.w.
62.5Ti–17.5Si–20Sn
Th.w.
[2]
M. Bulanova et al. / Journal of Alloys and Compounds 350 (2003) 164–173
169
Table 3. Continued Phase
Crystal structure
˚ Lattice periods (A)
5 / 3, Ti 5 Sn 3
Mn 5 Si 3 , hP16 –P63 /mcm Mn 5 Si 3 , hP16 –P63 /mcm
a58.049(2), c55.454(2) a58.05160.008 c55.4860.01 a58.03660.005 c55.48960.005 a58.05260.007 c55.47060.007 a58.02960.005 c55.46160.005 a58.04460.004 c55.46460.005 a58.06560.003 c55.48460.004 a510.16260.009 c55.09860.008 a510.23860.007 c55.08160.007 a510.2260.01 c55.05860.009 a510.3560.01 c55.1360.01 a510.2560.01 c55.1060.01 a510.2860.01 c55.1260.01 a510.43260.006 c55.12960.003
5 / 3, Ti 5 (Sn,Si) 3
T
W5 Si 3 , tI32 –I4 /mcm
a
Remarks
Refs. [9,10]
55Ti–5Si–40Sn
Th.w.
60Ti–10Si–30Sn
Th.w.
55Ti–10Si–35Sn
Th.w.
62.5Ti–12.5Si–25Sn
Th.w.
62.5Ti–7.5Si–30Sn
Th.w.
62.5Ti–2.5Si–35Sn
Th.w.
70Ti–5Si–25Sn
Th.w.
70Ti–10Si–20Sn
Th.w.
65Ti–10Si–25Sn
Th.w.
60Ti–10Si–30Sn
Th.w.
62.5Ti–27.5Si–10Sn
Th.w.
62.5Ti–17.5Si–20Sn
Th.w.
62.5Ti–12.5Si–25Sn
Th.w.
Th.w., results of this examination.
L1T↔a 2 1Z: L↔a 2 1T, L1Z↔T, L↔Z1a 2 ; L12 / 1↔a 2 1T: L↔a 2 12 / 1, L↔T12 / 1, L↔a 2 1T; L1T↔2 / 115 / 3: L↔T12 / 1, L↔T15 / 3, L↔2 / 115 / 3; L1Z↔T15 / 3: L1Z↔T, L↔T15 / 3, L↔T15 / 3. The two-phase fields Z1b, T12 / 1, a 2 12 / 1, Z1T and Z15 / 3 of the solidus surface have temperature maxima corresponding to the invariant three-phase equilibria L↔b1Z, L↔T12 / 1, L↔a 2 12 / 1, L1Z↔T, L↔Z15 / 3. The coordinates of the invariant points for the first two equilibria were estimated to be 1470 and 1580 8C, and |79Ti–11.5Si–9.5Sn and |62Ti–10.5Si–27.5Sn, respectively. The maximum of the b1Z solidus is located very close to the Z1b1a 2 three-phase field, almost on its boundary. The melting diagram (solidus1liquidus) is presented in Fig. 2b. The character of the equilibria and the location of invariant points and monovariant curves were determined from microstructure examinations (Fig. 3). It should be noticed that the b-phase was never observed. The X-ray patterns contained the reflections of the a-phase, resulting from the solid-state transformation b↔a. The alloys with 10 at.% Si containing 5 and 10 at.% Sn (Figs. 3a,b) are hypoeutectic with primary crystallization of the b-phase. In the alloy 80Ti–5Si–15Sn (Fig. 3f) the b-phase crystallizes first, as well. Meanwhile the striped
character of the primary grains indicates a transformation in the solid state. According to the EPMA results, light and dark regions of the primary grains correspond to the a 2 and Ti-phases, respectively. So, on cooling, the a 2 -phase precipitates from the b-phase due to the intersection of the (b1Z) /(b1Z1a 2 ) phase boundary. No traces of solid-state transformations of the primary grains were observed for the alloy 75Ti–5Si–20Sn (Fig. 3g). So the alloy is located in the field of primary crystallization of the a 2 -phase. The morphology of the primary grains in the alloy 70Ti–5Si–25Sn (Fig. 3h) is different. This allows to conclude that the 2 / 1-phase gives rise to primary crystallization for this alloy. In the alloy 75Ti–10Si–15Sn (Fig. 3c), no primary grains are found, while two types of eutectics were observed. This shows its location on the monovariant curve L↔Z1a 2 , the liquid-phase composition being moved during solidification towards the ternary eutectic L↔b1Z1a 2 . The alloys (65–60)Ti–10Si–(25–30)Sn (Fig. 3d,e) and 62.5Ti–(27.5–12.5)Si–(10–25)Sn (Fig. 3i–k) are hypereutectic with primary crystallization of the Z-phase. The amount of Z-phase in the alloy 65Ti–10Si–25Sn is extremely low, so the alloy is located very close to the monovariant curve L1Z↔T. In the alloys 62.5Ti–7.5Si–30Sn (Fig. 3l) and 62.5Ti–
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Table 4 Compositions of Ti-rich Ti–Si–Sn phases according to the EPMA examination of as-cast alloys Alloy
90Ti–10Si a 90Ti–7Si–3Sn a 90Ti–5Si–5Sn a 85Ti–10Si–5Sn
80Ti–10Si–10Sn
75Ti–10Si–15Sn
70Ti–10Si–20Sn
65Ti–10Si–25Sn
60Ti–10Si–30Sn
55Ti–10Si–35Sn
50Ti–10Si–40Sn
85Ti–5Si–10Sn
80Ti–5Si–15Sn
75Ti–5Si–20Sn
70Ti–5Si–25Sn
62.5Ti–27.5Si–10Sn
Chemical composition (at.%) Ti
Si
Sn
95.4 95.3 92.2 92.1 91.6 92.0 90.9 91.0 79.6 84.1 85.2 78.0 78.0 62.4 62.2 75.0 74.7 77.2 80.3 71.9 62.6 62.6 70.6 62.6 62.5 67.7 68.0 62.0 62.5 62.2 62.3 59.6 59.8 62.0 62.4 55.2 57.6 58.3 62.0 61.9 57.2 56.5 56.7 87.1 87.0 63.9 78.2 77.4 88.1 81.3 80.8 75.2 74.8 64.9 66.9 66.5 66.9 65.3 64.5 64.7
4.5 4.6 3.8 3.8 2.9 2.6 2.7 2.6 16.3 2.0 2.3 13.2 13.5 35.9 34.7 1.5 1.4 14.9 10.5 8.8 20.1 19.4 7.5 17.7 19.7 2.9 2.7 34.1 15.1 34.0 17.2 4.1 4.8 35.6 35.2 0.6 8.7 10.5 35.8 35.4 10.6 8.9 8.6 1.5 1.6 9.3 0.8 0.6 1.3 2.3 3.3 0.0 0.0 29.1 5.3 4.8 5.0 2.7 34.3 34.0
0 0 4.0 4.0 5.5 5.4 6.4 6.4 4.0 13.9 12.5 8.7 8.5 1.7 3.1 23.4 23.9 7.9 9.2 19.2 17.3 18.0 21.9 19.6 17.8 29.4 29.3 3.9 22.4 3.8 20.5 36.2 35.4 2.4 2.4 44.2 33.7 31.2 2.2 2.7 32.3 34.6 34.7 11.4 11.4 8.2 21.0 22.1 10.6 16.4 15.8 24.8 25.2 6.0 27.8 28.6 28.0 32.0 1.1 1.4
Table 4. Continued Alloy
Identification of the measurement
b b b b b b b b Mixture b b Mixture Mixture Z Z a2 a2 Mixture Mixture Mixture T T Mixture T T 2/1 2/1 Z T Z T 5/3 5/3 Z Z 6/5 5/3 5/3 Z Z 5/3 5/3 5/3 b b Mixture Mixture Mixture b Mixture Mixture a2 a2 Mixture Mixture Mixture Mixture 2/1 Z Z
62.5Ti–22.5Si–15Sn
62.5Ti–17.5Si–20Sn
b1Z
b1Z b1Z
62.5Ti–12.5Si–25Sn
62.5Ti–7.5Si–30Sn
b1Z b1Z a 2 1Z
a 2 1T
b1Z b1a 2 b1a 2 b1Z b1Z
a 2 1Z 2 / 11T 2 / 11T 2 / 11T
62.5Ti–2.5Si–35Sn
a
Chemical composition (at.%) Ti
Si
Sn
67.6 67.4 63.1 64.4 65.5 66.9 64.1 63.9 63.6 65.3 65.5 63.4 63.6 64.0 61.8 64.3 63.4 60.8 60.4 64.0 59.8 60.2 60.4
1.9 2.1 35.4 33.9 2.3 2.2 33.2 15.4 19.1 1.4 1.0 12.2 16.0 13.6 2.5 16.4 17.5 3.9 4.0 2.2 2.9 3.1 2.9
30.4 30.6 1.5 1.7 32.2 30.9 2.4 20.6 17.3 33.3 33.5 24.4 20.4 22.4 35.8 19.2 19.1 35.4 35.6 33.8 37.2 36.8 36.6
Identification of the measurement
2/1 2/1 Z Z 2/1 2/1 Z T T 2/1 2/1 Mixture 2 / 11T T Mixture 2 / 11T Mixture 2 / 115 / 3 T T kTi 5 Sn 3 l kTi 5 Sn 3 l Mixture 2 / 115 / 3 5/3 5/3 5/3
Data from Ref. [2]. All the other results are obtained from this study.
2.5Si–35Sn (Fig. 3m), the primary phases are T and 5 / 3, respectively. The complicated character of the solidification of the alloys with 62.5 at.% Ti, located in the field of Z-phase primary crystallization can be seen from the microstructure of the alloy 62.5Ti–27.5Si–10Sn (Fig. 3i,j). Fig. 3j shows enlarged fields between the primary grains. This allows to consider the following route of crystallization: Z (primary)→T (L1Z↔T)→2 / 1 (L1T↔2 / 1)→a 2 (L↔a 2 12 / 1). For the alloys with 5 at.% Si, the microhardness of the primary grains was measured versus the tin concentration in the alloys. The results are shown in Fig. 4. As it is seen, the microhardness of the primary b-grains, transformed on cooling to a (0–10 at.% Sn), increases when the tin concentration increases. The microhardness of the primary b-grains in the alloy with 15 at.% Sn has a substantially lower value. As discussed above, in this alloy (Fig. 3f) in addition to the b→a transformation, a 2 precipitates from the primary b-phase on cooling. This allows to suppose that the microhardness represents the average value between b (transformed into a) and a 2 . This is proved by the lower value of the microhardness for the alloy with 20 at.% Sn, where the primary phase is a 2 and no transformations occur on cooling (Fig. 3g). Thus, the microhardness of kTi 3 Snl is essentially lower than that of the ternary kTi,Si,Snl solid solution, while it is of the same level as for the binary kTi,Sil solid solution.
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Fig. 3. Microstructures of as-cast Ti–10Si–Sn alloys: (a) 85Ti–10Si–5Sn, 31000, b1eutectic (b1Z); (b) 80Ti–10Si–10Sn, 31000, b1eutectic (b1Z); (c) 75Ti–10Si–15Sn, 31000, eutectic (Z1a 2 )1eutectic (b1Z1a 2 ); (d) 65Ti–10Si–25Sn, 31000, T1Z1a 2 12 / 1; (e) 60Ti–10Si–30Sn, 31000, Z1T15 / 3; (f) 80Ti–5Si–15Sn, 31000, b1eutectic (b1Z); (g) 75Ti–5Si–20Sn, 31000, a 2 1eutectic (a 2 1Z); (h) 70Ti–5Si–25Sn, 34000, 2 / 11eutectic (a 2 12 / 1); (i) 62.5Ti–27.5Si–10Sn, 3600, Z1T12 / 11a 2 ; (j) 62.5Ti–27.5Si–10Sn, 32400, intergrain space, Z1T12 / 11a 2 ; (k) 62.5Ti–12.5Si–25Sn, 31000, Z1T12 / 115 / 3; (l) 62.5Ti–7.5Si–30Sn, 31000, T1eutectic (T15 / 3); (m) 62.5Ti–2.5Si–35Sn, 3600, 5 / 31eutectic (T15 / 3).
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M. Bulanova et al. / Journal of Alloys and Compounds 350 (2003) 164–173
Fig. 3. (continued)
4. Conclusions The character of phase equilibria at crystallisation is studied for the Ti-corner of the Ti–Si–Sn system. The
solidus projection, melting diagram (solidus1liquidus) and reaction scheme at crystallization are constructed. The most prominent feature of the system is the existence of the ternary compound T, which has been found for the first time. The crystal structure of the T-compound was shown to be tetragonal, of the W5 Si 3 type. A ternary eutectic equilibrium occurs in the system at a temperature of 1460 8C. The invariant point (E) is located at |77Ti–9Si–14Sn. The microhardness of the primary grains was measured for the alloys with 5 at.% Si. The microhardness of the kTil-grains increases versus tin concentration in the alloys. The microhardness of kTi 3 Snl was shown to be substantially lower than that of kTi,Si,Snl.
Acknowledgements
Fig. 4. Microhardness of the primary grains in the alloys with 5 at.% Si versus tin content in the alloys.
The work was performed within the Partner Project Agreement N P-060 between the Science and Technology Center in Ukraine, the I.N. Frantsevich Institute for
M. Bulanova et al. / Journal of Alloys and Compounds 350 (2003) 164–173
Problems of Materials Science of National Academy of Science and EOARD.
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