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TiAl–TiGa section of the Ti–Al–Ga system
Journal of Alloys and Compounds 264 (1998) 167–172
L
TiAl–TiGa section of the Ti–Al–Ga system N.V. Antonova*, L.A. Tretyachenko, T.Ya. Velikanova, P.S. Martsenyuk National Academy of Science of Ukraine, I.N. Frantsevich Institute for Problems of Materials Science, 3 Krzyzanovsky Str., Kyiv, 252180, Ukraine Received 20 September 1996; received in revised form 26 May 1997
Abstract Alloys on the basis of titanium aluminides are very important for modern industry. The influence of b-stabilizer of titanium on the structure and properties of Ti–Al alloys has been studied by many authors. The alloying of titanium by a-stabilizers has almost not been investigated. The purpose of this investigation was to determine the influence of gallium on the structure of Ti–Al based alloys at 50 at.% Ti. The TiAl–TiGa section has been investigated by means of metallographic, electron microprobe, differential thermal and XRD analyses. The alloys were prepared by arc-melting from pure metals with an unconsumable tungsten electrode on a water-cooled copper hearth and studied in both as-cast and annealed state. It is shown that continuous solid solutions g are formed between the equiatomic phases TiAl and TiGa with the same crystal structure (tetragonal CuAu-type) and similar lattice parameters. 1998 Elsevier Science S.A. Keywords: Ti–Al–Ga alloys; High-temperature alloys; Phase relationships
1. Introduction Attractive elevated-temperature properties and low density, good oxidation resistance make the titanium aluminides Ti 3 Al, TiAl and TiAl 3 very interesting for both engine and airframe application, particularly in airspace industry. However, they are extremely brittle near room temperatures. Alloying of Ti–Al alloys by the elements that retain the attractive high temperature properties whilst improving the low temperature ductility should solve this problem. The influence of isomorphous b-stabilizers of titanium, such as Mo, Nb, Ta, V, W, etc. and those which form eutectoids, such as Cr, Mn, Fe, on the structure and properties of the Ti–Al alloys has been studied by many authors [1]. The alloying of titanium with a-stabilizers are almost not investigated. Gallium is an element of the third period of the Periodic Table and as well as aluminium is an a-stabilizer of titanium. It shows a high solubility in a-Ti (up to 13 at.% Ti) and in b-Ti (up to 28 at.% Ti) [2] and increases the temperature of the a⇔b allotropic transformation of titanium from 882 to 940 8C [2]. Gallium forms a number of stable titanium gallides. Some of them are isostructural with titanium aluminides. They are Ti 3 X (X5Al,Ga) with hexagonal Ni 3 Sn-type, TiX with CuAu type, TiX 2 with tetragonal HfGa 2 -type and TiX 3 with tetragonal TiAl 3 *Corresponding author. 0925-8388 / 98 / $19.00 1998 Elsevier Science S.A. All rights reserved. PII S0925-8388( 97 )00257-0
type structures (Table 1). As far as aluminides and gallides have close lattice parameters so the formation of continuous solid solutions between isostructural aluminides and gallides would be expected. The phase diagram of the ternary Ti–Al–Ga system is not constructed yet. There are only a few works concerning the Ti-rich region of this system. The results of the investigations have been summarised by Perrot [7] taking into consideration the Ti–Al and Ti–Ga binary phase diagrams and experimental data on the structure of ternary alloys. The isothermal sections of the ternary Ti–Al–Ga phase diagram in the Ti-rich region up to 70 at.% Ti at 900 and 1000 8C have been constructed [7]. The formation of continuous solid solutions between Ti 3 Al and Ti 3 Ga phases with hexagonal structure (Ni 3 Sn-type) has been suggested. The aim of this study is the investigation of the structure of the alloys of the TiAl–TiGa section. According to the assessed Ti–Al phase diagram [8], the binary phase TiAl (g) is formed by the peritectic reaction L1(a-Ti)⇔g at 1462 8C. According to the data of the present study the g phase actually forms by such a reaction, but the temperature of the peritectic reaction was found to be somewhat higher |1485 8C. The titanium-poor boundary of the g homogeneity range is displaced towards aluminium but moves to the 1:1 stoichiometry when the temperature decreases, as it is accepted by Perrot [7] and Okamoto [8] too. Recently similar results have been found by Ding and
N.V. Antonova et al. / Journal of Alloys and Compounds 264 (1998) 167 – 172
168
Table 1 Crystal structure of the isostructural intermediate phases in the Ti–Al and Ti–Ga systems Phase
Ti 3 X
TiX
Structural type, Pearson symbol Ni 3 Sn, hP8
CuAu, tP4
X5Al
X5Ga
Lattice parameters (nm)
Reference
Lattice parameters (nm)
References
a50.5787, c50.4637 a50.5791 c50.4657
[3]
a50.5752 c50.4645 a50.5748 c50.4651
[4]
a50.4005, c50.4070 a50.4003 c50.4076
[5]
a50.3969, c50.3970 a50.3969 c50.3978
[4]
This work
This work
This work
This work
TiX 2
HfGa 2 , tI24
a50.3967, c52.4297
[6]
a50.3929, c52.4370
[4]
TiX 3
TiAl 3 , t18
a50.3849, c50.8598
[6]
a50.3789, c50.8734
[4]
co-workers [9] by means of diffusion couples with subsequent EMPA examination. ¨ According to Potzschke and Schubert [4], TiGa occurs as a result of the peritectic reaction L1 Ti 5 Ga 4 (r)⇔TiGa(g) at 1175 8C and the composition of g-phase is 50Ti–50Ga. The results of the present study confirms such type of TiGa formation reaction, but the temperature of the reaction was determined to be 1250 8C.
2. Experimental procedure Eight alloys of different compositions were prepared from commercially available metals of high purity (iodide titanium (99.98%), aluminium (99.995%) and gallium (99.99%)). Samples were prepared in an arc-furnace with nonconsumable tungsten electrode on a water-cooled copper hearth in an argon atmosphere gettered by titanium. The weight losses during melting were mostly small, therefore nominal composition was accepted. The ingots of 5 g were turned over and remelted four times to obtain homogeneous alloys. The alloys were annealed to achieve homogeneous state at the temperature 1200 8C for 53 h in aluminium oxide crucibles under purified argon. The alloys containing 50, 45, 40 and 30 at.% Al were annealed in addition at 1300 8C for 33 h. A study was made of as-cast and annealed alloys using metallographic (MSA), electron microprobe (EPMA), differential thermal (DTA) and X-ray diffraction (XRD) analyses. The microstructure of the alloys was examined by means of the MIM-8M optical microscope. For revealing the microstructure of alloys the polished samples were etched in a mixture of 10 ml HF, 20 ml HNO 3 and 30 ml H 2 O. XRD study was carried out on powder samples in the
Debay camera (d557.3 mm) by the URS-2.0 device or the DRON-UM diffractometer in monochromic Cu Ka radiation. The lattice parameters were refined with the computer program ‘‘Lattice’’ using the least squares fit. Composition of phases in some annealed and as cast alloys was determined using the EPMA by Superprobe 733 (JEOL LTD, Tokyo, Japan). The photographs were obtained with reflected electrons with this device and the MIM-8M optical microscope. The error in the results of the EPMA was observed to be within 60.5%. The temperatures of the phase transformations have been determined by DTA taking into consideration only heating curves owing to the effect of the supercooling which was observed on the cooling curves for both as cast and annealed alloys. DTA-curves were obtained in DTAanalyzer with a W/ W–Re string thermocouple in He atmosphere. The Al 2 O 3 crucibles were used. The heating 21 rate was 40 8C min . Calibrating by points of phase transformation of pure Fe and melting points of pure Au and Pt was used to obtain exact values of phase transformation temperatures in alloys which have been analysed.
3. Results and discussion The investigation results of the alloys in the TiAl–TiGa section are presented in Table 2. According to the MSA and EPMA data the structure of as cast alloys at low gallium content (5 and 10 at.%) is similar to the structure of TiAl alloy (Fig. 1(a)). As shown in this picture the primary titanium grains with lamellar structure are surrounded by peritectically formed g-matrix. Perhaps the crystallization in this concentration region starts with the
N.V. Antonova et al. / Journal of Alloys and Compounds 264 (1998) 167 – 172
169
Table 2 The phase composition and the temperatures of the phase transformations of the Ti–Al–Ga alloys along the section at 50 at.% Ti Composition of alloys (at.%)
Phase composition As-cast
Al 50 45 40 30 20 10 5 0 a b
Solidus Annealed
Temperature of the phase transformations (8C)
Ga 0 5 10 20 30 40 45 50
a
a b
g 1(a ) g1(a) g1(a) g g g1r a g1r g1r
1200 8C
1300 8C
Liquidus
g g g g g g1r a g1r g1r
g g g g
1505 1410 1370 1345 1315 1295 – 1250
1520 1460 1435 1380 1350 1375 – 1405
g – Ti (Al x Ga 12x )-phase, r – Ti 5 (Al x Ga 12x ) 4 -phase, a – solid solution on the a-Ti basis. () – small quantity of a phase.
formation of (b-Ti), but a and b phases are undistinguishable in this case by means of the used methods of investigation. According to the EPMA data the Ti-based
phase in the alloy with 5 at.% Ga was found to have the composition: 51.7 at.% Ti, 44.5 at. % Al and 3.8 at.% Ga and the composition of g-phase 48.8% Ti, 45.7 at.% Al
Fig. 1. Microstructure of the alloys in the TiAl–TiGa section of the Ti–Al–Ga system: (a) 50Ti–45Al–5Ga as-cast (a1g); (b) 50Ti–30Al–20Ga as-cast (g), (c) 50Ti–30Al–20Ga annealed at 1300 8C for 33 h (g), (d) 50Ti–45Ga–5Al as cast (r1g), (e) 50Ti–45Ga–5Al annealed at 1200 8C for 53 h (g1r).
N.V. Antonova et al. / Journal of Alloys and Compounds 264 (1998) 167 – 172
170
Table 3 Compositions of the phases in the annealed alloys of two-phase (g1r) region for the Ti–Al–Ga alloys along the section at 50 at.% Ti according to EMPA data Composition of alloys (at.%)
Phase
Compositon of the phases (at.%)
Compositon of g-phase Ti 12y (Al x Ga 12x ) 11y
Ga
Al Al
Ga
x
y
50
0
g r
47.7 54.1
– –
52.3 45.9
0 –
0.046
45
5
g r
48.7 54.7
5.8 2.2
45.5 43.1
0.113
0.026
40
10
g r
47.0 53.2
12.6 5.0
40.4 41.8
0.238
0.06
Ti
and 5.5 at.% Ga (Ti 0.98 (Al 0.93 Ga 0.07 ) 1.02 ). In both as-cast and annealed alloys with 20 and 30 at.% Ga only the g-phase is observed, which was crystallized directly from the melt (Fig. 1(b)). The alloys with a content of gallium up to 20 at.% annealed at 1300 8C have single-phase structure with polyhedral grains (Fig. 1(c)). The binary compound TiGa and the alloys with high gallium contents (up to 10 at.% Al) show a conglomerate structure, that consists of the primary Ti 5 Ga 4 (r) grains in g matrix (Fig. 1(d)). This structure is the result of the peritectic reaction L p 1r⇔g. Ti 5 Ga 4 (r) and TiGa (g) phases were identified by XRD, EMPA and MSA methods. The alloys with 45 and 40 at.% Ga as well as TiGa remained two-phase (g1r) after annealing at 1200 8C for 53 h, but the amount of r phase decreases in comparison with as cast alloys (Fig. 1(e)). The composition of r and g phases are given in Table 3 according to the EPMA data. The Ti-rich edge of the g homogeneity region in the Ga-rich alloys runs away from a 1:1 stoichiometry to lower Ti contents. It does not agree with the Ti–Ga phase ¨ diagram by Potzschke and Schubert [4]. The lattice parameters of g and r phases in annealed alloys are given in Table 4 without taking into consideration insignificant
deviation in its composition from stoichiometry. The concentration dependence of lattice parameters for g phase is shown in Fig. 2 in terms of the Ga to Al ratio in g phase. It shows a nearly linear decrease of the lattice parameters with increasing gallium content. The continuous variation of the g lattice parameters throughout the g existence region from TiAl to TiGa was accepted as evidence of the formation of a continuous solid solution between these compounds in the Ti–Al–Ga system. Fig. 3 shows the vertical section of the Ti–Al–Ga phase diagram at 50 at.% Ti according to the results of the present investigation. As it is seen the section contains not only a wide range of one-phase g-solid solutions between TiAl and TiGa, but also two-phase regions g1a at the subsolidus temperatures in the aluminium-rich region and g1r in the gallium-rich one when the gallium content exceeds 35 at.% Ga. The latter region is present because of the deviation of the Ti-rich phase boundary of the g homogeneity region from the 1:1 stoichiometry. Heating and cooling curves were obtained by means of the DTA for as-cast and annealed samples (Fig. 4). It is observed that the solidus temperature decreases with increasing gallium content. The solidus of the section consists of three segments. The first one for Al-rich alloys
Table 4 Lattice parameters of the g and r phases in alloys of the TiAl–TiGa section annealed at 1200 8C Composition of alloys (at.%)
Lattice parameters (nm) Ti 12y (Al x Ga 12x ) 11y (g)
Ti 5 (Al x Ga 12x ) 4 (r)
Ga
Al
a
c
a
c
0 5 10 20 30 40 45 50
50 45 40 30 20 10 5 0
0.400360.0003 0.399960.0001 0.399560.0002 0.398960.0002 0.397960.0002 0.397560.0002 0.396560.0003 0.396960.0004
0.407660.0003 0.406560.0002 0.405360.0004 0.403160.0003 0.401860.0002 0.400460.0004 0.399260.0004 0.397860.0003
– – – – – 0.786860.0008 0.785660.0004 0.785760.0008
– – – – – 0.533960.0008 0.544660.0006 0.546560.0009
N.V. Antonova et al. / Journal of Alloys and Compounds 264 (1998) 167 – 172
171
Fig. 2. Concentration dependence of lattice parameters of g-phase in the alloys of the TiAl–TiGa section. X(Ga)5Ga (at.%) /(Ga1Al) (at.%).
Fig. 3. The TiAl–TiGa section of the Ti–Al–Ga phase diagram. D DTA data, s single-phase alloy, g two-phase alloy.
is formed by intersection with the surface restricting the g1a volume from above, where the incongruent reaction L1a⇔g goes to completion. The last segment of the solidus curve for gallium rich alloys is formed by intersection of the tie-line surface g1r, where the incongruent reaction L1r⇔g goes to completion. The central part of the solidus curve corresponds to the beginning of the g-phase melting. The liquidus is also represented by three curves. For the Al-rich alloys it is the curve of primary crystallization of a-Ti, for the Ga-rich alloys it is the primary crystallization curve of the phase on the Ti 5 Ga 4 -basis. The central curve corresponds to g solid solution crystallization directly from the melt. As it is seen from Fig. 3, the peritectic temperature of TiGa (g) formation is 1250 8C, which is in disagreement with the results of [4]: 1175 8C. The liquidus temperature of the alloy with 50 at.% Ga, where the primary Ti 5 Ga 4 grains were found, is 1405 8C. This temperature considerably exceeds the Ti 5 Ga 4 melting point (1242 8C) in the binary Ti–Ga system according to data by [4]. This fact has forced us to carry out the re-examination of the Ti 5 Ga 4 melting point. According to the DTA results of this work the Ti 5 Ga 4 melting point was determined to be 1460 8C.
4. Conclusions
Fig. 4. DTA heating curves of the annealed alloys: (a) 45Al–5Ga, (b) 40Al–10Ga, (c) 30Al–20Ga, (d) 20Al–30Ga, (e) 10Al–40Ga, (f) 0Al– 50Ga.
The formation of continuous g-solid solutions between equiatomic phases TiAl and TiGa with the same tetragonal CuAu-type crystal structure and similar lattice parameters has been established by the methods of MSA, XRD, EMPA and DTA. The type of g-phase crystallization depends upon the composition of alloys. It is peritectic in aluminium- or gallium-rich regions owing to the peritectic type of TiAl and TiGa formation. g-Solid solutions crystallize directly from the melt in the concentration range |10–35 at.% Ga of the section at 50 at.% Ti. Lattice parameters of g-phase were found to decrease nearly linearly with Ga content.
172
N.V. Antonova et al. / Journal of Alloys and Compounds 264 (1998) 167 – 172
References [1] A.T. Tumanov (Ed.), Titanium Alloys, Metallurgia, Moskwa, 1976. [2] J.L. Murray, in: T.B. Massalski (Ed.), Binary Alloy Phase Diagrams, Vol. 2, 2nd ed., ASM International, Materials Park, OH, 1990, pp. 1866–1868. [3] P. Rogl, Titanium: Physico-chemical Properties of its Compounds and Alloys, IAEA, Vienna, 1983, pp. 201–369. ¨ [4] M. Potzschke, K. Schubert, Z. Metallk. 53 (1962) 74–489.
[5] P. Villars, L. Calvert, Handbook of Crystallographic Data for Intermetallic Phases, ASM, Metals Park, OH, 1985. [6] J.C. Schuster, H. Ipser, Z. Metallk. 81 (1990) 83–386. [7] P. Perrot, Ternary Alloys, Vol. 5, VCH, Weinheim, 1992, pp. 627630. [8] H. Okamoto, J. Phase Equil. 14 (1993) 120–121. [9] J.J. Ding, G.W. Gin, S.M. Hao, X.T. Wang, G.L. Chen, J. Phase Equil. 17 (1996) 117–120.
L
TiAl–TiGa section of the Ti–Al–Ga system N.V. Antonova*, L.A. Tretyachenko, T.Ya. Velikanova, P.S. Martsenyuk National Academy of Science of Ukraine, I.N. Frantsevich Institute for Problems of Materials Science, 3 Krzyzanovsky Str., Kyiv, 252180, Ukraine Received 20 September 1996; received in revised form 26 May 1997
Abstract Alloys on the basis of titanium aluminides are very important for modern industry. The influence of b-stabilizer of titanium on the structure and properties of Ti–Al alloys has been studied by many authors. The alloying of titanium by a-stabilizers has almost not been investigated. The purpose of this investigation was to determine the influence of gallium on the structure of Ti–Al based alloys at 50 at.% Ti. The TiAl–TiGa section has been investigated by means of metallographic, electron microprobe, differential thermal and XRD analyses. The alloys were prepared by arc-melting from pure metals with an unconsumable tungsten electrode on a water-cooled copper hearth and studied in both as-cast and annealed state. It is shown that continuous solid solutions g are formed between the equiatomic phases TiAl and TiGa with the same crystal structure (tetragonal CuAu-type) and similar lattice parameters. 1998 Elsevier Science S.A. Keywords: Ti–Al–Ga alloys; High-temperature alloys; Phase relationships
1. Introduction Attractive elevated-temperature properties and low density, good oxidation resistance make the titanium aluminides Ti 3 Al, TiAl and TiAl 3 very interesting for both engine and airframe application, particularly in airspace industry. However, they are extremely brittle near room temperatures. Alloying of Ti–Al alloys by the elements that retain the attractive high temperature properties whilst improving the low temperature ductility should solve this problem. The influence of isomorphous b-stabilizers of titanium, such as Mo, Nb, Ta, V, W, etc. and those which form eutectoids, such as Cr, Mn, Fe, on the structure and properties of the Ti–Al alloys has been studied by many authors [1]. The alloying of titanium with a-stabilizers are almost not investigated. Gallium is an element of the third period of the Periodic Table and as well as aluminium is an a-stabilizer of titanium. It shows a high solubility in a-Ti (up to 13 at.% Ti) and in b-Ti (up to 28 at.% Ti) [2] and increases the temperature of the a⇔b allotropic transformation of titanium from 882 to 940 8C [2]. Gallium forms a number of stable titanium gallides. Some of them are isostructural with titanium aluminides. They are Ti 3 X (X5Al,Ga) with hexagonal Ni 3 Sn-type, TiX with CuAu type, TiX 2 with tetragonal HfGa 2 -type and TiX 3 with tetragonal TiAl 3 *Corresponding author. 0925-8388 / 98 / $19.00 1998 Elsevier Science S.A. All rights reserved. PII S0925-8388( 97 )00257-0
type structures (Table 1). As far as aluminides and gallides have close lattice parameters so the formation of continuous solid solutions between isostructural aluminides and gallides would be expected. The phase diagram of the ternary Ti–Al–Ga system is not constructed yet. There are only a few works concerning the Ti-rich region of this system. The results of the investigations have been summarised by Perrot [7] taking into consideration the Ti–Al and Ti–Ga binary phase diagrams and experimental data on the structure of ternary alloys. The isothermal sections of the ternary Ti–Al–Ga phase diagram in the Ti-rich region up to 70 at.% Ti at 900 and 1000 8C have been constructed [7]. The formation of continuous solid solutions between Ti 3 Al and Ti 3 Ga phases with hexagonal structure (Ni 3 Sn-type) has been suggested. The aim of this study is the investigation of the structure of the alloys of the TiAl–TiGa section. According to the assessed Ti–Al phase diagram [8], the binary phase TiAl (g) is formed by the peritectic reaction L1(a-Ti)⇔g at 1462 8C. According to the data of the present study the g phase actually forms by such a reaction, but the temperature of the peritectic reaction was found to be somewhat higher |1485 8C. The titanium-poor boundary of the g homogeneity range is displaced towards aluminium but moves to the 1:1 stoichiometry when the temperature decreases, as it is accepted by Perrot [7] and Okamoto [8] too. Recently similar results have been found by Ding and
N.V. Antonova et al. / Journal of Alloys and Compounds 264 (1998) 167 – 172
168
Table 1 Crystal structure of the isostructural intermediate phases in the Ti–Al and Ti–Ga systems Phase
Ti 3 X
TiX
Structural type, Pearson symbol Ni 3 Sn, hP8
CuAu, tP4
X5Al
X5Ga
Lattice parameters (nm)
Reference
Lattice parameters (nm)
References
a50.5787, c50.4637 a50.5791 c50.4657
[3]
a50.5752 c50.4645 a50.5748 c50.4651
[4]
a50.4005, c50.4070 a50.4003 c50.4076
[5]
a50.3969, c50.3970 a50.3969 c50.3978
[4]
This work
This work
This work
This work
TiX 2
HfGa 2 , tI24
a50.3967, c52.4297
[6]
a50.3929, c52.4370
[4]
TiX 3
TiAl 3 , t18
a50.3849, c50.8598
[6]
a50.3789, c50.8734
[4]
co-workers [9] by means of diffusion couples with subsequent EMPA examination. ¨ According to Potzschke and Schubert [4], TiGa occurs as a result of the peritectic reaction L1 Ti 5 Ga 4 (r)⇔TiGa(g) at 1175 8C and the composition of g-phase is 50Ti–50Ga. The results of the present study confirms such type of TiGa formation reaction, but the temperature of the reaction was determined to be 1250 8C.
2. Experimental procedure Eight alloys of different compositions were prepared from commercially available metals of high purity (iodide titanium (99.98%), aluminium (99.995%) and gallium (99.99%)). Samples were prepared in an arc-furnace with nonconsumable tungsten electrode on a water-cooled copper hearth in an argon atmosphere gettered by titanium. The weight losses during melting were mostly small, therefore nominal composition was accepted. The ingots of 5 g were turned over and remelted four times to obtain homogeneous alloys. The alloys were annealed to achieve homogeneous state at the temperature 1200 8C for 53 h in aluminium oxide crucibles under purified argon. The alloys containing 50, 45, 40 and 30 at.% Al were annealed in addition at 1300 8C for 33 h. A study was made of as-cast and annealed alloys using metallographic (MSA), electron microprobe (EPMA), differential thermal (DTA) and X-ray diffraction (XRD) analyses. The microstructure of the alloys was examined by means of the MIM-8M optical microscope. For revealing the microstructure of alloys the polished samples were etched in a mixture of 10 ml HF, 20 ml HNO 3 and 30 ml H 2 O. XRD study was carried out on powder samples in the
Debay camera (d557.3 mm) by the URS-2.0 device or the DRON-UM diffractometer in monochromic Cu Ka radiation. The lattice parameters were refined with the computer program ‘‘Lattice’’ using the least squares fit. Composition of phases in some annealed and as cast alloys was determined using the EPMA by Superprobe 733 (JEOL LTD, Tokyo, Japan). The photographs were obtained with reflected electrons with this device and the MIM-8M optical microscope. The error in the results of the EPMA was observed to be within 60.5%. The temperatures of the phase transformations have been determined by DTA taking into consideration only heating curves owing to the effect of the supercooling which was observed on the cooling curves for both as cast and annealed alloys. DTA-curves were obtained in DTAanalyzer with a W/ W–Re string thermocouple in He atmosphere. The Al 2 O 3 crucibles were used. The heating 21 rate was 40 8C min . Calibrating by points of phase transformation of pure Fe and melting points of pure Au and Pt was used to obtain exact values of phase transformation temperatures in alloys which have been analysed.
3. Results and discussion The investigation results of the alloys in the TiAl–TiGa section are presented in Table 2. According to the MSA and EPMA data the structure of as cast alloys at low gallium content (5 and 10 at.%) is similar to the structure of TiAl alloy (Fig. 1(a)). As shown in this picture the primary titanium grains with lamellar structure are surrounded by peritectically formed g-matrix. Perhaps the crystallization in this concentration region starts with the
N.V. Antonova et al. / Journal of Alloys and Compounds 264 (1998) 167 – 172
169
Table 2 The phase composition and the temperatures of the phase transformations of the Ti–Al–Ga alloys along the section at 50 at.% Ti Composition of alloys (at.%)
Phase composition As-cast
Al 50 45 40 30 20 10 5 0 a b
Solidus Annealed
Temperature of the phase transformations (8C)
Ga 0 5 10 20 30 40 45 50
a
a b
g 1(a ) g1(a) g1(a) g g g1r a g1r g1r
1200 8C
1300 8C
Liquidus
g g g g g g1r a g1r g1r
g g g g
1505 1410 1370 1345 1315 1295 – 1250
1520 1460 1435 1380 1350 1375 – 1405
g – Ti (Al x Ga 12x )-phase, r – Ti 5 (Al x Ga 12x ) 4 -phase, a – solid solution on the a-Ti basis. () – small quantity of a phase.
formation of (b-Ti), but a and b phases are undistinguishable in this case by means of the used methods of investigation. According to the EPMA data the Ti-based
phase in the alloy with 5 at.% Ga was found to have the composition: 51.7 at.% Ti, 44.5 at. % Al and 3.8 at.% Ga and the composition of g-phase 48.8% Ti, 45.7 at.% Al
Fig. 1. Microstructure of the alloys in the TiAl–TiGa section of the Ti–Al–Ga system: (a) 50Ti–45Al–5Ga as-cast (a1g); (b) 50Ti–30Al–20Ga as-cast (g), (c) 50Ti–30Al–20Ga annealed at 1300 8C for 33 h (g), (d) 50Ti–45Ga–5Al as cast (r1g), (e) 50Ti–45Ga–5Al annealed at 1200 8C for 53 h (g1r).
N.V. Antonova et al. / Journal of Alloys and Compounds 264 (1998) 167 – 172
170
Table 3 Compositions of the phases in the annealed alloys of two-phase (g1r) region for the Ti–Al–Ga alloys along the section at 50 at.% Ti according to EMPA data Composition of alloys (at.%)
Phase
Compositon of the phases (at.%)
Compositon of g-phase Ti 12y (Al x Ga 12x ) 11y
Ga
Al Al
Ga
x
y
50
0
g r
47.7 54.1
– –
52.3 45.9
0 –
0.046
45
5
g r
48.7 54.7
5.8 2.2
45.5 43.1
0.113
0.026
40
10
g r
47.0 53.2
12.6 5.0
40.4 41.8
0.238
0.06
Ti
and 5.5 at.% Ga (Ti 0.98 (Al 0.93 Ga 0.07 ) 1.02 ). In both as-cast and annealed alloys with 20 and 30 at.% Ga only the g-phase is observed, which was crystallized directly from the melt (Fig. 1(b)). The alloys with a content of gallium up to 20 at.% annealed at 1300 8C have single-phase structure with polyhedral grains (Fig. 1(c)). The binary compound TiGa and the alloys with high gallium contents (up to 10 at.% Al) show a conglomerate structure, that consists of the primary Ti 5 Ga 4 (r) grains in g matrix (Fig. 1(d)). This structure is the result of the peritectic reaction L p 1r⇔g. Ti 5 Ga 4 (r) and TiGa (g) phases were identified by XRD, EMPA and MSA methods. The alloys with 45 and 40 at.% Ga as well as TiGa remained two-phase (g1r) after annealing at 1200 8C for 53 h, but the amount of r phase decreases in comparison with as cast alloys (Fig. 1(e)). The composition of r and g phases are given in Table 3 according to the EPMA data. The Ti-rich edge of the g homogeneity region in the Ga-rich alloys runs away from a 1:1 stoichiometry to lower Ti contents. It does not agree with the Ti–Ga phase ¨ diagram by Potzschke and Schubert [4]. The lattice parameters of g and r phases in annealed alloys are given in Table 4 without taking into consideration insignificant
deviation in its composition from stoichiometry. The concentration dependence of lattice parameters for g phase is shown in Fig. 2 in terms of the Ga to Al ratio in g phase. It shows a nearly linear decrease of the lattice parameters with increasing gallium content. The continuous variation of the g lattice parameters throughout the g existence region from TiAl to TiGa was accepted as evidence of the formation of a continuous solid solution between these compounds in the Ti–Al–Ga system. Fig. 3 shows the vertical section of the Ti–Al–Ga phase diagram at 50 at.% Ti according to the results of the present investigation. As it is seen the section contains not only a wide range of one-phase g-solid solutions between TiAl and TiGa, but also two-phase regions g1a at the subsolidus temperatures in the aluminium-rich region and g1r in the gallium-rich one when the gallium content exceeds 35 at.% Ga. The latter region is present because of the deviation of the Ti-rich phase boundary of the g homogeneity region from the 1:1 stoichiometry. Heating and cooling curves were obtained by means of the DTA for as-cast and annealed samples (Fig. 4). It is observed that the solidus temperature decreases with increasing gallium content. The solidus of the section consists of three segments. The first one for Al-rich alloys
Table 4 Lattice parameters of the g and r phases in alloys of the TiAl–TiGa section annealed at 1200 8C Composition of alloys (at.%)
Lattice parameters (nm) Ti 12y (Al x Ga 12x ) 11y (g)
Ti 5 (Al x Ga 12x ) 4 (r)
Ga
Al
a
c
a
c
0 5 10 20 30 40 45 50
50 45 40 30 20 10 5 0
0.400360.0003 0.399960.0001 0.399560.0002 0.398960.0002 0.397960.0002 0.397560.0002 0.396560.0003 0.396960.0004
0.407660.0003 0.406560.0002 0.405360.0004 0.403160.0003 0.401860.0002 0.400460.0004 0.399260.0004 0.397860.0003
– – – – – 0.786860.0008 0.785660.0004 0.785760.0008
– – – – – 0.533960.0008 0.544660.0006 0.546560.0009
N.V. Antonova et al. / Journal of Alloys and Compounds 264 (1998) 167 – 172
171
Fig. 2. Concentration dependence of lattice parameters of g-phase in the alloys of the TiAl–TiGa section. X(Ga)5Ga (at.%) /(Ga1Al) (at.%).
Fig. 3. The TiAl–TiGa section of the Ti–Al–Ga phase diagram. D DTA data, s single-phase alloy, g two-phase alloy.
is formed by intersection with the surface restricting the g1a volume from above, where the incongruent reaction L1a⇔g goes to completion. The last segment of the solidus curve for gallium rich alloys is formed by intersection of the tie-line surface g1r, where the incongruent reaction L1r⇔g goes to completion. The central part of the solidus curve corresponds to the beginning of the g-phase melting. The liquidus is also represented by three curves. For the Al-rich alloys it is the curve of primary crystallization of a-Ti, for the Ga-rich alloys it is the primary crystallization curve of the phase on the Ti 5 Ga 4 -basis. The central curve corresponds to g solid solution crystallization directly from the melt. As it is seen from Fig. 3, the peritectic temperature of TiGa (g) formation is 1250 8C, which is in disagreement with the results of [4]: 1175 8C. The liquidus temperature of the alloy with 50 at.% Ga, where the primary Ti 5 Ga 4 grains were found, is 1405 8C. This temperature considerably exceeds the Ti 5 Ga 4 melting point (1242 8C) in the binary Ti–Ga system according to data by [4]. This fact has forced us to carry out the re-examination of the Ti 5 Ga 4 melting point. According to the DTA results of this work the Ti 5 Ga 4 melting point was determined to be 1460 8C.
4. Conclusions
Fig. 4. DTA heating curves of the annealed alloys: (a) 45Al–5Ga, (b) 40Al–10Ga, (c) 30Al–20Ga, (d) 20Al–30Ga, (e) 10Al–40Ga, (f) 0Al– 50Ga.
The formation of continuous g-solid solutions between equiatomic phases TiAl and TiGa with the same tetragonal CuAu-type crystal structure and similar lattice parameters has been established by the methods of MSA, XRD, EMPA and DTA. The type of g-phase crystallization depends upon the composition of alloys. It is peritectic in aluminium- or gallium-rich regions owing to the peritectic type of TiAl and TiGa formation. g-Solid solutions crystallize directly from the melt in the concentration range |10–35 at.% Ga of the section at 50 at.% Ti. Lattice parameters of g-phase were found to decrease nearly linearly with Ga content.
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