Phase relationships in the ternary system Ti-Au-Al at 775 K

Journal of the Less-Common

Metals, 134 (1987) 99 - 107

PHASE RELATIONSH~S AT 775 K

IN THE TERNARY

99

SYSTEM

Ti-Au-AI

J. L. JORDA, J. MULLER and H. F. BRAUN* Dbpartement de Physique de la Mati&e Conder&e, Universite’ de Genkve, 24 quai Ernest-Ansermet, CH-1211 Gendve (Switzerland~ c. susz Usine Geneuoise (Switzerland)

de D&grossissage d ‘Or, Boulevard

Etienne

Marcinhes,

C’H-I 217 Meyrin

(Received December 22,1986)

Summary

The phase equilibria of the ternary system Ti-Au-Al were investigated at 775 K by means of X-ray powder diffraction, optical metallography, microhardness measurements and differential thermal analysis. Two ternary compounds, TiAuAl which melts congruently at 1453 K and TiAu*Al which forms by peritectic reaction at 1273 K, were identified, TiAuAl belongs to the hexagonal Ni,In structure with a = 4.410(2) A and c = 5.841(3) A. TiAu,Al is cubic with a = 3.198(Z) A. From the powder patterns it was not possible to establish whether the compound is of the ordered Heusler (MnCu,Al-type) structure or of the CsCl-type structure.

1. Introduction

Binary alloys involving gold, ~uminium and titanium are of considerable interest in metallurgy. For instance, compounds in the Ti-Al system are known for their good resistance to oxidation up to 1273 K; Ti3Au was reported [l] to absorb significant quantities of hydrogen. Ten years ago Marazza et al. [2] reported a ternary phase TiAu,Al with a CsCl- or Heuslertype structure. However, no attempt was made to determine the phase equilibria in the ternary Ti-Au-Al system. In this work we present the isothermal section at 775 K which was studied by means of X-ray diffraction, differential thermal analysis, metallographic and microhardness investigations. The binary systems Ti-Al, Ti-Au and Au-Al have been investigated previously and reported in the handbooks of Moffat [3], Shunk [4] and *Present address: Experimentalphysik F.R.G.

V, Universitiit Bayreuth, D-8530 Bayreuth,

@ Elsevier Sequoia~Printed in The Netherlands

100

Hansen [5] respectively. In Sections 1.1, 1.2 and 1.3 we briefly present the phases encountered at 775 K. 1.1. E-Al

The most recent version which appears in Moffat’s comp~ation [3] is due to Kubaschewski [6]. At the ~mperature of interest, four inte~e~~c compounds are present. Ti,Al is hexagonal, a = 5.79 a, c = 4.65 a and has a large honogeneity range (18 - 36 at.% Al). The TiAl phase extends from 50 - 58 at.% Al with a tetragonal cell, CuAu-type structure, a = 3.99 A and c = 4.07 a [5]. Tetragonal TiAlz (HfGa,-type structure, a = 9.976 A, c = 24.36 A) and tetragonal TiAl, (a = 3,849 a, c = 8.610 8) have limited homogeneity ranges of 1 or 2 at.% Al. All lattice parameters are taken from those compiled by Shunk [4]. 1.2. Ti-Au This binary [4] shows four intermetallic compounds with a very reduced homogeneity range. TiaAu is cubic with an A15-type structure (a = 5.0974 a). There are two allotropic forms of TiAu which may coexist at 775 K: y-TiAu is tetragonal, r-TX&type structure (a = 3.336 4. c = 6.028 A), whereas an orthorhombic AuCd-type structure (a = 2.94 A, b = 4.88 a and c = 4.63 A) is found for the gold-poor limit. TiAu, , a line compound (a = 3.419 8, c = 9.514 A) and TiAu, (a = 6.458 A, c = 3.983 A) are both tetragonal with MoSiz and MoNi4 structure types respectively. 1.3. Au-Al Au-Al is reported in Hansen [S]. Of the five intermediate phases Au,A12 is doubtful and has to be considered as an allotropic modification of Au,Al [7]. Apart from Au& which extends over 2 or 3 at.%, all the intermediate phases are reported as line compounds. &Au&l is cubic (a = 6.92 A), Au,Al is orthorhombic (a = 6.715 A, b = 3.219 a, c = 8.815 a) with the deformed MoSi,-type structure [7]. AuAl and AuAl, are cubic, of the ZnS-type structure (a = 6.05 a) and CaF,-type structure (a = 5.99 A) respectively .

2. Experimental details 53 alloys (Fig. 1) were prepared from the elements by arc melting in an argon atmosphere. Starting materials were gold pellets (purity, 99.99%; Usine Genevoise de Degrossissage d’Or), aluminium ingots (purity, 99.999%; Ventron) and titanium powder (purity, 99.9%; Ventron). The samples were turned over and remelted at’least three times in order to promote good mixing. Weight losses after melting were insi~ific~t (less than 0.2 %). All samples were annealed in a resistance furnace at 775 K for 7 days under flowing argon and subsequently quenched to room temperature.

101

Y

Ti

“I”

20

v

”

Lo

-

v 60

v

v 80

v

Au

Au Fig. 1. Composition triangle of the Ti-Au-Al single phase; 0, two phases; A, three phases.

system with the 53 alloys prepared:

0,

Powder X-ray photographs were taken using a Guinier-de Wolff camera and Cu Ka radiation. The position of the lines was evaluated using a scanner device [ 81. Lattice parameters were determined by least-squares refinement of the powder data, using silicon (a = 5.4108 A) as an internal standard. Differential thermal analysis was performed in two different apparatuses depending on the temperature range. In their common range the temperatures of the observed phase transformations did not differ by more than 5 K. For metallographic observations the alloys were etched after polishing in an HCl-HNOs-glycerine solution. Some of the alloys were examined by electron microprobe analysis using Ti Ka, Al Ko and Au La! radiations. Owing to the large difference in atomic number between gold and titanium or aluminium the ZAF correction programme proved inadequate even when compounds such as AuAlz or TiAls were used as standards. For this reason we relied mainly on microstructural examination of the samples for the determination of phase limits.

3. Results In Table 1 we report the results of differential thermal analysis (DTA), X-ray powder diffraction, optical metallography, Vickers hardness measurements and electron microprobe analysis. The symbol (L) in the DTA column signifies that the liquidus temperature was exceeded. The number of phases present (column 4) was determined from optical metallography and may

102 TABLE 1 Summary of investigations in the Ti-Au-Al Nominal Composition

(at.%)

Differential thermal analysis (K)

Ti

Au

Al

5.8

16

78.2

5

28

67

23

10

67

25

15

60

7

33

60

40

10

50

25

25

50

1178 (L)1433

17

33

50

10

40

50

20

40

40

1008 1193 (L)1303 913 1043 1223 (L)1343 1413 (L)1418

10

50

40

system Number of phases

Hardness Microprobe (kgf mmM2) analysis (at.%)

3

-

-

AuAI,

3

-

-

AuAlz TiAlz AuAlz TiAl AuAlz TiAuAl TiAl TiAuAl AuAlz TiAl TiAuAl AuA12 TiAuAl

2

-

-

-

-

-

-

-

-

3

-

-

3

-

2

-

-

AuAlz TiAuzAl

2

-

-

AuAlz TiAuAl TiAuzAl AuAl

3

-

-

2

175

Ti 0.2 Au 51.3 Al 48.5 Ti 21.6 Au 52.3 Al 26.1 Ti 33.1 Au 33.0 Al 33.9

X-ray

933

(L)1283 1253 (L)1273 1253 (L)1353 1178 (L)1143 1183 (L)1323

833 1008 (L)1343

245

TiAu*Al

TiAuAl

1

883 1080 (L)1343 1483 (L)1753

AuAl TiAuzAl

2

Ti2Al

1

525

30

1178

2

25

1013 1373 (L)1468

TiAuAl TiAuzA; TiAuzAf

364 200 250

33

33

34

1458

17

50

33

60

10

30

30

40

25

50

1

365

Ti 60.7 Au 10.3 Al 29.0 Ti 25.8 Au 49.7 Al 24.5 (continued)

103 TABLE

1 (continued)

Nominal Composition

(at.%)

Ti

Au

Al

67

10

23

50

25

25

Differential thermal analysis

X-ray

Number

phases

of

Hardness (kgf mm-‘)

Microprobe analysis (at.%)

(K) 1298 1343 (L)1753 1373 (L)1453

TizAl

1

TizAl TiAuAl

2

1443 (L)1533

AuTi

40

40

20

67 50

16 33

17 17

1413

35

50

15

1448

65

20

15

1423 (L)1453

55

30

15

1353 1413 (L)1473

75

12.5

12.5

50

40

10

40

50

10

30

60

10

10

80

10

943 1083 (L)1173 1563 (L)1653

793 818 (L)1303

360 425

-

Ti 38.6 Au 36.5 Al 24.8 -

-

-

375

Ti 45.5 Au 44.6 Al 9.9 Ti 31. Au 52.5 Al 16.5

2

270

TizAl Ti&u TiAu Ti*Al

-

215 -

T&Au Ti,Al ? TiAu

3

-

2

315

Ti 49. Au 45.5 Al 5.5

TiAu TiAuz TiAuz TiAuzAl TiAq Au&I

exceed the number of structures detected by X-ray powder diffraction. The Vickers microhardness (column 5) was determined with a load of 100 gf. The equilibrium phase diagram at 775 K is shown in Fig. 2. Our observations indicate that TiAuAl and TiAuzAl are the only ternary compounds occurring in this system. 3.1. TiAuAl The ternary compound TiAuAl melts congruently at 1453 K. A photomicrograph of an annealed specimen is shown in Fig. 3 where large grains

Ti

20 7

7’1, Au

10

TiAu

GXAU

60

1 Ti Au2

80

AU

Fig. 2. The isothermal (775 Rf section of the T&-Au-AI system.

Fig. 3. Singlesphase TiAuAl: grains of approximately 100 /.krnare easily grown from the liquid owing to the congruent melting of the compound (magnification: 108~f.

~~pproxima~ly 100 pm) associated with the congruentformation are a~p~ent. The phase has a homogeneity range centred at a gold concentration of 33 at.% and is elongated towards a titanium-rich faluminiumdeficient, 25 at.% abminium) composition.

105 TABLE 2 Observed and calculated X-ray powder pattern for TiAuAl (Cu KCW radiation, Ni&-type structure, space group P63/mmc, u = 4.4075(8) 8, c = 5.829(l) A, 2Ti in 2a: 0, 0,O; 2Au in 2d: +, $, p; 2Al in 2c: i, $, i) hkl

d (obs) (A)

d( talc) (4

I (obs)

I (talc)

100 101 002 102 110 200 201 112 103 202 004 210 211 203 104 212 300 301 114

3.8151 3.1935 2.9231 2.3166 2.2034 1.8126 1.7583 1.7317 1.5962 1.4000 a 1.2920 1.2729 1.2154

3.8170 3.1933 2.9145 2.3164 2.2037 1.9085 1.8138 1.7578 1.7316 1.5966 1.4572 1.4427 1.4004 1.3615 1.3614 1.2930 1.2723 1.2431 1.2155

245 1000 240 820 897 156 307 189 356 236 a 197 131 295

136 1000 192 553 688 24 223 303 197 195 74 25 232 109 23 236 168 0 315

aOverlap with silicon. -, not observed.

As a central compound in the ternary section, TiAuAl participates in six two-phase equilibria with AuAl,, TiAl, Ti,Al, TisAu, TiAu and the ternary phase TiAu,Al. The powder pattern of stoichiomet~c TiAuAl was indexed based on a hexagonal unit cell with a = 4.4075(8) A and c = 5.829(l) A. The observed intensities suggest isotypism with N&In, space group P63/mmc. Satisfactory agreement between observed and calculated [9] powder patterns was obtained with titanium in 2a: 0, 0, 0; gold in 2d: i, f, $; and aluminium in 2c: i, $, i positions. Calculated and observed d spacing and intensities are listed in Table 2. The data do not permit the exclusion of site exchange disorder between gold and ~uminium involving 5% of gold and ~uminium atoms or between titanium and aluminium. However, complete inversion between aluminium and titanium (aluminium in 2a, titanium in 2~) or the occupation of the 2a position by gold are excluded. The atomic cell volume decreases slightly from 16.39 A3 for the stoichiometric compound to 16.24 A3 for the titaniumpoor limit, owing to a contraction in the c direction. The hardness of the compound is 365 + 5 kgf mmm2.

106

We confirm the existence of the ternary compound TiAu,Al first observed by Marazza et al, [ 21. The compound forms by peritectic reaction from the mixture (L + TiAu). A micrograph of the stoichiometric annealed alloy is shown in Fig. 4(a). The peritectic formation is revealed in the cast alloy (Fig. 4(b)).

fbl

(a)

Fig. 4. (a) Nearly sing!e-phase TiAuzAl after annealing at 775 K (magnification: 80x); (b) the same alloy as cast, showing the peritectic formation and the equilibrium with the binary AuAls (dark inclusions).

The crystal structure of TiAuzAl is cubic, of the CsCl type with a = 3.198(2) a. On Guinier photographs, no superstructure lines were detected that would allow the assignment of the MnCu,AI-type structure to TiAu,Al. The lattice parameter is independent of annealing temperature and composition. Thus the compound is characterized by a very small homogeneity range. The intensities of the diffraction lines for TiAu,Al annealed for 20 days at 775 K and calculated values for the CsCl structure are compared in Table 3. TABLE 3 X-ray diffraction pattern of TiAusAl (Cu Ka radiation, CsCl-type structure, a = 3.198(2) A) hkE

d (obs)

d (talc)

100 110 111 200

3.200 2.262 1.845 1.601

3.1980 2.2613 1.8464 1.5990

210 211

1.430 1.307

1.4300 1.3056

%s, very strong; s, strong; m, medium.

Z (obs)a S

vs m m S S

z (talc)

823 1000 256 194 400 487

107

The situation concerning the alloy is similar to that of TiAg,Al CsCl-type compound. In addition to the ternary homogeneity ranges of two binary ternary section with a maximum 10 at.% aluminium in TiAu.

possible occurrence of an ordered Heusler which was reported by Dwight [lo] as a compounds, it must be noted that the phases, Ti.# and TiAu, extend into the solubility of 10 at.% gold in TizAl and

Acknowledgment The authors wish to Degrossissage d’Or and Mr. help in the metallographic Fonds National Suisse de la

thank Mrs. D. Sappey of Usine Genevoise de F. Liniger of the University of Geneva for their studies. Part of this work was supported by the Recherche Scientifique.

References 1 E. A. Aitken, in J. H. Westbrook (ed.), Intermetallic Compounds, R. E. Krieger, Huntington, NY, 1977, p. 512. 2 R. Marazza, R. Ferro and G. Rambaldi, J. Less-Common Met., 39 (1975) 341. 3 W. G. Moffat, Handbook of Binary Phase Diagrams, Genium, New York, 1976 and updates. 4 F. A. Shunk, Constitution of Binary Alloys, 2nd Suppl., McGraw-Hill, New York, 1969. 5 M. Hansen, Constitution of Binary Alloys, McGraw-Hill, New York, 1958. 6 0. Kubaschewski, in 0. Kubaschewski (ed.), Atomic Energy Review, SpeciuE Issue No. 9, International Atomic Energy Agency, Vienna, 1983, p. 77. 7 M. Pusseli and K. Schubert, J. Less-Common Met., 35 (1974) 259. 8 K. 2. Johansson, T. Palm and P. E. Werner, J, Phys. F, 13 (1980) 1289. 9 E. R. Hovestreydt, J. Appl. Crystallogr., 16 (1983) 651. 10 A. E. Dwight, in J. H. Westbrook (ed.), Intermetallic Compounds, Huntington, NY, 1977, p. 166.

R.

E. Krieger,