Carbide and hydride formation during mechanical alloying of titanium and aluminium with hexane

MATERIALS SCIENCE & ENGINEERING ELSEVIER

Materials Science and Engineering A196 (1995) 205-211

A

Carbide and hydride formation during mechanical alloying of titanium and aluminium with hexane J a r i K e s k i n e n a, A n d r e w

P o g a n y b, J i m R u b i n a, P e k k a R u u s k a n e n ~

VTT ManuJhcturing Technology, Technical Research Centre of Finland, PO Box 17031, FIN-33101 Tampere, Finland bRoyal Melbourne Institute of Techology, GPO Box 2476, Melbourne 3001, Australia Received 13 January 1994; in revised form 23 September 1994

Abstract

Mixtures of elemental titanium and aluminium powders of overall composition Tix AI~ +x (x = 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.75, 0.8) were mechanically alloyed in a planetary-type ball mill. Hexane was added as a process control agent to reduce powder agglomeration during milling. The as-milled powders were characterized using X-ray diffraction, differential scanning calorimetry, scanning electron microscopy and transmission electron microscopy. During milling, the hexane is partially dissociated, with the free carbon and hydrogen incorporated within the Ti, AI~_ x alloy powders in increasing amounts with increasing milling time. The amount of incorporated carbon increases with the initial Ti content of the powder mixture, reaching a maximum of 12 wt.% incorporated into an initial Ti0.sA10.2 powder mixture after 100 h of milling. The hydrogen is found to combine with elemental Ti to form Till 2 x, with an initial Ti0.5A105 powder mixture milled for 40 h incorporating 0.95 wt.% H. The milled (Tix AI~ _ x + C) powder mixtures form a large fraction of amorphous phase near x = 0.5. Annealing of the as-milled powders incorporating dissolved carbon and hydrogen produced a mixture of A12Ti4C2, TiC and TiA1.

Keywords: Carbide; Hydride; Alloying; Titanium; Aluminium; Hexane

1. Introduction

Intermetallic alloys have been widely investigated in recent years, particularly those such as Ti3A1 and TiA1 which combine a high melting point, excellent high temperature mechanical properties, good corrosion resistance and high strength-to-weight ratio. Mechanical alloying (MA), a non-equilibrium, low temperature, solid state alloying process, has been used successfully to synthesize numerous crystalline, microcrystalline and nanocrystalline [1,2] intermetallic compounds as well as amorphous alloys [3]. The non-equilibrium nature of the alloying process often results in extended solute elements in alloy powder mixtures compared with the same alloys produced by conventional (melt-derived) methods. This extended alloying range, along with the very fine grain size obtainable (typically in the nanometre scale), makes it possible to produce novel types of intermetallic materials with non-equilibrium crystal Elsevier Science S.A.

S S D I 0921-5093(94)09701 - 1

structures and improved mechanical and chemical properties using the MA process. The milling conditions experienced by the powder m i x t u r e - - s u c h as milling atmosphere, milling time and temperature; mill type; type and quantity of process control agents (PCAs); size and number of milling balls; e t c . - - a r e known to have strong effects on the alloying process and therefore on the structure and properties of the alloyed powder product. For example, amorphization of Ti,. A11 _ x alloys has been found to be dependent on the size of the milling balls [4]. Various PCAs such as dodecyl carbonyl have been used to enhance the effectiveness of milling, by reducing powder agglomeration, and to introduce new elements into the alloy [5]. Benn et al. [6] have used different PCAs to decrease the grain size and to produce oxides and carbides in mechanically alloyed Ti0.75A10.25 and Ti0.sA10.5 powder mixtures. Such chemical additions make it possible to tailor the mechanical and corrosion properties of the

206

J. Keskinen et al./ Materials Science and Engineering A 196 (1995) 205-211

milled material to specific applications. Srinivasan et al. [7] have used hexane as a PCA to prevent agglomeration of titanium and aluminium powders during MA. These authors found that carbon from the decomposition of the hexane reacted with elemental titanium to form a fine dispersion of carbides. In their experiments the maximum carbon content in the powder reached 2 wt.%. This amount of carbon was calculated to form 9 wt.% of TiC in the as-milled mixture. It has also been shown by Suzuki and Nagumo [8] that during the mechanical alloying of a Tio.sAl0.s powder mixture the total carbon content of the alloy increases as a function of ball-milling time owing to the decomposition of heptane used as a PCA. A carbon content of 5 wt.% in a Tio.sA10.s alloy was measured after 28 h of milling, with no apparent saturation as a function of milling time. X-Ray diffraction (XRD) showed the carbon reacting with Ti, through a mechanochemical reaction, to form TiC, while differential thermal analysis (DTA) measurements indicated the additional presence of Till 2. These authors found that elemental aluminium did not react with heptane, but that hot pressing of the (Ti0.sA10.s + C) powders produced a mixture of TiA1 and A12 T i 4 C 2 . In this study we report on the mechanical alloying of Tix All _ ,. (x = 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.75, 0.8) powder mixtures in a planetary-type ball mill and the formation of titanium carbide, titanium aluminium carbide and titanium hydride caused by the decomposition of hexane used as a PCA.

2. Experimental procedures Ti~All x (0.2 ~< x ~< 0.8) powder mixtures were mechanically alloyed using a planetary-type mill with vials and balls made of hardened steel. The rotation speed of the mill was 300 revmin -1 and the ball-to-powder weight ratio was 10:1. The weight of the powder mixture was 20 g, corresponding to 200 g of balls. The diameter of the balls was 10 mm. The purity of the elements Ti and A1 powders was greater than 99.8%. The elemental Ti and A1 powder mixtures were loaded into the vials inside an argonfilled glove-box (less than 10ppm oxygen). In the planetary-type mill hexane ( C 6 H 1 4 ) w a s used as a PCA. The initial amount of hexane added to the vial was 1 g per 20 g of powder. This was enough to only wet the powders. Samples for X R D analysis (Mo Ke, 2 = 0.7107 ~ ) were taken after 5, 10, 20, 40 and 100 h of milling. One gram of hexane was added into the vial each time an X R D sample was removed to compensate for evaporation. The microstructures of the mechanically alloyed powders were characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The hydrogen and carbon contents of the alloys were

measured by using a Leco RH-2 or Leybold Heraeus CSA 2003 analyser respectively.

3. Results and discussion

3.1. X-Ray diffraction Fig. 1 presents the X R D spectra of Tix All _ , alloys after 40 h of mechanical alloying in a planetary-type mill in the presence of hexane. For the Al-rich composition the as-milled powder contains unreacted AI as well as small amounts of Ti, TiA1, Till].924 and an amorphous phase, the latter characterized by a broad diffraction peak centred at 20 = 17°. Table 1 gives the results of a deconvolution analysis of the X R D spectum for this composition. As the initial composition approaches equiatomic, the diffracted intensities of the peaks due to A1 and Ti are reduced, while the total area of the amorphous diffraction peak grows until, for x ~ 0.5, the spectrum is dominated by this broad peak. For x > 0.6, peaks in the diffraction spectra corresponding to crystalline phases again appear, which were indexed to TiC. Table 2 gives the d spacings and diffracting planes obtained from a deconvolution of the X R D spectrum for the as-milled powder of initial composition Tio.sA10.2. For the initial compositions 0.5 ~
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0.6 0.7

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, , I 30

=

0.75 o.a

TiC I 40

(degrees)

Fig. 1. X-Ray diffraction spectra of Ti, AI, ., powders mechanically alloyed in a planetary-type mill as a function of composition (milling time 40 h).

J. Keskinen et al. / Materials Science and Engineering A196 (1995) 205 211

207

Table 1 d spacings and diffracting phases obtained from a deconvolution of the X R D spectrum of the as-milled TiozAl0s powder mixture shown in Fig. 1 20 (o)

d (A)

Peak height (counts)

Peak width (o) (FWHM-20)

Diffracting phase and (hkl)

15.928 17.490 17.697 17.860 18.201 20.083 20.290 26.112 28.894 30.739 30.793 32.048 33.998 35.536

2.565 2.337 2.310 2.289 2.247 2.038 2.017 1.573 1.424 1.341 1.338 1.287 1.216 I. 165

241 921 578 457 47 120 609 61 379 37 125 15 109 109

0.406 0.373 0.403 2.266 0.277 2.157 0.582 0.764 0.655 0.684 11.118 0.784 0.814 1.022

TiHn.924(111) + Ti(100) AI(111) TiAI(I 11) Amorphous alloy Ti(101) Al(200) TiAI(200) Till 1.924(202) A1(220) + TiAI(202) Till 1924 (202) Ti(103) Till 1.924 AI(311) A1(222)

Table 2 d spacings and diffracting phases obtained from a deconvolution of the X R D spectrum of the as-milled Tio.sAlo.2 powder mixture shown in Fig. 1 20 (°)

d (/~)

Peak height (counts)

Peak width (°) (FWHM-20)

Diffracting phase and (hkl)

16.410 17.719 18.902 20.030 26.916 30.009 31.632 33.065 38.340 42.007 43.231

2.490 2.307 2.164 2.043 1.527 1.373 1.304 1.249 1.082 0.991 0.965

438 184 326 56 225 57 134 68 19 38 53

0.937 2.707 0.892 0.923 1.189 9.365 1.020 1.429 1.322 1.438 1.299

TiC(lll) Amorphous alloy TIC(200) Fe ? TIC(220) Amorphous alloy TIC(311) TIC(222) TIC(400) TIC(331) TIC(420)

intensities of the peaks from these elemental phases continuously decreasing. It can be seen in this figure, however, that a diffraction peak at 20 = 16° remains relatively intense, indicating that it is not due to either elemental phase. Previous investigators have reported the presence of stoichiometric hydrides and/or carbides of titanium in Ti-A1 powder mixtures mechanically alloyed with organic PCAs. We therefore examined whether one or more of the observed diffraction peaks arises from a stoichiometric phase formed between Ti and either carbon and hydrogen. The formation of stoichiometric compounds between these elements and Ti (TIC, TiH2_ x) is difficult to establish from diffraction spectra, since these stoichiometric compounds produce similar diffraction spectra. Furthermore, many of the expected diffraction peaks for these compounds overlap those of elemental Ti. Table 3 gives the interplanar spacings for Ti, Till2 ,- and TiC, along with TiA1 and TiA13 taken from the JCPDS database [9]. It can be seen that the coexistence of TiH2_x and TiC

with Ti is difficult to verify owingto peak overlaps, and that differentiation between Till 2 x and TiC suffers from the same difficulty. Also, the reduction in grain size (and the possible generation of non-uniform lattice strain) during MA produces a reduction in maximum peak intensities and an increase in peak width for the Ti phase. Additional complications arise from shifts in d spacing due to possible off-stoichiometric compositions for T i l l 2 _ , and TiC, as well as solid solution formation involving elemental Ti and uniform crystallite strain. An examination of the diffraction spectra for the samples milled for 5, 10 and 20 h does reveal an extra, well-resolved peak at d = 1.55-1.56/k. Fig. 3 shows a portion of the X R D spectrum for the sample milled for 10 h which more clearly shows this peak. Comparison with Table 1 suggests that this peak could be indexed to Till2 x or TiC. A deconvolution of this same spectrum for the range 14 ° ~<20 ~< 10°, Fig. 4, was performed and the results are given in Table 4. The peaks at d = 2.48 and 2.21/~ provide additional evie¢

-

208

J. Keskinen et al. Materials Science and Engineering A 196 (1995) 205-211 ,

,.

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dence for the presence of Till 2 (compare with Table 3). The decrease in d spacing for the Ti(100) and Ti(101) reflections relative to the values given in Table 3 for pure Ti indicates a decrease in unit cell volume. This is

consistent with the dissolution of A1 into the Ti lattice, forming an c~-Ti(A1) solid solution [10,11]. The carbon liberated from the dissociated hexane would be expected to form TiC, but the absence of diffraction peaks suggests that either the TiC phase is dispersed as very small crystallites, as observed by Srinivasan et al. [7], or that the carbon is taken up into another phase. After 40 h of milling, the diffraction pattern indicates that the powder contains a large fraction of an amorphous phase. For 100 h of MA, crystalline phases reappear which can be indexed to TiC. Traces of TiA1 were also detected in the spectra. Fig. 5 shows DTA curves for the samples of Fig. 2. The formation of titanium hydride, deduced from the X R D spectra, is confirmed by endothermic events occuring over a broad range of temperatures centred near 400 °C, which is indicative of hydrogen desorption. Additional supporting evidence for hydride formation is obtained from an analysis of the sample milled for 40 h, which shows the powder to contain 0.95 wt.% H. An exothermic event at T ~ 660 °C appears in the DTA spectrum for the sample milled for 10 h, which becomes more prominent for samples milled for longer times. We associate this exothermic event with crystallization of the amorphous phase, which constitutes an increasing fraction of the total powder volume with increasing

Table 3 Interplanar spacings for Ti, Till 2 .... TiC, TiAI and TiA13 (2.6 >~d ~> 1.0 A) Ti (5-682)

Till2 (9-371)

2.557 (30)

TiHh924 (25-982)

TiHt ,924 (25-983)

2.569 (100)

2.567 (100)

2.500 (50)

TiC (32-1383)

TiAI (5-678)

TiAI 3 (37-1449)

2.310 (100)

2.301 (100)

2.499 (80)

2.342 (26) 2.244 (100)

2.244 (25) 2.200 (100) 2.100 (10)

2.234 (20) 2.220 (10) 2.163 (100)

2.146 (50) 2.040 (20) 1.990 (60) 1.926 (95) 1.790 (10)

1.725 (19) 1.689 (15) 1.550 (30)

1.573 (35)

1.583 (10) 1.569 (20)

1.530 (60)

1.474 (17)

1.332 1.276 1.247 1.233

(16) (2) (16) (13)

1.330 (35) 1.270 (8)

1.170 (2) 1.122 (2) 1.065 (3)

1.341 (25) 1.285 (8)

1.347 (15) 1.304 (5)

1.424 (60) 1.407 (20) 1.331 (5) 1.304 (30) 1.249 (17)

1.081 (10) 1.010 (10)

1.263 1.224 1.203 1.159

(5) (20) (60) (20)

1.071 (5) 1.015 (10)

1.476 (5) 1.434 (70) 1.362 (55) 1.266 (40)

1.171 (65)

J. Keskinen et al. / Materials Science and Engineering A 196 (1995) 205 211

209

10°C/min .~

vo

Z O O

I

,

,

,

,

0 15

I

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20 25 20 (degrees)

I

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5000 4000 -

11

II iI -

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1000 0

i

i

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1500

Fig. 5. DTA spectra of Ti0.sAl0. 5 powder mechanically alloyed in a planetary-type mill as a funtion of milling time.

6000

2000

i

500 1000 TemperGture (°C)

Fig. 3. X-Ray diffraction spectra of Tio.sA10. 5 powder mechanically alloyed in a planetary-type mill (14 ° ~<20 ~< 30°).

~5000 .o.

I

k 16

~

.~

18 20 20 (degrees)

22

Fig. 4. Deconvolution of X R D spectrum of Tio.sAl05 powder mechanically alloyed in a planetary-type mill (14.5 ° ~<20 ~<23°). The broken curves show the peaks obtained from the deconvolution, the full circles give the summation of the deconvoluted peaks and the full curve gives the measured diffraction profile.

milling time. The relatively high crystallization temperature would be due to a stabilization of a Ti,.A1,-x amorphous phase by carbon, whose overall composition would be described as (Tix A11 -x)y C, _y.

The carbon content of Tio.sA10.5, Tio.6A10.4 and Ti0.sAlo. 2 alloy powders mechanically alloyed in the presence of hexane was measured as a function of ball-milling time. As seen in Fig. 6, the carbon content of all three compositions increases continuously during milling with no apparent saturation up to 100 h, exceeding 12 wt.% for initial Tio.sA10.2 alloy powder. The carbon content as a function of milling time in our samples is of the same order as that reported previously by Suzuki and Nagumo [8]. They used n-heptane as a PCA during mechanical alloying and found a carbon content of 4.8 wt.% after 28 h of milling of an initial Tio.5 Alo.5 alloy. To determine the nature of the reactions occuring in the DTA curves, the Tio.sA10.5, Tio.6Alo.4 and Tio.8A10.2 alloy powders produced by mechanical alloying with hexane were annealed in an argon atmosphere at temperatures of 560, 700 and 900 °C for 30 min. Annealing of the samples at 560°C produced no noticeable changes in the X-ray diffraction spectra compared with those of the as-milled powders. After annealing the samples at 700 and 900 °C, the main crystalline compo-

Table 4 Diffraction peaks obtained from a deconvolution of the X R D spectrum shown in Fig. 3 20 (o)

d (~,)

Peak height (counts)

Peak width (°) (FWHM-20)

Diffmctihg phase and (hkl)

16.045 16.461 17.497 17.638 18.250 18.412 18.439 20.240

2.546 2.482 2.336 2.318 2.241 2.221 2.218 2.022

900 252 1287 840 2413 564 758 322

0.313 1.146 0.259 0.912 0.232 2.971 0.698 0.406

Ti( 100 ) Till2 (101) Ti(002) + AI(111) TiAI(111) Ti(101) Amorphous phase TiH2(110) A1(220)

210

J. Keskinen et al./ Materials Science and Engineerb N A 196 (1995) 205-211

14

I

I

I

I

I

12 (D

10 8

_,_;

6 4

//7

2 0

I" T!°.6AIo.41 ,

0

, I. T'i::";:21 '

20 40 60 80 1 O0 MILLING TIME (h)

Fig. 6. Carbon content as a function of milling time for alloy compositions Tio.sAlo.5, Tio.6AIo4 and Tio.sAlo2 (planetary-type mill).

Fig. 7. Scanning electron micrograph of Tio.sAlo.5 powder mechanically alloyed in a planetary-type mill for 40 h.

nents of the Tio.sAlo.5 and Ti0.6Alo. 4 powder mixtures were primarily TiA1, with minor fractions of AI2Ti4C2, TiC and elemental Ti. A small amount of TiA13 could not be ruled out. The large endothermic peak at T ~ 1440 °C seen in the D T A curves of Fig. 5 corresponds to the melting of TiA1 [12]. Annealing of the powder with the initial composition Tio.sAlo.2 resulted in the formation of TiC. 3.2. M i c r o s t r u c t u r e

Fig. 7 shows a scanning electron micrograph of the Tio.sAl0.5 powder milled in the planetary mill for 40 h in the presence of hexane. The powder particle size is 1-10 gm. Powder mixtures of Ti0.sA10. 5, Tio.6Alo. 4 and Tio.gAlo.2 milled for 100 h were examined using transmissin electron microscopy. The carbon content in all these samples is high, as shown in Fig. 6. The X-ray diffraction curves in Fig. 1 show that the as-milled structure of the Ti-rich compositions consists of singlephase TiC. Electron diffraction studies on all three compositions, however, reveal a phase with a simple f.c.c, structure. The calculated lattice parameter for this phase, ao = 4 . 3 6 A , matches well with the reported value of ao = 4.33 A for TiC. This observation is in agreement with the findings of Srinivasan et al. [7] in that TiC forms at all compositions but is of a crystallite size which does not give rise to well-resolved X-ray diffraction peaks. The electron diffraction patterns of the as-milled powder samples are presented in Figs. 8(a), 8(c) and 8(e). Based on the T E M studies, the average grain size in all the samples is 10-50 nm. These powder samples were annealed in situ inside the T E M up to a m a x i m u m temperature of 800 °C. Figs. 8(b), 8(d) and 8(f) show the corresponding diffraction patterns of the annealed and cooled powders. Annealing

Fig. 8. Electron diffraction patterns of mechanically alloyed powders: (a) Tio.5Alo.5, (c) Tio.6Alo.4 and (e) Tio., Alo.2 powders in the as-milled condition; (b) Tio.sAlos, (d) Tio6Alo4 and (f) Tio.sAlo.2 powders annealed at 800 °C.

J. Keskinen et al. / Materials Science and Engineering A 196 (1995) 205-211

the samples in situ produces an electron diffraction pattern consisting o f the original f.c.c, ring pattern corresponding to TiC, plus a second, superimposed spot pattern. The original f.c.c, pattern in all three samples becomes noticeably sharper, with the appearance o f some spots, indicating an increase in average grain size. The superimposed spot pattern for the equiatomic p o w d e r sample was indexed to TiA1 plus a m i n o r fraction o f Ti 3 A1. The superimposed spot pattern for the Ti0.sAl0. 2 sample was indexed to Ti3A1. These results were confirmed by annealing the samples at 900 °C for 30 min followed by X - r a y diffraction studies as mentioned above.

4. Conclusion W h e n hexane is used as a P C A in the mechanical alloying o f Tix Al~ x p o w d e r mixtures, there is a strong tendency for titanium carbide and titanium hydride formation, with the fraction o f both c o m p o u n d s increasing with milling time. F o r initial titanium concentrations x < 0 . 5 the mechanically alloyed p o w d e r consists primarily o f unreacted A1, with m i n o r fractions o f T i l l 2 .,., Ti and TiA1. Elemental aluminium does not f o r m carbides or hydrides during alloying but does partially dissolve in the Ti lattice. F o r x > 0 . 5 the as-milled p o w d e r consists o f single-phase f.c.c. TiC.

211

Annealing o f the mechanically alloyed powders at 900 °C results in the f o r m a t i o n o f hexagonal A12Ti4C 2 in addition to small a m o u n t s o f TiA1 and A13 Ti.

References [1] R.B. Schwarz, S. Srinivasan and P.B. Desch, Mater. Sci. Forum, 88 90 (1992) 595. [2] F.H. Froes, C. Suryanarayana, G.-H. Chen, A. Frefer and G.R. Hyde, J. Met. (May 1992) 26. [3] L. Schultz, in E. Artz and L. Schultz (eds.), New Materials by Mechanical Alloying, Calw-Hirsau, Oberursel. 1988, p. 53. [4] R. Watanabe, H. Hashimoto and Y.-H. Park, in L.F. Pease III and R.J. Sansoucy (eds.), Advances in Powder Metallurgy, Vol. 6, MPIF/APMI, Chicago, IL, 1991, p. 119. [5] A.P. Radlinski, A. Calca, B.W. Ninham and W.A. Kaczmarek, Mater. Sci. Eng. A, 134 (1991) 1346. [6] R.C. Benn, P.K. Mirchandani and A.S. Watwe, in A.H. Clauer and J.J. deBarbadillo (eds.), Solid State Powder Processing, Minerals, Metals and Materials Society, Indianapolis, IN, 1989, p. 157. [7] S. Srinivasan, S.R. Chen and R.B. Schwarz, Mater. Sci. Eng. A, 153 (1992) 691. [8] T. Suzuki and M. Nagumo, Scr. Metall., 27 (1992) 1413. [9] JCPDS, Swathmore, PA, 1989. [10] W.B. Pearson, Handbook of Lattice Spacings and Structure of Metals, Pergamon, Oxford, 1967. [11] W. Guo, A. Iasonna, M. Martelli and F. Padella, J. Mater. Sci., 29 (1994) 2436. [12] T.B. Massalski (ed.), Binary Alloy Phase Diagrams, ASM, Metals Park, OH, 1986.