the AI-Fe system by several investigators using MA in the composition range of ... are the same as reported for the AIsFe 2 phase in the JCPDS file # 29-43.
Pergamon
Scripta Metallurgica
et
Materialia, Vol. 31, No. 3, pp. 333-338, 1994 Copyright © 1994 Elsevier Science Ltd Primed in the USA. All rights reserved 0956-716X/94 $6.00 + 00
SYNTHESIS OF NANOCRYSTALLINE AIsFe z BY MECHANICAL ALLOYING
D.K. Mukhopadhyay, C. Suryanarayana, and F.H. Froes Institute for Materials and Advanced Processes (IMAP) University of Idaho, Moscow, ID 83844-3026 (USA).
(Received March 21, 1994) (Revised April 12, 1994) Introduction Mechanical alloying(MA), a solid state powder processing technique, has been employed to synthesize a variety of alloy phases starting from either blended elemental or prealloyed powders[1,2]. The repeated welding, fracturing, and rewelding of powder particles can lead to the formation of supersaturated solid solutions, crystalline and quasicrystalline intermediate phases, and metallic glasses[2,3]. The grain size of the crystalline phases produced by this technique is often of nanometer dimensions[4,5], and these nanocrystalline materials can exhibit an excellent combination of properties making them potentially attractive for a number of applications[6]. Recently there have been many investigations on the synthesis of intermetallic compounds by MA [7]. However, in a majority of these cases the synthesis of the intermetallics is achieved only on heat treatment of the MA powders. Only in a few cases is the formation of the intermetallics achieved directly by MA [8,9]; some of these phases are in the ordered state, although the ordering is far from complete [10]. Addition of Fe to AI can increase the use temperature of the AI and because of this there have been several investigations into the non-equilibrium processing of AI-Fe alloys to increase the solid solubility of Fe in AI and also to synthesize the stable and metastable phases in an economical way leading to improved properties. The rapid solidification (RS) and MA techniques have received considerable attention in this regard. The solid solubility of Fe in AI can be increased from the equilibrium limit of 0.025 at.% to 4.4 at.% by RS [11] and up to 4.5 at.% by MA[12], although even higher levels have been suggested using MA[13]. Formation of an amorphous phase which is not possible by RS techniques has been reported in the AI-Fe system by several investigators using MA in the composition range of 17-33 at.% Fe [12,1417]. Direct synthesis of the disordered AIzFe compound by MA has been reported only by Morris and Morris [15], whereas other investigators could synthesize the different intermetallics in the AI-Fe system only after heat treating the MA powders [12,14]. In the AI-rich side of AI-X systems (where X = Nb, Fe, Zr, and Ge) there are reports of synthesis of intermetallic compounds directly by MA [8,10,1 5,18,19]. The purpose of the present paper is to report on the successful direct synthesis of the ordered AIsFe 2 intermetallic by MA starting from blended elemental powders.
333
334
NANOCRYSTALLINE AlsFe2
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Experimental Procedure Pure elemental aluminum (-100 mesh, > 9 9 % pure) and iron (-325 mesh, > 9 9 % pure) were mixed to yield an average starting composition corresponding to the stoichiometric AI3Fe compound, i.e., AI-25 at.% (41 wt.%) Fe. MA was carried out at room temperature in a Spex 8000 shaker mill for times of up to 50 h. The grinding medium used was hardened 52100 steel balls of 4.76 mm diameter. About 1 wt.% stearic acid was added as a process control agent in order to prevent excessive sticking of the powder samples to the balls and to the vial walls. For each run about 10 g of powder and 100 g of steel balls were loaded into the steel canister in a glove box under a partially protective atmosphere of argon. Forced air cooling during milling prevented excessive temperature rise in the powder. The MA powder was removed from the canister and examined by x-ray diffraction with CuKo radiation at 40 kV and 30 mA settings in a Siemens D5000 x-ray diffractometer. The phases formed were identified by comparing the peak positions and intensities with those listed in JCPDS files. The MA powders were also characterized in a Philips EM 400 transmission electron microscope to measure the crystal size of the powder after milling and in an AMRAY 1830 scanning electron microscope to analyze the composition of the milled powder. Results Figure 1 shows the x-ray diffraction patterns of the blended elemental powders as a function of milling time. All the reflections from AI (fcc, a = 0.405 nm) can be clearly identified in Fig.l(a), but because of the closeness of lattice spacings between AI and Fe (bcc, a =0.286 nm), the latter are superimposed on AI with the (110) and (200) Fe reflections coinciding with those of (200) and (220) of AI, respectively. With increasing milling time, these x-ray diffraction peaks became broader and their intensities decreased as shown in Fig.l(b). Signs of alloying which resulted in the formation of a new phase (an intermetallic) can be clearly seen after milling for 15 h [Fig.l(c)]. The formation of this intermetallic, without any trace of either AI or Fe, is complete on milling for 30 h [Fig.l(d)]. The x-ray diffraction peaks observed in Fig. 1 (c) and 1 (d) could be indexed as belonging to the AIsFe 2 phase having an orthorhombic structure with the lattice parameters a = 0.767 nm, b = 0.64 nm and c = 0.42 nm. These lattice parameters are the same as reported for the AIsFe 2 phase in the JCPDS file # 29-43. Since all the reflections expected of the ordered AIsFe 2 compound appeared in the x-ray diffraction pattern, it was concluded that the compound synthesized by MA is in the ordered state. On continued milling up to 50 h the powder was completely amorphous [Fig. 1 (e)]. Thus, the sequence of phase evolution with milling time in this powder mix is:
AI + Fe
30 h
AIsFez
50 h ~
Amorphous
The crystal size of the as-milled powder was calculated for each condition from the x-ray diffraction patterns using the Scherrer formula [20] after subtracting the instrumental and strain broadening effects. Figure 2 shows the variation of the crystal size with milling time and suggests the presence of crystals of about 10 nm size after milling for 30 h, indicating that the Al~Fe2 compound produced has nanometer dimensions. The transmission electron micrograph recorded from the shock-consolidated powder (Fig.3) confirms that the crystal size of the powder milled for 15 h is about 15 nm, decreasing to as low as 10 nm after milling for 30 h. These observations suggest that the crystal sizes calculated directly from transmission electron micrographs and indirectly from x-ray line broadening studies are quite close. The powder milled for 30 h showed only the AIsFe 2 phase, with the peaks which are relatively broad due to the fine crystal size and the presence of strain in the powder. On annealing the powder milled for 30 h at 625°C for 324 h, sharp x-ray diffraction lines resulted (Fig.4), including a number with low intensities which did not show up in the as-milled powder. However, there is no difference either in the structure or the lattice parameters of this phase between the as-milled and annealed conditions.
Vol. 31, No. 3
NANOCRYSTALLINEAlsFe2
002 Of~ 110
200 020
O
O
335
130
)
O
~
_._d_~
111•
.220
sh
•200 b 311
111•
AI
•
•110
Fe • AI5 Fe20
20
• 220 • ,00 ~
0
30
40
50
6O
a
311 J~
70
8O
20---~ FIG.1 X-ray diffraction patterns of blended elemental nominal AI-25 at.% Fe powders mechanically alloyed for different times. (a) as mixed, and after milling for (b) 5 h, (c) 15 h, (d) 30 h and (e) 50 h. 30 '~
25
_
20
50 pm
O9 =¢ I- 15 O9 >¢1: (.,1 10
0
5
10
15
20
25
30
35
TIME (h] FIG.2 Plot showing the variation of crystal size with milling time for an AI-25 at.% Fe powder mix.
336
NANOCRYSTALLINE AlsFe2
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FIG.3 Transmission electron micrograph of the AI-25 at.% Fe powder milled for 15 h, and shock consolidated.
~[ "AI5Fe2
/110 200
["~
-i-
002° "130 020 , ~
I 310
/110 2 0 0 020
1112
I I
[
1
t. .400
t
240
b
.
331 ~ 241
240
151
53011 042 ~'
~[
a
[
70
80
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20
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20-FIG.4 X-ray diffraction patterns of the AI-25 at.% Fe powders, (a) mechanically alloyed for 30 h, (b) mechanically alloyed for 30 h and heat treated at 625°C for 324 h. Discussion In the present investigation, we have been successful in synthesizing the ordered AIsFe2 intermetallic directly by MA starting from blended elemental powders. The c/a ratio of the AIsFe2 compound formed directly by MA is the same as the equilibrium value. However, EIIner and Mayer [21] reported that the c/a ratio of the AIsFe2 synthesized by RS techniques was smaller by 1.4% than under equilibrium conditions. Since rapid solidification from the liquid state can lead to retention of a high
Vol. 3 l, No. 3
NANOCRYSTALL1NE A1sFe2
337
vacancy concentration, the reduction of c/a ratio in any lattice could result from a vacancy segregation preferentially on the basal planes. Since no liquid-to-solid state transformation is involved during MA, the c/a ratio of the AIsFe2 phase formed in the present investigation does not differ from the equilibrium value. Thus, the lattice parameters of the AIsFe 2 phase formed by MA match exactly with those reported in the JCPDS files. Although the starting composition of the powder mix was AI-25 at.% Fe, the expected formation of AI3Fe was not achieved; instead the AIsFe 2 intermediate phase formed after 30 h of milling. Chemical analysis of the powder in the scanning electron microscope (shown in Table I), indicates that the composition of the powder has actually changed during milling, and has about 26.5 at:% Fe after milling for 15 h. This can be either due to loss of AI due to sticking of the powder to the container wall and to the grinding balls and/or due to a gain of Fe from the steel balls and container wall. Since the AIsFe 2 intermetallic forms under equilibrium conditions in the composition range of about 27 to 29 at. % Fe [22] it is not surprising that formation of homogeneous AIsFe2 was observed in the present investigation at an Fe content of 26.5 at.%. TABLE I Chemical analysis of the AI-25 at.% Fe powder mechanically alloyed for 15 h. Element
AI
Fe
at.%
73.5
26.5
To determine whether AI3Fe can be synthesized directly using MA by compensating for the loss of AI, an extra amount of 1.5 at.% AI was added to the nominal AI-25 at.% Fe powder mix prior to MA. However, the AI3Fe phase did not form; instead a solid solution of Fe in AI formed at early milling times and an amorphous phase formed after 50 h of MA. The Miedema model predicts that an amorphous phase should form in the composition range of 25 to 60 at.% Fe in the AI-Fe system [12]. Accordingly, an amorphous phase was observed in the AI3Fe composition in the present work, which is one of the boundaries of the composition range predicted by Miedema model and by other investigators [ 12,14,16]. However, Morris and Morris [1 5] reported direct synthesis of the AI3Fe compound using a Fritsch mini planetary ball mill at a ball-to-powder ratio of 6:1. Continued milling by Morris and Morris [15] beyond the time for the formation of the AI3Fe compound may have resulted in the amorphous phase formation; although the lower milling intensity (and hence reduced energy in the system) used by these workers may have precluded formation of the amorphous phase, which normally requires severe deformation and accumulation of sufficient defect concentration. Conclusions Mechanical alloying of blended elemental AI and Fe powders corresponding to a nominal composition of AI-25 at.% Fe led to the direct synthesis of the ordered AIsFe 2 intermetallic and later to production of an amorphous phase after milling for 30 h and 50 h, respectively. A loss of AI and/or gain of Fe was observed in the milled powder which was responsible for the shift in the stoichiometry from AI3Fe to AIsFe 2. Synthesis of AI3Fe by MA was attempted by adding appropriate amount of extra AI; but a solid solution and at longer MA times an amorphous phase formed. Acknowledqements The authors wish to thank Dr. David G. Morris of the University of Neuchatel, Switzerland, for useful discussions and Ms. Susan Goetz for typing the manuscript.
338
NANOCRYSTALLINE AIsFe,=
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References 1. 2. 3. 4. 5. 6. 7. • 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.
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