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CRYSTALLIZATION OF AMORPHOUS ALLOYS BY HIGH ENERGY MECHANICAL DEFOFMATION
R. Schulz, M. Trudeau, D. Dussault, A. van Neste
To cite this version:
R. Schulz, M. Trudeau, D. Dussault, A. van Neste. CRYSTALLIZATION OF AMORPHOUS AL-
LOYS BY HIGH ENERGY MECHANICAL DEFOFMATION. Journal de Physique Colloques, 1990,
51 (C4), pp.C4-259-C4-264. �10.1051/jphyscol:1990431�. �jpa-00230791�
CRYSTALLIZATION OF AMORPHOUS ALLOYS BY HIGH ENERGY MECHANICAL DEFORMATION
R. SCHULZ, M.L. TRUDEAU, D. DUSSAULT* and A. VAN NESTE'
Institut de Recherche d'Hydro-Quebec. 1800 Montee Sainte Julie, yarennes, Quebec JOL 2P0, Canada
Universite Laval. DBpartement de Mines et ~ B t a l l u r ~ i e , Qu&bec G I K 7P4, Canada
Resume
-
Le broyage B billes B haute energie a et6 utilise frequemment ces dernieres annees pour produire des alliages amorphes en alliant mecaniquement des elements pures cristallins ou en deformant mecaniquement des composes intermetallics. Le processus de broyage B billes est normallement caracteris6 par une temperature effective locale au site d'impact qui controle les transformations de structure survenant lors du broyage. A des intensites de broyage Blevees, la temperature effective serait telle que la precipi- tation de composes intermetalliques pourrait survenir/l.Dans cette publication, le processus de broyage B bille est utilise pour cristalliser des alliages B base de £er dQjA amorphes. Nous comparons la stabilite structurale des alliages amorphes lors d'un processus thermique et lors de deformations mecaniques et les resultats indiquent que la temperature effective locale au site de la collision n'est pas le param&- tre important qui controle le processus de cristallization durant les deformations.
Abstract
-
High energy ball milling has been used extensively in the last few years to produce amorphous metallic alloys by mechanically alloying pure crystalline elements or by high energy mechanical deformations of intermetallic compounds. The structural transformations occurring during the milling process is usually ascribed to a local effective temperature at the collision sites. At too elevated milling intensities, the effective temperature would be such that the precipitation of intermetallic compounds would occur/l.In this paper, the process of ball milling is used to crystallize already amorphous iron based alloys. The structural stability of the amorphous alloys against a thermal and mechanical deformation process is compared and the results indicate that the local effective temperature at the collision site is not the important parameter that controls the crystallization process during the deformations.
1- INTRODUCTION
The crystallization of rapidly solidified metastable alloys is an irreversible structural transformation which is usually characterized by the temperature and the time. The influence of other parameters, such as the hydrostatic pressure, the strain in the sample or the influence of an applied stress or a shear deformation, has been overlook in the past.
Gopal and co-workers have shown that high pressure (of the order of several GPa) can induce the crystallization of rapidly solidified quasi-crystalline alloys/2. Kawamura et al. have shown that the crystallization temperature of an Fe78B13Sig amorphous alloy rise, under hydrostatic pressure, at a rate of ~ O O K / G P ~ /3. Koster has pointed out that, in a number of glasses, surface crystallization can take place only by scratching the surface /4. In that last instance, there was indication for the formation of post-critical nuclei in addition to the generation of additional nucleation sites during the scratching process. Miyoshi and Buckley / 5 observed that the crystallization of Fe67Col8B14Sil glasses was taking place during sliding friction with a relatively slow sliding velocity and small load. Under these conditions frictional heating was negligeable. Recently Eckert et al. have studied the amorphization reaction taking place during the ball milling of a mixture of Ni and Zr crystalline powders /l. They investigated the influence of the milling intensity on the glass formation. The amorphous phase formation was only possible below a certain threshold. At elevated milling intensities the precipitation of intermetallic compounds occured. They characterized the process by the effective temperature of the individual particles and estimated that the peak temperature can reach values above 400°c which is near the crystallization temperature of the amorphous alloy. Other authors estimate that the peak temperature during the collision is no more than 30 or 40 degrees above the average processing temperature /6. Recently, by probing the Ni content of Zr crystallites as a function of milling time using high spatial resolution Auger microscopy / 7 we estimated an average local Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990431
C4-260 COLLOQUE DE PHYSIQUE
effective temperature of 180°C at the collision site using a simple model for the interdiffusion process.
The effective temperature has been so far the only parameter used to characterized the process of ball milling. It is well known however that the strain plays an important role in the classical theory of nucleation. The strain affects the nucleation barrier as well as the critical nucleus size. Also the possibility of having local "shear band" at the site of a sub- critical nucleus during milling will certainly affect the surface energy term and therefore the local thermodynamic parameters controlling the nucleation and growth. A sub-critical nucleus at rest may for instance become over-critical during a shear deformation.
In this paper we pursue our recent investigation /8 of the parameters that influences the crystallization of amorphous alloys during mechanical deformations and show that the effective temperature is not the only important parameter of the process.
Two amorphous ribbons, the Metglas 26058-2 and the Metglas 2605C0 were obtained from Allied Metglas Products. The ribbons (6.2 g) were cut into pieces and put in an argon atmosphere cylindrical steel container (70 m1 maximum load) with two 1/2" and one 9/16" diameter steel balls. The milling was performed with a SPEX 8000 laboratory ball mill. For some experiments some amount of Ni (99.9% pure) and/or CO (99.8% pure) powders were incorporated at the beginning of the process. A Philips X-ray diffractometer with MoK, radiation has been used to study the structure of the powders and the thermal stability of the alloys was investigated using a Perkin Elmer DSc-4 calorimeter.
3-RESULTS and DISCUSSION
The Metglas 2605s-2 is a thermally fairly stable amorphous alloy. When this glass is heated in a DSc at a rate of 20°C/min it crystallizes in two steps. The first peak at 553OC (Ea=4.0 eV) corresponds to the crystallization of a-Fe(Si) while the second peak at 5 7 5 O ~ is associated to the precipitation of Fe2B (Ea=3.5 eV). When subjected to high energy mechanical deformation this glass also crystallizes. Fig.1 shows the X-ray diffraction scans of the Metglas 26058-2 before and after 24 hours of milling. The data reveal the presence of very small cl - Fe(Si) particles after such mechanical treatment. From the width of the X-ray peaks and-using the Scherrer formula we estimated the size of the crystallites to be on the order of 30 A while the corresponding width for the amorphous alloy gives an effective size of 14 A.
TEMPERATURE ( C ) TWO THETA K ~ M o
15 25 35 45
FIG. 1. X-ray and DSc data of Metglas 2605s-2 for the as-quenched amorphous ribbons and the powders after 0 and 24 h of high energy mechanical deformation. ( V ) U - Fe(Si)
I r t x ' I ' j ' o
METGLAS 2605s-2 -
-
. O h
-
Z -I m v,z
- < -l- v
-
r-
0 P-
1. C-
- m
-
and shifts to lower temperature by about 70°C. The shift of the second peak may be caused by several factors: the precipitation of alpha iron crystallites, the formation of structural defects during the milling process as well as the incorporation of impurities like oxygen and iron /9,10. Indeed there is a typical increase of about 0.5 wt% of oxygen and 2 wt% of iron in all the powders after 24 hours of milling.
The Metglas 2605CO contains Cobalt. Its thermal stability is inferior to the one of the cobalt free amorphous alloys. The first and second crystallization peaks are 441°C (Ea = 2.2 eV) and 521°c (Ea = 3.5 eV) respectively. Fig.2 shows the DSc scans measured on Metglas 260560 powders as a function of milling time. The first peak which corresponds to the crystallization of n
-
Fe(Co) disappears almost completely after 4 hours of milling. The second peak shifts, like in the previous case, from 521°C to about 460°C (maximum heat flow) after 10 hours of milling.Fig.3 shows the structural evolution of this alloy as a function of milling time. In agreement with the DSc results, we observe in the X-ray scans the beginning of the precipitation of the a
-
Fe(Co) after 3 hours. After 6 hours, all the Bragg peaks corresponding to the crystalline phase can easily be identify. Using the Scherrer formula and the half width at half maximum of the X-ray peaks we can estimate the size of the crystallites in the early stage of the mechanical crystallization. The result is 33 A after 6 hours of milling. The size of the first crystals during mechanical crystallization is smaller than the one usually observed during isothermal crystallization at tempef-atures near the crystallization temperature. This size is typically of the order of 50 to 75 A /11.The critical radius of nucleation is given by
where "S" is the surface free energy per unit area and " AG" is the bulk free energy difference between the crystal and the amorphous phase.
If we assume that
A
G is linearly proportional to the difference between the processing temperature and the melting temperature of the alloywhere Tm equals approximately 1200°C for this type of alloy and considering that the critical radius of the nuclei is between 50 and 75
A
for an annealing temperature near the crystallization temperature (Tx=440°C) we get, for the average local effective temperature of0 2 4 -
350 450 550 350 450 550
TEMPERATURE ( C )
METGLAS 2 6 0 5 C 0
1
FIG.
2. Differential scanning calorimetry of TWO THETA KaMcMetglas 2605C0 powders as a function of milling
time at a rate of 20°c/min. FIG. 3. X-ray spectra of Metglas 2605C0 powders as function of milling time.
( 7 ) a-Fe(Co); ( 0 ) (Fe(Co))2B.
C4-262 COLLOQUE DE PHYSIQUE
180°C estimated during ball milling /7, a critical radius between 37 and 56
A.
This issomewhat larger than what we observe experimentally and therefore it is reasonable to believe that the average surface energy term is lower during the mechanical deformation.
Therefore, the presence of Cobalt in an Iron-Silicon-Boron amorphous alloy seems to make the glass less stable against a thermal and a mechanical treatment. To prove this point we add 8 at% of crystalline CO to the Metglas 26059-2 before milling. The result is shown in Fig.4.
After a rapid dissolution of the Cobalt in the amorphous matrix during the first hour of milling we observe the precipitation of 0 -Fe(Co) already after two hours of milling. The crystallization of (Fe,Co)2B occurs like in the case of the metglas 2605C0 after about 1 0 to 12 hours of milling. The crystallite size of the a
-
Fe(Co) precipitates is, after 6 or 8 hours of milling, approximately the same as the one measured in the case of the Metglas 2605CO.If instead of adding CO we add Ni to the Metglas 26058-2 no crystallization occurs during milling. Fig.5 shows the structure of the alloy as a function of milling time when 17 at% of Ni is added to the Metglas 26058-2. The Ni diffuses in the amorphous alloy during the first few hours of the process and a complete amorphous mixture is observed after 24 hours of milling. We extended the milling time up to 36 hours in a case where 20 at% of Ni was added to the initial mixture and up to 44 hours for a 30 at% Ni alloy without seeing any crystallization taking place. Ni therefore stabilizes the Metglas 26059-2 against mechanical crystallization. It is interesting to note that, with the addition of 17 at% of Ni, the new amorphous phase crystallizes at a temperature of 3 7 5 O ~ instead of 553O~. Therefore the Ni based amorphous alloy is less thermally stable than the Ni free Metglas product but perfectly stable against mechanical crystallization.
This phenomena can not be explained if the effecf%ve temperature at the collision site is the only parameter controlling the structural transformation occurring during ball milling.
If only 4.3 at% of Ni is added to the initial mixture, the alloy crystallizes after 24 hours of milling like in the case of the Metglas 26058-2 alone. At 8.5 at% of Ni the alloy is still amorphous after 20 hours but seems to show some beginning of crystallization after 24 hours.
The limit of mechanical stability seems therefore to be around 9 or 10 at% Ni.
TWO THETA K= M O
-
2 4 h -
14 42 60
T W O THETA K, MO
FIG.
4 - X-ray t r a c e s showing t h e e f f e c t s of FIG. 5. X-ray t r a c e s showing t h e e f f e c t o f a d d i n g 8 a t . % o f C0 t o M e t g l a s 26055-2; t h e adding 17 a t . % o fr4i
t o M e t g l a 26055-2;Powder i s c o m p l e t e l y c r y s t a l l i n e a f t e r 24 h t h e powder i s c o m p l e t e l y amorphous a f t e r o f m i l l i n g . ( v ) CO peaks; ( 7 ) a-Fe(Co); 24 h o f m i l l i n g . ( V ) N i peaks.
( 0 ) ( F ~ ( C O ) ) ~ B .
various alloys.
TABLE
1. Summary of the crystallisation of the various iron based alloys by a mechanical and a thermal process.
When 10 at% of Ni and 20 at% of CO is added to the initial mixture, the crystallization of alpha iron during the milling is very rapid. At the "equiatomic" mixture MG80NiloColo the alpha iron still crystallizes but at a slower rate. Finally in the case of MG70Ni20Co10 the alloy is totally amorphous after 8 hours and stays amorphous upon further milling.
ALLOY
2605 S2 Fe7gSigBlg
2605 CO Fe66Co18SilB15
2605S2 + 8%co Fe72CoaSigB12
2635S2 + 17%Ni Fe65NiI7Si7Bl1
2 0 0
0 2 0 4 0 6 0 8 0 0 2 0 4 0 60
ATOMIC P E R CENT COBALT ATOllIC P E R CEliT NICKEL
FIG. 6. Binary phase diagram of Fe-CO and Fe-Ni from M. Hansen, Constitution of Binary Alloys (McGraw-Hill, New York, 1985).
PHASE
a
-
Fe(Si)Fe20
a
-
Fe(Co)(Fe.Co)gB
a-Fe(Si,Co) (Fe.Co)2B a-Fe(Si.Ni)
Kechanical Crystallization
t, (hours)
24 m
3 12
2 10-12
W
Mechanically stable
Thermal Crystallization unmilled
T~(OC) Ea(eV)
553 4.0 575 3 . 5
441 2.2 521 3.5
553 4.0 575 3.5
after 24h of milling Tx(Oc) Ea(eV)
505 < 3 . 5
c460 -
375 1.8 480 2.4
C4-264 COLLOQUE D E PHYSIQUE
In a last experiment-l0 at% of CO was added to the Metglas 26058-2 and milled for 6 hours. The crystalline mixture containing n
-
Fe(Co) crystallites was mixed with some Ni in order io get the following average concentration. MG70Ni20C01?. After 8 hours of milling the structure goes back to the amorphous phase. The crystall~zation-amorphization process is therefore reversible by adding more Ni or Co.These phenomena can be understood by looking at the Fe-Ni and Fe-CO phase diagrams shown in Fig. 6. The CO is almost totally miscible in alpha iron such that the precipitation of crystalline a
-
Fe(Co) from the amorphous'matrix can take place without chemical segregation while in the case of the Ni, the maximum solubility of Ni in alpha iron at the average effective temperature of 180°C is about 9 or 10 at%. If the concentration of Ni in the amorphous mixture is above this limit, the Ni atoms will have to diffuse out of the alpha iron critical nucleus in order for them to grow. This seems to be unlikely because ball milling is a process which tends to homogenize the chemical concentration.By high energy ball milling it is possible to crystallize entirely an amorphous alloys on a relatively short time scale and with a vTry fine microstructure. The nanocrystalline precipitates are typically of the order of 30 A. In order to produce such a microstructure thermally the glass would have to be anneal for several months at a few hundred degrees below its crystallization temperature /12,13. This, of course, would not be very practical. The process of crystallization by ball milling could be technologically important in areas such as catalysis where a fine microstructure is often associated with high catalytic activity. For instance, the Metglas 2605C0 in the amorphous state is not a good electrocatalyst for the hydrogen evolution reaction in alkaline solution. In order to activate the surface, an oxidation-reduction process which produces very small alpha iron particles on the surface of the electrode must be applied /14,15. The crystalline material obtained by ball milling shows the same level of activity without any surface treatment.
REFERENCES
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(1988) 415 /4/ U.Koster, Material Science and Eng.97
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118
(1984) 363 and "Sliding induced crystallization of metallic glasses" NASA Tech. Paper TP-2140,1983/6/ R.B.Schwarz and C.C.Koch, Appl.Phys.Lett. 49 (1986) 146
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