Cyclic Acta

Acta Materialia 50 (2002) 1113–1123 www.actamat-journals.com

Cyclic phase transformations of mechanically alloyed Co75Ti25 powders M. Sherif El-Eskandarany a

a,*

, K. Aoki b, K. Sumiyama c, K. Suzuki

d

Mining and Petroleum Engineering Department, Faculty of Engineering, Al-Azhar University, 11371 Nasr City, Cairo, Egypt b Department of Materials Science, Kitami Institute of Technology, Kitami, Hokkaido 090, Japan c Department of Materials Science and Engineering, Nagoya Institute of Technology, Nagoya 466-8555, Japan d Advanced Institute of Materials Science, Moniwadai 2-6-8, Taihaku-ku, Sendai 982-0252, Japan Received 31 January 2001; revised 24 October 2001; accepted 19 November 2001

Abstract Cyclic crystalline–amorphous–crystalline phase transformations have been investigated during high-energy ball milling of a mixture of elemental Co75Ti25 powder under an argon gas atmosphere. The results have shown that the single amorphous phase, which is obtained after 11 ks of mechanical alloying time, transforms into a new metastable phase of nanocrystalline bcc-Co3Ti upon milling for 86 ks. However, it transforms to the same amorphous phase upon further milling (360 ks). The present work shows the effects of the milling time and the milling speed on the structure of the mechanically alloyed powders.  2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Mechanical alloying; X-ray diffraction (XRD); Differential thermal analysis (DTA); Scanning electron microscopy (SEM); Transmission electron microscopy (TEM); Amorphous materials; Metastable phase; Powders; Microstructure; Lattice defects

1. Introduction Since 1983, the mechanical alloying (MA) [1] method has been successfully employed for the preparation of several amorphous alloy powders [2–10], carbides [11,12], nitrides [13,14] and nanocomposite materials [15,16], using the ball milling and/or rod-milling techniques. It has been reported

* Corresponding author. Present address: Inoue Superliquid Glass Project, Exploratory Research for Advanced Technology, Japan Science and Technology Corporation, Yagiyama-Minami, 2-1-1, Sendai 982-0807, Japan. Tel.: +81-22-243-0261; fax: +81-22-243-7616. E-mail address: [email protected] (M. Sherif ElEskandarany).

that further milling of amorphous Ti75Al25 [17], Al80Fe20 [18], Fe78Al13Si9 [19] and Ti50Al25Nb25 [20] alloy powders leads to amorphous–crystalline phase transformation (crystallization) and the formation of crystalline phases. The present study has been undertaken as part of an investigation into the structural changes that take place upon high-energy ball milling of a mixture of elemental Co75Ti25 powders at room temperature. For the purpose of the present work, Xray diffraction, scanning and transmission electron microscopes, and differential thermal analysis have been used to detect the structural changes and the thermal stability of the milled products. In addition, the change of magnetization for the ballmilled powders has also been measured to follow

1359-6454/02/$22.00  2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 6 4 5 4 ( 0 1 ) 0 0 4 1 2 - 8

1114

M. Sherif El-Eskandarany et al. / Acta Materialia 50 (2002) 1113–1123

the structural changes at the several stages of milling.

the milled powders. The gas contamination contents (oxygen, nitrogen and hydrogen) were determined by helium carrier fusion–thermal conductivity method.

2. Experimental procedure Pure elemental powders (99.9%) of Co (70 µm) and Ti (50 µm) were mixed to give the nominal composition of Co75Ti25 (at. %) in a glove box under purified argon atmosphere and sealed in a stainless steel vial (SUS 316, 250 ml in volume) together with 50 stainless steel balls (SUS 316, 10 mm in diameter). The ball-to-powder weight ratio was maintained as 17:1. The MA process was performed in a high-energy planetary ball mill (Fritsch P5) at a rotation speed of 4.2 s⫺1. However, some samples were obtained after milling at rotation speeds of 1.1, 2.1 and 3.3 s⫺1, using the same milling conditions. The MA experiments were performed under the same experimental conditions three times to confirm the reproducibility of the present investigation and to avoid any accidental results. In the all milling runs, the ball mill was interrupted when the temperature of the vial reached approx. 320 K (almost every 1.8 ks of continuous milling) and then resumed when the temperature decreased to 300 K. The structural changes with the milling time for the powders were followed by X-ray diffraction (XRD) with CuKα radiation and transmission electron microscopy (TEM), using a 200 kV microscope. The metallographical and morphological examinations of the milled powders were performed by means of light microscopy and scanning electron microscopy, using a 25 kV microscope. The samples were thermally analyzed with a differential thermal analyzer (DTA) at a heating rate of 0.33 K/s. The DTA measurements were performed at a constant heating rate of 0.33 K/s under an argon gas atmosphere. All the samples were heated up to 1200 K (first run) and cooled down to about 400 K. Then, second heating runs (dashed lines) were performed in order to get the base line. The magnetization of the milled powders was measured at room temperature in magnetic fields up 16 kOe using a vibrating sample magnetometer (VSM)). An induction coupled plasma emission method was used to analyze the concentration of Co and Ti, and the degree of Fe contamination in

3. Results 3.1. Ball-milling the elemental powders at a rotation speed of 4.2 sⴚ1 3.1.1. XRD analyses The XRD patterns of as-milled Co75Ti25 powders are shown in Fig. 1 after selected mechanical alloying times. In contrast to the initial mixture of polycrystalline hcp-Co and hcp-Ti [Fig. 1(a)], a broad diffuse and smooth halo appears after 11 ks of MA time [Fig. 1(b)], suggesting the formation of an amorphous phase. When this sample was annealed under argon gas in a DTA system at 1200

Fig. 1. XRD patterns of ball-milled Co75Ti25 powders after (a) 0, (b) 11, (c) 86, (d) 173, (e) 360, (f) 540 and (g) 720 ks of MA time, using a rotation speed of 4.2 s⫺1.

M. Sherif El-Eskandarany et al. / Acta Materialia 50 (2002) 1113–1123

K, it revealed an fcc structure [Fig. 2(a)] that corresponded to the equilibrium phase of Co3Ti [21]. Surprisingly, the obtained amorphous phase transforms into a new metastable phase of bcc-Co3Ti upon milling for 86 ks [Fig. 1(c)]. The lattice parameter, a0, of this new metastable phase was calculated to be 0.2855 nm, being smaller than that for the ordered phase of equiatomic bcc-CoTi (0.2987 nm) [22]. After 173 ks of MA time, the Bragg peaks that corresponding to the bcc-Co3Ti became broader [Fig. 1(d)]. When this sample is annealed at 1200 K in a DTA under argon gas flow, it reveals a polycrystalline structure corresponding to bcc-Co3Ti (metastable phase) coexisting with the ordered fcc-Co3Ti [Fig. 2(b)]. This is attributed to the crystallization of the existing amorphous phase in the ball-milled powders at this stage of milling.

1115

After 360 ks of MA time, this bcc phase transforms completely to an amorphous phase, suggested by the halo that is presented in Fig. 1(e). Increasing the MA time to 540 ks again leads to the formation of a nanocrystalline phase of bcc-Co3Ti coexisting with an amorphous phase, as shown in Fig. 1(f). In a cyclic-phase transformation, this metastable phase returns to the same amorphous phase of Co75Ti25 upon milling for 720 ks [Fig. 1(g)]. It is worth mentioned that such cyclic phase transformations have been observed for all the samples which were milled for three (and sometimes four) different milling runs. It is worth noting that this nonequilibrium phase of bcc-Co3Ti transforms into an ordered phase of fcc-Co3Ti upon annealing up to 1589 K, as presented in Fig. 2(c). 3.1.2. TEM analyses The bright field images (BFIs) and the corresponding selected area diffraction patterns (SADPs) for the ball-milled Co75Ti25 powders are shown in Fig. 3 after selected MA times. The BFI and the corresponding SADP for the sample taken after 18 ks of MA time are shown together in Fig. 3(a). Overall, the sample appears to have a homogeneous fine structure with no dominant structure contrast. In addition, the SADP [inset of Fig. 3(a)] shows a typical halo pattern of an amorphous phase. Contrary to this sample, the BFI [Fig. 3(b)] of the powders, which milled for 75 ks of MA time, shows the existence of numerous faults with

Fig. 2. XRD patterns of ball-milled Co75Ti25 powders that were heated up to 1200 in DTA under Ar gas atmosphere after milling for (a) 11 and 173 ks of MA time. The XRD pattern in (c) is for the sample that was milled for 86 ks and then heated up to 1589 K in DTA under Ar gas atmosphere.

Fig. 3. BFIs and the corresponding SADPs of as-ball-milled Co75Ti25 powders after (a) 18, (b) 75 and (c) 86 ks of MA time, using a rotation speed of 4.2 s⫺1.

1116

M. Sherif El-Eskandarany et al. / Acta Materialia 50 (2002) 1113–1123

grain boundary fringes and heavy dislocations in the boundary. Furthermore, some crystalline cells with an average diameter of approx. 55 nm can be seen. Moreover, the SADP shows a halo pattern coexisting with obvious sharp spots, indicating the existence of large crystals in the amorphous matrix of Co75Ti25. The BFI of the powders that milled for 86 ks of MA time is presented together with the related SADP in Fig. 3(c). The powders consist of nano-size grains (less than 30 nm in diameter) of polycrystalline bcc-Co3Ti, as indicated by the Debye–Scherrer rings. These nanocrystalline cells are subjected to a continuous shear and impact forces upon further milling for 386 ks so that the powders are heavily dislocated, as shown in Fig. 4(a). Such defects lead to a rise in the free energy

Fig. 4. BFIs of as-ball-milled Co75Ti25 powders after (a) 386 and (b) 720 ks of MA time, using a rotation speed of 4.2 s⫺1.

from the more stable phase (bcc-Co3Ti) to a less stable phase (amorphous), as indicated by the homogeneous and fine structure which is presented in Fig. 4(b). 3.1.3. Magnetic measurements Since the magnetization is sensitive to the structure, the magnetic properties of mechanically alloyed Co75Ti25 powders are considered as a powerful tool to monitor the phase transformations. The magnetizations of the milled powders for the three different milling runs are plotted as a function of the MA time in Fig. 5. The rapid decrease in the value of magnetization during the first few kiloseconds (3.6–7.2 ks) indicates a drastic decrease in pure Co particles in the mixture of Co75Ti25 powders and the formation of an amorphous phase. After 11 ks of MA time, it decreases down to a range of 65–70 emu/g. During the next stage of milling (14.4–22 ks), it is almost constant, suggesting the completion of the solid-state amorphization reaction. After 43 ks of MA time, the magnetization tends to increase slightly to a higher value, ranging from 79 to 87 emu/g, suggesting the precipitation of bcc-Co3Ti (a ferromagnetic phase) in the amorphous matrix of Co75Ti25 alloy powders. After 86 ks of the MA time, the amorphous phase

Fig. 5. Magnetization measured at room temperature of asmixed and ball-milled Co75Ti25 powders as a function of the MA time. The rotation speed of MA is 4.1 s⫺1 for sample preparation. The closed symbols display the results for the samples prepared through three independent milling runs. The solid, dotted and broken lines are drawn for eye guides.

M. Sherif El-Eskandarany et al. / Acta Materialia 50 (2002) 1113–1123

1117

transforms completely into bcc-Co3Ti [Fig. 1(c)] where the magnetization increases drastically to have higher values, ranging from 121 to 127 emu/g. It is worth noting that these values are about twice as large as that of fcc-Co3Ti (approx. 56 emu/g), which was obtained by isothermal annealing the amorphous Co75Ti25 alloy powders. This indicates that the mechanically alloyed bccCo3Ti phase has higher magnetization at room temperature. After 173 ks of MA time, the magnetization of bcc-Co3Ti slightly decreases due to the partial formation of the amorphous phase. After milling for 259–360 ks, the magnetization of the samples attains the same values (65–70 emu/g) as the amorphous samples. After 540 ks of MA time, nanocrystalline bcc-Co3Ti phase is formed where the magnetization increases again to a higher value, ranging from 110 to 118 emu/g. This value is slightly lower than that one of the bcc-Co3Ti samples, which obtained after 86 ks of MA time, probably due to the existence of amorphous Co75Ti25 phase [Fig. 1 (f)]. The amorphous phase that was obtained after 720 ks of MA time [Fig. 1(g)] has almost the same magnetization value of the mechanically alloyed 18 and 360 ks amorphous alloys. 3.1.4. Thermal stability The DTA curves of mechanically alloyed Co75Ti25 powders are presented in Fig. 6 after selected MA times. After 11 ks of MA time [Fig. 6(a)], a single sharp exothermic peak appears at 874 K. The low temperature broad exothermic reaction, which appears in the scan, is might be attributed to the relaxation of the obtained amorphous phase. This exothermic reaction is attributed to crystallization of the obtained amorphous phase into an ordered phase of fcc-Co3Ti [Fig. 2(a)]. No remarkable changes on the crystallization temperature can be detected in the sample that was milled for 22 ks [Fig. 6(b)]. After 86 ks of MA time, the exothermic reaction disappears and no further reactions can be detected, as illustrated in Fig. 6(c). This suggests the amorphous–crystalline (bccCo3Ti) phase transformation of the as-milled sample. This bcc-crystalline phase is thermally stable and does not change to any other phase(s) even at high temperatures, as high as 1500 K. It however,

Fig. 6. DTA curves of ball-milled Co75Ti25 powders that milled for (a) 11, (b) 22, (c) 86, (d) 173, (e) 360, (f) 540 and (g) 720 ks of MA time, using a rotation speed of 4.2 s⫺1.

transforms to the ordered phase of fcc-Co3Ti upon annealing at 1589 K [Fig. 2(c)]. A broad exothermic peak that is centered at about 880 K is observed for the sample, which is milled for 173 ks [Fig. 6(d)]. This exothermic reaction takes place due to the crystallization of the small mole fraction of the amorphous phase in the milled powders. The XRD pattern of the sample that was heated up to

1118

M. Sherif El-Eskandarany et al. / Acta Materialia 50 (2002) 1113–1123

1200 K [Fig. 2(b)] shows the co-existence of an fcc-Co3Ti with the bcc-Co3Ti phase that had already been formed during the ball milling process. This bcc-Co3Ti phase transforms to a single amorphous phase after 360 ks of the MA time, suggested by the sharp exothermic reaction which appears at 880 K in Fig. 6(e). The broad exothermic peak of the sample, which was milled for 540 ks, indicates the existence of a small amount of the amorphous phase [Fig. 6(f)]. After 720 ks of MA time, the bcc-Co3Ti phase transformed to an amorphous Co75Ti25 powders upon milling for 720 ks which crystallizes through a single sharp exothermic peak [Fig. 6(g)]. 3.2. Ball-milling the elemental powders at several rotation speeds In order to understand the effect of the milling rotation speed on the cyclic phase transformations, several samples of mechanically alloyed Co75Ti25 were prepared using rotation milling speeds of 1.1, 2.1 and 3.3 s⫺1. Fig. 7(a) shows the XRD patterns of the milled

powders at a rotation speed of 1.1 s⫺1. After 43– 86 ks of MA time, the milled powders are still a mixture of elemental Co and Ti without the existence of any reacted phases [Fig. 7(a) (1) and(2)]. No remarkable changes in the total structure can be observed for the powders that were milled for milling times as long as 173 ks [Fig. 7(a)(3)]. The powders at this stage of milling (173 ks) are heavily defected [Fig. 8(a)] and contain large grains of polycrystalline Co and Ti, as indicated by the sharp spot pattern in Fig. 8(b). The metallographical examination of the powders at this stage of milling shows that the particles are still have a layered structure morphology with an average particle size of approx. 500 µm in diameter [Fig. 9(a)]. Some broadening can however be seen, indicating the formation of an amorphous phase coexisting with the starting reactant materials of Co and Ti. As the milling time increases (360–720 ks), the intensities of the Bragg peaks for the unprocessed powders decreased [Fig. 7(a)(4–6)], suggesting a progress of the solid state amorphization reaction. The broad diffuse halo becomes more marked for the sample that was milled for 720 ks, indicating an

Fig. 7. XRD patterns of ball-milled Co75Ti25 powders that were milled at a rotation speed of (a) 1.1, (b) 2.1 and (c) 3.3 s⫺1. The samples were taken after (1) 43, (2) 86, (3) 173, (4) 360, (5) 540 and (6) 720 ks of MA time.

M. Sherif El-Eskandarany et al. / Acta Materialia 50 (2002) 1113–1123

Fig. 8. BFIs and the corresponding SADPs of ball-milled Co75Ti25 powders that were milled for 173 ks at three different rotation speeds, (a, b) 1.1, (c, d) 2.1 and (e, f) 3.3 s⫺1.

Fig. 9. SEM micrographs of the cross sectional view of ballmilled Co75Ti25 powders that were milled for 173 ks at three different rotation speeds, (a) 1.1, (b) 2.1 and (c) 3.3 s⫺1.

increase in the mole fraction of an amorphous phase [Fig. 7(a)(6)]. Fig. 7(b) shows the XRD patterns of the milled powders at rotation speeds of 2.1 s⫺1. When the powders are mechanically alloyed at this milling speed, the intensities of the Bragg peaks for the elemental starting materials have decreased during the first stage of milling (43–86 ks), whereas a halo pattern overlapped with the Bragg peaks of the starting materials is detectable for the sample that milled for 86 ks [Fig. 7(b)(2)], indicating the formation of an amorphous phase coexisting with the nano-size starting materials. The BFI and the corresponding SADP of the powders that milled for 173 ks are shown in Fig. 8(c,d), respectively. The overall matrix which has a fine structure is an

1119

amorphous Co75Ti25 alloy with clear visible halo diffraction pattern [Fig. 8 (d)]. Those small nanosize cells that are embedded into the amorphous matrix correspond to the unprocessed powders of Co and Ti. The SEM micrograph of the cross-sectional view for the powder which was milled for 173 ks is shown in Fig. 9(b). The powders have a lamellar structure, indicating the existence of a large mole fraction of the unreacted Co and Ti powders. An amorphous phase is formed after 360 ks of milling, characterized by a broad diffuse halo, as displayed in Fig. 7(b)(4). Increasing the milling time (540–720 ks) does not lead to any crystallization and/or phase transformations, as displayed in Fig. 7(b)(5) and (6). The XRD patterns of the milled powders at rotation speeds of 3.3 s⫺1 are displayed in Fig. 7(c). An amorphous phase is formed with small mole fraction of the starting powders after 43 ks of milling [Fig. 7c(1)]. Increasing the MA time (86 ks) leads to enhancement of the solid state amorphization reaction and a single amorphous phase is obtained, characterized by a broad diffuse and smooth halo, as displayed in Fig. 7(b)(2). This amorphous phase is suddenly crystallized into bccCo3Ti [Fig. 7(c)(3)]. The obtained metastable phase consists of nanocrystalline grains (less than 50 nm in diameter, [Fig. 8(e)] with a sharp diffraction ring pattern [Fig. 8(f)]. The cross-sectional view of the powders that milled for 173 ks have mirror-like metallography without precipitation of any other phase(s), indicating the formation of a single phase [Fig. 9(c)]. After 360–540 ks of MA time, this metastable phase have started to transform to an amorphous phase, characterized by the broadening in the Bragg peaks [Fig. 7(c)(4) and (5)]. A single amorphous phase is obtained after 720 ks, suggested by the smooth halo [Fig. 7(c)(6)]. The XRD patterns in Fig. 10 present the effect of the rotation speed of the mill on the cyclic phase transformations for mechanically alloyed Co75Ti25 powders. The powders of the starting materials [Fig. 10(a)] which had been already milled for 720 ks at a rotation speed of 1.1 s⫺1 [Fig. 7(a)(6)] were then milled at a higher rotation speed (4.2 s⫺1). In contrast to the starting materials (720 ks) that contain a small mole fraction of an amorphous phase

1120

M. Sherif El-Eskandarany et al. / Acta Materialia 50 (2002) 1113–1123

plotted in Fig. 11 as a function of the milling time. During the first stage of MA, the iron which comes from the stainless steel milling tool increases drastically to about 0.75 at.%. The gas contamination content that may be introduced to the milled powder during the ball-milling process and/or handling the sample outside the glove box increases during this stage of milling. This is attributed to the formation of active-fresh surfaces of the elemental powders (especially Ti) which are able to react with the surrounding atmosphere. As the MA time increases the milling tools are coated with the milled powder that works as iron-resistance wear. These coated balls play an important role to prevent introducing a further iron contamination to the milled powder. The iron contamination content of the end product is about 0.80 at.%, as illustrated in Fig. 12. In addition, no remarkable changes in the gas content can be detected for the samples that milled for longer times.

4. Discussion Fig. 10. XRD patterns of ball milled Co75Ti25 powders that were milled first for 720 ks of MA time at a rotation speed of 1.1 s⫺1 (a), and then milled at a rotation speed of 4.2 s⫺1 for (b) 10, (c) 90 and (d) 170 ks of MA time.

with elemental Co and Ti phases, an amorphous phase is predominant upon milling the powders for just 10 ks [Fig. 10(b)]. Increasing the milling time to 90 ks leads to crystallization of the amorphous phase into a metastable bcc-Co3Ti phase [Fig. 10(c)] which has the same lattice constant as the one which is obtained after 86 ks by milling the elemental powders at a rotation speed of 4.2 s⫺1 for 86 ks [Fig. 1(c)]. On an expected amorphous– crystalline–amorphous cyclic phase transformation, this metastable phase is retained to an amorphous phase after further milling (170 ks), as suggested by the clear halo in Fig. 10(d).

The present results confirm that the cyclic amorphization reaction takes place upon high-energy ball milling elemental powders of Co75Ti25 under an argon gas atmosphere at room temperature. Fig. 12 summarizes the results obtained by milling the powders at a rotation speed of 4.2 s⫺1. Obviously, a homogeneous amorphous phase is formed after

3.3. Chemical analysis The iron and gas (oxygen, nitrogen and hydrogen) contamination contents of the powders that were milled at a rotation speed of 4.2 s⫺1 are

Fig. 11. Dependence of the contamination content of ballmilled Co75Ti25 powders on the MA time, using a rotation speed of 4.2 s⫺1.

M. Sherif El-Eskandarany et al. / Acta Materialia 50 (2002) 1113–1123

Fig. 12. Schematic illustration of amorphous–crystalline– amorphous cyclic phase transformations that are took place during ball-milling elemental powders of Co75Ti25, using a rotation speed of 4.2 s⫺1.

only a few kiloseconds of milling [Fig. 1(b)]. The amorphous phase formed at this stage is not stable against the impact and shear forces, which are generated by the milling media (balls) and surprisingly transforms into a new nonequilibrium phase of nanocrystalline bcc-Co3Ti upon further milling [Fig. 1(c)]. Such a room temperature transformation between the amorphous and crystalline phases can be repeated upon longer milling (see Figs. 1 and 5), using a high rotation speed of milling, as high as 4.2 s⫺1. The obtained metastable phase transforms to an ordered fcc-Co3Ti upon annealing at 1589 K [Fig. 2(c)]. This amorphous phase transforms into an ordered fcc-Co3Ti upon annealing at 1200 K [Fig. 2(a)]. Whilst the powders are milled at a lower rotation speed of milling (1.1 and 2.1 s⫺1) such cyclic transformations could

1121

not be observed [Fig. 7(a,b)]. When the powders previously milled at a low rotation speed are remilled at a higher rotation speed (4.2 s⫺1), the cyclic amorphization is detected (Fig. 10). It is worth noting that the cyclic amorphization is not unique for Co75Ti25 alloy. It has been commonly observed for some mechanically alloyed powders such as, Al50Zr50 [10] and Co50Ti50 [23] so that the reversibility of phase transformations is not unique to a particular system. It has been reported [24] that hcp-Co transforms to fcc-Co phase by high-energy ball milling an elemental Co at room temperature. The obtained fcc phase was again transformed to hcp-Co by further milling. One possible factor, which leads to such cyclic crystalline–amorphous transformation of Co75Ti25, is the introduction of contamination in the ballmilled powders. The total impurity concentration measured at various MA times is less than 1.50 at. % for the milled powders (Fig. 11). It has been suggested by the chemical analyses, that there was no oscillation in the impurities values while the cyclic transformation took place (Figs. 5 and 12). These low values of the contamination contents plus the fact that they do not oscillate during the milling time [25] can make us neglect their effects for such cyclic amorphization. They therefore do not play any role in such reported cyclic phase transformations of Co75Ti25. Another possible factor, which can lead to the cyclic phase transformations, is the temperature rise during the milling. In order to examine the effect of temperature increase on the structure of the powder during milling, the samples that milled for 11 ks (amorphous phase of Co75Ti25alloy) and 86 ks (bcc-Co3Ti) were heated separately in the DTA to 700 K (well above the measured temperature of the vial). The XRD patterns of these samples prove that no phase transformation has taken place even at this relatively high temperature, having the same XRD patterns displayed in Fig. 1(b,c). In contrast, the amorphous Co75Ti25 alloy is transformed to an ordered fcc-Co3Ti upon heating to about 880 K, as shown by the DTA measurements (see Fig. 6). In addition, the bcc-Co3Ti phase transforms into fcc-Co3Ti when it is annealed at 1589 K [Fig. 2(c)]. In this context, the temperature recorded during the ball-milling process is far

1122

M. Sherif El-Eskandarany et al. / Acta Materialia 50 (2002) 1113–1123

below the temperatures required for structural changes in the milled powder. The TEM/EDX analyses for the obtained amorphous and bccCo3Ti phases [Fig. 3(a,c)] have not shown any concentration gradient or compositional fluctuation, indicating that both the amorphous and metastable phases are homogeneous. Since mechanical milling introduces many vacancies, lattice defects, grain boundaries and surfaces, milled powders store a large amount of mechanical-strain energy [26–31]. In the initial milling stage, crystalline elemental powders are mechanically crushed and fresh surfaces are highly created. Kneading of such ground powders enhances atomic diffusion of elemental atoms and to step-by-step local alloying (mechanical alloying). When the long-range periodic structure is destroyed by introduction of defects, an amorphous phase is obtained as a metastable state. In the milling process, friction between balls, balls and the surface of a vial also generate frictional heat. Such heat not only accelerates diffusion of constitutional atoms of powder mixture, but also contributes to thermal annealing of the amorphous phase. Without interruption of the milling process, the amorphous phase is transformed to the equilibrium crystalline fcc phase (over-milling). Hence the milling process was interrupted every 1.8 ks to reduce the vial temperature down to 300 K, the amorphous phase is moderately annealed, being transformed to another metastable bcc phase under mechanical shear and impact forces. Since further mechanical milling again introduces many defects in the crystalline phase and destroys the periodical structure, the bcc phase is transformed to the amorphous phase (mechanical disordering). These sequential transformations are observed as the cyclic transformation in the moderate milling process. Moreover, the present result suggests that the formation enthalpy of the metastable bcc phase is comparable to the amorphous phase and the energy barrier between these two phases is rather low to allow such transformation. 5. Conclusion In conclusion, we have demonstrated cyclic amorphous-to-crystalline phase transformations

during ball milling of elemental Co75Ti25 powder. After a few kiloseconds of milling, a solid-state reaction takes place at the fresh interfaces of Co/Ti layers and an amorphous phase of Co75Ti25 is formed after 11 ks. This amorphous phase transforms into an ordered fcc-Co3Ti upon heating to 880 K (crystallization). Further milling (86 ks) also leads to an amorphous–crystalline transformation (crystallization) and the formed phase is metastable bcc-Co3Ti nanocrystalline powders. We have attributed this transformation, which took place in the ball mill to the inability of the formed amorphous phase to withstand the impact and shear forces that are generated by the milling media (balls). When the milling time is increased (360 ks), the crystalline phase is subjected to several defects (points and lattice defects) that raise the free energy from the stable phase of bcc-Co3Ti to a less stable phase (amorphous). Here, the crystalline–amorphous transformation which takes place is similar to the mechanical grinding method [28] in which the amorphization occurs by relaxing the short-range order without any compositional changes. Further milling leads to the formation of crystalline and/or amorphous phases depending on the milling time. References [1] Koch CC, Cavin OB, MacKamey CG, Scarbourgh JO. Appl Phys Lett 1983;43:1017. [2] Politis C, Johnson WL. J Appl Phys 1986;60:1147. [3] Politis C, Johnson WL. J Appl Phys 1986;60:1147. [4] Schwarz R, Koch CC. Appl Phys Lett 1986;49:146. [5] Ozaki K, Matsumoto A, Sugiyama A, Nishio T, Kobayashi K. Mater Trans JIM 2000;41:1495. [6] Sherif El-Eskandarany M, Aoki K, Suzuki K. J Appl Phys 1992;72:2665. [7] Seidel M, Eckert J, Schultz L. Mater Sci Forum 1997;235–238:29. [8] Sherif El-Eskandarany M. J Alloys Comp 1999;284:295. [9] Sherif El-Eskandarany M, Bahgat AA, Gomaa NS, Eissa NA. J Alloys Comp 1999;290:181. [10] Sherif El-Eskandarany M, Aoki K, Sumiyama K, Suzuki K. Met Trans 1877;A 1999:30. [11] Sherif El-Eskandarany M. J Alloys Comp 2000;305:219. [12] Krivoroutchko K, Kulik T, Matyja H, Portnoy VK, Fadeeva VI. J Alloys Comp 2000;308:230. [13] Wexler D, Calka A, Mosbah AY. Mater Sci Forum 2000;343–346:399. [14] Sherif El-Eskandarany M, Omori M, Konno TJ, Sumiyama K, Hirai T, Suzuki K. Met Trans A 1973;1998:29.

M. Sherif El-Eskandarany et al. / Acta Materialia 50 (2002) 1113–1123

[15] Krasnowski M, Matyja H. Mater Sci Forum 2000;343– 346:302. [16] Sherif El-Eskandarany M. J Alloys Comp 2000;296:175. [17] Cocco G, Soletta I, Battezzati L, Baricco M, Enzo S. Phil Mag B 1990;61:473. [18] Morris MA, Morris DG. Mater Sci Forum 1992;88– 90:529. [19] Huang B, Perez RJ, Crawford PJ, Sharif AA, Nutt SR, Lavernia EJ. NanoStruct Mater 1995;5:545. [20] Chen G-H, Suryanarayana C, Froes FH. Met Trans A 1995;26:1379. [21] ASTM card No. 15-806. [22] ASTM card No. 18-0429. [23] Sherif El-Eskandarany M, Aoki K, Sumiyama K, Suzuki K. Scripta Metall 1997;36:1001.

1123

[24] Huang JY, Wu YK, Ye HQ. Acta Metall 1996;44:1201. [25] Sherif El-Eskandarany M, Aoki K, Sumiyama K, Suzuki K. Appl Phys Lett 1997;70:1679. [26] Ermakov AE, Yurchikov EE, Barinov VA. Phys Met Metall 1981;52:50. [27] Schwarz RB, Koch CC. Appl Phys Lett 1986;49:146. [28] Weeber AW, Bakker H, Boer FR. Europhys Lett 1986;2:445. [29] Fecht HJ, Hellstein E, Fu Z, Johnson WL. Metall Trans A 1990;21:2333. [30] Harris SR, Pearson DH, Garland CM, Fultz B. J Mater Res 1991;6:1. [31] Sluiter M, Turchi PEA. Phys Rev B 1993;46:14.