Investigation of phase composition and nanoscale microstructure of high-energy ball-milled MgCu sample
© Ma et al.; licensee Springer. 2012
Received: 12 June 2012
Accepted: 1 July 2012
Published: 13 July 2012
The ball milling technique has been successfully applied to the synthesis of various materials such as equilibrium intermetallic phases, amorphous compounds, nanocrystalline materials, or metastable crystalline phases. However, how the phase composition and nanoscale microstructure evolute during ball milling in various materials is still controversial due to the complex mechanism of ball milling, especially in the field of solid-state amorphization caused by ball milling. In the present work, the phase evolution during the high-energy ball milling process of the Mg and Cu (atomic ratio is 1:1) mixed powder was investigated. It was found that Mg firstly reacts with Cu, forming the Mg2Cu alloy in the primary stage of ball milling. As the milling time increases, the diffracted peaks of Mg2Cu and Cu gradually disappear, and only a broad halo peak can be observed in the X-ray diffraction pattern of the final 18-h milled sample. As for this halo peak, lots of previous studies suggested that it originated from the amorphous phase formed during the ball milling. Here, a different opinion that this halo peak results from the very small size of crystals is proposed: As the ball milling time increases, the sizes of Mg2Cu and Cu crystals become smaller and smaller, so the diffracted peaks of Mg2Cu and Cu become broader and broader and result in their overlap between 39° and 45°, at last forming the amorphous-like halo peak. In order to determine the origin of this halo peak, microstructure observation and annealing experiment on the milled sample were carried out. In the transmission electron microscopy dark-field image of the milled sample, lots of very small nanocrystals (below 20 nm) identified as Mg2Cu and Cu were found. Moreover, in the differential scanning calorimetry curve of the milled sample during the annealing process, no obvious exothermic peak corresponding to the crystallization of amorphous phase is observed. All the above results confirm that the broad halo diffracted peak in the milled MgCu sample is attributed to the overlap of the broadened peaks of the very small Mg2Cu and Cu nanocrystalline phase, not the MgCu amorphous phase. The whole milling process of MgCu can be described as follows: .
The mechanical alloying (MA) process developed by Benjamin et al. [1, 2] in the early 1970s is now recognized as a versatile technique for obtaining oxide dispersion-strengthened superalloys, equilibrium intermetallic phases, amorphous compounds, nanocrystalline materials, or metastable crystalline phases. Due to the complicated ball milling environment, how the nanoscale microstructure evolutes during the ball milling in various materials is still under discussion, and some conclusions on the final phases after ball milling are controversial, especially in the field of solid-state amorphization caused by ball milling [3, 4].
On the other hand, in the past two decades, Mg-based amorphous alloys (Mg-Cu-Y, Mg-Ni, Mg-Cu, etc.) are regarded as a new family of promising materials with excellent specific strength, improved hydrogen storage, and good corrosion resistance . Considering the large differences in melting points and vapor pressures between Mg and other alloying elements, it is a great challenge to obtain Mg-based amorphous alloys by traditional casting techniques. MA is a low-temperature process; therefore, it overcomes the disadvantages of conventional alloying and allows forming amorphous samples for compositions which cannot be amorphized by casting techniques. In fact, a number of binary or ternary Mg-based amorphous alloys, such as Mg-Ni  and Mg-Cu-Y [7, 8], have been synthesized by mechanical alloying of the crystalline elemental powders. Some previous studies [9, 10] reported that the MgCu amorphous alloys could also be prepared by ball milling. They considered the final product of a milled MgCu sample as amorphous alloy based on the broad halo peak in the X-ray diffraction pattern alone. However, it should be noted that it is not possible to distinguish among the materials which are (a) truly amorphous and (b) extremely refined grain by observing the broad X-ray peaks alone , especially in the ball-milled samples. Hence, the above conclusions on the ball-milled MgCu sample might not be very valid.
Based on above background, from the perspective of the development of Mg-Cu amorphous alloys, and also on the understanding of solid-state amorphization mechanism during ball milling process, it makes sense to clarify the phase composition and nanoscale microstructure of the high-energy ball milled MgCu. Hence, in the present work, the phase evolution during the high-energy ball milling process of the Mg and Cu mixed powder was investigated. Furthermore, microstructure observation and annealing treatment of the milled MgCu sample were also carried out.
The Mg powder (99.8cs% purity, 325 mesh) and Cu powder (99.9% purity, 625 mesh) were mixed in a molar ratio of Mg50Cu50. Then, tungsten carbide milling balls and the mixed powder were put into the tungsten carbide vessel with the ball-to-powder weight ratio of 5:1 in the argon box. The high-energy ball milling was performed on a SPEX 8000 M mill (Thomas Scientific, Swedesboro, NJ, USA) under argon atmosphere. The milling process was performed in a discontinuous way consisting of 1 milling h followed by rest period of 0.5 h. The powders after milling for several different times were characterized by X-ray diffraction (XRD) in the Bruker D8 Advance X-ray diffractometer (Bruker Optik GmbH, Ettlingen, Germany). The evolution of grain size of Mg and Cu during the ball milling was estimated using the single-line method of diffraction line-broadening analysis based on the XRD data.
The microstructure was investigated using high-resolution transmission electron microscopy. The differential scanning calorimetry (DSC) measurements of the milled MgCu powders were carried out using the power-compensated PerkinElmer Pyris-1 (PerkinElmer, Waltham, MA, USA) from 50°C to 450°C with the heating rate of 20°C/min under argon gas protection. Accordingly, several annealing temperatures were determined, and the annealing experiment is carried out at these temperatures. After that, the phase composition of the annealed samples is also studied by X-ray diffraction.
Results and discussion
Based on the above discussion, what is the real origin of the broadened halo peak in the milled sample? Are nanocrystals or amorphous phase present in the ball-milled MgCu sample? To answer these questions, it is needed to further investigate the ball-milled sample with the aid of microstructure observation and annealing experiment.
The microstructure of the particle in Figure 5 seems different from the particle in Figure 4. The rings in the SAD pattern (see Figure 5b) are more diffused, and no clear ring can be observed. Only a halo is present in the SAD pattern. This type of pattern is always identified as amorphous in the literature. However, in the corresponding dark-field image of this particle (see Figure 5a), the presence of bright dots indicates that many nanocrystals still exist in this particle (about 10 nm). Observing Figure 5b more carefully, it can be found that the halo is located in almost the same position of the rings belonging to Cu and Mg2Cu (see Figure 4b). Hence, it is speculated that the diffraction of a large amount of very small Cu and Mg2Cu nanocrystals possibly results in the amorphous-like pattern in the Figure 5b. Different microstructures in the Figures 4 and 5 from the same milled sample also imply that the size distribution of grains after ball milling is not uniform.
Based on the above analysis, it is concluded that the long-time milled MgCu sample consisted of Cu and Mg2Cu nanocrystals, and the halo peak in the XRD pattern of the 18-h milled sample ought to be attributed to the overlap of the broadened peaks of the Cu and Mg2Cu nanocrytals. The whole milling process of the MgCu system can be described as follows: . According to the present research, it is worth noting that for the preparation of amorphous alloys from different kinds of metal using ball milling, it is not precise to consider the diffused halo peak appearing in the XRD pattern of milled samples as amorphous phase without the careful investigation of the microstructure. Even during the observation of the microstructure, the appearance of the diffused ring in the SAD cannot guarantee that the sample consisted of the amorphous phase. The diffraction of a large amount of nanocrystals with a very small size might also result in the amorphous-like SAD pattern.
The authors are grateful to the National Natural Science Foundation of China (Grant No. 51077099 and 50901049), Program for New Century Excellent Talents in University, and Seed Foundation of Tianjin University for grant and financial support.
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