Transmission X-ray diffraction of single-crystal nickel-base superalloys
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Fig. 5—Dependence of the contamination content of the Al50Zr50 powder that ball milled at a milling speed of 8 on the MA time.
atmosphere. As MA time increases, the milling tools are coated with the milled powder that tends to reduce wear of the grinding balls. These coated balls play an important role in preventing further iron contamination to the milled powder. The iron contamination content of the end product is less than 0.35 at. pct, as illustrated in Figure 4. In addition, no remarkable changes in the gas content can be detected for the samples that milled for longer times (150 hours). Hence, we can conclude that these low contamination values are not considered to be responsible for the cyclic phase transformations presented here. Another possible factor, which may lead to the phase changes during the ball milling, is the increase of the temperature during MA.[8] To minimize this factor, the ball-milling process was interrupted when the temperature of the vial reached about 320 K (almost every 30 minutes of continuous milling) and then resumed when the temperature decreased to 300 K. In order to assess the effect of the temperature increasing on the structure of the powder during milling, the samples milled for 24 hours (amorphous phase) and 48 hours (ordered phase) were heated separately in the DTA to 800 K (well above the measured temperature of the vial). The XRD patterns of these samples were the same as those displayed in Figures 1(c) and (d). However, the amorphous Al50Zr50 alloy is transformed to a crystalline orthorhombicAlZr upon heating to about 1000 K, as shown by the DTA measurements (Figure 3). We can conclude that the temperature increase during the ball-milling process is far below that required to effect structural changes in the milled powder. In order to study the so-called interrupted combustion effect[15] in the Al-Zr system, individual samples are milled for 12, 24, 48, 72, 100, and 150 hours at I 5 8 millings with interrupting. The structure of the powder of each MA time was investigated by XRD. The powders are either amorphous or crystalline phases, depending on the MA time. Here also, the maximum-recorded values of the vial’s temperature were about 352 K, being far below that required for amorphous-crystalline transformation (Figure 3). In conclusion, we have demonstrated cyclic amorphous– crystalline amorphous phase transformations in mechanically alloyed Al50Zr50 powder. The milling times and the milling speed govern this reaction. The results show that a single amorphous phase is obtained after 24 hours of the 1880—VOLUME 30A, JULY 1999
The authors thank Dr. K. Takada for performing the chemical analyses. This work was partially supported by Grantin-Aid for General Scientific Research No. 06452323, given by the Ministry of Education, Science, Sports and Culture, Japan. REFERENCES 1. J.S. Benjamin: Metall. Trans., 1970, vol. 1, pp. 2943-51. 2. C.C. Koch, O.B. Cavin, C.G. MacKamey, and J.O. Scarbourgh: Appl. Phys. Lett., 1983, vol. 43, pp. 1017-19. 3. R. Schwarz and C.C. Koch: App
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