Effects of iron impurities in mechanical alloying using steel media
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Mechanical alloying, a high-energy ball-milling technique, is now widely used for preparing alloy powders with metastable phases (crystalline or amorphous). The technique, however, may contaminate the powder with material eroded from the vial and milling media. We report on the analysis and effects of iron contamination on Al25Ge75 powders that we prepared by mechanically alloying mixtures of aluminum and germanium powders, using different mechanical alloying apparatuses. Mechanical alloying (MA) is a high-energy ballmilling technique developed in the 1970s1 to prepare nickel-based oxide-dispersion-strengthened alloys. More recently, MA has been used to produce alloy powders with metastable phases (crystalline or amorphous).2 For these applications, MA usually starts from a mixture of elemental powders. MA has been used to prepare amorphous metallic powders in a number of alloy systems. The glass-forming range (GFR), or range of compositions where the process leads to the formation of a single-phase amorphous alloy, can be significantly expanded over the GFR obtained with other techniques such as rapid solidification. In addition, the MA method is well suited to producing large quantities of material and thus has the potential for practical use. Accompanying MA is the danger of contaminating the alloyed powder with material from the milling media. However, with few exceptions,3"5 researchers have paid little attention to the composition of the mechanically alloyed product, and to possible iron contamination from the balls and vial. The main reason for this oversight is that in many cases the iron contamination does not affect the properties of the mechanically alloyed powder. For example, a few atomic percent of iron in an equimolar alloy of an early-transition metal and a late-transition metal, such as Ni-Zr or Cu—Ti, would not change the structure of the alloyed powder; for long MA times, the product would be amorphous in both cases. We have learned, however, that the aluminum-germanium system is very sensitive to iron contamination from the balls and that a small iron contamination drastically alters the alloy phases obtained by MA. Aluminum and germanium form a simple eutectic system, with no stable intermetallic phase and very little mutual solubility.6 For an equilibrium alloy M\-xGex, and for compositions other than the eutectic composition at x = 0.3, we expect a differential calorimetry curve to show two endothermic peaks upon heating, the first at the eutectic temperature of 420 °C, and the second at the crossing of the liquidus line. We expect supplementary exothermic peaks if the mechanically alloyed Ali-jGe*
powder has metastable phases. However, after the sample is melted in the calorimeter, a second heating of the same alloy should show only the two endothermic peaks. We did not observe this behavior with powders mechanically alloyed in iron media, and we linked the anomalous results to iron contamination. We studied Al-Ge alloys with compositions close to Al25Ge75. Our starting materials were 99.99
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