men of the as milled powder was prepared by compact ing the powder into a 3 mm

Men of the as milled powder was prepared by compact

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men of the as-milled powder was prepared by compact- ing the powder into a 3 mm diameter disk. Thinning was accomplished using a twin-jet electropolisher with a solution of perchloric acid and methanol. TEM anal- ysis was conducted using a JEOL 2000FX TEM at an accelerating voltage of 200 kV. The density functional theory (DFT) calculations were performed by the package of the exact muffin- tin orbital method combined with coherent potential approximation or EMTO-CPA.[ 14 ] This computational method has been widely used for solid solution systems, for example, stainless steels [ 15 ] and NiFeCrCoMn.[ 16 ] The results of our mechanical alloying of the selected alloy, Al 20 Li 20 Mg 10 Sc 20 Ti 30, must be divided into two materials—one which contained mainly the five components intentionally added, and another, which besides these components had significant impurity con- tent of nitrogen and oxygen. The material prepared by mechanical alloying which did not have the high N, O impurity levels was observed to have a single-phase fcc crystal structure in the as-milled condition, as illustrated in Figure 1 . It had a nanocrystalline grain size estimated by the Scherrer for- mula to be about 12 nm. The lattice parameter of this sample determined by the method of Cohen was found to be 0.4323 nm. The mechanical hardness of this sam- ple was very high 5.8 GPa. After annealing this sample at 500°C for 1 h, the crystal structure changed as shown Figure 1. XRD pattern of as-milled alloy. Figure 2. XRD pattern of uncontaminated alloy after anneal- ing at 500°C. in Figure 2 . The new structure was indexed as hcp with c/a ratio of 1.588. The grain size, again estimated by the Scherrer method, was 26 nm and the mechanical hard- ness dropped to 4.9 GPa. TEM measurements of the as-milled material, shown in Figure 3 , generally agree with the XRD results, giving an average grain size of 20 nm and showing no second phases in the diffraction pattern. The material with the higher N and O concentra- tions (0.4 at% N, 1.39 at.% O as determined by chemical analysis) apparently obtained this contamination from a batch of Sc powder which was contaminated during the process of converting the as-received pellets into powder by cryomilling. Subsequent batches did not contain these impurities and were used in the sample described above. The contaminated sample exhibited an as-milled struc- ture essentially identical to that of the un-contaminated sample, that is, single-phase fcc with a grain size of about 12 nm. However, it had a slightly higher mechan- ical hardness of 6.1 GPa. After annealing at 500°C and 800°C, the structure remained similar to fcc, but some splitting of the diffraction peaks was observed in the XRD results, suggesting either a slight distortion from a perfect cubic lattice or separation into two similar phases with a lattice parameter difference < 1%. Annealing at 500°C decreased the hardness to only 5.9 GPa, while annealing at 800°C resulted in a hardness of 5.75 GPa, consistent with a nanocrystalline grain size. Because of the diffraction peak splitting, the grain size cannot
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