BALL MILLING OF INTERMETALLIC PHASES IN THE Nb-Al SYSTEM

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Abstract
A powder consisting mainly of the intermetallic A15 phase Nb3Al and a small amount of the σ phase Nb2A1 was milled in a planetary mill. By X-ray diffractometry it is observed that the A15 phase can almost completely be transformed into a supersaturated bcc NbA1 solid solution. The results are discussed with respect to the mechanism of the transformation. In order to estimate the enthalpy stored by ball milling the average crystallite size, the root mean square strain and the long-range order parameter are determined from X-ray diffractograms. It is concluded that the transformation of the A15 phase mainly originates from a decrease of its long-range order which raises its free energy above the one of the bcc solid solution. This result will also be discussed with respect to a transformation of an intermetallic compound into an amorphous structure

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