Phase transformations in Al{sub}87Ni{sub}7Ce{sub}6 and Al{sub}87Ni{sub}7Nd{sub}6 amorphous alloys

摘要

This paper reports on studies of the effect of different rare earth (RE) elements on the devitrification behaviour of alloys of general composition Al{sub}87Ni{sub}7RE{sub}6 (here RE = Ce, Nd). We have evidenced two crystallisation mechanisms as a function of the type of rare earth clement. When RE = Ce the transformation proceeds in two steps as shown by Differential Scanning Calorimetry (DSC) traces. When RE = Nd the transformation sequence has an additional step. There is also evidence of a calorimetric continuous background for all alloys which is attributed to diffusional homogenisation of the matrix. The glass transition becomes manifest for both alloys when fast enough rates are used in DSC (higher than 10K/min and 40K/min, respectively for Al{sub}87Ni{sub}7Ce{sub}6 and Al{sub}87Ni{sub}7Nd{sub}6). The first crystallisation step implies the precipitation of nanocrystalline Al. The kinetics of this process is influenced by the simultaneous occurrence of the glass transition. In Al{sub}87Ni{sub}7Ce{sub}6 when it takes place above T{sub}g (i.e. at high heating rates) it also implies the formation of an intermetallic compound. Kissinger plots for the first transformation display a kink at temperatures corresponding to the glass transition range marking the change in mechanism for the transformation. In Al{sub}87Ni{sub}7Nd{sub}6 the glass transition remain visible after partial crystallisation showing that the matrix is readily homogenised. Therefore the molten state of these alloys appears rather fragile.