Phase Transformations in Al₈₇Ni₇Ce₆ and Al₈₇Ni₇Nd₆ Amorphous Alloys

Abstract

This paper reports on studies of the effect of different rare earth (RE) elements on the devitrification behaviour of alloys of general composition Al₈₇Ni₇RE₆ (here RE=Ce, Nd). We have evidenced two crystallisation mechanisms as a function of the type of rare earth element. 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 10 K/min and 40 K/min, respectively for Al₈₇Ni₇Ce₆ and Al₈₇Ni₇Nd₆). 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 Al₈₇Ni₇Ce₆ when it takes place above T_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 Al₈₇Ni₇Nd₆ 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.