Devitrification of Al-Ni-rare earth amorphous alloys

The effect of rare earth (RE) elements on the devitrification behaviour of Al87Ni7RE6 alloys (RE = La, Sm) is reported. Two crystallisation mechanisms are found as a function of the type of element and, therefore, size: when RE = La the transformation occurs in two steps, when RE = Sm the transformation has an additional step. The glass transition becomes manifest for both alloys when fast enough rates are used in Differential Scanning Calorimetry. The first crystallisation step implies precipitation of nanocrystalline Al in Al87Ni7Sm6 and of a metastable intermetallic phase in Al87Ni7La6, relatively coarse grained. Its formation is suppressed by substitution of 1% at La with Ti or Zr. Interestingly, the amorphous matrix in Al87Ni7La6 is stabilised by annealing at temperatures below the glass transition so that the second crystallisation peak is shifted to higher temperature. In Al87Ni7Sm6 the glass transition remains visible after partial crystallisation showing that the matrix is readily homogenised. As a consequence, it transforms abruptly with a polimorphic-like reaction. There is evidence of a calorimetric continuous background for all alloys which is attributed to diffusional homogenisation of the alloy and grain growth. The results are discussed in terms of nucleation and growth mechanisms.