Electron beam induced crystallization of amorphous Al-based alloys in the TEM

We have measured aluminum-like medium range order (MRO) in amorphous Al92Sm8 using fluctuation electron microscopy (FEM). Here we show similar but not identical medium-range structure in amorphous Al88Y7Fe5. Both compositions begin to crystallize in less than 7 min under a 120 kV TEM electron beam. Beam-induced heating of these samples is minimal, so we suggest that crystallization is caused by beam-induced atomic displacements (knock-on) that introduces free volume or localized mixing, both of which will induce crystallization. If this is the case, the higher beam energies used for high-resolution TEM will exacerbate, not reduce, the problem. Studying these materials in the TEM therefore requires low-dose FEM. Since FEM is an inherently statistical technique, the goal of which is to characterize the entire sample structure, we can reduce the dose to any one area of the sample by spreading the total dose across more material. The only experimental limit is the noise of the electron detector, which makes FEM experiments possible even on metastable materials that suffer from beam damage.