Crystallization under electropulsing suggesting a resonant collective motion of many atoms and modification of thermodynamic parameters in amorphous alloys

For melt-spun amorphous (a-)Cu50Ti50 and a-Pd80Si20, crystallization under electropulsing was studied by means of the discharge of a condenser with initial current density id0 of the order of 109 A/m2 and decay time τ in between 2 and 0.1 ms, where a specimen was sandwiched by AlN/BN substrates to minimize an effect of joule heating. The crystallization proceeds during electropulsing when id0 is higher than the threshold id0,c. Here id0,c is a function of τ and shows a minimum value of 1.4×109 A/m2 at τ∼2 ms for a-Cu50Ti50 and 2.6×109 A/m2 at τ∼0.9 ms for a-Pd80Si20, where the maximum increase in temperature during electropulsing is about 120 K for a-Cu50Ti50 and 50 K for a-Pd80Si20, respectively. One-half of the specimen volume crystallizes after a few repetitions of electropulsing with id0 beyond id0,c for a-Cu50Ti50 and after several repetitions for a-Pd80Si20. We surmise that for the density fluctuations existing in amorphous alloys, under electropulsing a high-density region undergoes a resonant collective motion as a whole, which induces migrational motions of atoms in the low-density matrix around it. For a-Pd80Si20, it is observed that an unknown phase was formed in the early stage of the crystallization under electropulsing and disappeared after further electropulsing. It is also found for a-Cu50Ti50 and a-Pd80Si20 that for electropulsing with high id0, the electrical resistivity of a specimen decreased at the early stage of the crystallization and then turned to increase for further electropulsing. These phenomena may be associated with changes in the thermodynamic free energy of phases under an electric current predicted by the theoretical works. We surmise that present electropulsing excites a resonant collective motion of many atoms and modifies the thermodynamic free energy of phases too