| Research Abstract |
The resonant-electropulsing-induced low temperature crystallization (r-e-LTC) of amorphous (a-) Cu_<50>Ti_<50>, a-Cu_<50>Zr_<50>, a-Pd_<80>Si_<20> and a-Zr_<60>Cu_<30>Al_<10> was investigated by means of electropulsing at RT and that in liquid nitrogen (LN2). Resonant (r-) electropulsing was made by means of discharge of a condenser which is characterized by the initial current density, i_<a0>, and the decay time, τ, where the frequency of the principal Fourier constitutional component of r-electropulsing is 1/2πτ. The range of i_<a0> was between 10^8 A/m^2 and 10^<10> A/m^2 and that of τ was between 0.05 ms and 20. ms. For all the a-alloys, the r-e-LTC took place during single r-electropulsing with i_<a0> higher than the threshold current density, i_<a0>,c, where i_<a0>,c was a function of τ. The maximum specimen temperature during the r-e-LTC was, e.g., 400 K for r-electropulsing at RT and 200K for r-electropulsing in LN_2, respectively, indicating that the r-e-LTC is associated with
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an athermal process. The dependence of i_<a0>,c, on τ found for r-electropulsing in LN_2 was essentially the same to that observed for r-electropulsing at RT, indicating that the density fluctuation associated with the resonant collective motion responsible for the r-e-LTC was that frozen at the glass transition temperature. The transmission electron microscopy observation revealed that crystallites formed by the r-e-LTC showed crystallographic alignment with each other and the size of the crystallites was, e.g., the order of ten nm. A possible mechanism for the aligned crystallization is a direct transformation of the relatively high density region (RHDR) to a crystalline phase, where cooperative motions of many atoms such as an optical phonon like motions of atoms should take place. In other words, the amplitude of displacement during the resonant collective motion should be a function of atomic species and the compositional fluctuation in this region should be inside in a permissible range for the crystalline phase formed too. In order to realize such cooperative motion of RHDR, a capacity for the anelastic strain in the relatively low density regions should be considerably large. The anelasticity study of nanocrystalline metals supported this point of view. The present work demonstrated the realization and application of excitation of collective motion of many atoms in amorphous alloys. Less
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