Rapid melting and quenching with microsecond current pulses

Abstract

We report the use of microsecond current pulses to transform layered crystalline Ni---Zr films to amorphous alloys. The starting materials were electron-beam- or sputter-deposited multilayers with a composition modulation wavelength of 34 nm, an average composition of Ni63Zr37 and a total thickness of 680 nm. The electrical pulses were approximately rectangular and about 3 ms in duration, with an intensity of several hundred amperes, directly coupling 1.6-3 J of energy uniformly into the film. By monitoring current and voltage, the reaction and melting of the sample were observed, and the total energy of the pulse was easily computed. A sharp threshold in pulse energy for sample transformation was observed. A simple heat flow calculation demonstrated that the chemical energy released by the reaction and the change in diffusion kinetics as the sample temperature exceeded the glass transition temperature of the amorphous alloy are responsible for this sudden onset. The maximum temperature estimated from this calculation is below the melting point of the constituents, and the cooling rate is 107 - 108 K s-1 which is in agreement with the formation of amorphous alloys.