Enhanced growth of intermetallic phases in the Ni-Ti system by current effects

The effect of direct current upon interfacial reactions in the Ni-Ti system was investigated. Isothermal diffusion couple experiments were conducted under varying current densities to de-couple Joule heating from intrinsic effects of the current flux. Current densities of up to 2546 A cm(-2) were used in the temperature range of 625-850 degreesC. All of the intermetallic compounds (NiTi, Ni3Ti and NiTi,) present in the equilibrium phase diagram were identified in the product layer. In addition, beta-Ti solid solutions formed in samples annealed above the alpha-->beta temperature, 765 degreesC. The growth of all product layers was found to be parabolic and the applied current was found to significantly increase the growth rate of the intermetallic layers. Using Wagner's analysis the present results were compared to published results on current-free diffusion couples. The intrinsic growth rate constant of the NiTi2 intermetallic was found to be 43 times higher under the influence of 2546 A cm(-2) than that obtained without a current at 650 degreesC. The effective activation energy for the formation of all phases was found to decrease with increasing current density. The effect was strong for all phases but the decrease was most marked for Ni3Ti. In this case, the activation energy decreased from 292 kJ mol(-1) under the influence of a current density of 1527 A cm(-2) to 86 kJ mol(-1) when the current density was 2036 A cm(-2). The results are explained in terms of current induced changes in the growth mechanism arising from changes in the concentration of point defects or their mobility. (C) 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.