On the Diffusion Behaviour in Stressed Ni-Zr Couples
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ON THE DIFFUSION BEHAVIOUR IN STRESSED NI-ZR COUPLES
G. MAZZONE, A. MONTONE and M. VITTORI ANTISARI ENEA, Divisione Scienza dei Materiali, CRE Casaccia, C.P. 2400, 00100 ROMA A.D. ITALY
ABSTRACT Solid state reactions at the interface of bulk Ni-Zr couples have been induced at several temperatures by compressive plastic deformation. The reaction product is an amorphous phase whose thickness increases with applied load and sample temperature. In addition, similar samples have been thermally reacted in order to measure the thermal interdiffusion coefficent in the same conditions. Measurements on stressed couples show that the interdiffusion coefficent (several orders of magnitude larger than the corresponding thermal value) follows a dual regime Arrhenius behaviour. The activation energy is independent of load and of the order of 0.2 eV in the low temperature regime extending up to 550 K. A different behaviour characterized by a single value of the activation energy in the whole temperature range has been observed in thermally treated samples. INTRODUCTION In previous papers (1)(2) we have proposed a novel approach to the study of the kinetics of solid state reactions (SSR) induced by plastic deformation in elemental diffusion couples. The experimental method is based on the compressive plastic deformation of bulk trilayers prepared by placing an elemental foil between two sheets made of the second element of the couple. The main advantage of this method over mechanical alloying or cold rolling is that the temperature of the interface during deformation can be measured and varied independently of applied load. In fact, good thermal contact between sample and pistons of the fatigue machine used to deform the trilayer keeps the temperature increase due to mechanical deformation as low as a few tens of K. The temperature can then be varied by heating sample and pistons with a suitable RF coil. The experimental work was performed on Ni-Zr couples which show good adhesion when deformed with a high enough load. For temperatures lower than about 700 K the reaction product was an amorphous layer, whose thickness was used to derive an approximate interdiffusion coefficent under the hypothesis that diffusion in the already grown glass is the rate limiting step of the reaction. As a result of these experiments, it was possible to measure the interdiffusion coefficent under different processin& conditions, The width of the glassy layer was consistent with values of the diffusivities several order of magnitude larger than those observed in thermal experiments and increased with both the value of applied load and the rate at which the load was applied. The temperature dependence of the interdiffusion coefficent was described by a dual regime Arrhenius behaviour with a change in the apparent activation energy at about 570 K. The purpose of this paper is to present further experimental data which may contribute to the understanding of this behaviour. EXPERIMENTAL Square samples of 25 mm were prepared by placing a 0.5 mm thick 99.9
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