Nanocomposites and nanomaterials

Influence of modulation period of Ni/Al nanolayered foils on phase transformations initiated by their heating
Demchenkov S.O., Kulinich M.V., Ustinov A.I.
Inorganic materials vapour phase technology Department, E.O. Paton Electric Welding Institute of the NAS of Ukraine. 11 Kazymyr Malevych St., Kyiv-03150 Ukraine. 
E-mail: s_ demchenkov@ukr.net

Thermal and plastic effects, that accompany phase transformations, initiated by heating of Ni/Al nanolayered foil, make these materials promising for application as interlayers in the technologies of permanent joining of materials. According to theoretical models, both the intensity of phase transformations and thermal effect should rise monotonically at reduction of modulation period (MP) in the foil structure. However, in the case of nanolayered foils, prepared by the method of electron beam deposition, this regularity is observed only at MP reduction to a value of about 200 nm. Further reduction of the multilayer foil MP results in lowering of the intensity of heat evolution at initiation of phase transformations both by continuous and pulsed heating. This work is a study of the influence of MP value on the phase transformations and effects, caused by formation of intermetallic compounds in different modes of foil heating.
It is shown that the nonmonotonic dependence of the intensity of heat evolution on the multilayer foil MP is due to formation of a metastable Al9Ni2 phase on the interfaces of nickel and aluminium layers during interlayer production and ageing. Its volume fraction increases with the MP reduction. Formation of interlayers of metastable Al9Ni2 phase enables realization of a two-channel schematic of phase transformations at continuous (slow) heating of the foil. The formed interlayers of the metastable Al9Ni2 phase act as a barrier for further diffusion of nickel into the aluminium layers, leading to a significant reduction of the velocity of propagation of self-propagating high-temperature synthesis (SHS). The established regularities of the MP effects on the sequence and thermal characteristics of phase transformation in nanolayered Ni/Al foils were the basis to propose a procedure for determination of their microstructural parameters, which would provide effective application of such materials as interlayers for local heating of the joint zone at brazing, or for activation of diffusion processes in the joint zone at diffusion welding of materials.