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Transformation of Ti/Al multilayers to the Ti3Al phase aimed at bonding of titanium alloys DOI:10.15199/28.2016.5.4

  The freestanding Ti/Al multilayer (with 3:1 the chemical composition ratio) was applied to bond the parts from Ti-6Al-4V alloy. The as-deposited multilayer and the joint obtained with it were characterized with the use of transmission electron microscopy (TEM) method including selected area electron diffraction (SAED) and energy dispersive X-ray spectroscopy (EDS). It was observed that the annealing at 800°C for 1 h in vacuum of 1 Pa helped to transform Ti/Al multilayer into fine grains (grain size < 1 μm) of Ti3Al acting as a filler material between pieces of Ti-6Al-4V. The resulting joint shows generally good connection with the presence of small porosity both at the filler material/Ti-6Al-4V base material interface (larger) and on the Ti-6Al-4V side (smaller). Additionally, near surface zone of the joint, the parts were strongly depleted of aluminium. Key words: diffusion bonding, Ti/Al multilayers, Ti-6Al-4V alloys, TEM, EDS.1. INTRODUCTION Heating of Ti/Al multilayers could result in the formation of various titanium aluminides, such as Al3Ti, Ti3Al or TiAl. Due to negative enthalpy of mixing their formation is accompanied by a strong exothermic effect [1, 2]. The type of a final product depends on the bilayer thickness ratio, i.e. averaged multilayer chemical composition. However, slow heating of the whole multilayer causes that the reaction is controlled through a solid state diffusion (SSD) across neighboring interfaces being an example of one of phase transformations in solid state [3]. The level and extent of the local intermixing, both increasing with lowering of the multilayer period, decides on the phase precipitation sequence taking place during this process [4]. Fast local temperature rise leads to a reaction known as self-propagating hightemperature synthesis (SHS) occurring along the multilayers in the form of roughly planar front. In Ti/Al multilayers it is accompanied by melting of aluminium layers and conseque[...]

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