TY  - THES
AU  - Cavaleiro, Andre
TI  - Ni/Ti Reactive Multilayers for Joining
PB  - University of Coimbra
VL  - Dr.
M1  - PUBDB-2015-02094
SP  - 218
PY  - 2015
N1  - University of Coimbra, Diss., 2015
AB  - Reactive multilayers are interesting for a variety of applications, including near-net shape form- ing of parts/systems of intermetallic compounds, dissimilar/similar joining, ignition origin and as highly localised heat sources. In the last decade, the production of intermetallic compounds, from multilayer thin films, became a widespread process. Due to its moderate enthalpy of reaction, the Ni-Ti system has not yet received special attention in the context of low temperature joining, in spite of its reaction product, NiTi, having promising mechanical properties, like superleasticity and shape memory effect. The aim of this PhD thesis was to study the reaction between Ni/Ti multilayers, by thermal treatment/pressure or activated using femtosecond laser pulses and the possibility to use them as joining mechanism.Ni/Ti reactive nanomultilayers with different modulation period (Λ =5 nm, 12 nm, 25 nm and 75 nm) and a total thickness of around 2.5 μm were deposited, using a magnetron sputtering equipment (double cathode). These multilayer thin films exhibit a columnar growth, independently of the substrates, and a roughness value that is directly proportional to the modulation period. Structural evolution was followed during thermal treatment in situ using synchrotron x-ray diffraction. The multilayers evolve from Ni/Ti to B2-NiTi, in one step, without the formation of intermediate phases and independently of selected material and modulation period. The reaction temperature of the phase evolution varies with the period and the heating rate. When the multilayer thin films are deposited onto Ti6Al4V substrates, the nickel from the NiTi (final phase), at temperatures higher than ≈450 ◦C, diffuses to the substrate.Sound joints between NiTi and Ti6Al4V were achieved utilising Ni/Ti multilayers with 12 and 25nm periods at temperature as low as 600◦C, during 30min, under a pressure of 10MPa. However, the loss of nickel for the material poor in Ni was always observed. This element promotes the occurrence, in the selected alloy (Ti6Al4V), of the formation of titanium in the beta phase. In situ synchrotron x-ray diffraction in transmission mode, during the joining process, revealed that the formation of the NiTi2 is reduced in the joints produced at lower temperatures. Nevertheless, the bonds presented good interface quality, without pores or other significant defects.Ignition experiments were conducted on several multilayers using femtosecond laser pulses with high energy density. However, the substrates’ thermal properties play a very important role: while metallic substrates quench the reaction confining it to the volume interacted by laser, for ceramic substrates like zirconia, the reaction is more significant in dimension. In truth, for conventional film thickness (2-5 μm) it was demonstrated that even when multilayers are nanocrystaline, the reaction continues to be difficult to start. Thicker films (60 μm) are incompatible with the adhesion of the bonding material. Using femtosecond laser pulses it was possible to produce a nanopattern with lower periodicity than the laser wavelength (LIPSS), while maintaining the multilayer structure.
KW  - Dissertation (GND)
LB  - PUB:(DE-HGF)11
UR  - https://bib-pubdb1.desy.de/record/220063
ER  -