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@PHDTHESIS{Cavaleiro:220063,
      author       = {Cavaleiro, Andre},
      title        = {{N}i/{T}i {R}eactive {M}ultilayers for {J}oining},
      school       = {University of Coimbra},
      type         = {Dr.},
      reportid     = {PUBDB-2015-02094},
      pages        = {218},
      year         = {2015},
      note         = {University of Coimbra, Diss., 2015},
      abstract     = {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.},
      keywords     = {Dissertation (GND)},
      cin          = {DOOR},
      cid          = {I:(DE-H253)HAS-User-20120731},
      pnm          = {6G3 - PETRA III (POF3-622) / FS-Proposal: I-20120017 EC
                      (I-20120017-EC) / CALIPSO - Coordinated Access to
                      Lightsources to Promote Standards and Optimization (312284)},
      pid          = {G:(DE-HGF)POF3-6G3 / G:(DE-H253)I-20120017-EC /
                      G:(EU-Grant)312284},
      experiment   = {EXP:(DE-H253)P-P07-20150101},
      typ          = {PUB:(DE-HGF)11},
      url          = {https://bib-pubdb1.desy.de/record/220063},
}