Follow-up structural evolution of Ni/Ti reactive nano and microlayers during diffusion bonding of NiTi to Ti6Al4V in a synchrotron beamline

Abstract

Reaction-Assisted Diffusion Bonding (RADB) of NiTi to Ti6Al4V using either magnetron sputtered Ni/Ti nanomultilayers or Ni/Ti commercial microfoils as filler material was studied. The joining process takes advantage of the exothermal reactive character of the Ni-Ti system to provide extra energy during the bonding process. Therefore, sound joints could be achieved at lower thermal conditions. The oven with load capabilities at the High Energy Materials Science beamline (P07) of the Deutsch Synchrotron (DESY) is ideal to follow the structural evolution of the materials involved in the bonding process. Prior to RABD, Ni/Ti multilayers with a 2.5 μm total thickness and with 12 or 25 nm of modulation period were deposited onto the materials being joined. In alternative, up to 20 alternated thin μ-foils were placed in between the base materials. The materials were heated by induction to the selected temperature during 30 min and quenched to room temperature by blowing helium. During the thermal cycle a 10 MPa pressure was applied. Using thin μ-foils, 650 °C was required to promote joining, while using multilayer coated materials sound joints were obtained at 600 °C. Such low temperatures are attractive from the application/economic point of view, and are crucial to reduce the formation of undesired intermetallic phases, such as NiTi2. The nanoindentation experiments of the joints processed using Ni/Ti nanomultilayers confirm that the presence of the NiTi2 phase is more pronounced at 650 °C than when the joints are processed at 600 °C.