2008 Volume 49 Issue 9 Pages 1998-2005
We propose a high-temperature actuator composite material composed of a high temperature shape memory alloy (HTSMA) TiAu with a high martensitic transformation temperature (Ms=880 K) and ferromagnetic cobalt with a high Curie temperature (TC=1388 K). This actuator material can be driven by magnetic field in a bending mode due to ferromagnetic force acting on the Co-layer and generates a large actuation strain which originates from the HTSMA. The purposes of this work are (1) to fabricate the composite materials laminated as TiAu/Co/TiAu by a diffusion bonding method through hot pressing, (2) to characterize the microstructure near the bonding interface between TiAu and Co layers, (3) to evaluate growth behavior of the diffusion layer, and (4) to determine the optimum condition for the fabrication. The composite materials were fabricated by hot pressing at 1073, 1173 and 1273 K for 10 h. The bonding interface between TiAu and Co was observed by a scanning electron microscope and concentration profiles were measured by an energy-dispersive X-ray spectroscopy. In order to evaluate the growth behavior of the diffusion layer, the TiAu/Co composites were aged at 773, 1073, 1173 and 1273 K for 24 h. It was found that, after the hot pressing, TiAu and Co layers were successfully bonded, and that two reactant intermetallic compounds were formed near the TiAu/Co interface. The intermetallic compounds were identified to be C11b Ti(Au,Co)2 and C36 (Ti,Au)Co2. As for the growth behavior, the thickness of the diffusion layer was not changed by aging at 773 K. However, the thickness was increased by increasing the aging temperature above 1073 K. The apparent activation energy for the growth of the diffusion layer was estimated to be 280±20 kJ/mol in a temperature range of 1073–1273 K. Using the values of the activation energy and the diffusion constant, the thickness of the diffusion layer was predicted to be sufficiently thin: 12 μm by the hot pressing at 1073 K for 10 h. This predicted value was in good agreement with the experimental result of 7 μm.
