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Tuesday, December 4, 2007 - 9:40 AM
16.3

High Temperature Shape Memory Effect Due to FCC/HCP Martensitic Transformation in the Co-Al Binary and Co-Al-X Ternary Systems

K. Ando, T. Omori, Y. Sutou, K. Oikawa, R. Kainuma, K. Ishida, Tohoku University, Sendai, Japan

Shape memory alloys operating at temperatures higher than 100°C have been required in various fields such as automotive industry.  Recently, the present authors have reported that Co-Al alloys containing Al composition over 10at% exhibit a good shape memory effect associated with the hcp→fcc reverse martensitic transformation at about 200°C.  In this study, the martensitic transformation and the shape memory effect were investigated in Co90Al10 and (Co90Al10)100-yXy (X= Si, Ti, V, Cr, Mn, Fe, Ni, Nb, Mo, Ru, Ta, W, Re and Ir) alloys.  The addition of Ti, V, Mn, Fe, Ni, Nb, Mo, Ta or W decreases the volume fraction of the ε martensite phase (Vm), resulting in the improvement of the ductility at room temperature due to the stabilization of the γ phase.  On the other hand, while increasing the transformation temperature, Si, Cr, Ru, Re or Ir known as hcp stabilizer slightly decrease the Vm.  Although the transformation temperatures and the ductility can be controlled by the alloying elements, the shape recovery decreases by the decrease of Vm and the ternary alloys show shape recovery lower than the Co-Al binary alloys.  The shape recovery temperatures of (Co90Al10)100-yXy alloys (X=Si, Cr, Ru, Re or Ir) can be increased up to 697°C, and a good thermal stability is obtained at temperatures below 400°C.

Summary: Effects of alloying elements on γ (fcc) / ε (hcp) martensitic transformation and shape memory (SM) properties of Co90Al10 alloy were investigated. The addition of Ti, V, Mn, Fe, Ni, Nb, Mo, Ta or W decreased the volume fraction of the ε martensite phase (Vm), resulting in improvement of the ductility due to the stabilization of the γ phase and decrement of the SM effect. On the other hand, Si, Cr, Ru, Re and Ir known as hcp stabilizer increased the transformation temperatures. The shape recovery temperatures of (Co90Al10)100-yXy alloys (X=Si, Cr, Ru, Re or Ir) can be increased up to 970K, and a good thermal stability was obtained at temperatures below 673K.