| The Ni-rich β-NiAl alloys are considered as potential materials for high temperature shape memory alloys because of their higher thermoelastic martensite transformation temperature. But the transformation to Ni_5Al_3 phase occurring at 450∼550℃ during heating of NiAl martensite interrupts the reverse martensitic transformation, and causes the shape memory effect in NiAl martensite to disappear. In the present study, the phase transformation process in binary Ni-(33∼37 at.%)Al martensite was investigated by DTA, and it was found that the blocker of reversible martensitic transformation was not the β→Ni_5Al_3 transformation but the M→Ni_5Al_3 transformation occurring at 250∼300℃. Therefore, the transformation temperature of M→Ni_5Al_3 determined the critical temperature for operation of shape memory effects. In addition, for verifying the critical temperature, the phase transformation process was investigated for various ternary Ni-33Al-X alloys (X = Cu, Co, Fe, Mn, Cr, Ti, Si, Nb). As a result, Ti, Si and Nb were very effective to lower the A_s, temperature of Ni-33Al-X alloys, facilitating the shape memory effect in Ni-33Al-X alloys. Especially, Si and Nb additions were beneficial for increment of the transformation temperature of M→Ni_5Al_3, resulting in higher operating temperatures of NiAl-based shape memory alloys. |
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