| The microstructure and mechanical properties of 55 Nitinol and those with 1.0, 2.5, and 5.5 at. % cobalt substituted for nickel respectively have been investigated. First, alloys are melted by non-consumable electrode arc melting furnace, under helium. After several remelting, these alloys are cast in a high frequency induction vacuum furnace. Any other secondary phases by substituting Co for Ni are not observed. The values of hardness, yield strength, tensile strength, and compressive strength of alloys are increased by Co addition the maximum values occurring at 2.5% Co, and more Co substitution shows rapid decrease. The martensitic transformation temperatures are reported to vary within wide limits by changing the composition of alloy. Cobalt is an effective substitution element for lowering the transformation temperature. The relations between the transformation temperature and Co substitution were obtained as follows, M_S
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