A summary of recent research on the nitinol alloys and their potential application in ocean engineering

Abstract

The Nitinol alloys are based on the intermetallic compound TiNi. Alloys on the nickel-rich side of stoichiometry contain a certain portion of the compound TiNi 3 in equilibrium with TiNi and are hardenable. In other alloying variations some of the nickel is replaced with cobalt. When a near stoichiometric specimen is mechanically deformed below its martensitic transition temperature it retains this deformed shape indefinitely. Upon heating above its martensitic transition temperature, however, it rapidly reverts to the shape it had before deformation. Thus the material exhibits a “mechanical memory” effect. The martensitic transition temperature may be varied within wide limits by changing the composition of the alloy. Cobalt, in the formula TiNi x Co 1− x , is an effective substitution element for lowering the transition temperature. The resistance of some of the Nitinol alloys to corrosion in sea water has been evaluated by conducting high velocity impingement, cavitation-erosion, stress corrosion and crevice corrosion measurements. The results of these tests have shown these alloys to be quite resistant to marine corrosion. Combination of the unusual mechanical and corrosion properties exhibited by these alloys make them especially suitable for applications in ocean engineering.