If alloys & metal expand to heat, why does the alloy Nitinol contract from an AC current/heat?
wikipedia: Nickel titanium, also known as nitinol, is a metal alloy of nickel and titanium, where the two elements are present in roughly equal amounts. Nitinol alloys exhibit two closely related and unique properties: shape memory and superelasticity (also called pseudoelasticity). Shape memory refers to the ability of Nitinol to undergo deformation at one temperature, then recover its original, undeformed shape upon heating above its “transformation temperature”. Superelasticity occurs at a narrow temperature range just above its transformation temperature; in this case, no heating is necessary to cause the undeformed shape to recover, and the material exhibits enormous elasticity, some 10-30 times that of ordinary metal. Nitinol’s extraordinary ability to accommodate large strains, coupled with its physiological and chemical compatibility with the human body have made it one of the most commonly used materials in medical device engineering and design. Rest of explanation can be foun
