Method of modifying the transition temperature range of TiNi base shape memory alloys

In the process of forming an article with a shape change memory from a nickel-titanium based shape change memory alloy by annealing the object at a temperature above the transition temperature range (TTR) while the object is restrained in its permanent shape and then reshaping the object into another shape at a temperature below the transition temperature range, the improvement comprising:

• determining the annealing temperature which produces the desired transition temperature range for the article by performing the following steps in order:
• (1) forming the alloy into the desired permanent shape of the article;
• (2) restraining the article in this permanent shape;
• (3) annealing the article at a temperature above the TTR;
• (4) cooling the alloy down to a temperature below the TTR;
• (5) removing the restraint from the article;
• (6) forming the article into an intermediate shape taking care not to cause more than 7 percent deformation in the material;
• 7) determining the TTR by slowly heating up the article and observing the temperature range over which it recovers its permanent shape;
• and
• (8) deciding the next steps as follows:
• (a) if the TTR is lower than that desired, steps (2) through (8) are repeated using a higher annealing temperature in step (3);
• (b) if the TTR is higher than that desired, steps (1) through (8) are repeated using fresh alloy and a lower annealing temperature in step (3);
• but
• (c) if the TTR is that desired, the annealing temperature last used in step (3) is used in the process.