TiNi Base alloy shape memory enhancement through thermal and mechanical processing

Claim 1

A process for improving the shape change memory properties of a titanium-nickel base alloy comprising the following steps in order: (1) forming a favorable atomic-order in the alloy by (a) heating the alloy at a temperature in the range of from just above 700° C. to 800° C. until the TiNi phase has stabilized into disordered body centered cubic (A2) crystal structure; (b) slowly cooling the alloy to a temperature in the range of from 600° C. to 700° C.; and (c) annealing the alloy in the temperature range of from 600° C. to 700° C. until the TiNi phase has been substantially converted from disordered body centered cubic (A2) crystal structure to CsCl(B2)-type crystal structure; and then (d) slowly cooling the alloy to a temperature in the range of from 450° C. to 550° C.; (2) refining the micro-twinning of the alloy crystal structure by (a) cold working the alloy in the temperature range of from room temperature to less than 600° C.; and then (b) annealing the alloy in the temperature range of from 500° C. to less than 600° C.; and (3) orienting the micro-twins by (a) cooling the alloy to a temperature below the shape transition temperature range; (b) placing a sufficient load on the alloy to cause from a 5 to less than 8 percent strain in the alloy; (c) heating the alloy while still under load to a temperature above the shape transition temperature range; (d) cooling the alloy while still under load to a temperature below the shape transition temperature range; (e) increasing the load back up to the load applied in substep (3)(b); (f) heating the alloy while still under load to a temperature above the shape transition temperature range; and (g) repeating substeps (3)(d), 3(e), and (3)(f) until the desired degree of micro-twin orientation has been achieved.