Long fatigue life nitinol

Abstract

A high fatigue life superelastic nickel-titanium (nitinol) wire, ribbon, sheet, tubing, or the like is disclosed. The nitinol has a 54.5 to 57.0 weight percent nickel with a balance of titanium composition and has less than 30 percent cold work as a final step after a full anneal and before shape setting heat treatment. Through a rotational beam fatigue test, fatigue life improvement of 37 percent has been observed.

1 - 14 . (canceled) 
   
   
       15 . A process for improving the fatigue life of a superelastic metal wire, ribbon, sheet or tubing, comprising:
 forming an ingot having a composition of approximately 54.5 to 57.0 wt. % nickel with a balance of titanium and trace elements, with an ingot A f  of approximately −15° C.±25° C.;   cold working and heat treating the ingot to form a wire, ribbon, sheet or tubing; and   in a final cold working step, cold working the wire less than approximately 30%.   
   
   
       16 . The process of  claim 15 , wherein the trace elements in the nickel-titanium alloy ingot includes approximately:
 ≦0.300 wt. % (3000 ppm) Fe,   ≦0.050 wt. % (500 ppm) Cu,   ≦0.050 wt. % (500 ppm) O,   ≦0.035 wt. % (350 ppm) C,   ≦0.003 wt. % (30 ppm) H; and   wherein a total amount of trace elements is <0.4 wt. %.   
   
   
       17 . The process of  claim 15 , wherein after the final cold working step the process includes mounting the wire, ribbon, sheet or tubing on a fixture and shape setting the wire, ribbon, sheet or tubing at approximately 250-600° C. for 1 to 60 minutes. 
   
   
       18 . The process of  claim 15 , wherein the wire, ribbon, sheet or tubing has a fatigue life > approximately 20,000 mean cycles to failure under alternating compressive and tensile strain from −0.75% to +0.75% in a rotary beam test. 
   
   
       19 . The process of  claim 15 , wherein the process includes electropolishing the wire, ribbon, sheet or tubing. 
   
   
       20 . The process of  claim 15 , wherein the wire, ribbon, sheet or tubing includes an ultimate tensile strength ≧ approximately 150 ksi, and elongation at failure ≧ approximately 15%, as measured at a temperature of approximately 23±2° C., at a strain rate of approximately 0.001/sec. 
   
   
       21 . The process of  claim 15 , wherein the wire, ribbon, sheet or tubing has a fatigue life > approximately 38 million cycles to failure under alternating, loading and unloading forces to produce 80% to 120% stretch ratio and corresponding to strain levels of approximately 0.9% to 1.4%. 
   
   
       22 - 23 . (canceled)