A Comparison between NiTi and NiTiCr Alloys

The role of alloy additions in nitinol on mechanical properties and fatigue resistance has been reported in the open literature.  This paper is primarily concerned with the addition of Chromium (~0.25wt%).  Alloy additions are added to nitinol to obtain the desired mechanical and transformation properties.  Chromium decreases the transformation temperature, while increasing the strength, with decreasing ductility.  The fatigue and mechanical properties of the alloys will be compared to standard binary nitinol, to give some understanding of the effect of alloying.  The binary and alloyed wires tested had an A_f of:  -35^oC, -15^oC and 0^oC.  Each wire tested, had a diameter of 1mm and had been cold worked 45%.

 

A commercially available wire fatigue tester was utilized throughout this study.  This machine offers a simple, rapid, and highly accurate method of obtaining the fatigue properties, while performing the tests under controlled temperatures.  Metallographic analysis was used to evaluate any microstructure changes difference that might be a benefit or hindrance to mechanical or fatigue properties.  A MTS tensile machine was used to obtain the mechanical properties, including plateau stresses and elastic modulus.