Phase transformation and energy absorption of Nitinol alloys

Nitinol shape memory alloys are known to exhibit mainly three different phases: the B2 high temperature austenitic phase (A), the low temperature monoclinic martensitic phase (M), martensite and the intermediate rhombohedral phase known as the R-phase formed during the transformation of A->M and/or M->A. All transformations involving the three phases mentioned are martensitic in nature and involve lattice distortions. However, the transformations are sensitive to variations in metallurgical and mechanical conditions as well as thermal and chemical conditions. Depending upon the thermal and mechanical treatment of the alloy, various combinations of transformations are exhibited. This study compares the superelastic behavior of a mechanically alloyed Nitinol alloy with tantalum as the ternary element with that of “as cut” and electropolished Nitinol stents.