Load-biased martensitic transformation strain of Ti50-Ni47-Co3 strip obtained by a twin-roll casting technique

Mechanical properties of the Ti50-Ni47-Co3 (at%) strips produced by twin-roll casting are analyzed, focusing on the maximum recoverable strain obtained by inducing the martensitic transformation under constant tensile load. The recoverable strain achieves a value close to 5.5% for a load of ~120 MPa. The microstructure shows a typical cellular solidification structure, where the austenite grains tend to a columnar array morphology with strong {100}B2 fiber texture aligned with ND (perpendicular to the strip surface). On the basis of the measured texture, the maximum B2→B19' transformation strain was estimated at 5.6%, using a Sachs-type upper bound model, which averages the most favorable martensite variants in each grain, disregarding interactions between them. The agreement between the measured maximum recoverable strain and the model's outcome is ascribed to special properties of Ti-Ni alloys, which allow them to overcome incompatibilities in shape changes of transforming grains in this particular polycrystalline environment.