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

Abstract

Mechanical properties of the Ti50?Ni47?Co3 (at%) strips produced by twin-roll casting are analyzed, focusing on the maxim umrecoverable 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 120MPa. 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%, usinga 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.