Load-biased martensitic transformation strain of Ni-Ti-Co strip obtained by Twin-Roll Casting technique

ISOLA, LUCIO M.; LA ROCA, PAULO; ROATTA, ANALIA; VERMAUT, PHILIPPE; OCHIN, PATRICK; MALARRÍA, JORGE A.

Paris

Encuentro; Colloque Plasticité 2013; 2013

The twin-roll casting (TRC) is a powerful technique, which allows us to obtain semi- finished strips in one-step procedure. Moreover, a favorable microstructure with a columnar grain morphology and small grain size can be achieved [1]. The NiTi alloys of near-equiatomic composition are recognized as the most attractive shape memory alloy (SMA) for a wide variety of applications. A small amount of Co atoms are introduced in order to promote the separation of the B2 → R and R → B19′ transformations, due to a shift of Ms(B19′) downwards lower temperatures range, which is more appropriate for biomedical applications. The aim of this work is to analyze the mechanical properties of the Ti−47Ni−3Co (at.%) strips, in particular the maximum recoverable shape memory strain. The microstructure was characterized by optical microscopy, while the texture was measured using X-ray diffraction. The experiments show the strong texture of the columnar grain structure, were the austenite grains are oriented with the {001}B2 ND, and their average diameter size is of around 3 µm. 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. This value is in good agreement with the maximum recoverable strain measured in thermal cycles under constant stress experiments.