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

L. ISOLA, P. LA ROCA, A. ROATTA, PH. VERMAUT, P. OCHIN, J. MALARRÍA

Paris

Simposio; Colloque Plasticité 2013; 2013

Société Française de Métallurgie et de Matériaux

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. The NiTi alloys of near-equiatomic composition are recognized as the most attractive shape memory alloy 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. The aim of this work is to analyze the mechanical proprieties of the Ti-47Ni-3Co (at.%) strips, in particular the maximum recoverable shape memory strain. The microstructure was characterized by optical and electronic 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.