Microstructure Evolution of a Fully Martensitic TM6Nb Shape Memory Alloy during Thermomechanical Treatment

摘要

Ti-Nb based alloys have acquired considerable attention in the last decade as biomedical shape memory alloys (SMAs) due to an excellent combination of properties, such as nontoxic elements, low elastic modulus, superior biocompatibility and pronounced workability. These alloys undergo the reversible martensitic transformation from a cubic β phase to an orthorhombic α" phase, which is essential for the shape memory effect (SME) and superelasticity (SE). To minimize the change in shape during martensitic transformation, the six lattice correspondence variants (CVs) of a" martensite that have the crystallographically equivalent orientation relationships with the β parent are generally organized themselves in a self-accommodating manner. Such self-accommodated martensite variants are alternatively in the {1 1 1} type I or the (2 1 1) type II twin relations, which has been well established by experimental observations and theoretical calculations. Thermomechanical process is proved to be an effective way that improves both the mechanical properties and the shape memory behavior of Ti-Nb based alloys. Generally speaking, the accommodated structure of martensite varies significantly during the thermomechanical processing. For example, the morphology of martensite varies from the accommodated structure to the variant-crossed and the variant-hatched structure in an equiatomic Ti-Ni alloy with 20-40% cold rolling [1]. Up to now, most of the thermomechanical routes have been performed at the β phase, while little attempt has been made on the martensitic Ti-Nb based alloys [2, 3]. Moreover, study on the microstructure of martensite and the structure of intervariant interface in the Ti-Nb based alloys subjected to thermomechanical process is insufficient, which play a crucial role in the shape memory behavior in return.