Abstract
Deformation of a B19′ martensitic, polycrystalline Ni49.9Ti50.1 (at. %) shape memory alloy and its influence on the magnitude and stability of the ensuing two-way shape memory effect (TWSME) was investigated by combined ex situ mechanical experimentation and in situ neutron diffraction measurements at stress and temperature. The microstructural changes (texture, lattice strains, and phase fractions) during room-temperature deformation and subsequent thermal cycling were captured and compared to the bulk macroscopic response of the alloy. With increasing uniaxial strain, it was observed that B19′ martensite deformed by reorientation and detwinning with preferred selection of the (1¯50)M and (010)M variants, (201¯)B19′ deformation twinning, and dislocation activity. These mechanisms were indicated by changes in bulk texture from the neutron diffraction measurements. Partial reversibility of the reoriented variants and deformation twins was also captured upon load removal and thermal cycling, which after isothermal deformation to strains between 6% and 22% resulted in a strong TWSME. Consequently, TWSME functional parameters including TWSME strain, strain reduction, and transformation temperatures were characterized and it was found that prior martensite deformation to 14% strain provided the optimum condition for the TWSME, resulting in a stable two-way shape memory strain of 2.2%. Thus, isothermal deformation of martensite was found to be a quick and efficient method for creating a strong and stable TWSME in Ni49.9Ti50.1.
Highlights
Near-equiatomic NiTi shape memory alloys (SMAs) undergo a reversible martensitic phase transformation between a cubic (B2) austenite phase and a monoclinic (B190) martensite phase
The tensile stress-strain-temperature response of the Ni49.9Ti50.1 alloy is shown in Fig. 2 for 7 samples deformed at room temperature to strains between 6% and 22%, unloading to zero stress, followed by thermal cycling
Room-temperature deformation of a martensitic, polycrystalline Ni49.9Ti50.1 shape memory alloy, and its subsequent effect on the two-way shape memory effect (TWSME) was investigated by combining ex situ thermomechanical experiments and in situ neutron diffraction measurements
Summary
Near-equiatomic NiTi shape memory alloys (SMAs) undergo a reversible martensitic phase transformation between a cubic (B2) austenite phase and a monoclinic (B190) martensite phase. This first-order phase transformation gives SMAs their unique shape recovery capabilities, exemplified by two behaviors: the shape memory effect (temperature-induced phase transformation) and superelasticity (stress-induced phase transformation). When used in such applications, the SMA elements remember the original austenitic shape and are used in conjunction with a biasing force to complete the actuation cycle. This is known as biased shape memory or one-way shape memory behavior. TWSME is not an inherent behavior of SMAs but can be obtained after specific thermomechanical training procedures
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