Role of B19′ martensite deformation in stabilizing two-way shape memory behavior in NiTi

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.

Mechanical and functional behavior of a Ni-rich Ni50.3Ti29.7Hf20 high temperature shape memory alloy

“…Loading below A s or above A f , where the deformation was not fully reversible (e.g., 140 or 260 C test temperatures), is expected to result in internal stress fields that can be caused by a number of sources including dislocation arrays. However, the magnitude of the TWSME was less than 0.2% when initiated through isothermal deformation, which is an order of magnitude less than that observed in binary NiTi under comparable conditions [24].…”

“…Since this Ni-rich alloy is a twophase material containing a fine dispersion of second phase precipitates, an increasing externally applied stress is essential in continuing the reorientation and detwinning process. Thus, a distinct plateau is not observed as in some binary, equiatomic NiTi alloys [24]. In addition, this isothermal deformation did not generate any TWSME, likely due to the absence of permanent plastic deformation.…”

Mechanical Performance of Nickel-titanium Archwires

“…The stress can be created by mechanisms such as dislocation arrays resulting from the thermomechanical treatment. The results should be retention of the martensite, some point defects, aligned coherent precipitates or a stabilized stress induced martensite 8,9 . Fine and coherent Ni 4 Ti 3 precipitates formed during aging of Ni-rich NiTi alloys affect the transformation sequence, the transformation temperature and even the mechanical properties of the alloy 2 .…”

“…Different wire deflections were reported in literature up to 5 mm; some authors have used deflections between 2 mm and 4 mm and it was stated that although these values were greater than the 5% deflection limit for the span length used, they reproduce the clinical use of the material and are regularly used in the oral environment [3][4][5][6][7][8][9][10]13,14,17,18 . Mallory et al 19 reported that during 5-mm deactivation of all heat activated wires, there was a sharp decline in force production.…”

“…The R s and R f temperatures for transformation involving the R-phase are defined in a similar manner. The first-order transformation of NiTi alloy is responsible for the shape recovery ability demonstrated by the super/pseudoelasticity and shape memory effects, which are stress-induced and temperature-induced phase transformations, respectively 8 . The stress-strain behavior of a NiTi archwire under constant temperature (above A f ) is schematically shown in Figure 2.…”