Simulation of the thermally induced austenitic phase transition in NiTi nanoparticles

The reverse martensitic ("austenitic") transformation upon heating of equiatomic nickel-titanium nanoparticles with diameters between 4 and 17 nm is analyzed by means of molecular-dynamics simulations with a semi-empirical model potential. After constructing an appropriate order parameter to distinguish locally between the monoclinic B19' at low and the cubic B2 structure at high temperatures, the process of the phase transition is visualized. This shows a heterogeneous nucleation of austenite at the surface of the particles, which propagates to the interior by plane sliding, explaining a difference in austenite start and end temperatures. Their absolute values and dependence on particle diameter are obtained and related to calculations of the surface induced size dependence of the difference in free energy between austenite and martensite.Comment: 6 pages, 4 figures, accepted for publication in "The European Physical Journal B

EFFECT OF ANODIC OXIDATION ON THE CORROSION BEHAVIOR OF NICKEL-TITANIUM SHAPE MEMORY ALLOYS IN SIMULATED BODY FLUIDS (SBF)

The effect of anodic oxidation of a NiTi shape memory alloy in sulfuric acid electrolyte on its surface characteristics was studied. Surface roughness was measured by roughness tester. Surface morphology was studied using optical microscopy (OM) and scanning electron microscopy (SEM). Corrosion behavior was specified by recording Potentiodynamic polarization curves and measuring the content of Ni ions, released into a SBF solution using atomic absorption spectroscopy (AAS). Fourier transformation infrared radiation (FT-IR) and energy dispersive spectroscopy were employed to verify the biocompatibility of the anodized and bare alloys after submersion in SBF. It was shown that anodic oxidation in sulfuric acid significantly increases corrosion resistance and biocompatibility. This layer improves corrosion resistance and Ni ion-release resistance by impeding the direct contact of the alloy with the corrosion mediums i.e. Ringer and SBF solutions. The TiO2 oxide layer also decreases the releasing of Ni ions in to SBF solutio

On the relation between Ni4Ti3-precipitation in Ni-rich NiTi shape memory alloys (during aging and creep) and martensitic transformations

The influence of aging and creep on the martensitic transformation of a Ti-50.7 at.%Ni shape memory alloy was investigated. The particle precipitation which occurs during aging of the solution annealed and water quenched material at 500°C is strongly affected by the presence of small external stresses (pre-loads). Stress free aging results in local precipitation near grain boundaries while aging under a small pre-load (2 MPa) results in a homogeneous precipitation of particles. The corresponding microstructures are associated with specific types of DSC charts. As compared to the solution annealed material, the material after 11 hours of creep (530°C, 120 MPa) and after stress free aging for 11 hours at 530°C exhibit an 18°C increase in phase transition temperatures. The accumulated creep strain influences the shape, spatial distribution and volume fraction of Ni$_4$Ti$_3$ precipitates; it does not strongly affect the transformation behavior. This experimental result is discussed in terms of the counteracting effects of Ni-depletion due to the precipitation of Ni$_4$Ti$_3$ particles and the formation of a dislocation substructure. Dynamic precipitation moreover has an effect on the shape of creep curves, which exhibit sharp creep rate minima at small creep strains

Superior multifunctional polypyrrole anticorrosion coating modified by polydopamine decorated barium titanate nanoparticles on NiTi shape memory alloys

The enhancement of corrosion protection capabilities through the use of conducting polymer coatings has become a necessity in order to safeguard metallic materials for biomedical purposes. In the present study, the hydroxylated barium titanate nanoparticles (BT-OH) were functionalized with polydopamine (PDA) to promote the dispersion of nanoparticles in a polypyrrole (PPy) matrix. The TEM analysis confirmed the formation of the PDA layer on the BT nanoparticles (BT NPs). Then, PPy coating was successfully electropolymerized on NiTi alloy in the presence of various concentrations (1, 3, 5, and 10 wt %) of PDA functionalized BT (PBT). The morphology, structure, and topography of PPy and composite coatings were investigated utilizing the field emission scanning electron microscope (FESEM), Fourier transforms infrared spectroscopy (FTIR), and atomic force microscope (AFM). Surface morphology analysis exhibited that the augmentation of the PBT nanoparticles changed it from cauliflower-like for PPy to nodular-like for composite coatings. Nano-indentation evaluations of the coatings have demonstrated that the hardness and elastic modulus of composite-coated NiTi was raised favorably by PBT enhancement, reaching 0.86 ∓ 0.2 (GPa) and 16.06 ∓ 3.23 (GPa) in PPy with 5 wt % PBT (PPy/5PBT), respectively. Electrochemical examinations demonstrated that PPy/5PBT coating revealed superb corrosion protection in Ringer solution, serving as a physical barrier and preventing the pitting corrosion of NiTi alloy. Finally, the constructive effect of PBT inclusion in the PPy coating on the corrosion performance of NiTi substrate was authenticated by lower Ni ion release from composite-coated NiTi compared with PPy coating

On the role of chemical and microstructural heterogeneities in multistage martensitic transformations

The present paper considers chemical and microstructural reasons for unusual multiple step transformations as observed using differential scanning calorimetry (DSC) for Ni-rich NiTi-alloys on cooling from the B2 regime. In a simplified two-particles/matrix system a multiple step transformation can be explained based on two elements: (1) The composition inhomogeneity which evolves during aging as Ni$_4$Ti$_3$, precipitates grow. (2) The difference between nucleation barriers for R-phase (small) and B19$'$ (large). These two elements explain the features of the evolution of DSC charts during aging, which change from two transformation peaks on cooling after short aging times, to three after intermediate aging times and finally again back to two peaks after long aging times (2-3-2 transformation behavior). Another reason for multiple step transformations in Ni-rich NiTi alloys may be that Ni$_4$Ti$_3$, particles precipitate heterogeneously near grain boundaries. Then the first two transformation peaks can be attributed to the formation of R-phase and B19$'$ in the grain boundary regions while the third peak corresponds to a transformation of B2 to B19$'$ in the precipitate free interior of the grains

A neutron diffraction study of the martensitic transformations in aged Ni-rich NiTi alloys

The transformation of a Ni-rich NiTi alloy of nominal composition 50.7 atomic percent nickel which was solution annealed, water quenched and aged at 500 °C for 20 hours was investigated. The material shows a two-step transition on cooling with differential scanning calorimetry peaks at 180 °C and 7 °C. On heating, a single peak occurs at 43 °C. The aging leads to the formation of Ni$_4$Ti$_3$ precipitates in the microstructure, and in the presence of the precipitates the martensitic transformation occurs from the cubic B2 phase via the R-phase to B19$'$ martensite. However, the first peak on cooling is not due to the formation of R-phase alone. Below the peak temperature, R-phase is present along with some B19$'$ and with residual B2 and the precipitates. The second peak is related to the transformation of R-phase and B2 to B19$'$. This result confirms our earlier conclusions which were obtained for the same alloy aged at 400 °C for 20 hours