Shanshan Cao

Optimization of a FIB/SEM slice-and-view study of the 3D distribution of Ni4Ti3 precipitates in Ni–Ti

The 3D morphology and distribution of lenticular Ni4Ti3 precipitates in the austenitic B2 matrix of a binary Ni51Ti49 alloy has been investigated by a slice‐and‐view procedure in a dual‐beam focused ion beam/scanning electron microscope system. Due to the weak contrast of the precipitates, proper imaging conditions need to be selected first to allow for semi‐automated image treatment. Knowledgeable imaging is further needed to ensure that all variants of the precipitates are observed with equal probability, regardless of sample orientation. Finally, a volume ratio of 10.2% for the Ni4Ti3 precipitates could be calculated, summed over all variants, which yields a net composition of Ni50.27Ti49.73 for the matrix, leading to an increase of 125 degrees for the martensitic start temperature. Also, the expected relative orientation of the different variants of the precipitates could be confirmed.

3D Reconstruction of Ni4Ti3 Precipitates in a Ni51Ti49 Alloy in a FIB/SEM Dual-Beam System

Ni4Ti3 precipitates play an important role in the shape memory and superelastic behaviour of thermo-mechanically treated Ni-Ti material. The 3D morphology and distribution of such precipitates with lenticular shape and rhombohedral atomic structure in the austenitic B2 matrix of a binary Ni-rich Ni-Ti alloy has been elucidated via a slice & view procedure in a Dual-Beam FIB/SEM system. With the sequence of cross-section SE images obtained from the SEM, a 3D reconstruction has been achieved after proper alignment and image processing, from which both qualitative and quantitative analysis can be performed. Careful imaging is needed to ensure that all variants of the precipitates are observed with equal probability, regardless sample orientation. Moreover, due to the weak contrast of the precipitates, proper imaging conditions need to be selected to allow for semi-automated image treatment. Finally, a volume ratio of 10.2% for the Ni4Ti3 precipitates could be calculated, summed over all variants, which yields a net composition of Ni50.36Ti49.64 for the matrix, leading to an increase of 113 degrees for the martensitic start temperature Ms. Also, the expected relative orientation of the different variants of the precipitates could be confirmed. In the near future, other quantitative measures on the distribution of the precipitates can be expected.

Anisotropic Negative Thermal Expansion Behavior of the As-Fabricated Ti-Rich and Equiatomic Ti–Ni Alloys Induced by Preferential Grain Orientation

The present study focuses on the anisotropic negative thermal expansion (NTE) behaviors of Ti-rich (Ti54Ni46) and equiatomic Ti–Ni (Ti50Ni50) alloys fabricated by vacuum arc melting and without subsequent plastic deformation. Both alloys exhibit NTE responses in vertical and horizontal directions, and the total strains and CTEs of the NTE stage along the two mutually perpendicular measuring directions are obviously different, indicating obvious anisotropic NTE behavior of the alloys. Besides, the numerical differences between the starting temperature of NTE and austenitic transformation and between the finishing temperature of NTE and austenitic transformation are very small, which indicates that an apparent relationship exists between the NTE behavior and the phase transformation. The microstructure in the vertical cross sections shows obviously preferential orientation characteristics: Ti2Ni phases of both alloys grow along the vertical direction, and B19′ martensite of Ti50Ni50 alloy has distinct preferential orientation, which results from a large temperature gradient between the top and the bottom of the button ingots during solidification. The microstructure with preferential orientation induces the anisotropic NTE behavior of the samples.

Reversible Negative Thermal Expansion Response and Phase Transformation Behavior of a Ti-Rich Ti 54 Ni 46 Alloy Prepared by Rapid Solidification

In this study, a Ti-rich Ti–Ni alloy (Ti54Ni46) was prepared by rapid solidification technique through vacuum suction casting into a water-cooled copper mold. The microstructure, thermal expansion, and phase transformation behavior of the alloy were studied systematically. The results show that the rapidly solidified Ti54Ni46 alloy exhibits negative thermal expansion (NTE) response in both vertical and horizontal measuring directions upon heating and cooling. The discrepancy in the NTE response between the two mutually perpendicular directions of the alloy is small, indicating an implicit anisotropic NTE behavior. A one-to-one correspondence exists between the characteristic temperatures of phase transformation and NTE, as well as between their changes during thermal cycling. It is conclusive that the NTE strains generated upon heating and cooling originate from the volume changes accompanying the forward and reverse martensitic transformations in Ti54Ni46 alloy. Characteristic temperatures of both phase transformation and NTE of the alloy rapidly shift to lower temperatures due to the multiplication of dislocations during the initial approximately 20 thermal cycles, and then tend to be relatively unchanged in subsequent thermal cycling as the transformation-induced defects reach saturation. The absolute values of the coefficient of thermal expansion of the NTE stage upon heating and cooling decrease rapidly during the initial approximately 20 thermal cycles, and thereafter become relatively stable with the increase of thermal cycle number, which is mainly attributed to the decrease of the effective fraction of the B19′ martensite participating in the forward and reverse martensitic transformations.

Optimization of Automated Crystal Orientation Mapping in a TEM for Ni4Ti3 Precipitation in All-Round SMA

Automated crystal orientation and phase mapping in TEM are applied to the quantification of Ni4Ti3 precipitates in Ni–Ti shape memory alloys which will be used for the implantation of artificial sphincters operating using the all-round shape memory effect. This paper focuses on the optimization process of the technique to obtain best values for all major parameters in the acquisition of electron diffraction patterns as well as template generation. With the obtained settings, vast statistical data on nano- and microstructures essential to the operation of these shape memory devices become available.

Thermal Cycling Induced Instability of Martensitic Transformation and the Micro-Mechanism in Solution-Treated Ni 51 Ti 49 Alloy

The effect of the maximum temperature of thermal cycling (Tmax) on the instability of martensitic transformation (MT) in the solution-treated Ni51Ti49 alloy was investigated by differential scanning calorimetry (DSC). Results manifest that the peak temperature of martensitic transformation (Mp) decreases linearly with the increase of cycle number, while the transformation hysteresis (H) increases linearly. The instability of MT is promoted by increasing Tmax from 20 to 100 °C, with variation of Mp increasing from 0.9 to 12.3 °C and variation of H increasing from 0.4 to 4.3 °C after 10 thermal cycles. Transmission electron microscopy (TEM) study demonstrates the appearance of transformation-induced dislocations in the NiTi matrix, which are responsible for the instability of MT. Moreover, the dislocation multiplication is obviously enhanced with the increase of Tmax during thermal cycling, as a result of the interaction between dislocations and quenched-in point defects (QIDs) in the solution-treated Ni-rich Ni51Ti49 alloy, which consequently leads to the temperature dependence of MT instability during thermal cycling.

Porous Ni–Ti–Nb Shape Memory Alloys with Tunable Damping Performance Controlled by Martensitic Transformation

Porous Ni–Ti–Nb shape memory alloys (SMAs) with designed porosities and compositions were prepared by the powder metallurgy technique. The phase-transformation-controlled damping behavior of the porous alloys was investigated by dynamic mechanical analysis (DMA). Systematic microstructural study indicates that both the pore configuration and Nb distribution in the matrix of porous Ni–Ti–Nb alloys have significant influence on the damping performance of the alloys. Increase in both pore size and porosity in micro-scale leads to obvious decrease of the internal friction, while addition of Nb brings dramatic increase in the damping capacity of the porous alloys. The damping capacity can be optimized by adjusting the Nb/NiTi ratio, which balances the competitive contribution of Nb/matrix and B2/B19′ interfaces to the internal friction during martensitic transformation. Moreover, β-Nb phase of lamellar structure in the matrix plays a greater role in the internal friction than that of granular shape by offering a large amount of interfaces.

Functional Stability of the Ni 51 Ti 49 Two-Way Shape Memory Alloy as Artificial Anal Sphincter During Thermo-Mechanical Cycling

Ni51Ti49 alloy strip with optimal two-way shape memory effect to be potentially used in a purpose-designed artificial anal sphincter (AAS) was prepared by means of rapid solidification process followed by constraint-aging treatment. The functional stability in terms of phase transformation behavior and deformation performance during thermo-mechanical cycling (TMC) was studied. Results show that the forward and reverse R-phase transformation temperatures of the alloy remain in the required operation temperature range of 35–55 °C with small shifts during TMC. The alloy strip exhibits stable deformation performance with steady recovery ratio during TMC, and its microstructure after TMC remains featured fine and stable Ni4Ti3 precipitates together with limited number of dislocations, indicating that the external force and cycling temperature have no influence on the size of Ni4Ti3 precipitates and coherent stress field. Such excellent stability of microstructure and corresponding functionalities are attributed to the stabilized Ni4Ti3 precipitates formed through optimal constraint-aging treatment and the small lattice distortion of R-phase and reverse R-phase transformations during TMC. Nevertheless, the Ni51Ti49 alloy strip has the maximum displacement of at least 10 mm within 35–55 °C and an irreversible displacement of 4 mm.

3D Microstructures of Sb2Te3 precipitates in PbTe matrix with prediction by a weak compatibility condition

We propose that a weak compatibility condition predicts the elongated directions for Widmanstatten type precipitates. The distribution of the elongated directions of precipitates lies on a family of crystallographically equivalent cones in 3D determined by a certain transformation stretch matrix obtained independently. A 3D visualization and digitization method is developed to show how the cone variants control the preferred growth directions during precipitation of Sb2Te3 in a (5 µm)3 PbTe matrix. A series of two-dimensional secondary electron images are acquired along the direction perpendicular to the imaging plane. By pixelating all the images and calculating the position vectors on the surface of each precipitate, the elongation directions are calculated using a 3-dimensional ellipsoidal fitting for 182 precipitates. The 3D plot of the elongation directions shows that their spacial orientations are close to four predicted cones with a standard deviation of 5.6°. The length along the elongation directions reveals an asymmetric distribution with a mean value of about 240 nm. The total volume fraction of the precipitates is 8.3 %. The average area of the precipitates per volume is 0.68 µ-1 by one point statistical calculation. These results build on our study presented in [1] by analyzing a significantly bigger data set and by including the length distribution and 1-point statistics.

3D Reconstruction of Ni4Ti3 Precipitates in Ni-Ti by FIB/SEM Slice-and-View

Ni4Ti3 precipitates with lenticular shape and rhombohedral atomic structure growing in the austenitic B2 matrix of binary Ni-rich Ni-Ti alloys upon proper annealing treatment have an important influence on the shape memory effect which originates from the martensitic transformation of B2 — B19’ [1]. Figure 1 shows a typical 2D distribution of such precipitates as obtained by TEM under conventional imaging conditions.