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Compared with the conventional laser cutting and braided method, this research aimed to produce unibody NiTi vascular stents by metal injection molding (MIM), simplifying the manufacturing process. MIM NiTi alloys and stents with a low oxygen content of 0.17 % were obtained via strict oxygen control. Sintered Ni4Ti3 precipitates can be observed in both micron and nanometer scales. The MIM NiTi alloy has a tensile strength of 812 MPa, an elongation of 5.7 %, and full recovery at 5 % pre-strain. During 3.96 × 108 cycle-simulated in vitro fatigue tests, the MIM NiTi stents essentially maintained a constant diameter without any marked damage, although local pitting was observed. The biocompatibility of MIM NiTi was evaluated based on toxicity, migration, and adhesion of the human umbilical vein endothelial cells. The hemolysis rate was as low as 0.4 %. This study provides a new manufacturing strategy for NiTi self-expanding vascular stents. Additionally, it aimed to develop porous and gradient porous NiTi vascular stents using the characteristics of MIM process.

Elemental powder sintering is an important low-cost preparation method for NiTiNb shape memory alloys. Usually, the performance is poor due to high impurity content and macroscopic defects. In this study, high-strength NiTiNb alloys without macroscopic defects were successfully prepared using mixed elemental powders based on the previous investigation. Higher sintering temperature and Nb addition brought about faster atomic diffusion, which strongly affected the densification, grain size, phase composition and Nb solid solution, and further determined the transformation temperature and mechanical properties. The transformation temperature decreased with increasing Nb solid solubility. A high thermal hysteresis of 72–75 °C was obtained by nano-Nb precipitation. The compressive strength reached 1662–2185 MPa. The recovery properties gradually decreased with the increase of large Nb particles in the matrix, which should be optimized in further studies. Overall, the addition of Nb drastically reduced the sintering temperature of elemental NiTi powders, which was conducive to suppressing the loss of liquid phase and promoting homogenization. It provides a new idea for the preparation of high-performance, fully dense and low-cost NiTi-based alloys.

Generally, porous Ni–Ti shape-memory alloys prepared by the hydrogenation–dehydrogenation process have inferior load-bearing properties and recoverable strains. In this work, high-strength porous Ni–Ti alloys with stabilized cyclic properties were prepared by homogenizing sintering at a high temperature (1250 ℃) under high vacuum conditions (10−4 Pa) using NaCl as the space holder. High vacuum levels are essential to reduce the risk of sample oxidation during sintering. The sintering process was optimized to ensure the homogenization of the components and densification of the pore wall matrix at 1250 ℃. The alloys with different porosities were studied for their microstructures, phase transformations, mechanical properties, cycle stabilities, and cytotoxicities. Upon increasing the NaCl content from 15% to 40% (volume fraction), the porosities of the samples increased from 14% to 37%, and the average pore size increased from 60 µm to 124 µm, while the oxygen content gradually increased from 0.23% to 0.36% (mass fraction). The porous Ni–Ti alloys predominantly comprised austenite (B2) with a small amount of martensite (B19′) and Ti2Ni at room temperature (25 ℃). Furthermore, the spontaneous formation of Ni4Ti3 nanoprecipitates without heat treatment was observed. The size of the precipitates grew from 20 nm to 145 nm with increasing oxygen content. The martensitic transformation showed multiple peaks in DSC curves attributed to the inhomogeneous distribution of the precipitates. The compressive strengths of the porous Ni–Ti alloys were 1236–1600 MPa. Compared to the porous Ni–Ti alloys prepared by powder metallurgy, the porous Ni–Ti alloys prepared in this study exhibited ultrahigh strength due to matrix strengthening owing to the process optimization. The results of the compression loading–unloading test with 8% strain revealed that the samples exhibited superelasticity as well as shape-memory properties. After heating, the samples’ shape recovery rates exceeded 99%. Under 50 loading–unloading cycles at a constant stress level approaching 8% strain, the irreversible strains of the samples increased with an increasing number of cycles. As the porosity increased, the final residual strains toward the end of the cycle measurements were 1.4%, 1.55%, and 1.66%. These low values of irreversible strains indicated that the porous Ni–Ti samples had excellent cyclic stabilities, which is ascribed to the strengthening effect of Ni4Ti3 precipitation in the matrix. To test the cytotoxicity of the porous Ni–Ti alloys, the proliferation of MC3T3E1 cells was tested by the Cell Counting Kit-8 method. The results showed that the cell proliferation rate decreased with increasing porosity, which was due to the release of more Ni ions. Compared to the control group, the proliferation of cells cultured with the Ni–Ti alloys with different porosities in the extracting liquid was optimal. Accordingly, it was shown that the alloys had low cytotoxicity.

Herein, powder metallurgy gas-solid nitriding was used to prepare a high-nitrogen Ni-free austenitic stainless steel with more than 1.0 wt.% N. Study of the microstructures and corrosion resistances of the as-prepared samples revealed that the corrosion weight loss, size, number of corrosion pits, and corrosion depth decreased as the N content increased from 1.19 to 1.37 wt.%. The self-corrosion current densities of the alloys were determined from their potentiodynamic polarization curves. The changes in the capacitive arc radius as demonstrated by the Nyquist curves were consistent with the conclusions of the immersion corrosion analysis; that is, a high N content leads to a high corrosion resistance.