高强度高应力循环稳定的HDH多孔NiTi形状记忆合金

以氢化脱氢(Hydrogenation dehydrogenation, HDH)钛粉和镍粉为原料制备的多孔NiTi形状记忆合金普遍承载性能与可恢复应变较差. 本研究以NaCl为造孔剂，通过在高真空(10⁻⁴ Pa)下高温(1250 ℃)均匀化烧结制备出了高强度、高应力循环稳定的多孔NiTi合金，研究了不同孔隙率下的微观结构、相变行为、力学性能以及细胞毒性. 研究发现，随着NaCl添加量的增加，样品的孔隙率和孔径增大，同时氧含量略有增加. 在样品中观察到无热处理自发形成的Ni₄Ti₃沉淀相，沉淀相尺寸随样品氧含量增加而增加. 所有样品的马氏体相变均呈现多峰现象，主要归因于非均匀分布的Ni₄Ti₃沉淀相引发的多步相变效应. 孔隙率为14% ～ 37%的多孔NiTi合金的压缩强度为1236 ～1600 MPa. 与其他粉末冶金法制备多孔NiTi合金的抗压强度相比，本研究所获得的合金表现出超高的强度. 样品在8%应变压缩加载–卸载后同时表现出超弹性和形状记忆效应，经加热处理后形状恢复率超过99%. 在循环压缩实验中，多孔NiTi样品在接近8%应变的高应力下承受了50次循环. 样品的残余应变随着周期数的增加而增加. 随着孔隙率的增加，循环结束时的最终残余应变为1.4%、1.55%和1.66%. 低的残余应变说明多孔NiTi样品在高应力压缩环境中具有较好的稳定性，这归因于Ni₄Ti₃沉淀相对基体的强化作用. 使用MC3T3E1 细胞评估了样品的细胞毒性，结果表明多孔NiTi样品具有较低的细胞毒性.

Porous NiTi for bone implants: A review

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.