High-strength porous Ni–Ti shape-memory alloys with stabilized high-stress cyclic properties
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摘要: 以氫化脫氫(Hydrogenation dehydrogenation, HDH)鈦粉和鎳粉為原料制備的多孔NiTi形狀記憶合金普遍承載性能與可恢復應變較差. 本研究以NaCl為造孔劑，通過在高真空(10?4 Pa)下高溫(1250 ℃)均勻化燒結制備出了高強度、高應力循環穩定的多孔NiTi合金，研究了不同孔隙率下的微觀結構、相變行為、力學性能以及細胞毒性. 研究發現，隨著NaCl添加量的增加，樣品的孔隙率和孔徑增大，同時氧含量略有增加. 在樣品中觀察到無熱處理自發形成的Ni4Ti3沉淀相，沉淀相尺寸隨樣品氧含量增加而增加. 所有樣品的馬氏體相變均呈現多峰現象，主要歸因于非均勻分布的Ni4Ti3沉淀相引發的多步相變效應. 孔隙率為14% ～ 37%的多孔NiTi合金的壓縮強度為1236 ～1600 MPa. 與其他粉末冶金法制備多孔NiTi合金的抗壓強度相比，本研究所獲得的合金表現出超高的強度. 樣品在8%應變壓縮加載–卸載后同時表現出超彈性和形狀記憶效應，經加熱處理后形狀恢復率超過99%. 在循環壓縮實驗中，多孔NiTi樣品在接近8%應變的高應力下承受了50次循環. 樣品的殘余應變隨著周期數的增加而增加. 隨著孔隙率的增加，循環結束時的最終殘余應變為1.4%、1.55%和1.66%. 低的殘余應變說明多孔NiTi樣品在高應力壓縮環境中具有較好的穩定性，這歸因于Ni4Ti3沉淀相對基體的強化作用. 使用MC3T3E1 細胞評估了樣品的細胞毒性，結果表明多孔NiTi樣品具有較低的細胞毒性.Abstract: 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.
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圖 1 原材料粉末的SEM形貌圖(a) Ti, (b) Ni, (c) NaCl; (d) 壓制樣品和燒結態樣品; (e) 真空燒結過程的示意圖; (f) 用于壓縮測試的樣品
Figure 1. Scanning electron microscopy images of (a) Ti, (b) Ni, and (c) NaCl raw powders; (d) green and sintered samples; (e) schematic diagram of the vacuum sintering process; (f) as-sintered samples for compressive and compressive recovery tests
圖 2 細胞毒性測試中用于制備浸提液樣品
Figure 2. Samples for preparing the extract-solution for the cytotoxicity tests
圖 3 多孔NiTi合金的二次電子圖像(a) P15, (b) P25, (c) P40; (d) P15在高倍率下的背散射圖
Figure 3. Secondary electron images of the porous Ni–Ti alloys (a) P15, (b) P25, and (c) P40; (d) high-magnification backscattered electron image of the P15 sample
圖 4 室溫下的多孔NiTi合金的XRD圖譜
Figure 4. X-ray diffraction patterns of the porous Ni–Ti alloys at room temperature
圖 5 TEM形貌圖(a) P15, (b) P25, (c) P40; (a1～c1) 對應黃色虛線框區域的選區電子衍射（SAED）圖; (d) P40樣品中的高分辨透射圖(HRTEM)，d為原子面間距; (e), (d)中黃色方框區域的快速傅里葉變換圖(FFT); (f) P40在HAADF模式下的圖像; (g)和(h)為(f)的EDS面掃描圖; (i) EDS面掃描Ni、Ti元素原子數分數
Figure 5. Transmission electron microscopy (TEM) images of (a) P15, (b) P25, and (c) P40 samples; (a1–c1) selected area electron diffraction patterns of the corresponding yellow dotted areas; (d) high-resolution TEM lattice image of the interface in (f), and d is the distance between atomic surfaces; (e) fast-Fourier transform of (d) ; (f) high-angle annular dark-field image of the P40 sample; (g, h) energy-dispersive X-ray spectroscopy (EDS) mapping analysis of (f); (i) EDS results of elemental Ni and Ti contents
圖 6 DSC曲線. (a) P15; (b) P25; (c) P40
Figure 6. Differential scanning calorimetry curves: (a) P15; (b) P25; (c) P40
圖 8 8%應變加載–卸載的應力應變曲線. (a) P15; (b) P25; (c) P40
Figure 8. Stress–strain curves of 8% strain loading–unloading for samples: (a) P15; (b) P25; (c) P40
圖 9 應力控制的循環加載–卸載應力應變曲線及殘余應變. (a) P15; (b) P25;(c) P40; (d) 循環過程中的殘余應變
Figure 9. Cyclic loading–unloading stress–strain curves for stress controls: (a) P15; (b) P25; (c) P40; (d) residual strain during cycling
圖 10 細胞在浸提液中培養3 d和7 d的相對增殖率
Figure 10. Relative proliferation rates of cells in the extract-solution for three and seven days
表 1 多孔NiTi合金的雜質含量、孔隙參數以及實際Ni當量
Table 1. Impurity contents, pore parameters, and calculated Ni contents of the porous Ni–Ti alloys
Sample Porosity/% Open porosity/% Pore size/μm Carbon mass fraction/% Oxygen mass fraction/% Calculated Ni atomic content/% P15 14.0 ± 0.52 11.0 ± 0.52 60 ± 15 0.08 ± 0.01 0.23 ± 0.05 51.64 P25 22.0 ± 1.41 21.8 ± 0.87 91 ± 20 0.08 ± 0.01 0.29 ± 0.06 51.94 P40 37.0 ± 1.15 36.7 ± 1.03 124 ± 22 0.09 ± 0.02 0.36 ± 0.04 52.53 
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表 2 多孔NiTi合金馬氏體和奧氏體相變峰值溫度及相變潛熱
Table 2. Peak temperatures of martensitic and austenitic phase transformations and related latent heat values in the porous Ni–Ti alloys
Samples Cooling Heating MP1/℃ MP2/℃ MP3/℃ Exo./(J·g?1) AP1/℃ AP2/℃ AP3/℃ End./(J·g?1) P15 33.7 1.6 ?28.6 20.0 0.1 10.9 34 ?24.7 P25 34.3 3.2 — 18.7 1.0 14.6 16.4 ?23.2 P40 35.5 4.8 — 15.3 2.4 16.4 34.7 ?23.0
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表 3 8%應變壓縮加載–卸載測試結果
Table 3. Results of the 8% strain compressive loading–unloading tests
Sample Stress of 8% strain / MPa Superelastic recovery strain/% Memory recovery stain/% Residual strain/% P15 615 ± 13 6.74 ± 0.12 1.25 ± 0.24 0.02 ± 0.01 P25 568 ± 12 6.68 ± 0.11 1.28 ± 0.30 0.04 ± 0.02 P40 338 ± 13 4.97 ± 0.22 2.43 ± 0.27 0.60 ± 0.08
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表 4 MC3T3E1 細胞與多孔NiTi合金共培養3 d和7 d的吸光度值（490 nm特征波長）
Table 4. Absorbance values of the MC3T3E1 cell co-culture with the porous Ni–Ti alloys for three and seven days (characteristic wavelength of 490 nm)
Samples OD490 3 d 7 d Background 0.1917 ± 0.0008 0.1962 ± 0.0011 Control 2.5466 ± 0.0432 2.9316 ± 0.0382 P15 2.2361 ± 0.0397 2.8422 ± 0.0091 P25 2.0532 ± 0.0228 2.8342 ± 0.0321 P40 1.8191 ± 0.0221 2.7751 ± 0.0143 Ingot Ni–Ti 2.1512 ± 0.0346 2.8168 ± 0.0359
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