富鈦TiNi形狀記憶合金燒結行為、機械性質與形狀記憶之研究

Abstract

本研究主要探討利用粉末冶金方法製作出高密度與高相變態溫度的鈦鎳形狀記憶合金。利用較細的初始粉末、Ti51Ni49成分、二段式升溫與1280oC液相燒結，燒結後的試片密度可以達到95.3%，相變態潛熱也可達到31.9J/g，已遠遠超過一般文獻中利用粉末冶金製造TiNi所得到的值，同時，與鑄鍛方式得到的TiNi結果相近。最主要的原因是，Ti51Ni49的組成會在TiNi基地內形成適量的Ti2Ni相，而Ti2Ni會吸收TiNi基地內的碳、氮與氧，來純化TiNi基地，使TiNi基地內的Ti/Ni比例保持在50.5/49.5。在燒結後，儘管在母相晶界上析出Ti2Ni，所得到的拉伸強度、延性與五次訓練後形狀回復率能可分別達到638MPa、14.6%與99.1%。 由於晶界上連續的Ti2Ni，使燒結後試片的拉伸與形狀回復性質不如一般鑄鍛所得到的TiNi產品。當燒結試片在經過熱均壓和球化處理後，因為密度的提升與Ti2Ni的球化，拉伸與形狀回復性質都有明顯的改善。在添加0.5和1at.%硼，TiB在升溫過程中形成，提供Ti2Ni冷卻凝固時異質成核位置，因為TiB的形成，TiNi晶粒粗化被抑制，晶界上連續的Ti2Ni無法形成。使試片的拉伸強度與延性都有很明顯的提升，而相變態行為與形狀回復性質並沒有下降。 多孔TiNi形狀記憶合金由於和人體骨頭有相似的壓縮性質，因此被廣泛地應用於生醫用途上。本研究中，Ti、Ni與NaCl混合粉在高真空下，1050oC燒結30分鐘後，NaCl即被移除，使TiNi試片在同時擁有高孔隙度下，又能維持試片尺寸。麻田散體相變態起始溫度、結束溫度與潛熱分別為74、48與28J/g，相變態行為與試片孔隙度無關。而所得到的多孔TiNi具有均勻的組織、控制良好的孔洞結構與高的相變態溫度。

This study demonstrates that a high density and a high transformation heat (ΔH) can be obtained for powder metallurgy TiNi shape memory alloys (SMAs). With the use of fine elemental powders, a composition of Ti51Ni49, two-step heating, and persistent liquid phase sintering at 1280oC, a 95.3% sintered density is attained for compacts with a green density of 66%. A transformation heat of 31.9J/g is also achieved, which is much higher than that reported previously for sintered TiNi and is approaching the highest ΔH reported to date, 35J/g, for wrought TiNi with low C, O, and N contents. The main reason for having these properties in powder metal TiNi with higher amounts of C, O, and N is that the extra Ti, that over the equiatomic portion in the Ti-rich Ti51Ni49, forms Ti2Ni compound, which traps most of the C, O, and N. This results in low interstitial contents and a high Ti/Ni ratio of 50.5/49.5 in the TiNi matrix. The tensile strength, elongation, and shape recovery rate after 5 training cycles are 638MPa, 14.6%, and 99.1%, respectively, despite the presence of Ti2Ni compounds at grain boundaries. Due to the presence of a semi-continuous Ti2Ni network, the shape recovery and tensile properties in the martensitic state at room temperature, determined using bending tests, are lower than those of cast TiNi counterparts. Through hot isostatic pressing and annealing above the peritectic temperature on sintered specimens, these properties are improved due to the increase in density and spheroidization of the Ti2Ni compound. The phase transformation temperature and enthalpy are also enhanced due to the continuing carbon absorption by the Ti4Ni2X(X=C,O) phase, which decreases the carbon content in the TiNi matrix. With 0.5 and 1.0at.% B additions, fine TiB forms during heating and sintering and acts as an inoculant for Ti2Ni to precipitate within the grain during cooling. The resultant uniform distribution of TiB and Ti2Ni impedes grain growth and prevents the formation of continuous Ti2Ni networks at grain boundaries. As a result, significant improvement in elongation, and not reduction, as in most as-cast titanium alloys, is obtained due to the changes in the morphology of the Ti2Ni intermetallic compounds. The tensile strength also increases, without deterioration of the shape memory characteristics. The tensile strength and elongation are close to those of wrought TiNi alloys. Porous TiNi shape memory alloys with a porosity similar to that of human bones have become a focus of research for biomaterials applications. In this study, net-shaped TiNi foams with well-controlled porosity, pore size, and pore shape were produced by pressing and sintering mixed fine Ti and Ni powders with coarse NaCl powders. When sintered at 1050°C for 30 min in high vacuum, the NaCl space holder was removed during heating, and the remaining TiNi powders were then sintered with about 2.3vol.% liquid phase. The Ti51Ni49 prepared in this study had porosities of 26, 64, 70, 78, and 85%, and no distortion was observed. The martensitic phase transformation starting temperature (MS), finishing temperature (Mf), and enthalpy (ΔH) are 74°C, 48°C, and 28J/g, respectively, irrespective of the porosity. These porous Ti51Ni49 sintered compacts exhibit homogeneous microstructures, good pore structures, and high martensitic phase transformation characteristics.