Sintering Behavior, Mechanical Properties, and Shape Memory Effect of Ti-rich TiNi Shape Memory Alloys
Date Issued
2012
Date
2012
Author(s)
Yen, Fu-Cheng
Abstract
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.
Subjects
TiNi
Shape memory alloys
Powder metallurgy
HIPing
Annealing
Boron
NaCl.
Type
thesis
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