Tae Hyun Nam

Functionally Graded Ti-Ni Shape Memory Alloys

Shape memory characteristics and superelasticity of an temperature gradient annealing(TGA) treated equiatomic Ti-Ni alloy have been investigated by means of differential scanning calorimetry(DSC), thermal cycling tests under constant load and tensile tests. By annealing 25% cold worked alloy under the temperature gradient from 658 K to 466 K, 7 K variation in TR*and 19 K variation in Ms* were obtained along the length of sample(150mm). Temperature dependence of transformation elongation(dε/dT) of TGA treated Ti-Ni alloy wires was in the range of 0.05 %/K and 0.01 %/K depending on annealing temperature ranges. The dε/dT obtained from TGA treated sample under the temperature gradient from 658 K to 466 K was 0.03 %/K. TGA treated alloy showed the clear superelastic recovery.

Consideration of Fe Nanoparticles and Nanowires Synthesized by Chemical Vapor Condensation Process

Various physical, chemical and mechanical methods, such as inert gas condensation, chemical vapor condensation, sol-gel, pulsed wire evaporation, evaporation technique, and mechanical alloying have been used to synthesize nanoparticles. Among them, chemical vapor condensation(CVC) represents the benefit for its applicability to almost materials because a wide range of precursors are available for large-scale production with a non-agglomerated state. In this work, iron nanoparticles and nanowires have synthesized by chemical vapor condensation(CVC) process, using iron pentacarbonyl(Fe(CO)5) as precursor. The effects of processing parameters on the morphology, microstructure and size of iron nanoparticles and nanowires were studied. Iron nanoparticles and nanowires having various diameters were obtained by controlling the inflow of metallic organic precursor. Both nanoparticles and nanowires were crystallized. Characterization of obtained nanoparticles and nanowires were investigated by using a field emission scanning electron microscopy, transmission microscopy and X-ray diffraction.

Transformation Behavior and Shape Memory Characteristics of Ti-45Ni-5Cu(at%) Alloy Ribbons Fabricated by Melt Spinning

Transformation behaviors and shape memory characteristics of Ti-45Ni-5Cu alloy ribbons fabricated by melt spinning were investigated by means of optical microscopy, differential scanning calorimetries(DSC), X-ray diffraction and thermal cycling tests under constant load. They depended largely on temperatures of liquid metal. The B2-B19-B19x92 two-step transformation occurred in the ribbons fabricated with the liquid whose temperature was higher than 1723 K, while the B2-B19x92 one-step transformation occurred in the ribbons with the liquid at 1673 K. The stabilization of the B19 martensite in Ti-45Ni-5Cu alloy ribbons was ascribed to the high density of dislocations which made strong resistance to large lattice deformation associated with a formation of the B19x92 martensite.

The Effect of Rapidly Solidified Microstructures on the Martensitic Transformation in Ti50-Ni45-Cu5 Shape Memory Alloys

Transformation behaviors and shape memory characteristics of Ti–Ni45-Cu5 alloy ribbons prepared by melt spinning were investigated by means of DSC, XRD and OM. In these experiments particular attention has been paid to change the ejection temperature of the melt from 1400°C to 1600 °C. As the results, the thickness of ribbons could be controlled. An increase of the superheat of the melt leads to a reduced ribbon thickness and a refinement of grains. The microstructural refinement and the increased internal strains achieved by controlling the melt-spinning temperature decreased Ms significantly. It was also found that two-step transformation (B2-B19-B19’) occurred in the ribbons fabricated at higher melt-spinning temperatures than 1450°C.

Thermal Sulfidation Behavior of Ti-Ni Alloys

Ti and Ni sulfides were formed on the surface of Ti-Ni alloys by isothermal annealing at various sulfur pressures, and then microstructures of the surface sulfide layers were investigated by means of scanning electron microscopy and X-ray diffractions. Morphology of sulfurized surface changed from granular shape to porous shape at the sulfur pressure of 80 kPa, which is related to the change in sulfide from NiS1.97 to NiS. Two-layered sulfide was observed in which the inner layer was mainly Ti8.2S11 , and the outer layer was a mixture of NiS1.97 and NiS. The discharge curve of the Ti and Ni sulfides cathode formed on the Ti-Ni current collector at the first cycle showed a plateau voltage of 1.6 V, and the discharge capacity was found to be about 530 mAh/g-NiS1.97.

CRYSTALLIZATION BEHAVIOR OF AMORPHOUS TI-NI-CU ALLOY RIBBONS

Amorphous Ti50Ni(50-x)Cux (at.%) (x = 15, 20 and 25) alloy ribbons were prepared by melt spinning, and then their crystallization behavior was investigated by optical microscopy, transmission electron microscopy, X-ray diffraction and differential scanning calorimetry. Wavenumber (Qp) decreased from 29.40 nm-1 to 29.29 nm-1 and ΔT(Tg - Tx) increased from 31 K to 36 K with increasing Cu content from 15 at.% to 25 at.%, suggesting that glass forming ability of Ti–Ni–Cu alloy ribbons increased with increasing Cu content. Activation energy for crystallization decreased from 211.5 kJ/mol to 136.4 kJ/mol with increasing Cu content from 15 at.% to 25 at.%, suggesting that a stability of Ti–Ni–Cu amorphous decreased with increasing Cu content.

Cooling and Heating Characteristics of Ti-Ni Based Shape Memory Alloy Wire Actuators

Ti-51Ni(at%) and Ti-40Ni-10Cu(at%) alloy wires with diameters of 0.3mm, 0.5mm and 0.7mm were prepared by drawing the alloy ingots fabricated by vacuum induction melting. Heating rates of the wires were investigated by measuring changes in temperatures of them while applying currents in the range of 1 A and 6 A to them and cooling rates were investigated by measuring changes in temperatures of them after cutting currents. Heating rate increased with increasing the amount of current, while cooling rate was kept constant. Both heating rate and cooling rate increased with decreasing diameter of wire. This suggested that high amount of current and small wire diameter were required for high heating and cooling rate. Comparing Ti-50Ni alloy wires with Ti-40Ni-10Cu alloy wires, heating rates of the latter was faster than that of the former, although cooling rates were almost same. This suggested that Ti-40Ni-10Cu alloy wires is better than Ti-50Ni alloy wires for the applications requiring high actuating rates.

Clinical Application of TiNi Shape Memory Alloy Bone Fastener

Titanium-Nickel shape memory alloy (TiNi SMA) has great potential as a biomaterial in orthopaedic applications due to its unique thermal shape memory effects, superelasticity and high damping properties. We designed and manufactured bone fasteners using newly developed TiNi SMA wire (Af, 35 2.C). Several types of bone fastener designs were prepared for the application of orthopedic treatment of bone fracture. We applied this fastener to 82 fracture patients. Fracture types according to the anatomic location included distal fibular (21), femur shaft (periprosthetic fracture, 17), distal tibia (15), distal femur (10), metacarpal bone (9), and subtrochanter of femur (5), distal clavicle (5). Serial radiographs, complete blood count (CBC) and urine analysis were performed postoperatively. Radiological union was achieved without complications at due time after operation. There were no abnormal findings on follow-up CBC or urine analysis. On a subjective level, use and application of the TiNi SMA fastener was not as demanding as conventional fixation methods, such as circumferential wiring (cerclage) or the Dall-Miles technique. The efficacy of SMA bone fastener in this study is very excellent as demonstrated in this clinical study. It gives the new armament to orthopedic surgeon.

SHAPE MEMORY PROPERTIES OF RAPIDLY SOLIDIFIED Ti50Ni50-XCuX (X = 20, 25) ALLOY STRIPS

The shape memory alloy strips of Ti50Ni30Cu20 and Ti50Ni25Cu25 have been fabricated by arc melt overflow technique. Their microstructures and shape memory characteristics were investigated by means of X-ray diffraction (XRD), optical microscopy and differential scanning calorimetry (DSC). The microstructure of the as-cast strips exhibited columnar grains normal to the strip surface. XRD analysis showed that the martensitic transformation of B2–B19 occurred in the alloy strips. During thermal cyclic deformation with the applied stress of 60 MPa, transformation hysteresis and elongation associated with the B2–B19 transformation were observed to be 4.9°C and 1.8% in Ti50Ni30Cu20 alloy strip and 3.5°C and 1.7% in Ti50Ni25Cu25 alloy strip. The as-cast strip of Ti50Ni25Cu25 alloy also showed a perfect superelasticity and its stress hysteresis was as small as 14 MPa. These mechanical properties and shape memory characteristics of the alloy strips were ascribed to B2–B19 transformation and the controlled microstructures produced by rapid solidification of the arc melt overflow process.

New approach to synthesis of carbon nanotubes

Carbon nanotubes (CNTs) have been synthesized through chemical vapor deposition in argon gas atmosphere using Fe–2.5%Mo alloyed nanoparticles as a catalyst and H2/CH4 gas mixture as a reaction gas. Fe–2.5 wt.%Mo alloyed nanoparticles with average diameter of 7, 20, 45 and 85 nm are prepared by the chemical vapor condensation process using the pyrolysis of iron pentacarbonyl (Fe(CO)5) and molybdenum hexacarbonyl (Mo(CO)6). The morphologies of the CNTs are controlled by adjusting the nanoparticle size, reaction gas ratio and reaction temperature. With decreasing nanoparticle size under the same experimental conditions, the degree of crystalline perfection increases gradually and the morphologies of the carbon nanotubes vary from multi wall carbon nanotubes to single wall carbon nanotubes. Also, the ratio of reaction gas has an effect on the morphology and the degree of crystallinity of CNTs. In this work, it is suggested that morphology, diameter and degree of crystallinity of CNTs could be controlled by adjusting the reaction gas ratio, reaction temperature and catalyst size.