Kwang-Koo Jee

Damping mechanism and application of Fe-Mn based alloys

Abstract The damping mechanism and its application in Fe-Mn based high damping alloys are studied in this work. The mechanism of damping capacity is proposed by correlating it with the microstructure, which is varied through changes in grain size. The Fe-Mn based alloys are successfully applied as a part of a stone cutter, reducing the noise by 10–25 dB.

Damping capacity in Fe-Mn based alloys

Abstract 1. 1) Stacking faults and e martensite both act for damping with the latter much more effective. The damping capacity by e martensite is proposed to be caused by the phase boundary movement with the static hysteresis on vibration. 2. 2) We show that damping capacity of Fe-Mn based alloys depends on the area and mobility of Y/E boundary by varying microstructure. At a low strain amplitude, the area is more dominant than the mobility, and reverse is also true.

Shape memory effect in NiAl and NiMn-based alloys

Profound understanding of crystallography, deformation and memory behavior is important to develop new high temperature shape memory alloys (SMAs). Structural aspects of NiAl and NiMn base alloys have been studied in detail. At the same time, although the shape memory effect has been demonstrated in NiAl and NiMnTi/Al, such important memory characteristics as recovery curves, one-way and two-way strains and the effect of prestrain are not well documented. The purpose of the present paper is to compare the SME of NiAl-base and NiMn-base alloys. Since the polycrystals are greatly susceptible to intergranular fracture in tension, prestrain is given by compression. To avoid the different influence of thermally activated processes, the alloys were chosen to have similar transformation temperatures.

Cyclic fatigue resistance, torsional resistance, and metallurgical characteristics of V taper 2 and V taper 2H rotary NiTi files

The aim of this study was to compare the cyclic fatigue resistance, torsional resistance, and metallurgical characteristics of conventional NiTi wire (V taper 2, V2) and CM wire (V taper 2H, V2H)-based files. Cyclic fatigue and torsional resistance of V2 and V2H were investigated by measuring the number of cycles to fracture, maximum torque at fracture, and maximum angle at fracture. The typical patterns of fatigue and torsional fractures were investigated using a scanning electron microscope (SEM). The metallurgical characteristics were investigated by differential scanning calorimetry (DSC) from -100 °C to 100 °C. The austenite finishing temperature (Af) of each instrument was also measured. The microstructures of the instruments were investigated by a transmission electron microscope (TEM) along with selected area diffraction pattern analysis. The results were statistically analyzed by Mann-Whitney U-test (p = 0.05). V2H showed significantly higher cyclic fatigue resistance and torsional resistance than V2. SEM images of the fractured surfaces showed typical patterns of fatigue and torsional fracture. The DSC analysis of V2 showed one small peak in both the heating and cooling curves. The Af of V2 was -0.32 °C. V2H showed two remarkable peaks in the heating curve and one remarkable peak in the cooling curve. The Af of V2H was 33.25 °C. The TEM analysis showed that both V2 and V2H are mainly composed of austenite. In conclusion, V2H showed higher cyclic fatigue resistance and torsional resistance than V2. The superior properties of V2H could be attributed to the annealing effect and possibly the martensite phase. SCANNING 38:564-570, 2016.

Interfacial microchemistry and microstructure of Al-Mg-Si alloy matrix composites reinforced with Al2O3 particulates

In discontinuously reinforced metal matrix composites produced by ingot metallurgy, the ceramic particles are introduced in the molten alloy which provides adequate opportunity for reactions to quickly occur between the particles and the molten alloy. Similar reactions, although less rapid, can occur in composites produced by powder metallurgy routes. Using transmission electron microscopy (TEM) and X-ray spectroscopy, matrix and interfacial microchemistry can be analyzed in the metal matrix composites and correlated with the observed characteristics. In this study, the microstructure and the microchemistry of Al-Mg-Si alloy matrix composite reinforced with 10 vol.% Al{sub 2}O{sub 3} were characterized in order to examine the effect of the reinforcements on the distribution of alloying elements.

Measurement of volume fraction of stress-induced εs martensite in Fe-32Mn-6Si alloy

Fe-Mn-Si alloy has drawn much attention due to its shape memory effect and damping capacity which are based on {gamma} {leftrightarrow} {var_epsilon} non-thermoelastic transformation. Thermoelastic martensitic alloys like Ni-Ti undergo complete transformation if they are cooled below Mf temperature. In a Fe-Mn-Si alloy, however, the volume fraction of {var_epsilon} martensite below Mf depends on its heat treatment condition because of the incomplete non-thermoelastic transformation. Thus measurement of the volume fraction of {epsilon} martensite is necessary to correlate it with some physical and mechanical properties. In this work, the authors propose a new way of measuring the volume fraction of stress-induced {epsilon} martensite based on the volume expansion accompanying {var_epsilon} {yields} {gamma} transformation. Cold rolling is applied to generate anisotropic {var_epsilon} martensite, and the length change is measured in the RD, TD and ND during heating. Dilatometric behavior in the three directions and variation in the volume fraction are investigated with various degrees of cold rolling.

Fracture Resistance of K3 Nickel-Titanium Files Made from Different Thermal Treatments

The purpose of this study was to compare fracture resistances of K3 nickel-titanium files made from different thermal treatments. K3 (SybronEndo, Orange, CA), K3XF (SybronEndo), and experimentally heat treated K3 (K3H) were used. For the cyclic fatigue test, the samples were rotated with up-and-down motion in the artificial canal with the curvature of 60 degrees until the fracture occurred. The number of cycles to fracture (NCF) was measured. For the torsional fracture test, the samples were tightly bound and rotated until the fracture occurred. Elastic modulus (EM), ultimate torsional strength (UTS), and angle of rotation to fracture (ARF) were measured. The results were statistically analyzed by one-way ANOVA. The NCF of K3H was higher than those of K3 and K3XF (P < 0.05). The EM of K3XF and K3H was lower than that of K3 (P < 0.05). There was no significant difference in UTS. The ARF of K3XF was higher than that of K3 (P < 0.05). K3XF and K3H showed more flexibility than K3. The maximum torsional angle of K3XF was higher than that of K3, but there was no significant difference on the UTS in all three groups.

Effect of heat treatment on cyclic fatigue resistance, thermal behavior and microstructures of K3 NiTi rotary instruments

Abstract Objective. The aim of this study was to investigate the effect of heat treatment on the cyclic fatigue resistance, thermal behavior and microstructural changes of K3 NiTi rotary instruments. Materials and methods. Twelve control (as-received) and 12 experimental (heat-treated) K3 NiTi rotary instruments were compared in this study. Those experimental K3 instruments were heated in a furnace for 30 min at 450°C and then quenched in water. The cyclic fatigue resistance was measured with a fatigue tester. The thermal characteristic and the microstructures of both instruments were investigated by differential scanning calorimetry (DSC) and transmission electron microscopy (TEM), respectively. Results. There was a significant increase in the cyclic fatigue resistance between the heat-treated instruments and the as-received instruments (T-test, p < 0.05). DSC showed that the as-received and heat-treated samples were different, with an increased Af (austenite-finish temperature) for the latter. TEM analysis revealed that both as-received and heat-treated instruments were composed mainly of an austenite phase. However, the heat-treated samples had an increased appearance of larger grains, twinning martensite, TiO2 surface layer and a Ni-rich inner layer. Conclusions. Heat treatment increased the cyclic fatigue resistance of NiTi files and changed the thermal behavior of the instruments without marked changes in the constituting phases of NiTi alloy.

Electron beam deposition and characterization of thin film Ti-Ni for shape memory applications

Abstract Thin film of Ti-Ni alloy has a potential to perform the microactuation functions required in the microelectromechanical system (MEMS). It is essential, however, to have good uniformity in both chemical composition and thickness to realize its full potential as an active component of MEMS devices. Electron beam evaporation technique was employed in this study to fabricate the thin films of Ti-Ni alloy on different substrates. The targets used for the evaporation were first prepared by electron beam melting. The uniformity of composition and microstructure of the thin films were characterized by electron probe microanalysis (EPMA), Auger electron spectroscopy (AES), X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and transmission electron microscopy (TEM). The mechanical property of the thin films was evaluated by the nano - indentation test. The martensitic transformation temperature was measured by differential scanning calorimetry (DSC). It is confirmed that the chemical composition of deposited thin films is identical to that of the target materials. Furthermore, results from depth profiling of the chemical composition variation reveal that the electron beam evaporation process yields better compositional homogeneity than other conventional methods such as sputtering and thermal evaporation. Microstructural observation by TEM shows that nanometer size precipitates are preferentially distributed along the grain boundaries of a few micron size grains. The hardness and elastic modulus of thin films decreases with an increase in Ti contents.

Mechanical and Metallurgical Properties of Various Nickel-Titanium Rotary Instruments

The aim of this study was to investigate the effect of thermomechanical treatment on mechanical and metallurgical properties of nickel-titanium (NiTi) rotary instruments. Eight kinds of NiTi rotary instruments with sizes of ISO #25 were selected: ProFile, K3, and One Shape for the conventional alloy; ProTaper NEXT, Reciproc, and WaveOne for the M-wire alloy; HyFlex CM for the controlled memory- (CM-) wire; and TF for the R-phase alloy. Torsional fracture and cyclic fatigue fracture tests were performed. Products underwent a differential scanning calorimetry (DSC) analysis. The CM-wire and R-phase groups had the lowest elastic modulus, followed by the M-wire group. The maximum torque of the M-wire instrument was comparable to that of a conventional instrument, while those of the CM-wire and R-phase instruments were lower. The angular displacement at failure (ADF) for the CM-wire and R-phase instruments was higher than that of conventional instruments, and ADF of the M-wire instruments was lower. The cyclic fatigue resistance of the thermomechanically treated NiTi instruments was higher. DSC plots revealed that NiTi instruments made with the conventional alloy were primarily composed of austenite at room temperature; stable martensite and R-phase were found in thermomechanically treated instruments.