Chang-Sup Oh

A Study on the Preferred Orientation Characteristics of AlN Thin Films by Reactive Evaporation Method using NH3

Aluminum nitride(AlN) is a compound (III-V group) of hexagonal system with a crystal structure. Its Wurzite phase is a very wide band gap semiconductor material. It has not only a high thermal conductivity, a high electrical resistance, a high electrical insulating constant, a high breakdown voltage and an excellent mechanical strength but also stable thermal and chemical characteristics. This study is on the preferred orientation characteristics of AlN thin films by reactive evaporation using NH3. We have manufactured an AlN thin film and then have checked the crystal structure and the preferred orientation by using an X-ray diffractometer and have also observed the microstructure with TEM and AlN chemical structure with FT-IR. We can manufacture an excellent AlN thin film by reactive evaporation using NH3 under 873 K of substrate temperature. The AlN thin film growth is dependent on Al supplying and NH3 has been found to be effective as a source of N2. However, the nuclear structure of AlN did not occur randomly around the substrate a particle of the a-axis orientation in fast growth speed becomes an earlier crystal structure and is shown to have an a-axis preferred orientation. Therefore, reactive evaporation using NH3 is not affected by provided H2 amount and this can be an easy a-axis orientation method. (Received July 5, 2011)

RETRACTED ARTICLE: Precipitation behavior of Al-Ti-Ag alloy system

Transmission electron microscopy (TEM) examinations of Al3Ti with an L12-ordered structure have revealed the precipitation of D023-Al11Ti5 and L10-TiAl upon aging after quenching from higher temperatures. TEM observations revealed that fine uniform precipitation of Al2Ti occurs when the supersaturation is sufficiently high, and, a preferential precipitation at the antiphase boundaries can be observed in alloy with a low supersaturation. When L12-Al3Ti is supersaturated with DO22-Al3Ti, DO23-Al11Ti5 with a multidomain structure is formed during aging. On the other hand, plate-like L10-TiAl precipitates lie on the {001} planes of (Al,Ag)3Ti matrix in the short aging period and the habit plane changed from {001} to {hhl} after a long period aging or higher temperature aging and finally to {225} of the matrix lattice. The Ll2 phase field in the Al-Ti-Ag system is severely skewed with respect to the temperature axis and is restricted into a much smaller field at lower temperatures. Appreciable hardening and overage softening during aging can be explained in terms of microstructural variations.

Mechanical Properties of B-Doped Ni 3 Al-Based Intermetallic Alloy

The mechanical behavior and microstructural evolution during high temperature tensile deformation of recrystallized Ni3Al polycrystals doped with boron were investigated as functions of initial grain size, tensile strain rate and temperature. In order to obtain more precise information on the deformation mechanism, tensile specimens were rapidly quenched immediately after deformation at a cooling rate of more than , and were then observed by transmission electron microscopy (TEM). Mechanical tests in the range of 923 K to 1012 K were carried out in a vacuum of less than Pa using an Instron-type machine with various but constant cross head speeds corresponding to the initial strain rates from to . After heating to deformation temperature, the specimen was kept for more than 1.8 ks before testing. The following results were obtained: (1) Flow behavior was affected by initial strain size; with decreasing initial grain size, the level of a stress peak in the true stress-true strain curve decreased, the steady state region was enlarged and elongation increased. (2) On the basis of TEM observation of rapidly quenched specimens, it was confirmed that dynamic recrystallization certainly occurred on deformation of fine-grained () and intermediate-grained () specimens at an initial strain rate of and at 973 K. (3) There were some dislocation-free grains among the new recrystallized grains. The obtained results suggest that both dynamic recrystallization and grain boundary sliding are operative during high temperature deformation.

Retraction note: Microstructure of Co Precipitation Strengthened B2-Ordered NiAl

A transmission electron microscopy investigation on the phase decomposition of B2-ordered (Ni,Co)Al supersaturated with Ni and Co has revealed the precipitation of (Ni,Co)2Al which has not been expected from the reported equilibrium phase diagram. The (Ni,Co)2Al phase has a hexagonal structure and takes a rodlike shape with the long axis of the rod parallel to the 〈111〉 directions of the B2 matrix. By aging at temperatures below 873 K, a long period superlattice structure appears in the hexagonal (Ni,Co)2Al phase. The orientation relationship between the (Ni,Co)2Al precipitates and the B2-(Ni,Co)Al matrix is found to be (0001)p//(111)B2 and [\( \bar 1 \)2\( \bar 1 \)0]p//[\( \bar 1 \)10]B2, where the suffix p and B2 denote the (Ni,Co)2Al precipitate and the B2-(Ni,Co)Al matrix, respectively. (Ni,Co)Al hardens appreciably by the fine precipitation of the (Ni,Co)2Al phase.

Hydrogen Reduction of NiO Particles in a Single-Stage Fluidized-Bed Reactor without Sticking

A commercial NiO (green nickel oxide, 86 wt% Ni) powder was reduced using a batch-type fluidized-bed reactor in a temperature range of 500 to 600 oC and in a residence time range of 5 to 90 min. The reduction rate increased with increases in temperature; however, agglomeration and sintering (sticking) of Ni particles noticeably took place at high temperatures above 600 oC. An increasing tendency toward sticking was also observed at long residence times. In order to reduce the oxygen content in the powder to a level below 1 % without any sticking problems, which can lead to defluidization, proper temperature and residence time for a stable fluidized-bed operation should be established. In this study, these values were found to be 550 oC and 60 min, respectively. Another important condition is the specific gas consumption rate, i.e. the volume amount (Nm3) of hydrogen gas used to reduce 1 ton of Green NiO ore. The optimum gas consumption rate was found to be 5,000 Nm3/ton-NiO for the complete reduction. The Avrami model was applied to this study; experimental data are most closely fitted with an exponent (m) of 0.6 ± 0.01 and with an overall rate constant (k) in the range of 0.35~0.45, depending on the temperature.

RETRACTED ARTICLE: Microstructure and strengthening mechanism of Ni3Al intermetallic compound

Structural studies have been performed on precipitation hardening found in Ni3Al-base ordered alloys using transmission electron microscopy. The γ′ phase hardens appreciably by the fine precipitation of disordered γ. The strength of γ′ increases over the temperature range of experiment by the precipitation of fine γ particles. The peak temperature where a maximum strength was obtained shifted to higher temperature. Superlattice dislocations dissociate into fourfold Shockley partial dislocations in a uniform supersaturated solid solution of the γ′ phase. Dislocations are attracted into the disordered γ phase and dissociate further in the particles. At any stage of aging, dislocations cut through the particles and the Orowan bypassing process does not occur even in the overaged stage of this alloy system. When the applied stress is removed, the dislocations make cross slip into (010) plane, while those in γ precipitates remain on the (111) primary slip plane. The increase of high temperature strength in γ′ containing γ precipitates is due to the restraint of cross slip of dislocations from (111) to (010) by the dispersion of disordered γ particles. The orientation dependence of strength is decreased by the fine precipitation of a disordered γ phase.

Retraction Note: Microstructures and morphologies of B2-ordered NiAl(Co) and FeAl(Co)

Fine dispersion of disordered phases is obtained in a Ni-Al-Co and Fe-Al-Co ternary system. A transmission electron microscopy investigation has been performed in the present work on the precipitation of supersaturated B2-ordered (Ni,Co)Al and α-Fe in B2-ordered FeAl(Co) with different stoichiometries. Precipitation behavior and hardening were investigated by measuring the hardness variation. The hardness of (Ni,Co)Al and B2-FeAl(Co) increased appreciably by the fine precipitation of (Ni,Co)2Al, α-Fe, and overage softening occurred after prolonged aging. In case of B2-ordered (Ni,Co)Al, the (Ni,Co)2Al phase had a hexagonal structure and took a rod-like shape with the long axis of the rod parallel to the 〈111〉 directions of the B2 matrix. By aging at temperatures below 873 K, a long period superlattice structure appeared in the hexagonal (Ni,Co)2Al phase. The orientation relationship between the (Ni,Co)2Al precipitates and the B2-(Ni,Co)Al matrix was (0001)p//(111)B2 and \([\bar 12\bar 10]_p //[\bar 110]_{B2}\), where the suffix p and B2 denote the (Ni,Co)2Al precipitate and the B2-(Ni,Co)Al matrix, respectively. (Ni,Co)Al hardened appreciably by the fine precipitation of the (Ni,Co)2Al phase. On the other hand, in case of B2-FeAl(Co), the disordered α-Fe phase was present as a precipitate in a B2-FeAl(Co) matrix and had a cubic-cubic orientation with the matrix. At the early aging periods, prismatic dislocation loops formed in the B2-FeAl(Co) matrix. B2-FeAl(Co) matrix was typically hardened by the precipitation of α-Fe.

알루미나입자로 강화된 알루미늄합금 복합재료의 미세조직과 기계적 성질

The mechanical properties and microstructures of aluminum-matrix composites fabricated by the dispersion of fine alumina particles less than 20μm in size into 6061 aluminum alloys are investigated in this study. In the as-quenched state, the yield stress of the composite is 40~85 MPa higher than that of the 6061 alloy. This difference is attributed to the high density of dislocations within the matrix introduced due to the difference in the thermal expansion coefficients between the matrix and the reinforcement. The difference in the yield stress between the composite and the 6061 alloy decreases with the aging time and the age-hardening curves of both materials show a similar trend. At room temperature, the strain-hardening rate of the composite is higher than that of the 6061 alloy, most likely because the distribution of reinforcements enhances the dislocation density during deformation. Both the yield stress and the strain-hardening rate of the T6-treated composite decrease as the testing temperature increases, and the rate of decrease is faster in the composite than in the 6061 alloy. Under creep conditions, the stress exponents of the T6-treated composite vary from 8.3 at 473 K to 4.8 at 623 K. These exponents are larger than those of the 6061 matrix alloy.