Dong-Wha Kum

Analysis of X-ray diffraction patterns from mechanically alloyed Al-Ti powders

X-ray diffraction (XRD) method is one of the most versatile tools to characterize various forms of materials. Simplicity and wealth of information from the spectrum makes it attractive for the evaluation of mechanical alloyed powders. However, careful interpretation of the solubility of minor phase is necessary due to the effect of particle size on the detection limit in XRD method. In this study, we demonstrate the inaccuracy of solubility from XRD analysis of nanosized particle system using Al-Ti as a model. Using transmission electron microscopy (TEM), it is confirmed that large amount of nanosized Ti in Al matrix is not detected by XRD. The peak disappearance of minor phase can not be used to determine the solubility of mechanically alloyed powders. Lattice parameter change of the major phase should be used to assess the solubility limit of the minor phase in nanosized particle system. In addition, the possible sources of error are addressed when mechanically alloyed powders of Al-Ti system are characterized by the XRD method. Proper XRD analysis methods are suggested to determine the lattice parameter, solubility of minor elements, crystallite size and strain variance in the MA Al-Ti samples. Pure A1 is used as an internal standard to correct instrumental broadening, the Al {111} peak is used to determine lattice parameter of A1, and the lattice parameter of Al is recommended to estimate the solubility of Ti in Al. The calculation of crystallite size and strain variance in the MA powders using Williamson-Hall equation is also discussed in detail.

Simple procedure for phase identification using convergent beam electron diffraction patterns

The use of the primitive cell volume and the zero order Laue zone (ZOLZ) pattern is proposed as a means to identify phases in a complex microstructure. A single convergent beam pattern, containing a higher order Laue zone ring, from a nanosized region is sufficient to calculate the primitive cell volume of the phase, while ZOLZ pattern is used to determine the zone axis of the crystal. A computer program is used to screen out possible phases on the basis of the value of measured cell volume. The indexing of the ZOLZ pattern follows in the program to find the zone axis of the identified phase. Combination of these two methods ensures accuracy and reliability of phase identification from a single CBED pattern. An example of the analysis is given from the rapidly solidified Al‐Al3 Ti system.

Low temperature buffer growth to improve hydride vapor phase epitaxy of GaN

Abstract Two-step growth of hydride vapor phase epitaxy (HVPE) was optimized to grow high-quality, thick GaN film on the (0001) sapphire substrate using ammonia, chlorinated gallium and nitrogen carrier gas. Chlorinated Ga and NH 3 were used to grow GaN-buffer layers at a temperature range of 550–650°C for 1 to 7 min. The main growth of approximately 30 μm thick GaN film was performed at 1125°C for 30 min. Surface roughness after the low temperature buffer growth was measured by atomic force microscopy (AFM), and its effect on thick GaN film was characterized by double crystal X-ray diffractometry (DCXRD) and electron microscopy techniques (SEM and TEM). Direct correlation between AFM roughness (in terms of the RMS value) of the buffer layer surface and crystalline quality of the GaN film was observed. It is suggested that the smooth surface of low temperature grown GaN is critical in obtaining good quality GaN film in HVPE.

Structural evolution during mechanical alloying and annealing of a Nb-25at%Al alloy

Mixtures of pure elemental Al and Nb powders of Nb-25at%Al composition was mechanically alloyed, and structural evolution during high energy ball milling has been examined. Al dissolved in Nb from the early stage of the ball milling, and amorphization became noticeable after longer than five hours of milling. However the dissolution of Al in Nb was not completed before the amorphization. No intermetallic phase formed during the mechanical alloying. Before complete amorphization, metastable nitride of Nb4.62N2.14 (i.e., β-NbN) with hexagonal structure has formed in nanocrystalline size through nitrogen incorporation from ambient environment. The lattice parameter of Nb increased significantly (up to 3.3433 Å after 5 hours of milling) during the milling. Upon annealing above 950 °C, Nb2Al became the dominant feature with the β-NbN, and Nb3Al did not form from the samples milled at ambient environment. Nb3Al appeared only from a sample milled at Ar environment. Structural evolution during mechanical alloying of the Nb-Al system is critically dependent the upon milling environment.

High strain rate superplasticity of an ultra-fine grained Al-Ti-Fe alloy

Transition metals have limited solubility in aluminum, and alloying content higher than the solubility limit results in high melting point aluminides. When Fe is added into a hyper peritectic Al-Ti alloy, Al{sub x}Fe(X = 3 or 6) aluminide forms by an eutectic reaction at 655 C, and thus the Al{sub x}Fe phase could be an additional reinforcement to the Al-Ti system. The presence of the secondary Al{sub x}Fe particles is expected to enhance superplasticity by promoting finer grain size during PM processing and by retarding grain growth during deformation at elevated temperatures. The purpose of this study is to investigate HSRS behavior of an Fe-added Al-10wt%Ti alloy prepared by powder metallurgy and to compare it with that of similarly processed Al-10wt%Ti alloy.

Microstructure ofin situ MoSi2/SiC nanocomposite coating formed on Mo substrate by displacement reaction

The microstructure of an in-situ Mosi2/β-SiC nanocomposite coating formed by the solid-state displacement reactions of Si deposited by chemical vapor deposition (CVD) with Mo-carbide layers at 1100°C, which had previously been formed on the surface of a Mo substrate by a CVD process, was investigated. The Mo-carbide layers formed by the simultaneous CVD of Mo and carbon at 900°C for 5 h using a gas mixture of C2H4−MoCl5−Ar consisted of two layers, an inner layer of Mo2C and an outer layer of MoC. While the monolithic MoSi2 coating showed a typical colummar microstructure perpendicular to the Mo substrate, the MoSi2/β-SiC nanocomposite coating formed by the solid-state displacement reactions between the Mo-carbide layers and Si was composed of equiaxed MoSi2 grains with an average size of 150–500 nm and β-SiC particles with an average size of 80–105 nm. The β-SiC particles exhibited an oblate-spheroidal shape and were located mostly at the grain boundaries of MoSi2. The volume percentage of β-SiC particles ranged from 18.5 to 29.2% with respect to the carbon concentration in Mo-carbide layers.

New Pretreatment Method of Sapphire for GaN Deposition

It has been confirmed that the reactive ion (N+2) beam (RIB) pretreatment of the sapphire substrate at room temperature is an alternative pretreatment method. The chemical and physical status of RIB treated sapphire surface results in the etching of the surface and the formation of a very thin amorphous-like disordered AlON layer under the sapphire surface. The threading dislocation density of GaN on Al2O3(0001) with RIB pretreatment was decreased due to the partial crystallization of the RIB layer during high temperature main growth of GaN. The crystallized region may contribute to the preferential nucleation site for GaN, promoting the 2-D growth mode. In addition, the remaining amorphous layer may absorb lattice strain originating from the lattice misfit between sapphire and GaN film. The optical properties of GaN films have improved with RIB pretreatment. Current observation clearly shows that the RIB pretreatment of the sapphire surface can be used to improve the GaN films grown by metalorganic chemical vapor deposition (MOCVD).

Determination of titanium solubility in alpha-aluminum during high energy milling

Recent studies on mechanical alloying (MA) have drawn special attention due to two major reasons. Firstly, significantly higher solid solubility is achieved compared to what rapid solidification could do. Secondly, formation of various metastable phases, or even amorphous phase, is observed in certain systems at different stages of mechanical alloying. The most common method to evaluate the formation of solid solution during MA is the X-ray diffraction method (XRD). In this study, Ti solubility in alpha-Al is carefully estimated utilizing XRD spectra, and reliability issue of the methods to characterize MA powders with nanosized particles is addressed. For the analysis, hyper peritectic compositions of Al-Ti are ball milled up to 15 hours, in which time the MA process is considered to reach certain steady state.

Temperature dependence of threshold stress for aluminum-based materials exhibiting high strain rate superplasticity

High strain rate superplasticity has been mostly demonstrated in aluminum-base powder metallurgy materials. The superplasticity has been illustrated by incorporating threshold stress, which decreases apparent flow stress, and is commonly interpreted as an extension of usual fine-grain superplasticity. That is, majority of deformation takes place in grain boundaries, meaning grain boundary sliding is the governing mechanism. Dispersion particles or whiskers seem to cause the threshold stress, whose value shows strong temperature dependence. In order to understand the exact nature of the temperature dependence, threshold stress data appeared in literature has been reanalyzed using linear and exponential functions of temperature. The modulus-compensated threshold stress data fit well with the Arrhenius plot, and two modes of thermally activated process are suggested. One is dominant at lower temperature ranges, and has an activation energy of about 50 kJ/mole and an interaction between moving dislocations and particles or interfaces is considered as the origin of the threshold stress. Another one is operational at very high temperatures near incipient melting condition, which exhibits considerably higher activation energy. Referring to the high activation energy and serrated flow curves, solute drag against gliding dislocations has been suggested as a possible source for the second threshold stress.

The effect of Ca addition on viscosity and electrochemical properties of Mg-alloys produced by casting

Summary form only given. The composition of different Mg alloys is known to affect their current capacity, potential, and anode efficiency. Many alloying elements have been used in attempts to improve the electrochemical properties of magnesium anodes. Significant improvements of electrochemical properties have been achieved by controlling the adverse effects of impurity elements such as Fe, Ni, Cu with alloying elements. Out of many elements, Ca is considered as a very effective element that can improve the electrochemical properties of Mg-alloys because of its relatively low potential in comparison with specified elements such as Mn, Al, Zn in high Mn alloys or AZ63 alloys with the effect of grain refining. Ca has recently been used as a common inhibitor for the ignition of molten Mg alloys. However, the viscosity of pure Mg is markedly increased with increasing Ca content. Ca is responsible for making the casting of Mg alloys from Mg melt difficult at desirable pouring temperatures. In the present study, the effect of Ca addition on the viscosity and electrochemical properties of Mg-Ca alloys is investigated. Viscosity as well as electrochemical data will be correlated with chemical composition of impurities, and the microstructural change before and after Ca is added.