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Growth behavior and microstructure of oxide scales formed on WSi 2 coating by isothermal oxidation between 800 and 1300 °C were investigated. At and below 1000 °C, lamellar WO 3 embedded in the amorphous SiO 2 matrix was formed from the direct oxidation of WSi 2 by inward diffusion of oxygen through short-circuit paths. At the intermediate temperatures between 1100 and 1200 °C, fastest oxidation rate was observed. The oxide scale consisted of the spheroidal WO 3 particles embedded in the amorphous SiO 2 matrix. WSi 2 was initially oxidized to form W 5 Si 3 and SiO 2 followed by the oxidation of W 5 Si 3 to form WO 3 and SiO 2 phases. At 1300 °C, oxide scale consisted of continuous cristobalite SiO 2 and W 5 Si 3 layer underneath, leading to the slowest isothermal oxidation behavior. It is concluded that the interplay of phase stability of WSi 2 , defect formation process, and viscosity of SiO 2 gives rise to unique and complex oxidation behavior especially at intermediate temperatures.

An attempt was made to coat bio-active calcium phosphate on a titanium substrate by room temperature blasted-coating process as an alternative for conventional thermal spray or ion beam-assisted coating techniques. Thickness, surface roughness, crystallinity, and Ca and P-dissolution behavior of the coated layer were investigated as a function of pressure and time of the blasted coating process. The blasted coating process offered comparable coating characteristics with those of conventional techniques, including a thickness of 1.3 pm∼ 6.3 pm, and a surface roughness of 1.5∼ 2.3 μm. Furthermore, the blasted coating layer showed higher crystallinity and more favorable dissolution behavior (i.e., constant and long-lasting release of Ca and P) than those of the convention thermal processes. Based on the comparable or superior characteristics of the blasted coating process at room temperature, it was suggested as a useful technique for calcium phosphate coating on Ti-based medical device.

Dense nanostructured TiSi 2 –SiC composite was synthesized by high-frequency-induced heated combustion synthesis method within 1 min in one step from TiC and 3Si powders. Simultaneous combustion synthesis and densification were accomplished under the combined effects of an induced current and mechanical pressure. Highly dense TiSi 2 –SiC with relative density of up to 99% was produced under the simultaneous application of a 60 MPa pressure and the induced current. The mechanical properties of the composite were investigated.

Bulk metallic glasses are adequate for micro-forming because of the viscous flow in the supercooled liquid state. The deformation behavior of Zr 41 Ti 14 Cu 12.5 Ni 10 Be 22.5 alloy under compression in the supercooled liquid state were investigated by using DSC and TMA. Based on the deformation behavior, micro-forming process of Zr 41 Ti 14 Cu 12.5 Ni 10 Be 22.5 alloy was designed and carried out. The surface of the deformed specimens were analyzed by SEM, XRD, and AFM. The roughness of the specimen surface exhibited a correlation with the total deformation and a new parameter reflecting roughness evolution of the deformed surface was suggested as the relative comparison among different forming conditions.

The crystallization behavior of Cu and Zr-base Bulk Metallic Glass were investigated using the electrical resistivity measurements in isothermal annealing. Electrical resistivity evolutions exhibited one or two stage of resistivity reduction according to additional elements respectively. In order to analyze the electrical resistivity reduction, micro structure evolutions were analyzed using DSC, XRD, and TEM.

The microstructure and oxidation resistance of MSi2-SiC or MSi2-Si3N4 nanocomposite coatings (M = Mo, W, Nb, Ta) on M substrates formed by displacement reactions between M-carbides or M–nitrides and silicon, respectively, was investigated. Present study demonstrated that the crack density formed in the MSi2-base nanocomposite coatings due to mismatch in the coefficient of thermal expansion between nanocomposite coatings and M substrates could be controlled by adjusting the volume fraction of the SiC or Si3N4 reinforcing particles with the low CTE values. The high- and low-temperature oxidation resistance of nanocomposite coatings was superior to that of monolithic MSi2 coatings.

Plasma electrolytic oxidation (PEO) has drawn attention and been studied intensively all through the world. The thick ceramic coatings fabricated by the technique exhibit excellent properties, including hardness and wear resistance, thermal and electrical insulation, and corrosion resistance, due to the characteristic phase composition and microstructure of the coating layers. However, most of the studies have dealt with manufacturing process itself and the apparent properties of coating layers and researches on the microstructural basis including transmission electron microscopy analysis are limited so far. In this investigation, a basic approach to PEO process was tried, adapting time-potential behavior analysis under constant current mode (galvanostatic) oxidation, and microstructural analysis on the coating structure, including X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The time-potential behavior analysis under constant current DC was carried out, and the resultant evolution of the microstructure was characterized..

The crystallization behavior of Cu 43 Zr 43 Al 7 Ag 7 (numbers indicate at.%) bulk metallic glass was investigated using the isothermal electrical resistivity measurements at 450 °C in the supercooled liquid region. The crystallization process is a single step phase transformation. To analyze the electrical resistivity reduction, microstructure evolutions were analyzed using differential scanning calorimetry, X-ray diffraction, transmission electron microscopy and small-angle X-ray scattering. The Avrami parameter of the electrical resistivity reduction step was 1.73, indicating that the crystallization process is a diffusion-controlled growth of intermetallic compounds with decreasing nucleation rate.

By the high frequency induction heated combustion synthesis (HFIHCS) method, dense nanostructured TaSi2-SiC composite was synthesized within 2 minutes and in a single step from powders of TaC and 3Si. Simultaneous combustion synthesis and densification were accomplished under the combined effects of an induced current and mechanical pressure. Highly dense TaSi2-SiC with relative density of up to 97% were produced under a 60MPa pressure and induced current. The average grain size and mechanical properties (hardness and fracture toughness) of the composite were investigated.

Dense nanostructured NbSi 2 –SiC composite was synthesized by high-frequency induction heated combustion synthesis (HFIHCS) method within 2 min in one step from powders of NbC and 3Si. Simultaneous combustion synthesis and densification were accomplished under the combined effects of an induced current and mechanical pressure. Highly dense NbSi 2 –SiC with relative density of up to 99.8% were produced under simultaneous application of a 60 MPa pressure and the induced current. The average grain size and mechanical properties (hardness and fracture toughness) of the composite were investigated.

The microstructure and oxidation property of NbSi2/α-Si3N4 nanocomposite coating formed on Nb substrate by a prior nitridation process followed by pack siliconizing process were investigated. The Nb-nitride layers formed by nitridation process at 1300 °C consisted of two layers, i.e. the outer layer of mixed nitrides of NbN and Nb4N3 and the inner layer of Nb2N. While the monolithic NbSi2 coating showed the typical columnar microstructure perpendicular to the Nb substrate, the NbSi2/α-Si3N4 nanocomposite coating formed by the solid-state displacement reaction of the outer Nb-nitride layer with Si was composed of the equiaxed NbSi2 grains and the α-Si3N4 particles with the oblate-spheroidal shape, which were mostly located at the grain boundaries of NbSi2. The average size of equiaxed NbSi2 grains and α-Si3N4 particles were about 44–125 and 33–45 nm, respectively. The nanocomposite coating formed by the solid-state displacement reaction of the inner Nb-nitride layer with Si showed the columnar microstructure. The average diameters of NbSi2 grains and the α-Si3N4 particles were about 255 and 43 nm, respectively. The volume percentage of α-Si3N4 particles ranged from 16.8 to 24.4% with respect to nitrogen concentration in Nb-nitride layers. No cracks were observed in the nanocomposite coating, indicating that its thermal expansion coefficient was close to that of Nb substrate. The isothermal oxidation resistance of NbSi2/α-Si3N4 nanocomposite coating in 80% Ar–20% O2 atmosphere at 1100 °C was superior to that of monolithic NbSi2 coating.

The crystallization behavior of the Zr 41 Ti 14 Cu 12.5 Ni 10 Be 22.5 bulk metallic glass was investigated using the isothermal electrical resistivity measurements at 390 °C in the super-cooled liquid region. The crystallization process is completed through two sequential phase transformations. The first phase transformation was the formation of icosahedral phases, which is followed by the formation of intermetallic compounds.

Growth behavior and microstructure of oxide scale formed on MoSi2 coating by cyclic oxidation testing in air at 500 °C were investigated using field emission scanning electron microscopy, cross-sectional transmission electron microscopy, glancing angle x-ray diffraction, and x-ray photoelectron spectroscopy. MoSi2 coating was prepared by chemical vapor deposition of Si on a Mo substrate at 1100 °C for 5 h using SiCl4–H2 precursor gas mixtures. After the incubation period of about 454 cycles, accelerated oxidation behavior was observed in MoSi2 coating and the weight gain increased linearly with increasing oxidation cycles. Microstructural analyses revealed that pest oxide scale was formed in three sequential processes. Initially, nanometer-sized crystalline Mo4O11 particles were formed with an amorphous SiO2 matrix at MoSi2 interface region. Inward diffusing oxygen reacted with Mo4O11 to form Mo9O26 nano-sized particles. At final stage of oxidation, MoO3 was formed from Mo9O26 with oxygen and growth of MoO3 took place forming massive precipitates with irregular and wavy shapes. The internal stress caused by the growth of massive MoO3 precipitates and the volatilization of MoO3 was attributed to the formation of many lateral cracks into the matrix leading to pest oxidation of MoSi2 coating.

The mechanically alloyed (Al + 12.5 at.% Cu) 3 Zr powders were consolidated by cold isostatic pressing (CIP) and subsequent sintering. Effects of CIP pressure and sintering temperature on the stability of metastable L1 2 phase and nanocrystalline structure were investigated. Before sintering, the powders were CIPed at 138, 207, 276, and 414 MPa. The relative densities of the CIP compacts were not greatly affected by the CIP pressure. However, the L1 2 phase of the specimen CIPed at pressures greater than 276 MPa was partially transformed into D0 23 . The optimum consolidation conditions for maintaining L1 2 phase and nanocrystalline microstructure were determined to be CIP at 207 MPa and sintering at 800 °C for 1 h for which the grain size was 34.2 nm and the relative density was 93.8%. Full density specimens could be prepared by sintering above 900 °C, however, these specimens consisted of L1 2 and D0 23 phases. The grain sizes of all the specimens were confirmed by TEM and XRD, and were found to be less than 40 nm. This is one of the smallest grain sizes ever reported in trialuminide intermetallic compounds.

Effects of Mn addition on microstructure, phase stability and mechanical properties of (Al+ x at.%Mn) 3 Ti intermetallic compounds fabricated by mechanical alloying and spark plasma sintering were investigated. As-milled ternary powders consist of homogeneous distribution of fine crystallites in amorphous matrix. (Al+0.08 Mn) 3 Ti showed the most stable thermal stability with average grain sizes of less than 60 and 100 nm after heat treatments at 900 and 1100 °C, respectively. The reduction of grain size down to nano-scale was not sufficient enough to increase the fracture toughness of trialuminide. Fracture toughness was also remained relatively constant and showed no significant dependence on Mn concentration. Microhardness was generally increased with increasing Mn concentration and decreased with increasing heat treatment temperatures due to the grain size effects. The formation of L1 2 phase played a major role in increased fracture toughness and (Al+0.08 Mn) 3 Ti showed the highest fracture toughness of 4.12 MPa m 1/2 with only L1 2 phase formation.

The low-temperature cyclic oxidation resistance of MoSi 2 /19.3 vol% SiC nanocomposite coating formed on a Mo substrate in air at 500°C was investigated and compared with that of the monolithic MoSi 2 coating using field emission-scanning electron microscopy (SEM) and crosssectional transmission electron microscopy (XTEM). The nanocomposite coating was produced by a prior carburizing process followed by chemical vapor deposition of Si on a Mo substrate. While the accelerated oxidation behavior was observed for the monolithic MoSi 2 coating after the incubation time of about 454 cycles, no pest oxidation was observed in the nanocomposite coating. The excellent low-temperature cyclic oxidation resistance of nanocomposite coating resulted from the deceleration of further inward diffusion of oxygen by formation of relatively dense SiO 2 and Mo 9 O 2 6 composite oxide scale through the preferential oxidation of SiC particles followed by oxidation of MoSi 2 phase.

A WSi2/(19.3–32.9) vol.% β-SiC nanocomposite coating was formed by chemical vapor deposition of Si on the W-carbide layers. The nanocomposite coating consisted of the equiaxed WSi2 grains with the average size of 88–153 nm and the β-SiC particles with the average size of 37–65 nm, which were mostly located at the grain boundaries of WSi2.

The microstructures and growth kinetics of the Mo 5 Si 3 and Mo 3 Si layers in the MoSi 2 /Mo diffusion couple was investigated using optical microscopy, field-emission scanning electron microscopy, electron probe microanalyzer, and cross-sectional transmission electron microscopy. The MoSi 2 /Mo diffusion couple was made by chemical vapor deposition CVD of Si on a Mo substrate at 1100 °C and annealed at temperatures between 1250 and 1600 °C in an Ar atmosphere. Simultaneous parabolic growth of the Mo 5 Si 3 and Mo 3 Si layers was observed at the early annealing stage of the MoSi 2 /Mo diffusion couple. From the marker experiments using ZrO 2 particles, the growth mechanism of the Mo 5 Si 3 and Mo 3 Si layers and the dominant diffusion element in each phase were revealed. The difference in the growth rates of Mo 3 Si layer between the MoSi 2 /Mo diffusion couple and the Mo 5 Si 3 /Mo diffusion couple was explained by a multiple layer growth model.

This paper reports the results of experimental studies on the relationship between grain boundary character distribution and the fracture toughness in the polycrystalline NiAl. The fracture toughness has been investigated by the ring on ring disk bend test in conjunction with the acoustic emission (AE) measurement, and the grain boundary character distribution is measured by the electron back-scatter diffraction pattern (EBSD or EBSP) technique. The fracture toughness of the annealed specimen (1473 K, 20 h) is 6.42±0.88 (MPa m−1/2) and that of the forged (1473 K) and annealed (1473 K, 20 h) specimen is 5.08±0.36 (MPa m−1/2). The strong boundary and weak boundary were decided by comparing the distribution of grain boundary characteristics at the intergranular cracks to the distribution in general population. Σ1, Σ3 and Σ5 boundaries have strong and the good crack resistance, but Σ7, Σ11, Σ13, Σ21 and Σ23 boundaries are relatively weak boundary in the polycrystalline NiAl. Consequently, the annealing process makes the fraction of Σ1 and Σ5 boundaries higher than forging and annealing process. The high fraction of Σ1 and Σ5 boundaries that have the good crack resistance seem to affect the increase of fracture toughness in the annealed specimen.

Structural evolution during mechanical alloying and subsequent heat treatment of Al65Cu20Fe15 and Al65Cu20Fe10Si5 was studied by X-ray diffractometry (XRD), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The Al65Cu20Fe15 and Al65Cu20Fe10Si5 powders milled for 10 h showed a layered structure consisting of Al-, Cu- and Fe-rich layers. DSC traces obtained from the powder during heating, up to 600°C, showed two exothermic peaks with peak temperatures of 330 and 440°C in Al65Cu20Fe15 and 330 and 500°C in Al65Cu20Fe10Si5 powders. The lower exothermic peaks correspond to the formation of Al2Cu and Al7Cu2Fe phases from the layered structure. The second high temperature exotherms correspond to the formation of Al(Fe,Cu) and 1/1 cubic approximant in the Al65Cu20Fe15 powder and to the formation of Al13Fe4 and 1/1 cubic approximant in the Al65Cu20Fe10Si5 powder. The Al65Cu20Fe15 and Al65Cu20Fe10Si5 powders annealed for 5 h at 750°C showed microstructure consisting of Al(Cu,Fe) and Al13Fe4 phases, and Al13Fe4, icosahedral and new approximant phases, respectively. The partial substitution of Fe by Si increased the stability of the 1/1 cubic approximant and icosahedral phases.

The high thermal conductivity, high melting point and low density of NiAl alloys compared to Ni-based superalloys make attractive materials for high temperature structural applications. The lack of high temperature strength, however, has been one of the barriers which limits the use of NiAl alloys. The use of these alloys in structures will be severely hampered, unless this problem can be overcome. Consequently, a large amount of effort has been devoted to the improvement of the various measures of high temperature strength, e.g., precipitation strengthening. In order to obtain better understanding of the effects of iron on the properties of NiAl, a systematic study of its effects on both physical and mechanical properties of NiAl has ben undertaken. Of specific interest are the correlation of point defects (thermal vacancies, constitutional vacancies and anti-site atoms) and Fe precipitates and the variation of the concentration of point defects along with iron content, substitution scheme (for Ni, for Al and for both Ni and Al) and temperature. The effects of vacancies on Fe precipitation in NiAlFe have been studied by using magnetic susceptibility measurements and transmission electron microscopy.

The phenomenon of cyclic creep acceleration (CCA) is well known. Also, it has been observed experimentally that there exists a peak stress level above which CCA begins to occur. For copper the value of this peak stress is known to be independent of cyclic stress conditions such as stress amplitude and frequency. This stress level or the threshold stress was observed to increase with temperature in the range 0.4 to 0.5T m. A model is suggested to predict the threshold stress level and to explain the temperature dependence of the stress. This model is based on the dislocation core diffusion process by the athermally generated excess vacancies. The concept of critical length of dislocation core is used for core diffusion fast enough to enhance dislocation climb. This critical length is expressed as a function of stress and temperature. From these concepts, the threshold stress is expressed asτ/G - A exp (-ΔQ/RT). The experimentally observed values of the threshold stress are in good agreement with this model, and the temperature dependence is also experimentally proved.

The difference in the grain boundary deformation between statically and cyclically crept specimens of Al-Mg solid solution has been investigated at the temperature of 580 K and for the peak stress level of 15 to 20 M Pa. In statically crept specimens, the grain boundaries deform irregularly and no crack is formed either at the triple point or along the serrated boundaries. However, in cyclically crept specimens, where the stress frequency, stress amplitude and the ratio of on-load to off-load time are 3 cycles per minute, 90% of maximum peak stress and less than 1, respectively, the grain boundaries remain smooth and wedge-type cracks are formed at the triple points, which results in intercrystalline fracture. On the basis of the experimental observations it is believed that cyclic stressing enhances grain boundary sliding through an accelerated recovery with the help of mechanically generated excess vacancies during cycling. However, due to the constraints of the grain alignment, boundary sliding becomes very difficult and creates an intercrystalline fracture at a triple point. On the other hand, under static stress, since the grain boundary is serrated to decrease the stress concentration at a triple point, a crack hardly forms at the triple point.