C.G. Park

Chemical driving force for dislocation motion in hydrogen embrittlement

Abstract Two major effects of hydrogen in metals, particularly iron and its alloys, are proposed here. The first is the effect of hydrogen on plastic behavior. It is proposed that during charging or discharging a large chemical driving force can generate and move dislocations to facilitate hydrogen transport. The second is the effect of hydrogen on fracture. It is proposed that the dislocations emitted by the chemical driving force can exert a sufficient stress intensity factor at the crack tip to cause propagation along the slip plane, even without any externally applied stress.

Microstructure and amorphization induced by frictional work in Fe–Cr–B alloy thermal spray coatings

Abstract The microstructure and friction-induced amorphization of Fe–Cr–B alloy thermal spray coatings were investigated. Thermal spraying was carried out using a detonation gun with various fuel gas contents. The Fe–Cr–B alloy coatings were made up of Cr-rich boride particles embedded in the matrix of an Fe–Cr solid solution phase. Microstructural investigations using transmission electron microscopy (TEM) and Auger elemental analysis indicate that a friction-induced amorphization reaction occurs in the Fe–Cr solid solution phase which is supersaturated with boron. The boron concentration in the Fe–Cr solid solution phase may play an essential role in the stabilization of the amorphous phase formed by frictional work. The formation of oxide inclusions seems to impede the crystalline-to-amorphous transformation by reducing the boron concentration in the Fe–Cr solid solution phase.

Microstructure and wear-resistance of Fe-Cr-B base metamorphic alloys

Metamorphic (crystalline-to-amorphous) transition and consequent microstructural change were investigated in relation with the wear-resistance during the wear process of Fe-base metamorphic coating layer. The spray coating was accomplished using a detonation gun with various fuel gas conditions. The spray coated layer exhibited significantly improved wear resistance, which is about 45 times higher than that of the non-coated substrate. The significantly low friction coefficient and high hardness, resulting from the presence of amorphous surface film, seems to be the main cause of the improved wear-resistance. Electron microscopy on the cross-section of the coated layer exhibited intra-particle micro-segregations associated with rod-shape Cr and B rich particles and Fe-Cr solid solution phase. From the observations using TEM and EDS, Fe-Cr solid solution with supersaturated B and Si content seems to be the main phase contributing to the crystalline-to-amorphous transition during wear.

Friction-induced amorphous phase formation observed in Fe-Cr-B-Ni-Mo alloy thermal spray coatings

Almost all surfaces prepared by mechanical preparation techniques contain a modified surface layer which is caused by mechanically induced deformation and heating. The formation of a modified surface layer is of great importance because this layer usually exhibits unique mechanical properties quite different from those of the original bulk materials and most material failures originate at the surface either by fatigue or wear. The purpose of the present study is to investigate the effects of the microstructure of the coating, the phase fraction and the phase composition, on the amorphization induced by frictional work. Fe-Cr-B-Ni-Mo alloy powder with particle sizes 5 to 45 {micro}m in diameter was used in the present work.

Precise determination of fatigue crack closure in Al alloys

Abstract The crack closure phenomenon is important when evaluating the effective driving force for crack growth. Hence, the precise measurement of the crack closure load is an essential preprequisite for the determination of the effective stress intensity factor range (ΔKeff). In the present investigation, an acoustic emission (AE) technique was employed to measure crack closure during the fatigue cycling of Al alloys and the results are compared with those obtained by the crack opening displacement (COD) method, the back face strain (BFS) gauge method and the surface strain (SS) gauge method. Results show that the intrinsic fatigue crack growth rates (da/dN vs. ΔKeff) obtained by the AE technique gave the best fit for the high stress ratio (R = 0.8) test, which implies that the AE technique should be considered as a reliable crack closure measurement method.

Theoretical expectation of strip thickness in planar flow casting process

Abstract The objective of this study is to obtain a simple analytical solution that will predict the strip thickness with given planar flow casting (PFC) processing variables. Mass balance and the Bernoulli equation are used to formulate the final analytical solution. The focus is on the evaluation of the friction loss (dissipation of mechanical energy over the volume of the entire flow field) in the Bernoulli equation. Friction loss in the PFC process can be attributed to (1) a sudden contraction of the melt path (crucible to slit), (2) flow through a slit and, finally (3) an abrupt change in the size and direction of the melt path at the gap between the nozzle and the roll. Among these causes, friction loss resulting from an abrupt change in the size and direction of the melt path at the gap is the most significant factor under PFC conditions. Our experimental results show that the effects of the gap on the friction loss factor can be expressed as ef,gap = 0.172 (h/b)−2.5, where h and b represent the gap size between the nozzle and the roll, and the slit breadth respectively. The accuracy of our formulation is evaluated by comparing the experimental data with theoretical expectation.

A microstructural study of indium-indium oxide composite films

We have investigated the microstructure of indium‐indium oxide composite films prepared by thermal vapor deposition in the oxygen atmosphere. The films show granular‐amorphous structural phase transformation near the oxygen concentration of 2 × 10−4 Torr. Although the thermal evaporation generates films with indium grains much larger in size than the reactive sputter deposition, the film resistivity shows the characteristics of the homogeneous films as long as the thickness is larger than the grain size.

A low-resistance Pd/Ge/Ti/Au ohmic contact to a high-low doped GaAs field-effect transistor

Abstract Pd/Ge/Ti/Au ohmic contacts have been studied for application to high-low doped GaAs metal-semiconductor field-effect transistors (MESFETs). The interfacial reaction of the Pd/Ge/Ti/Au contact is investigated using X-ray diffraction, Auger depth profile, and transmission electron microscopy. The good Pd/Ge/Ti/Au ohmic contact with the lowest contact resistivity of 2.8 × 10 −6 Ωcm 2 is obtained after annealing at 340 °C, which is two times lower than that of the Pd/Ge contact. This is due to formation of AuGa through fast in-diffusion of Au toward the GaAs substrate. The AuGa compound enhances creation of more Ga vacancies, followed by incorporation of Ge into the Ga vacancies, and it allows the contact to be formed directly on the buried high-doped GaAs layer. The MESFET with the Pd/Ge/Ti/Au ohmic contact displays good d.c. characteristics. This supports the fact that the Pd/Ge/Ti/Au ohmic contact is well suitable for application to high-low doped GaAs MESFETs due to its low-resistance characteristics and good surface morphology.

Dislocation structure associated with deformation behavior of Fe3Al alloys

Abstract The aim of the present investigation is to correlate the change of tensile yield strength with the dislocation structure of Fe-23Al and Fe-30Al alloys tested at room and high temperatures. The results showed that the yield strength of Fe-23Al alloy increased with increasing the quench and test temperatures. This was mainly attributed to the increase in the partition of imperfect superdislocations, leaving NNN and NN type APB trails behind during the deformation. In Fe-30Al alloy, the yield strength increased with increasing the test temperature, but decreased with increasing the quench temperature. The lower yield strength of the specimen quenched at 500 ° C was thought to be due to relatively lower NNN APB energy. It is considered that the yield strength of Fe-30Al alloy is governed by two competing factors; the partition of perfect and imperfect variants of dislocations and the shear stress required for the dislocation movement.

The interactions between Si/Co films and GaAs(001) substrates

Interfacial reactions of Si/Co films on (001) oriented GaAs substrate, in the temperature range 300–700°C for 30 min, have been investigated using a combination of x-ray diffraction, Auger electron spectroscopy, and transmission electron microscopy. Cobalt starts to react with GaAs and Si at 380°C by formation of Co2GaAs, and Co2Si phases, respectively. At 420°C, the entire layer of Co is consumed, and the layer structure is observed with the sequence Si/CoSi/CoGa(CoAs)/Co2GaAs/GaAs. Contacts produced in this annealing regime are rectifying and the Schottky barrier heights increase from 0.69 eV(as-deposited state) up to 0.81 eV (420°C). In the subsequent reaction, CoSi grows at the expense of the decompositions of CoGa and CoAs at 460°C. In addition, the ternary phase also is decomposed and only the CoSi phase remains upon the GaAs surface at 600°C. Contacts produced at higher temperature regime (>460°C) have low barriers. The interface between CoSi and GaAs is stable up to 700°C. The results of interfacial reactions can be understood from the calculated Si−Co−Ga−As quaternary phase diagram.