발간논문

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Mechanical alloying of Al-8wt%Fe alloy powders has been studied to understand the mechanical alloying process. Various analytical techniques including microvickers hardness test, SEM, optical microscopy, and X-ray diffraction were used to find optimum conditions for mechanical alloying. On the bases of these optimum conditions, Al-8wt%Fe powders with Mo and Si have been mechanically alloyed to investigate the effects of alloying elements. The mechanically alloyed powders were hot pressed and heat treated from 250℃ to 650℃ for various times to study the morphologies, sizes, and the kinds of precipitates. In the case of Al-8wt%Fe and Al-8wt%Fe-1.5wt%Mo, the precipitate was Al₃Fe, and in the case of Al-8wt%Fe-1.5wt%Si alloy were Al₃Fe and silicide. High temperature (250℃, 350℃) tensile strengths of the hot extruded mechanically alloyed powders were superior to those of conventional precipitation hardened Al alloy. This superiority was due to effects of thermally stable intermetallic compounds, oxide, and carbide.

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As the residual stress in the ZrO₂ coating affects adhesion strength, toughness and strength of the plasma spray coated layer, it is important to predict the variation in residual stress distribution. The residual stresses and distortions are important parameters to determine optimum spraying condition. A stress model is proposed to predict the trend for residual stress developed in the plasma spray coating. Residual stresses based on this model are computed by finite element method. Results of the analysis of the plasma sprayed ZrO₂ coating by finite element analysis were compared with XRD experimental observations in PSZ coated layer. Residual stresses (radial, circumferential and axial stresses) which remains compressive mainly decrease with coating depth and turns into tensile state near the substrate/coating interface. In the direction of axes, relatively large shear stress develops in the ZrO₂ coating.

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A smooth specimen subjected to cyclic loading undergoes different stages of crack growth behavior; three distinct crack growth regimes exist, namely the regime of microstructually small cracks, the regime of physically small cracks and the long crack regime. The limits of these different crack growth regimes can be evaluated in terms of characteristic crack lengths which are functions of material and mechanical parameters. A log △σ-log a diagram is used to derive estimates of these characteristic. crack lengths. The approach presented relies on relations between material strength and grain size, on the one side, and on fracture mechanics quantities depending on the applied load such as the size of the plastic zone ahead of the crack tip, on the other side.

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To clarify the character of the relationship between the threshold stress and coherence, Al-Al₃Zr alloy with coherent particles was chosen in this work. High temperature creep test particle strengthened Al-Al₃Zr alloy was carried out by means of stress relaxation test at 573K, 623K and 673K. For the experiments, an Al-0.55wt%Zr alloy was melted in Ar gas atmosphere and then chilled to an ingot. The alloy was strengthened by coherent particles, β`(Al₃Zr) with Ll₂ structure. It was found that there exist the threshold stress during stress relaxation test and the threshold stress, σ_(th) results from the presence of β`(Al_Zr) particles. It was also obtained that the stress are approximately equal to Orowan stress, σ_(or) at 573K but 0.4σ_(or), at 673K. The temperature dependence of threshold stress might be due to the interaction between dislocations and particles. The ratio of σ_(th) and σ_(or) decreases gradually with increasing temperature. During local climb a considerable amount of length of dislocation increase lead to the threshold stress which depends on particle shape and is roughly estimated by σ_(th) =(0.4∼0.9)σ_(or). The activation energy for relaxation creep is found to be very close to that for pipe diffusion of pure Al. Observation of deformed specimens by transmission electron microscopy(TEM) suggests that dislocation and particles interact as a repulsive type.

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The mechanical alloying is an unique process in that it is an entirely solid state process, permitting dispersion of insoluble phases such as refractory oxides and addition of reactive alloying elements into base alloys. Fine grain size and high density dislocation substructure produced during mechanical alloying process promote the inter diffusion kinetics through defect regions such as dislocations and grain boundaries. In this study, the pipe diffusion and grain boundary diffusion are found to be more dominant than lattice diffusion. The grain boundary diffusion in Al-4Mg system becomes the controlling mechanism during mechanical alloying process because the process makes the grain size of the alloy particles as small as possible as the process time becomes longer.

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Various yield criteria for describing densification and deformation processes of powder compacts and sintered metals have been reviewed and compared with a yield criterion proposed by the present authors. Various parameters involed in the criterion have been discussed. The yield criterion satisfies experimental yield curves from combined tension and torsion tests of sintered porous iron specimens.

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Joining of 304 stainless steel to Si3N4 ceramic using Cu-34.8%Ag-6.2%Ti brazing alloy was made in vacuum furnace. Effects of Al and Sn addition in Cu-Ag-Ti alloy, brazing temperature and brazing time on interfacial reactions and bonding strength of Si3N4-304 joints were investigated. Bonding strength was determined by fracture shear loading and high shear strength value of more than 202 MPa were obtained for joints brazed with Cu-Ag-Ti-Al alloy at 1163 K for 1.8ks. The strong bonding is attritutable to the formation of M1.5Ti(M; Fe, Cr, Fe/Cr=6.4) and Ti5Si4 by redox reaction between Si3N4 and Ti at Si3N4 ceramic interface. Thickness of interfacial reaction layer was decreased and bonding strength was increased as brazing temperature and brazing time were increased. Types of fracture observed at the strength test of Si3N4/304 joints was classified and schematic diagram of fracture mode with joint strength was proposed.

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Joining of 304 stainless steel to Si₃N₄ ceramic using Cu-Ag-Ti-Al braze alloy was made in vacuum furnace. For the joint showing shear strength value of more than 202 ㎫, reaction products formed at the interface between Si₃N₄ and brazing alloy were investigated by O. M., SEM, EDS, WDS, XRD and X-AEM. The redox reaction of Si₃N₄ with Ti results in the formation of particulate TiN and continuous Ti_5Si₄ layers at the nearest interface. For a Cu-Ag-Ti alloy the FeTiSi on those layers was uniformly formed by solid-state reaction of Ti_5Si₄ with Fe segregated/diffused from stainless steel. But for a Cu-Ag-Ti-Al alloy the M_(1.5)Ti(M : Fe, Cr, Fe/Cr=6.4) compounds were locally formed on the Ti_5Si₄ layer and in the brazement. Titanium irregularly erodes the Si₃N₄ surface, resulting in the formation of local intrusions of TiN particles into the substrate. EDAX analyses showed that Fe and Cr diffused through the glassy phase of Si₃N₄ in the neighborhood of interface.

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Cu-based shape memory alloys are likely to occur intergranular fracture at low strain due to the high elastic anisotropy (A=2 CQ_(44)/(C_(11)-C_(12))=13-15), so their mechanical properties such as fracture strength, ductility and fatigue strength are poor. But a study to improve their properties has been doing extensively because of their easy of manufacturing and relatively low cost than that of Ni-Ti alloys. It has been known that the methods to improve properties of Cu-based shape memory alloys are additional elements method, powder metallurgical method and rapid solidification method etc. In the present work, both of additional elements method added Mn, Ti and powder metallurgical method used to HIP technology are applied simultaneously to improve the properties of the alloy and get the basic data for practical application.

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The precipitation behavior of SiC whisker reinforced 2124 aluminum metal matrix composites was investigated, especially the influence of existing oxides on strengthening was emphasized. Two kinds of composites, one made by powder metallurgical process and the other by squeeze casting one, were used to study the differences in precipitation behavior. The specimens were heat-treated with an appropriate conditions, and microstructural changes were observed by using electron microscopy, DSC and micro hardness test. In the case of PM specimens, the influence of precipitates and hardness were increased by the amount of oxides formed on aluminum powder surface. TEM study shows that the oxides serve as an anchoring point for the mobile dislocation and diffusion path of precipitate formation. Therefore, it is suggested that the presence of oxides formed on aluminum powders, strongly affect the amount of precipitate in Al-SiCw composite.

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