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It has been well known that physical property such as heat resistance and electric conductivity of Al-Zr alloys are closely related with precipitation behavior of Al₃Zr phase. In the present work, the effects of misch metal(Mm) addition on the precipitation process and mechanical property in Al-Zr alloys were studied. The results showed that accelerated ageing at low temperature(623K) and smaller sized Al₃Zr phases in comparison with Mm-free alloy were observed by the Mm addition, resulting in hardness increase. From the DVRC analysis indicated that the precipitation of Al₃Zr phase at low temperature(T $lt; 623K) is promoted by Mm addition. It was attributed to the heterogeneous precipitation of Al₃Zr phase at low temperature.

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A study on the phase transformation of martensite in metastable beta phases induced by sub-zero quenching after solid solution treatment has been done. TEM examination was carried out to characterize the structure of martensite and metastable phase in a commercial titanium alloy, β-C. The results showed that the martensite had a twinned hcp structure. It is believed that this structure is generated initially by a martensitic transformation from the metastable bcc phase which is consisted of tweed structure in matrix.

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The ordered phase was tried to disappear in 6.5% Si steel to reduce the power loss and enhance the economic interests. They were prepared by the conventional casting or spray forming method, and they were investigated using the transmission electron microscopy(TEM). The variation in the methods of preparing the specimen did not result in any difference in suppressing the ordered B2 phase in the Fe-Si binary system. It was almost impossible to suppress this ordered phase keeping the silicon level as high as 6.5% even after the heat treatment at 1000℃ for 24 hr or after hot rolling. It was necessary to change the silicon level and control the cooling rate to prevent the ordering reaction, especially, in cooling after heat treatments. The silicon level of 5.8% was obtained as a maximum desirable value.

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The Mg₂Si precipitates in A356 alloy have been characterized by high resolution transmission electron microscopy with the combined use of a computer-aided image processing system. We observed that G.P.zones, β$quot;, β`, and B` phases aged at 155℃, 175℃, 195℃ and 250℃, respectively. It was found that the orientation relationship between the precipitates and the matrix. The β$quot; phase has a monoclinic structure. The crystallographic orientation relationship is found as follows : (001)_(Al) // (010)_(β$quot;), [3￣10]_(Al) // [001]_(β$quot;), [230]_(Al) // [100]_(β$quot;) and β◎105°. The β` phase has two types of the crystal structure; the hexagonal structure with the lattice parameter a = 0.704 ㎚ and the orthorhombic structure with lattice parameters a = 0.734 ㎚, b = 0.658 ㎚ and c = 0.405 ㎚. The B` phase has a hexagonal structure. The angle between the rod direction of B` phase and the [010] direction of the matrix is 10°.

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This study is concerned with a correlation between fracture toughness and carbide distribution in the ductile-brittle transition temperature region of an SA 508 steel used for nuclear reactor pressure vessels. The micromechanism of cleavage fracture processes involved in void and microcrack formation was identified, and tensile properties and elastic-plastic fracture toughness were measured in the transition temperature region. The fracture toughness data were interpreted using a simple fracture model in which the carbide size distribution quantified using a Weibull distribution was included. This modeling study indicated that the critical nearest-neighbor distance between coarse carbides was an important microstructural factor affecting elastic-plastic fracture toughness, since it had a linear relationship with the critical distance from a precrack tip to a cleavage initiation site. These findings suggested that reducing coarse M₃C carbides and homogeneously distributing fine M₂C carbides were useful ways to improve the fracture toughness in the transition temperature region.

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The relationship between creep deformation and microstructural changes in martensitic 10Cr-MoW steel has been studied. Transmission electron microscopy and image analyser were used to determine the variation of precipitates and martensite lath width size during creep deformation and aging. As precipitates are coarsened during creep deformation, dislocations become easy to move and the recovery proceeds rapidly. This leads to the growth of lath width. The average size of precipitates was linearly increased with creep time. On the other hand the growth rate of lath width is constant until tertiary creep, but the growth of lath width is accelerated during tertiary creep. It has been concluded that the growth behavior of lath width are consistent with creep deformation. Because the growth of lath width is controlled by the coarsening of precipitates it is important to form more stable precipitates in creep condition for improvement of creep properties of martensitic steel. Microstructure of martensitic steel is thermally very stable, so the size of precipitates and martensite lath width are hardly changed during aging.

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Fatigue tests were conducted on a lower bainite steel, which is applicable to vehicle components due to good resistance against crack propagation, under mode I and mixed mode conditions. The fatigue crack propagation path and fracture criterion were investigated from the results of the tests. The crack growth rates were evaluated in terms of effective stress intensity factor range, using both the extended and projected crack lengths. The initial crack branching angles with variation of the ratio of K_I/K_(II) are found to be in good agreement with the maximum tangential stress criteria. The effective stress intensity factor range with the result of growth rate under mode I loading condition can predict the crack growth rate under mixed mode loading conditions. Extended crack length rather than projected crack length is appropriate for the presentation of the crack growth rate under the mixed mode loading conditions.

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High temperature deformation of heavily drawn Cu-24 wt.%Ag filamentary microcomposite wires have been investigated. The filamentary microstructure tends to break up at high temperatures by spheroidization, grooving and/or recrystallization because of its high interface energy and high cold-work energy. Above 400℃, extensive recrystallization occurred and the grains with 300-500 ㎚ in size were observed in Cu matrix. As the deformation proceeds, the elongated silver lamellae and/or the region with numerous filaments break up and rearrange into chevron patterns which mostly lie perpendicular to the loading axis. The elongation up to 1000% was obtained at 500℃ at the strain rates of 7×10^(-4)/sec ∼ 3×10^(-3)/sec and the elongation up to 600% was obtained at 400℃ at the strain rate of 1×10^(-4)/sec. Arrays of dislocations in the matrix and near grain and phase boundaries were observed after superplastic deformation, supporting the slip-accommodated grain and/or phase boundary sliding.

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Attempts have been made to evaluate the relative stability of surface layer reactions of cast metal with mold material in commercially pure titanium and Ti-6Al-4V alloy castings. For melting of titanium alloys, plasma arc melting process was used and the extent of the surface reaction of castings was determined by optical metallography and microhardness profiles. The mold materials being examined included ZrO₂, Al₂O₃, CaO stabilized ZrO₂, CaZrO₃ and CaO. The machined graphite mold was used for comparison. Moreover, it was to investigate the effect of the binder on metal-mold reaction in case of Al₂O₃ mold. The results showed that titanium castings with ZrO₂, Al₂O₃ mold had clear reaction layer(α-case), whereas negligible reaction with titanium occurred in CaO stabilized ZrO₂, CaO, CaZrO₃ and graphite mold. The grading of thermal stability of mold material does not follow the free energy data of the formation of these oxide. CaZrO₃ is regarded as a promising mold material for titanium investment castings. In additions, metal-mold reaction of titanium castings was affected sufficiently by the binder as well as the mold material.

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The mechanical properties of heavily drawn bundled Cu-Nb filamentary microcomposite was examined as a function of Nb content. In order to predict the variation of the yield strength with Nb content, the interfilamentary spacing was calculated as a function of Nb content based on the assumption that Nb filaments are distributed regularly along the sides of a triangular unit cell in the transverse section. The yield stress consists of the substructure strengthening component due to elongated grains, subgrains and/or cells, the phase boundary strengthening term associated with the Hall-Petch type interaction between dislocations and phase boundaries and precipitate strengthening component. The contributions from phase boundary strengthening σ_(P.B.)(Cu-Nb), and precipitate strengthening σ_(ppt), increases with increasing Nb content. However, the contribution from substructure strengthening, σ_(sub)(Cu-Nb), decreases with increasing Nb content since more grain or subgrain boundaries are absorbed at Cu/Nb phase boundaries with increasing Nb content. The good agreement between the prediction and the experimental data suggests that the increase of the strength in Cu-Nb filamentary microcomposite with increasing Nb content results mostly from an increasing volume fraction of Nb filaments, which act as barriers to plastic flow.

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