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Fe, Mn alloying effects on the shape memory behavior in two phases NiAl base shape memory alloys have been investigated. The γ phase formed by the alloying Fe or Fe+Mn affected drastically not only on the hot workability but also on the tensile ductility of these alloys at room temperature. The transformation temperature increased with increase in annealing temperature because of the change in composition of the β matrix. Martensite transformation temperature (M_s) of the alloys annealed at 1300℃ was above 150℃. And the shape recovery ratio of the alloys is about 70%. Since a part of shape recovery energy of β(β′) phase was consumed on shape recovery of γ phase plastically deformed, the less γ phase, the better shape memory effect. Therefore these results mean that NiAl alloys with Fe or Fe+Mn are potential shape memory alloys for applications above the temperature of 150℃.

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The main emphasis of the present study has been placed on investigating the formability and understanding the effects of retained austenite on the formability of a 0.15C-1.5Si-1.5 Mn multi-phase cold-rolled steel sheet (ECO-1 steel sheet). The steel sheet was intercritically annealed at 800℃, and isothermally treated at 400℃ and 430℃. Microstructural observation, tensile tests, and limiting dome height (LDH) tests were conducted, and the change of retained austenite volume fractions as a function of tensile strain was measured. The tensile properties and the formability of the ECO-1 steel sheets were superior to the conventional low carbon cold-rolled steel sheets, due to maintaining high strain hardening rate in the high strain region by the strain induced transformation of retained austenite to martensite. The formability of the ECO-1 steel sheets was dependent on the stability of retained austenite. If the stability of retained austenite was high, the strain induced transformation of retained austenite to martensite can be stably progressed, resulting in the delay of necking to high strain region and the improvement of formability. The effects of retained austenite on formability of the multi-phase cold-rolled steel sheet could be clearly understood in terms of the stability of retained austenite.

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The effects of Mo addition on the crystallographic factors and chemistry of y phase, the as-cast and heat-treated microstructures, and the mechanical properties of Ti-48at%Al intermetallic compound have been investigated. The Mo addition has been found to have an effect on stablizing fine β particles in the heat-treated structure. The Mo addition also tends to enhance work hardening rate by stabilizing the lamellar structure in the heat-treated condition. These results lead to the fact that the compressive yield stress increases from 349㎫ to 617㎫ as Mo content increases from 0 to 3at.%Mo in Ti-48at.%Al. The Mo additon increases c/a ratio as well as Al and Mo contents of γ phase. However although the increment of Al content and c/a ratio in γ phase is relatively small, a significant decrease in ductility is observed. The fracture tensile strain has been calculated using Chan`s model and the deterioration in ductility with Mo addition is predicted primarily to be due to the high intrinsic yield stress in Mo-added alloys. We also suggest the modified Ashby`s work hardening theory considering the effect of volume fraction of second phase in α (lamellar)-γ two-phase structure. Using this model, the dependence of work hardening rate on the lamellar volume fraction has been calculated. The result shows that the separate lamellar volume fraction contributes additionally to the enhancement of the work hardening rate in α (lamellar)-γ two-phase structure.

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The effect of nitrogen on the nucleation of a low cycle fatigue (LCF) crack was studied at high temperature using type 316L stainless steel with different nitrogen contents (0.04-0.15%). Strain-controlled LCF tests were conducted in the temperature range of R.T. ∼600℃ and in air. The waveform of LCF was a symmetrical triangular with a total strain amplitude of 1.0% and in a constant strain rate of 2×10^(-3)/s for all tests. The sites for crack nucleation, crack population, and crack depth from the surface of the specimens were measured after LCF testing up to 40% of fatigue life. The preferred sites for crack nucleation were grain boundaries, but some of cracks also nucleated at twin boundaries and slip bands. Crack population on the surface of the specimen decreased but the maximum depth of the crack from the surface of the specimen did not change with the addition of nitrogen at high temperatures. The cycles for the nucleation (100㎛) of LCF crack increased with the addition of nitrogen. So, the increase of LCF life with the addition of nitrogen at high temperatures may be explained by the fact that nitrogen increases the resistance of crack nucleation.

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In this study, the deformation and fracture behavior of the Cu-Nb microcomposites fabricated by the bundling and drawing process was investigated. The yield strength of a Cu-20% Nb microcomposite was predicted by modifying the model of Verhoeven et al. It was assumed that the substructural strengthening in pure Cu and Nb phase fully contribute to the yield strength of Cu-Nb microcompoaite, σ_(Cu-Nb), at low draw strains (η $lt; 5.5). At high drawing strains (η $gt; 5.5) where the microstructural scale of the Cu matrix is limited by Nb filaments, the contribution of the partial grain boundaries (connecting the edge of Nb filaments) to the strength was assumed to be proportional to (λ/W_(Nb))^½ The good agreement between the yield stress predicted in this study and the experimental data at drawing strains between 6 and 9.5 supports the suggestion of the present study that the effectiveness of substructure strengthening of Cu and Nb in heavily deformed Cu-Nb microcomposites is much lower than that expected from the yield strength of heavily deformed Cu and Nb. The yield stress of the Cu-Nb microcomposite fabricated by the bundling and drawing process was found to be in good agreement with the predicted yield stress of the present model. At drawing strains above 10, Nb filaments reached a thickness of approximately 10 ㎚ and there after further working resulted in random rupture of the filaments rather than continued plastic deformation and thinning, which may result in the slower increase of the strength than the predicted values with drawing strain.

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Formation range, thermal and mechanical properties of amorphous phase in liquid quenched Al_(100-(X+Y))Ni_xMm_Y ((X+Y)≤20at%) alloys were investigated. Mixed phases of (Al+comp) and (Al+Am) were obtained in the range of (Ni+Mm)≤6at% and 6at%≤(Ni+Mm)≤10at%, respectively. Amorphous single phase was formed in the range of 10at%≤(Ni+Mm)≤20at%. An onset temperature(Tx) for crystallization of amorphous phase increased with increasing (Ni+Mm) content, indicating an enhancement of the thermal stability. Glass transition phenomenon was occurred in the composition of Al((90-x))Ni_(10)Mm_x (X=5-10at%) and Al_((92-x))Ni_8Mm_x (X=6-10at%). Tensile fracture strength(σ_f) and hardness(H_v) for the mixed phase alloys of (Al+Am) showed higher values than those of other phases. In the amorphous single phase region, σ_f and H_v increased with solute (Ni+Mm) content. The strengthening was presumed to be caused by an increase of Al-Ni or Al-Mm atomic pairs with strong bonding nature in the amorphous matrix. (Received September 1, 1998)

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In this paper, TiNiCo/Al shape memory composites were fabricated by powder metallurgy method. TiNi and TiNiCo were used as fiber for reinforcement. To investigate the fatigue crack propagation behavior due to the compressive residual stress which exists in the composites, fatigue test was performed at room temperature and above the inverse transformation temperature A_f of TiNi and TiNiCo shape memory alloy. Results are as follows; At 363 K, both composites have lower fatigue crack propagation rate than pure matrix in the same ΔK area and the fatigue crack propagation of TiNi/Al composite is lower than that of TiNiCo/Al composite.

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Self-propagating high-temperature synthesis (SHS) of Ti_5Si₃, Ti_5Si₄ and TiSi has been investigated by ball milling of Ti-37.5at%Si, Ti-44.4at%Si and Ti-50.0at%Si powder mixtures. Ti and 60min pre-milled Si powders were milled by a high-energy ball mill. SHS reactions were observed to produce three Ti silicide phases during ball milling. Each critical milling period for the formation of Ti_5Si₄, Ti_5Si₃ and TiSi was found to be 16.5, 18.5 and 19.5 min, respectively, which were shorter than that of the ball milling using as-received and Si powders. The powders ball-milled after the critical milling period showed a rapid increase in micro hardness. The Hv values were found to be higher than 1000 kgf/㎟. The SHS reaction rates for the formation of the Ti silicides appeared to depend on the refinement of Si powder as well as the heat of formation of the products (Ti_5 Si₄→-81.0 kJ/㏖.at., Ti_5Si₃→-72.5 kJ/㏖.at., TiSi → -64.9 kJ/㏖.at).

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Plasma spraying technique was used to fabricate functionally graded coating (FGC) of NiCrAlY/YSZ (8 wt%Y₂O₃-ZrO₂) on a Co-base superalloy (HAYNES 188) substrate. Six layers of gradual compositional variation was achieved for FGC. Conventional thermal barrier coating (TBC) of NiCrAlY/YSZ with sharp interface was also fabricated. The prepared FGC and TBC were exposed at the temperature of 1100℃ for 10, 50, 100 hours. The coatings were examined the change in microstructure, oxidation behavior, phase transformation and adhesive strength. Adhesive strength was measured before and after the exposure at high temperature. Results showed that pores and microcracks were formed in YSZ due to thermal internal stress development sintering shrinkage. The amount of pores and microcracks in YSZ were increased with exposure time to high temperature. High temperature oxidation of coatings were occurred at the NiCrAlY/YSZ interface. The increased area of the NiCrAlY/YSZ interface in FGC comparing that in TBC is attributed to the higher amount of oxide formation. Adhesive strength measurement revealed that strength of FGC in early oxidation time increased with formation of oxides due to the mechanical locking at the interface. However, adhesive strength of FGC decreased by weakening of YSZ according to the pores and microcracks which becomes main reason of the failure of the FGC as exposure time increased.

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STS403 stainless steel substrate was treated by duplex treatment of chromizing and boronizing. Metallographic cross sections of the treated specimens were consisted of duplex layer, layer, Cr diffusion layer, and matrix. Microstructures of duplex layer and others layers formed on STS403 were examined by OM, SEM, EDS, XRD and surface properties of duplex layer were evaluated by micro-vickers hardness tester, wear tester and corrosion tester. From the XRD analysis of the duplex layer, various phases such as FeB, Fe₂B, CrB were identified. The results from EDS analysis showed that intermedium layer contained much more Cr than any other layers. The activation energy for the diffusion of boron in the chromium diffusion layer in the duplex treated STS403 steel was calculated to be approximately 220 kJ/mole, which was higher than the diffusional activation energy of boron in the STS403 steel, 160 kJ/mole. Microhardness of duplex layer was measured to be approximately Hv1800-2000 (50 gf). On high temperature wear testing at 600℃, duplex treated specimen exhibited an excellent wear resistance, showing that the wear volume of the duplex treated specimen was approximately five times smaller than that of untreated specimen. From the result of anodic polarization curves in 1M, H₂SO₄ solution, duplex treated specimen showed an excellent corrosion resistance than the specimen treated by boronizing only.

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