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Formation mechanism and control of pores in a gamma alloy synthesized by elemental powder metallurgy(EPM) was investigated. The chemical composition of the experimental alloy was Ti-46.6Al-1.4Mn-2Mo(at.%) and elemental powders were used as raw materials to synthesize the alloy by hot extrusion. Retained molybdenum was observed in as-extruded alloy, and formation of pores at molybdenum-rich regions was confirmed in heat treated alloy by back scattered electron imaging as well as by energy dispersive spectroscopy, The porosity increased with the average particle size of Mo powder. Depending on the processing condition the density of pores varied. Pores could not be easily detectable in as-extruded condition ; however, subsequent heat treatment brought them out. Pre-extrusion treatment affected the severity of porosity in extruded and heat-treated condition. Extended soaking or partial sintering prior to extrusion was found to be effective for suppressing porosity during the heat treatment to produce a fully lamellar structure.

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Al-Mg-Si system alloys are capable of providing a good combination of formability and paint bake response. which has favored with automotive panel applications. Be is a useful element to improve the bake hardenability in Al-Mg-Si alloy. Two step aging and RRA(retrogression and reaging) are very useful to increase the bake hardenability in Al-Mg-Si system alloys. Cr is also known to increase strength and the bake hardenability in Al-Mg-Si system alloys. In this paper, the effects of Be and Cr addition, two-step aging and RRA treatment on the bake hardenability in Al-Mg-Si-Be-Cr alloys are investigated by hardness, tensile test, and differential scanning calorimetry. Age hardening is accelerated and bake hardening is ameliorated by the addition of Be and Cr in Al-Mg-Si alloy. The bake hardening is improved by preaged at 60℃ and 150℃ followed by baked at 180℃ for 30 minutes in Al-Mg-Si-Be-Cr alloy rather than Al-Mg-Si alloy. And the bake hardening is also improved by RRA(150℃/20min→225℃/3min→180℃/30min) treatment in Al-Mg-Si and Al-Mg-Si-Be-Cr alloys.

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In-situ straining experiments in the TEM have been performed on thin foils of polysynthetically twinned(PST) crystals of Ti-49.3 at% Al. As expected, the deformation behavior of PST crystals of TiAl is anisotropic, depending on the angle between the lamellar boundaries and tensile loading axis. Shear deformation proceeds, in a zigzag fashion, across the lamellar boundaries when tensile loading axis is parallel to the lamellar boundaries. For tensile loading perpendicular to the lamellar boundaries, deformation occurs across the lamellar boundaries, whereas crack propagates along the lamellar boundaries. When tensile loading axis is inclined at an intermediate angle to the lamellar boundaries, shear deformation occurs parallel to the lamellar boundaries. The α₂ phase plays different roles depending on the orientation. Roles of the various types of interfaces in the locallized deformation and fracture behavior have been evaluated to understand the overall behavior of the PST crystals.

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In the present study, tensile properties of continuously cast and swaged Cu-9Ni-6Sn alloys were examined. Solutionized and aged Cu-9Ni-6Sn alloys, with 95% swaging, showed a multi-stage change in tensile strength with aging time, probably due to the sequential occurrence of spinodal decomposition, formation of γ` and recrystallization. The change in tensile strength with aging time for the aged specimens without solution heat treatment was, on the other hand, minimal. A complex interaction between the decrease in dislocation density, formation of equilibrium phases and refinement of Sn-rich phases is believed to be responsible for such a lean sensitivity. Spinodal decomposition in Cu-Ni-Sn alloys appears to be restricted with increasing the amount of cold deformation.

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The main emphasis of the present study has been placed on understanding the effects of heat-treatment (intercritical annealing and isothermal treatment) on the microstructures and mechanical properties of a cold-rolled 0.15C-1.5Si-1.5Mn steel sheet. The steel sheet was intercritically annealed, and isothermally treated at bainitic region following air cooling. Microstructural observation and tensile tests were conducted, and also the volume fractions of retained austenite were measured. The volume fractions of retained austenite and the mechanical properties of the steel sheet was changed dramatically by heat-treatment conditions, and the volume fractions of retained austenite increased with decreasing intercritical annealing temperature and isothermal treatment temperature, resulting in the improvement of tensile elongation and strength-ductility balance (TS×Elong.) without huge decrease of the yield or tensile strength. From the results of the present study, it is reached a conclusion that the conditions (temperature and time) of intercritical annealing and isothermal treatment should be established in consideration of the stability of austenite with the solubility of alloying elements in the austenite formed during the intercritical annealing.

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The cavitation erosion behavior of wear resistant hardfacing alloys, Fe-base NOREM 02, Ni-base Deloro 50 and Co-base Stellite 6, were investigated up to 30hrs using a 20kHz vibratory apparatus. It was shown that the mean erosion depths of Stellite 6. NOREM 02 and Deloro 50 after 30hrs test were 15㎛, 37㎛ and 75㎛, respectively and the nominal incubation time of both NOREM 02 and Deloro 50 were about 1hr while it was about 8hrs for Stellite 6. It is considered that the lower cavitation erosion resistance of NOREM 02 than Stellite 6 is mainly due to the undevelopment of the strain-induced phase transformation. It was also considered that the lowest cavitation erosion resistance of Deloro 50 among the three hardfacing alloys is due to the easy formation of cracks at the interface between matrix and hard second phase as well as the high stacking fault energy of Ni-base matrix.

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The microstructure and mechanical properties of high strength 9Cr-0.5Mo-1.8WVNb steel have been investigated to identify the effects of W addition into the 9Cr-1MoVNb steel. The W-added Cr-Mo steel (9Cr-0.5Mo-1.8WVNb) revealed better high-temperature tensile strength regardless of tempering temperature, but exhibited somewhat brittle fracture at higher tempering temperature region, in comparison to those of 9Cr-1MoVNb steel. Transmission electron microscopy also revealed that the evolution of polygonized substructures was delayed by the W addition. The EDS analysis on the structural change of the precipitates, especially (Cr,Fe)₂(C,N), indicated that the W addition increased the ratio of Cr/Fe of (Cr,Fe)₂(C,N), resulting in the stabilization of (Cr,Fe)₂(C,N) through the lattice expansion. It is believed that the stabilized (Cr,Fe)₂(C,N) can enhance the pinning effects on both the movement of dislocation and the growth of lath boundaries. Therefore, it is concluded that the enhanced high-temperature strength of the 9Cr-0.5Mo-1.8WVNb steel is mainly attributed to the retardation of dislocation recovery and the stabilization of carbo-nitrides, caused by the W addition.

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The second generation DS alloy CM186LC has comparable creep-rupture life to first generation single crystal alloy CMSX-2 and CMSX-3 in the as-cast and double aged condition. In the assembly of nozzle guide vanes, DS components are joined into pairs by brazing. In order to study mechanical behavior and microstructural evolution of the material at the same condition as production, DS bars were subjected to brazing simulated treatments. Compared with standard treatment a small drop in creep-rupture life was observed but the degree of fall in life was not serious. Creep cracks initiated at the transverse components of the grain boundaries, which were normal to the applied stress direction. Every crack was associated with the γ-γ` eutectic phase at grain boundaries. γ-γ` eutectic was not only located at grain boundaries but also in the interdendritic regions. It also acted as crack initiation site during creep. Low cycle fatigue tests were conducted under fully reversed trapezoidal strain wave at a frequency of 0.25Hz. Relatively large total strain range showed multiple initiation sites. The depth of surface cracks was comparable to that of the secondary dendrite arms from the surface of the specimen. Cracks initiated at all of the interdendritic regions exposed at the surface, especially at the γ-γ` eutectic phase. LCF crack initiation and propagation is therefore proposed to be microstructally dependent.

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The superplastic deformation of Zn-0.3wt.%Al alloys were investigated at room temperature under various strain rate. These alloys were highly elongated up to 1400% at the initial strain rate of 2×10^(-4)/s, revealing the rate sensitivity parameter of about 0.4. In order to investigate the microstructural change during superplastic deformation, the transmission electron microscopy(TEM) studies were performed on the specimen elongated up to the 100% tensile strain at various strain rate. The microstructure was composed of zinc rich matrix phase, with 1㎛ in grain size, and Al rich second phases precipitated generally at grain boundaries and/or triple point. In region III(5×10^(-2)/s), the high density of dislocations were observed within the grains and deformation twins were also formed within the grains. In region II(2^＊10^(-4)/s), however, the dislocations were highly localized along the grain boundaries and deformation twins were not observed. Observation of severe deformation along the grain boundaries and grain boundary migration(GBM) suggest that grain boundary sliding(GBS) is a dominant mechanism of superplastic deformation in this alloy.

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Al-TM(transition metal) or Al-RE(rare earth element) MMC`s(metal matrix composite) exhibit high stability and strength at elevated temperatures because of low diffusion rate and solubility of alloying elements in Al. Particularly, Al-Ti and Al-Fe alloys have high strength, but low ductility. In this study, Al-Nb alloy was developed by rod mill as an alternative material. It was intended to produce Al MMC containing fine Al₃Nb intermetallics by hot extrusion of mechanically alloyed powders. Metastable BCC phase was found to form during mechanical alloying. This phase had similar lattice parameter and the same crystal structure with pure Nb. The structures of the hot extruded bars consisted of fine metastable BCC`s and equilibrium Al₃Nb`s in Al matrix. Al-5at%Nb alloy showed the strength of 567MPa and 154MPa at room temperature and 400℃, respectively.

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