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In the previous study, the highest phase transformation temperature among Ti-Ni-Hf alloys has been obtained in the alloy containing 15at.%Hf. In the present study, the Ni-content dependence of phase transformation behavior and microstructures in Ti-Ni-Hf shape memory alloys containing 15at.%Hf is investigated by means of differential scanning calorimetry, X-ray diffraction and transmission electron microscopy. Transformation temperatures decreased with increasing Ni-content and a martensitic transformation did not occur in a 51at.%Ni alloy. In alloys whose Ni-content is less than 51at.%, both the B19(orthorhombic) and the B19`(monoclinic) martensites coexisted over the temperature ranges between 77K and 623K, indicating that the B2-B19 and the B19-B19` transformations were not separated. The (001) twin existed in the B19` martensite as an internal defect and became finer with increasing Hf-content, while did not exist in the B19 martensite.

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The dynamic behavior of a tungsten heavy alloy (WHA) have been studied using an exploding bridgewire (EBW) detonator to load stress pulse onto the alloy plate. The particle velocity at the rear free surface of WHA was measured with varying the specimen thickness by a velocity interferometer system for any reflector (VISAR). Simple experimental technique has been introduced herein to obtain the Hugoniot elastic limit and spall strength of metals. The peak pressure generated by the EBW detonator has been exponentially decayed with increasing the sample thickness. The fracture behavior of WHA caused by the tensile stress pulse is similar to that due to the Charpy impact test.

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The 7000 series, Al-Zn-Mg and Al-Zn-Mg-Cu aluminium alloys have been known to be strengthened by precipitation hardening with aging treatment. But, among of defects in 7000 Al alloy, casts defects such as gas porosity and shrinkage defect are formed during solidification because of wide mushy zone. This study was undertaken to investigate the effects of applied pressures on aging precipitating behavior, micro and macro-structures and mechanical properties in 7000 series Al alloys. The microstructure of 25, 50, 75 MPa specimens was more fine and dense than that of non-pressurized cast, because of the greater cooling rate of billet by the application of pressure. GP zone solvus was estimated to be 130℃ by DSC results. Growth of precipitates at 180℃ aging treatment was faster than 120℃ aging treatment. Hardness values of as cast specimens was increased by fine grain size and increase in solute amount within matrix with increase in applied pressure. In case of 180℃ aging treatment, aging time obtained highest hardness valuers was shortened than in case of 120℃ aging treatment. In case of 120℃ aging treatment, tensile strength and hardness was represented a similar increasing tendency. Due to fine grain size, tensile strength values of 7075 Al alloy was higher than that of 7050 Al alloy.

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The crystallographic features of sigma phase precipitation in superaustenitic Fe-22Cr-21Ni-6Mo-(0.3N) stainless steels during isothermal aging were investigated utilizing transmission electron microscopy. The sigma phase precipitated along the austenite grain boundaries even after solution treatment due to higher Mo contents and remained stable throughout aging at 900℃ up to 168 hours. The sigma phase observed in thin study was found to be ternary Fe-Cr-Mo sigma phase and had tetragonal structure with lattice parameters of a=9.17Å and c=4.74Å. The orientation relationships between the sigma phases and austenite were determined from the analyses of selected area diffraction patterns taken by various zone axes and stereographic analyses. The orientation relationships between sigma and austenite phases obtained in this study were as follows; 1) (110)_γ ∥ (110)_σ, [1￣11]_γ, ∥[001￣]_σ and (11￣2)_γ ∥(1￣10)_σ, [1￣11]_γ ∥[001￣]_σ and 2) (1￣10)_γ∥(1￣10)_σ, [11￣2]_γ∥[113]_σ and (1￣11)_γ∥(332￣)_σ, [1￣12]_γ∥[113]_σ. However, the former orientation relationship was predominant throughout aging and the latter orientation relationship was scarcely observed in very limited aging condition.

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The effect of small addition of Mm(Ce, La, etc) on the mechanical properties and electrical resistivity of Cu-6Ni-4Sn(wt%) alloy has been investigated by means of microstructural observation as well as the tensile and electrical resistivity tests. The Cu-6Ni-4Sn alloy was found not to strengthen by an aging treatment after cold working but the tensile strength of this alloy was decreased with increasing the aging temperature, indicating that the spinodal decomposition didn`t occur in the α supersaturated solid solution. The grain growth was significantly suppressed by the small addition of Mm, resulting into the substantially enhanced mechanical properties. The tensile strength and elongation of the Cu-6Ni-4Sn-0.05Mm alloy was found to increase with increasing aging temperature at low temperature range possibly due to the spinodal decomposition and CeNi₄Sn₂ precipitation. The electrical resisitivity was significantly reduced by the small addition of Mm. In all of the alloys examined, the electrical resistivity was reduced by raising the aging temperature. The Mm addition is found to reduce the electrical resistivity more rapidly as the aging temperature increases.

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The effects of Mn substitution for Ni on the decomposition behavior of the supersaturated α and properties in Cu-x%Ni-y%Mn(x＋y＝9)-6%Sn alloys were investigated. When the alloys with less than 2 wt% Mn were aged below 350℃, the supersaturated α was decomposed in three stages. The spinodal decomposition occurred in the first stage and the metastable γ` phase with D0_(22) type superlattice nucleated from the modulated structure in the second stage. In the final stage of transformation, the γ` phase decomposed into the equilibrium γ phase. For the alloys with more than 3 wt% Mn, however, the spinodal decomposition and the γ` were not observed at all the aging temperatures. All the alloys were found to strengthen exhibiting very high tensile strength larger than 1,000 MPa by aging at the temperatures below 350℃. However, the strengthening mechanisms were changed with the amount of Mn substitution for Ni. The less than 2 wt% Mn alloys were strengthened by the spinodal decomposition and γ` formation, while the more than 3 wt% Mn alloys were strengthened by the precipitation of Ni₂MnSn phase. When the specimens were aged at above 500℃, the discontinuous precipitate consisting of α and γ formed at the initial stage of transformation in all the alloys examined. All the alloys were not strengthened by aging at temperatures above 450℃. In all of the alloys examined, the electrical resistivity was decreased by aging. With increasing the amount of Mn substitution for Ni, the formation of discontinuous precipitate accelerated and the electrical resistivity increased. The maximum amount of Mn substitution for Ni to ensure the spinodal decomposition in the Cu-xNi-yMn(x＋y＝9)-6Sn alloys appears to be about 2 wt%.

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Hypereutectic Al-16Si-5Fe based alloys fabricated by hot extruding air atomized powders were investigated for microstructure and tensile deformation behavior up to 520℃. TEM, SEM and EDS analyses revealed that very fine Al_5FeSi intermetallics and Si particles are uniformly distributed in the matrix which has the average grain size of 800nm. Elongation to failure increased with increasing temperature, with accompanying decrease in both yield and tensile strength at the initial strain rate of 1.4×10^(-3)s^(-1). Above 450℃, a high strain-rate superplasticity was observed, and the maximum elongation of 377% was achieved in the specimen deformed at 520℃ with the initial strain rate of 1.4×10^(-1)s^(-1). The superplastic deformation was considered to occur by a grain boundary sliding due to the extremely fine grains. Retarded grain growth during deformation owing to the uniformly distributed fine-precipitates was also considered to be an important factor for the superplasticity. A superplastically deformed specimen at 520℃ showed an evidence of a partial melting in the vicinity of grain boundary, suggesting that the liquid phase formed by the partial melting in the vicinity of grain boundary contributed as an accommodation helper, which relieved the stress concentration arised from the grain boundary sliding.

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Compression tests were carried out on Alloy 718 to investigate the microstructure development following dynamic recrystallization, meta-dynamic recrystallization and grain growth. Tests were performed in the temperature range of 927∼1066℃ and strain rate range of 5∼5×10^(-4)s^_(-1), with true strains up to 0.8. Dynamically recrystallized grain size decreases consistently with increasing Zener-Hollomon parameter (Z=ε exp(Q/RT)) which is function of strain rate and temperature. The extent of recrystallization increases with a decrease in Zener-Hollomon parameter due to an increase in the critical strain (ε_c) corresponding to the initiation of dynamic recrystallization. The fraction of meta-dynamically recrystallized grain, however, increases with increasing the strain rate from 0.01s^(-1) to 1s^(-1). This paper outlines the microstructural evolution of Alloy 718 during the high temperature deformation.

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A study has been made of the effects of strip casting process variables on the microstructure of strip cast SUS 304 stainless steel. The main impetus for the present investigation is to suggest the optimum process variables for the strip casting of SUS 304 stainless steel based on the systematic studies of microstructural evolution. Results slow that there is a variation of microstructure through the thickness of strip. Along the surface of strip, the microstructure consists of fine dendrite and cellular structure, and there is also a formation of α` martensite and mechanical twin due to the deformation by roll contact. On the other hand, microstructure in the middle of strip consists of equiaxed grains due to slow solidification rate and recalescence. One of the most important finding is that there is a depletion of Mn in the interdendrite region and the presence of amorphous Si-rich particle.

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Electromagnetically stirred Al-4.5 wt%Cu alloy was reheated into semi-solid state by precisely temperature-controllable gold image furnace. Evolution of microstructures in the semi-solid state was investigated with fixed reheating rate, various holding time and holding temperature. The interface of cells consisting of primary particle formed by electro-magnetic stirring disappeared due to particle regrowth during reheating the alloy. Particle with entrapped eutectic appeared by the coarsening of dendrite which was preciously formed by rapid cooling of residual liquid after stirring stopped and entrapped eutectic changed into K phase by diffusion mechanism as holding time increased. Particles were coarsened, showing polygonal shape and the solute content of solid particle(K phase) increased by redistribution of solute between solid particle and liquid phase, as holding time increased more. Liquid fraction was 11.6% at 637℃ when the alloy was held for 420 minutes, although it is expected 50% by Scheil equation. It was thought that particle coarsening resulted from simple particle growth, coalescence, Ostwald ripening during reheating the electromagnetically stirred Al-4.5 wt%Cu alloy.

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