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The microstructural changes of Mg-Al-Zn-X(X＝Si and Sr) alloys during semi-solid state processing has been investigated in the present study. The equiaxed α-Mg primary dendrites are broken into nearly spherical primary particles at the early stage of isothermal shearing, i.e. after isothermal shearing at f_s≒0.5 for ∼3-5 minutes. Once the non-dendritic structure has formed, the primary particle morphology and size distribution remain nearly unchanged for upto 40 minutes, due to the competing mechanisms of particle dissociation and restructuring. Microstructure and composition analysis shows that particle dissociation and restrucuring occur in various mechanisms such as recrystalization or fracture of dendritic structurers. Ostwald ripening and particle sintering due to isothermal shearing in semisolid state.

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Microstructural changes in rapidly solidified Mg-Al-Zn-Nd during heat treatment at 100℃, 250℃, 350℃ for 1hr and 24hrs have been investigated by using TEM, EDS, DSC, and X-ray diffraction analysis and hardness measurements. As-extruded Mg-Al-Zn-Nd consists of 1∼2㎛ size fine grains with the homogeneous distribution of 10∼50㎚ size Al₂Nd phase particles and 0.1∼0.3㎛ size quasicrystal-related particles. No microstructural changes occur during heat treatment at 100℃ and 250℃ for 1hr and 24hrs. After heat treatment at 350℃, the Al₂Nd phase particles are still stable, but the quasicrystal-related phase particles are replaced with Al_(11)Nd₃ phase particles. The amount of hardness decrease after heat treatment at 350℃ for 24hrs corresponds to only 10% of the hardness of as-extruded material, which suggests high thermal stability of rapidly solidified Mg-Al-Zn-Nd due to the relatively fine grain size and, particularly, the presence of thermally stable 50㎚ size Al₂Nd phase particles.

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Removal of hafnium from zirconium way carried out using solvent extraction with Versatic Acid-10 in order to produce the high purity zirconia. The results showed that zirconium was effectively extracted at the pH range of 2.1∼2.3, while hafnium was hardly extracted at this pH range. On the other hand, the stripping effect of zirconium was more effective in the case of using HCl as a stripping solution. When the zirconium oxychloride solution containing zirconium of 700ppm and hafnium of 18.9ppm was treated by Versatic Acid-l0 of 1M at a phase ratio of A/O=1/3 with six stages in the extraction and also by HCl of 1M at a phase ratio of O/A=1 with five stages in the stripping, the content of zirconium and hafnium in the stripping solution was 210ppm and <0.3ppm, respectively. High purity zirconia(99.84% ZrO₂) could be obtained by hydrolysis followed by the calcination of precipitates.

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A new process, named ERC (Evaporation and Rapid Condensation) method, had been developed for the production of a large amount of isolated ultrafine particles(UFP). This method is based on a phenomenon of evaporation and condensation, but can be also said to be a kind of enhanced evaporation technique because of sucking the metal vapor generated from a molten metal. Therefore, in this paper, in order to investigate whether ERC method can be applied in preparation of alloy ultrafine particles, Fe-Ni alloys were tested. And the shape, the content and the morphology of obtained UFP were observed by TEM-EDX, SEM-EDX and X-ray diffraction analysis. Most of the obtained alloy particles were spherical, and the mean particle size was 15㎚. And the content of the obtained Fe-Ni alloy UFP almost agreed with that of equilibrium vapor of mother alloy, and the lattice parameter of the Fe-Ni alloy UFP was equal to that of mother alloy. Therefore, it was found that this method can be applied in preparation of alloy UFP.

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Cracking behavior of laser welded high strength alloy steel during solidification was investigated. Bead-on-plate welding was carried out with high power carbon dioxide laser system. Test coupons were prepared from hot rolled 2.4㎜ thick plates. Two important welding parameters such as travel speed and applied laser power were varied. Results showed that input energy for the welding had nothing to do with the solidification crack susceptibility. Under the tensile pre-stress of 10㎏/㎟ and 2.1㎾ laser output condition, the crack ratio was slightly decreased with increase of travel speed. Threshold stress for initiation of hot crack shows linear relation with travel speed over the test range of 700-1,100㎜/min which was the condition for full penetration; the more the speed goes up the higher the applied threshold stress is needed to develop weld hot crack.

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Electrochemical techniques have been applied to study on the pitting behavior of ion-nitrided 304L(304LSS) and 316L(316LSS) stainless steels at 25℃ in 0.5M H₂SO₄, 0.1M HCl, 0.5M H₂SO₄+ 0.1M HCl and 0.5M H₂SO₄+ 0.5M HCl solutions. Ion-nitriding was carried out at 550℃ for 10, 20, and 30hrs under pressure of 6 torr. The structure and thickness of nitrided layer and the corrosion morphology of nitrided surface were investigated using optical microscopy, SEM, TEM and EDX. Nitriding at 550℃ produced a regular surface morphology with the spherical nitrides. The ε-phase of compound layer thickened as nitriding time increased, resultting in the corrosion resistance. Electrochemical measurements showed that the corrosion potential, passive film breakdown potential and repassivation potential of ion-nitrided austenitic stainless steels decreased with increasing nitriding tune but the critical anodic current density at active region and the current density at passive region increased. The passive film stability of the 10hr nitrided 316LSS was higher than that of the nitrided 304LSS, whereas the rates of pit nucleation and growth were lower. When nitrided at 550℃ for 30hrs, larger pits were observed in the of 304LSS case rather than the 316LSS one.

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Microstructures and mechanical properties of Ti-modified HY180 ultrahigh strength steels have been investigated by means of optical microscopy, energy dispersive X-ray spectrometry, tensile tests, Charpy V-notch impact tests and scanning electron microscopy, and then compared with those of the typical Ti-unmodified HY180 steel. Ti-modificztion refined grain sizes of the HY180 steels and formed Ti and S containing inclusions in the steels. The tensile strength of the HY180 steels was not almost changed(about 1350 MPa) by Ti-modification when Ti content was less than 0.1 wt%. The Charpy V-notch energy absorption increased from 75 J to 145 J by Ti-modification, and this was attributed partly to the grain size refinement.

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The transition temperature-hydrogen content relationships and microstructural refinement of Ti-6Al-4V alloy have been studied by thermochemical hydrogenation processing. As the hydrogenation temperature increased, the total hydrogen in-diffusion decreased. Also, hydrogen content was independent of the hydrogenation time. In specimens hydrogenated at various temperatures, as the hydrogen content increased, β_T, transition temperature decreased linearly. In the hydrogenation at 700 and 800℃, microstructures showed the fine equiaxed α phase, lamellar of α and β phase and hydrides. However, the microstructure after hydrogenation at 900℃ showed the formation of orthorhombic martensite α″_H and the heterogeneous precipitation of the black plates at prior β grain boundaries. The black plates were a mixture of tetragonal hydride, β and α phase. The broadening of α peaks could be attributed to the build-up of a high stress field, resulting from the formation of fine hydride particles. In the hydrogenation at 700℃ for 8hr, the finest microstructures were obtained. Accordingly, the significant variations in a unique class of microstrucuures of Ti-6Al-4V alloy could be achieved by charging with hydrogen as a temporary alloying element and subsequently dehydrogenating.

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This study is concerned with the halo formation and the shape of dendrites of chromium primary phase in the solidified Cu 2.3%Cr alloy and the dendritic growth directions of zinc during solidification of the Zn-3.8%Al alloy. Cu-Cr hypereutectic alloys show the well developed copper halo around chromium primary phase, because the coupled zone is skewed to the chromium side of the phase diagram. In nf-nf type eutectic system, the coupled zone is skewed due to the large difference of melting temperature between α phase and β phase. The shapes of chromium primary phase imply that <111> directions are the preferred growth directions in chromium dendrites. These directions minimize the strain energy in the dendrite arm. The dendritic growth direction of zinc has been found to be the primary <0001> and secondary <101￣0> directions. The results have been explained based on the factors suggested by the present anthers.

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Vibration damping capacity(i.e Specific Damping Capacity, SDC) of extruded bars of both casted and Powder Metallurgy(PM) Al-Zn alloys was investigated in relation to their Zn compositions and heat treatment conditions. The effect of α/β interface area on SDC was studied. The SDC is found to increase with increasing Zn content. The SDC of specimens heat treated at 250℃ for 1 hour is smaller than that of as-extruded bars because their α/β interface area decreases. In the case of extruded bars of PM Al-78wt%Zn alloy heat treated at 250℃ and 310℃ for 1 hour, SDC and lamellar spacing are 12.8% and 0.5㎛, 25.6% and 0.1㎛, respectively. This large difference in SDC is believed due to the disappearance of retained α phase as well as much finer lamellar spacing by heat treating at 310℃. The SDC of extruded bars of PM alloys is about 3% larger than that of extruded bars of casting.

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