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The mechanical properties of M5 type (Zr-1Nb) and ZIRLO type (Zr-1Nb-1Sn-0.3Fe) alloys were evaluated for the final annealing process at the annealing temperature regime of 450∼800℃. The microstructures were observed by OM and TEM. The mechanical properties were investigated by the tensile and creep test. The β-Zr was observed at annealing temperature above 700℃. It was observed from the tensile test that the mechanical strength decreased rapidly with increasing the annealing temperature, but maintained the constant value above the fully recrystallization temperature. The tensile and creep strength of ZIRLO type alloy was higher than those of M5 type alloy. The strengthening effect of solid solution was dominant of ZIRLO type alloy. Also, it was believed that both alloys were strengthened with the appearance of β-Zr. The trend of creep strength with annealing temperature was similar to that of tensile strength. The both alloys showed good creep resistance due to the β-Zr at the annealing temperatures of 700℃ and 800℃.

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J-R fracture resistance tests were performed at the temperature range of RT-288℃ for three kinds of Korean-made SA508-Gr.3 nuclear reactor pressure vessel steels which were classified by steel refining process. The Si+Al refined steel showed the best J-R fracture resistance while the VCD refined steel showed the worst J-R fracture resistance among three kinds of RPV steels. Microstructural analyses were conducted for the materials to correlate microstructure and J-R fracture resistance. Sub-grain sizes were measured for three steels using the EBSD technique. Sub-grain size, as well as prior austenite grain size, of the VCD refined steel was remarkably larger than that of the other two steels. The carbide size and distribution of the three kinds of RPV steels was investigated through SEM and an image analyzer. The result showed that more and bigger carbides precipitated in the VCD refined steel than in the other two steels. These results matched the previously reported analysis for the relationship between microstructure and ductile fracture resistance.

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The mechanical properties of thin film materials are known to be different from those of bulk materials, it is generally overlooked in practice. The difference in mechanical properties can be misleading in the estimation of residual stress states in micro-gas sensor with multi-layer structure during manufacturing and in service. In this study the residual stress of each film layer in micro-gas sensor was measured according to the five difference sets of film stacking structure used for the sensor. The Pt thin film layer was found to have the highest tensile residual stress, which may affect the reliability of micro-gas sensor. For the Pt layer the changes in residual stress were measured as a function of processing variables during thermal cycling. In addition the effect of heat treatment on the microstructure was investigated during processing of the sensor. The residual stresses in the Pt layers caused by microstructural changes were also investigated.

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Using equal-channel angular pressing(ECAP) at room temperature, the grain size of 1080 Al(99.8% pure) was refined from ∼107 ㎛ to the submicrometer level. The effect of grain size refinement on the mechanical properties and recrystallization behavior was evaluated by the tensile test, micro-hardness test and microstructural observations. The grain size was reduced to 0.9 ㎛ after a single pass and the strength and the Vickers hardness was increased significantly. But the microstructures after a single pass were consisted of bands of elongated subgrains divided by low angle boundaries. With the pressing numbers, the microstructures were evolved into the grain structure with an equiaxed grains divided by high angle boundaries. As the strains induced into the samples were increased, the grain size recrystallized after annealing treatment became more refined.

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In this study, the flow properties of various structural steels were estimated by analyzing the indentation load-depth curve obtained by ball indentation and the pile-up or sink-in phenomenon around indentation. In deriving the true stress-true strain curve, the representative strain ε_R was obtained by considering the deformation behavior beneath the indenter. And the representative stress σ_R was obtained by using the relation P_m / σ_R = Ψ for fully plastic stage, where P_m is the mean contact pressure. The constraint factor Ψ was then related to the work-hardening characteristics of material in this study. For various steels such as TMCP, SM50, SA508, SA213, SA106, and AISI1025, the results of indentation test were compared to those of tensile test, which shows that the present analysis can predict successfully the plastic flow properties of structural steels.

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Notch effect and loading rate dependency on fracture toughness were considered when evaluating fracture toughness of small notched specimens using the instrumented impact test. Notch effect was analyzed into stress redistribution effect and stress relaxation with a viewpoint of stress triaxiality. Stress redistribution effect was corrected by introducing effective crack length, which was the sum of actual crack length and plastic zone size. Stress relaxation effect was also corrected using elastic stress concentration factor, which would decrease if plastic deformation occurred. As a result, corrected fracture toughness of the notched specimen was very consistent with the reference fracture toughness obtained using precracked specimen. In addition, limiting notch root radius, below which fracture toughness was independent of notch radius, was observed and discussed. Loading rate dependency on fracture toughness, which was obtained from the static three point bending test and the instrumented impact test, was also discussed with stress field in plastic zone ahead of a notch and fracture based on stress control mechanism.

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Microstructural evolution of AZ91D Mg alloy during partial remelting was evaluated as a function of isothermal holding temperature and time in the semisolid state, with special attention to the prior solidification and reheating conditions before and during partial remelting, respectively. The shape and size of the solid phase and the liquid fraction were particularly examined. The results show that the globularization of the solid phase occurs, and the total liquid fraction increases with isothermal holding time while the grain-interior liquid fraction decreases. It is also reported that the size of the solid phase in the semisolid state is independent of holding temperature, indicating that the size of the solid phase is a function of isothermal holding time with the initial size determined by the initial grain size. The grain-interior liquid is induced by the coalescence of dendrite arms with the liquation of small grain boundaries and segregated regions, and the shape of it is almost globular due to the crystallographic related surface energy consideration. The fact that the average diameter of the solid globules is reduced at a relatively high temperature is mainly due to the preferential remelting.

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An experimental investigation concerning the thixoformability, in the continuation of the previous research for control of liquid fraction and microstructure of semisolid AZ91D Mg slurry in the repeated semisolid state, was carried out with grain-refined AZ91D Mg alloy. The grain-refined AZ91D Mg alloy billet was produced by superheating with close control of solidification conditions, and thixoforming was performed using a step die cavity for evaluating thixoformability and using a testing die cavity for thixoforming mechanical testing specimens. Special attention was paid to the variation of hardness with regard to microstructural evolution. All slugs filled the step mold cavity with essentially laminar flow even in the incomplete mold fillings, and the thixoformability was improved by increasing the mold temperature and plunger speed enough to avoid premature freezing. The strength increase of the thixoformed AZ91D Mg alloy was not obtained and the source of this low strength is not yet clearly explained, but it may be due in part to the presence of low strength globular solid phase of about 120 ㎛. Although little work has so far been carried out, the preliminary results showed the improvement of the mechanical properties of thixoformed AZ91D alloy could be achieved by increasing mold temperature and by the heat treatment.

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Spontaneous infiltration and strengthening behaviors were analyzed in terms of microstructures and tensile properties of Al/BN composite fabricated by pressureless infiltration technique, compared with control alloy without BN fabricated by the same method. The Mg₃N₂ formed by the reaction of Mg vapor and nitrogen gas, which coated the particles in the powder bed, is believed to induce spontaneous infiltration through a great enhancement of wetting via the following reaction, Mg₃N₂+ 2Al→2AlN + 3Mg. This was identified by the finding of AlN particle layers on the surfaces of old Al particles in the powder bed, which contacted with the infiltrating melt. In addition, unreacted Mg₃N₂ was observed outside the composite, where the Al melt directly did not come into contact. Fine AlN particles formed in-situ resulted in a significant strengthening even in the control alloy with no addition of BN. In the composite reinforced with BN, additional AlN was formed by the interfacial reaction of the BN and Al melt as well as AlN by the in-situ reaction. Consequently, both the BN particles and the additional AlN particles formed by the interfacial reaction led to a further strengthening in the composite, as compared to control alloy strengthened by the AlN particles formed in-situ.

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BN/Al(-Mg) metal matrix composite(MMC) was fabricated using pressureless infiltration technique, in order to clarify the reaction mechanism. The phase characterizations of the composite were analysed using the scanning electron microscopy, energy dispersive X-ray spectroscopy, Auger electron spectroscopy and electron probe micro analysis on reaction products after the electrochemical dissolution of the matrix. It is confirmed that aluminum nitride (AlN) was formed by the reaction of Mg₃N₂ and Al alloy melt. Plate type AlN and polyhedral type Mg(-Al) boride were formed by the reaction between Mg₃N₂, BN and molten Al in the BN reinforced metal matrix composite. The reaction mechanism in the fabrication of BN/Al(-Mg) metal matrix composite was derived from the phase analysis results and thermodynamic investigation.

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