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This study evaluated the microstructure and mechanical properties enhancement of cross roll rolled Ni-10Cr alloy, comparing with conventional rolled material. Cold rolling was carried out to 90% thickness reduction and subsequently annealed at 700 °C for 30 min to obtain the fully recrystallized microstructure. Particularly, cross roll rolling was carried out at a tilted roll mill condition of 5º from the transverse direction in RD – TD plane. Application of cross roll rolling on Ni-10Cr alloy contributed to the notable grain refinement due to higher effective strain than that of conventional rolling, consequently, average grain size was refined from 135 µm in initial material to 4.2 µm in cross roll rolled material. Furthermore, //ND texture in CRR material was well developed than that of CR material, which contributed to the mechanical properties and formability enhancement.

This study evaluated the microstructure and mechanical properties enhancement of cross roll rolled Ni-10Cr alloy, comparing with conventional rolled material. Cold rolling was carried out to 90% thickness reduction and subsequently annealed at 700 °C for 30 min to obtain the fully recrystallized microstructure. For annealed materials after rolling, to investigate the grain boundary characteristic distributions, electron back-scattering diffraction technique was introduced. Application of cross roll rolling on Ni-10Cr alloy contributed to the notable grain refinement, consequently, average grain size was refined from 135 μm in initial material to 4.2 μm in cross roll rolled material. These refined grain size led to an enhanced mechanical properties such as yield and tensile strengths. Furthermore, //ND texture in CRR material was well developed than that of CR material, which contributed to the mechanical properties and formability enhancement.

To evaluate the effect of rolling process on microstructures and textures development, this work was carried out using pure Ta sheets. For this work, conventional rolling (CR) and cross roll rolling (CRR) as a deformation process were introduced, pure Ta sheets were cold rolled to 90% in thickness reduction. After this, to obtain the recrystallized microstructure on cold rolled materials, the recrystallization heat-treatment was performed at 1200°C for 60 min under the high vacuum state. Also, to analyse the grain boundary character distributions of the annealed sheets, electron back-scattered diffraction method was introduced. As an experimental result, increase in strain rate led to the development of texture with its texture component of at conventional rolling and cross roll rolling, in common. Therefore, in this study, we systematically discussed the texture development due to increase in effective strain.

In order to evaluate the microstructure and mechanical properties of cross roll rolled Ni-30Cr alloy, comparing with conventionally rolled material, this work was carried out. The materials were cold rolled to 90% in thickness reduction by conventional rolling and cross roll rolling methods and subsequently annealed at 700°C for 30 min. After this work, in order to evaluate the grain boundary character distributions of the materials, electron back-scattered diffraction technique was introduced. The application of cross roll rolling was more effective to develop the microstructure and mechanical properties than those of conventional rolling. As a result, the grain size was significantly refined to 1.3 μm in conventional rolling and 0.6 μm in cross roll rolling, compared to initial material (30 μm), respectively. Also, these grain refinements directly affected an increase in mechanical properties. In the present study, we systematically discussed the relationship between grain size and mechanical properties in terms of an increase in effective strain.

In order to investigate the texture development in cold rolled and annealed tantalum (Ta), the present study was carried out. For this work, cold rolling was performed up to ~ 96% reduction in thickness, and annealing was performed at elevated temperature up to ~ 1200 °C for 60 min under the high vacuum to prevent the surface oxidation. As a result, increase in the reduction ratio of the cold rolled Ta was effective to develop the a fibre texture, with its texture components such as (001) and (112). Also, resulting grain size was more refined by increase in reduction ratio, consequently, 96% cold rolled and annealed material showed the best refined grain size. However, in case of annealed material, g fibre texture with its texture component, such as (111) and (111), was developed by increase in annealing temperature. In this study, we systematically discussed the texture development due to the increase in reduction ratio.

Microstructures and pseudoelastic behavior of Ti-Nb-Ge alloys were investigated in order to correlate the pseudoelasticity and microstructure together with martensite transformation. XRD results and transmission electron microscope revealed that stress-induced martensitic transformation takes place during room temperature deformation in the present alloys. Recrystallization heat treatment of the present alloy displayed pseudoelastic behavior to be prominent. It is concluded that the pseudoelastic behavior of a metastable-β Ti-Nb-Ge alloy is correlated to the stress-induced martensite transformation.

The effect of Ge and oxygen content on microstructural formation and mechanical properties of Ti-Nb alloys were investigated in order to design a desirable Ti based alloy through casting process. Three phase mixtures consisting of bcc-structured β phase, orthorhombic structured α" phase and intermediate ω phase were found depending on Nb, Ge, oxygen content in the present alloy system. The volume fraction of α" phase and ω phase decreased with increasing Ge or oxygen content. This microstructural information may indicate that both Ge and oxygen act to increase the stability of β phase rather than α" phase in metastable β-Ti based alloys prepared by water quenching. Elastic modulus values were sensitive to phase stability of constituent phases.

This study is to examine wear properties of Ti-Nb-Si alloys under dry condition and to investigate its wear mechanism. A ball-on-disc type wear testing machine was used to evaluate the wear factor. Optical microstructure observation revealed that the microstructure appeared to mixture appearance consisting of β phase and small amount of α″ martensite phase. Yield strength increased with increasing Nb and Si content. Wear resistance of the present alloy are strongly dependent upon yield strength and elastic modulus. Energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) analysis confirmed the tribo-chemical reaction between the alumina ball and the present alloy occurs due to decomposition of alumina to aluminum.

Pseudoelastic behavior of Ti-xNb-yGe alloys, where x=22~28at.% and y=0.5~2.0at.%, was investigated by controlling martensite start temperature and phase stability of β phase. Cyclic tensile test was carried out to display a pseudoelastic behavior at room temperature. Determination of the martensitic transformation temperature (Ms and Mf) and reverse transformation temperature (As and Af) of the alloys were carried out using differential scanning calorimetry (DSC). Optical microscopy, X-ray diffraction (XRD) and DSC results revealed that Ge is stronger stabilizer of β-phase than Nb. XRD spectra of the deformed specimens confirmed that the crystal structure of stress-induced martensite phase is orthorhombic structured α″. It is concluded that pseudoelasticity of the present Ti-Nb-Ge alloy is closely associated with phase stability, and metastable β-phase is better to increase pseudoelasticity than stable one.

Texture and cyclic tensile behavior of Ti-26Nb-0.5Si (denoted as atomic percent) alloys in which the microstructures were varied by quenching, cold rolling and recrystallization heat treatment were investigated in order to understand the relationship between pseudoelastic behavior and texture formation. Three phase mixtures consisting of bcc-structured β phase, orthorhombic structured α" phase and hcp-structured intermediate ω phase were characterized to display the constituent phases. The volume fraction of constituent phases was found to be insensitive to the given materials processing. Two-stage yielding, one at low stress with low strain hardening rate and the other one at high stress with high strain hardening rate, appeared to exhibit a characteristic flow behavior in the present alloys. It is revealed that stress-induced martensite transformation resulting in two-stage yielding was closely associated with pseudoelasticity. On the basis of texture analyses, we have suggested that pseudoelasticity of the present alloys is hindered by the development of {001} rotated cube component.

Microstructure and texture of Ti-Nb-Si based alloys, prepared by water quenching from β-phase field, cold rolling and recrystallization heat treatment followed by water quenching, were investigated in terms of optical microstructure and analysis of X-ray pole figure result. Optical microstructure observation and X-ray diffraction analysis revealed that the microstructure of as-quenched sample appeared to mixture appearance consisting of mostly bcc-structured β phase and small amount of orthorhombic-structured α″ phase. After cold rolling elongated structure parallel to the rolling direction was observed, and equiaxed structure with the average grain size of about 20~30μm was developed for the sample after recrystallization heat treatment. In as-cold rolled sample we have found well-developed α-fiber texture components which are frequently observed in bcc-structured metals and alloys. In recrystallized sample, rotated cube texture component was weakly detected. The variation of elastic modulus values was interpreted in terms of changes in texture components depending on thermomechanical processing.

Microstructure and texture formation behavior of a gold rod and fine wire which were produced by continuous casting process and drawing, respectively were investigated by means of optical micrographs, TEM images and X-ray pole figure. Well-developed unidirectional structure aligned to casting direction was found in 7 mm gold rod. Higher casting speed was found to be favor in developing the directional microstructure. In the sample with unidirectional microstructure developed in partial it is found that dislocation substructure with nano-size appeared to display a characteristic feature, while no dislocation substructure is seen in the sample with complete unidirectional microstructure. Interface stability between solid and liquid may be responsible for this difference in dislocation substructure. fiber component was observed to be well developed over the whole microstructure in unidirectional sample. With decreasing casting speed fiber component became weak. Two texture components consisting of fiber and fiber were seen in as-drawn samples. For the development of fiber texture component initial texture component plays a more significant role than subsequent annealing process. Annealing heat treatment would be effective way to control the formation of fiber component. On the basis of results obtained it is suggested that both the initial texture and final annealing are important in controlling the texture of gold bonding wire.

The microstructures and mechanical properties of the bulk Al-Fe-(Mo, V, Zr) alloy produced by melt spinning process and subsequent hot extrusion at 693K in the extrusion ratio of 25 to 1 were investigated. TEM observation revealed an equiaxed grain structure with the average grain size of 200 nm for the extruded bulk alloy. Extremely fine dispersoids based on Al-Fe phases, Al-Fe-(Mo, V) phases and Al-Zr phases were observed to be distributed uniformly within grains and at grain boundaries. The size distribution of the binary Al-Fe and the Al-Fe-(Mo, V) phases were ranged from 20 nm to 50 nm, whereas the Al-Zr phase was less than 10 nm. The very high tensile strength of about 800MPa was achieved at room temperature for the extruded bulk alloy.

In this study, we have investigated a texture and microstructure of gold bonding wire by transmission electron microscope, X-ray diffraction and electron back scatter diffraction according to process parameters, such as casting, drawing and annealing processes. The gold was cast into 7mm diameter rods by vertical continuous casting technique, the cast rods were drawn to a final wire size of 25μm, and then the fine wires were annealed. A unidirectional solidification structure having fiber texture of cast rod was strongly developed depending on casting route and speed. In the sample with unidirectional microstructure developed in partial it is found that dislocation substructure with nano-size appeared to display a characteristic feature, while no dislocation substructure is seen in the sample with complete unidirectional microstructure. Interface stability between solid and liquid may be responsible for this difference in dislocation substructure. With decreasing casting speed fiber component became weak. Initial texture plays important role on development of fiber texture at drawing and final annealing stages. On the basis of results obtained it is suggested that both the initial texture are important in controlling the texture of gold bonding wire.

Elastic modulus and mechanical property of Ti-Nb-O alloys prepared by arc melting and subsequent water quenching were investigated in order to correlate the result of microstructural observations and phase stability. In quenched state, it was evidently observed that the volume fraction of bcc-structured β phase increased with increasing content of oxygen, and the occurrence of intermediate ω phase was suppressed in the present Ti-Nb based alloys. This microstructural result was supported by changes in martensite transformation temperature, which decreased with increasing content of oxygen within the chemical composition range investigated. Therefore, it is suggested that oxygen plays a role as a β phase stabilizer rather than α stabilizer in metastable β Ti-Nb alloys. Yield strength increased with increasing content of oxygen without a large consumption of ductility in Ti-Nb based alloys with Nb content of 24% while elongation value decreased with increasing content of oxygen in Ti-Nb based alloys with Nb content of 28%. The variation of mechanical property was explained by the phase stability, phase formation and microstructure in correlation with oxygen and Nb content.

The effects of oxygen content on microstructures, elastic modulus and tensile properties of quenched Ti-Nb alloys were investigated in order to design a desirable Ti based alloy through casting process. From the microstructural and phase analysis, it is evidently revealed that the volume fraction of β phase increased with increasing content of oxygen, and the occurrence of intermediate ω phase was suppressed in metastable β Ti-Nb based alloys. Martensite transformation temperature decreased with increasing content of oxygen. Therefore, it is suggested that oxygen acts to stabilize β phase rather than α stabilizer in quenched state. Yield strength increased with increasing content of oxygen without a large consumption of ductility in metastable β Ti-Nb based alloys. The variation of mechanical property was explained by the phase stability, phase formation and microstructure in correlation with oxygen and Nb content.

Bulk Al-8Fe-2Mo-2V-1Zr (wt.%) alloys were produced by melt spinning which can give rise to develope a nano crystalline structure in terms of rapid cooling and subsequent hot extrusion. The bulk alloys exhibited multi-phase microstructures consisting of ultra fine equiaxed grains with the average grain size of 100nm and a fine intermetallic Al-Fe, Al-V and Al-Zr based phase having less than 50nm in particle size. From compression test, it was revealed that the bulk alloys have very high yield strength at both room temperature (942MPa) and elevated temperatures (651MPa at 473K, 500MPa at 573K, respectively).

Mechanical Property and elastic modulus of Ti-Nb based alloys with various Si content, prepared by water quenching from high temperature β phase field, cold rolling and recrystallization heat treatment followed by water quenching, were investigated in terms of tensile test and resonance vibration method. TEM observations revealed that in addition to orthorhombic structured α" phase and bcc structured β phase, an intermediate ω phase is characterized in the microstructure. The volume fraction ratio of constituent phases was dependent upon Nb and Si contents. In as-quenched samples yield strength increased with increasing Si content. This propensity was similar to the cold rolled alloys. In recrystallized samples however, yield strength decreased slightly with increasing Si content even though the alloy containing higher Si content showed smaller grain size. From these results, it is found that strength of the present alloy influences largely on solid solution hardening and phase stability but does weakly on grain size. Yield strength of cold rolled sample was higher than as-quenched and recrystallized samples. Elastic modulus values were measured to 55GPa, 63GPa and 44GPa for as-quenched, cold rolled and recrystallized samples, respectively. The variations of yield strength and elastic modulus values were interpreted in terms of changes in microstructure as well as grain size and phase stability.

We have newly designed a metastable β Ti-Nb-Si based alloy with biocompatible alloying elements without containing toxic V or Al for orthodontic applications. Microstructures and pseudoelastic behavior of β Ti-Nb-Si alloys were investigated in order to correlate the pseudoelasticity and microstructure together with martensite transformation. Nb and Si alloying to the present alloy make β phase to be stable. It is found that metastable β phase is favorable to display pseudoelastic behavior than stable or unstable β phase. Optical microscope (OM) revealed that stress-induced martensitic transformation takes place during room temperature deformation in the present alloys. Within the alloys having β (bcc) phase studied the alloy with low content of Si appeared to exhibit a dominant behavior for stress-induced martensitic transformation than that with high content of Si. After recrystallization heat treatment pseudoelasticity of the present alloy appeared to be prominent. The pseudoelastic behavior of this alloy was correlated to the stress-induced martensite transformation. Pseudoelasticity of the present alloys is hindered by the development of {001} rotated cube component.

This work describes the effect of microstructures on elastic modulus in Ti-26Nb-xSi alloy (x=0.5~1.5at.%) prepared by arc melting, cold rolling and recrystallization heat treatment. OM observation and x-ray diffraction analysis revealed that the microstructure of as-quenched sample appeared to mixture appearance consisting of mostly bcc-structured β phase and small amount of orthorhombic-structured α″ phase. After cold rolling, elongated structure parallel to the rolling direction was observed, and equiaxed structure with the average grain size of about 20~30μm was developed for the sample after recrystallization heat treatment. In as-quenched sample randomly distributed feature of pole figure was characterized without showing a specific texture component. In cold-rolled sample α-fiber, γ-fiber and rotated cube texture components were detected. After recrystallization heat treatment the intensity of α-fiber texture component was markedly decreased, while the rotated cube component becomes sharpened and γ-fiber component remains relatively unchanged. The elastic modulus increased by cold rolling and then decreased by recrystallization over the entire chemical composition range investigated. The variation of elastic modulus values was interpreted in terms of changes in texture components depending on thermomechanical processing.

Microstructures and mechanical properties of Ni3Al based intermetallic alloys produced by vacuum arc melting and vacuum induction melting were investigated in terms of phase analysis using scanning electron microscope (SEM) equipped with energy dispersive X-ray spectroscopy (EDS), X-ray diffractometer and tensile test machine. The duplex microstructural feature consisting of γ’ matrix phase and small intermetallic dispersoids was observed to be distributed over the whole microstructure in the Zr and/or Mo-added samples. From the SEM-EDS analysis of the alloys, it is clearly confirmed that the Mo is solved both into γ’ matrix phase and intermetallic phase while Zr has a role to form an intermetallic Ni5Zr phase for the entire alloys investigated. The ultimate tensile strength of the present alloy was superior to iron-based and Ni-based die materials especially in the high temperature region. The mechanical results obtained will be discussed in correlation with microstructural observations, phase analyses.

Microstructures and mechanical properties including elastic modulus were investigated in terms of ternary alloying elements Si addition, Nb content variations and tensile test. Martensite structure with α'(hcp) or α"(orthorhombic) was observed in Ti-xNb-1.5at.%Si, where x=10-20at.%. The crystal structure of martensite formed from water quenching process was largely dependent upon Nb content but does not on Si content. On the basis of experimental results obtained, it is suggested that Si has an effective role to suppress the precipitation of ω phase leading to reduction in elastic modulus in the metastable β phase region. Metastable β phase region was superior to reduce the elastic modulus than stable β phase region in the present alloy system. The minimum value of elastic modulus was measured to 48GPa. We have found that stress-induced martensitic transformation takes place during the deformation in the present alloys. Within the alloys having β(bcc) phase studied Nb-poor region appeared to exhibit a dominant behavior for stress-induced martensitic transformation than Nb-rich region. This result suggests that metastable β phase is superior to stable β phase for the occurrence of stress-induced martensitic transformation in the present alloy system.

Al-8Fe-2Mo-2V-1Zr alloy powders were prepared by gas atomization and melt spinning method. In melt spinning technique, melt spun ribbons were pulverized by a speed rotor mill to make a powder shape. In order to produce a bulk form, powders were canned and hot extruded in the extrusion ratio of 25 to 1 at 693K. For the gas atomization and hot extrusion processed bulk material, equiaxed grains with the average size of 400 nm and finely distributed dispersoids with their particle sizes ranging from 50nm to 200nm were observed to display a characteristic nano-structured feature over the entire region. For the melt spun and hot extrusion processed alloy, a refined microstructural feature consisting of equiaxed grains with the average size of 200 nm and fine dispersoids with their particle sizes under 50 nm appeared to exhibit a difference in microstructure. Yield strength of the latter alloy was higher than that for the former alloy up to elevated temperatures. The maximum yield strength was measured to about 800 MPa at room temperature for the latter alloy.

Two atomized alloy powders, those chemical compositions are Al-10Si-5Fe-1Zr and Al- 10Si-5Fe-4Cu-2Mg-1Zr, were pre-compacted by cold pressing with 350MPa and subsequently hot forged at temperatures ranging from 653K to 845K and at an initial strain rate of 10-2/s in order to produce bulk cylindrical type alloys with the diameter of 10 mm. The addition of Cu and Mg into the present alloy causes a decrease in the eutectic reaction temperature of Al-10Si-5Fe-1Zr alloy from 841K to 786K and results in a decrease of flow stress at the given forging temperature. TEM observation revealed that in addition to Al-Fe based intermetallics, Al2Cu and Al2CuMg intermetallics appeared to display the alloying effect additionally. The volume fraction of intermetallic dispersiods increased by the addition of Cu and Mg. Compressive strength of the present alloys was closely related to the volume fraction of intermetallic dispersoids.

We report on advanced Ni3Al based high temperature structural alloys with refractory alloying elements such as Zr and Mo to be apllied in the fields of die-casting and high temperature press forming as die materials. The duplex microstructure consisting of L12 structured Ni3Al phase and Ni5Zr intermetallic dispersoids was observed to display the microstructural feature for the present alloys investigated. Depending on alloying elements, the volume fraction of 2nd phase was measured to be different, indicating a difference in solid solubility of alloying elements in the matrix γ’ phase. Lattice parameter of matrix phase increased with increasing content of alloying elements. In the higher temperature region more than 973K, the present alloys appeared to show their higher strength compared to those obtained in conventional superalloys. On the basis of experimental results obtained, it is suggested that refractory alloying elements have an effective role to improve the high temperature strength in terms of enhanced thermal stability and solid solution hardening.

High strength Al-8Fe-2Mo-2V-1Zr (wt.%) alloys fabricated by a melt spinning and a hot extrusion process were produced to correlate the microstructure and mechanical property. Melt spun ribbon prepared by single roll melt spinner showed a cellular structure with an average size of 10nm and Al-Fe based intermetallic dispersoid of less than 10nm in particle size. The melt spun ribbon obtained was then pulverized to make a powder shape followed by hot extrusion at 648K, 673K, 723K and 773K in extrusion ratio of 5 to 1, respectively. Equiaxed grain structure containing Al-Fe based intermetallic phase was observed in all extruded specimens. According to increasing extrusion temperature, the grain size increased and particle size of intermetallic dispersoid. The lattice parameter increased from 0.4051nm to 0.4059 nm with increasing extrusion temperature from 648K to 773K, those values were larger than that obtained in pure Al (0.4049nm). Yield strength of the specimen extruded at 648K measured to 956MPa at room temperature, 501MPa at 573K and 83MPa at 773K, respectively. With increasing extrusion temperature yield strength decreased significantly at room temperature and even in the intermediate temperature range, while no noticeable difference in yield strength was observed at 773K.

In the present study, we have attempted to refine a microstructure of conventional AZ31 magnesium alloy using a new combination process including hot extrusion followed by a cold equal channel angular pressing (ECAP). ECAP die was specially designed with an inner die corner angle of 135 degree, the fillet angle of 45 degree and dimensional thermo-coupled elastro-plastic material model in order to understand the change of stress and strain of the deformed material after a cold ECAP. ECAP for the AZ31 alloy, which was extruded in the extrusion ratio 20 to 1 and heat-treated at 623K, was successfully carried out at room temperature. The uniform shear band obtained from experiment was well matched with the zone of effective strain more than 0.533 estimated from calculation. On the basis of the results, it is suggested that the room temperature ECAP makes microstructure to be refined and the basal plane to be rotated slightly from extrusion direction to axis direction. Compressive yield strength of AZ31 alloy can be enhanced up to twice in applying ECAP process. Hall-Petch relations do not fit to the experimental data This can be ascribed to the texture effect. Room temperature ECAP process is very promising in improving mechanical properties of AZ31 alloy in terms of grain refinement and texture control.

We report on advanced β-titanium alloys having excellent biocompatibility without containing V or Al that has been known as a toxic element for human body, and on a low elastic modulus to be used in the fields of artificial joint and dental implant in the replacement of natural human bone. Martensite structure with α’ (hcp) or α”(orthorhombic) was observed in Ti-xNb-1.5at.%Si, where x=10-20at.%. The crystal structure of martensite formed from water quenching process was largely dependent upon Nb content but does not on Si content. On the basis of experimental results obtained, it is suggested that Si has an effective role to suppress the precipitation of ω phase leading to reduction in elastic modulus in the metastable β phase region. Metastable β phase region was superior to reduce the elastic modulus than stable β phase region in the present alloy system. The minimum value of elastic modulus was measured to 48GPa.

The mechanical properties of pre-sintered Al-10Si-5Fe-1Cu-0.5Mg-1Zr (wt%) alloy were investigated in the temperature range from 673K to 813K and at initial strain rates from 10-4 to 100 s-1. In the high temperature range of 793K and 813K, the strain rate sensitivity index was close to that for superplasticity (0.3). Stress exponent was estimated to be 2 and 5 in the temperature range from 793K to 813K and from 673K to 773K, respectively. The activation energies for the plastic flow were calculated to 77kJ/mol for the n=2 region and 127kJ/mol for the n=5 region. These values were close to that for grain boundary diffusion and self-diffusion in pure aluminum, respectively. We found that a dynamic recrystallization (DRX) and grain boundary sliding (GBS) occur depending on the test temperature and stain rate. A filament-like phase containing Cu and Mg was observed in the cracked surface of the specimen deformed at 793K and 813K.

Microstructure and mechanical property at room temperature and at 1773 K of Nb-Si based refractory intermetallic alloys were investigated in terms of compression and fracture toughness test. Mo and V were chosen as ternary alloying elements because of their high melting points, atomic sizes smaller than Nb. Both ternary alloying elements were found to have a significant role in modifying the microstructure from dispersed structure to eutectic-like structure in Nb solid solution/Nb5Si3 intermetallic composites. The 0.2% offset yield strength at room temperature increased with increasing content of ternary elements in Nb solid solution and volume fraction of Nb5Si3. At 1773 K, Mo addition has a positive role in increasing the yield strength. On the other hand, V addition has a role in decreasing the yield strength. The fracture toughness of ternary alloys was superior to binary alloys. Details will be discussed in correlation with ternary alloying, volume fraction of constituent phase, and the microstructure.

A recrystallized Co-Ni-Cr-Mo based superalloy was produced by cold working of 72% and subsequent recrystallization heat treatment. Microstructural observation revealed that a full recrystallization of the cold-worked alloy occured when heat treatment was performed at and above 1273K for 1h. So that, recrystallization heat treatment was carried out in a temperature range from 1273K to 1473K for 1h~24h, by which the average grain size was controlled to 28µm～238µm. Tensile tests were carried out from room temperature (RT) to 1073K in order to understand the effect of grain size on the mechanical properties of the Co-Ni-Cr-Mo based superalloy. At RT and 943K, yield strength, tensile strength and elongation of the recrystallized alloy were improved with decreasing grain size. The alloy having a grain size less than 42µm exhibited a steady-state flow behavior in the true stress-true strain curve at 943K. However, the alloy having a grain size of 28µm showed lower yield strength than that of 42µm at 1073K. It was found that the steady state flow is closely related to the occurrence of {111} deformation twinning in the Co-Ni-Cr-Mo based superalloy.

The porous Ti-45at.%Al-1.6at.%Mn intermetallic alloys were fabricated by reactive sintering process using warm extruded mixtures consisting of elemental titanium and aluminum alloy powders and subsequent heat treatment. At the extrusion stage, three different extrusion ratios were tried in order to provide the various types of porous structure in the microstructure. It was clearly found that the shape of pore and porosity are largely dependent on extrusion ratio. With increasing extrusion ratio the porosity decreased, on the other hand, aspect ratio of porous structure increased. The porosity measured was in the range from 35% to 45%. The maximum porosity of 45% can be obtained when the specimen was reactively sintered with the extrusion ratio of 1. A directionality of pores oriented parallel to the extrusion direction was found when the extrusion ratio of powder mixtures is 6 and above.

The microstructure and mechanical property of Al 5083 alloy produced by spray forming and then subsequent hot extrusion at 693 K were investigated. In order to compare the mechanical property and microstructure permanent casting and hot extrusion was also carried out in air. The spray formed Al 5083 alloy showed the microstructure consisting of Al matrix phase, MnAl6, FeAl6 and Mg2Al3 dispersoids. This microstructural feature analyzed is quite similar to Al5083 alloy prepared by conventional casting. However, a noticeable difference in grain size appeared between those alloys with different processing. It is found that spray forming plays an effective role to reduce the grain size of the present Al 5083 alloy. At room temperature, the yield strength of spray formed alloy was higher than that of permanent casting alloy. At high temperature, spray forming appeared to display a higher elongation value than conventional casting. It is therefore suggested that spray forming of the present Al 5083 alloy promotes to increase yield strength at room temperature and to increase elongation at high temperature. Details will be discussed in relation with the results obtained.

Compressive deformation behavior of pre-sintered Al-10Si-5Fe-1Cu-0.5Mg-1Zr (wt%) alloy containing 15% of porosity was investigated in the temperature range from 753 K to 793 K and at strain rates from 10-4 to 100 s-1. From the microstructural observation, it was revealed that the occurrence of grain boundary sliding accomodated by dynamic recrystallization during the compressive deformation was closely associated with the considerable decrease in the porosity of the pre-sintered alloy. In the specimens deformed at 793 K with 10-4~100 s-1 and at 773 K with 10- 4~10-2 s-1, we have found an evidence of the occurrence of a liquid phase during compressive deformation in the microstructure. The liquid phase was considered to promote particle boundary sliding and hinder the reduction of the pore.

Mechanical property of carbon-free Co-Ni-Cr-Mo based superalloy was investigated at room temperature and at 943 K. Cold working and subsequent recrystallization heat treatment was carried out to produce an equiaxed grain structure. The average grain size was controlled to 28 and 238 µm in order to understand the effect of grain size on mechanical property and flow behavior. At room temperature and 943 K, 0.2% offset yield strength increased with decreasing grain size to exhibit grain size dependence. At 943 K, a specimen with the average grain size of 28 µm showed higher yield strength than that obtained with a specimen having the average grain size of 238 µm. A steady-state like behavior leading to an extensive ductility was observed in the fine-grained specimen. {111} deformation twinning was found to correlate with the steady-state like behavior in the true stress-true strain curve. It was suggested that grain refinement is favor to enhance the fracture strength and allow the deformation twinning to occur.

Phase relation and mechanical property of Cr2Zr Laves phase alloy systems were investigated in terms of ternary alloying based on geometric concept of atomic sizes of constituent alloying elements. Ternary phase diagrams at 1573 K of Cr-Zr-Nb and Cr-Zr-Hf alloy systems were examined using SEM-EPMA analysis in order to understand the site occupation behavior of ternary elements and the phase stability of Laves phase. In the Cr-Zr-Nb alloy system, Laves phase had a broad off-stoichiometry range in the middle of Cr2Zr-ZrNb2 pseudo-binary line, while a limited and uniform range of Laves phase field was found in the Cr-Zr-Hf alloy system. For both Laves phase alloys, 0.2% offset yield strength decreased with increasing content of allying element, on the other hand compressive ductility increased with increasing content of alloying elements. Details will be discussed in conjunction with microstructure, phase stability and atomic size considerations.

The effects of temperature and strain rate on porosity variation have been studied in order to obtain an optimum processing condition in the powder forging of pre-sintered Al-Si-Fe based alloys. Compressive test was carried out at temperatures from 673 K to 813 K and at initial strain rates from 10 0 s -1 to 10 -3 s -1 . The prior porosity of 15% in the pre-sintered alloy decreased with compressive deformation depending on temperature and strain rate. We have found that the amount of liquid phase containing Cu and Mg is closely associated with porosity changes of the present alloys deformed. It is suggested that the occurrence of liquid phase during the compressive deformation may facilitate the particle boundary sliding resulting in decreasing porosity in the pre-sintered Al-Si-Fe based alloys.

Room temperature tensile behavior of nano-structured Al-8Fe-2Mo-2V-1Zr alloy prepared by spray forming and subsequent hot extrusion (as-received) was investigated by tensile test in relation to microstructure and processing step. The microstructure of spray formed and extruded sample is composed of equiaxed grain structure of about 500 nm with finely dispersed intermetallic phases such as Al 3 (Fe, Mo, V, Zr) and Al 6 (Fe, Mo, V, Zr) at grain boundaries and grain interiors. The average particle size of dispersoids was measured to be 40 nm. Warm rolling of extruded sample accelerates an occurrence of dispersoids less than 20 nm. The elongation of warm rolled alloy is higher than that of as-received one, although yield strength decreases by warm rolling. It is suggested that warm rolling of as-received alloy causes the increase in elongation and decrease in yield strength due to decreasing content of solid solution strengtheners by activated diffusion process to form a precipitate in supersaturated Al matrix phase. Details will be discussed in correlation with XRD, SEM, TEM analyses.

Microstructure, precipitation behavior and mechanical property of the supersaturated single-phase Co-based superalloy are investigated in terms of materials processing, and aging condition. Mechanical twining is found to occur at the cold rolling stage. A second phase has been found to precipitate in the cold-rolled alloy with an aging treatment at 1023 K and been revealed to be the Co7Mo6 phase. Hardness values increase with increasing aging time up to 4 h and then decrease with prolonged time of aging. The maximum value of hardness is obtained to be 686 Hv (micro Vickers hardness).