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The effect Si addition on structural evolution during heat treatment was studied in mechanically alloyed Al_(65)Cu_(20)Fe_(15) and Al_(65)Cu_(20)Fe_(10)Si_5 Powders using X-ray diffractometry, transmission electron microscopy and differential scanning calrorimetry. Both alloy powders processed for 10 hr showed a layered microstructure of Al, Cu and Fe. Mechanical alloyed powders for 10 hr transformed into several different phases depending on heat treatment conditions: Both alloy powders, heat treated for 10 hr at 335℃, showed a mixed structure of Al₂Cu, Al_7Cu₂Fe and unreacted Fe; Both alloy specimens heat treated for 10 hr at 550℃ showed 1/1 cubic approximant phase as a major phase; Al_(65)Cu_(20)Fe_(15) and Al_(65)Cu_(20)Fe_(10)Si_5 alloy powders heat treated for 5 hr at 750℃ showed a mixture of Al(Fe, Cu) and Al_(13)Fe₄ and 1/1 cubic approximant phase as a major phase with some icosahedral phase and other approximant phases, respectively. Addition of Si promotes the stability of quasicrystalline and approximant phases. No icosahedral phase or approximant phases were observed in both alloy powders heat treated either at 550℃ or at 750℃ after 120 hr Mechanical alloying.

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To investigate the effect of cooling rate and Nb-content on the microstructure and corrosion characteristics of Zr-Nb alloys, four kinds of Zr-xNb(x = 0.5, 1.0, 2.0, 3.0 wt.%) alloys were designed and manufactured. The corrosion test and microstructural study were performed for the specimens which were heat-treated at 1020℃ for 30 minutes and then cooled by water quenching, oil quenching and air cooling. When the cooling rate was the same, the addition of Nb enhanced the change of microstructure from Widmansta¨tten to Martensite and decreased the corrosion resistance. In the equal Nb-content, the lath width and corrosion resistance increased with decreasing the cooling rate. It is considered that corrosion resistance would be related with the distribution of supersaturated Nb in Zr matrix.

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Shape memory effect(SME) and martensitic transformation kinetics of Co-X%Ni alloys were studied. SME decreased with increasing nickel content, and the SME was proportional to the initial ε martensite content, suggesting that the SME of the Co-X%Ni alloys is associated with coalescence of the pre-existing ε plates. Thermal cycling(ε↔α) increases the ε martensite content, and the SME becomes larger with increasing number of thermal cycles. The martensitic transformation kinetics of the Co-X%Ni alloys can be expressed as Y=1-exp[-0.00526(M_s-25)], where Y is volume fraction of ε martensite, and M_s is the martensite start temperature.

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The effect of cooling rate on the metastable liquid phase separation(MLPS) in Cu-Fe binary and Cu-Fe-Co ternary alloys was investigated using an electromagnetic levitation technique. The cooling rate during phase separation was controlled by dropping the specimens to copper substrate which was in 15 ㎝ apart from levitation coil. The cooling rate in levitation state was 5-20 Ｋ/sec, and those of free falling and after splash were 130 Ｋ/sec and 2300 Ｋ/sec respectively. In the specimen solidified at medium cooling rate(about 130 Ｋ/sec), secondary phase separation was observed in both L1(Fe- or Fe, Co-rich phase) and L2(Cu-rich phase) phases that have been formed by primary phase separation. In this case the phase separation occured by nucleation and growth. Spinodal decomposition is considered as a dominent phase separation mechanism in L1 droplets solidified at fast cooling rate (about 2300 K/sec). There was no evidence of spinodal decomposition in L2 phase.

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The effects of TRansformation Induced Plasticity(TRIP) phenomena on the plastic deformation of 0.2C-1.5Si-1.5Mn multiphase steel have been investigated at various heat treatment and stress conditions. In order to estimate a deformation behavior under multi-axial stress rotate, the hole expansion(HE) tests were carried out. It is shown that the formability evaluation from the uni-axial tensile tests was quite different from the formability measured from multi-axial HE-tests because of different stress state. The formability in the multi-axial stress state decreased due to the extinction of the transformable retained austenite at relatively earlier deformation stage and the production of irregular α` martensite. The voids of TRIP steel were initiated exactly at the interface between transformed martensite and ferrite matrix regardless of stress state. In addition, new form of experimental formula is proposed in order to predict the multi-axial formability of the TRIP steel from the results of uniaxial tensile test.

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Fracture toughness of the weldment of a pressure vessel steel was evaluated by ABI(automated ball indentation) tests in the transition temperature region. The evaluation procedure was based on the semi-empirical IEF(indentation energy to fracture) theory that has been recently developed using two basic assumptions. One is that the triaxial stress state beneath the ball indenter is similar to that of a crack tip ahead. Second, the fracture stress of a material is almost temperature-independent in case the stress-controlled cleavage fracture occurs. The fracture toughness characteristics determined by ABI tests were comparable to those obtained by standard 3-point bending tests. The multi-passed weldment was divided into two parts by their macroscopical appearances; as-solidified region and recrystallized one. However, both regions did not show a significant difference in fracture toughness as well as hardness and flow properties in the tested material.

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The present study aims to investigate the wear and fracture properties of the hardfacing alloys with a variation of volume fraction of complex carbides. The hardfacing alloys were deposited twice on a low-carbon steel place by a submerged arc welding method. In order to investigate the effect of complex carbides, different fractions of complex carbide powders included inside the hardfacing electrode were employed. As the volume fraction of complex carbides, particularly that of cuboidal carbides, increased, hardness and wear resistance increased. In-situ observation of fracture process showed that microcracks were initiated at complex carbides and that shear bands were formed between them, leading to ductile fracture. Hardness wear resistance, and fracture toughness were improved in hardfacing alloys reinforced with complex carbides in comparison with high-chromium white iron hardfacing alloys. This was mainly associated with the homogeneous distribution of fine complex carbides in the bainitic matrix.

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The objective of this study is to investigate the dynamic deformation and fracture behavior of an oxide-dispersed tungsten heavy alloy fabricated by mechanical alloying. The tungsten alloy was processed by adding 0.1 wt.% Y₂O₃ powders during mechanical alloying in order to form fine oxides in tungsten/matrix interface. Dynamic torsional tests were conducted for this alloys, and then the test data were compared with those of a conventionally liquid-phase sintered specimen. The refinement of tungsten particles could be obtained after mechanical alloying and multi-step heat-treatment without an increase in volume fraction of the interfacial area between tungsten particles occurred over broad deformed areas in this alloy , expecting the low possibility of the shear band formation. Also, the oxide dispersion strengthening was effective in promoting the interfacial fracture since fine oxides acted as a initiation site of interfacial fracture. These findings suggested that the idea of forming fine oxides would be useful for improving the self-sharpening effect and the penetration performance of tungsten heavy alloys.

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Effect of W on creep properties of martensitic heat resistant HT9M steel have been investigated at 600 and 650℃. The creep rupture strength of W added HT9M steel was higher than that of HT9M steel. The addition of W decreased the self-diffusion rate of iron atom and delayed the microstructural degradation during creep. As seen in dispersion-strengthened alloys, high stress exponent, n, and activation energy, Q_c, are found in these steels. During creep deformation, formation of subgrain, growth and agglomeration of precipitates and precipitation of Laves phase occurred. These microstructural changes made the steels soften. When the stress was applied, hardness values were abruptly decreased at an early stage and remained constant with time. The degree of softening of W added HT9M steel was less than that of HT9M. But when the stress was not applied, the decrease of hardness value was not occurred in both steels.

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A TFS(Ti-xFe-ySi) alloy system has been newly designed for a high strength titanium alloy with low cost. In this study, Fe was selected as β-stabilizer to replace expensive β-stabilizing elements such as V, Mo. In addition, Si was selected as third element to compensate the high temperature mechanical properties. Fe contents of over 4 wt% stabilize β phase even at room temperature. Coarse silicide chain was formed at prior β grain boundary in TFS alloys containing over 2 wt% of Si, and deteriorated the mechanical properties. Fe was dissolved in Ti matrix as solid solution and increased α/β phase boundaries. Si was also dissolved as solid solution or precipitated finely in Ti matrix. The optimum composition range was Ti-4Fe-(0.5∼1)Si. Ultimate tensile strength was a range of 799∼831 ㎫ at room temperature and 532∼585 ㎫ at 400℃ which are comparable to Ti-6Al-4V.

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