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Nb solubility and phase boundary in Zr-rich region of Zr-Nb binary alloy system were investigated using sponge Zr containing 700 ppm Fe and 700 ppm O. Characteristics of microstructures of the isothermally annealed specimens was analyzed by hardness measurement, OM, SEM and TEM. The transformation temperature from alpha to beta in the pure Zr and the monotectoid temperature were 930±3℃ and 585±3℃, respectively. It was not consistent with the previous result by Cox where transformation temperature and the monotectoid temperature are 863℃ and 610℃, respectively. The Nb solubility measured using TEM in this study was 0.2 wt.%, which was lower than 0.6 wt.%, measured using OM by Cox. It can be concluded that the oxygen contained in sponge Zr played a role to increase the transformation temperature of alpha to beta and to decrease the monotectoid transformation temperature.

keyword : Zr , Zr-Nb , Fuel cladding , Phase diagram , Phase transformation

The effect of Mn contents in 3000 aluminum alloys on formation of intermetallic compounds and extrusion pressure was investigated. The eutectic intermetallic compounds α-Al(Fe, Mn) Si were precipitated when Mn was added in the range of 0.3-1.5 wt.% in 3000 aluminum alloys. The concentration of Mn in the eutectic intermetallic compounds was increased with the increase of Mn addition. Also, the primary intermetallic compound of Al_6(Fe, Mn), which Mn atoms were replaced completely by Si atoms, was precipitated in the addition of 1.5 wt.% Mn. The TEM analysis represents that the eutectic intermetallic compounds have bee structures and primary intermetallic compounds has rhombohedral structure. The amounts of intermetallic compounds were increased with the amounts of precipitated compounds, resulting in the increase of yield stress as well as extrusion pressure.

keyword : Intermetallic compounds , α-A1(Fe , Mn)Si , Al6(Fe , Mn) , Extrusion pressure

The effects of pressing temperatures and initial microstructures on the formability during equal channel angular pressing (ECAP) of Ti-6Al-4V alloy were investigated in this study. ECAP above 600℃ was successful without producing any noticeable segments at the specimen surfaces after a single pass of pressing. Also, the equiaxed microstructure showed more uniform material flow at the surface than the Widmanstatten microstructure. However, this microstructural influence was diminished with the increase of the number of ECA pressing. A flow localization parameter that quantifies the flow non-uniformity was used to explain the microstructural effect.

keyword : Equal channel angular pressing , Ti-6Al-4V alloy , Flow - localization parameter , Flow softening

This study is concerned with the analysis of fracture toughness in the transition region of a Mn-Mo-Ni low-alloy steel according to ASTM E1921 standard test method. Elastic-plastic cleavage fracture toughness, K_Jc was determined by 3-point bend tests, using precracked Charpy V-notch (PCVN) specimens, and then the measured K_Jc, values were interpreted by the 3-parameter Weibull distribution with a theoretical slope of 4. Fractographic observation indicated that the critical distance from a precrack tip to a cleavage initiation site linearly increased with increasing the critical J(J_c) value, and that the stretch zone width had a good correlation with K_Jc value, irrespective of testing temperature. Relationship between J_c and critical distance, local fracture stress, and plane strain fracture toughness were discussed on the basis of the cleavage fracture behavior in the transition temperature region.

keyword : Mn-Mo-Ni low - alloy steel , ASTM E1921 stand test method , Elastic - plastic cleavage fracture toughness , Weibull distribution , Critical distance , Stretch zone width

The objective of this study was to investigate the effect of AE activities during fatigue test. Laboratory experiment was carried out to identify AE characteristics of fatigue cracks for compact tension specimen. The relationship between a stress intensity factor and AE signals activity was discussed. From the experimental results, the features of specific parameters such as the length of crack growth, the AE energy and the AE peak amplitude, showed similar trend in their increase as the stress intensity factor increased. The fracture surface of test specimens revealed both intergranular fracture and transgranular fracture. Size and density of slip band were varied as the stress intensity factor increased and AE activities were different according to the slip band shape.

keyword : Acoustic emission , Slip band , Fatigue crack , SWS 490B

Room temperature deformation behavior of a Fe-base intermetallic alloy with the composition of Fe-24 at.% Al has been investigated in this study. Phase composition and degree of order in Fe-24 at.% Al alloy were varied by aging at temperatures corresponding to the various phase fields of DO_3, α+DO_3, α+B2, and B2 to elucidate the effects of microstructure and degree of order on the room temperature deformation behavior. Load relaxation tests were conducted at room temperature and the results were analyzed based on the internal variable theory of inelastic deformation. Room temperature strength of the Fe-24 at.% Al alloy appeared to increase with decreasing degree of order and the hardening effect of disordered α precipitation was found to depend on the shape. All flow curves could be successfully described by the constitutive equations of the internal variable theory and the strain rate sensitivities of the Fe-24 at.% Al alloy at room temperature were very small governed by the frictional stress element.

keyword : Fe-24 at.% Al alloy , Degree of order , α - precipitation , Load relaxation , Internal variable theory , Strain rate sensitivity

Hot compression test was performed on Al-10%Si-2%Cu-1%Mg-0.5%Mn ingot in the temperature range 440∼500℃ and in the strain rate range 0.1∼10/sec in order to understand the hot deformation features and to explore the possibility of uniform distribution of eutectic Si particles. A typical stress-strain curve was well established where the stress increases sharply in the beginning of the deformation, and then decreases gradually in the strain rate of 0.5∼10/sec. Apparent activation energy of 65 ㎉/㏖ and stress exponent of 8.4 were evaluated from the peak stress. Deformation at low strain rate and high temperature is favorable in view of low flow stress and relatively uniform distribution of Si particles. Hard Si eutectics between primary Al grains begin to deform after soft Al grains are easily elongated in an early stage of the deformation. This type of deformation mode strongly demands at least 0.7 true strain to break the net-work structure of eutectic Si effectively. It is suggested that suitable heat treatment prior to the hot deformation be employed to ensure uniform distribution of fine Si particles.

keyword : Hot deformation , Al-Si alloy , Eutectic Si , Uniform distribution , Modification , Activation energy , Stress - strain curve

The microstructure and mechanical properties of the gas atomized and extruded Al-20Si-5Fe-2x (x=Cr, Zr or Ni) alloys were evaluated. The alloy powder was degassed under different vacuum levels in advance of the extrusion. It was shown that the addition of a quaternary element resulted in the formation of very fine particles that contributed to the improved mechanical properties of the alloys, especially at elevated temperatures. Of these additions, nickel was found to be the most effective in increasing the UTS of the alloy both at room and elevated temperatures. Degassing the rapidly solidified powders under better vacuum improved the properties further.

keyword : Rapid solidification , Hypereutectic Al-Si alloy , Transmission electron microscopy ( TEM ) , Tensile strength , Thermal expansion coefficient , Degassing , Extrusion

Valve seat inserts are used in the valve train area of internal combustion engines. Good wear resistance is needed since repeated stress is applied by the valves under combustion gas atmosphere of high temperatures. Currently, most of the valve seat inserts are being manufactured by P/M route. In this study, the characteristics of materials especially the wear resistance with the addition of Cr-hard particles were investigated. The results of this study are as follows: increase in the Cr-hard particles resulted in a decrease of the density of specimens. Pores formed by Cr diffusion from Cr-hard particles was attributed to the reduction in density. As a result, radial crushing strength was decreased with increase of the Cr-hard particals. Engine simulation test indicated that matrix can be hardened with the addition of Cr-hard particles. The excess addition of Cr-hard particles, however, degrades the wear resistance by the increase of residual pores. Therefore, the appropriate amount of Fe-Cr-C hard particles is 1∼5 wt%.

keyword : Valve seat inserts , Sintering , Cr , Microstructure , Mechanical properties , Wear resistance

Pretreatment process is very important in plating process because failure of deposits almost comes from inadequate pretreatment. Recently, in leadframe manufacturing, plasma is used for metal cleaning and modification of polymeric materials. In this study we investigated a possibility of plasma pretreatment for coating technology. Plasma pretreatment was performed for pretreatment in vacuum chamber using Ar and H_2 gas. Specimens were prepared with copper plate. Then, specimens were plated in Ni sulfamate bath using DC and pulse current. To prevent reoxidation of speciemens, plating was conducted immediatly after pretreatment. The surface morphology and roughness were examined by AFM, SEM. And surface residual oxide was investigated with XRD. Also adhesion test was performed by acoustic emission. In the test results optimum gas ratio was 1(Ar):4(H_2).

keyword : Pretreatment , Cleaning , Plasma , Reoxidation