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The effect of the phases of y, α` and ε martensites, which constitute the structure of the 316L stainless steel with austenite structure at room temperature, on the damping capacity at room temperature has been investigated. As a result, various fractions of these phases were formed depending on the degree of cold work. The largest volume fraction of a martensite phase is obtained by 20% cold rolling and showed the highest damping capacity. The damping capacity is strongly controlled by the volume fraction of ε martensite with linear relationship, while the other phases of α` and r have actually no effect on the damping capacity.

keyword : Damping capacity, Deformation-induced martensite, ε martensite phase, Internal friction, Noise, Vibration, Austenitic stainless steel

A previously developed fixed grid finite volume method was used to investigate hydrogen removal process involving surface reaction, diffusion, β/α phase boundary movement and phase transformation that occurs during dehydrogenation of cylindrical β-titanium specimens. Effects of temperature, initial hydrogen content and radius of specimen on times for dehydrogenation were studied. It was found that dehydrogenaion occus through the following 4 steps: (1) lowering of hydrogen concentration at the surface of the β specimen to equilibrium value, (2) lowering of hydrogen concentration gradient to a value matching the surface reaction flux. (3) the (β/α phase transformation by inward movement of the β/α phase boundary and (4) further removal of hydrogen in the a phase specimen to final value. Time for hydrogen removal in the first step increases with increase of temperature and initial hydrogen content, due to the rapid increase of amount of hydrogen to be removed comparing with the increase of hydrogen removal rate. It increases with specimen radius. Time for hydrogen removal in 2nd step decreases with increase of temperature due to increase of hydrogen removal rate and decrease of amount of hydrogen to be removed, but it increases with specimen radius and initial hydrogen content. Time for hydrogen removal in 3rd step (β/α phase transformation) decreases with increase of temperature due to decrease of diffusion and reaction resistance and due to increase of partition coefficient, but it increases with specimen radius due to increase of diffusion and reaction resistance. It increases with initial hydrogen content. Time for hydrogen removal in 4th step decreases with increase of temperature due to decrease of diffusion and reaction resistance as well as equilibrium concentration, but it increases with specimen radius due to increase of diffusion and reaction resistance.

keyword : Thermo hydrogenation, Phase transformation, Numerical analysis, Titanium

The present study is concerned with fabrication and microstructural analysis of Cr₃C₂/SUS316L surface composites using high-energy electron-beam irradiation. Three kinds of powder mixtures, i.e., 50Cr₃C₂-50CaF₂(flux), 100Cr₃C₂, and 87.5Cr-12.5C (wt.%), were placed on an SUS₃16L stainless steel substrate, which was then irradiated with electron beam. In the specimens fabricated without flux powders, the surface composite layer of 3.5-4.8 mm in thickness was successfully formed without defects by the self-fluxing effect, and contained a large amount (up to 18 vol.%) of Cr_(7)C₃ carbides in the austenite matrix. The composite layer fabricated with Cr and C powders having lower melting points than Cr₃C₂ powders was thicker than that fabricated with Cr₃C₂ powders because of more effective melting of powders during electron-beam irradiation. The hardness and wear resistance of the surface composite layer were directly influenced by hard primary Cr_(7)C₃ carbides, and thus were about two times greater than those of the untreated stainless steel substrate.

keyword : Surface composite, High-energy electron beam irradiation, SUS316L stainless steel, Chromium carbide

The goal of the present investigation was to visualize the propagating crack in a mechano-luminescence (ML) material to enable the measurement of instantaneous R-curves and directly observe the bridging (shielding) stress in a fast-propagating crack system. The well-known ML compound, SrAl₂O₄: Eu^(2+), was used as a model test material. Two additional trivalent rare earth elements, Dy and Nd, were introduced as co-dopants to improve the luminescent efficiency. The initiation and growth of a crack from the mechanically machined sharp notch tip in a disc shaped compact tension (CT) specimen at a relatively high loading rate were found to be associated with the extent of light emission around the crack. An in-situ measurement of crack length and applied load for 0.3 sec yielded an instantaneous R-curve at the conventional crack propagation speed.

keyword : Mechano-luminescence (ML) material, Crack propagation, Bridging (shielding) stress

Thermal stability of the lamellar microstructure in TiAI-Nb alloys containing Si and C was investigated by partial melting experiments. The proper compositions which have enough thermal stability to be used as a seed material were found for TiAl-Nb-Si-C alloy system. The lamellar microstructure of the Ti-44.5A1-3Nb-0.6Si-0.2C and Ti-45Al-2Nb-0.6Si-0.2C alloys was successfully aligned parallel to the growth direction by directional solidification using Ti-44.5A1-3Nb-0.6Si-0.2C alloy as a seed material. The DS ingots of these alloys exhibited a excellent combination of room-temperature ductility (8.5%) and high-temperature yield strength (700 MPa at 800℃). Nb was not an effective element to improve the lamellar stability. However, adding a small amount of Si and C to the TiAl-Nb system was very effective to improve the thermal stability of the lamellar microstructure.

keyword : TiAl, Seeding technique, Directional solidification, Lamellar orientation, Thermal stability

This study was focused on verifying quenching embrittlement property of Zircaloy-4 cladding in the simulated Loss Of Coolant Accident (LOCA) environment as well as finding suitable embrittlement criteria. Effect of Nb on the thermal embrittlement behavior was also investigated. Claddings were oxidized at given temperature and given time followed by cold water injection. Ring compression test and microstrucutal analysis on oxidized cladding were carried out. The results showed that transformed phase which formed at quenching stage, rather than oxygen-saturated a phase, has an influence on cladding property. In case of Nb effect, Nb retards cladding oxidation, thus enhances LOCA resistance.

keyword : Zirconium cladding, Loss of Coolant Accident, LOCA, Oxidation, Niobium

A tensile test is the most widely used mechanical property testing method to measure inherent material behaviour including full flow curves as well as Young`s modulus, yield strength, tensile strength and elongation. In the present study, tensile deformation and necking behaviour was investigated using the finite element method code MSC.MARC and the plasticity instability theory. It is shown that the finite element analysis for the tensile test of cylindrical samples can reproduce necking instability by applying a radial constraint at the loading points without assuming initial imperfections. The predicted finite element results compared to the Hart`s instability criterion shows that the Hart`s analytical solution overestimates necking strains a little in strain hardening materials. The finite element analysis of tensile tests with the geometric constraint is useful for interpreting deformation and developing new materials in that it can successfully reproduce not only uniform elongation and necking initiation but also post necking behaviour.

keyword : Tensile test, Finite element method, Instability, Necking, Strain hardening

Effects of fabrication condition on the formation of A1N by means of gas/melt in-situ reactions were investigated through injection of mixture gas of 95% N₂＋5%H₂into the commercial grade A5083 alloys. In this investigation, the fabrication was performed in the temperature range of 1000 to 1200℃ for up to 4 hours. The fabricated composites were characterized using X-ray diffraction, scanning electron microscopy with dispersive X-ray microanalysis and transmission electron microscopy. The volume fraction of MN formed in the specimens increased with increasing reaction time and reaction temperature as well, while the trend of increment of volume fraction of AIN per hour is different with the reaction temperature. The formation of A1N by means of gas/melt in-situ reactions was examined through the thermodynamic and kinetic considerations.

keyword : AIN, Al-Mg alloys, In-situ reaction, Nitrogen, Direct nitridation, Indirect nitridation

The effects of Ca addition on the liquid oxidation behaviors of AZ91 alloys have been investigated. The oxide layers formed on the molten Ca-containing AZ91 alloys were investigated by using scanning electron microscope and Auger electron spectroscopy in order to elucidate the effect of Ca additions on the ignition and burning of the molten alloys. The thermogravimetric measurements indicated that Ca-containing AZ91 alloys have good oxidation resistance. AES depth analysis revealed that the oxide layers on the molten Ca-containing AZ91 alloys consisted of several layers; the outer layer mainly consisted of CaO and the inner layer was mainly a mixture of CaO-MgO and Al₂O₃. The formation of protective CaO in the oxide layer was directly responsible for improved oxidation resistance of the Ca-containing AZ91 alloys.

keyword : AZ91, Non-combustible Mg alloys, Oxidation, Calcium

The volume-averaged hydrostatic parts of thermo-mechanical stresses in the metal interconnects determined by X-ray diffraction (XRD) and finite element analysis are compared with each other. Al-Cu lines with PE-SiN(plasma enhanced CVD nitride) and FOx(Flowable Oxide) passivations and with various line aspect ratios are selected for this study. For the numerical calculation, the stress concentration effect around the edge of Al-Cu lines and elastic-plastic behavior of the metal line following its hardening rule are considered. Volume-averaged stresses measured by XRD method are compared to the calculated values. The probable difference between the hydrostatic stresses obtained from XRD method and finite element method is finally analyzed.

keyword : Thermal stress, Metal interconnect, Aspect ratio, Passivation dielectrics, Al-Cu, reliability