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This study examined the effect of Si addition in (α+β) Ti alloy on the phase stability about the Ti silicide precipitation from b matrix. The changes in the volume fraction and shape of precipitates with different Si contents were correlated with the tensile properties of the present alloys based on the detailed TEM micrographic observation. In Ti-6Al-4Fe-0.5Si alloy, the precipitates with rectangular or round shape were found at inter-grain of α/β interface and intra-grain of β phase. The precipitates were generally known as Ti5Si₃ with enough stabilizing thermo-mechanically. However, in this study, a new type silicide of Ti_(5)Si₄was mostly observed in the as-cast specimens, representing the metastable or unstable state phase thermo-mechanically compared to Ti_(5)Si₃. It was consistent with the phase transformation processes of TiSi₂TiSiTi_(5)Si₄Ti5Si₃ in the ternary alloy system of Ti-Al-Si. The improvement of mechanical properties including high temperature ones was to be induced by the fine precipitation of Ti silicides such as Ti5Si₄ and Ti5Si₃ in the ductile matrix of b phase. The precipitates of Ti_(5)Si₄ (P1) and Ti_(5)Si₃ (P2) have a coherent relationship with bcc β matrix of (200)_(β) //(220)_(P1) //(2111)_(P2) and (011)_(β) //(001)_(P1)//(0110)_(P2).

keyword : Ti-6Al-4Fe, Silicon addition, Ti-Si precipitates, Ti5Si4 , Ti5Si3

Microstructural characteristics and corrosion properties were evaluated for the intermediate products in the manufacturing process of advanced nuclear fuel cladding tubes containing high Nb. The microstructures of the intermediate products at each manufacturing steps were found to be almost identical due to the same annealing condition. The precipitates observed in the intermedate products were β-Nb and Zr₃Fe-type precipitate containing Nb. Texture measurement reveals that the (0002) pole intensity in the radial direction increased as the manufacturing step proceeded. It was found that the corrosion behavior was not significantly affected by the texture but highly dependent on the annealing temperature. Therefore, it is indicated that the corrosion behavior of Zr alloy containing Nb more than solubility was controlled mainly by the Nb contents and the Nb-containing precipitates.

keyword : Zr alloys, Manufacturing process, Precipitate, Texture, Corrosion

Interest in the L1₂ phase in the Al-Ti-Cr system has increased with respect to the demand for light weight materials at elevated temperature because of its enhanced mechanical properties that arise through the substitution of Cr for Al in Al₃Ti intermetallic compounds. Additionally, recent work has indicated that the application of L1₂- based alloys was extended to oxidation resistance coating materials for γ-based TiAl alloy because the L12 phase could form a protective Al₂O₃ scale under oxidizing environments. However, L1₂ phase decomposes partially or completely to a more brittle phase when exposed to temperatures in the range of 800~1000℃, resulting a serious degradation of the cracking resistance in L1₂-based alloys. In this study, Zr was substituted for Ti in Al-21Ti-23Cr alloys to improve the phase stability around 800℃. As a result of Zr addition in L12-based Al-Ti-Cr alloys, the fraction of L12 phase decomposition at 800℃ was decreased. Furthermore, temperature range, in which L1₂ phase was decomposed to other brittle phase, was shrunken when Zr was added in the Al-Ti-Cr alloys. These phenomena resulted from the fact that the L1₂ phase field was less shifted in the direction of the Al-rich corner at 800oC in the Al-Ti-Cr system and that atomic diffusion for the L1₂ phase decomposition was reduced. Therefore, the addition of Zr is required to increase the possibility for potential usage of L1₂-based Al-Ti-Cr alloys at elevated temperatures around 800℃ because of the improvement of phase stability.

keyword : L12 phase, Phase stability, Phase decomposition, Zr addition

Fracture properties of various structural steels were determined non-destructively using the advanced indentation system (AIS), which is developed on the basis of continuous indentation technique. In this study, conventional critical strain model was modified to predict the ductile fracture toughness from the AIS data. In deriving the ductile fracture toughness model, fracture strain and characteristic length were estimated using the stress triaxiality and the work-hardening exponent. AIS tests were conducted on carbon steels, stainless steels, aluminum alloys and copper alloys. For all these materials, the results of indentation test were compared to those of the standard fracture toughness test methods, which shows that the present analysis can predict successfully the ductile fracture toughness of structural steels.

keyword : Continuous indentation test, Ductile fracture toughness, Critical strain model, Stress triaxility, Work-hardening exponent

Tensile behavior of friction stir welded Al6061-T651 alloy plate joined with different tool rotating and welding speeds was examined in the present study. Within the range of processing parameters where the welding defects were not observed, the tensile elongation of friction stir welded Al6061-T651 alloy plate was greatly improved with increasing welding speed and/or decreasing rotating speed. The change in the strength level was, however, marginal. For those specimens with low tensile elongation values, clusters of coarse Mg2Si precipitates were often observed in the vicinity of fractured area. Severe plastic flow occurred during friction stir welding, particularly for the specimens joined with low welding speed, appeared to be responsible for the clustering and eventually lower the tensile elongation.

keyword : Friction stir welding, Al 6061-T651, Tensile behavior

The objective of this study is to investigate delayed hydride cracking velocity (DHCV) of Zr-2.5Nb pressure tubes with yield strength. DHC tests were conducted at an initial K_(I) of 15 to 19 MPa √m and 250℃ on the compact tension specimens containing up to 100 ppm hydrogen that were taken from the Zr-2.5Nb tubes with different yield strengths. The crack growth was determined with the direct current potential drop method. A nice correlation of the DHC velocity and the yield strength was obtained at all temperatures by normalizing the DHCV with the terminal solid solubility and diffusion of hydrogen: the DHCV increased exponentially with increased yield strength of the Zr- 2.5Nb tubes. In contrast, the texture and the distribution of the β-Zr phase were found to have a comparatively minor effect on the DHCV of the Zr-2.5Nb tubes. Yield strength of the Zr-2.5Nb tubes determines a plastic zone size with a steep gradient of hydrogen concentration ahead of the crack tip where hydrides can be precipitated. The yield strength effect is analyzed by correlating the plastic zone and the striation spacing observed on the fractured surfaces as a function of the yield strength.

keyword : Zr-2.5Nb, DHC Velocity, Yield strength, Texture, β-Zr, Striation spacing

The creep rupture tests were carried out on actual welded joint specimens of 12Cr-Mo-V steel. It was revealed that the creep rupture life of actual welded joint was approximately 41% of that of base metal and the creep damage of it occurred mainly at intercritical heat-affected zone(HAZ). On the basis of the heat input during the welding, microstructure, microhardness and grain size of the actual intercritical HAZ, simulated HAZ was made in order to evaluate its material properties. The tensile tests and interrupted creep test were carried out on the simulated HAZ specimens. It was found that the tensile properties of the simulated HAZ were slightly inferior to those of the base metal and the creep rupture strength of it was approximately 46% of that of base metal and the creep strain rate of it was higher than that of base metal. The creep cavities in simulated HAZ were found at an early stage of creep life and the number of creep cavities increased exponentially and the average diameter of cavities increased linearly with creep life fraction. Plasticity constants of simulated HAZ can be available for the analysis of creep crack growth and the number and average diameter of creep cavities can be available for the evaluation of creep damage for welded joint.

keyword : 12Cr-Mo-V steel, Welded joint, Simulated heat affected zone, Creep cavities, Plasticity constant, Creep strain rate

The microstructure of Cu-15 wt%Nb filamentary nanocomposite fabricated by the bundling and drawing process were characterized using high resolution electron microscopy and the computer-aided image processing. Nb filaments were randomly distributed throughout the copper matrix and appeared to be straight or slightly curved in the bundled wires. The copper grain is observed in a <110> zone axis while two differently oriented grains are found in the niobium filaments, a first grain being observed in a <111> direction, the other in a <001> zone axis. The semi-coherent interface with periodic misfit dislocations was formed to have {002}_(Cu)||{110}_(Nb), <100>_(Cu)||<110>_(Nb) orientation relationship and the interface was parallel to <100>_(Cu). The micro-twins having {112} twin planes were observed in a few niobium filaments.

keyword : Cu-Nb filamentary nanocomposite, High resolution electron microscopy, Image processing, Interphase interface structure, Misfit dislocation, Twin structure

A free-standing tungsten/copper composite plate, which may be used as heat sink material for electronic packaging, was produced with air plasma spray by using spherically agglomerated W/Cu composite powder. The model for particle vaporization process was studied to predict the particle behavior in the high temperature of plasma jet. At the surface of the deposit layer, copper became oxidized to cuprite (Cu₂O). The degree of oxidation decreased towards the interface between deposit and substrate. Almost no Cu₂O was detected at the inner layer after grinding off 70 ㎛ from the surface. Most pores were found in the vicinity of tungsten-rich region. The microstructures of W/ Cu deposits were also discussed.

keyword : Plasma spray, Tungsten/copper composite, Deposit, Heat sink

When stainless steels are used in the human body, the nickel ion released from the alloys over a short or long term can cause such adverse effects as allergies, dermatitis, asthma. Although the higher the corrosion resistance of stainless steels and the lower the nickel content, the better it is for the human body, since nickel has an essential role in the stabilization of the austenitic phase and the corrosion resistance of stainless steels, it is difficult to remove nickel completely. In this regard, the purpose of this study was to investigate corrosion resistance, cytotoxicity and mechanical properties of super stainless steels, and upon this basis, to evaluate their potential for orthodontic applications. Super austenitic stainless steels had higher content of nickel(17~20 wt%) than super duplex stainless steels (7~10 wt%) or super ferritic stainless steels (lower than 3 wt%). Super austenitic and duplex stainless steels had nitrogen content of above 0.2 w%. These steels had high Mo or W contents of which synergistic effect together with N increases localized corrosion resistance. In terms of hardness, super stainless steels exhibited higher value than 316L S. S. and two times higher than cp Ti(grade II). Austenitic and duplex stainless steels exhibited better mechanical properties. Super stainless steels exhibited low passive current density (3~4 μA/㎠) and similar potentiodynamic polarization behaviors among themselves. Thus, these steels are considered to have high corrosion resistance in the human body-simulated solution. Pitting corrosion occurred in 316L S. S.. A3 containing the highest nickel content, as well as SAF2507 exhibited none cytotoxicity, and the others, slightly. This indicates that the cytotoxicity has no direct relation with nickel content itself in these alloys. Super stainless steels with both high corrosion resistance and good biocompatibility are expected to be used for orthodontic applications by virtue of the passive film attribute enhanced by synergistic effect of Mo+N or by high addition effect of Cr+W.

keyword : Super stainless steels, Nickel content, Passive film, Corrosion resistance, Cytotoxicity, Biocompatibility, Orthodontic application