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Precipitation behavior of high-nitrogen duplex Fe-24Cr-7Mn-4Ni-4Mo-0.43N stainless steel aged at 850℃ was investigated using scanning transmission electron microscopy. Based on the analyses of selected area diffraction patterns, four kinds of precipitates (intermetallic sigma (σ) and chi (χ), Cr2N and secondary austenite) were identified. At the ferrite/austenite phase boundary, the σ phase and secondary austenite were formed via α→γ + σ eutectoid reaction. The precipitation of Cr2N occurred at the austenite grain boundary as well as the interior of the ferrite. The intermetallic χ phase also formed within the ferrite and showed a cube-cube orientation relationship with the ferrite. Further aging produced a lamellar structure composed of Cr2N and austenite along the ferrite/austenite boundary and enhanced the precipitation of the χ phase. The crystallographic features of the precipitates were also examined in terms of the orientation relationship with the austenite or ferrite matrix.

keyword : alloys, aging, precipitation, transmission electron microscopy, TEM, high-nitrogen duplex stainless steel

An ultrafine grained Mg-9Al-1Zn magnesium alloy with the mean grain size less than 1 μm was produced by using high-ratio differential speed rolling. The processed alloy exhibited excellent superplasticity at relatively low temperatures. The micro-forming tests were carried out using a micro-forging apparatus with micro V-grooved shaped dies made of silicon and the micro-formability was evaluated by means of micro-formability index, Rf (=Af/Ag, Af: formed and inflowed area into the V-groove, Ag: area of the V-groove). The Rf value increased with temperature up to 280℃ and then decreased beyond 300℃. The decrease of the Rf value at 300℃ was attributed to the accelerated grain coarsening. Increasing the micro-forging pressure increased the Rf values. At a given die geometry, die filling ability decreased as the die position moved away from the die center to the end. FEM simulation predicted this behavior and a method of improving this problem was proposed.

keyword : metals, severe plastic deformation, mechanical properties, electron backscattering diffraction, EBSD, micro-forming

The out-of-phase thermo-mechanical fatigue (OP TMF) in a <001> oriented single crystal nickelbased superalloy CMSX-4 has been studied. OP TMF life was less than a half of low cycle fatigue(LCF) life in spite of a small hysteresis loop area of OP TMF compared to that of LCF. The failure was caused by the initiation of a crack at the oxide-layered surface followed by its planar growth along the <100> γ channel in both LCF and OP TMF. However, deformation twins appeared near the major crack of OP TMF. The multiple groups of parallel twin plates on {111} planes provided a preferential path for crack propagation, which caused a significant decrease in OP TMF life. Additionally, the analysis on the surface crack morphology revealed that the tensile strain at the minimum temperature of OP TMF was found to accelerate the crack propagation.

keyword : alloys, fatigue, deformation, twinning, transmission electron microscopy, TEM

The mechanical properties of the various heat treatment conditions on Ti-15Mo-3Nb-3Al-0.2Si alloy plates were examined. XRD patterns from the surface of Ti-15Mo-3Nb-3Al-0.2Si were analyzed as a solution-treated Ti alloy has the single-phase β structure whereas the aged Ti alloys have the β matrix embedded with α needles. High strength (~1500 MPa) with decent ductility (7%) was obtained by the Ti alloy double aged at 300℃ and 520℃ for 8 hours each. The double-aged alloy exhibits the finer structure than the single-aged alloy at 300℃ for 8 hours because of the higher nucleation rate of α needles at an initial low aging temperature (320℃). TEM observation revealed that the fine nanostructure with α needles in the β matrix ensured the excellent mechanical properties in the double aged Ti-15Mo-3Nb-3Al-0.2Si alloy. In the solution treated alloy, the yield drop, stress-serrations and the ductility minimum typically associated with dynamic strain aging can be attributed to the dynamic interaction between dislocations and oxygen atoms. The yield drop and the stress serration were not observed in aged samples because the geometrically introduced dislocations due to phase precipitates suppressed the dynamic strain aging.

keyword : alloy, aging, mechanical properties, tensile test

Equal channel angular pressing (ECAP) with intermediate heat treatment was employed to optimize the strength of Cu-15 wt.%Ag. Changes in microstructure, electrical properties and mechanical properties were studied as a function of pressing methods and heat treatment. ECAPed Cu-15wt.%Ag exhibited ultrafine-grained microstructures with the shape and distribution of Ag-rich lamellae dependent on the processing routes. For route A in which the sample was pressed without rotation between each pass, the initial dendrites of Ag-rich phase were elongated along the shear direction and developed into elongated filaments. For route C in which the sample was rotated by 180 degree after each pass, the morphology of initial dendrites of Ag-rich phase was not much modified and the networked structure remained even after 8 passes of ECAP. For route Bc in which the sample was rotated by 90 degree after each pass, the initial dendrites became finer by fragmentation with no pronounced change of the shape and distribution of Ag-rich lamellae. The strength of Cu-15wt.%Ag ECAPed using route Bc was found to be greater than those ECAPed using route A, suggesting that the substructural strengthening is more effective in strengthening than the interface strengthening.

keyword : composite, severe plastic deformation, mechanical properties, ECAP, conductivity

Mechanical and property corrosion resistance of Mg-Zn-Y alloys with an atomic ratio of Zn/Y of 6.8 are investigated using optical microscopy, scanning electron microscopy, transmission electron microscopy, uniaxial tensile test and corrosion test with immersion and dynamic potentiometric tests. The alloys showed an in-situ composite microstructure consisting of α-Mg and icosahedral phase (I-phase) as a strengthening phase. As the volume fraction of the I-phase increases, the yield and tensile strengths of the alloys increase while maintaining large elongation (26~30%), indicating that I-phase is effective for strengthening and forms a stable interface with surrounding α-Mg matrix. The presence of I-phase having higher corrosion potential than α-Mg, decreased the corrosion rate of the cast alloy up to I-phase volume fraction of 3.7%. However further increase in the volume fraction of the I-phase deteriorates the corrosion resistance due to enhanced internal galvanic corrosion cell between α-Mg and I-phase.

keyword : quasicrystals, casting, corrosion, tensile test, transmission electron microscopy, TEM

The AISI 216L stainless steel with a composition of Fe-16Cr-6Ni-6Mn-1.7Mo (wt.%) was oxidized at 700~900℃ in air for 100 h. At 700℃, a thin Mn1.5Cr1.5O4 oxide layer with a thickness of 0.4 μm formed. At 800℃, an outer thin Fe2O3 oxide layer and a thick inner FeCr2O4 oxide layer with a total thickness of 30 μm formed. The non-adherent scale formed at 800oC was susceptible to cracking. At 900℃, an outer thin Fe2O3 oxide layer and a thick inner Mn1.5Cr1.5O4 oxide layer formed, whose total thickness was 10~15 μm. The scales formed at 900℃ were non-adherent and susceptible to cracking. 216 L stainless steel oxidized faster than 316 L stainless steel, owing to the increment of the Mn content and the decrement of Ni content.

keyword : metals, hot working, oxidation, thermal analysis, 216 L stainless steel

The aim of this study was to determine the effect of high-energy mechanical milling (HEMM) time and sintering temperature on microstructure and mechanical properties of the TiAl composite fabricated by pulse current activated sintering. TiAl intermetallic powders were milled by HEMM for 1h, 4h, and 8h respectively. Thermal analysis was used to observe the phase transformation of the milled TiAl powders. The sintering time decreased with increase of milling time. The hardness and fracture toughness of the sintered specimens also was improved with increasing milling time. The grain size of the sintered specimens which was milled for 4h was in the range of 50~100 nm.

keyword : intermetallics, sintering, microstructure, scanning electron microscopy, SEM, TiAl alloy

Nickel cobalt ferrite(Ni0.5Co0.5Fe2O4) powder was prepared through the ceramic route by the calcination of a stoichiometric mixture of NiO, CoO and Fe2O3 at 1100℃. The pressed pellets of Ni0.5Co0.5Fe2O4 were isothermally reduced in pure hydrogen at 800~1100℃. Based on the thermogravimetric analysis, the reduction behavior and the kinetic reaction mechanisms of the synthesized ferrite were studied. The initial ferrite powder and the various reduction products were characterized by X-ray diffraction, scanning electron microscopy, reflected light microscope and vibrating sample magnetometer to reveal the effect of hydrogen reduction on the composition, microstructure and magnetic properties of the produced Fe-Ni-Co alloy. The arrhenius equation with the approved mathematical formulations for the gas solid reaction was applied to calculate the activation energy(Ea) and detect the controlling reaction mechanisms. In the initial stage of hydrogen reduction, the reduction rate was controlled by the gas diffusion and the interfacial chemical reaction. However, in later stages, the rate was controlled by the interfacial chemical reaction. The nature of the hydrogen reduction and the magnetic property changes for nickel cobalt ferrite were compared with the previous result for nickel ferrite. The microstructural development of the synthesized Fe-Ni-Co alloy with an increase in the reduction temperature improved its soft magnetic properties by increasing the saturation magnetization(Ms) and by decreasing the coercivity(Hc). The Fe-Ni-Co alloy showed higher saturation magnetization compared to Fe-Ni alloy.

keyword : magnetic materials, chemical synthesis, hydrogen, X-ray diffraction, reduction kinetics

We investigated the surface morphology changes of a lithium/sulfur battery using multi-walled canbon nanotube added sulfur electrode during charge-discharge cycling. The Li/S cell showed the first discharge capacity of 1286 mAh/g-S, which utilized is 71% of the theoretical value. It decreased to 328 mAh/g-S at the 100th cycle, which corresponds to about 19% utilization of the total sulfur in the cathode. The spherical lumps of the reaction product were observed on the surface of the sulfur electrode. This material was verified as lithium sulfide by X-ray diffraction measurement. The pores in the separator were filled with reaction product. Thus the diffusion of the Li+ ion decreased, which resulted in the decreased capacity of the Li/S cell.

keyword : energy storage materials, mechanical alloying/milling, electrochemistry, SEM, Lithium/sulfur battery