발간논문

Home > KJMM 논문 > 발간논문

The short range ordering (SRO) reaction is investigated in 316L stainless steel through differential scanning calorimeter (DSC) using thermo-mechanically treated specimens. It is interpreted that the exothermic reaction and the endothermic reaction during DSC analysis are due to the ordering and disordering, respectively. The activation energy for the exothermic reaction is determined to be 234 kJ/mol. This suggests that the exothermic reaction is governed by substitutional diffusion. It supports that the nature of the exothermic reaction is the SRO reaction. The cold work affects the kinetics of SRO significantly and shifts the SRO region from 500-570 ℃ to 200-600 ℃. The exothermic energy due to the SRO reaction increases with the amount of cold work. The fact that the SRO is an unavoidable reaction below 570 ℃ is very important.

keyword : 316L stainless steel, thermal analysis, ordering reaction, exothermic reaction, kinetics of ordering, activation energy for ordering

To improve the surface harndess of the STS304, Yttria stabilized zirconia (YSZ) films with nano-sized grain were deposited by an aerosol-deposition (AD) method. Coating layers showed dense structure and had ~5μm thickness. When 3 mol% YSZ powders with tetragonal phase were deposited on STS304 substrate, tetragonal structure was transformed to cubic structure due to the high impact energy during the AD process. At the same time, strong impact by YSZ particles allowed the austenite phase in STS304 to be transformed into martensite phase. Surface hardness measured with nano indentor showed that YSZ coated film had 11.5 GPa, which is larger value than 7 GPa of STS304.

keyword : oxides, coating, mechanical properties, indentation, aerosol deposition

A novel approach is established to produce oxide dispersion-strengthened steels with a fine distribution of oxide nanoparticles. This approach involves cryo-milling followed by ball-milling at room temperature. This complementary milling technique allows such steels to exhibit a remarkable combination of strength and ductility. Based on our experimental findings, the associated microstructural factor contributing to the strength is likely to be Y-Ti-O complex oxide particles. On the other hand, an additional milling process performed at room temperature is not effective with regard to deformationinduced sintering. Therefore, it is concluded that milling at room temperature after cryo-milling does not improve the tensile ductility of oxide dispersion-strengthened steels.

keyword : oxide dispersion strengthened, mechanical alloying, oxide particles, cryomilling

In the case of cemented WC, Ni or Co is added as a binder for the formation of composite structures. However, the high cost and the low hardness of Ni or Co, and the low corrosion resistance of the WC-Ni and WC-Co cermets have generated interest in alternative binder phases recently. In this study, TiAl was used as a novel binder and consolidated by the high frequency induction heated sintering (HFIHS) method. Nanopowders of WC and TiAl were made by high energy ball milling. Highly dense WC and WC-10 vol% TiAl with a relative density of up to 99 % were obtained within one min by HFIHS under a pressure of 80 MPa. The addition of TiAl to WC enhanced the toughness without great decrease of hardness due to the crack deflection and nanostructured phase.

keyword : composites, nanostructured materials, sintering, mechanical properties

In this study, the microstructure and mechanical characteristics of WC-3 wt% Ni and WC-3 wt% Fe hard materials by spark plasma sintering (SPS) for friction stir-processing tool applications were evaluated. Sintered bodies with a diameter of 66 mm were fabricated with a relative density up to 99% after the simultaneous application of a pressure level of 60 MPa and electric current for 35 min mostly without grain growth during sintering. Only the initial phase existed in the initial powers and in the ball-milled powders, but the phases of W2C and WC1-x in all sintered bodies appeared after the sintering process. The Vickers hardness and fracture toughness of the WC-3 wt% Ni hard material were approximately 2,240 kg/mm2 and 9.1MPa·m1/2, respectively, while those of the WC-3 wt% Fe sintered body were correspondingly 2,242 kg/mm2 and 6.8 MPa·m1/2.

keyword : spark plasma sintering, friction stir process, WC-3 wt% Ni, WC-3 wt% Fe, hard material

A laser surface-melting alloying process using a laser beam is a new surface-hardening process of obtaining an alloying layer that melts the surfaces of substrates and alloying powder at the same time. This study used SKD61 hot die steel as a substrate and SKH51 powder as an alloying powder. The laser beam speed and the laser power were fixed at 70 mm/sec and 2 kW. The power feeding rate was changed from 0 rpm to 6 rpm (step: 1 rpm). The alloying layer showed high hardness (710~830 Hv), but cracks occur at a high powder feeding rate. Cracks occur at more than 5 rpm, and the lengths of cracks become longer as the powder feeding rate increases. Moreover, cracks were observed at the dendrite boundary, and dendrite protrusions were observed on the fracture surfaces. As the powder feeding rate increases, the concentration of the Mo, V, and W in alloying layer increase. The liquidus and solidus temperatures decreased by as much as 6 ℃ and 26 ℃. As a result of calculating the aspect ratio (penetration depth/width) of the molten zone, it was found that there is no difference as the powder feeding rate increases. Therefore, strain by solidification contraction was constant with an increase in the powder feeding rate, but cracks occur, and the number of cracks increases because the solidus temperature decreases and the ductility of alloying layer reduces.

keyword : surface modification, melting, optical microscopy, laser surface alloying hardening

This article shows how the adoption of polishing as the back-lapping process of semiconductor wafers influences the flexural fracture strength of their individually diced chips. According to the experimental results of this study, a polishing powder with a particle size of approximately 1 um in diameter can meaningfully reduce the fracture strength of chips with a thickness of 50 um. Particularly, when the diced chips have polishing-induced defects formed along a (110)-dicing plane, they reveal the lowest fracture strength. An in-situ examination details how polishing causes sharp notches on the back surface of the chips which provide preferential sites for chip cracking during a flexural test.

keyword : semiconductor, silicon wafer, chip, fracture, reliability

A specimen with a composition of 80 wt% Mg+14 wt% Ni+6 wt% NbF5 (termed Mg-Ni-NbF5) was prepared by mechanical alloying. Its hydrogen absorption and release characteristics were then examined. Mg2NiH4 was formed by a reaction of Mg with nickel and hydrogen during mechanical alloying and hydrogen absorption-release cycling. The particles after hydrogen absorption-release cycling became smaller than those after mechanical alloying owing to the expansion and contraction of the particles upon cycling. The cycled Mg-Ni-NbF5 had a fairly high hydrogen absorption rate and a large hydrogen storage capacity, with an effective hydrogen storage capacity close to 5.6 wt% at 593 K. The cycled Mg-Ni-NbF5 absorbed 5.58 wt% H in 12 bar H2 and released 4.81 wt% H in 1.0 bar H2 at 573 K for 60 min. At 593 K, the cycled specimen absorbed 4.53 wt% H for 5 min, 5.27 wt% H for 10 min, 5.50 wt% H for 25 min, and 5.58 wt% H for 60 min in 12 bar H2, and released 1.28 wt% H for 5 min, 2.26 wt% H for 10 min, 4.13 wt% H for 30 min, and 4.81 wt% H for 60 min in 1.0 bar H2.

keyword : hydrogen-storage materials, mechanical grinding/milling, scanning electron microscopy, SEM, X-ray diffraction, Ni and NbF5 addition

The plastic deformation behavior of pure Ti during equal channel angular pressing (ECAP) is simulated using the three-dimension finite volume method and is experimentally investigated. The calculated effective strain and effective stress distributions and histories are analyzed to understand the local and global deformation characteristics. The predicted plastic deformation behavior of the Ti workpiece during the ECAP process was compared with the theoretical total strain for every pass at RAM speed v of 10 mm/s and at constant temperature of 400 ℃. The simulated strain and stress distributions are homogenous in the central region of the ECAP processed Ti. The experimental ECAP performed with four, eight, and twelve passes at 400 ℃ results in refined grain sizes of approximately 5-10 μm, 0.4-0.5 μm and 0.1-0.2 μm, respectively.

keyword : metals, titanium, severe plastic deformation, microstructure, computer simulation

Hydrogen pre-charged Zr-1.0Nb alloy was prepared to study the effect of hydrogen uptake on its mechanical properties. The apparent activation energy of the Zr-1.0Nb alloy for hydriding was about 27 kcal/mol. The tensile strength of the Zr-1.0Nb alloy with a hydrogen content of up to 700 ppm was similar to that of an as-fabricated specimen, in that its elongation was drastically decreased after 400 ppm and became nearly zero above about 1200 ppm. The critical hydrogen content range needed to produce the ductile-brittle transition was above 400 ppm.

keyword : Zr-1.0Nb based alloys, cladding, hydride, mechanical properties, ductility, tensile test