발간논문

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The effect of texture and {10-12} twin on the low cycle fatigue properties of rolled AZ31 magnesium alloy was investigated by varying the loading direction and the amount of twin volume fraction, respectively. The hysteresis loops of the specimens loaded parallel to the rolling direction (RD) showed asymmetric nature in tension and compression, and were quite different from those of the specimens loaded normal to the rolling plane (ND). ND specimens showed superior fatigue properties than RD specimens due to the compressive mean stress. It was also found that the fatigue resistance could be modified by controlling its twinning- detwinning characteristics via a pre-straining process. Initial {10-12} twin generated by pre-compression along the RD improved fatigue resistance in proportion to the amount of pre-compression. Meanwhile, the pre-tension along the ND caused an increase of mean stress, and thus deteriorated the fatigue resistance of the material.

keyword : metals, deformation, fatigue, optical microscopy, twinning

To develop 7075 aluminum (Al7075) alloy with low residual stress and high strength, a modified cryogenic treatment was proposed. In this study, the effects of two additional steps on residual stress in Al7075 were investigated. Unlike the conventional age-hardening process, the first step of the modified treatment is cryogenic quenching, followed by the second step of uphill quenching of the samples. The mechanical properties and residual stress of samples were characterized by Vickers hardness and X-ray diffraction with sin2 method, respectively. Structural properties such as size and formation of the precipitates of the samples were characterized by transmission electron microscopy and electron energy loss spectroscopy. Three different uphill quenching methods, boiling water, room temperature water, and air, were applied to the samples. Samples treated by the boiling water uphill quenching method showed the lowest residual stress and highest hardness. Results clearly demonstrated that the modified cryogenic treatment with boiling water uphill quenching method was a very useful heat treatment to achieve an age-hardened Al7075 alloy with low distortion shape, low residual stress and high hardness.

keyword : alloys, aging, residual stress, transmission electron microscope, cryogenic treatment

The effect of high-energy ball milling on sintering, microstructure and mechanical properties of ZrO2 was investigated. The starting temperature of consolidation decreased with high-energy ball milling because the driving force for sintering, and the contact points of powders for atomic diffusion, increased. The mechanical properties (hardness and fracture toughness) and relative density of ZrO2 sintered from high-energy ball milled powder were higher than those of ZrO2 sintered from unmilled powder. A highly dense nanostructured ZrO2 was produced using high-energy ball milled powder with simultaneous application of 80 MPa pressure and pulsed output current of 2800 A within 2 min.

keyword : compound, nanostructure, sintering, mechanical properties, pulsed current activated combustion

The microstructure and mechanical properties characteristics in AISI 436 weld metal were investigated for different ranges of chemical composition of stabilization element Ti Three new filler wires were designed for flux cored arc welding (FCAW) process, and the Ti contents and solidification path of each of the weld metals was changed. Each specimen of AISI 436 transformed to fully ferrite or ferrite and martensite. Ti suppressed the formation of martensite and had a grain refinement effect. The tensile strength varied according to the content of additional Ti. The effects were negligible when 0.5 wt% Ti was added, but the tensile strength increased by 41 MPa with the addition of 0.8 wt% Ti. When the amount of added Ti was greater than 0.5%, some Ti dissolved. The increase in tensile strength is attributed to the remaining undissolved Ti. Cracking occurred in the specimens contained added Ti. Fracture monograph observed the Transgranular Cleavage Fracture that is a commonly occurring phenomenon in Medium- chromium Ferritic stainless steel. This crack may be influenced by stabilization elements and cooling speed.

keyword : metals, welding, fracture, scanning electron microscopy, ferritic stainless steels

In this study, Precipitation behaviour and their corrosion properties about three types of weld metals adjusted the Ti content of AISI 436 stainless steel welding wire flux were evaluated by examining the microstructure of the welding metal and by conducting an anodic polarisation test after two hours of heat treatment at 850℃ were produced. Microstructural examinations revealed that the sigma phase in the Ti-free specimen was formed by the segregation of Cr-Mo. However, in the Ti-added specimens, the sigma phase did not precipitate instead, TiN formed within the grain matrix. Ti expanded the ferrite region and delayed the formation of the sigma phase. EDS and mapping analyses revealed that the segregation of Cr-Mo occurred where the sigma phase precipitated. Anodic polarisation revealed that the potentials of passive film formation were nearly identical for all samples, but the pitting potential increased as Ti was added, improving the corrosion resistance. Examination of the starting point of pitting corrosion revealed that pitting in the Ti-free specimen began around the Cr-deficient area that surrounded the sigma phase, whereas in the Ti-added specimens, pitting began in the vicinity of the Ti precipitates that were formed within the grain matrix.

keyword : metals, welding, precipitation, scanning electron microscopy, Cr-segregation

We prepared ZnO-Bi2O3-Sb2O3-Mn3O4-NiO(Sb/Bi=0.5, 1.0, and 2.0) varistors to examine the co-doping effect of 1/6 mol% Mn3O4 and 0.5 mol% NiO (Mn:Ni = 1:1) on the reaction, microstructure, and electrical properties, such as J-E characteristics, bulk defects and grain boundary properties. The sintering and electrical properties of the ZnO-Bi2O3-Sb2O3-Mn3O4-NiO varistor were controlled by Sb/Bi ratio. Pyrochlore (Zn2Bi3Sb3O14) was decomposed and promoted densification at lower temperature in Sb/Bi = 1.0. Pyrochlore reproduced on cooling, α-spinel (Zn7Sb2O12), and β-Bi2O3 were formed in all systems above 1000℃. More homogeneous microstructure was obtained in all systems by Mn doping due to the α-spinel formation. The varistor performance was drastically improved as non-linear coefficient of α = 22~84. It was found to form Zni (0.20 eV) and V0 (0.33 eV) as dominant defects. The grain boundaries consisted of an electrically single type (0.90~0.93 eV) by the co-doping effect of Mn and Ni.

keyword : ZnO varistor, Mn3O4, NiO, sintering, electrical properties

In the present study, the influence of desorption-recombination (DR) speed during hydrogenation- decomposition-desorption-recombination (HDDR) process on the microstructure and magnetic properties of Nd-Fe-B based magnetic powders has been studied. Nd-Fe-B based alloys were made by mold casting and homogenization heat treatment. They were then subjected to HDDR treatment. During DR reaction, pumping speed and time of hydrogenation were systematically changed to control the speed of DR reaction. It was found that the slower hydrogen desorption during DR reaction can result in a smaller grain size and significantly increases the magnetic properties of the powder. The coercivity and the remanence of the HDDR powders in the size range of 200-300 μm was 12.4 kOe and 11.4 kG, respectively. In addition, the remanence was further increased to 12.0 kG by milling the powders down to 25-90 μm, which resulted in (BH)max of 26.5 MGOe.

keyword : magnetic materials, HDDR, grain refinement, ribrating-sample magnetometer, anisotropy

Metallic biomaterials such as titanium and its alloys have been widely used as permanent implant materials due to their excellent corrosion resistance, fatigue property, and strength. Calcium pyrophosphate (CPP) is an attractive bioceramic for human tissue implantation due to its good biocompatibility and similar composition to human bone. High energy mechanical milling and spark plasma sintering (SPS) can be used to fabricate a sound composite of the Ti-Nb-Zr-CPP. In a biomaterial such as the Ti-6Al-4V ELI alloy, the coating property between the bioceramic and Ti-based alloy is very important. By addition of CPP to the Ti-Nb-Zr alloy, the Ti-Nb-Zr alloy can improve the wetability between the Ti alloy and bioceramic as a coating material. Also the porous surface of the biomaterials can improve the symphyseal ability with osteoblast. In this study, a porous surface can be made by using several different leaching solutions such as aqueous diluted H3PO4. Ti-Nb-Zr-CPP composite was fabricated by spark plasma sintering (SPS) at 1000 ℃ under 70 MPa using high energy mechanical milled powder. A porous surface was successfully fabricated by using a 40 vol%H3PO4+ 60 vol.%H2O leaching solution at room temperature for 24 h.

keyword : biomaterials, sintering, microstructure, SEM, calcium pyrophosphate

In the present study, Al-CNT pellets were investigated to understand the effect of Mg addition and manufacturing conditions on the interfacial reactions between Al and CNTs in Al-CNT pellets. The pellets were heated and held at 700 ℃ and 800 ℃ for 2 hours under nitrogen (N2) atmosphere. To confirm the reactions between Al and CNT in the pellets under different manufacturing conditions, the microstructures were observed by optical microscopy (OM) and field emission scanning electro microscopy (FESEM). And, the composition and reaction phases were analyzed by energy dispersive X-ray spectroscory (EDXS) and X-ray diffractometry (XRD). The presence of oxidation products and Mg on the surface of Al powder in the pellets appeared to prevent the formation of Al4C3. But, Al4C3 reaction products were increased due to the high temperature of 800 ℃, which produced a high amount of reduced aluminium and increased the reaction areas between Al and CNT. The Al-CNT pellets compacted under air atmosphere prohibited the reaction between Al and CNT because of the high amount of oxidation products, such as MgO and MgAl2O4.

keyword : Al-CNT, Al4C3, Mg contents, N2 atmosphere, oxidation products