발간논문

Home > KJMM 논문 > 발간논문

The microstructure, tensile properties at room and elevated temperatures, and high-cycle fatigue behavior of an extruded Mg-7Sn-1Al-1Zn (TAZ711) alloy were investigated in comparison with those of an Mg-8Al-0.5Zn (AZ80) alloy extruded under the same conditions. The extruded AZ80 alloy exhibited a fully recrystallized structure, whereas the extruded TAZ711 alloy had a bimodal structure consisting of coarse nonrecrystallized grains and fine recrystallized grains. The TAZ711 alloy had smaller recrystallized grains and a larger amount of fine dynamic precipitates than the AZ80 alloy. Both alloys showed the typical texture of extruded Mg alloys in which the basal planes of the grains are arranged parallel to the extrusion direction, but the texture intensity of the TAZ711 alloy was greater than that of the AZ80 alloy. The yield strength of the TAZ711 alloy was higher than that of the AZ80 alloy at room temperature because of the finer grain size, stronger texture intensity, and greater amount of fine precipitates in the TAZ711 alloy. However, at an elevated temperature of 150 °C, the yield strength of the TAZ711 alloy was lower than that of the AZ80 alloy because the climb-controlled dislocation creep, grain boundary sliding, and diffusional creep behaviors were more easily generated in the TAZ711 alloy owing to its finer grain structure. As the applied tensile strain rate was reduced, the decrease in the strength of the TAZ711 alloy was greater than that of the AZ80 alloy owing to the higher strain rate sensitivity of the former alloy. Although the yield strength of the TAZ711 alloy at room temperature was higher than that of the AZ80 alloy, the TAZ711 alloy exhibited lower high-cycle fatigue resistance, which can be attributed to the occurrence of fatigue cracks at the twins formed in the coarse non-recrystallized grains remaining in the TAZ711 alloy. (Received May 31, 2018; Accepted August 20, 2018)

keyword : magnesium alloy, extrusion, microstructure, tensile properties, fatigue resistance

For proper maintenance and inspection scheduling of aircraft, it is extremely important to measure the resistance to stress corrosion cracking (SCC) behavior of aluminum alloys used for aircraft structures. The SCC resistances of high-strength 7xxx-series aluminum alloys, including 7050-T7451, 7475-T7351 and 7140- T7451, were evaluated along longitudinal (L) and transverse (T) directions in 3.5% NaCl solution, using a constant testing method without polarization, and a slow strain rate testing method under a cathodic applied potential. It was found that the SCC resistance of the 7xxx series aluminum alloys in 3.5% NaCl solution varied with different testing method, chemical composition and temper condition. 7475-T7351 showed a slightly lower sensitivity to SCC than 7050-T7451 and 7140-T7451 in both anodic dissolution and hydrogen embrittlement conditions, even though the difference was not significant. The present result suggests that the Cu content and the ratio between Zn and Mg are important metallurgical factors affecting the SCC behavior of 7xxx series aluminum alloys. It was also found that pits served as initiation sties for SCC in the environment of anodic dissolution, while the small areas of hydrogen embrittlement acted as SCC initiator in the hydrogen-generating environment. The SCC behavior of each specimen was discussed based on the detailed fractographic and micrographic analyses. (Received August 7, 2018; Accepted September 4, 2018)

keyword : stress corrosion cracking, 7xxx series aluminum alloy, 3.5% NaCl solution

In this work, we studied the formation of partially stabilized zirconia by a high energy ball milling (HEBM) approach. Zirconia powder was mixed with 8 mol% MgO and the powder mixture was milled for 10 hours. The structural and morphological evolutions of the powder particles were studied using X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The Williamson-Hall method was used to calculate the crystallite size of the zirconia powder particles. The results show that the tetragonal zirconia phase (t-ZrO₂) can be very well stabilized after the addition of 8 mol% MgO. The formation of t-ZrO₂ was identified by XRD analysis after 3 hours of ball milling. In addition, SEM results demonstrated a great refinement in the size of the ZrO₂ particles whose distribution was in the sub-μm to nm range indicating better microstructural characteristics. (Received July 18, 2018; Accepted August 16, 2018)

keyword : ball mill, zirconia, microstructure, ceramics, crystallite size, strain

This study investigated the microstructure and room temperature mechanical properties of W-7Ni- 3Fe (wt%) tungsten heavy alloy (as-fabricated) which was manufactured by metal injection molding (MIM). To identify the effect of heat treatment on the W-7Ni-3Fe alloy, its properties were compared after heattreatment (1,100 oC for 1 hour) in vacuum atmosphere. The initial microstructural observations confirmed that both the as-fabricated and heat-treated specimens were composed of W phase and γ(Ni, Fe) phase. Vickers hardness measurements were 363.7 Hv for the as-fabricated specimen and 387.2 Hv for the heat-treated specimen. Room temperature tensile strength was tested, and the yield strength of the as-fabricated and heattreated specimens were 698.3 and 714.0 MPa, and the ultimate tensile strengths were 760.8 and 960.0 MPa, respectively. The elongation of the two materials was 2.4% (as-fabricated) and 20.3% (heat-treated). It was found that additional heat treatment after MIM significantly improved the alloy’s mechanical properties. In addition, fracture surface observation results confirmed that the main crack propagated along the interface between the spherical W phase and γ(Ni, Fe) phase in the as-fabricated specimen, and traversed the W phase in the heat-treated specimen. Based on the above findings, this study also discusses the fracture mechanisms of tungsten heavy alloy manufactured with MIM. (Received August 9, 2018; Accepted August 31; 2018)

keyword : W-7Ni-3Fe, metal injection molding, tungsten heavy alloy, microstructure, tensile properties

The microstructure, texture evolution, and room temperature formability of warm-rolled and coldrolled AZ31 magnesium sheets were investigated in this study. The localized deformation regions, i.e., shear bands, formed during warm-rolling and cold-rolling mainly provide nucleation sites for recrystallization, but the orientation of the recrystallized grains nucleated on the shear bands changes depending on the rolling procedure. EBSD analysis clearly indicated that the large accumulated strain produced by cold-rolling can activate basal and non-basal slip and lead to the rotation of the recrystallized grains from their original orientation. In this study, the resulting texture of the cold-rolled AZ31 sheet was weaker than that of the warm-rolled sheet, and as a consequence, the room temperature formability of the cold-rolled AZ31 sheet doubled from 2.2 to 4.2 mm in the Erichsen index. Interestingly, the simple cold-rolling and subsequent recrystallization annealing produced a ‘rare earth texture’, in which the peaks of the basal pole figure were split and spread in the rolling direction. This texture was obtained without the addition of rare earth elements or severe plastic deformation. (Received August 27, 2018; Accepted September 7, 2018)

keyword : magnesium alloy, cold-rolling, texture, stretch formability, recrystallization

The CMSX-4 superalloy including refractory elements was directionally solidified using the seed of a [001] single crystal to control the growth direction to [001], where solidification velocity was changed from 25 to 160 μm/s. In the log-log plot between spacing and solidification velocity, the primary dendrite arm spacing and secondary dendrite arm spacing linearly decreased with slopes of -0.288 and -0.318, respectively, as solidification velocity increased. A γ/γ′ eutectic structure was observed in the interdendritic region and the size linearly decreased with increasing solidification velocity. However, the volume fraction of the γ/γ′ eutectic structure did not change significantly with solidification velocity. Micro-segregation of the refractory elements was found in the primary dendrite arm region. These elements were not uniformly distributed even after solid-solution heat treatment, and an irregularly coarse γ′ -phase was observed in the interdendritic region after heat-treatment. (Received July 11, 2018; Accepted August 16, 2018)

keyword : superalloy, directional solidification, seed, single crystal

This study was conducted to obtain engineering design data for a process to recover lithium in the form of Li₃PO₄ from the waste water of a lithium ion battery (LIB) recycling process. The influence of temperature, pH, and concentration on the solubility of Li₃PO₄ and the reaction rate in the Na₃PO₄-Li₂SO₄-H₂O system was investigated. As a result, the solubility of Li₃PO₄, 372 mg/L, increased to 417 mg/L as the reaction temperature increased from 20 to 90 ℃. At the same time, the reaction rate increased from 0.0174 × 10-4 to 2.32 × 10-4 mol/L·s. The yield of lithium phosphate rises in proportion to increases in the concentration of salt, temperature and pH, in the reaction of 3Li₂SO₄ + 2Na₃PO₄ → 2Li₃PO₄ + 3Na₂SO₄. That is, the higher the concentration, temperature and pH solution are, the more Li₃PO₄ precipitates. The temperature must be 70 ℃ or higher to obtain 90% or more Li₃PO₄ as a precipitate. The lithium phosphate particle formed at 20 ℃ are composed of secondary particles or their agglomerate, which is composed of primary particles of 0.01 μm or less size. The size of the primary particle becomes bigger when the reaction temperature is raised and at 90 ℃ it becomes a large columnar particle, with a 10~20 μm length and 5~10 μm breadth. (Received June 19, 2018; Accepted August 13, 2018)

keyword : lithium, recovery, lithium phosphate, lithium ion battery, waste water

The present paper deals with liquid-liquid extraction studies and the possible separation of light rare earths (LREs) from chloride solutions. Aromatic diluents such as benzene, toluene and xylene were tested for LREs extraction and possible separation from chloride solutions. Bis(2,4,4-triethylpentyl)phosphinic acid (trade name Cyanex^® 272) was used as an extractant and dissolved in the diluents. Various experimental parameters such as time effect, influence of pH, extractant concentration variation and maximum loading capacity of the target LREs to Cyanex^® 272 were examined. Finally, the separation potential of each of the other LREs was calculated, and a suitable diluent system was proposed. The highest separation factors were found using the present systems. (Received July 18, 2018; Accepted September 3, 2018)

keyword : light rare earth elements, chloride solutions, extraction, separation, Cyanex^® 272

Partially La/Pr-double-filled and Co-substituted (La_1-zPr_z)_0.8Fe_4-xCo_xSb₁₂ skutterudites were synthesized by encapsulated melting and vacuum hot pressing. (La,Pr)Sb₂ phases were not formed because the partial filling did not exceed the filling fraction limit, and the amount of secondary phase FeSb₂ also decreased as the Co content increased. Electrical conductivity decreased with increasing Pr filling and Co substitution because of the decreased carrier concentration. The (La_0.75Pr_0.25)_0.8Fe₄Sb₁₂ skutterudite, which was not charge-compensated with Co, showed the highest electrical conductivity of 2.44 × 10⁵ Sm^-1 at 323 K. In contrast, the Seebeck coefficient increased with increased Co substitution, and the (La_0.25Pr_0.75)_0.8Fe₃CoSb₁₂ exhibited the maximum Seebeck coefficient of 168.2 μVK^-1 at 723 K; (La_0.75Pr_0.25)_0.8Fe_3.5Co_0.5Sb₁₂ exhibited the highest power factor of 2.67 mW m^-1 K^-2 at 823 K; and (La_0.25Pr_0.75)_0.8Fe₃CoSb₁₂ exhibited the lowest thermal conductivity of 2.09 W m^-1 K^-1 at 523 K. Consequently, (La_0.25Pr_0.75)_0.8Fe_3.5Co_0.5Sb₁₂ exhibited the maximum dimensionless figure of merit, ZT = 0.79 at 723 K. (Received August 13, 2018; Accepted August 27; 2018)

keyword : thermoelectric, skutterudite, partial double filling, charge compensation