발간논문

Home > KJMM 논문 > 발간논문

The dynamic precipitation evolution of Mg-3 wt% Zn alloy during extrusion was evaluated in this study. Precipitation evolution during aging has been well investigated for the Mg-Zn alloy, which is known as static precipitation. However, little attention has been given to dynamic precipitation in Mg-Zn alloy. In order to evaluate the dynamic precipitation evolution, Mg-3 wt% Zn alloy was prepared in an electrical resistance melting furnace and extruded at 180 ℃ by direct extrusion. The microstructure of the as-extruded Mg-3 wt% Zn alloy was investigated by Optical Microscopy (OM), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). A bi-modal crystal structure was observed after extrusion, which consisted of dynamically recrystallized (DRX) regions and deformed regions. The 2^nd phases that precipitated during the extrusion were distributed in the dynamically recrystallized regions. The precipitates were mainly Mg₇Zn₃ phase, which is known as a metastable phase, and it was determined that the factor influencing the precipitates’ formation and distribution during extrusion was dominantly dislocation. Also, the corresponding mechanical properties were investigated by measuring the Vickers hardness and compared with as-cast Mg- 3 wt% Zn alloy after aging at 160 ℃. The Vickers hardness of the as-extruded Mg-3 wt% Zn alloy was 81.5 HV, which was 36.7% higher than that of the as-cast one after aging at 160 ℃. This was attributed to grain refinement and dynamic precipitation during severe plastic deformation. (Received May 13, 2019; Accepted July 8, 2019)

keyword : dynamic precipitation, extrusion, Mg-Zn alloy, Mg₇Zn₃ phase

The effect of annealing temperature on shape memory properties in the Ti-44.5Ni-5Cu and Ti- 45.2Ni-5Cu(at%) alloys was investigated by differential scanning calorimeter (DSC) and tensile testing. The martensitic transformation temperature of Ti-44.5Ni-5Cu(at%) increased from 308K to 324K with increasing annealing temperature and did not change with further increasing annealing temperature. This is because dislocations decreased with increasing annealing temperature due to recovery and recrystallization. However, the martensitic transformation temperature of Ti-45.2Ni-5Cu(at%) increased from 286K to 317K with increasing annealing temperature until reaching a maximum, and then decreased with further increasing annealing temperature. This is understood to be due to the decrease of Ni content in the matrix by the formation of Ti(Cu, Ni)2 at the lower annealing temperature, and the increase in Ni content in the matrix by the re-solution of Ti(Cu, Ni)2 at the higher annealing temperature. The critical stress for slip of Ti-45.2Ni- 5Cu(at%) was higher than that of Ti-44.5Ni-5Cu(at%) at the lower annealing temperature. This is because of precipitation hardening of the Ti(Cu, Ni)₂. (Received May 27, 2019; Accepted July 3, 2019)

keyword : Ti-Ni-Cu, shape memory alloy, martensite, annealing, transformation temperature

In this study, an eco-friendly pretreatment process was developed to recycle titanium turning scraps and ferro-titanium ingots with low gaseous impurities were prepared using recycled titanium turning scraps. The amount of gaseous impurities of titanium scraps before removing the machining oils on the surface was as high as 20000 ppm. To remove these oils, the eco-friendly pretreatment solution excluding acid solution or organic solvent was developed. As a result, the respective gaseous impurities of oxygen, nitrogen and carbon showed reduction ratio of 54.0%, 96.2%, and 98.4% as compared to the raw material under the optimum condition of NaOH + Na₄P₂O₇. When a Fe-Ti ingot was prepared with the eco-friendly pretreatment scraps under the optimum conditions, the total content of gaseous impurities in the Fe-Ti ingot with pretreated Ti scraps was reduced by 81% (O, N, C = 1365 ppm) compared to the Fe-Ti ingot with raw Ti scraps. (Received March 18, 2019; Accepted July 23, 2019)

keyword : titanium, ferro-titanium, scrap, recycling, gaseous impurity

Alloys of Fe-12Cr-0.3C-4Mn-(13, 14, 15)Cu (wt%) were cast using an induction furnace. They were structurally heterogeneous composites that consisted of a relatively stiff α-Fe matrix and a soft (Cu, Mn)-rich secondary phase. They were developed as self-lubricating, wear-resistant metals. Since the formation of oxides plays an important role in wear, understanding the oxidation behavior of two-phase, multi-component composites is of practical interest. This study was undertaken to understand the effect of varying Cu concentration on the formation of oxide scales. When the concentration of Cu in the alloys was increased to 13, 14, and 15 wt%, the volume fraction of the secondary phase also increased from 10.3, 11.2, and 13.3 vol%. When oxidized at 700 ℃ for 30 h, the secondary phase preferentially oxidized to oxide nodules due to Cu and Mn, which could not form a protective scale. The matrix was uniformly oxidized, forming a thin surface scale, due mainly to oxidation-resistant Cr in the matrix. When oxidized at 800-900 ℃ for 30 h, not only the secondary phase but also the α-Fe matrix oxidized rapidly, because of the increased reaction and diffusion rates, resulting in the formation of thick, bi-layered oxide scales. An Fe-rich outer scale was formed by the outward diffusion of Fe, Cr, Mn, and Cu, while a Cr-rich inner scale was formed by the inward diffusion of oxygen. Mn and Cu were rather uniformly distributed in the outer and inner oxide scales. The oxidation rates increased with increasing amounts of Cu, and significantly more with increasing temperature. (Received April 18, 2019; Accepted June 28, 2019)

keyword : composite, alloy steel, copper, oxidation, oxidation scale

Chemical mechanical polishing (CMP) of bulk AlN was performed with colloidal silica slurry at pH 9 for different times. The result shows that colloidal silica slurry at pH 9, which has the relatively high surface charge of -50.7 mV is most stable, and it was selected as chemically optimum condition in this study. The ultra-smooth surface was shown in CMP 90 min with the roughness average (Ra) value of 0.172 nm. It was demonstrated that the damaged layers including subsurface defects and micro scratches in the whole machining process were successfully removed and atomically flat surface can be shown. With increasing process time, the zeta potential and mean particle size of the colloidal silica decreased and increased by -35.07 mV and 143.4 nm, respectively. While the silica particles agglomerated and densely packed slurry particles were formed by mechanical shearing. These increased the Ra value above 0.5 nm of AlN substrate and generated additional surface damages. In terms of the surface chemistry, the carbon compounds and organic impurities adsorbed on the substrate during mechanical polishing (MP) can be removed and aluminum oxidehydroxide; AlOOH and Al(OH)₃ were observed during the CMP. It was determined that the chemically polished AlN substrate was continuously hydrated with generating the AlOOH and Al(OH)₃ on the surface. (Received June 4, 2019; Accepted July 15, 2019)

keyword : aluminum nitride, chemical mechanical polishing, colloidal silica, material removal rate, surface chemistry, slurry agglomeration

Metal-organic frameworks are used in many applications, such as gas separation, catalysis, and the synthesis of nanomaterials. Metal-organic frameworks show superior stability at high temperature, which makes them potentially useful for the adsorption of metal ions from leaching solutions at high temperature. In this work, UIO-66-NH₂, a zirconium(IV) based metal-organic framework, was employed for the adsorption of Pd(II) from hydrochloric acid solutions. The adsorption percentage of Pd(II) decreased rapidly as HCl concentration increased from 0.2 M to 1 M and was negligible beyond 3 M. It was found that some of the UIO- 66-NH₂ was disrupted in the strong HCl solution and released Zr(IV) into the solution. The effect of chloride ion concentration on the adsorption of Pd(II) was investigated. The adsorption percentage of Pd(II) by UIO- 66-NH₂ decreased rapidly as chloride ion concentration increased. The adsorption of Pd(II) onto UIO-66-NH₂ followed the Langmuir isotherm and reached equilibrium within 2 hours under these experimental conditions. The kinetic data for the adsorption of Pd(II) onto UIO-66-NH₂ agreed well with a pseudo-second order kinetic model. The initial rate constant at 0.2 M HCl was 15.2 mg/g·min and decreased to 2.7 mg/g·min at 1.0 M HCl. More work is necessary to improve the stability of UIO-66-NH₂ in strong HCl solution and to make the metalorganic framework at lower cost. (Received May 21, 2019; Accepted July 22, 2019)

keyword : metal-organic framework, palladium(II), HCl, adsorption

Spent lithium ion batteries contain cobalt, copper, lithium, manganese and nickel. Recovery of these metals with high purity from spent lithium ion batteries is of importance. A sulfuric acid solution is generally employed for the treatment of spent lithium ion batteries, but it is difficult to separate these metal ions from the leaching solution because the divalent metal ions have similar extraction behaviors. Once Cu(II) is separated from the sulfuric acid leaching solution, the traditional solvent extraction process for the separation of Co(II) and Ni(II) can be applied to the raffinate. Therefore, selective extraction of Cu(II) was investigated in this work using Cyanex 301. The effect of the concentration of sulfuric acid and Cyanex 301 was investigated. Within the sulfuric acid concentration range (pH 5-2.0 M) employed in this work, Cu(II) was completely extracted by Cyanex 301, while the extraction percentage of other metal ions decreased to negligible when the sulfuric acid concentration increased to 2.0 M. The addition of TBP to Cyanex 301 depressed the extraction of other metal ions except Cu(II) at an initial pH of 3, facilitating the selective extraction of Cu(II). The McCabe-Thiele extraction diagram indicates that most of the Cu(II) was extracted by two count-current extraction at an A/O ratio of 2. The Cu(II) loaded in Cyanex 301 was completely stripped by using 5% aqua regia. (Received June 14, 2019; Accepted July 17, 2019)

keyword : spent lithium ion batteries, sulfuric acid, Cyanex 301, copper(II), extraction

We report the effect of interface control using reduced graphene oxide (RGO) on the thermoelectric transport properties of Bi₂Te_2.7Se_0.3 (BTS)-RGO composites. The interface-controlled BTS-RGO composites were prepared by the consolidation of BTS-RGO hybrid powders using spark plasma sintering (SPS). The RGO-hybridization reduced the grain size in the BTS-RGO composites because the formation of an RGO network at the grain boundaries inhibited the grain growth during the SPS process. The carrier concentration was almost independent of the RGO content, however, mobility was reduced by the RGO network and it led to a decrease in the electrical conductivity of the BTS-RGO composites. Although the Seebeck coefficient was enhanced by the RGO-hybridization due to an increase in effective mass, the hybridization did not lead to an increase in the power factor of the BTS-RGO composites due to the reduction of the electrical conductivity. On the other hand, thermal conductivity was reduced significantly by the RGO-hybridization, and it led to the enhancement of ZT in the BTS-RGO composite. The effects of interface control using RGO on the thermoelectric transport properties of BTS-RGO composite are discussed in detail, and our results based on the RGO-hybridization strategy will be useful for developing thermoelectric materials with high ZTs. (Received January 10, 2019; Accepted March 5, 2019)

keyword : thermoelectric, Bi₂Te₃, interface control, reduced graphene oxide

We employed spark plasma sintering (SPS) in the presence of sintering aids to fabricate highquality Ho³⁺,Yb³⁺-co-doped Y₂O₃ transparent ceramics suitable for use as a laser gain medium, and analyzed their microstructure and optical properties. Ho³⁺,Yb³⁺-co-doped Y₂O₃ transparent ceramics with different contents of Yb³⁺ were fabricated by SPS and exhibited high transparency in both near-infrared and visible regions. All specimens showed high transparency in both the near-infrared and visible-light regions, and sample microstructure was almost unaffected by the amount of doped Yb³⁺. The transmission spectra of 1 mol% Yb³⁺ doped specimens exhibited transmissivities of 73.4 and 81.0% at wavelengths of 700 and 1500 nm, respectively. Relatively high transmissivity was quickly achieved through the addition of a sintering aid (1 mol% La₂O₃) which accelerated mass transport during sintering. The effect of Yb³⁺ doping on the upconversion photoluminescence of Y₂O₃:Ho³⁺,Yb³⁺ was examined. Irradiation at 980 nm resulted in strong green photoluminescence (552 nm) and weak red photoluminescence (669 nm). Excitation at 447 and 980 nm resulted in strong green emission (⁵F₄, ⁵S₂ → ⁵I₈) from Ho³⁺. The effects of Yb³⁺ content and laser input power on emission intensity and luminescence decay were investigated in detail. Under excitation at 980 nm, the presence of Yb³⁺ increased emission intensity over the entire range, while the opposite behavior was observed under excitation at 447 nm. (Received May 3, 2019; Accepted July 10, 2019)

keyword : Y₂O₃, transparent ceramics, spark plasma sintering, upconversion, downconversion