발간논문

Home > KJMM 논문 > 발간논문

This study investigated hydrogen induced cracking (HIC) of X70 steel, used for gas pipeline material, with a bainitic microstructure. The effect of hydrogen on the steel was investigated by tensiletesting in a high pressure hydrogen atmosphere, using OM, SEM and TEM analyses. The mechanisms of hydrogen-related cracking were represented as stress-induced hydride formation, hydrogen enhanced localized plasticity and decohesion. While hydrogen-trapped sites increased the HIC susceptibility, a kind of surface corrosion occurred in the tensile tested gauge length, due to the high pressured hydrogen atmosphere. The surface corrosion was composed of HIV (hydrogen induced voids) and HIC occurred by linking of the voids. Typical SOHIC (stress oriented HIC) propagated to the inner areas by linking of planar cracks normal to the primary propagation direction. Carbides at grain and lath boundary of the X70 bainite acted as initiation sites of the voids, followed by cracking without any phase transformation. The relation of hydrogen induced defects and grain boundary carbide is discussed in relation to hydrogen corrosion and safe application in hydrogen system. (Received March 27, 2017; Accepted July 28, 2017)

keyword : hydrogen induced crack (HIC), hydrogen induced void (HIV), X70 steel, bainite, grain and lath boundary carbide, hydrogen corrosion

The mechanical properties of nickel-base superalloys used in disk and blade materials for gas turbine engines can be maximized by controlling their microstructure, such as the size of precipitates. In this study, we investigated the size of gamma prime precipitates after age hardening heat treatment at 650-980 ℃ for 1-48 hours. As the aging temperature and time increased, the grain size of gamma prime increased. The growth constants were 0.11, 0.24, 15.5 and 167 nm3/sec at 650, 760, 870, and 980 ℃ respectively. Activation energy for the growth of the gamma prime precipitates was determined to be 261±54 kJ/mol and reference studies have confirmed that Ni, Al, or Ti lattice diffusion in Ni or Ni3Al dominates the rate. The hardness increased with the decreasing size of gamma prime precipitates, and reached maximum at 24 nm, and decreased thereafter. The data on hardness vs. precipitate size followed the Hall-Petch relation over a precipitate size range from 24 nm to hundreds of nanometers. The maximum hardness at each annealing time increased with a reduction in the annealing temperature time. The optimum condition was determined to be 727 ℃ and 24 hours by using an approximation curve. The maximum hardness (520HV) was experimentally confirmed under this condition. The reduction in hardness with decreasing size might be due to changes in the misfit strain at the interface, decrease in the gamma prime volume fraction, or an increase in particle cutting events by dislocation. (Received August 4, 2017; Accepted September 29, 2017)

keyword : Ni-base superalloy, aging, activation energy, hardness

High entropy alloys (HEAs) are a relatively new class of metallic materials. HEAs lead to high mixing entropy and the formation of a simple solid solution rather than intermediate phases. This might be explained by the fact that the configurational entropy outweighs the enthalpy of formation of intermetallic compounds. Meanwhile, bulk metallic glasses (BMGs) are representative of multicomponent systems which have (1) low thermodynamic driving force for crystallization, and (2) low atomic mobility associated with viscosity. Recently, it was reported that high entropy BMG (HE-BMG) can be fabricated by combining the concepts of both BMGs and HEAs. These alloys usually consist of at least five constituent elements with an equi-atomic or near equi-atomic ratio. In the present study, we focused on the development of new rare-earth element (RE)-based HE-BMGs by changing both the number and the kinds of the RE elements in the REAl-TM (transition metal) BMGs. We carefully considered the following factors for alloy design: 1) heavy RE-based BMGs exhibit a relatively high glass-forming ability and high thermal stability; 2) the elements of the RE group have many common properties, with only very small differences in solubility and complex formation, which makes it difficult to separate or even distinguish among them. We evaluated the glass-forming ability and unique properties of the newly developed RE-based HE-BMGs, and compared their properties with those of other conventional multi-component alloys. This report will provide a novel alloy design strategy for HEBMGs that possess the characteristics of both HEA and BMG. (Received August 2, 2017; Accepted September 20, 2017)

keyword : alloy design, high entropy bulk metallic glass, microstructure, thermal property, mechanical property

The effect of carbon addition on the cast and rolled microstructures of CoCrFeMnNi high entropy alloys were studied. Both as-cast CoCrFeMnNi and CoCrFeMnNiC_0.1 alloys have a dendritic microstructure. Small carbide particles were observed at the inter-dendritic region in the as-cast CoCrFeMnNiC_0.1 and elongated-grained structure with aligned carbides were developed at grain boundaries in CoCrFeMnNiC_0.1 after homogenization. The grain size decreased from 9.2 μm to 5.8 μm with the addition of 0.1 wt% carbon. The decrease in grain size is due to its high stored cold work energy during deformation processing, resulting in more active recrystallization in CoCrFeMnNiC_0.1 and the pinning effect of carbides. The strengthening in CoCrFeMnNiC_0.1 is most likely attributed to solid solution strengthening and nanoscale carbide strengthening, because grain size strengthening was found to be not as effective. (Received May 23, 2017; Accepted September 25, 2017)

keyword : high entropy alloys, carbon, interstitial, CoCrFeMnNi, carbide

A model based on Irwin's approach to elastic-plastic deformation was applied to predict the tensile ductility of a sintered iron. The evaluation of this model using experimental results for porous sintered iron showed that tensile ductility depends not only on the pore volume fraction, but also the pore size and the mechanical properties of the matrix. It was shown that this model agrees well with experimental data and resolves some of the discrepancies of the Brown-Embury model. This model suggested that the ductility of the porous metal can be improved by actively controlling the metallurgical parameters, such as the mechanical properties of the matrix and the pore size. It can be seen that the fracture true strain of the base metal has the same effect as the change in pore radius, because it acts as a linear function in the ductility prediction of porous iron. On the other hand, the σ^m_y/ΔK_th,eff ratio varies from the quadratic function. The calculation results based on this model show that effect of the change in the σ^m_y/ΔK_th,eff ratio on the ductility of the porous metal is greater than the effect of the change in the pore radius and fracture true strain. This model also shows it is possible to improve tensile ductility in porous metals by controlling the mechanical properties of the base metal or controlling the pore radius, in addition to reducing the pore volume fraction. In other words, increasing the ductility of the porous metal is possible by increasing the pore radius, or using a base metal with high yield strength and low ΔK_th,eff, or using a base metal with high fracture true strain. (Received July 24, 2017, Accepted October 18, 2017)

keyword : theory and modeling, porous materials, tensile ductility, sintered iron, powder metallurgy processing

The effect of Al addition on the microstructure and mechanical properties of extruded Mg-Sn based alloy is investigated by adding 1 wt% Al to the Mg-7Sn-1Zn alloy and indirectly extruding the billets with and without the Al addition. The Al addition promotes dynamic recrystallization (DRX) behaviour during hot extrusion, which results in an increase in the area fraction of dynamically recrystallized (DRXed) grains and a decrease in the width of unDRXed grains of the extruded alloy. In addition, dynamic precipitation during extrusion is also accelerated by the Al addition owing to the decrease in Sn solubility in the Mg matrix, thereby increasing the amount of Mg₂Sn precipitates and decreasing their size. The increased amount of fine precipitates, and the reduced amount of coarse unDRXed grains, lead to a reduction in the average grain size of the extruded alloy. The Al addition significantly improves tensile properties (i.e., strength and ductility) and 3 point bending strength of the extruded alloy. The improvement in the tensile and bending strengths is attributed to the combined effects of grain-boundary hardening by the reduced grain size, precipitation hardening by more abundant precipitates, and solid-solution hardening by dissolved Al atoms. The increase in the tensile elongation results from the decrease in the amount and size of the coarse unDRXed grains because {10-11} contraction and {10-11}-{10-12} double twins, which can act as initiation sites of microcracks, easily occur under tension along the extrusion direction in the coarse grains. However, the increased amount of precipitates and the reduced grain size accelerate the formation of shear bands and suppress the activation of {10-12} twinning in the upper side of the bending sample, which consequently results in a deterioration in the bending formability of the extruded alloy. (Received August 21, 2017; Accepted October 31, 2017)

keyword : magnesium alloy, extrusion, recrystallization, precipitation, tensile properties

A bauxite of low quality containing 6.75% reactive silica and 37.4% Al₂O₃ as gibbsite was roasted with additives including NaOH (caustic/total alumina molar ratio of 2-4:1) and CaO (CaO/total SiO₂ molar ratio of 0.5-3:1) at 300-1000 ℃, producing calcines containing sodium aluminate (Na₂O.Al₂O₃), silicates and hematite. Optimum conditions included roasting at 350-400 ℃ with CaO added at a CaO/total SiO₂ molar ratio of 1:1 for 2-3 h. Dissolving the calcine in water at 90 ℃ produced a sodium aluminate liquor from which aluminum hydroxide (Al₂O_3.3H₂O - alumina trihydrate or ATH) was precipitated. The addition of CaO reduced the silica extraction to <15% at optimum conditions, whereas the total organic carbon(TOC) was ~200 mg/L as carbon in all cases. Losses of alumina and caustic were experienced, as different phases of Al-Ca-Si or Na-Ca-Al-Si minerals were formed during roasting and/or leaching. The extracted alumina in the leached solution were precipitated at 60 ℃ for 20 h to produce a fine aluminum hydroxide having D₅₀ of 1.9-2.8 μm with a whiteness index of 96.8-97.5, better than commercially available products. (Received May 11, 2017; Accepted September 29, 2017)

keyword : fine aluminium hydroxide, high silica, bauxite, caustic roasting, high whiteness

In this work, Mg₂Ni and Ni were added to MgH₂ in order to improve the hydrogen-storage properties of Mg. A 94 wt% MgH₂+5 wt% Mg₂Ni+1wt% Ni (named 94MgH₂+5Mg₂Ni+1Ni) sample was prepared by milling in a hydrogen atmosphere in a planetary ball mill for 5 h. The Mg₂Ni was hydrided during milling in a hydrogen atmosphere. The 94MgH₂+5Mg₂Ni+1Ni had an effective hydrogen-storage capacity (the quantity of hydrogen absorbed for 60 min) of near 5 wt%. At n=1, the sample released 0.18 wt% for 2 min, 2.14 wt% for 5 min, 4.65 wt% for 10 min, and 5.46 wt% for 60 min at 648 K. The reactive mechanical grinding of MgH₂ with Mg₂Ni and Ni is believed to facilitate nucleation (by creating defects, which serve as active sites for nucleation, on the surfaces and inside the Mg particles), increase reactivity with hydrogen (by making clean surfaces), and decrease the diffusion distances of hydrogen atoms (by reducing the particle size of Mg). (Received April 10, 2017; Accepted July 28, 2017)

keyword : hydrogen absorbing materials, mechanical milling, hydrogen, X-ray diffraction, Mg₂Ni and Ni addition

Bi_2-xSb_xTe₃ (x = 1.4-1.7) solid solutions were synthesized by mechanical alloying (MA) and consolidated by hot pressing (HP), and their charge transport and thermoelectric properties were examined. The relative densities of the hot-pressed specimens were higher than 96% on average. As the Sb content was increased, the lattice constants decreased, which confirmed that mechanical alloying using a planetary mill was successful in synthesizing solid solutions. The carrier concentration increased with increasing Sb content, and the specimens with x ≥ 1.5 behaved as degenerate semiconductors. All specimens showed p-type conduction, which was confirmed from the positive values of the Seebeck coefficient and the Hall coefficient. The increased Sb content caused a shift in the peak values of the Seebeck coefficient to higher temperatures and enhanced the power factor. As the Sb content increased, the electronic thermal conductivity increased, and the lattice thermal conductivity decreased. Bi_0.3Sb_1.7Te₃ hot-pressed at 698 K exhibited a maximum power factor of 3.4 mWm^-1K^-2 at 323 K and a low thermal conductivity of 0.8 Wm^-1K^-1. The maximum dimensionless figure of merit (ZT_max = 1.4) and the average performance (ZT_ave = 1.2) were obtained at 323 K. (Received July 5, 2017; Accepted October 30, 2017)

keyword : thermoelectric, bismuth telluride, solid solution, mechanical alloying, hot pressing

An inorganic binder is a good way to mitigate the environmental issues of organic binders containing phenol, formaldehyde, etc. One of the most representative materials in the field of inorganic moulding sand binders is a group of binder systems based on alkali silicate compounds, however, many aspects need to be improved, such as flowability, usable time and moisture resistance. In order to develop and improve inorganic binders, it is necessary to understand the characteristics of raw materials and utilize them appropriately. In this study, FT-IR analysis and thermal properties of aqueous sodium silicate solution were investigated. The FT-IR analysis results indicated, that asymmetric vibrations had higher wavenumber than their symmetric vibrations and also the stretching vibrations appeared at higher energies than the deformation vibrations. FT-IR analysis of Na₂SiO₃ showed that the asymmetric stretching vibration absorption band of Si-O(Na) was indicated at around 1030 cm^-1, and the absorption bands due to hydrated unit molecules were observed in the sodium silicate solution. As a result of TGA analysis of the dried sample, the weight loss of about 16% which was confirmed to be due to pyrolysis of the silanol group up to 500 ℃, and an XRD analysis of the dried sample after heat treatment was confirmed that quartz crystals were formed at 900 ℃ or higher. (Received August 31, 2017; Accepted October 31, 2017)

keyword : sodium silicate solutions, FT-IR, thermal properties