• 소성∙가공
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• 제28권5호
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• pp.247-256
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• 2019

In this study, the development of annealing textures in cold rolled and annealed tantalum sheets was analyzed using electron backscatter diffraction. At $900^{\circ}C$, the textures of the recrystallized grains in the partially and completely recrystallized microstructures displayed significant similarities. The average diameter of the recrystallized grains with ${\gamma}-fiber$ orientations exceeded that of grains with different orientations, and the average growth rates were unrelated to the orientations after an initial stage of recrystallization. Additional cold rolling and annealing was done for controlled initial microstructures and textures inherited from various processes of prior cold rolling and annealing. This second cycle of the process resulted in stronger textures with major ${\gamma}-fiber$ orientations as a result of the enhanced ${\gamma}-fiber$ orientations in the preceding textures. A coarse-grained prior microstructure resulted in a weaker annealing texture than a fine grained one regardless of the stronger previous texture, which was occasioned by the sub-structures of the minor orientations at local deformation inhomogeneities such as sharp in-grain shear bands.

• Nuclear Engineering and Technology
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• 제54권8호
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• pp.2801-2808
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• 2022

The synergistic effect of ZrC nanoparticle pining and Re solution in W matrix on the thermal stability of tungsten was studied by investigating the evolution of the microstructure, hardness and tensile properties after annealing in a temperature range of 1000-1700 ℃. The results of metallography, electron backscatter diffraction pattern and Vickers micro-hardness indicate that the rolled W-1wt%Re-0.5 wt% ZrC alloy has a higher recrystallization temperature (1600 ℃-1700 ℃) than that of the rolled pure W (1200 ℃), W-0.5 wt%ZrC (1300 ℃), W-0.5 wt%HfC (1400-1500 ℃) and W-K-3wt%Re alloy fabricated by the same technology. The molecular dynamics simulation results indicated that solution Re atoms in W matrix can slow down the self-diffusion of W atoms and form dragging effect to delay the growth of W grain, moreover, the diffusion coefficient decrease with increasing Re content. In addition, the ZrC nanoparticles can pin the grain boundaries and dislocations effectively, preventing the recrystallization. Therefore, synergistic effect of solid solution Re element and dispersed ZrC nanoparticles significantly increase recrystallization temperature.

• 소성∙가공
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• 제31권3호
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• pp.151-159
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• 2022

Ti alloys are used in a wide range of applications, especially for aviation and medical purposes, because of their high specific strength and excellent corrosion properties. When subjected to various manufacturing processes, one of the most popular Ti alloys, Ti-6Al-4V, exhibits a variety of microstructural and mechanical properties that makes it an attractive lightweight metal. The purpose of this study was to analyze the microstructure and mechanical properties of Ti alloy wires. Subsequently, the microstructure and electrochemical behavior of Ti alloy bolts produced from these wires were analyzed. The Ti alloy wires are manufactured with different diameters (6.22, 7.81 mm alloys), and their microstructures are measured using electron backscatter diffraction. Recrystallization was observed to occur significantly in the 7.81 alloy than in the 6.81 alloy, and the strain distribution of 7.81 alloy is seen to be likely more uniform than 6.22 alloy. Ti alloy bolt was then forged under moderate temperature by using the 7.81 alloy. Results of the electrochemical analysis indicate that the Ti alloy bolt has excellent corrosion resistance.

• Archives of Metallurgy and Materials
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• 제66권3호
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• pp.759-763
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• 2021

The effects of the sintering holding time and cooling rate on the microstructure and mechanical properties of nanocrystalline Fe-Cr-C alloy were investigated. Nanocrystalline Fe-1.5Cr-1C (wt.%) alloy was fabricated by mechanical alloying and spark plasma sintering. Different process conditions were applied to fabricate the sintered samples. The phase fraction and grain size were measured using X-ray powder diffraction and confirmed by electron backscatter diffraction. The stability and volume fraction of the austenite phase, which could affect the mechanical properties of the Fe-based alloy, were calculated using an empirical equation. The sample names consist of a number and a letter, which correspond to the holding time and cooling method, respectively. For the 0A, 0W, 10A, and 10W samples, the volume fraction was measured at 5.56, 44.95, 6.15, and 61.44 vol.%. To evaluate the mechanical properties, the hardness of 0A, 0W, 10A, and 10W samples were measured as 44.6, 63.1, 42.5, and 53.8 HRC. These results show that there is a difference in carbon diffusion and solubility depending on the sintering holding time and cooling rate.

• 한국재료학회지
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• 제27권10호
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• pp.524-529
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• 2017

The effect of strain aging on the tensile properties of API X70 linepipe steel was investigated in this study. The API X70 linepipe steel was fabricated by controlled rolling and accelerated cooling processes, and the microstructure was analyzed using optical and scanning electron microscopes and electron backscatter diffraction. Strain aging tests consisting of 1 % pre-strain and thermal aging at $200^{\circ}C$ and $250^{\circ}C$ were conducted to simulate U-forming, O-forming, Expansion(UOE) pipe forming and anti-corrosion coating processes. The API X70 linepipe steel was composed of polygonal ferrite, acicular ferrite, granular bainite, and bainitic ferrite whose volume fraction was dependent on the chemical composition and process conditions. As the thermal aging temperature increased, the steel specimens showed more clearly discontinuous type yielding behavior in the tensile stress-strain curve due to the formation of a Cottrell atmosphere. After pre-strain and thermal aging, the yield and tensile strengths increased and the yield-to-tensile strength ratio decreased because yielding and aging behaviors significantly affected work hardening. On the other hand, uniform and total elongations decreased after pre-strain and thermal aging since dislocation gliding was restricted by increased dislocation density after a 1 % pre-strain.

• 한국표면공학회지
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• 제54권5호
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• pp.267-277
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• 2021

Nitriding layers developed during gaseous nitriding of AISI4115 steels for the application of steel bushing part were investigated. The compound layer thickness of about 10㎛, 0.3mm of case depth under the same conditions, and conventional nitriding, nitrocarburizing, and controlled nitriding were performed in three methods. In the controlled nitriding, K_N was controlled by measuring the hydrogen partial pressure. The nitrided samples were analyzed by micro Vickers hardness test, optical microscopy and scanning electron microscopy. The phases of compound layer were identified by X-ray diffraction and electron backscatter diffraction. The controlled nitriding specimen indicated the highest surface hardness of about 860 HV_0.1. The compound layer of the conventional nitriding and nitrocarburizing specimen was formed with about 46% porous layer and 𝜺 + 𝜸' phase, and about 13% porous layer and about 80% 𝜸' phase were formed on the controlled nitriding specimen. As a result of the Ball-on-disk wear test, the worn mass loss of ball performed on the surface of the controlled nitriding specimen was the largest. The controlled nitriding specimen had the highest surface hardness due to the lowest porous percentage of compound layer, which improved the wear resistance.

• 한국재료학회지
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• 제28권12호
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• pp.702-708
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• 2018

This study deals with the effect of microstructure factors on the strain aging properties of API X70 pipeline steels with different microstructure fractions and grain sizes. The grain size and microstructure fraction of the API pipeline steels are analyzed by optical and scanning electron microscopy and electron backscatter diffraction analysis. Tensile tests before and after 1 % pre-strain and thermal aging treatment are conducted to simulate pipe forming and coating processes. All the steels are composed mostly of polygonal ferrite, acicular ferrite, granular bainite, and bainitic ferrite. After 1 % pre-strain and thermal aging treatment, the tensile test results reveal that yield strength, tensile strength and yield ratio increase, while uniform elongation decreases with an increasing thermal aging temperature. The increment of yield and tensile strengths are affected by the fraction of bainitic ferrite with high dislocation density because the mobility of dislocations is inhibited by interaction between interstitial atoms and dislocations in bainitic ferrite. On the other hand, the variation of yield ratio and uniform elongation is the smallest in the steel with the largest grain size because of the decrease in the grain boundary area for dislocation pile-ups and the presence of many dislocations inside large grains after 1 % pre-strain.

• 소성∙가공
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• 제27권6호
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• pp.339-346
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• 2018

This study investigated the microstructure and wear properties of extruded hyper-eutectic Al-Si (15wt.%) alloy in an engine oil environment. The wear mechanism of the material was also analyzed and compared to conventional gray cast iron. In microstructural observation results of Al-15wt.%Si alloy, primary Si phase ($45.3{\mu}m$) and eutectic Si phase ($3.1{\mu}m$) were found in the matrix, and the precipitations of $Mg_2Si({\beta}^{\prime})$, $Al_2Cu({\theta}^{\prime})$ and $Al_6(Mn,Fe)$ were also detected. In the case of gray cast iron, ferrite and pearlite were observed. It was also observed that flake graphite ($20-130{\mu}m$) were randomly distributed. Wear rates were lower in the Al-Si alloy as compared to those of gray cast iron in all load conditions, confirming the outstanding wear resistance of Al-15wt.%Si alloy in engine oil environment. In the $4kg_f$ condition, the wear rate of gray cast iron was $6.0{\times}10^{-5}$ and that of Al-Si measured $0.8{\times}10^{-5}$. The microstructures after wear of the two materials were analyzed using scanning electron microscope (SEM) and electron backscatter diffraction (EBSD). The primary Si and eutectic Si of Al-Si alloy effectively mitigated the abrasive wear, and the Al matrix effectively endured to accept a significant amount of plastic deformation caused by wear.

• Journal of Welding and Joining
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• 제34권1호
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• pp.75-81
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• 2016

This study applied laser surface melting process using CW(Continuous wave) Yb:YAG laser and cold-work die steel SM45C and investigated microstructure and hardness. Laser beam speed, power and beam interval are fixed at 70 mm/sec, 2.8 kW and $800{\mu}m$ respectively. Depth of Hardening layer(Melting zone) was a minimum of 0.8 mm and a maximum of 1.0 mm that exceeds the limit of minimum depth 0.5 mm applying trimming die. In all weld zone, macrostructure was dendrite structure. At the dendrite boundary, Mn, Al, S and O was segregated and MnS and Al oxide existed. However, this inclusion didn't observe in the heat-affected zone (HAZ). As a result of interpreting phase transformation of binary diagram, MnS crystallizes from liquid. Also, it estimated that Al oxide forms by reacting with oxygen in the atmosphere. The hardness of the melting zone was from 650 Hv to 660 Hv regardless of the location that higher 60 Hv than the hardness of the HAZ that had maximum 600 Hv. In comparison with the size of microstructure using electron backscatter diffraction(EBSD), the size of microstructure in the melting zone was smaller than HAZ. Because it estimated that cooling rate of laser surface melting process is faster than water quenching.

• 한국재료학회지
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• 제26권12호
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• pp.688-695
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• 2016

Recrystallization behavior has been investigated for commercial purity AA1050 (99.5wt%Al) and high purity 3N Al (99.9wt% Al). Samples were cold rolled with 90% of thickness reduction and were annealed isothermally at 290, 315, and 350o C for various times until complete recrystallization was achieved. Hardness measurement and Electron Backscatter Diffraction(EBSD) analyses, combined with Grain Orientation Spread(GOS), were employed to investigate the recrystallization behavior. EBSD analysis combined with GOS were distinctly revealed to be a more useful method to determine the recrystallization fraction and to characterize the recrystallization kinetics. As the annealing temperature increased, recrystallization in AA1050 accelerated more than that process did in Al 3N. Both AA1050 and Al 3N showed the same temperature dependence of the n value of the Johnson-Mehl-Avrami-Kolmogorov equation(JMAK equation), i.e., n values increased as annealing temperature increased. Activation energy of recrystallization in AA1050 is about 176 kJ/mol, which is comparable with the activation energy of grain boundary migration in cold-rolled AA1050. This value is somewhat higher than the activation energy of recrystallization in Al 3N.