• Transactions of Materials Processing
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• v.31 no.3
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• pp.160-166
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• 2022

The purpose of this study is to investigate the effects of combined addition of Ca and Y on the precipitation and age-hardening behavior of an extruded AZ91 alloy by conducting the aging treatment at 200 ℃ for hot-extruded AZ91 and AZ91-0.3Ca-0.2Y alloys. In the AZ91 alloy, many Mg₁₇Al₁₂ discontinuous precipitate (DP) bands formed during air cooling immediately after extrusion are present, whereas in the AZ91-0.3Ca-0.2Y alloy, a few DP bands and numerous Al₂Y, Al₈Mn₄Y, and Al₂Ca phase particles are distributed along the extrusion direction. The peak-aging time of the AZ91-0.3Ca-0.2Y alloy is 16 hours, twice that of the AZ91 alloy. Although both alloys have similar hardness before aging treatment, the hardness after peak-aging treatment (i.e., peak hardness) of the AZ91-0.3Ca-0.2Y alloy is higher than that of the AZ91 alloy, as 93.1 and 88.7 Hv, respectively. The microstructures of both peak-aged alloys comprise DPs and continuous precipitates (CPs). However, the peak-aged AZ91-0.3Ca-0.2Y alloy has a smaller amount of DPs and a larger amount of CPs than the peak-aged AZ91 alloy. Additionally, the inter-particle spacings of DPs and CPs in the former are significantly narrower than those in the latter. These results demonstrate that the addition of small amounts of Ca and Y to a commercial AZ91 alloy considerably affects the formation rate, size, and amount of CPs and DPs during aging and resultant age-hardening behavior.

• Journal of the Korean Society for Heat Treatment
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• v.12 no.2
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• pp.129-135
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• 1999

The effects of Ag addition on the microstructures and mechanical properties of 7050 Al alloy were investigated. Various homogenizing and aging treatments were carried out to analyze the controversial effects of Ag in 7050 Al alloy. Transmission electron microscopy(TEM) was used for microstructural analysis. The hardening precipitates(${\eta}^{\prime}$) become finer with Ag addition. It suggests that Ag promotes easier nucleation of ${\eta}{\prime}$. The strength of overaged Ag bearing alloys are higher than that of Ag free alloy. Hardening precipitates(${\eta}^{\prime}$) in Ag bearing alloys are smaller than that of Ag free alloys, because the growth rate of ${\eta}^{\prime}$ during overaging stage is lower in Ag bearing alloys.

• Korean Journal of Metals and Materials
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• v.47 no.2
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• pp.71-78
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• 2009

The precipitation behavior of solute carbon and nitrogen strongly affects the mechanical properties of low-carbon automotive panel. In the present study, the effects of aluminum and solute nitrogen on the bake hardenability and strain aging of extremely low-carbon steel with carbon content below 15 ppm has been investigated. The ferrite grain size and distribution of precipitates were varied with the amount of aluminum content of 0.003 to ~ 0.100 wt% in a constant solute carbon and nitrogen. With increasing the aluminum content, the ferrite grain size is increased and strain aging is delayed. The strain aging is also delayed by increasing the annealing temperature, although the ferrite grain size is not much changed.

• Nuclear Engineering and Technology
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• v.55 no.1
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• pp.80-99
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• 2023

This paper presents the results of investigating the change in welding residual stresses of the core shroud, which is one of subcomponents in reactor vessel internals, performing finite element analysis. First, the welding residual stresses of the core shroud were calculated by applying the heat conduction based lumped pass technique and finite element elastic-plastic stress analysis. Second, the temperature distribution of the core shroud during the normal operation was calculated by performing finite element temperature analysis considering gamma heating. Third, through the finite element viscoelastic-plastic stress analysis using the calculated temperature distribution and setting the calculated residual stresses as the initial stress state, the variation of the welding residual stresses was derived according to repeating the normal operation. In the viscoelastic-plastic stress analysis, the effects of neutron irradiation on mechanical properties during the cyclic normal operations were considered by using the previously developed user subroutines for the irradiation agings such as irradiation hardening/embrittlement, irradiation-induced creep, and void swelling. Finally, the effect of neutron irradiation on the welding residual stresses was analysed for each irradiation aging. As a result, it is found that as the normal operation is repeated, the welding residual stresses decrease and show insignificant magnitudes after the 10^th refueling cycle. In addition, the irradiation-induced creep/void swelling has significant mitigation effect on the residual stresses whereas the irradiation hardening/embrittlement has no effect on those.

• Journal of the Korean Society of Propulsion Engineers
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• v.9 no.2
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• pp.46-53
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• 2005

We have analyzed the temperature and humidity aging test results of a composite solid propellant. The temperature aging test was performed to evaluate the storage life of a propellant, while the humidity aging test could provide the hygroscopicity of Ammonium Perchlorate(AP) exposed to .elative humidity (RH) 10, 30, $50\%$ environment. A specimen was used in the temperature test, and a block of propellant from the actual motor was used in the humidity test. We report that the 4-month storing at 60 degree is equivalent to the 10-year 60 degree condition. The composite solid propellant with HTPB binder showed signs of hardening with time lapse but the effect of humidity up to RH $50\%$ was not noticeable.

• Journal of the Korean Society for Nondestructive Testing
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• v.26 no.6
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• pp.417-426
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• 2006

A Digital X-ray imaging system using Compton backscattering has been developed to obtain a cross-sectional profile and mass loss of corroded lap-splices of aging aircraft from density variation. A slit-type camera was designed to focus on a small scattering volume inside the material, from which the backscattered photons are collected by a collimated scintillator detector for interpretation of material characteristics. The cross section of the lap-joint is scanned by moving the scattering volume through the thickness direction of the specimen. The mass loss of each layer has been estimated from a Compton backscatter A-scan to obtain the thickness of each layer including the aluminum sheet, the corrosion layer and the sealant. Quantitative information such as location and width of planar corrosion in the lap splices of fuselages is obtained by deconvolution using a nonlinear least-square error minimization method(BFGS method): A simple reconstruction model is also introduced to overcome distortion of the Compton backscatter data due to attenuation effects attributed to beam hardening and quantum noise.

• Journal of the Korean Society for Heat Treatment
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• v.22 no.6
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• pp.345-353
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• 2009

The precipitates of $L2_1$-type $Ni_2AlHf$ phase in B2-ordered NiAl system has been observed by using transmission electron microscope (TEM). The hardness of as-quenched NiAl-Hf alloys is high due to the larger strengthening. However, age hardening of this alloy is not main effect to increase hardness compared to the large microstructural variations during aging. At the beginning of aging, the $L2_1$-type $Ni_2AlHf$ precipitates keep a lattice coherency with the NiAl matrix. The orientation relationship between the $Ni_2 AlHf$ precipitate and the NiAl matrix is <100>$_{Ni2AlHf}$//<100>$_{NiAl}$, {001}$_{Ni2AlHf}$//{001}$_{NiAl}$. By aging treatment for long time $Ni_2AlHf$ precipitates lost their coherency and change their morphology to the spherical ones surrounded by misfit dislocations. The orientation relationship between the NiAl matrix and the $Ni_2AlHf$ precipitates, however, has been kept even after longer aging time. The lattice misfit between the $Ni_2AlHf$ precipitate and the NiAl matrix has been calculated by the selected electron diffraction patterns, and the spacings of misfit dislocations is about 4.5% at 1173 K.

• Korean Journal of Materials Research
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• v.27 no.10
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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.

• Journal of the Korean Society for Heat Treatment
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• v.35 no.2
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• pp.88-95
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• 2022

Titanium alloy for bio-medical applications have been developed to reduce the toxicity of alloying elements and avoid the stress-shielding effect which is caused by relatively high elastic modulus compared to bone. Ti-39Nb-6Zr (TNZ40) alloy of elastic modulus exhibits around 40 GPa in the case of beta single phase. However, the strength of this alloy is lower than the other types of titanium alloys. Many research found that adding oxygen to beta-titanium alloys is beneficial for improving the strength through solid solution strengthening. In this study, TNZ40 ingots with addition of O were prepared by an arc remelting process (Ti-39Nb-6Zr-0.16O (wt.%), Ti-39Nb-6Zr-0.26O (wt.%)). Thermo-mechanical processing (i.e., heat treatment, cold swaging and aging heat treatment) has been performed under various conditions. Therefore, the aim of this study is to investigate the effect of oxygen content and ω phase formation on microstructure and mechanical properties.

• Korean Journal of Metals and Materials
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• v.46 no.8
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• pp.477-481
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• 2008

The secondary hardening and fracture behavior in P/M high speed steels bearing V content of 9 to 10 wt% have been investigated in terms of austenitizing temperature and precipitation behavior. Austenitizing was conducted at 1,100 and $1,175^{\circ}C$ of relatively low and high temperatures. Coarse primary carbides retained after austenitization were mainly V-rich MC type. They give a significant influence on hardeness and toughness, as well as wear resistance. Tempering was performed in the range of $500{\sim}600^{\circ}C$. The peak hardness resulting from the precipitation of the fine MC secondary carbides was observed near 520, irrespective of austenitizing temperature. Aging acceleration(or deceleration) did not occur with increasing austenitizing temperature because it mainly influences contents of V and C of matrix through the dissloution of coarse primary MC containing lots of V and C. The precipitation of secondary MC carbides, which also contain V and C, did not change the aging kinetics itself. In the 10V alloy containing much higher C content, the impact toughness was lower than 9V alloy, because of the larger amount of primary carbide and high hardness.