• Transactions of Materials Processing
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• v.33 no.1
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• pp.12-17
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• 2024

Aluminum-zinc-magnesium alloy is a well-known alloy that is both strong and lightweight. Precipitation strengthening plays a significant role in the strength mechanism of this alloy, with nano-sized η-based precipitates being the representative precipitates. However, the growth of η precipitates can lead to a decrease in strength, necessitating research into ways to control their growth. In this study, we observed the atomic-level behavior of η₂ precipitates and discovered that the precipitates grew through a combination with magnesium after a zinc segregation layer was formed around them.

• Journal of the Korean Society for Nondestructive Testing
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• v.30 no.5
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• pp.451-457
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• 2010

In the present study, the influences of grains and precipitates of microstructural evolution on the ultrasonic nonlinearity have been experimentally investigated. The prior-austenite grain and precipitate size are controlled by the variation in austenitizing and tempering conditions in reactor pressure vessel materials of nuclear power plant, SA508 Gr.3 low alloys. The ultrasonic nonlinearity was found to have strong correlations with grains and precipitates since the ultrasonic nonlinear parameter $\beta$ shows decrease trend with coarsening of grains and precipitates. Although the prior-austenite grain size increased, the $\beta$ changed little due to the effects of subgrains, packets and laths. For the preciptate effects, the $\beta$ decreased sharply due to decrease in $Mo_2C$ causing the coherency stain in addition to the precipitate size. The results in this study may provide a potential for characterizing the microstructural evolution, grains and precipitates, by measuring the ultrasonic nonlinearity.

• Journal of the Korean Vacuum Society
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• v.1 no.1
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• pp.206-211
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• 1992

SIMOX SOI structures were formed by oxygen ion implantation with a dose of 2 1018 ions/cm2 at 180kev and post-implantation annealing at $1250^{\circ}C$ for 6 hours in nitrogen ambient. The oxygen redistribution process during post-implantation annealing was examined by AES and TEM. The electrical property of the structure was investigated by SRP method. We could find oxygen precipitates in SOI layer was discussed. And the limiting factor to the decrease of the precipitates during post-implantation annealing was discussed also.

• Journal of Ocean Engineering and Technology
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• v.4 no.2
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• pp.262-262
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• 1990

In order to check the effect of dislocation behavior on creep rate in 12% Chromium steel, 14 samples of different compositions were examined by creep rupture test, and subgrain sizes, distribution of dislocations and precipitates were checked. And, authors reviewed the behaviors of dislocations, the formation and growth of subgrains and precipitates during creep. The results are as the following: 1) Creep rates calculated by .epsilon. over dot = .rho.bv show 10-15% higher values than actual data measured. However, authors conclude that the density and velocity of dislocations together with subgrain size are important factors governing deformation during creep in 12% chromium steel. 2) The values of the strength of obstacles in the mobility of dislocations are more clearly depended on the effective stress in the range of $10{\pm}5kgf/mm^{2}$ and increase with the increase of temperature. 3) Creep rates decrease with the smaller sizes of subgrains formed and can result in the longer creep rupture lives(hours). The smaller subgrains can be made by forming shorter free gliding distances of dislocations with very fine precipitates formed in the matrix during creep by applying proper alloy design. 4) Dislocation mobility gets hindered by precipitates occurring, which are coarsened by the softening process governed by diffusion during long time creep.

• Journal of Ocean Engineering and Technology
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• v.4 no.2
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• pp.112-120
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• 1990

In order to check the effect of dislocation behavior on creep rate in 12% Chromium steel, 14 samples of different compositions were examined by creep rupture test, and subgrain sizes, distribution of dislocations and precipitates were checked. And, authors reviewed the behaviors of dislocations, the formation and growth of subgrains and precipitates during creep. The results are as the following: 1) Creep rates calculated by .epsilon. over dot = .rho.bv show 10-15% higher values than actual data measured. However, authors conclude that the density and velocity of dislocations together with subgrain size are important factors governing deformation during creep in 12% chromium steel. 2) The values of the strength of obstacles in the mobility of dislocations are more clearly depended on the effective stress in the range of $10{\pm}5kgf/mm^{2}$ and increase with the increase of temperature. 3) Creep rates decrease with the smaller sizes of subgrains formed and can result in the longer creep rupture lives(hours). The smaller subgrains can be made by forming shorter free gliding distances of dislocations with very fine precipitates formed in the matrix during creep by applying proper alloy design. 4) Dislocation mobility gets hindered by precipitates occurring, which are coarsened by the softening process governed by diffusion during long time creep.

• Nuclear Engineering and Technology
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• v.53 no.10
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• pp.3398-3405
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• 2021

This study observed the effect of neutron irradiation and ageing on the microstructure, hardness, and corrosion resistance of SAV-1 (Al-Mg-Si) alloy. The investigated material was irradiated with neutrons to fluences of 10²¹-10²⁶ n/m² in the WWR-K research reactor and kept in dry storage. Long-term irradiation led to an increase in hardness of the alloy and a deterioration of pitting corrosion resistance. Post-irradiation ageing for 1 h at 100-300 ℃ resulted in a decrease in microhardness of the irradiated SAV-1. The effect of post-irradiation ageing on pitting corrosion was made clear through the formation of Guinier-Preston zones and secondary precipitates in the Al matrix. Ageing at 250 ℃ corresponded to the development of stable microstructure and the highest corrosion resistance for the irradiated samples. Mg₂Si, Si, and needle-shaped β" precipitates were formed in SAV-1 alloy that was irradiated with low fluences. β" and clusters of rod-shaped B-type precipitates were observed in highly irradiated samples. The precipitates were similar to those seen in non-irradiated pseudo-binary Al-Mg₂Si alloys with Si excess.

• Korean Journal of Metals and Materials
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• v.48 no.12
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• pp.1070-1077
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• 2010

The creep rates of polycrystalline $L1_2$-ordered $Co_3Ti$ and B2-ordered NiAl-Hf intermetallics decrease appreciably with the fine precipitation of the coherent disordered fcc Co-rich phase and $Ni_2AlHf$ phase. With B2-ordered NiAl containing $L2_1-Ni_2AlHf$ precipitates, transmission electron microscope observations of the interaction between dislocations and spherical precipitates revealed that the dislocations tend to be strongly attracted to the particle interfaces during the creep deformation. On the other hand, with $L1_2$-ordered $Co_3Ti$, the significance of the threshold stress is discussed based upon the transmission electron microscope observations of the interaction between dislocations and precipitates. The superdislocations produced during deformation tend to be strongly attracted and dissociated as they meet the coherent disordered precipitates because the anti-phase boundary energy in the disordered phase was zero. An extra force required to pull the dislocations out of the disordered particles during the creep deformation establishes the threshold stress which is beneficial for improving creep strength under lower stresses.

• Korean Journal of Materials Research
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• v.21 no.4
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• pp.187-191
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• 2011

Prior austenite grain size plays an important role in the production of high strength hot-rolled steel. This study investigated the effect of Ti and C contents on the precipitates and prior austenite grain size. Steel with no Ti solutes had prior austenite grain size of about 620 ${\mu}m$. The addition of Ti ~ 0.03 wt.% and 0.11 wt.% reduced the prior austenite grain size to 180 ${\mu}m$ and 120 ${\mu}m$, respectively. The amount of Ti required to significantly decrease the prior austenite grain size was in the range of 0.03 wt.%. However, the amount of carbon required to significantly decrease the prior austenite grain size was not present from 0.04 wt.% to 0.12 wt.%. Oxides of Ti ($Ti_2O_3$) were observed as the Ti content increased to 0.03 wt.%. The specimen containing 0.11 wt.% of Ti exhibited the complex carbides of (Ti, Nb) C. The formation of Ti precipitates was critical to reduce the prior austenite grain size. Furthermore, the consistency of prior austenite grain size increased as the carbon and Ti contents increased. During the reheating process of hot-rolled steel, the most critical factor for controlling the prior austenite grain size seems to be the presence of Ti precipitates.

• Korean Journal of Materials Research
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• v.5 no.8
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• pp.945-952
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• 1995

The effect of Zr content on the thermal stability of mechanically alloyed Al-8wt.% (Ti+Zr) alloys was investigated. As the Zr to Ti addition ratio increased the decrease of hardness due to the long time exposure at high temperature reduced so that the thermal stability of the alloy was improved. From the TEM work it was found that the coarsening of precipitates was responsible for the decrease of hardness and the coarsening of precipitates could be suppressed by the addition of Zr, XRD, SAD and EDS analyses confirmed that these precipitates were consisted of DO$\sub$22/, and DO$\sub$23/ type Al$_3$(Ti+Zr) ternary in termetallic compounds. Especially for the DO$\sub$23/ Al$_3$(Ti+Zr), the lattice parameter changed toward the smaller lattice mismatch between the precipitate and Al matrix as the Zr content increased. Therefore, it was considered that the improvement of thermal stability of Al-8wt.% (Ti+Zr) alloys was due to the formation of the ternary Al$_3$( Ti + Zr) intermetallic compounds.

• Journal of Welding and Joining
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• v.13 no.4
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• pp.55-64
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• 1995

A study was made to examine the effects of postweld heat treatment(PWHT) on the toughness and microstructures in the weld heat affected zone(HAZ) of Cu-bearing HSLA-100 steel. The Gleeble thermal/mechanical simulator was used to simulate the weld HAZ. The details between toughness and PWHT of HAZ were studied by impact test, optical microscopy(O.M.), scanning electron microscopy (SEM), transmission electron microscopy(TEM) and differential scanning calorimetry(DSC). The decrease of HAZ toughness in single thermal cycle comparing to base plate is ascribed to the coarsed-grain formed by heating to 1350.deg.C. The increase of HAZ toughness in double thermal cycle comparine to single thermal cycle is due to the fine ferrite(.alpha.) grain transformed from austenite(.gamma.)formed by heating to .alpha./.gamma. two phase region. Cu precipitated during aging for increasing the strength of base metal is dissolved during single thermal cycle to 1350.deg.C and is precipitated little on cooling and heating during subsequent weld thermal cycle. It precipitates by introducing PWHT. Thus, the decrease of toughness in triple thermal cycle of $T_{p1}$ = 1350.deg.C, $T_{p2}$ = 800.deg.C and $T_{p3}$ = 500.deg.C does not occur owing to the precipitation of Cu. The behaviors of Cu=precipitates in HAZ is similar to that in base plate. PWHT at 550.deg.C shows highest hardness and lowest toughness, whereas PWHT at 650.deg.C shows reasonable toughness, which improves the toughness of as-welded state.state.