Chong-Sool Choi

Refinement of Cast Microstructure of Hypereutectic Al-Si Alloys Through the Addition of Rare Earth Metals

Microstructural observation and thermal analysis of Al-21 wt % Si alloys with different rare earth metals were performed to examine the effect of rare earth metal on the refinement of primary silicon phase. Simultaneous refinement of both primary and eutectic silicon morphology is achieved with the addition of rare earth and its effect increases with the amount of rare earth addition and cooling rate. Depression of 12–17 °C in primary reaction temperature and 2–7 °C in eutectic temperature is measured with the addition of rare earth. Rare earth bearing compounds were not believed to act as a nucleation agent of primary silicon phase. Some rare earth bearing compounds determined to AlCe were around primary silicon in the matrix. The twin density of eutectic silicon remains same regardless of the addition of rare earth. The refinement of silicon in rare earth treated hypereutectic Al-Si alloys is supposed to be due to the suppression of the nucleation temperature of silicon phase.

Amount of retained austenite at room temperature after reverse transformation of martensite to austenite in an Fe–13%Cr–7%Ni–3%Si martensitic stainless steel

Abstract Below a heating rate of 10°C/s, the reverse transformation (α ′ →γ) of a low carbon Fe–13%Cr–7%Ni–3%Si martensitic stainless steel occurs by diffusion, whereas it occurs in a diffusionless manner above 10°C/s. The amount of retained austenite at room temperature is proportional to both the volume fraction of reversed austenite and its stability.

Variation of stacking fault energy with austenite grain size and its effect on the MS temperature of γ → ε martensitic transformation in Fe-Mn alloy

Abstract The change in MS temperature of γ→e martensitic transformation with austenite grain size (AGS) has been investigated in relation to the stacking fault energy (SFE) in an Fe–18%Mn alloy. With increasing AGS, the MS temperature increases steeply up to 35 μm, and gradually increases in larger grains. The SFE, on the other hand, decreases abruptly until 35 μm, over which it is slightly lowered. A good linear relationship is established between MS temperature and inverse of SFE, i.e. stacking fault probability (SFP). This result suggests that the variation of MS temperature with AGS depends strongly on the change in SFE.

Effect of ε martensite content on the damping capacity of Fe-17%Mn alloy

In previous studies the authors have shown that a Fe-17% Mn alloy possesses the highest damping capacity in the Fe-Mn system. It was suggested that the damping is related to the amount of {epsilon} martensite. Thus the objective of this study has been to investigate the effect of {epsilon} content on the damping capacity of Fe-17% Mn alloy. To increase the amount of {epsilon} martensite, two different heat treatments were applied: One was cooling to various temperatures between room temperature and {minus}196 C, and the other was cyclic transformation, {gamma}{leftrightarrow}{epsilon}, between room temperature and 255 C above A{sub f}. The two heat treatments led to the different dependences of damping capacity on {epsilon} martensite content. The first heat treatment showed that the damping capacity is gradually increased with increasing {epsilon} martensite volume fraction, while the second heat treatment showed that the damping capacity is steeply decreased with increasing {epsilon} martensite content. In the present paper, the reason has been elucidated clearly.

Effect of isothermal transformation temperature on amount of retained austenite and its thermal stability in a bainitic Fe–3%Si–0.45%C–X steel

Abstract Film-like morphology of retained austenite is observed at low isothermal temperatures, whereas blocky morphology of retained austenite appears at high temperatures in a bainitic Fe–3%Si–0.45%C–X steel. Amount of retained austenite increases with increasing temperature, exhibiting a peak at around 350 °C, above which it decreases with increasing temperature.

Growth kinetics of three Mo-silicide layers formed by chemical vapor deposition of Si on Mo substrate

Abstract The growth kinetics of three Mo-silicide layers formed by chemical vapor deposition (CVD) of Si on a Mo substrate using SiCl 4 -H 2 gas mixtures were investigated at temperatures between 950 and 1200 °C. Three Mo-silicide layers (Mo 3 Si, Mo 5 Si 3 , and MoSi 2 ) grew simultaneously with a parabolic rate law after an initial nucleation period, indicating the diffusion-controlled growth. The activation energy (130 kJ/mol) for the MoSi 2 layer were in a good agreement with the previous results having low activation energy (130±20∼157 kJ/mol), but its growth rate was higher than the previous results with high activation energy (209∼241±25 kJ/mol). A possible explanation about this difference may be the detrimental effect of impurities such as oxygen on the growth rate of the MoSi 2 layer. The activation energy (350 kJ/mol) for growth of the Mo 5 Si 3 layer was consistent with the prior values (297∼360 kJ/mol) obtained by annealing of the MoSi 2 /Mo diffusion couples, but its growth rate was an order of magnitude lower than the rate measured in the MoSi 2 /Mo diffusion couples. The activation energy (223 kJ/mol) for the growth of the Mo 3 Si layer was similar with the value (199 kJ/mol) obtained from annealed Mo 5 Si 3 /Mo diffusion couple at temperatures between 1250 and 1350 °C. This value was lower than the value (326 kJ/mol) reported at higher temperatures from 1500–1715 °C. This suggests that the rate-limiting step for growth of the Mo 3 Si layer is the grain boundary diffusion-controlled process at low temperatures but volume diffusion-controlled process at high temperatures. The growth rates of the Mo 3 Si layer measured at condition of the simultaneous parabolic growth of three Mo-silicide layers were approximately two orders of magnitude lower than the rates measured in the Mo 5 Si 3 /Mo diffusion couples. The differences in the growth rates of the Mo 5 Si 3 and Mo 3 Si layers depending on the type of diffusion couples were well explained by the multiple layer growth model.

Strain amplitude dependence of the damping capacity in Fe-17%Mn alloy

Abstract The damping capacity of Fe-17%Mn alloy depending on the e martensite content represents three different trends in the range of 1 × 10 −4 to 10 × 10 −4 strain amplitude. A linear relationship between damping capacity and volume fraction of e martensite is established from 1 × 10 −4 to 3 × 10 −4 strain amplitude. It implies that the stacking fault boundaries in ϵ martensite and the ϵ martensite variant boundaries dominantly give rise to the damping capacity. In the range of 4 × 10 −4 to 6 × 10 −4 strain amplitude, however, the variation of damping capacity with ϵ martensite content is very similar to that of relative length of γ/ϵ interface. This result suggests that movement of γ/ϵ interfaces is introduced as an additional damping mechanism above 4 × 10 −4 strain amplitude. The damping behavior depending on the ϵ martensite volume percent in the 7 × 10 −4 to 10 × 10 −4 strain amplitude is nearly the same as that in the 4 × 110 −4 to 6 × 10 −4 strain amplitude range, except for the abnormally high damping capacity of the specimen with 35% of ϵ martensite. This is probably caused by microslip deformation in the austenite phase.

Effects of amount of ε martensite, carbon content and cold working on damping capacity of an Fe–17% Mn martensitic alloy

Abstract Damping capacity of an Fe–17% Mn alloy has been studied with respect to several factors such as volume fraction of e martensite, carbon content, and cold rolling. In the case of subzero cooling, the damping capacity of an Fe–17% Mn alloy increases with increasing e martensite content. In case of thermal cycling, however, the damping capacity decreases with increasing e martensite content, because the dislocations, which act as barriers to operation of damping sources, are introduced during the thermal cycling. The carbon contents above 0.06 wt.% deteriorate the damping capacity of the Fe–17% Mn alloy, which is ascribed to an interaction between damping sources and carbon atoms and to the decrease in volume fraction of e martensite with the increase in carbon content. The damping capacity of the Fe–17% Mn alloy shows a maximum value around 10% reduction in thickness, and decreases with further deformation. The increase in damping capacity up to 10% deformation is attributed to an increase in e martensite volume fraction with increasing deformation, and a decrease in damping capacity above the 10% deformation is ascribed to the stress-induced α′ martensite and dislocations formed during deformation, which act as barriers to operation of damping sources.

The influence of Mn content on microstructure and damping capacity in Fe–(17∼23)%Mn alloys

Abstract This study is aimed to investigate the microstructural evolution with respect to Mn content and its contribution to the damping capacity in Fe–(17∼23)%Mn alloys. The volume fraction of e martensite in these alloys increases with decreasing Mn content and cooling temperature, which gives rise to the increase of damping capacity. For the same amount of e martensite, the higher the Mn content, the lower the damping capacity. The microstructural examinations and tensile tests have revealed that the decrease in values of stacking fault density in e martensite and γ austenite, length of γ / e interface in unit area, and mobility of γ / e interface, is responsible for the deterioration of damping capacity.

Study on reaction and diffusion in the Mo–Si system by ZrO2 marker experiments

The dominant diffusing element in the MoSi2, Mo5Si3 and Mo3Si was found to be Si from marker experiments using ZrO2 particles. Based on these marker experiments, the formation mechanism of the Mo5Si3 and Mo3Si layers by reaction and diffusion was also discussed.