• 한국주조공학회지
• /
• 제41권6호
• /
• pp.505-515
• /
• 2021

일방향응고로 제조된 니켈기 초내열합금 CM247LC의 다양한 온도 및 응력조건에서 크리프 특성에 대해 고찰하였다. 열처리 후 일부 공정조직이 남아 있었으며 비교적 균일한 육면체의 γ'이 수지상 내부와 수지상간 영역에서 관찰되었다. 상대적 저온인 750℃의 고응력 크리프 영역에서는 1차 크리프 동안 많은 변형이 발생하였으나 고온으로 갈수록 3차 크리프 구간이 크리프 변형시간의 대부분을 차지함을 알 수 있었다. 저온 고응력에서는 부분 전위가 γ'으로 진입하며 적층결함을 γ'내에 생성시켰으며 초기 크리프 변형속도가 증가하는 부분이 있었으나 850℃ 이상의 온도에서는 γ' 내부에서 적층결함이 관찰되지 않았으며 이는 적층결함에너지의 온도의존성 때문인 것으로 판단된다. 고온이어서 확산속도가 빠른 950℃와 1000℃에서는 γ'의 래프팅이 관찰되었다. 온도가 좀 더 낮은 850℃에서는 변형기구가 응력에 따라 다르게 나타나서 상대적으로 크리프 시간이 긴 저응력에서만 래프팅이 관찰되었다.

• 한국안전학회지
• /
• 제13권2호
• /
• pp.45-53
• /
• 1998

A method to predict the creep behavior of fiber-reinforced ceramic composites at high temperatures was suggested based on finite element modeling using constituent creep equations of fiber and matrix and showed good agreement with the experimental results. The effects of matrix creep behavior, fiber volume fraction, and residual stresses on the composite creep behavior were also investigated. The results showed that the primary behavior of composites was greatly affected by that of matrix but post-primary behavior was governed by fiber creep characteristics. The increase of fiber volume fraction from 15 vol% to 30 vol% caused the 50% and 40% decrease of steady-state creep rates and total creep strains at $1200^{\circ}C$, 180MPa, respectively. Feasible compressive residual stresses in the matrix caused by different thermal expansion coefficients between the fiber and the matrix could significantly reduce total creep strains of the composite. The creep deformation mechanism in the fiber-reinforced ceramic composites could be explained by the stress transfer and redistribution in the fiber and matrix due to different creep characteristics of its constituents.

• 소성∙가공
• /
• 제17권5호
• /
• pp.364-372
• /
• 2008

In this study, the circumferential creep behaviors ofpilgered advanced Zirconium alloy tubes such as Zr-Nb-O and Zr-Nb-Sn-Fe were investigated in the temperature range of $400\sim500^{\circ}C$ and in the stress range of 80$\sim$150MPa. The test results indicate that the stress exponent for the steady-state creep rate of the Zr-Nb-Sn-Fe alloy decreases with the increase of stress(from 6$\sim$7 to 4), while that of the Zr-Nb-O alloy is nearly independent of stress(5$\sim$6). The activation energy of creep deformation is found to be nearly the same as the activation energy for Zr self diffusion. This indicates that the creep deformation may be controlled by dislocation climb mechanism in Zr-Nb-O. On the other hand, the transition of stress exponent(from 6-7 to 4) in Zr-Nb Sn-Fe strongly suggests the transition of the rate controlling mechanism at high stresses. The lower stress exponent at high stresses in Zr-Nb-Sn-Fe can be explained by the dynamic deformation aging effect caused by interaction of dislocations with Sn substitutional atoms.

• 동력기계공학회지
• /
• 제10권3호
• /
• pp.45-50
• /
• 2006

In this paper, creep tests of Mg-Zn-Mn and Mg-Zn-Mn-Ca alloy casted by mold under the temperature range of 473.00-573.00K, and the stress range of 23.42-87.00Mpa were done with the equipment of automatic controlled temperature and computer for data acquisition. The activation energies were obtained by relationship between creep rate and temperature, and the stress exponents were obtained by relationship between creep rate and stress. From the experiment results, the activation energies of Mg-Zn-Mn and Mg-Zn-Mn-Ca alloy were 149.87kJ/mol, 147.97kJ/mol, respectively, and the stress exponents of those alloy were 5.13, 5.59, respectively, under the temperature of 473.00-493.00K and the stress range of 62.43-78.00Mpa. And the activation energies of those alloy were 134.41kJ/mol, 129.22kJ/mol, respectively, and the stress exponent of those alloy were 3.48, 3.77, respectively, under the temperature of 553-573Mpa and the stress range of 23.42-39.00Mpa. Also the lifes of Mg-Zn-Mn-Ca alloy were higher than those of Mg-Zn-Mn alloy, and the results of SEM showed fracture surfaces under low temperature had smaller dimples than those under high temperature.

• 한국세라믹학회지
• /
• 제28권10호
• /
• pp.785-792
• /
• 1991

An investigation of the crack propagation behavior of Al2O3-33Vol.% SiCw at 140$0^{\circ}C$ was conducted with various loading frequencies. Higher crack propagation was observed in lower frequency and higher load ratios. Interface sliding fracture due to glassy phase from the oxidation of SiCw and cavitation along grain boundary of diffusional creep appeared to be the main mechanism of fatigue fracture in slower crack propagation while interface sliding and whisker pull out aided by glassy phase formation played main role of fatigue fracture for higher crack growth condition. The frequency effect on deformation behavior was discussed with a Maxwell model.

• 한국재료학회지
• /
• 제4권3호
• /
• pp.349-356
• /
• 1994

재료의고온소성변형과 수명예측 및 수명향상을 위하여 재료의 변형기구를 규명하는 것이 매우 중요하다. 이를 위하여 전위환모델이 자주 사용되며, 현재 실험적인 결과를 토대로 한 두개의 중요한 전위환모델로서, Orlova등고 Mills등이 제시한 모델들이 있다. 이들은 모두AI-5.5at.%MG을 사용하였으나 상호 상반된 전위환모델을 설명하고 있다. 그러므로 본 연구에서는 상반된 전위환 모델을 확인하기 위하여 AI-5.5at.%MG을 사용하여 573K의 약 30MPa에서 $\varepsilon$=0.03까지 크\ulcorner시험을 하고, 이러한 크\ulcorner시험후 이어서 각각 약 15,10 및 oMPa의 응력감소 시험도 수행하였으며, 동시에 응력감소 시험 전과 후의 전위구조를 관찰하여 전위환모델을 고찰하였다.