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The effects of TRansformation Induced Plasticity(TRIP) phenomena on the plastic deformation of 0.2C-1.5Si-1.5Mn multiphase steel have been investigated at various heat treatment and stress conditions. In order to estimate a deformation behavior under multi-axial stress rotate, the hole expansion(HE) tests were carried out. It is shown that the formability evaluation from the uni-axial tensile tests was quite different from the formability measured from multi-axial HE-tests because of different stress state. The formability in the multi-axial stress state decreased due to the extinction of the transformable retained austenite at relatively earlier deformation stage and the production of irregular α' martensite. The voids of TRIP steel were initiated exactly at the interface between transformed martensite and ferrite matrix regardless of stress state. In addition, new form of experimental formula is proposed in order to predict the multi-axial formability of the TRIP steel from the results of uniaxial tensile test.
This investigation was conducted to clarify the effects of process variables such as spraying distance and fuel flow rate on the formation of the new amorphous coating using Zr_(65)Al_(7.5)Ni_(10)Cu_(17.5) alloy by a HVOF(High Velocity Oxygen-Fuel) spraying process. Powder used for spraying was prepared by vacuum gas atomization and centrifugal-mill. HVOF spraying experiments were carried out using a Tafa JP-5000 spraying gun. The amorphous content was measured up to about 65% depending on the spraying parameters. The amorphous fraction is decreased with increasing the spray distance. Microstructural observations and XRD analysis reveal that the change of amorphous fraction is attributed to the degree of solidification of droplets and the oxide (ZrO₂) formation in coatings. The higher fuel flow rate resulted in the decrease of amorphous fraction, and this decreased amorphous fraction was caused by the increase of oxide fraction. Therefore, flame temperature and spray distance that can control the oxide formation in coating and undercooling effects of droplets are the most crucial factors for the formation of amorphous coating using this alloy system.
Growth kinetics of Nb(CN) precipitates have been studied in the 0.03wt.% Nb bearing HSLA steel which was compressively rolled at high temperature(850℃∼1000℃) up to 0.3 strain and isothermally held. Interrupted compression test was also carried out to observe the recrystallization phenomena associated with the precipitation at high temperature. The precipitates were analyzed by the TEM. Observed precipitates revealed that they were nucleated heterogeneously at dislocations and spherical shape. The mean particle size of the precipitates were found to increase with increasing time and temperature. The slope obtained from log plot of precipitate diameter versus time was about 0.5, indicating that the growth behavior followed the Zener's diffusion controlled growth theory. The slope was about 0.3 for the Ostwald ripening region at longer holding time. The nucleation of precipitates was found to retard softening phenomena owing to pinning effect. In ripenening region of precipiates, restart of softening was followed with eliminating of precipitate pinning effect. This result clearly indicate that the growth kinetics of Nb(CN) precipitates affect both the softening and the recrystallization behavior.
V, N을 함유하고 있는 공석강의 펄라이트 조직을 TEM으로 관찰한 결과 다음과 같은 결론을 얻었다 1) 펄라이트내 페라이트에서 펄라이트 엣지방향과 수직에 가까운 각도를 이루고 있는 띠형태의 계면석출물을 관찰할 수 있었다. 하나의 펄라이트 콜로니내에서 모든 계면석출물(IBP)들의 결정학적 방위가 동일하였고, 석출물 열들의 간격 및 방향이 동일하였다. 그리고 하나의 석출물 열은 인접한 2개이상의 페라이트에 걸쳐 연속적이었다. 2) 계면석출물(IBP)의 열은 콜로니 경계 및 노듈 경계에 대하여 평행하거나 수직하지 않고 일정한 각도로 기울어져 있다. 3) 펄라이트의 라멜라가 끊어지는 지점이나 라멜라 홀에서도 계면석출물(IBP)의 열은, 다른 정상 부위와 동일하게, 연속성을 유지하고 있었다. 4) 동일 관찰부위에서도 띠 형태의 석출물이 tilting에 따라 무질서한 형태의 석출물로 바뀐다. The detailed transmission electron microscopy(TEM) was conducted to study the growth mechanism of pearlite. The experiments were performed using vanadium carbonitrides, which precipitate during the pearlite growth in the Fe-C-V and Fe-C-V-Mn alloys, in order to trace the sequences of the pearlite growth. The growth mechanism based on the present experimental results are presented in the following two papers. In this first paper, the interphase boundary precipitation(IBP) accompanied with the pearlite growth is presented. The main features of IBP are : (a) IBP arrays are observed not only in the normal pearlite area but also in the lamellae fault area. In addition, IBP arrays are also found at the lamellar holes; (b) The characteristics of IBP, such as spacing, direction and the crystallographic orientation with the pearlitic ferrite matrix, remain unchanged during the pearlite growth. It was also found that the perturbation of the lamellar structure did not alter the characteristics of IBP; (c) The rows of IBP arrays are not parallel but inclined with respect to the nodule boundaries. Based on the present experimental observations, the integrated growth mechanism of pearlite will is presented in the following paper.