Ni-Ti 형상기억합금의 상변능에 미치는 외부응력의 영향

홍종휘,성윤막,지광구,갑명철 대한금속재료학회(대한금속학회) 1988 대한금속·재료학회지 Vol.26 No.8

The effect of external stress on the phase transformation behavior of annealed and aged Ni-Ti alloys has been studied by the tensile test and the measurement of electrical resistance. Martensitic (M_s) and austenitic transformation starting temperatures (A_s) increased gradually with increasing stess up to 129 MPa and increased rapidly at a rate of 0.2℃/MPa above this stress level. However, incommensurate (T'_R) and commensurate phase transformation starting temperatures (T_R) increased very slowly at a rate of 0.06℃/MPa with the increasing stress. Martensitic transformation temperature range, ΔT(M_s∼M_f), and the stress at which ΔT was kept constant were larger for the aged specimen than for the annealed specimen. The stress at which the strain accompanied by incommensurate and commensurate phase transformation began to decrease (ε-1c) was higher for the annealed specimen than for the aged specimen. these facts imply that the incommensurate and commensurate phases are more stable in the annealed specimen than in the aged specimen. While the residual stress (ε_p) in the annealed specimen remained low regardless of the stress level, ε_p in the aged specimen began to increase rapidly at 240 MPa. This means that slip deformation is more difficult for the annealed specimen than for the aged specimen.

용탕교반법에 의한 AC8A/SiCp 복합재료의 제조조건에 따른 기계적 성질

홍종휘,홍영환,한준현 대한금속재료학회(대한금속학회) 1994 대한금속·재료학회지 Vol.32 No.6

Microstructure, wettability, tensile strength and wear properties were examined in AC8A/SiC_p composites. Increasing Mg content in matrix improved wettability probably due to decreasing surface tension of the melt. By increasing the stirring time and particle size, the particles were more uniformly distributed in the matrix. And the particle distribution of as-cast state had a significant influence on that of extruded state, that is, particles were rarely aligned in the extrusion direction in the composites stirred for 20 mimutes. Tensile strength was increased as stirring time increased and particle size decreased, but significant increase of high temperature strength due to the presence of particles was not obtained. Also wear resistance did not vary significantly with the particle size. It is clear that SiC particle reinforcement improved both tensile strength and wear resistance.

아르니코형 주조자석에서 결정조직에 미치는 알미늄과 티탄의 영향

홍종휘 대한금속재료학회(대한금속학회) 1963 대한금속·재료학회지 Vol.1 No.3

Magnetic property in Alnico V type permanent magnet is improved by adding special elemets, heat treatment in magnetic field and producing columnar structres. This experiment is a part of an investigation into methods of producing columnar structure in Fe-Ni-Al alloy of high Co content, with consequent improvement in their magnetic properties. Alnico V type alloys are coarse grained and columnar growth is readily induced by casting into suitably chilled molds. As increasing amounts of Ti are added to this type of alloy, the grain size becomes progressively smaller, and the production of columnar castings more difficult. Consequently, high coercivity alloys with the composition 14.5% Ni, 28∼35% Co, 4.5% Cu, 7% Al and 5% Ti are extremly fine grained and, when chill cast, form only very small columnar crystals near the chilled face. The occurrence of fine equi-axed crystals in Fe-Ni-Co-Cu-Al-Ti alloys is a function of Al and Ti contents and not Ti contents alone.

구상흑연주철의 흑연확산에 의한 강화기구

홍종휘,안근영,신평우 대한금속재료학회(대한금속학회) 1985 대한금속·재료학회지 Vol.23 No.5

The effects of the holding temperature, heating rate and nickel content on the formation of the send phase were investigated for the ductile cast iron having ferrite matrix. In order to vary the heating rate, specimens were prepared from 1 to l0㎜ in thickness and nickel was added up to 1% to study the effect of nickel content. The main results obtained from this study were as follows; 1. Austenitizing velocity increased with high holding temperature and rapid heating rate but decreased with nickel amount. 2. Incubation period decreased with high holding temperature, rapid heating rate, and high nickel content. 3. Radius of the second phase formed by the graphite diffusion into ferrite matrix increased by the rapid heating but decreased by addition of nickel. 4. Uniform spheroidal second phase was formed when the specimen were held at high temperature and heated rapidly to the high temperature. 5. The discrepancies between Paxton-Judd model and this study were mainly arised from the condition of the holding temperature, the carbon, and the impurities. 6. After rapid increase of the hardness in early stage, the hardness increased slowly with the amount of the second phase.

내부반응법에 의한 Ti 합금기 복합재료의 제조 및 기계적 성질

홍종휘,홍영환,박종일 대한금속재료학회(대한금속학회) 1995 대한금속·재료학회지 Vol.33 No.12

In this study powder metallurgy method was used to fabricate the TiB reinforced Ti alloy composite materials, and the matrix Ti alloy composition was basically Ti-5Al-2.5Fe. Especially the reinforcing particle TiB was obtained from TiB₂ particles by the internal reaction during sintering process. Sintering at 1300℃ in argon atmosphere, no TiB formation was observed with any addition of TiB₂. And at 1250℃ in vacuum, TiB formation wasn't also observed. At 1300℃ in vacuum, however, TiB formation was observed for the specimens added 2∼9wt% TiB₂. The TiB particles formed have a shape of acicular structure, and have a size of 10∼30 ㎛. Microhardness test showed that specimens with TiB particles have much higher hardness value than that of unreinforced alloy. Therefore internal reaction process, which also has an advantage of a near-net-shape technology, can be said to be a potential method to produce high strength Ti alloy composites using low-cost additive elements.

용탕교반법에 의한 Al2o3(p)/AC8A 복합재료의 제조 및 기계적 성질

홍종휘,홍영환,김상길 대한금속재료학회(대한금속학회) 1996 대한금속·재료학회지 Vol.34 No.2

Among various processes in fabricating metal matrix composites, melt-stirring technology is much more attractive route in industrial application because it is simpler and less expensive compared to squeeze casting or powder metallurgy. In the present study, effects of size and volume fraction of reinforcing particles on tensile strength and thermal expansion coefficients of α-Al₂O₃_(p)/AC8A composite, which was made by melt-stirring process, were investigated. And the effect of Mg addition on the above properties was also investigated. Composites with 3%Mg showed better wettability and distribution of particles than that of composites with no Mg. The addition of Al₂O₃ particles was not effective in increasing the tensile strength of the composite except the composite materials with 3%Mg, which showed an increased strength in the case of 5vol% addition of 16 ㎛ α-Al₂O₃ particles. On the other hand, thermal expansion coefficients of the composites decreased with increasing the volume fraction of reinforcing particles and Mg contents.

Ti-Ni 형상기억합금의 반복변형에 의한 성능렬화현상

홍종휘,신명철,오명훈,지광구,장우양 대한금속재료학회(대한금속학회) 1989 대한금속·재료학회지 Vol.27 No.1

The effect of deformation on the transformation behavior in Ti-Ni alloy was studied. Coil spring of Ti-Ni alloy used as element of tilt plate type heat engine, was cyclically expanded and contracted by temperature difference. The Results obtained in this study are as follows. 1. By the cyclic deformation, M_s and A_f decreased with increasing the number of cycles, however, M_s remained constant. The cycling effect was caused by the generation of dislocation during cyclic deformation 2. The cycling effect (decrease of M_s and A_f) increased with the increase of the heat treatment temperature and deformation strain. When the cyclic deformation was conducted in the temperature range of B2↔R-phase, transformation temperature changed little. 3. That unrecovered strain appeared to be proportional to the amount of decrease of M_s and A_f confirmed that cycling effect was caused by introduction of dislocation during deformation.

Fe-15Mn-5Cr-5Co-3Si 합금의 열처리조건에 따른 상변태거동 및 형상기억효과

홍종휘,신명철,지광구,장우양 대한금속재료학회(대한금속학회) 1991 대한금속·재료학회지 Vol.29 No.4

The effect of heat treatment condition on the martensitic transformation behavior and shape memory effect in Fe-15Mn-5Cr-5Co-3Si alloy has been studied by microscopy and calorimetry. The grain size increased with the increase in heat treatment temperature and holding time. Three types of transformation behavior, such as no martensitic transformation, r→ε and r→ε→α' were observed in increasing order of grain size. The r phase did not seem to transform directly to α', for α' martensite was found only in the a martensite plates. The orientation relationships between the ε and the α' martensites were [011] ε∥ [311]α', (1￣00) ε∥ (1￣12) α' and [011] ε∥ [111] α', (1￣11￣) ε ∥ (1￣01) α'. The Ms temperature of r→ε transformation remained almost constant, while that of ε→α' transformation increased rapidly with the increase in grain size. As the heat treatment temperature and holding time increased, the shape memory effect became worse. This could be attributed to the reduction of r phase contributing to the shape memory effect and the easiness of movement of dislocations during the deformation.

Fe-15Mn-5Cr-50Co-3Si 합금에서 변형에 따른 미세조직변화와 형상기억효과

홍종휘,신명철,지광구,장우양 대한금속재료학회(대한금속학회) 1991 대한금속·재료학회지 Vol.29 No.7

The relationship between mircrostructural changes during deformation and shpae memory effect(SME) has been studied in Fe-15Mn-5Cr-5Co-3Si alloy. Dislocations generated as well as ε-α' transformation during deformation were revealed to deteriorate the SME. Teh γ→ε transformation contributing to SME was hindered by the multiplication of the dislocations during the deformation. The transformation of stress-induced ε into α' was found to be another cause for making SME worse due to the reduction in the amount of the ε. It is proposed that SME could be improved not only by the increase in strength but by the prevention of ε→α' transformation during the deformation.

2024 알루미늄 합금의 열간추출 변형 기구에 관한 연구

홍종휘,신명철,맹선재 대한금속재료학회(대한금속학회) 1980 대한금속·재료학회지 Vol.18 No.4

2024 알미늄 合金을 對象으로 熱間押出에 있어서의 變形機構를 調査하였다. 즉 熱間押出時의 strain rate (ε·), flow stress (σ) 및 押出溫度 사이의 關係를 調査한 바 creep時에 成立되는 ε·=A-sinh(ασ)〕ⁿexp(-ΔH/RT)의 關係가 그대로 成立됨을 알았다. 이 硏究에서 押出比를 一定하게 했으며 押出溫度는 300℃에서 443℃사이, strain rate는 0.07내지 2.7/sec의 範圍에서 變化시켜 試驗하였다. 試驗 結果를 上記式에 適用하여 押出에 對한 activation energy ΔH를 求한 바 33.6Kcal/mole로 計算 되었다. 押出材의 顯微鏡 組織은 光學顯微鏡上으로는 纖維狀 組織을 이루고 있었으며 그 纖維狀 組織을 電子 顯徵鏡으로 觀察한 바 等軸型의 subgrain이 形成되어 있었다. Subgrain의 크기와 Zener-Hollomon parameter (Z) 사이의 關係를 調査한 바 subgrain의 크기는 "Z"값의 對數에 逆比例하였다. 2024 aluminum alloy was deformed by extrusion over the temperature range of 300-443℃ and the strain rates between 0.07-2.7/sec. Flow stress and strain rates were calculated from the ram pressure and ram speeds. The stress (σ)-strain rate (ε·)-temperature (T) relationship was found to be similar to that in creep. The experimental data were shown to fit the relationship ε·=A[sinh(ασ)ⁿexp(-ΔH/RT) where A,α, and n are material constants. The activation energy, ΔH, for 2024 aluminum extrusion were determined to be 33.6 Kcal/mole. The dislocation substructures produced during hot extrusion were studied by transmission electron microscopy. The equilibrium subgrain size was found to vary inversely with the logarithm of the Zener-Hollomon parameter, ε·exp(ΔH/RT).