• 한국재료학회지
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• 제25권6호
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• pp.269-273
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• 2015

The effect of adding Ca on the microstructural and mechanical properties of as-cast Mg-11Li-3Zn-1Sn(wt%) alloys were investigated. Mg-11Li-3Zn-1Sn-0.4Mn with different Ca additions (0.4, 0.8, 1.2 wt%) were cast under an $SF_6$ and $Co_2$ atmosphere at $720^{\circ}C$. The cast billets were homogenized at $400^{\circ}C$ for 12h and extruded at $200^{\circ}C$. The microstructural and mechanical properties were analyzed by OM, XRD, SEM, and tensile tests. The addition of Ca to the Mg-11Li-3Zn-1Sn-0.4Mn alloy resulted in the formation of $Ca_2Mg_6Zn_3$, MgSnCa intermetallic compound. By increasing Ca addition, the volume fraction and size of $Ca_2Mg_6Zn_3$ with needle shape were increased. This $Ca_2Mg_6Zn_3$ intermetallic compound was elongated to the extrusion direction and refined to fine particles due to severe deformation during hot extrusion. The elongation of the 0.8 wt% Ca containing alloy improved remarkably without reduction strength due to the formation of fine grain and $Ca_2Mg_6Zn_3$ intermetallic compounds by Ca addition. It is probable that fine and homogeneous $Ca_2Mg_6Zn_3$ intermetallic compounds played a significant role in the increase of mechanical properties.

• 한국재료학회지
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• 제26권3호
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• pp.154-159
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• 2016

A roll-bonding process was applied to fabricate an AA1050/AZ91/AA1050 laminate complex sheet. Two AA1050 and one AZ91 magnesium sheets of 2 mm thickness, 30 mm width and 200 mm length were stacked up after surface treatment that included degreasing and wire brushing; material was then reduced to a thickness of 3 mm by one-pass cold rolling. The laminate sheet bonded by the rolling was further reduced to 2 mm in thickness by conventional rolling. The rolling was performed at 623K without lubricant using a 2-high mill with a roll diameter of 210 mm. The rolling speed was 15.9 m/min. The AA1050/AZ91/AA1050 laminate complex sheet fabricated by roll bonding was then annealed at 373~573K for 0.5h. The microstructure of the complex sheets was revealed by electron back scatter diffraction (EBSD) measurement; the mechanical properties were investigated by tensile testing and hardness testing. The strength of the complex sheet was found to increase by 11 % and the tensile elongation decreased by 7%, compared to those values of the starting material. In addition, the hardness of the AZ91 Mg region was slightly higher than those of the AA1050 regions. Both AA1050 and AZ91 showed a typical deformation structure in which the grains were elongated in the rolling direction; however, the mis-orientation distribution of grain boundaries varied greatly between the two materials.

• 한국레이저가공학회지
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• 제11권2호
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• pp.15-19
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• 2008

Electron beam (EB) weldability of pure grade Nb sheet was studied. One of Nb sheets was as-annealed and the other was cold rolled. Microstructures, Vickers hardness, and transverse weld tensile test were carried out for the base metal, the heat affected zone (HAZ) and weld metal. In the case of the EB welds made using the annealed Nb sheeet, fine equiaxed grains and coarse grains were dominant at the base metal and the HAZ, respectively, and columnar grains were observed at the weld metal. For the EB welds made using the cold rolled Nb sheet, elongated grains in the rolling direction at the base metal, and the microstructures of the weld metal and the HAZ are similar to those of the EB welds made using the annealed Nb sheet, respectively. For both annealed and cold rolled Nb sheet, the width of the HAZs are unusually wide in spite of using high density heat source, i.e. electron beam, and the grain sizes of both HAZs are similar. When tensile test was carried out using the transverse weld specimens, the failure occurred at the HAZ for both EB welds made using Nb sheets annealed and cold rolled, respectively and the tensile strengths of both specimens were 161MPa. Vickers hardness of EB welds made using annealed Nb was 56-57 Hv at both base metal and weld metal, 52-53 Hv at the HAZ. On the other hand, Vickers hardness of EB welds made using cold rolled Nb was 97-99 Hv at the base metal, but the hardness values of weld metal were similar to those obtained at the weld metal of annealed Nb.

• 대한금속재료학회지
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• 제50권7호
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• pp.477-485
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• 2012

The effect of aging heat treatment on microstructure evolution of the Ti-6Al-4Fe-0.25Si alloy with an initial microstructure of an elongated alpha was investigated. Aging treatments of the samples were carried out at $550^{\circ}C$ for up to 100 hours. The microstructure of the 5 hours heat-treated sample consisted of alpha grains, beta matrix and some TiFe intermetallic compounds that were precipitated from the beta matrix. Increasing the aging time to 10 hours, most of the beta matrix was decomposed to very fine alpha grains (${\sim}0.5{\mu}m$) and TiFe, and thus the volume fraction of the beta matrix was significantly decreased. EBSD analysis revealed that newly formed tertiary-alpha-grains in the vicinity of TiFe had high angle boundaries with respect to the primary and secondary alpha grains. As a result of these phase transformations during aging, the fraction of the alpha/alpha grain boundary was increased while that of the alpha/beta phase boundary was decreased.

• 대한금속재료학회지
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• 제46권9호
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• pp.545-553
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• 2008

Dynamic deformation behavior of ultra-fine-grained pure coppers fabricated by equal channel angular pressing (ECAP) was investigated in this study. Dynamic torsional tests were conducted on four copper specimens using a torsional Kolsky bar, and then the test data were analyzed by their microstructures and tensile properties. The 1-pass ECAP'ed specimen consisted of fine dislocation cell structures elongated along the ECAP direction, which were changed to very fine, equiaxed subgrains of 300~400 nm in size as the pass number increased. The dynamic torsional test results indicated that maximum shear stress increased with increasing ECAP pass number. Adiabatic shear bands were not found at the gage center of the dynamically deformed torsional specimen of the 1- or 4-pass ECAP'ed specimen, while some weak bands were observed in the 8-pass ECAP'ed specimen. These findings suggested that the grain refinement according to the ECAP was very effective in strengthening of pure coppers, and that ECAP'ed coppers could be used without serious reduction in fracture resistance under dynamic torsional loading as adiabatic shear bands were hardly formed.

• 한국재료학회지
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• 제33권12호
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• pp.565-571
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• 2023

AA1050/AA6061/AA1050 layered sheet was fabricated by cold roll-bonding process and subsequently T4 and T6 aging-treated. Two commercial AA1050 sheets of 1 mm thickness and one AA6061 sheet of 2 mm thickness were stacked up so that an AA6061 sheet was located between two AA1050 sheets. After surface treatments such as degreasing and wire brushing, they were then roll-bonded to a thickness of 2 mm by cold rolling. The roll-bonded Al sheets were then processed by natural aging (T4) and artificial aging (T6) treatments. The as roll-bonded Al sheets showed a typical deformation structure, where the grains are elongated in the rolling direction. However, after the T4 and T6 aging treatments, the Al sheets had a recrystallized structure consisting of coarse grains in both the AA5052 and AA6061 regions with different grain sizes in each. In addition, the sheets showed an inhomogeneous hardness distribution in the thickness direction, with higher hardness in AA6061 than in AA1050 after the T4 and T6 age treatments. The tensile strength of the T6-treated specimen was higher than that of the T4-treated one. However, the strength-ductility balance was much better in the T4-treated specimen than the T6-treated one. The tensile properties of the Al sheets fabricated in the present study were compared with those in a previous study.

• 한국결정성장학회지
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• 제9권2호
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• pp.231-239
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• 1999

세라믹 복합체의 미세구조는 이차상의 종류 및 양, 소결 조제, 그리고 소결 조건(즉, 소결온도, 압력, 유지시간)에 영향을 받는다. 또한, 내마모 특성은 미세구조와 작업 조건이 내마모 특성에 크게 영향을 미친다. 본 실험에서는 질화규소에 마그네시아의 양을 변화시켜 가압 소결(Hot Press) 방법으로 시편을 제조하였다. 마그네시아 양에 따른 미세구조, 기계적 특성(경도, 강도, 파괴인성), 그리고 다양한 분위기(공기, 물, paraffin oil)에서의 내마모 특성을 조사하였다. 질화규소에 마그네시아의 양이 증가하였을 때, 입계의 유리상이 $\beta$-상의 장주형 입자를 커지게 하고, 또한 Hertzian contact damage를 저하시킨다. 대기 중에서 내마모 거동은 강도뿐만 아니라 파괴인성과 관련이 있고, 물과 paraffin oil에서는 경도와 밀접하다. 물속에서 마모시험 중 많은 유리상이 물과 반응하는 것을 생각할 수 있다. 그러므로 마찰 화학적 반응(tribochemical reaction)은 마모도 저하시킨다. Paaraffin oil에서 높은 하중을 부하할 경우, Hertzian contact damage 때문에 초기 마모가 마모량에 지배적이다.

• Applied Biological Chemistry
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• 제20권1호
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• pp.147-155
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• 1977

본도(本稻)에 심층추비(深層追肥)를 하여 초기무효분벽(初期無效分蘗)을 억제(抑制)하고 유효경비율(有效莖比率)을 높여 생육후기(生育後期)까지 지속적(持續的)인 질소공급(窒素供給)으로 수량증수(收量增收)를 위하여 품종(品種)은 동일(統一)과 진흥(振興), 토양(土壤)은 식양토(埴壤土)와 사양토(砂壤土), 배수조건(排水條件)은 무배수(無排水) 배수(排水)로 구분(區分)하여 질소질비료(窒素質肥料)를 표준시비(표층추비)標準施肥(表層追肥)와 대비(對比)하여 추비비율(追肥比率)을 50%액비(液肥), 50%단자비(團子肥) 80%단자비(團子肥)로 하여 출수(出穗) 35일전(日前) 12cm의 토양(土壤)깊이로 심층추비(深層追肥)하여 얻은 결과(結果)를 요약(要約)하면 다음과 같다. 1. 가) 생육시기별(生育時期別) 경수변화(莖數變化)를 보면 진흥(振興)은 표층추비(表層追肥)에서 경수(莖數)가 급격(急激)히 증가(增加)하다가 감소경향(減少傾向)도 심(甚)하여 유효경비율(有效經比率)이 낮으며 심층추비(深層追肥)에서는 경수(莖數)의 증감(增減)이 완만(緩慢)하여 유효경비율(有效經比率)이 현저(顯著)하게 높았다 나) 통일(統一)에서는 진흥(振興)처럼 표층추비(表層追肥)와 심층추비간(深層追肥間)의 경수증감(莖數增減)의 폭(幅)이 크지않으나 80%단자심층추비(團子深層追肥)는 유효경비율(有效經比率)이 $83{\sim}93%$로 가장 높았다. 다) 진흥(振興)에서 80%단자심층추비(團子深層追肥)는 시비량(施肥量)이 많은 것 같다. 라) 유효경비율(有效經比率)은 통일(統一)보다 진흥(振興)이 높은 경향(傾向)이다. 2. 주당총립수(株當總粒數)는 심층추비(深層追肥)로 증가(增加)하였으며 입수(粒數)가 증가(增加)할수록 등숙비율(登熟比率)은 감소(減少)하는 경향(傾向)이다. 3. 진흥(振興)에서 심층추비(深層追肥)로 지엽(止葉)의 길이가 길며 수량(收量)과는 유의(有意)한 정(正)의 상관(相關)을 보였다. 4. 심층추비(深層追認)는 수확기(收穫期) 고(藁)중의 질소함량(窒素含量)을 증가(增加) 시켰으며 수량(收量)과도 정(正)의 상관(相關)이 있었다. 5. 조고비율(粗藁比率)이 심층추비(深層追肥)로 증가(增加)하였으며 통일(統一)보다 진흥(振興)이 높았다. 6.정조(精粗收量)은 80%단자심추(團子深追)>50%단자심추(團子深追)>50%액심추(液深秋)>표층추비(表層追肥)의 순(順)으로 심층추비(深層追肥)의 효과(效果)가 있었으며 증수요인(增收要因)으로는 표층추비(表層追肥)에 비(比)하여 유효경비율(有效經比率)의 증가(增加) 및 수수증가(穗數增加)와 진당총립수(振當總粒數)의 증가(增加)로 본다. 7. 토양별(土壤別) 심층추비효과(深層追肥效果)는 통일(統一)은 사양토(砂壤土)에서 진흥(振興)은 식양토(埴壤土)에서 증수(增收)하였다. 8. 통일(統一), 진흥(振與) 다같이 식양토(壇壞土)에서는 80%단자심층추비(鬪子深層追肥)를 제외(除外)하고는 무배수(無排水)에서 증수(增收)하였다. 9. 본(本) 실험실시중(試驗實施中) 출수기(出穗期) 등숙기(登熟期)에 저온(低溫)이 계속(繼續)되어 통일(統一)에서 등숙비율(等熟比率)이 낮았으나 표층추비(表層追肥)에 대(對)한 증수효과(增收效果)는 진흥(振興)보다 통일(統一)이 높았다.

• 한국세라믹학회지
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• 제36권1호
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• pp.43-49
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• 1999

0~5 wt%의 $\beta$-Si3N4 whisker를 첨가한 질화규소 가스압 소결체를 제작하였다. 첨가된 whisker들은 tape casting을 응용하여 tape 내에서 일방향으로 배열하였으며, 제작된 tape로부터 절취된 sheet들의 적층 과정을 변화하여 다양한 미세구조를 갖는 성형체를 제작하였다. 가스압 소결을 통하여 치밀화된 소결체의 조대결정립은 성형체의 whisker와 같은 방향성을 가졌으며, 소결수축률과 기계적 특성 등도 이러한 다양한 미세구조상의 특징과 일관된 결과를 나타내었다. Whisker를 일방향으로 배열하였을 경우, 소결수축률은 whisker 배열 방향과 평행한 방향보다 수직한 방향으로 크게 일어났을 때, whisker 함량이 증가함에 따라 수축률의 차이가 증가하였다. Indentationi crack length는 whisker 함량이 증가함에 따라 whisker 배열 방향과 수직한 방향의 균열 길이는 더 짧아졌고, 그와 평행한 방향의 균열 길이는 길어졌다. Whisker 첨가에 의해 결정립이 더 크게 성장하였으나 강도는 whisker를 첨가하지 않을을때보다 낮지 않았다. Whisker 배열 방향을 90$^{\circ}$ 및 45$^{\circ}$ 간격으로 엇갈리게 적층하였을 경우에는 특성의 이방성이 나타나지 않았다.

• 한국재료학회지
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• 제11권6호
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• pp.502-507
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• 2001

일방향응고법으로 IN792+Hf 초내열합금의 응고속도에 따른 응고거동의 변화에 대해 연구하였다. 조직관찰을 통해 각 상의 응고과정과 석출거동을 분석하였다 일방향응고시 응고속도가 감소하면 문자형의 탄화물은 면상 탄화물로 변화하였고 ${\gamma}$상과 탄화물의 결합은 탄화물의 수지상 성장에 의한 것임을 확인할 수 있었다. 긴 막대형상의 탄화물이 0.5$\mu\textrm{m}$/s의 응고속도에서 입계를 따라 형성되었으며 잔류액상지역에서 ${\gamma}$'형성원소가 풍부한 구역과 고갈된 구역이 발견되었다. 공정 ${\gamma}$/${\gamma}$'은 형성원소가 풍부한 구역에서 핵생성하였으며 공정 ${\gamma}$/${\gamma}$'의 형성은 잔류액상지역의 (Ti+Hf+Ta+W)/Al 비율을 높여 η상의 석출을 유발하였다. 느린 응고속도에서는 잔류액상지역으로부터의 충분한 역확산으로 (Ti+Hf+Ta+W)/Al 비율이 낮아져 η상의 석출이 억제되었다.