• Journal of Korea Foundry Society
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• v.19 no.6
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• pp.472-483
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• 1999

The effects of rapid solidification and Cr addition to the microstructure variations of the rapid solidified Ti-48Al-xCr(X = 2,4,6) alloys have been investigated using X-ray diffractometry, optical microscopy, scanning electron microscopy and transmission electron microscopy. The segregated coarse ${\gamma}$ phase was eliminated and the microstructure was refined by rapid solidification of the cooling rate of $10^4-10^6\;^{\circ}C/sec$. The lattice parameters and tetragonality of ${\gamma}$ phase decrease with the increase Cr content and by the rapid solidification. Non-equilibrium phase ${\alpha}$ remains at room temperature condition, which would be resulted from the restriction of phase transformation ${\alpha}$ to ${\alpha}_2+{\gamma}$.

• Journal of Powder Materials
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• v.11 no.3
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• pp.193-201
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• 2004

1960년 Au-Si계 합금에서 처음으로 비정질상이 급속 응고법에 의해 보고된 이래/sup 1)/ 지난 40년 간 많은 합금계에서 비정질상이 보고되어졌다. 대표적으로 Fe-, Ni-, Co기 합금 등 많은 합금계에서 비정질상이 보고되었으나, 비정질상의 형성을 위해서는 약 105 K/s이상의 높은 냉각속도를 필요로 하였다. 1980년대 수백 K/s의 낮은 냉각속도 하에서도 비정질상이 형성될 수 있는 다원계 합금(multi-component alloy)이 Mg-Ln-(Ni, Cu, Zn), Ln-Al-TM 합금에서 보고되어 졌으나 많은 관심을 받지 못하다가 1993년 Zr-Ti-Ni-Cu-Be 합금에서 수 ㎝ 크기의 비정질합금 제조가 보고되면서 전 세계적으로 많은 관심을 받게 되었다. Zr-Ti-Ni-Cu-Be계 벌크 비정질 합금이 보고된 후 Zr-(Nb,Pd)-Al-TM, Pd-Cu-Ni-P, Fe-Co-Zr-Mo-W-B, Ti-Zr-Ni-Cu-Sn등 여러 합금계에서 벌크 비정질 합금이 보고되었다. (중략)

• Journal of Powder Materials
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• v.11 no.3
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• pp.233-241
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• 2004

In this study, bottom-up type powder processing and top-down type SPD (severe plastic deformation) approaches were combined in order to achieve both full density and grain refinement of Al-20 wt% Si powders without grain growth, which was considered as a bottle neck of the bottom-up method using the conventional powder metallurgy of compaction and sintering. ECAP (Equal channel angular pressing), one of the most promising method in SPD, was used for the powder consolidation. The powder ECAP processing with 1, 2, 4 and 8 passes was conducted for 10$0^{\circ}C$ and 20$0^{\circ}C$ It was found by microhardness, compression tests and micro-structure characterization that high mechanical strength could be achieved effectively as a result of the well bonded powder contact surface during ECAP process. The SPD processing of powders is a viable method to achieve both fully density and nanostructured materials.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2008.05a
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• pp.448-449
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• 2008

Light Mg alloy stands on the center of investigation due to the high potential of industrial application not only to the structural, but also to the functional fields. However, the intrinsic low strength and corrosion resistance have limited to extend its industrial use. In order to overcome the disadvantage, various attempts have been come to the modification of composition, resulting in finding Mg-Zn-Y alloys. The cast Mg-Zn-Y alloy leads to the high strength and hardness, low friction coefficient and low interfacial energy in both the ambient and elevated temperature.

• Journal of Welding and Joining
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• v.7 no.3
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• pp.55-62
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• 1989

Variation of the spray droplet velocity with spraying distance and the microstructural characteristics of spray deposits fromed by oxy-fuel thermal spraying with Ni-base alloy powder contained chrome boride for hard facing were examined. Measurements of spray droplet velocity as a function of distance from the nozzle tip were inexcellent agreement with computer simulated predictions. Optimum condition for thermal spray deposits in this experiment was found to be under #10kg/cm^2$ of acceleration gas pressure with 15cm of spraying distance. Fine microstructure and higher microhardness of the initial part of the deposits due to rapid solidification were found to be able to maintained in a thickness up to 0.4mm, and this initial microstructure and properties could be maintained throughout the thickness of a thick spray deposits by performing the multipass spraying with 0.4mm thickness of each pass.

• Korean Journal of Materials Research
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• v.9 no.9
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• pp.883-889
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• 1999

The pure Al or-(1,2,3wt%)Cu alloy internal chill with 4,6,8,12 and 15mm ø, respectively, was inserted at the center of a graphite mold with the size of 95mm ø$\times$200mm H. The molten metal with the same composition as the internal chill was poured into the mold at the pouring temperature of $750^{\circ}C$ and the cooling rates, solidified structures and chemical segregation were analyzed. The results represented that there was remarkable increased in cooling rate as well as decrease in grain size, secondary dendrite arm spacing and chemical segregation as the ratio of ingot to internal chill diameter was increased to 8. However there was a considerable drop of the internal chill effect when this ratio exceeded 8, resulting from incomplete melting of internal chills. The optimum ratio for the maximum internal chill effect of pure Al and-(1,2,3wt%)Cu allolys was 8 at the given pouring temperature.

• Korean Journal of Materials Research
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• v.5 no.1
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• pp.22-35
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• 1995

In this study, crystal structures and magnetic properties of as-ast, annealed and rapidly solidified Mn-A1-M( M=Cu, Fe) alloys have been investigated. In $Mn_{0.56}Al_{0.44}$ alloys, the largest fraction of $\tau$ phase and values of magnetic properties was obtained in Mnl, i6Alo or alloy. And this alloy was used as the basic composition. In $Mn_{0.56-X}M_{X}Al_{0.44}$ alloys, when annealed, $\tau$- and $\beta$-Mn phase appeared at x< 0.08, $\tau$- and $\kappa$ phase at 0.10 $\leq x \leq$ 0.12 and $\kappa$- phase only at 0.15 $\leq x \leq$0.20 . When rapidly solidified, specimens showed similar phases as when annealed except that $\varepsilon$ phase appeared at x=0.04. In Mnu FexAlo 44 alloys, asyast specimens showed $\tau$-, $\beta$-Mn and $\gamma_2$- phase at x<0.08 and K and $\beta$-Mn phase at x>0.10. When rapidly solidified, Mn-Fe-Al specimens showed $\varepsilon$-, $\gamma_2$- and small amount of $\tau$- and $\kappa$ phase at x<0.08 and $\kappa$- phase only at 0.$\leq x \leq$0.20. All the alloys investigated were ferromagnetic. The Curie temperature of annealed specimens and rapidly solidified of Mno 5sAlu 44 alloy were -650K and -644K. Spontaneous magnetization( UII of annealed and rapidly solidified specimens were 40-45 (emu/g) and 50-52(emu/g), respectively. Remanent (M,) to saturation magnetization( Ms) ratio was -0.7. M, of rapidly solidified specimen was about 48(emu/g). Magnetic properties of $Mn_{0.56}Al_{0.44}$ alloys were found to be determined by the relative fraction of ferromagnetic r- and K- phase. When M= Cu and x=0.15, maximum as($\sigma_{0.0}$) was obtained by about 64.3 emu/g), and when M=Fe and x=0.15, 66.4( emu/g). The Curie temperature decreased as x increased.

• Korean Journal of Materials Research
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• v.4 no.8
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• pp.847-854
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• 1994

An amorphous single phase and coexistent amorphous and hcp-Mg phases in Mg-Zn-Ce system were found to form in the composition ranges of 20 to 40% Zn, 0 to 10% Ce and 5 to 20% Zn, 0 to 5% Ce, respectively. A $Mg_{85}Zn_{12}Ce_{3}$ amorphous alloy containing nanoscale hcp-Mg particles was found to form either by melt spinning or by heat treatment of melt -spun ribbon. The particle size of the hcp-Mg phase can be controlled in the range of 4 to 20 nm. The mixed phase alloy prepared thus has a good bending ductility and exhibits high ultimate tensile strength($\sigma_{B}$) ranging from 670 to 930 MPa and fracture elongation($\varepsilon_{f}$) of 5.2 to 2.0%. The highest specific strength($\sigma_{B}$/density =$\sigma_{s}$)$3.6 \times 10^5N \cdot m/kg$. It should be noted that the highest values of flB, US and ？1 are considerably higher than those (690MPa,$2.5 \times 10^5N \cdot m/kg$and 2.5%) for amorphous Mg-Zn-Ce alloys. The increase of the mechanical strengths by the formation of the mixed phase structure is presumably due to a dispersion hardening of the hcp supersaturated solution which has the hardness higher than that of the amorphous phase with the same composition.

• Proceedings of the Korean Powder Metallurgy Institute Conference
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• 2001.11a
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• pp.28-28
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• 2001

$Bi_2Te_3$계 열전반도체 재료는 200 ~ 400K 정도의 저온에서 에너지 변환 효율이 가장 높은 재료로서 열전냉각 및 발전재료로 제조볍 및 특성에 관한 많은 연구가 진행되어 왔다. 전자냉각 모듈의 제조에는 P형 및 N형 $Bi_2Te_3$계 단결정이 주로 사용되고 있으나. $Bi_2Te_3$ 단결정은 C축에 수직한 벽개면을 따라 균열이 쉽게 전파하기 때문에 소자 가공사 수윤 저하가 가장 큰 문제점으로 지적되고 있다. 이에 따라 최근 열전재료의 가공방법에 따른 회수율 증가 및 열전특성 향상에 관한 열간압출, 단조와 같은 연구가 활발히 이루어지고 있다. 본 연구는 가스분사법(gas atomizer)을 이용하여 용질원자 편석의 감소, 고용도의 증가,균일고용체 형성, 결정립미세화 둥 급속응고의 장점을 이용하여 화학적으로 균질한$Bi_2Te_3$계 열전재료 분말을 제조하고, 제조된 분발을 압출가공하여 기계적성질, 소자의 가공성 및 열전 성능 지수율 향상시키는데 연구 목적이 있다. 본 설험에서는 99.9%이상의 고순도 Bi. Te. Se. Sb를 이용하여, 고주파 유도로에서 Ar 분위기로 용융하고, 가스분사법를 이용하여 균질한 $Bi_2Te_3$계 열전재료 분만을 제조하였다. 분말표면의 산화막을 제거하기 위하여 수소분위기에서 환원처리를 행하였고, 된 분말을 Al 캔 주입하여 냉간성형 한 후 진공중에서 압출온도를 변화시켜 열간압출 가공을 행하였다. 압출 온도변화에 따른 압출재의 미세조직 및 열전특성에 중요한 영향을 미치는 C면 배향에 대한 결정방위 해석, 압출재의 압축강도 등을 분석하였으며, 압출온도에 따삼 미세조직 변화와 결정방위의 변화에 따른 열전특성의 관계를 해석하였다성시켰고 이들이 산인 HNO3에서 녹았기 때문이다. 본 연구에서 개발된 새로운 에칭 용액인 90H2O2 - 10HNO3 (vol%)의 에칭 원리가 똑같이 적용 가능한 다른 종류의 초경 합금에서도 사용이 가능할 것으로 판단된다.로 판단된다.멸과정은 다음과 같다. 출발물질인 123 분말이 211과 액상으로 분해될 때 산소가스가 배출되며, 이로 인해 액상에서 구형의 기공이 생성된다. 이들 중 일부는 액상으로 채워져 소멸되나, 나머지는 그대로 남는다. 특히, 시편 중앙에 서는 수십-수백 마이크론 크기의 커다란 기공이 다수 관찰된는데, 이는 기공의 합체로 만들어진 것이다. 포정반응 열처리 시 기공 소멸로 만들어진 액상포켓들은 주변 211 입자와 반응하여 123 영역으로 변한다. 이곳은 다른 지역과 비교하여 211 밀도 가 낮기 때문에, 미반응 액상이 남거나 211 밀도가 낮은 123 영역이 된다. 액상으로 채워지지 못한 구형의 기공들 중 다수가 123 결정 내로 포획되며, 그 형상은 액상/ 기공/고상 계면에너지에 의해 결정된다.단의 경우, 파단면이 매끄럽고 파변상의 결정립도 매우 미세하였으며, 산확물 의 용집도 찾아보기 어려웠 나, 접합부 파단의 경우에는 파변의 굴곡이 비교척 심하고 연성 입계파괴의 형태를 보였￡며, 결정립도 모채부 파단의 경우에 비해 조대하였다. 조대하였다. 셋째, 주상기간 중 총 에너지 유입률 지수와 $Dst_{min}$ 사이에 높은 상관관계가 확인되었다. 특히 환전류를 구성하는 주요 입자의 에너지 영역(75~l13keV)에서 가장 높은(0.80) 상관계수를 기록했다. 넷째, 회복기 중에 일어나는 입자들의 유입은 자기폭풍의 지속시간을 연장시키는 경향을 보이며 큰 자기폭풍일수록 현저했다. 주상에서 관측된 이러한 특성은 서브스톰 확장기 활동이 자기폭풍의

• Korean Journal of Materials Research
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• v.7 no.12
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• pp.1063-1069
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• 1997

For me1t-spun Fe-Nd-C alloys, variation of phase development and magnetic properties with the variety of alloy compositions and production conditions were investigated. To find out whether hard magnetic $Fe_{14}Nd_2C$ is crystallized direct1y from the melt by rapid quenching, the phase development of the as-spun ribbons spun at various speed was a1so studied. For the ribbons spun at 10m/s, ${\alpha}-Fe$ was the primary crystallization phase followed by the secondary $Fe_{17}Nd_2C$. At 20m/s ${\alpha}-Fe$ was suppressed so that the primary $Fe_{17}Nd_2C$ coexisted with the secondary ${\alpha}-Fe$ and the amorphous phase. Above 30m/s the ribbons were a1most amorphous, and the amorphization was complete at 40m/s. $Fe_{14}Nd_2C$ therefore was not found in as-spun state but obtained after heat treating the ribbons. The effective temperature range ($700{\sim}900^{\circ}C$) in which $Fe_{14}Nd_2C$ can be obtained was wider than that of a cast alloy. An alloy made with the wheel speed of 20 or 30m/s yielded higher coercivities after heat treatment. In iron-rich Fe-Nd-C, the composition range in which a high coercivity (more than 10kOe) is expected is narrow, i.e., 77~78 Fe and 7~8 C(at.%).