1,720,957 research outputs found
Development and oxidation resistance evaluation of Al-rich surface layer on Alloy 617
No full textSurface treatment was carried out for Alloy 617 to improve the high temperature oxidation resistance. After high purity Al was pre-deposited on the surface, electron beam (EB) was applied to form Al-rich micro-alloying zone with the depth of similar to 50 mu m. In the micro-alloying zone, cellular and dendritic structures with very fine arm spacing were developed. In the oxidation test performed in air at 900 degrees C up to 1000 h, a transient oxidation behavior was observed for the surface-treated Alloy 617. During the short initial oxidation stage (<1 h), Al-rich external oxide layers with the outermost spinel and inner alumina were quickly formed. However, during the steady-state oxidation stage, the oxidation rate of EB surface-treated Alloy 617 was substantially lower than that of the as-received Alloy 617. (C) 2013 Elsevier B.V. All rights reserved
Effects of aging in high temperature helium environments on room temperature tensile properties of nickel-base superalloys
No full textThe influence of high temperature aging treatment on room temperature tensile properties of wrought nickel-base superalloys Alloy 617 and Haynes 230 was investigated. A significant decrease in elongation was observed for Alloy 617 exposed to a heavily oxidizing and decarburizing condition because of coarsening of grain boundary carbides and extensive inter-granular oxidation. On the other hand, Haynes 230 showed much lower ductility when exposed to a heavily carburizing condition, especially at 1000 degrees C because extensive carburization occurred due to a reaction with tungsten. Considerable loss of ductility for Alloy 617 and Haynes 230 was also observed in He-H(2)-H(2)O-CO-CO(2)-CH(4) and He-H(2)O-CO-CO(2) environments, which were the slightly oxidizing and decarburizing conditions. Loss of ductility was predominantly associated with brittle inter-granular cracking, while the extent of loss of ductility decreased depending on the decarburization depth. Decarburization was observed more extensively in helium with H(2)-H(2)O-CO-CO(2)-CH(4) than helium with H(2)O-CO-CO(2), and for Alloy 617 than for Haynes 230. Finally, the role of H(2) in accelerating decarburization is discussed. (c) 2010 Elsevier B.V. All rights reserved,This work was supported by the MEST/NRF (Nuclear R&D Program,2005-2004718 and 2009-0083392) of Korea. Part of the funding was provided by the Second Phase BK21 Program of the Ministry of Education, Science and Technology of Korea
오스테나이틱 Fe- 및 Ni-기 합금의 확산접합에 관한 연구
No full text학위논문(박사) - 한국과학기술원 : 원자력및양자공학과, 2015.2
,[viii, 111 p. :]철계 및 니켈기 합금은 소듐냉각로 (SFR) 및 초고온가스로 (VHTR) 중간열교환기 (IHX) 후보재료로 고려되고 있다. 본 연구에서는 미세유로채널 타입의 열교환기 제작에 필요한 과정 중 하나인 확산접합에 대하여 연구를 수행하였다. 또한, 고온재료의 산화/부식 저항성을 증가시키기 위한 목적으로 표면개질에 관한 내용을 포함하고 있다.
다양한 종류의 Fe-Cr, Fe-Cr-Ni, Fe-Ni-Cr, Ni-Cr-Fe 합금에 대하여 확산접합을 수행하였으며, 미세조직 및 상온/고온 인장 실험을 수행하였다. Fe-Cr-Ni 합금의 경우는 접합면을 가로질러 결정립계가 이동하며 상온과 고온에서 모두 우수한 인장특성을 확인할 수 있었다. 그러나, 나머지 합금들의 경우는 확산접합 이후에 상온/고온 인장 특성이 모재에 비해 현저하게 감소하는 것을 보였으며, 이를 개선하기 위해 후열처리 연구를 제안하였다. Fe-Cr 합금의 경우는 템퍼드 마르텐사이트 기지 조직을 형성하기 위한 열처리를 수행하였으며, Fe-Ni-Cr 합금 및 Ni-Cr-Fe 합금은 어닐링 열처리 조건에 유사한 온도/시간 조건에서 실험을 수행하여 접합면을 가로질러 구성 원자들간의 추가적인 확산을 도모하기 위함이다. 후열처리 이후에 실험에 사용된 후보재료들은 상온/고온 인장특성이 모재 수준으로 어느 정도 회복되는 것을 확인할 수 있었으나, VHTR 적용을 목적으로 하는 고온 (900 oC)에서는 모재의 절반수준에 그쳤다.
표면개질의 경우 산화/부식저항성을 향상시키기 위하여 물리적기상증착 (PVD)과 가속전자 (EB)를 이용하여 표면근처에 대해 화학조성을 변화시키는 것에 관한 연구를 수행하였다. 대기환경에서 산화실험을 수행한 결과, 모재에 비해 무게증가률이 현저하게 감소하는 것을 확인할 수 있었으며 이는 산화막/모재 경계에서 관찰된 Al2O3 외부 산화막 때문으로 보인다. 한편, 가속전자 조사 이후 미세합금화 영역에 대해 국부적으로 전위가 다량 형성되기에 초기 무게증가률은 모재에 비해 더 크지만, 산화막/모재에서 충분히 연속적인 외부 산화막이 형성된 이후에는 무게증가률이 감소하게 된다는 산화기구를 제안하였다.
한편, 표면개질의 또 다른 효과적인 방법인 확산열처리 (IDHT)의 경우는 우수한 산화저항성에도 불구하고 모재에 비해 크립-파단 수명이 짧아지는 현상이 관찰되었다. IDHT열처리 과정에서 표면으로부터 석출되는 Ni3Al, M23C6, σ 상 중 M23C6의 석출은 결정립계에 불연속적으로 존재하던 Cr 탄화물의 용해 및 확산에 의한 것이다. 이 과정에서 줄어든 결정립계 불연속적인 탄화물은 결정립계 미끌림을 효과적으로 막지 못하기 때문이라는 기구를 제안하였다.
마지막으로, 앞서 살펴본 연구를 결합하여 표면개질된 Alloy 617의 확산접합에 관한 연구를 수행하였다. 표면개질 방법으로 사용된 가속전자빔의 경우는 접합면에서 소량의 불연속적인 Al2O3가 존재하나 그 양이 적어 중간삽입물 혹은 후열처리를 통해 인장특성이 모재 수준으로 회복되는 것을 확인할 수 있었다. 그러나, 확산열처리를 통한 표면개질의 경우는 표면근처에 존재하는 다량의 Al으로 인한 Al2O3 형성과 금속간화합물 (Ni3Al)의 형성으로 인하여 중간삽입물 혹은 후열처리의 효과를 확인할 수 없었다. 성능개선을 위해 확산열처리와 확산접합을 동시에 수행하는 방법 (단일공정)을 제안하였으나, IDZ 영역에서 석출된 금속간화합물과 탄화물의 기계적특성 차이로 인하여 접합과정에서 크랙이 발생하여 접합성능을 평가할 수 없었다. 추후에 확산열처리 과정에서 수반되는 2차상들의 고온 (600 및 1150 oC) 기계적특성에 대한 충분한 이해를 바탕으로 최적 조건도출에 관한 연구가 요구된다.한국과학기술원 :원자력및양자공학과
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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