Influence of heat treated microstructures on the dynamic deformation characteristics of Ti-6Al-4V alloy

서용석,우성충,김태원,이영신 대한기계학회 2015 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.29 No.12

We investigated the influence of heat treated microstructures, namely, equiaxed, bimodal and lamella types of Ti–6Al–4V alloy on thedynamic deformation characteristics. Four different heat treatment conditions were employed for the development of the microstructures. Static tensile and compressive deformation tests were preliminarily performed with hydraulic test equipment. Dynamic deformation testsat a high level of strain rate, 2700 s-1 ~ 6400 s-1, together with high velocity impact tests were, respectively, conducted on the specimensthrough a compressive Split Hopkinson pressure bar (SHPB) and a high pressure gas gun system. The dependence of flow stress on thestrain rate associated with the corresponding microstructure was examined. The microstructural factors on the dynamic fracture characteristicswere analyzed by scanning electron microscopy. The static compressive tests showed that the flow stress was greatest in thelamella microstructure and decreased in the order of lamella, bimodal and equiaxed microstructures, whereas the ductility was largest inthe bimodal microstructure and smallest in the lamellar microstructure. In dynamic compressive tests, a similar dependency of the flowstress on microstructures was observed: highest in the lamellar microstructure and lowest in the equiaxed microstructure. The ductility,such as strain at maximum stress or at failure, was highest in the equiaxed microstructure and lowest in the lamellar structure. In addition,the ductility for individual microstructure decreased as the strain rate increased. Every microstructure exhibited ductile fracture surfaces,and it seems that a large shear crack on the lateral surface in the specimen was the main factor inducing the final failure. The result ofhigh velocity impact test exhibited that the resistance to fracture of equiaxed microstructure with superior dynamic toughness was muchhigher than that of lamella microstructure with inferior dynamic toughness. The results obtained help provide a fundamental idea andguide to improve the dynamic mechanical properties of Ti-6Al-4V alloy through the microstructure control.

중국어 사전 평가를 위한 평가 문항 제작 연구 ᐨ 온라인 중국어 학습사전의 미시구조 평가를 중심으로 ᐨ

이미향 한국사전학회 2019 한국사전학 Vol.- No.33

In learning a foreign language, in particular, the dictionary plays an important role in self-directed learning. However, the study and publishing of Chinese dictionaries have been in place for a long time, and the Chinese dictionary now has a lot of problems in its organization and composition, and it limits the self-directed learning of learners. The evaluation and study of the dictionary are essential for improving the quality of Chinese dictionaries and for the efficiency of Chinese learning. However, there is still a lack of research on systematic dictionary evaluation models in Korea, making it difficult to conduct objective evaluations of Chinese dictionaries. Therefore, this study set the evaluation criteria and evaluation items to evaluate the overall microstructure of the dictionary used mainly by domestic Chinese learners and created specific evaluation items. To evaluate a dictionary, it is necessary to identify the type of dictionary to be evaluated first. Provided that the type of dictionary is analyzed by using the dictionary classification method, the dictionary to study in this paper is a bilingual dictionary using Chinese and Korean, and a learners’ dictionary for Korean learners who are learning Chinese, and as an online dictionary using online media, this paper is called ‘Online Chinese learning dictionary’. Lexicographers have distinguished the structure of the dictionary from different perspectives, which in common include macrostructures and microstructures. In this paper, the discussion has been emphasized how the entries of the dictionary organize and provide information with a focus on its microstructure. Although there is still insufficient research on the dictionary evaluation in Korea, Western and Chinese lexicographers have already studied criteria for evaluating various types of dictionaries. However, since existing evaluation criteria are insufficient in system and unsatisfactory to evaluate microstructure as a whole. it was necessary to establish the appropriate evaluation criteria to evaluate the microstructure of the online Chinese learners’ dictionary. This paper refers to the evaluation criteria in the previous study and designed the criteria for the creation of evaluation items to evaluate the microstructure of the online Chinese learning dictionary from a macro perspective based on the lexicography, Chinese Linguistics and evaluation theory. And based on these criteria, specific evaluation items has been created to evaluate the microstructure of the online Chinese learners’ dictionary. First, five items have been created to evaluate the overall composition of the microstructure, and the vocabulary grade, spelling, pronunciation, semantic distribution, part of speech, definition of a word and example were set as essential information items of the microstructure, and 36 evaluation items have been created to evaluate them. The selection information items of the microstructure were made up of 18 evaluation items, divided into grammatical information, pragmatic information, reference words information, and other information. The evaluation items created in this paper can actually be a standard for evaluating the online Chinese learning dictionary in Korea, and the dictionary researcher and lexicographers can refer to compiling and updating online Chinese learners’ dictionaries, and can also be used to evaluate and study other foreign language dictionaries.

이종 입자복합재의 미세구조 생성과 계층적 모델의 선형 탄성적 응답특성 해석

조진래 한국전산구조공학회 2018 한국전산구조공학회논문집 Vol.31 No.3

본 논문은 로 구성된 금속-세라믹 이종 입자복합재의 2차원 미세구조(microstructure) 생성과 미세구조 스케일(scale)에 따라 정의되는 계층적 모델들의 역학적 특성 분석에 관한 내용이다. 이종 입자복합재의 미세구조는 수학적인 RMDF(random morphology description functions) 모델링기법을 복합재의 2차원 RVE(representative volume element) 영역에 적용하여 생성하였다. 그리고 미세구조 생성에 필요한 가우스 함수들의 개수에 따라 미세구조의 계층적 모델을 정의하였다. 한편 임의 미세구조 내 금속과 세라믹 입자가 차지하는 체적분율(volume fraction)은 RMDF 함수의 레벨을 조정함으로서 설정하였다. RMDF기법에 의한 미세구조들은 가우스 함수들의 개수가 일정할지라도 랜덤하게 생성된다. 이렇게 랜덤하게 생성되는 미세구조들을 2차원 보(beam) 모델에 적용하여 미세구조의 스케일에 따른 수직응력과 전단응력의 계층적 변동을 수치 해석적으로 고찰하였다. 또한, 균열해석을 통해 RMDF의 랜덤성과 가우스 함수들의 개수가 균열선단에서의 응력값에 미치는 영향을 고찰하였다. This paper is concerned with the 2-D micostructure generation for dual-phase composite materials and the numerical analysis of mechanical characteristic of hierarchical models of microstructure which are defined in terms of the scale of microstructure. The microstructures of dual-phase composite materials were generated by applying the mathematical RMDF(random morphology description functions) technique to a 2-D RVE of composite materials. And, the hierarchical models of microstructure were defined by the number of Gaussian points. Meanwhile, the volume fractions of metal and ceramic particles were set by adjusting the level of RMD functions. The microstructures which were generated by RMDF technique are definitely random even though the total number of Gaussian points is the same. The randomly generated microstructures were applied to a 2-D beam model, and the variation of normal and shear stresses to the scale of microstructure was numerically investigated. In addition, through the crack analyses, the influence of RMDF randomness and Gauss point number on the crack-tip stress is investigated.

Microstructure and Mechanical Properties of TA15 Alloy Produced by Electron Beam Melting

Jiangtao Ran,Fengchun Jiang,Zhuo Chen,Hong Zhao 대한금속·재료학회 2022 METALS AND MATERIALS International Vol.28 No.3

A series of TA15 specimens with different process parameters were built by electron beam melting in this paper to systematicallyinvestigate the relationship of material-process-microstructure-mechanical properties. A quantitative mathematicalmodel of yield strength and Vickers hardness with respect to α lath thickness was established. Microstructure analysis resultsshowed that the microstructure of TA15 titanium alloy were basketweave microstructure, lamellar microstructure and bimodalmicrostructure depending on process. In general, the yield strength, Vickers hardness and α plate thickness are correlated withthe Hall-Petch equation, but the mathematical model established in this paper has a better fitting effect. Process parametersaffect the mechanical properties of the TA15 titanium alloy by affecting α lath thickness, and speed factor has the greatestinfluence on α lath thickness. The tensile properties of TA15 titanium alloy are as well as, if not better than, that of TA15titanium alloys with tri-modal microstructure.

Ti-6Al-4V 합금의 동적변형특성과 탄도충격성능 : 2. 동적비틀림 특성과 탄도충격성능 2. Dynamic Torsional Properties and Ballistic Performance

이동근,이유환,이성학,이종수,이충열,허선무 대한금속재료학회 2003 대한금속·재료학회지 Vol.41 No.10

The effects of microstructural morphology on dynamic deformation and fracture behavior and ballistic performance of a Ti-6Al-4V alloy were investigated in this study. Ballistic impact tests were conducted on equiaxed and bimodal microstructures, which were processed by different heat treatments, and then the test data were analyzed in relation to microstructures, tensile properties, and dynamic torsional test data. In the impacted region of the equiaxed microstructure, a number of adiabatic shear bands and cracks were observed to be formed along plastic flow lines, and delamination occurred because of cracking along the plastic flow lines or shear bands. In the case of the bimodal microstructure, shear bands were found in limited area near the impacted region without occurring delamination, and their number was fewer than that of the equiaxed microstructure. Thus, ballistic performance of the bimodal microstructure was better than that of the equiaxed microstructure. The ballistic performance of the two microstructures was also discussed by comparing the possibility of adiabatic shear band formation obtained from dynamic torsional test data.

성형미세구조가 반응소결 탄화규소체의 소결미세구조 및 기계적 특성에 미치는 영향

박현철,김재원,백운규,최성철 한국세라믹학회 1999 한국세라믹학회지 Vol.36 No.1

탄화규소와 카본의 이성분계에서 pH, 계면활성제 및 출발원료의 입도 분포를 이용하여 성형체의 기공크기 및 기공을 제어에 관한 연구를 수행하였다. 성형체는 각각 다른 분산조건을 가지는 슬러리를 이장성형법을 이용하여 제조하였으며, 입자간의 응\ulcorner제어에 따른 상이한 성형미세구조를 가지도록 하였다. 제조된 성형체에 대하여 반응소결 공정은 1$600^{\circ}C$, 진공분위기에서 20분간 응용 실리콘 침윤 공정을 행하였다. 초기 성형 미세구조가 반응소결체의 미세구조에 미치는 영향을 관찰하기 위하여 광학 현미경 및 SEM 분석을 통하여 제조된 소결체의 미세구조를 분석하였으며, 그리고 이에 따른 반응 소결체의 기계적 특성을 평가하였다. 각각의 분산조건에 따라 성형미세구조는 다르게 나타났으며, 미세한 탄화규소를 입자를 사용하였을 경우에 기공크기가 현저하게 작아짐을 확인하였다. 제조된 반응소결체의 미세구조 분석결과 일반적인 반응소결 탄화수소에서 발견되는 bimodal 미세구조는 관찰되지 않았으며, 이는 초기 탄화규소의 카본의 비가 중요항 변수인 것으로 분석하였다. 반응소결 탄화규소의 3-점 곡강도 값은 입자간의 분산이 잘 이루어진 성형체를 사용하였을 경우 310$\pm$40 MPa으로서 응집이 일어난 성형체를 사용한 반응소결체의 260$\pm$50MPa 보다 높았다. In the binary system of SiC and carbon, porosity and pore size distribution of green body was controlled by varying pH, by the addition of polyelectrolyte dispersants, and by using different particle size of starting powders. The preforms having different green microstructure were fabricated by slip casting from suspensions having different dispersion condition. The reaction bonding process was carried out for these preforms. The condition of reaction bonding was 1600$^{\circ}C$ and 20 min. under vacuum atmosphere. The analyses of optical and SEM were studied to investigate the effect of green microstructure on that of reaction bonded silicon carbide and subsequently the mechanical properties of sintered body was investigated. Different green microstructures were obtained from suspensions having different dispersion condition. It was found that the pore size could be remarkably reduced for a fine SiC(0.5$\mu\textrm{m}$). The bimodal microstructure was not found in the present study, which is frequently observed in the typical reaction bonded silicon carbide. It is considered that the ratio between SiC and C was responsible for the formation of bimodal microstructure. For the preform fabricated from the well dispersed suspension, the 3-point bending strength of reaction-bonded silicon carbide was 310${\pm}$40 MPa compared to the specimen fabricated from relatively agglomerated particles having lower value 260${\pm}$MPa.

고강도 API 라인파이프강의 저온 인성에 미치는 베이나이트계 미세조직의 영향

이승완,이상인,황병철 대한금속·재료학회 2020 대한금속·재료학회지 Vol.58 No.5

In this study the correlation between bainitic microstructure and the low-temperature toughness of high-strength API pipeline steels was discussed in terms of crack initiation and propagation in the microstructure. Three types of API pipeline steels with different bainitic microstructures were fabricated using varying alloying elements and thermo-mechanical processing conditions, and then their microstructure was characterized by optical and scanning electron microscopy, and electron backscatter diffraction (EBSD). In particular, the effective grain size and microstructure fraction of the steels were quantitatively measured by EBSD analysis. Although all the steels were composed of polygonal ferrite (PF), and complex bainitic microstructures such as acicular ferrite (AF), granular bainite (GB), and bainitic ferrite (BF), they had different effective grain sizes and microstructure fraction, depending on the alloying elements and thermomechanical processing conditions. Charpy impact test results showed that when the martensite-austenite constituent fraction was lowest, it resulted in higher upper-shelf energy, and absorbed energy at room temperature due to the decrease in crack initiation. In contrast, excellent low-temperature toughness, such as lower ductile-brittle transition temperature and higher absorbed energy at low temperatures, could be achieved with a bainitic microstructure with fine effective grain size and high fraction of high-angle grain boundaries, which act as obstacles to prevent cleavage crack propagation.

AISI 521000 강의 미세조직 변화(펄라이트와 베이나이트)에 따른 미끄럼마멸 특성

윤나래(N. R. Yoon),김종철(J. C. Kim),김용석(Y.-S. Kim) 한국소성가공학회 2010 한국소성가공학회 학술대회 논문집 Vol.2010 No.5

Dry sliding wear behavior of AISI 52 100 steel was characterized to explore the effect of microstructure on the wear of the steel. Isothermal heat treatment of the bearing steel was carried out to obtain full pearlitic and bainitic microstructures. Pin-on-disk wear tests of the steel were performed at a fixed load of 100 N in the air. Sliding speed and wear distance were 0.1 m/sec and 300 m, respectively. The alumina ball was utilized as a counterpart. Worn surfaces, wear debris and cross-sections of the worn surfaces were examined with SEM to investigate the wear mechanism of the steel. Tensile properties and hardness of the steel were also evaluated. Wear rate of the steel was correlated with the measured mechanical properties and effect of the microstructure was explored. It was found that neither the hardness nor the strength solely does characterize the wear of the steel. The microstructure strongly influenced the wear rate, and the specimen with the pearlitic microstructure showed higher wear resistance than the bainitic microstructure.

상담학사전 편찬 연구 Ⅰ-미시구조를 중심으로-

김춘경,권희영,정종진,이윤주,김계원 한국상담학회 2012 상담학연구 Vol.13 No.2

본 연구는 각 학문분야별 전문 학술 용어 정리 및 사전편찬의 필요성에 대한 논의가 교육 현장이나 학계 내에서 활발하게 이루어지는 시점에서, 최근 들어 급격한 성장을 거듭하고 있는 상담학의 성과를 체계적으로 정리하기 위한 상담학사전 편찬에 대한 연구결과를 제시하였다. 상담학사전을 편찬하기 위한 여러 연구들 중에서 그 구조에 관한 연구는 거시구조와 미시구조로 나눌 수 있는데, 본 연구에서는 표제어의 표기, 개념 기술의 방법, 관련 정보의 표시 등의 미시구조를 중심으로 연구한 결과들을 제시하였다. 상담학사전의 미시구조에 대한 연구를 위해 2009년부터 2011년까지 다양한 문헌 자료를 수집하고, 기존에 출판된 상담관련 전문용어 사전들의 미시구조를 분석하였다. 또한 이러한 기초 조사를 바탕으로 상담학사전 연구팀의 상담관련 전문가 7명이 회의와 세미나 등을 통해 의견을 교환하였으며, 구체적이고 세부적인 미시구조의 지침들을 합의하에 확립하였다. 이렇게 합의된 상담학사전의 미시구조는 한글, 영어(원어), 한문 순의 표제어 표기, 실질적 정의항의 삽입, 하위 소 분야의 표시, 기술적 정의 그리고 참조정보의 표시로 요약될 수 있다. 또한 각 항목에는 사전 집필진들이 표제어의 의미기술 시 참고할 수 있는 세부적인 지침을 확정하여 제시하였다. 상담학사전의 미시구조를 과학적으로 연구하고, 그 구조를 체계화하여 사전 집필에 적용하는 작업은 상담학의 학문적 발전성과를 체계적으로 정리하기 쉽게 하고, 사전을 읽는 독자들에게 보다 용이한 개념 설명을 제시할 수 있도록 한다. 또한 상담학 관련 전문어를 정리하여 전자사전의 개발을 용이하게 하는 효과가 있다. As technical intellectual work of counseling now has become prolific and various, rapid evolution of technical terms needs to collect and manage them systematically and efficiently. This study intended to represent the results of research on the microstructure for systematic definitions of definiendum in terminological dictionaries of counseling. The research consisted of the writing system of headword, the description style of meanings and the mark of connected informations. In order to achieve the purpose of the study, documentary survey and analysing problems of microstructure in a current terminological dictionary of counseling from 2009 to 2011. As the result, the detailed microstructure of the counseling dictionary and adopted for compilation of encyclopedia of counseling. The developed microstructure would help to systematize of the present academic achievements of counseling and to offer easier understanding for the readers of the dictionary of counseling. Furthermore, the systemic microstructure of the dictionary could stimulate the development of the electronic dictionary.

Prediction of fracture toughness of ceramic composites as function of microstructure: I. Numerical simulations

Li, Y.,Zhou, M. Pergamon Press 2013 Journal of the mechanics and physics of solids Vol.61 No.2

The evaluation of macroscopic material parameters such as fracture toughness as functions of microstructural attributes is a fundamental issue in material science. The task requires the quantification of both microstructure and material response. Currently, no systematic approach other than experiments exists for establishing microstructure-fracture toughness relations for materials. In this paper, we present a multi-scale computational framework based on the cohesive finite element method (CFEM) for predicting fracture toughness of materials as a function of microstructure. This framework provides a means for evaluating fracture toughness through explicit simulation of fracture processes in microstructures. The approach uses the J-integral, allowing fracture toughness to be calculated for microstructures with random heterogeneous phase distributions and fracture processes with arbitrary crack paths or micro-crack patterns. Calculations carried out concern two-phase Al<SUB>2</SUB>O<SUB>3</SUB>/TiB<SUB>2</SUB> ceramic composites and focus on the effects of constitute behavior, phase morphology, phase distribution, phase size scale, and interphase bonding on fracture toughness. Results show that microstructure and constituent properties can significantly influence fracture behavior and combine to determine the overall fracture toughness through the activation of different fracture mechanisms. In particular, a combination of fine microstructure size scale, rounded reinforcement morphology, appropriately balanced interphase bonding strength and compliance can best promote desirable crack-reinforcement interactions and lead to enhanced fracture toughness.