• 비파괴검사학회지
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• 제28권6호
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• pp.477-482
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• 2008

The influence of dislocation substructure of metallic materials on ultrasonic velocity has been experimentally investigated. The test materials of pure Cu, brass (Cu-35Zn), 2.25Cr-1Mo steel, and AISI 316 with different stacking fault energy (SFE) are plastically deformed in order to generate dislocation substructures. The longitudinal wave velocit $(C_L)$ decreases as a function of tensile strain in each material. The $C_L$ of Cu-35Zn and AISI 316 decreases monotonously with tensile strain, but $C_L$ of Cu and 2.25Cr-1Mo steel shows plateau phenomena due to the stable dislocation substructure. The variation of ultrasonic velocity with the extent of dislocation damping and dislocation substructures is discussed.

• 비파괴검사학회지
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• 제31권1호
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• pp.32-39
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• 2011

본 연구에서는 Cu와 Cu-Zn 합금의 저주기 피로 동안 발달한 전위 하부조직의 변화를 비파괴적으로 구분하고 평가하고자 하였다. 비파괴시험으로 초음파속도, 전기비저항 그리고 양성자소멸시간을 측정하였다. 서로 다른 적층결함 에너지를 갖는 Cu와 Cu-Zn에 대해 반복피로시험을 수행하고 이들 재료에서의 전위거동과 비파괴평가 파라미터와의 상관성을 연구하였다. Cu는 전위셀 하부구조를 형성하였지만, Cu-Zn 합금은 피로 사이클에 따라서 전위밀도는 증가하고 단지 평면배열의 전위구조를 형성하였다. 상온에서의 반복적인 피로에 의해 발달한 격자결함인 전위와 공공으로 인해 초음파속도의 감소, 전기비저항의 증가 그리고 양성자 소멸시간이 증가하였다. 비파괴평가파라미터의 지속적인 변화를 보이는 평면배열의 전위구조를 갖는 Cu-Zn에서와 달리, Cu에서는 전위셀구조가 발달하면서 더 이상의 큰 변화를 보이지 않았다.

• Interaction and multiscale mechanics
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• 제1권4호
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• pp.437-448
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• 2008

This paper examined the stability of high-dense dislocation substructures (HDDSs) associated with martensite laths in High Cr steels supposed to be used for FBR, based on a series of dislocation dynamics (DD) simulations. The DD simulations considered interactions of dislocations with impurity atoms and precipitates which substantially stabilize the structure. For simulating the dissociation processes, a point defect model is developed and implemented into a discrete DD code. Wall structure composed of high dense dislocations with and without small precipitates were artificially constructed in a simulation cell, and the stability/instability conditions of the walls were systematically investigated in the light of experimentally observed coarsening behavior of the precipitates, i.e., stress dependency of the coarsening rate and the effect of external stress. The effect of stress-dependent coarsening of the precipitates together with application of external stress on the subsequent behavior of initially stabilized dislocation structures was examined.

• 열처리공학회지
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• 제10권2호
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• pp.81-92
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• 1997

The effect of alloying elements, precipitate size, its distribution, and dislocation substructure resulted from warm rolling or cold rolling in the superplastic Al-Mg alloy system was investigated. One of the major requirements for fine structure superplasticity is that the grain size should be very small. Fine grain structure is controlled by the dislocation substructure and the dynamic recrystallization during hot or warm working. The recovery of Al-Mg base alloys was constrained resulting in relatively high dislocation density when the alloys were warm rolled. In particular, Al-Mg-Zr alloy exhibited the smallest sub-grain size among Al-Mg alloys containing Mn, Cu, Zr as a third element. The Al-Mg-Mn alloy cold rolled 80% after hot rolling showed the maximun strain rate sensitivity exponent, m, of 0.75 under strain rate of $7.1{\times}10^{-4}/s$ at $500^{\circ}C$. The elongation of the alloys was limited in spite of high m values due to large dispersoids containing appreciable amount of Fe impurities.

• 비파괴검사학회지
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• 제28권3호
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• pp.285-291
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• 2008

We have experimentally investigated the cyclic deformation in copper using ultrasonic nonlinearity. The observation and characterization of dislocation substructure have been conducted using transmission electron microscope and electron backscattered diffraction technique. The ultrasonic nonlinearity (${\beta}/{\beta}_0$) was measured by the harmonic generation technique after various fatigue cycles. The microstructural effect on the nonlinearity was discussed regarding the extent of dislocation substructures evolved from low cycle fatigue. The ultrasonic nonlinearity of copper monotonically increased with the fatigue cycles due to the evolution of dislocation cell substructures.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2007년도 추계학술대회 논문집
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• pp.352-355
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• 2007

Polycrystalline materials such as steels(BCC) and aluminum alloys(FCC) show the strain hardening and the strain rate hardening during the plastic deformation. The strain hardening is induced by deformation resistance of dislocation glide on some crystallographic systems and increase of the dislocation density on grain boundaries or inner grain. However, the phenomenon of the strain rate hardening is not demonstrated distinctly. In this paper, tensile tests for various strain rates are performed in the rage of $10^{-2}$ to $10^2s^{-1}$ then, specimens are extracted on the same strain position to investigate the microscopic behavior of deformed materials. The extracted specimen is investigated by using the electron backscattered diffraction(EBSD) and transmission electron microscopy(TEM) results which contain grain size, grain shape, aspect ratio and dislocation substructure.

• 한국재료학회지
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• 제10권11호
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• pp.784-788
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• 2000

The Vickers microhardness was measured on the basal (0001) plane of sapphire single crystals in the temperature range from 25$^{\circ}C$to 1000$^{\circ}C$. The substructure surrounding the indents was investigated using selective chemical polishing and etching, optical microscopy, and trasmission electron microscopy (TEM). At room temperature, cracks were predominant, and at intermediate temperatures (400$^{\circ}C$and 600$^{\circ}C$), extensive rhombohedral twinning was observed. On the other hand, at higher temperatures, prism plane slip bands on prism plane {1120}(원문참조) were dominant in the microstructure. TEM observations revealed that the dislocation substructure at the vicinity of the indents consisted of fairly straight dislocations lying in basal and/or prism planes and aligned along the <1100> and <1120> directions. The details of the glide dissociation of perfect <110> screw dislocations into three collinear 1/3<1100> partials on the prism plane and the Peierls potential for sapphire single crystals were discussed.

• 한국정밀공학회지
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• 제25권7호
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• pp.131-138
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• 2008

The microstructural effects on the ultrasonic nonlinearity were investigated in isothermally degraded ferritic 2.25Cr-1Mo steel and low cycle fatigued copper. The variation in ultrasonic nonlinearity (${\beta}/{\beta}_0$) was interpreted as resulting from microstructural changes supported by the electron microscopy and X-ray diffraction, in addition to the mechanical test (Victor's hardness and ductile-brittle transition temperature). The ultrasonic nonlinearity of 2.25Cr-1Mo steel increased abruptly in the initial 1,000 h of degradation, and then changed little due to the coarsening of carbide and precipitation of stable $M_6C$ carbide during isothermal degradation. The ultrasonic nonlinearity of copper increased with the fatigue cycles due to the evolution of dislocation cell substructure.

• 비파괴검사학회지
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• 제30권5호
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• pp.444-450
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• 2010

본 연구에서는 전류 4단자 전위차법을 이용한 전기비저항을 측정하여 반복피로손상을 받은 구리와 구리합금의 미시적 열화를 평가하였다. 서로 매우 다른 적층결함 에너지를 갖는 구리(Cu)와 구리합금(Cu-35Zn)에 대해 반복피로손상을 가하고 이들 재료에서 발달한 전위구조와 전기비저항 간의 관계를 연구하고자 하였다. Cu는 전위셀 하부구조를 형성하였지만, Cu-35Zn 합금은 피로사이클에 따라서 전위밀도는 증가하고 평면배열의 전위구조를 형성하였다. 전기비저항은 두 재료 모두에서 피로변형 초기 단계에서 급격하게 증가하였다. 더욱이, 피로시험 후 구리는 약 7 % 그리고 구리 합금은 약 6.5 % 변하였다. 이러한 일관적인 결과들로부터, 반복적인 피로에 의해 발달한 전위 셀구조는 평면배열의 전위구조보다도 전기비저항에 매우 민감한 것으로 판단된다.

• 열처리공학회지
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• 제10권3호
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• pp.188-197
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• 1997

Effect of martensite morphology on damping capacity and hardness of Fe-Ni-C martensitic alloys were studied. The morphologies of martensite such as lath, butterfly, lenticular and thin plate were prepared by adjusting nickel content and austenite grain size. The hardness increased in order of lath, lenticular, thin plate, butterfly. The damping capacities of the lath and butterfly martensites were higher than those of the other two morphologies, indicating that the dislocation substructure is more effective in the damping capacity than the twin substructure. Especially, the butterfly martensite showed the highest damping capacity among these morphologies because of presence of not tangled but free dislocations in there.