Joon-Woong Noh

The effect of yttrium oxide on the sintering behavior and hardness of tungsten

In this study, the relative density and hardness of Y2O3 dispersed tungsten alloy were investigated as functions of the Y2O3 content and sintering temperature. The sintering temperature and the amount of the second phase were varied from 1800 to 2500°C and 0 to 2.0 weight pct, respectively. The relative density of the alloys is higher than that of pure tungsten in the range from 2000 to 2500°C, whereas the density is lower at 1800°C. As the Y2O3 content increases, the sintered density increases at a given temperature. The transition temperature (Ttr), where the relative density of the alloys exceeds that of pure tungsten, is reduced with increased Y2O3 particle content. In order to examine the effect of the second phase on the mechanical property, the hardness of pure tungsten and the alloys are measured. The hardness is mainly dependent upon the relative density of the alloys, rather than the amount of the second phase and tungsten grain size. The relationship between hardness and density is discussed according to the plasticity theory of porous materials.

Matrix penetration of w/w grain

The matrix phase of 93W-5.6Ni-l.4Fe heavy alloy has penetrated into W/W grain boundaries during a newly developed heat treatment which consists of repeated isothermal holdings at 1150 °C and water quenching between them. With the matrix penetration, the impact energy increases from 57 to 170 J, whereas the ultimate tensile strength and elongation remain un- changed. The tensile fracture surface of specimens appears similar regardless of the matrix penetration: predominant cleavage of W grains and dimple fracture of matrix. The fracture sur- face by impact, on the other hand, reveals a change of W/W interface failure toward ductile failure of penetrated matrix with many dimples by increasing the matrix penetration. The in- crease in the fraction of ductile failure and observed crack blunting is attributed to the increase in impact energy with the matrix penetration.

Dynamic and quasi-static torsional behavior of tungsten heavy alloy specimens fabricated through sintering, heat-treatment, swaging and aging

Abstract Dynamic and quasi-static torsional deformation behavior of a 93W–4.9Ni–2.1Fe tungsten heavy alloy (WHA) was investigated in order to evaluate the possibility of forming adiabatic shear bands. Dynamic torsional tests were conducted using a torsional Kolsky bar for the four WHA specimens fabricated through each processing condition, e.g. sintering, heat-treatment, swaging, and aging, and then the test data were compared with those of the quasi-static tests. The dynamic torsional test results indicated that the shear stress increases, while the shear strain decreases, with the maximum shear stress increasing in the order of the as-sintered, the as-heat-treated, the as-swaged, and the as-aged specimens. The as-swaged and the as-aged specimens showed a higher possibility of the adiabatic shear band formation because of the abrupt drop of shear stress between the maximum shear stress and the final fracture points and because of the radical reduction of shear strain at the maximum shear stress point. The observation of the deformed areas of the dynamically fractured torsional specimens revealed that the shear deformation was homogeneously distributed in a wide area in the as-sintered and the as-heat-treated specimens, whereas it was concentrated on the central area of the gage section in the as-swaged and the as-aged specimens. This torsional behavior correlated well with the shear stress–shear strain curves, suggesting that the torsional Kolsky bar technique is a good tool for helping evaluate the possibility of the adiabatic shear band formation.

Collapse of interconnected open

The closing behavior of interconnected open pores has been investigated in W-Ni systems where the dominant sintering mechanism is grain boundary diffusion. The samples were prepared by conventional powder metallurgy technique with large W particles containing Ni contents of 0.03 to 0.1 wt pct. The microstructure analysis indicated that pore collapse predominantly occurs at the three grain junction midpoints and produces closed pores at the four grain corners. The observed critical porosity, below which all the pores exist in isolated form, was found to be about twice as great as the critical porosity predicted by morphological perturbation theory. From these results, it is concluded that the grain boundary diffusion flux across the pore surface, as well as the capillary force, plays an important role in the closing behavior of interconnected open pores.

Udulation of W/matrix interface by resintering of cyclically heat-treated WNiFe heavy alloys

Abstract When liquid-phase sintered W Ni Fe alloys were cyclically heat-treated at 1100 °C adn resintered at 1485 °C, undulation of W/matrix interface resulted. The irregularity of the interface increased with the number of heat-treatment cycles. The residual thermal stress of W grains measured by X-ray diffraction increased with the number of heat-treatment cycles and exceeded the yield stress of W single crystal in certain crystallographic directions. A calculation by the finite element method also showed non-uniform distribution of thermal stress on W grains. Local yielding of W grains is believed to occur during the cyclic heat treatment. The observed undulation of W/matrix interface appears therefore to result from preferential dissolution of material from regions with higher strain energy and precipitation of material at regions with lower strain energy at the resintering temperature. The undulation disappeared with the grain growth during prolonged resintering.

Microstructural Evolution during Hot Deformation of Molybdenum using Processing Map Approach

Abstract The hot deformation characteristics of pure molybdenum was investigated in the temperature rangeof 600~1200°C and strain rate range of 0.01~10.0/s using a Gleeble test machine. The power dissipation map forhot working was developed on the basis of the Dynamic Materials Model. According to the map, dynamic recrys-tallization (DRX) occurs in the temperature range of 1000~1100°C and the strain rate range of 0.01~10.0/s, whichare the optimum conditions for hot working of this material. The average grain size after DRX is 5 μm. The mate-rial undergoes flow instabilities at temperatures of 900~1200°C and the strain rates of 0.1~10.0/s, as calculated bythe continuum instability criterion. Keywords : Molybdenum, Hot working, Processing map, Dynamic recrystallization 1. 서론 몰리브데늄은 밀도(10.20 g/cc)가 높고 음속(5.0km/s)이 매우 빠르기 때문에 기존 구리를 대체할 차세대 성형작약 라이너 소재로 연구되고 있다[1]. 현재까지 개발된 몰리브데늄 라이너 제조 방법은 단결정, 고에너지 변형성형(high energy rate forming),단조, 압출 및 열간등압성형(hot isostatic pressing)등이 있다[1, 2]. 이 중에서 열간 단조 공정 후, 열처리를 통해 미세조직을 제어하는 기술이 가장 일반적으로 사용되고 있다. 그러나 몰리브데늄의 경우, 상온취성이 있고 강도가 매우 높기 때문에, 냉간이 아닌열간 성형 방법으로 단조를 수행하여야 한다[3]. 일반적으로 성형성(workability)이란, 단조, 압출, 압연 등과 같은 가공 공정 중에 균열이 발생하지 않고쉽게 변형되는 정도, 즉 소성 변형의 용이성을 말한다[4]. 이러한 성형성은 응력 상태(state-of-stress)와고유(intrinsic) 특성으로 구분된다. 우선 전자의 경우,소재의 내부 특성, 즉 미세조직 등과는 무관하며, 변형 영역 및 금형 형상 등과 관계가 있다. 따라서이는 가공 공정 방법에 따라 크게 차이가 나게 된다.또한 후자의 경우, 특정 변형 조건에서의 미세조직변화와 관계가 있으며, 모재의 초기 미세조직과 큰연관성이 있다. 최근에는 고유 성형성을 이론적으로해석하기 위해 변형지도(processing map)이라는 개념이 도입되어, 현재 많은 소재에 적용되고 있다. 이개념은 Frost와 Ashby[5]에 의해 처음 도입 되었으며, 주로 낮은 변형률 속도에서의 크립 거동을 해석*Corresponding Author : [Tel : +82-42-821-2909; E-mail : ymkim78@yahoo.com]