기계적 합금화법으로 제조된 (Ti52Al48)100-xBx 합금의 미세조직 형성과정

표성규,김낙준,정현호,김상혁,황승준 대한금속재료학회(대한금속학회) 1998 대한금속·재료학회지 Vol.36 No.11

The main objective of this study is to investigate the microstructural evolution during mechanical milling of prealloyed (Ti_(52)Al_(48))_(100-x)B_x powders and also to investigate the effects of B contents and heat treatment on the microstructure of mechanically milled TiAl alloys. Microstructure of binary Ti_(52)Al_(48) Powders consists of grains of hexagonal phase whose structure is very close to Ti₂Al. (Ti_(52)Al_(48))_(95)B_5 powders contain TiB₂ in addition to matrix grains of hexagonal phase. The grain sizes in the as-milled powders of both alloys are nanocrystalline. The mechanically alloyed powders were consolidated by vacuum hot pressing (VHP) at 1000℃ for 2 hours, resulting in a material which is fully dense. Microstructure of consolidated binary alloy consists of γ-TiAl phase with dispersions of Ti₂AlN and A1₂O₃ phases located along the grain boundaries. Binary alloy shows a significant coarsening in grain and dispersoid sizes. On the other hand, microstructure of B containing alloy consists of γ-TiAl grains with fine dispersions of TiB₂ within the grains and shows the minimal coarsening during annealing.

기계적 분쇄화법으로 제조된 (Ti52Al48)100-xBx(X=0,0.5,2,5) 합금의 미세조직 및 기계적 성질

표성규,김낙준,정현호,황승준 대한금속재료학회(대한금속학회) 1998 대한금속·재료학회지 Vol.36 No.12

The present study is concerned with γ-(Ti_(52)Al_(48))_(100-x)B_x(x=0, 0.5, 2, 5) alloys produced by mechanical milling/vacuum hot pressing using melt-extracted powders. Microstructure of as-pressed alloys exhibit dual phase equiaxed microstructure of α₂ and γ with a mean grain size of 200 ㎚. Besides α₂ and γ phase, binary and 0.5%B alloys contains Ti₂AlN and Al₂O₃ phases located along the grain boundaries and show appreciable coarsening in grain and dispersoid sizes during annealing treatment at 1300℃ for 5 h. On the other hand, 2%B and 5%B alloys contains fine boride particles within the γ grains and show minimal coarsening during annealing. Room temperature compression tests of as-pressed alloys reveal low ductility, but very high yield strength >2100 ㎫. After annealing treatment, mechanically milled alloys show much higher yield strength than conventional PM and IM processed alloys, with equivalent ductility to IM processed alloys. 5%B alloy with smaller grain size shows higher yield strength than binary alloy at room temperature. When test temperature is increased to 850℃ 5%B alloy shows much lower strength than binary alloy, indicating the deformation of fine 5%B alloy is dominated by the grain boundary sliding mechanism.

용탕단조법으로 제조한 Ti 합금 강화 AM60 Mg 복합재의 미세조직 및 파괴거동

김재중,김낙준 대한금속재료학회(대한금속학회) 1999 대한금속·재료학회지 Vol.37 No.8

Microstructure and fracture behavior of Ti alloy reinforced AM60 Mg composite have been investigated in the present study. To clarify the fracture behavior, in-situ fracture test was conducted. The manufactured composite consists of 45.1 vol.% reinforcement and does not show the pore presence. No reaction layers or reaction products are observed at the Ti/Mg interface. However, there is a presence of fine Ti particles in the matrix. It has been suggested that the Ti particles form by diffusion during squeeze casting. It also has been shown that there is a diffusion of Mg into Ti. Microcracks are nucleated mainly along the reinforcement/matrix interface, but they are less sharp than those found in the ceramic reinforced composites. Considering the high volume fraction and large size of the reinforcement used in the present study, the manufactured composite shows the strong potential for further improvement in mechanical properties.

Strip Casting 법으로 제조된 304 Stainless Steel 의 응고 미세조직

김재중,표성규,김낙준 대한금속재료학회(대한금속학회) 1999 대한금속·재료학회지 Vol.37 No.10

A study has been made of the effects of strip casting process variables on the microstructure of strip cast SUS 304 stainless steel. The main impetus for the present investigation is to suggest the optimum process variables for the strip casting of SUS 304 stainless steel based on the systematic studies of microstructural evolution. Results slow that there is a variation of microstructure through the thickness of strip. Along the surface of strip, the microstructure consists of fine dendrite and cellular structure, and there is also a formation of α' martensite and mechanical twin due to the deformation by roll contact. On the other hand, microstructure in the middle of strip consists of equiaxed grains due to slow solidification rate and recalescence. One of the most important finding is that there is a depletion of Mn in the interdendrite region and the presence of amorphous Si-rich particle.

Melt-Overflow 법에 의해 제조한 Al-Fe-V-Si 합금의 미세조직 연구 (2) : 2차상 결정구조 및 상분해 거동 Crystallography and Phase Decomposition Behavior of the second Phases

고향진,김낙준 대한금속재료학회(대한금속학회) 1998 대한금속·재료학회지 Vol.36 No.12

Second phases in the melt-overflowed Al-Fe-V-Si alloy have been investigated, and found to be classified into three different types : e.g. icosahedral phase, hexagonal phase, and bcc α-AlFeSi. Crystallographic orientation relationships among the phases were determined by electron diffraction. All the phases have similar atomic arrangements, which are constructed by several (Fe+V) and (Al+Si) icosahedra connected through chains of (Al+Si) octahedra, and thereby providing close orientation relationships among them. Phase decomposition behavior of the second phases has been also investigated by means of hot-stage in-situ TEM technique. During heating icosahedral phase transforms to thermally more stable bcc phase, and such transformation is controlled by Si diffusion. On the other hand, transformation of the hexagonal phase to bcc phase is not controlled by diffusion.

PST-TiAl 금속간화합물의 변형 및 파괴거동의 이방성 : TEM 직접관찰

표성규,김낙준 대한금속재료학회(대한금속학회) 1998 대한금속·재료학회지 Vol.36 No.9

In-situ straining experiments in the TEM have been performed on thin foils of polysynthetically twinned(PST) crystals of Ti-49.3 at% Al. As expected, the deformation behavior of PST crystals of TiAl is anisotropic, depending on the angle between the lamellar boundaries and tensile loading axis. Shear deformation proceeds, in a zigzag fashion, across the lamellar boundaries when tensile loading axis is parallel to the lamellar boundaries. For tensile loading perpendicular to the lamellar boundaries, deformation occurs across the lamellar boundaries, whereas crack propagates along the lamellar boundaries. When tensile loading axis is inclined at an intermediate angle to the lamellar boundaries, shear deformation occurs parallel to the lamellar boundaries. The α₂ phase plays different roles depending on the orientation. Roles of the various types of interfaces in the locallized deformation and fracture behavior have been evaluated to understand the overall behavior of the PST crystals.

정밀주조된 Ti-6AI-4V 합금 용접부의 피로성질

이성학,김낙준,오진근,이의휘 대한금속재료학회(대한금속학회) 2001 대한금속·재료학회지 Vol.39 No.10

The effects of microstructural factors on high-cycle fatigue properties and fatigue crack propagation behavior of welded regions of a cast Ti-6Al-4V alloy were investigated in this study. High-cycle fatigue test and fatigue crack propagation test were conducted for the welded regions, which were processed by two different welding methods, i.e., tungsten inert gas (TIG) welding and electron beam (EB) welding, and then the test data were analyzed in relation to microstructures, tensile properties, and fatigue fracture mode. The base metal was composed of Widmansta¨tten structure, while the EB and TIG welds consisted of basket-weave type structure having thin α platelets. The high-cycle fatigue results indicated that fatigue strength of the EB weld was lower than that of the base metal or the TIG weld because of the existence of coarse pores formed during welding, although it had highest yield strength. In the case of the fatigue crack propagation, the EB weld composed of very thin α platelets had the faster crack propagation rate than the base metal or the TIG weld. The effective microstructural factor determining the fatigue crack propagation rate was found to be the width of α platelets because it was well matched with the reversed plastic zone size calculated at near-threshold ΔK.

초 고순도 TiAl 금속화합물의 미세조직 및 변형거동에 미치는 Al 조성의 영향

표성규,김낙준,오진근,Yamaguchi, M . 대한금속재료학회(대한금속학회) 2000 대한금속·재료학회지 Vol.38 No.5

The present study is concerned with the effect of Al composition on the deformation behavior of ultra high pure (UHP) Ti-(48,50,52) at.%Al alloys using ultra high pure Ti with 30 wt. ppm oxygen. It is suggested that the deformation mechanism of UHP binary TiAl is strongly dependent on Al compositions. The deformation substructure of UHP Ti-48 at.%Al is shown to be dominated by ordinary dislocation with b = 1/2$lt;11￣0] as well as deformation twinning and small portion of 1/2$lt;112￣] superdislocation. Deformation mechanism in UHP Ti-50 at.%Al alloy consists of 1/2$lt;11￣0] type ordinary dislocation and 1/2$lt;112￣] superdislocation and a small portion of $lt;01￣1] superdislocation, but deformation twinning is not observed. Most of dislocation structure in UHP Ti-52 at.%Al alloy consisted of the faulted dipoles. Major deformation mode of UHP Ti-48 at.%Al and UHP Ti-50 at.%Al alloys was ordinary dislocation in deformation orientation, which take an advantage of ordinary dislocation slip. However, the major deformation of UHP Ti-52 at.%Al alloys was superdislocation slip in this orientation. It seems that there is little difference between major deformation mode of nominal purity TiAl and that of TiAl contained ultra low oxygen.

Ti-(43-54)Al-xMn-yNb(x=0,3,4, y=0,4) 합금의 미세조직 및 변형거동 (1) : 미세조직 및 기계적 성질

표성규,김낙준,오진근 대한금속재료학회(대한금속학회) 1999 대한금속·재료학회지 Vol.37 No.11

The main objective of the present paper is to study the effect of heat treatment and temperature on mechanical properties of Ti-(43-54)Al-xMn-yNb (x=0, 3, 4, y=0, 4) alloys. The compressive yield strength as well as fracture strain was correlated with composition, microstructure, tetragonality, and strain rate. The controlled microstructures obtained by heat treatments were introduced in order to correlate the microstructural parameters with mechanical properties. The variation behavior of compressive yield strength of Ti_(49)Al_(51) base alloys showed the positive temperature dependence and the highest compressive yield strength among the measured six alloys was obtained in (Ti_(49)Al_(51))_(92)Mn₄Nb₄. The c/a ratio of (Ti_(53)Al_(47))_(97)Mn₃ alloy was measured using hot stage in-situ XRD technique at different temperatures. The c/a ratio was decreased significantly at 1273K. The c/a ratio of Ti-(43-54)Al-xMn-yNb (x=0, 3, 4, y=0, 4) base alloys also varied with the compositions. The compressive fracture strain of Ti_(53)Al_(47) base alloys was increased with decrease of the c/a ratio, while Ti_(49)Al_(51) base alloys had no correlation with the c/a ratio.

고강도 7075 Al 합금의 가공 열처리와 미세조직의 상관관계연구

표성규,김재중,김낙준 대한금속재료학회(대한금속학회) 1999 대한금속·재료학회지 Vol.37 No.8

The main objective of the present investigation is to study the effect of the thermomechanical treatment on the structure-property relationships of 7075 plate. Particular emphasis has been placed on clarifying the origin of the anisotropy of mechanical properties. It shows that the application of thermomechanical treatment improves the microstructure and overall tensile properties of 7075 plate. Grain size becomes finer and the morphology of coarse intermetallic particles become favorable as the 7075 plate is thermomechanically processed. This improvement of microstructure directly influences the tensile properties of 7075 plate. While the thermomechanically treated (TMT) 7075 plate don't show any significant change in tensile properties with specimen orientation, there exists a large variation of tensile properties, in especially elongation, of heat treated (HT) 7075 plate with specimen orientation. HT 7075 plate in LT orientation has a much lower elongation than that in I. orientation. Detailed fractographic study reveals that poor ductility in LT oriented specimen is due to the large effective grain size and also due to the occurrence of cleavage fracture of coarse intermetallic particles along grain boundaries. In short, it has been found that the application of thermomechanical treatment improves the tensile properties of 7075 plate and it can be directly applied for the alloy processing for forged harts.