발간논문

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Microstructural changes and variation in microhardness as a function of depth under the raceway during rolling contact fatigue of high-carbon chromium bearing steel (STB 2 or AISI 52100) have been studied to clarify rolling contact fatigue in the bearing steel. It was observed that dispersed spherical carbides in tempered martensitic matrix consisting of twinned martensite were produced by partial austenitizing, oil quenching and tempering of spheroidized annealed specimens. Platelike carbides were formed during rolling contact fatigue, at an angle of almost 90 deg to the rolling direction, along the depth of between 300 and 600 ㎛ below the raceway. The initial matrix subjected to cyclic rolling contact stress, transformed to ferrite and decayed martensite, resulted in local softening between 300 and 600 ㎛ below the raceway. In the present research, these structural changes and softening caused by rolling contact fatigue revealed that the region of maximum shear stress was the narrow zone between 300 and 600 ㎛ below the raceway.

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The purpose of this study is to explore the effects of the matrix alloy composition on fracture properties of the 2124 and the 2009 Al/SiCw composites and to attempt to assess the corresponding role of second phase particles such as coarse Mn-containing intermetallic particles. The contents of Mn and Fe were significantly reduced in the composition of the 2009 matrix alloy with respect to that of the 2124 alloy. Tensile tests, fracture tougness test and in-situ SEM fracture observations were conducted on these two composites. Experimental results indicated a higher toughness in the 2009 Al/SiC composite, although strengths were slightly decreased. The observations of fracture processes showed that coarse Mn-containing particles were cracked first and accentuated the formation of shear bands. Thus the improvement of fracture toughness in the 2009 Al/SiCw composite could be interpreted by the increase of the spacing between effective fracture initiation sites due to the absence of coarse Mn-containing particles. It is also suggested that another possible way of improving fracture toughness is to use SiC particulates as reinforcements instead of whiskers in order to modify the composite microstructure.

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The effects of melt flow rate and alloy composition on the microstructure of spray formed Al-Fe billets have been studied by using a combination of optical and transmission electron microscopy. Three transient regions in void distribution and solidification microstructure appear throughout the billet thickness : initial transient region dominated by chill effect by cold substrate ; quasi-steady state of microstructure beyond the initial transient region ; and final transient region due to decrease in melt flow rate. Large inter-connected void and fine microstructure developed first near the substrate and changes into different types of void and coarsened microstructure depending on thermal condition on the deposit top surface. In the deposit of Al-Fe alloy, monoclinic Al_(13)Fe₄precipitate is often observed with occasional observation of metastable orthorhombic Al_6Fe precipitate formed during flight.

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Wrought superalloy 718 is the most widely used nickel-base superalloy for high temperature applications. This study is aimed at illustrating deformation behavior and microstructural changes under hot forging conditions. Compression tests were conducted under argon atmosphere up to the strain 1.0, at temperatures between 930℃ and 1030℃, and at strain rates between l0^(-3)s^(-1) and 10¹s^(-1). Deformation mode is greatly affected not by the temperature, but by the strain rate under the test conditions. The specimens deform uniformly and dynamic recrystallization occurs prominently at the strain rates below 10^(-1)s^(-1). but the localized shearing governs the deformation mode and the microbands are formed at the strain rates above 10^(-1)s^(-1). To refine microstructures through dynamic recrystallization, the strain rate should be controlled to be below 10^(-1)s^(-1) and the temperature to be 930℃∼1030℃. The activation energy for dynamic recrystallization is 238∼325kJ/㏖.

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Effect of phosphorus on the creep-fatigue interaction of 304L stainless steel was investigated at the test temperature of 823K. The microstructural observation was very distinctive. As the content of phosphorus increases, the density of grain boundary carbide decreases. The results of creep-fatigue test for the three materials show that the effect of phosphorus is beneficial for the fatigue life under creep-fatigue loading condition. Since the segregation of phosphorus on grain boundary lowers grain boundary diffusivity and the density of grain boundary carbides, the nucleation site and growth rate of cavity is considered to be decreased as the content of phosphorus increases. Therefore, the addition of phosphorus to 304L stainless steel is believed to prolong the creep-fatigue life.

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The liquid phase diffusion bonding of the 2090 Al-Li alloy has been investigated and effects of surface treatment, composition of insert metals and homogenizing heat treatment on the microstructure of bonded interlayer have been studied. Main results obtained in this study are as follows. 1) Good joint can be obtained by using Al-10% Si-1.5% Mg alloy as insert metal, but the eutectic structures in the grain boundary of base metal are observed. 2) When no cleaned base metals were bonded at 793K using Al-4.9% Zn as insert metal, oxide film(β-Al₂O₃and α-LiAlO₂) was seperated from base metal and subsequently moved toward the center of bonded interlayer. Properties of joint were poor. In joint of acid-cleaned base metal, properties of joint were improved. 3) Al concentration at the center of bonded interlayer was very high, because of dissolution phenomenon of base metal by liquid insert metal. 4) When acid-cleaned base metal was bonded for 3.6ks at 793K and homogenized for 86.4ks at 813K, the composition of bonded interlayer was similar to that of base metal, but the hardness of bonded interlayer was even lower than that of base metal. If homogenized joints were reheated for 1.8ks at 813K and then quenched, the hardness of bonded interlayer equivalent to base metal has been achieved and properties of joint were reasonable.

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