발간논문

Home > KJMM 논문 > 발간논문

For a better understanding of the elastic behaviour of the crystalline aggregate of cold-rolled steel having preferred orientations, the lattice strains of ideally oriented crystallite groups under uniaxial loading and residual microstresses after unloading have been determined by x-ray measurements. It is necessary to understand the elastic behaviour of a single crystal in a polycrystalline aggregate in order to understand the problems with the X-ray elastic constants and furthermore the origin of the residual microstresses after plastic deformation. The elastic and plastic anisotrophies of the crystallites of the textured steel are observed in the curves of the applied stresses versus the lattice strains of the crystallite groups. Internal microstresses of one of the crystallite groups of the textured steel under various uniaxial loadings and the residual microstresses after unloading could have been determined by the X-ray stress measurements. A model is proposed to present a better understanding of the elastic behaviour of the crystalline aggregate. From the analysis by the model it is seen that the elastic behaviours of the crystalline aggregate of the cold-rolled steel for the rolling and transverse directions are different. The difference may explain the dependence of Young`s modulus on the relative direction of the sample cut to the rolling direction of cold-rolled materials.

keyword :

The objective of the present study is to investigate the adiabatic shear band of an HY-100 steel. This steel was deformed at high strain rate by ballistic impact and the adiabatic shear bands revealed white-etching bands with evidence of highly deformed flow connected with the cracks near :he impacted region. Transmission electron microscopy was used to investigate the dynamic transformation mechanisms including dynamic deformation and phase transformation in the adiabatic shear bands. The shear band microstructure is composed of very fine and highly dislocated equiaxed cells, together with coarser and dislocated grains with diffused boundaries, while thin elongated grains are found in the interface region between the matrix and the shear band. These microstructures are formed in an extremely short time by the combined effects of the temperature rise and highly localized deformation in the adiabatic shear band, suggesting that formation of adiabatic shear band might be explained by two possible mechanisms, i.e., a dynamic strain-induced transformation process and a dynamic martensite nucleation process from heavily deformed austenite.

keyword :

The impurity levels, shapes and solidification characteristics of PREPed Ti-51at%Al powders were investigated. Oxygen and carbon contents were varied very much with particle size. The appearances and microstructures of powders were found to be depended on cooling rate which is in turn particle size. The estimated cooling rate of powders ranging 200 to 50㎛ in size was 1.6×10⁴∼1.7×10^5 K/s. The coarse powders were solidified as primary α dendrite. From about 30% of ◎50㎛ in size martensite -like microstructure was found, which was believed to result from the solid state transformation of primary β phases during cooling.

keyword :

The influence of initial pH on the formation of iron oxyhydroxide has been investigated. As the initial pH of reactant solution increased, β-FeOOH(pH=4.5), mixture of β-FeOOH and α-FeOOH(4 5$lt;pH$lt;5.5), mixture of α-FeOOH and Fe₃O₄(6.4$lt;pH$lt;12), Fe₃O₄(pH=12), α-FeOOH(pH$gt;13) were formed in order. At the end of reaction, the product was strongly acidic(pH$lt;3) when the initial pH was lower than 13. When the initial pH was higher than 13, the product was strongly basic(pH$gt;12). In order to synthesize only iron oxyhydroxide, we propose either precipitation after the oxidation of dissolved ion is completed perpectly or oxidation simultaneously with the precipitation. Oxidation of oxygen promotes not only the oxidation of ferrous ion but also dissolution and precipitation in the range of strong basicity.

keyword :

In this study, experimental study on the atomization process is taken. Variations of melt flow rate and partice size distribution with operating conditions such as aspiration pressure, gas and melt flow rate, outer diameter and protrusion length of orifice and gas pressure have been studied. The melt flow rate is determined mainly with aspiration pressure P_a and expressed as following : mfr=K√2πD²_i/4·[gh+·P_a/p]^½, where K is constant, D_i, g, h, and p is orifice inner diameter, gravitational acceleration, height of melt head and density of the melt respectively. With increasing protrusion length, the shape of free film has changed from conical to cylindrical form, and the spray cone become more stable. The thinner the free film, the finer powder is obtained. Powder size decreases with increasing the ratio of gas folw rate to melt flow rate and the outer diameter of orifice, and with decreasing the protrusion length of orifice.

keyword :

The texture of Zircaloy-4 sheets induced by phase transformation was analyzed by Kearns number f_n and CODF(crystallite orientation distribution function). In order to improve the accuracy of the program which computers CODF, a two-step method for calculating the coefficients W_(lmn) of CODF from the incomplete pole figure data was introduced into the program. The odd-1 terms which cannot be acquired from the pole figure data were calculated by the Gaussian model function. The texture change accompanying phase transformation was studied with the misorientation distribution function(MODF) An MODF analysis revealed that the non-diffusional transformation phase had an orientation relationship with the diffusional transformation phase which formed before the non-diffusional transformation. The orientation relationship was attributed to the variants selection of transformation due to the anisotropy of thermal contraction, which could be explained in terms of the Schmid factor.

keyword :

Microstruture of squeeze cast 2024/20% SiC whisker composite was studied using transmission electron microscopy with energy dispersive spectrometry. Microstructure was oberserved at as-cast, solution treated. In as-cast condition, S`, θ`, S phase was observed in the matrix and at the interface Al_(15)(Fe, Mn)₃Si₂, θ, S phase was observed. After solution treatment, previous phase in the matrix were dissolved and Al_(20)Cu₂Mn₃ shape precipitate was observed in the matrix. Al_(15)(Fe, Mn)₃Si₂ phase which was formed in the casting stage was unchanged and Al_(20)Cu₂Mn₃ precipitate was observed at the interface.

keyword :

The main objective of this study is to investigate the effect of interface characteristics on the fracture behavior in a SiC continuous fiber reinforced 6061 Al composite. Microstructure observations, SEM in-situ fracture tests and AES analyses were conducted on the composite to identify the microfracture processes. Detailed microstructural analyses showed that Al matrix contained a significant amount of coarse second phase particles near the matrix/fiber interfacial region. It was also found from in-situ observations of crack initiation and propagation that the interfacial debonding occurred at the interfaces between the SiC fiber and the carbon coating layer in the early loading stage, providing easy crack propagation path. In order to improve mechanical properties of the Al/SiCr composite, thus, it is suggested that a new fiber coating method should be developed during the composite processing to prevent the critical interfacial debonding.

keyword :

CVD blanket tungsten films have been formed by the hydrogen reduction of WF_6 in a high pressure regime (10∼20 torr) at temperatures of 300∼400℃. The effects of pressure and reaction temperature on the deposition rate and the characteristics of the films were investigated. The deposition rate of tungsten increased with the total pressure in the reactor at pressures below 40 torr, while it decreased when the total pressure exceeded 40 torr. The deposition rate also showed a maximum value at 360℃ regardless of the pressure. Films with a good step coverage have been also formed at 40 torr and 360℃. It was found that the deposition mechanism changes with pressure and temperature : at low pressures and low temperatures, surface reaction determines the overall reaction rate, while at high pressures(＞40 torr) and high temperature(＞360℃), the deposition is controlled by the transportation of reactive gas molecules.

keyword :

W heavy alloys with different composition of matrix were sintered at various temperatures to investigate the microstructure and mechanical properties. The Ni/Fe ratios in the composition of 93W-7 (Ni-Fe) heavy alloys were varied from 7/3 via 8/2 to 9/1 by weight. The results showed that, with increased in Ni contents, the volume fraction of matrix was increased due to increased solubility of W in the matrix. In trun, it lowered dihedral angle and facilitated matrix peneratration along the grains to lower contiguity. The increased volume fraction of ductile matrix and decreased contiguity contributed to a higher ductility and toughness. The specimen with 9/1 of Ni/Fe ratio showed the lowest impact energy even though it had the largest volume fraction of matrix and the lowest contiguity. This was attributed to the highly dissolved W in the matrix. The optimum composition in the matrix is 80% Ni and 20Fe.

keyword :