Papers by Author: Ji Soon Kim

Abstract: Al-Fe-Y amorphous alloys of Al₈₄Fe₁₆, Al₈₂Fe₁₈ and Al₈₂Fe₁₆Y₂ composition were prepared by mechanical alloying in a planetary ball mill P100. A nearly complete amorphization could be achieved for the Al₈₄Fe₁₆, Al₈₂Fe₁₈ and Al₈₂Fe₁₆Y₂ powder alloys after 100h of milling at a rotational speed of 350 rpm in hexane medium. Differential scanning calorimetry (DSC) analyses revealed three-stage crystallization processes for Al₈₂Fe₁₈ and Al₈₂Fe₁₆Y₂ alloys and four-stage crystallization processes for Al₈₄Fe₁₆ alloy, respectively. Taking into account the DSC data, the thermal stability increased in the order of Al₈₄Fe₁₆, Al₈₂Fe₁₈, and Al₈₂Fe₁₆Y₂ composition. The Al₈₂Fe₁₆Y₂ alloy exhibited a relatively better thermal stability than the other two alloys.

Abstract: In this study, Cu-TiC nanocomposites were fabricated from a mixture of Cu and 5÷15% wt. TiC powders by ball milling and subsequent spark-plasma sintering. The morphology of Cu-TiC nanocomposite powders were observed by field emission scanning electron microscopy. Only characteristic peaks of Cu and TiC phases were detected from X-ray diffraction patterns of milled powder mixture. Sintered compacts showed a highly densified compacts (∼95% relative density) while retaining fine grains in the matrix. The hardness, wear resistance, and fracture surface of the sintered specimens were also investigated.

Abstract: Preparation of titanium diboride reinforced copper matrix composites with high conductivity and mechanical strength was developed based on in situ produced powders. The effect of the titanium diboride content on the mechanical properties of the bulk material produced from the powders by Spark Plasma Sintering technique was studied. Increasing titanium diboride content from 2.5 up to 7.5 wt.% resulted in a 1.5-fold increase in yield strength, tensile strength and hardness and 5-fold increase in wear resistance with only 10% decrease in conductivity.

Abstract: An attempt was made to produce nickel aluminides with fine microstructure from nanoscale nickel and aluminum powders produced by wire electric explosion (WEE). The powders were mixed and spark plasma sintered (SPS) for simultaneous reaction and densification of the material. Various mixing methods were tested to give the best homogeneity of the nanopowders mixtures. Utrasonic pre-treatment of the powders did not result in any positive effect on mixing homogeneity. Sintered compact contained NiAl and Ni3Al phases, showed fine grain size and unexpectedly high hardness possibly due to the presence of aluminum oxide in the starting nanopowders.

Abstract: Phase transformation induced by ball-milling was studied in this work. It was found that amorphous Fe90Zr10 ribbons undergo crystallization into BCC α-Fe(Zr) under milling in an AGO-2 mill. The decomposition degree of the amorphous phase increased with increasing milling time and intensity. Analyses of samples milled at different speeds suggested that the observed crystallization is a deformation-induced process rather than a thermally induced one. In addition, the decomposition behavior of a FeSn intermetallic under ball-milling was carefully studied. Upon milling a large amount of the FeSn intermetallic decomposed into Fe5Sn3 and FeSn2, where the average grain size of the product phases stayed nearly constant with milling-time. It is suggested that the mechanically driven decomposition of FeSn results from local melting of powder particles due to high temperature pulses during ball collisions.

Abstract: In the present work, Cu-TiB2 nanocomposite powders were synthesized by combining high-energy ball-milling of Cu-Ti-B mixtures and subsequent self-propagating high temperature synthesis (SHS). Cu-40wt.%TiB2 powders were produced by SHS reaction and ball-milled. The milled SHS powder was mixed with Cu powders by ball milling to produce Cu-2.5wt.%TiB2 composites. TiB2 particles less than 250nm were formed in the copper matrix after SHS-reaction. The releative density, electrical conductivity and hardness of specimens sintered at 650-750°C were nearly 98%, 83%IACS and 71HRB, respectively. After heat treatment at 850 to 950°C for 2 hours under Ar atmosphere, hardness was descedned by 15%. Our Cu-TiB2 composite showed good thermal stability at eleveated temperature.

Abstract: Dispersion-strengthened copper with TiB2 was produced by ball-milling and spark plasma sintering (SPS).Ball-milling was performed at a rotation speed of 300rpm for 30 and 60min in Ar atmosphere by using a planetary ball mill (AGO-2). Spark-plasma sintering was carried out at 650°C for 5min under vacuum after mechanical alloying. The hardness of the specimens sintered using powder ball milled for 60min at 300rpm increased from 16.0 to 61.8 HRB than that of specimen using powder mixed with a turbular mixer, while the electrical conductivity varied from 93.40% to 83.34%IACS. In the case of milled powder, hardness increased as milling time increased, while the electrical conductivity decreased. On the other hand, hardness decreased with increasing sintering temperature, but the electrical conductiviey increased slightly

Abstract: TiB2-43vol.%Cu nanocomposite powders with titanium diboride particle size 50-100 nm were cold and detonation sprayed in order to fabricate coatings on a copper substrate. The powders were produced by self-propagating high-temperature synthesis (SHS) followed by mechanical milling. The temperatures during spraying were calculated and the change in the nanostructure of the powders during spraying was studied: in cold sprayed coatings the size of TiB2 particles was well retained, in detonation sprayed coatings the growth of the particles was observed, the mode of spraying greatly affecting the microstructure and the size of the particles. The hardness of cold sprayed coatings was higher compared to detonation sprayed coatings. This research shows the future potential for development of coatings with submicron and nanostructure by cold and detonation spraying of powders produced by mechanical milling.

Abstract: TiB2-Cu composites in a nanostructured state are candidates for high-strength conductive and erosion-resistant materials. In this work, we studied formation of nanostructured TiB2-Cu composites under shock wave conditions. We investigated the influence of preliminary mechanical activation (MA) of Ti-B-Cu powder mixtures on the peculiarities of the reaction between Ti and B under shock wave. In the MA-ed mixture the reaction proceeded completely while in the nonactivated mixture the reagents remained along with the product – titanium diboride. The size of titanium diboride particles in the central part of the compact was 100-300 nm. This research shows that shock wave synthesis in mechanically activated powder mixtures with simultaneous compaction of the composite is a promising way to materials with submicron and nanostructures.

Abstract: The aging behavior of sintered Al composites with various ceramic contents was investigated. 2xxx series blended powder was used as the starting powder. Ceramic contents were 0 wt.% and 5 wt.%. The blended powders were compacted at 250 MPa. The sintering process was performed at 620oC for 60 min in a N2 atmosphere. After sintering, each part was solution-treated at 518oC for 60 min and aged at 180oC. The Rockwell hardness value at the peak aging time increased with increased ceramic contents. However, the peak aging time at maximum hardness was reduced with increased ceramic contents. The changes in aging behavior may have been caused by the acceleration of diffusion due to the strain induced by differences in the thermal expansion coefficients of the matrix and the ceramic phases.