Young-Hee Cho

The Effect of Alloy Addition on the High Temperature Properties of Over-Aged Al-Si(CuNiMg) Cast Alloys

The role of alloying elements in the improvement of the high temperature strength of Al-12Si(CuNiMg) cast alloys used for automotive piston applications was investigated. The addition of alloying elements such as Mn, Cr, Ti and Ge was studied and the detailed characterization of the composition and morphology of the constituent phases after over aging at 350 for 1000 hrs was performed. The compositions and volume fractions of the equilibrium phases determined by thermodynamic calculation were compared with the experimental results. The addition of transition elements, including Mn, Cr and Ti, increased the volume fraction of the intermetallic phases, which effectively enhanced the high temperature strength of the alloys. Among these transition elements, Mn turned out to be the most effective alloying element. After adding up to 0.5wt% of Mn, a large number of intermetallic phases, α-Al(Mn,Fe)Si as well as fine Al6(Mn,Fe) particles were precipitated and a significant improvement in the elevated temperature properties was achieved. The addition of Ge promoted the precipitation of the θphase (metastable phase, θ-Al2Cu), due to the formation of GeSi precipitates, thereby improved the mechanical properties of the alloy after T6 heat treatment. However, the presence of these GeSi precipitates did not affect the coarsening of the θ phase to form Qphase( Al5Cu2Mg8Si6) during aging and, thus, the elevated temperature properties were not improved by the addition of Ge.

Feasible process for producing in situ Al/TiC composites by combustion reaction in an Al melt

In situ Al/TiC composites with a homogeneous distribution of TiC reinforcements were prepared by adding a reactant mixture of Al-Ti-C to an Al melt. A certain amount of CuO addition facilitates a combustion reaction of the Al-Ti-C system and thereby enables the formation of in situ TiC at a reasonably low temperature range of 750–920 °C. Synthesised TiC particles with sizes of 1–2 μm are present in the Al matrix along with Al3Ti. Besides the CuO addition, the melt temperature plays a significant role in the final microstructure of the composites. Increase in the melt temperature up to 920 °C with CuO provides more external heat input and initiates the combustion reaction within a few seconds. Pellet microstructure evidently shows that the polygonal Al3Ti originates from the unreacted layer of which the distance is significantly shortens by increasing the melt temperature. The suppression of the Al3Ti formation is the most likely to occur at 920 °C, with producing a large volume fraction of TiC in situ synthesised.

Chymotrypsin linked to poly(ethylenimine) derivatives: Perturbation of lonization of active site groups

Abstract α-Chymotrypsin (ChT) is covalently linked to various poly(ethylenimine) (PEI) derivatives by using a carbodiimide as a coupling reagent. The PEI derivatives contain cationic or anionic microenvironments. In addition, hydrophobic microdomains are created on the PEI backbones in conjunction with the ionic environments. Stabilization of the tertiary structure of ChT by the cross-linking PEI derivatives is reflected by much greater resistance of the PEI-bound ChTs to thermoinactivation. Depending on the structural elements incorporated into the PEI derivatives, the activity of ChTs linked to PEI derivatives in either/both acidic or/and basic pH ranges is much greater than that of native ChT. The nature of microdomains introduced to the globular backbone of PEI affects the p K a values of the active-site groups of ChT sensitively. Cationic microenvironments created on the PEI backbone retards protonation of His-57 more than that of Ile-16. Anionic microenvironments of the PEI backbone stabilize the ammonium ion of Ile-16 more than the imidazolium ion of His-57. Moreover, substrate binding by the active site appears to render His-57 more sensitive to the change in the microenvironments.

Characterization of hydrogen sorption properties and microstructure of cast Mg-10wt%Ni alloys

New Mg-10wt%Ni hydrogen storage alloys were fabricated by casting which is a very simple and cost effective production process. Alloying elements such as Nb and Ti, which have relatively high melting temperatures and very low solubility in solid Mg, were successfully dissolved into the liquid Mg-Ni alloy. The Mg-Mg2Ni alloys contain a well-refined lamellar eutectic microstructure after solidification with a large interfacial area between the Mg and Mg2Ni phases which provides for good hydrogen sorption properties. This is considered to be due to the high diffusivity of hydrogen along the interphase boundaries. Addition of transition metals such as Nb and Ti results in the formation of intermetallic phases with a size about 10-20μm during solidification. Furthermore, Ti was found to be strongly segregated to the eutectic Mg-Mg2Ni interface. In the presence of Nb and Ti, the hydrogen sorption kinetics of the Mg-Mg2Ni alloy is further improved. This suggests that the transition metals act as active catalysts that eases and accelerates the hydrogen diffusion during hydrogenation and dehydrogenation. In this paper, we present the hydrogen storage properties and their relationship to the microstructure of the cast Mg-10wt%Ni alloys. Detailed microstructural analysis was carried out in order to further understand the hydrogen diffusion and storage mechanisms.

Influence of Ultrasonic Treatment on the Microstructure of Hypereutectic Al-17 Wt%Si Alloys

The effect of ultrasonic melt treatment (UST) on the microstructure of hypereutectic Al-17 wt.% Si alloys was investigated. UST applied to a melt at a temperature range of 750 - 800 °C refines the primary Si dramatically but has little influence on the grain size of primary Al. The solidification behaviour was characterised by thermal analysis and a mechanism responsible for the primary Si refinement was suggested. Whilst UST has no effect on the grain refinement, a significant increase in the matrix hardness as well as the tensile strength in the as cast condition is possibly associated with solid solution hardening. Detailed microstructure analysis was carried out and characteristics of the intermetallic formation in the Al-17 wt.% Si alloys were further discussed in the view point of Cu solubility in the Al matrix, which is considered to increase with UST.

Effects of processing parameters on the fabrication of in-situ Al/TiC composites by thermally activated combustion reaction process in an aluminium melt using Al-TiO 2-C powder mixtures

A feasible way to fabricate in-situ Al/TiC composites was investigated. An elemental mixture of Al-TiO2-C pellet was directly added into an Al melt at 800-920°C to form TiC by self-combustion reaction. The addition of CuO initiates the self-combustion reaction to form TiC in 1-2 μm at the melt temperature above 850°C. Besides the CuO addition, a diluent element of excess Al plays a significant role in the TiC formation by forming a precursor phase, Al3Ti. Processing parameters such as CuO content, the amount of excess Al and the melt temperature, have affected the combustion reaction and formation of TiC, and their influences on the microstructures of in-situ Al/TiC composites are examined. (Received March 15, 2012)

Two-step infiltration of aluminum melts into Al-Ti-B4C-CuO powder mixture pellets

Aluminum matrix composites with a high volume fraction of B4C and TiB2 were fabricated by a novel processing technique - a quick spontaneous infiltration process. The process combines a pressureless infiltration with the combustion reaction of Al-Ti-B4C-CuO in molten aluminum. The process is realized in a simple and economical way in which the whole process is performed in air in a few minutes. To verify the rapidity of the process, the infiltration kinetics was calculated based on the Washburn equation in which melt flows into a porous skeleton. However, there was a noticeable deviation from the calculated results with the experimental results. Considering the cross-sections of the samples at different processing times, a new infiltration model (two step infiltration) consisting of macro-infiltration and micro-infiltration is suggested. The calculated kinetics results in light of the proposed model agree well with the experimental results.

Sustaining the compact shape during the quick spontaneous infiltration process with Al-Ti-B4C-CuO powder mixtures

Aluminum matrix composites with a high volume fraction of reinforcements are fabricated using a quick spontaneous infiltration process through a combustion reaction of an Al-Ti-B4C-CuO powder mixture in molten aluminum. A cold-compacted powder mixture in a cylindrical shape was used as a preform. The effects of the composition of the initial powder mixture on the sustaining of the compact shape were systematically examined and thereby an optimal composition for fabricating sound aluminum matrix composites was suggested. The compact shape was greatly affected by the initial composition of the powder mixture. A sufficiently high relative volume fraction of the solid particles in the compact is critical for sustaining the compact shape during the combustion reaction. The volume fraction of Al and Ti powders in the initial mixture affects whether crumbling of the compact occurs during the ignition delay time. This study can be beneficial to utilizing the Al-Ti-B4C system as to fabricate components with any desired shape.

Catalysing Effect of Intermetallic Compounds on Hydrogen Desorption Kinetics in Cast Magnesium Alloys

Magnesium based hydrogen storage materials were prepared by a conventional melting and casting technique. Characterisation of microstructure and hydrogen sorption properties of the alloys was carried out. Additions of Al, Cu and Ni lead to the formation of eutectic mixtures, Mg-Mg17Al12, Mg-Mg2Cu and Mg-Mg2Ni, respectively, with an inter-lamellar spacing of a few hundred nanometers. 3d and 4d transition metals were also added to Mg based alloys and were found to form intermetallic compounds that were homogeneously dispersed in the alloys. The dehydrogenation rate of the Mg alloys was quantitatively analysed in order to determine the rate-limiting step for the hydrogen desorption kinetics. The catalysing role of each intermetallic compound for the hydrogen desorption kinetics is further discussed.

A Feasibility Study on Aluminum Nitride Pieces for an Application to the Nuclear Power Plant Flux Detector

The sensitivity of a solid-state flux detector is studied using a 3mm× 3mm× 0.635mm size aluminum nitride piece. We describe the results of the gamma measurements in the high flux Co 60 irradiation facility and the neutron measurements in a beam port of the HANARO research reactor where the gamma flux is about 1% of the neutron. The measured results are compared with the calculated results based on the MCNP4B and EGS4 code. The current generated by the neutron flux is calculated by using the MCNP4B code and the current generated by the gamma flux is computed by using the EGS4 code. The calculation results for the expected current in the full power PWR nuclear power plants are included.