• Journal of Biomedical Engineering Research
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• v.15 no.4
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• pp.389-394
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• 1994

The age-hardening behavior and the structural changes in a commercial dental Au-Ag-Cu-Pd alloy were investigated by means of hardness test, optical and scanning electron microscopic observation, energy dispersive spectroscopy and X-ray diffraction study. The drastic reduction in hardness by prolonged aging occurred after a rapid increase in hardness at the initial stage by the isothermal aging at $350^{\circ}C$. This softening was due to the broad precipitates formation of the lamellar structure which was composed of the f.c.t. AuCu I ordered f.c.t. phase containing Pd and the f.c.c. Ag-rich $\alpha$1 solid solution f.c.c.phase containing Au.

• Journal of Korea Foundry Society
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• v.37 no.6
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• pp.207-216
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• 2017

This study investigated the effects of the addition of Zn, Ca, and SiC on the microstructure and mechanical properties of Mg-Al alloys. The tensile properties of homogenized Mg-xAl (x = 6, 7, 8, and 9 wt.%) alloys increased with increasing Zn content by the solid-solution strengthening effect. However, when the added Zn content exceeded the solubility limit, the strength and ductility of the alloys decreased greatly owing to premature fracture caused by undissolved coarse particles or local melting. Among the Mg-xAl-yZn alloys tested in this study, the AZ74 alloy showed the best tensile properties. However, from the viewpoints of the thermal stability, castability, and tensile properties, the AZ92 alloy was deemed to be the most suitable cast alloy. Moreover, the addition of a small amount (0.17 wt.%) of SiC reduced the average grain size of the AZ91 alloy significantly, from $430{\mu}m$ to $73{\mu}m$. As a result, both the strength and the elongation of the AZ91 alloy increased considerably by the grain-boundary hardening effect and the suppression of twinning behavior, respectively. On the other hand, the addition of Ca (0.5-1.5 wt.%) and a combined addition of Ca (0.5-1.5 wt.%) and SiC (0.17 wt.%) increased the average grain size of the AZ91 alloy, which resulted in a decrease in its tensile properties. The SiC-added AZ92 alloy exhibited excellent tensile properties (YS 125 MPa, UTS 282 MPa, and EL 12.3%), which were much higher than those of commercial AZ91 alloy (YS 93 MPa, UTS 192 MPa, and EL 7.0%). The fluidity of the SiC-added AZ92 alloy was slightly lower than that of the AZ91 alloy because of the expansion of the solid-liquid coexistence region in the former. However, the SiC-added AZ92 alloy showed better hot-tearing resistance than the AZ91 alloy owing to its refined grain structure.

• Journal of Korea Foundry Society
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• v.7 no.2
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• pp.108-113
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• 1987

Al-Cr alloy with composition in the range from 1.5 wt% to 10 wt% Cr were rapidly solidified from the melt by the single roller method. The supersaturated solid solution was obtained up to 6 wt% Cr in Al-Cr alloy for $20{\mu}m$ thickness. Lattice parameter decreased with increasing Cr content at the rate of 0.00456A per wt% Cr up to 6 wt% Cr. Microhardness increased with increasing Cr content at the rate of $10\;Kg/mm^2$ per wt% Cr up to 6 wt% Cr. Microhardness measurements on the Al-6 wt% Cr supersaturated solid solution annealed isothermally showed no sign of age hardening. Decomposition temperature, determined by lattice parameter changes and microhardness changes, was $470^{\circ}C$ for Al-6 wt% Cr supersaturated solid solution. Transmission electron microscopy showed that decomposition within one hour below $400^{\circ}C$ occurred at grain boundaries only, and also the additional decomposition within grains being evident at $450^{\circ}C$ The coarse precipitate structure showed at $500^{\circ}C$ and $550^{\circ}C$, respectively. The coarse precipitate structure is considered $Al_7Cr$.

• Korean Journal of Materials Research
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• v.24 no.4
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• pp.201-206
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• 2014

TiC-21 mol% Mo solid solution (${\delta}$-phase) and TiC-99 mol% Mo solid solution (${\beta}$-phase), and TiC-(80~90) mol% Mo hypo-eutectic composite were deformed by compression in a temperature range from room to 2300 K and in a strain rate range from $4.9{\times}10^{-5}$ to $6.9{\times}10^{-3}/s$. The deformation behaviors of the composites were analyzed from the strengths of the ${\delta}$- and ${\beta}$-phases. It was found that the high strength of the eutectic composite is due primarily to solution hardening of TiC by Mo, and that the ${\delta}$-phase undergoes an appreciable plastic deformation at and above 1420 K even at 0.2% plastic strain of the composite. The yield strength of the three kinds of phase up to 1420 K is quantitatively explained by the rule of mixture, where internal stresses introduced by plastic deformation are taken into account. Above 1420 K, however, the calculated yield strength was considerably larger than the measured strength. The yield stress of ${\beta}$-phase was much larger than that of pure TiC. A good linear relationship was held between the yield stress and the plastic strain rate in a double-logarithmic plot. The deformation behavior in ${\delta}$-phase was different among the three temperature ranges tested, i.e., low, intermediate and high. At an intermediate temperature, no yield drop occurred, and from the beginning the work hardening level was high. At the tested temperature, a good linear relationship was held in the double logarithmic plot of the yield stress against the plastic strain rate. The strain rate dependence of the yield stress was very weak up to 1273 K in the hypo-eutectic composite, but it became stronger as the temperature rose.

• Journal of Ceramic Processing Research
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• v.22 no.5
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• pp.590-596
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• 2021

The widespread use of TiC-based cermets as cutting tools, thin-film, ultracapacitors, nozzles, and bearings is primarily due to exhibit combination of excellent mechanical properties such as low density, high hardness, and stiffness. The TiC cermets were synthesized by high energy ball milling, which includes binder metal (Ni), carbides (WC and Mo2C), wherein the present study focus on the relationship between the core-rim structure, phase constitution, and mechanical properties. Here, using in situ TEM, we clearly observed the behavior of adjacent core-rim formation from the solid-phase reaction with grain refinement of the TiC phase control of both the milling time and lattice formation. Also, we proposed that mechanically alloyed core-rim structure can affect oxidation resistance of TiC-Mo2C-WC-Ni cermets strongly related to activation energy attributed to TiC particle size. The mechanical properties of TiC-Mo2C-WC-Ni cermets suggest the hardening effect is not considered only grain refinement, but rather is solid solution strengthening and particle-dispersion hardening. The present study paves the relation to the formation behavior of both TiC hard phase and core-rim structure due to the mechanical powder synthesis of novel TiC-based cermets.

• Journal of Powder Materials
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• v.9 no.5
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• pp.303-306
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• 2002

P/M high speed steels with various Co contents were fabricated by gas atomization and Canning/HIP process. As Co content in P/M high speed steel increased, hardness, transverse rupture strength and yield strength in compressive testing increased due to solid solution hardening of Co in matrix. Especially, PM high speed steels with Co have high deformation resistance to repeated compressive loading.

• 연구논문집
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• s.29
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• pp.163-171
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• 1999

A new titanium alloy system. Ti-xFe-ySi (x,y=0-4 wt%). was designed and characterized with the point at low cost and high strength for casting applications. Fe improved room and elevated temperature mechanical properties owing to solid solution hardening and beta phase stabilization. Si yielded titanium silicides and Si addition over 1 wt% resulted in poor ductility due to coarse silicide chains at prior beta boundaries. The optimum composition was found to be Ti-4Fe-(0.5-1)Si in the viewpoint of tensile strength and ductility which are comparable to the Ti-6Al-4V. The metal-mould reaction was also examined for Ti-xFe and Ti-xSi binary alloy system. The thickness of surface reaction layer w as not affected significantly with Fe content, while it was decreased with Si content. In the Ti-4Si alloy, no reaction layer was found. The depth of surface hardening layer was about $200\mum$ regardless of the mould materials.

• Journal of the Korean Society for Heat Treatment
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• v.20 no.4
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• pp.187-194
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• 2007

A transmission electron microscope (TEM) investigation has been performed on the precipitation of $L2_1$-type $Ni_2AlTi$ phase in B2-ordered NiAl system. The hardness after solution treatment is high in NiAl-Ti alloys suggesting the large contribution of solid solution strengthening in this alloy system. However, the amount of age hardening is not large as compared to the large microstructural variations during aging. At the beginning of aging, the $L2_1$-type $Ni_2AlTi$ precipitates keep a lattice coherency with the NiAl matrix. By longer periods of aging $Ni_2AlTi$ precipitates lose their coherency and change their morphology to the globular ones surrounded by misfit dislocations. Misfit dislocations, which are observed on {100} planes of H-precipitates have the Burgers vector of a <100> with a pure edge type. The lattice misfits of NiAl-$Ni_2AlTi$ system is estimated from the spacings of misfit dislocations to be 1.1% at 1273 K. The lattice misfits decrease with increasing aging temperature in this system.

• Journal of the Korean Society for Heat Treatment
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• v.19 no.5
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• pp.249-256
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• 2006

Structural studies have been performed on precipitation hardening found in $L1_2$ ordered ${\gamma}^{\prime}-Ni_3(Al,Ti)$ alloys using transmission electron microscopy. A uniform solid solution of ${\gamma}^{\prime}-L1_2$ ordered phase supersaturated with Ni can be obtained by solution annealing in a suitable temperature range. The ${\gamma}^{\prime}$ phase hardens appreciably by the fine precipitation of disordered ${\gamma}$. The shape of ${\gamma}$ precipitates formed during aging is initially spherical or round-cubic and grow into platelets as aging proceeds. High resolution electron microscopy revealed that the ${\gamma}$ precipitates are perfectly coherent with the matrix ${\gamma}^{\prime}$ as long as the ${\gamma}$-precipitates are plates. The loss of coherency initiates by the introduction of dislocations at the ${\gamma}/{\gamma}^{\prime}$ interface followed by the step formation at the dislocations. The ${\gamma}$ precipitates become globular after the loss of coherency. The strength of ${\gamma}^{\prime}-Ni_3(Al,Ti)$ increases over the temperature range of experiment by the precipitation of fine ${\gamma}$ particles. The peak temperature where a maximum strength was obtained shifted to higher temperature.

• Korean Journal of Materials Research
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• v.17 no.8
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• pp.420-424
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• 2007

Precipitation behavior has been studied in NiTi-based ordered alloy using transmission electron microscopy. The hardness after solution treatment is high in NiTi alloy suggesting the large contribution of solid solution strengthening in this alloy system. However, the amount of age hardening is not large as compared to the large microstructural variations during aging. At the beginning of aging, the $L2_1-type$ $Ni_2AlTi$ precipitates keep a lattice coherency with the NiTi matrix. By longer periods of aging $Ni_2AlTi$ precipitates lose their coherency and change their morphology to the globular ones surrounded by misfit dislocations. Misfit dislocations, which are observed on {100} planes of H-precipitates have the Burgers vector of a <100> with a pure edge type. The lattice misfits of $NiTi-Ni_2AlTi$ system is estimated from the spacings of misfit dislocations to be 1.3% at 1273 K. The lattice misfits decrease with increasing aging temperature in this system.