Kang Cheol Kim

Microstructural Features of Multicomponent FeCoCrNiSi x Alloys

Recently, one of the new types of multicomponent alloys, high entropy alloy with major alloying elements in equiatomic or near-equiatomic ratios, receives a great attention as a new potential alloys due to their unique microstructural features and outstanding properties (Ranganathan, 2003; Cantor et al., 2004; Hsu et al., 2004; Huang et al., 2004; Yeh et al., 2004). The most eminent property enhancement including wear resistance, corrosion resistance, oxidation resistance and hardness has been reported previously (Chen et al., 2004; Huang et al., 2004; Chen et al., 2005; Wu et al., 2006). These findings have brought new insight in design of alloys when compared with the alloys which are currently being developed, and has provided a new concept in designing of modern alloys. As a result, a series of new multicomponent systems has been investigated (Wang & Zhang, 2008). Among the alloying elements available for new alloy design, Fe, Co, Ni, Ti, Cu, and Al have been used most frequently to synthesize the high entropy alloys. Among the alloying elements which have been mostly used, Fe, Co, Ni, Ti, Cu are transition metal elements. It has been shown that the addition of small amount of alloying element can occasionally induce a dramatic property enhancement in traditional alloys (Itabashi & Kawata, 2000; Lin & Chang, 2003; Kondo & Takahashi, 2006). While there are quite few reports on the effect of alloying element such as Si with small atomic radius on the microstructure development and property enhancement in high entropy alloys. Therefore, the aim of the present study is to investigate the effect of addition of alloying elements Al, Si on the microstructural development in FeCoCrNi high entropy alloys. The addition of Al in FeCoCrNi high entropy alloy has been reported to cause the change of the crystal structure of solid solution from fcc to bcc (Wang et al., 2012). In the present study, effect of Al addition has been investigated to confirm the previous result. Then, the effects of Si addition on the microstructural development in FeCoCrNi high entropy alloy has been investigated systematically. The result shows that the addition of Si element changes the alloy structure evidently, which further provides the potential for improvement of properties. pISSN 2287-5123·eISSN 2287-4445 http://dx.doi.org/10.9729/AM.2015.45.1.32

Effect of Aging Time and Temperature on the Aging Behavior in Sn Containing AZ91 Alloy

Effects of aging temperature and time on the aging behavior in AZ91 alloy and Sn containing AZ91 alloy (AZT915) have been investigated in the present study. The mode of precipitation, i.e. discontinuous and continuous precipitation in both alloys is strongly affected by the aging temperature. At low aging temperature of 403 K, only discontinuous precipitation occurs at the grain boundaries, whereas at high aging temperatures of 573 and 623 K only continuous precipitation occurs inside the grains. At intermediate temperature range (443 or 498 K) both discontinuous and continuous precipitation reactions occur. In AZT915, the Mg2Sn particles at the grain boundary effectively reduce the available nucleation sites for discontinuous β precipitates, and slow down the movement of the grain boundary, resulting in suppression of discontinuous precipitation. In addition, increased local lattice strain by the presence of Sn in the α-Mg solid solution matrix accelerates the nucleation of the continuous precipitates at the early stage of aging treatment. Therefore, significantly higher peak hardness can be obtained within a shorter aging time in AZT915.

Effect of minor addition of Zr on the oxidation behavior of Ti-Cu metallic glasses

The oxidation behavior of (TiCu)100-XZrX (x=3, 5, 7 at%) amorphous alloy during continuous heating up to 1073 K has been investigated. The weight-gain due to oxidation occurs in a two-step mode. The first-step oxidation (630~730 K) is associated with supercooled liquid region. The oxidation layer consists of intermixed TiO2 + ZrO2 layer at outer side and amorphous oxide layer at inner side. The oxidation rate at the second-step above 880 K becomes significantly different depending on the Zr content. In x=3 alloy, the oxidation layer consists of TiO2 layer at outer side, intermixed TiO2 + ZrO2 layer, and Ti3Cu3O layer at inner side. As the fraction of ZrO2 increases in x=5 and 7 alloys, the growth of oxide layer is accelerated due to provision of easier diffusion path in the intermixed oxide layer, forming intermixed multi-phase (TiO2 + ZrO2 + Ti3Cu3O + Cu51Zr14) oxide layer.

Crystallization Behavior of Al-Ni-Y Amorphous Alloys

The crystallization behavior in the and amorphous alloys has been investigated. As-quenched amorphous phase decomposes by simultaneous formation of Al and intermetallic phase at the first crystallization step, while as-quenched amorphous phase decomposes by forming Al nanocrystals in the amorphous matrix. The density of Al nanocrystals is extremely high and the size distribution is homogeneous. Such a microstructure can result from rapid explosion of the nucleation event in the amorphous matrix or growth of the preexisting nuclei embedded in the as-quenched amorphous matrix. The final equilibrium crystalline phases and their distribution at 873 K are exactly same in both and alloys.

Optimum Combination of Thermoplastic Formability and Electrical Conductivity in Al–Ni–Y Metallic Glass

Both thermoplastic formability and electrical conductivity of Al–Ni–Y metallic glass with 12 different compositions have been investigated in the present study with an aim to apply as a functional material, i.e. as a binder of Ag powders in Ag paste for silicon solar cell. The thermoplastic formability is basically influenced by thermal stability and fragility of supercooled liquid which can be reflected by the temperature range for the supercooled liquid region (ΔTx) and the difference in specific heat between the frozen glass state and the supercooled liquid state (ΔCp). The measured ΔTx and ΔCp values show a strong composition dependence. However, the composition showing the highest ΔTx and ΔCp does not correspond to the composition with the highest amount of Ni and Y. It is considered that higher ΔTx and ΔCp may be related to enhancement of icosahedral SRO near Tg during cooling. On the other hand, electrical resistivity varies with the change of Al contents as well as with the change of the volume fraction of each phase after crystallization. The composition range with the optimum combination of thermoplastic formability and electrical conductivity in Al–Ni–Y system located inside the composition triangle whose vertices compositions are Al87Ni3Y10, Al85Ni5Y10, and Al86Ni5Y9.

High thermal stability of the amorphous oxide in Ti44.5Cu44.5Zr7Be4 metallic glass

The oxidation behavior of Ti44.5Cu44.5Zr7Be4 metallic glass has been investigated. The oxide layer with a fully amorphous structure forms when heated up to the SCL temperature region, indicating that the presence of Be in the oxide layer improves the thermal stability of the amorphous oxide. The amorphous oxide is stable even when heated above the crystallization onset temperature. The thickness of the amorphous oxide layer reaches to ∼160 nm when heated up to 773 K. The oxide layer grows in both inward and outward directions, leaving Cu-enriched crystalline particles at the middle section of the oxide layer.

Synthesis of Amorphous Matrix Nano-composite in Al-Cu-Mg Alloy

The microstructure of as-quenched alloy has been investigated in detail using transmission electron microscopy. Al nano-crystals about 5 nm with a high density are distributed in the amorphous matrix, indicating amorphous matrix nano-composite can be synthesized in Al-Cu-Mg alloy. The high density of Al nano-crystals indicates very high nucleation rate and sluggish growth rate during crystallization possibly due to limited diffusion rate of solute atoms of Cu and Mg during solute partitioning. The result of hardness measurement shows that the mechanical properties can be improved by designing a nano-composite structure where nanometer scale crystals are embedded in the amorphous matrix.

Formation of Amorphous Oxide Layer on the Crystalline Al-Ni-Y Alloy

The oxidation behavior of the crystallized alloy has been investigated with an aim to compare with that of the amorphous alloy. The oxidation at 873 K occurs as follows: (1) growth of an amorphous aluminum-yttrium oxide layer (~10 nm) after heating up to 873 K; and (2) formation of crystalline oxide (~220 nm) after annealing for 30 hours at 873 K. Such an overall oxidation step indicates that the oxidation behavior in the crystallized alloy occurs in the same way as in the amorphous alloy. The simultaneous presence of aluminum and yttrium in the oxide layer significantly enhances the thermal stability of the amorphous structure in the oxide phase. Since the structure of aluminum-yttrium oxide is dense due to the large difference in ionic radius between aluminum and yttrium ions, the diffusion of oxygen ion through the amorphous oxide layer is limited thus stabilizing the amorphous structure of the oxide phase.