Hyoung Wook Kim

Microstructure and Mechanical Properties of ZK60 Alloy Sheets during Aging

Variations in microstructure and mechanical properties of ZK60 alloy sheets were investigated with aging time. ZK60 alloy sheets with a thickness of 1mm were prepared from a casting ingot followed by homogenization and warm-rolling. Artificial aging process after solution heat treatment (T6) affected both hardness variations and precipitates distributions with aging time. Hardness variations were related to precipitates, i.e. rod-shaped ( 1 β ′ ) or disc shaped ( 2 β ′ ) particles. Rod-shaped ( 1 β ′ ) precipitates mainly consist of Mg and Zn without Zr.

Effect of Heat Treatment on Microstructure and Mechanical Properties in ZK60 Alloy Sheet

The sheet of ZK60 alloy with a thickness of 1mm was prepared from a casting ingot followed by homogenization and warm-rolling. Variations in microstructure and mechanical properties of ZK60 alloy sheets were investigated during T6 treatment. Especially artificial aging after solution heat treatment affected both precipitates distribution and mechanical properties with aging treatment. Variations of mechanical properties were related to precipitates, i.e. rod-shaped ( 1 β ′ ) or disc shaped ( 2 β ′ ) particles. Around the peak of hardness values, regularly distributed rod-shaped ( 1 β ′ ) precipitates were found. The rod-shaped ( 1 β ′ ) precipitates were oriented with a growth direction of [0001]. When over-aged, rod-shaped ( 1 β ′ ) precipitates were expected to decrease and the density of disc-shaped ( 2 β ′ ) precipitates to change. The rod-shaped ( 1 β ′ ) precipitates mainly consist of {Mg, Zn}, while disc-shaped ( 2 β ′ ) precipitates, {Mg, Zn, Zr} or {Mg, Zn}. In this study the optimum T6 treatment was determined as solution treatment at 430 °C for 6 hours and subsequently aging treatment at 175 °C for 18 hours. At this T6 condition the tensile strength, yield strength and elongation are 321MPa, 280MPa and 16%, respectively.

Microstructure and Tensile Properties of Al-(5~10)wt%Mg Alloy Sheets Fabricated by Twin Roll Casting and Rolling Process

High strength Al-Mg alloy strips with high Mg contents (5-10wt%Mg) were successfully fabricated by twin roll casting. In order to get a good surface quality of Al-Mg strips, an optimum process condition was investigated in this experiment. The morphology of the cast nozzle and the roll separate force during twin roll casting was important to improve the surface quality of the strip and homogeneity of the cast structure through the thickness. The size of intermetallic particle like Al-Fe compounds was reduced down to 1~2uf06dm due to a high cooling rate of Al melt during strip casting. In addition, the dendrite structure was very fine and the segregation of Al8Mg5 phase between grains was remarkably reduced. Therefore, the Al-Mg strips have good workability during additional cold/warm rolling processes. After annealing, the rolled sheets have superior tensile properties to a commercial high strength Al-Mg alloy sheet.

Fabrication and Characterization of 4H-SiC pn Diode with Field Limiting Ring

4H-SiC pn diodes with field limiting rings(FLRs) were fabricated and characterized. The dependences of reverse breakdown voltage on the number of FLRs, the distance between p-base main junction and first FLR, and activation temperatures, were investigated. Al and B ions were implanted and activated at high temperature to form p-base and p+ region in the n-epilayer, respectively. We have obtained up to 1790V of reverse breakdown voltage in the pn diode with two FLRs on 10μm thick epilayer. The differential on-resistances of the fabricated diode are 5.3mΩcm at 100A/cm and 2.7mΩcm at 1kA/cm, respectively. All pn diodes with FLRs have higher avalanche breakdown voltages than those without an FLR. This study reveals that the FLR edge termination techniques for SiC pn diodes can provide high blocking voltage as well as low on-resistance.

Damage Relaxation Pre-Activation Anneal in Al-Implanted SiC

The effect of pre-activation annealing on the final surface morphology f the Al implanted 4H-SiC has been investigated using atomic force microscopy (AFM). The implanted SiC was annealed in dual steps; the pre-activation annealing which was done at 1100~1500 C earlier, and the activation one subsequently done at 1600 C. The implanted SiC showed the well-developed macrosteps after the activation annealing, while the unimplanted ones showed smoother surfaces. The pre-activation annealing at 1400 C was very effective to reduce the macrostep formation during the activation annealing. The surface damage induced from the ion implantation ha s been relaxed during the pre-activation annealing, which seemed to suppress the macrostep f ormation on the surface of SiC.

Influence of misch metal addition and deformation processes on microstructure and mechanical properties in AZ61 alloy

Magnesium alloys have been known as the best lightweight metallic materials for various applications of electronic equipments and automobile parts due to high specific strength and stiffness. The needs for wrought magnesium alloys have been increased for the application to structural parts in the form of sheets and bars. However, magnesium has a hexagonal closed-packed (HCP) crystal structure with a limited number of operative slip systems at room temperature, and its formability is restricted to mild deformation. The improvement of the formability of magnesium sheets for real applications is important. In order to increase formability of magnesium sheets at elevated temperature, one promising way is a grain refinement.

Tensile Deformation Behaviors of Ultrafine Grained Al-Fe-Si Alloy Sheets

Ultra-fine grained AA8011 alloy sheets manufactured by the accumulative roll-bonding (ARB) process exhibited unique tensile deformation behavior. Tensile strength of the ARB processed AA8011 sheets increased up to three cycles, but then showed nearly the same value after three cycles. Meanwhile, the total elongation grew significantly with an increasing nember of ARB cycles. It was found that the strain-rate sensitivities (m) of the AA8011 sheets increased up to 0.047 by the ARB process. A large number of high-angle boundaries were introduced by the ARB process and the fraction of high-angle boundaries reached 70% after eight ARB cycles. In this paper, we discusse the increase in total elongation on the basis of strain-rate sensitive deformation of the material, which is also correlated with dynamic recovery.

Simulation of Texture Evolution during Hot Rolling Deformation in FCC Materials

The evolution of hot rolling texture in FCC materials has been simulated numerically using a visco-plastic self-consistent (VPSC) polycrystal model. A finite element (FE) analysis with ABAQUS/StandardTM was conducted to evaluate the deformation gradients during hot rolling deformation. In order to capture crystallographic rotation during hot rolling deformation, an octahedral slip system was considered in a microscopic hardening model. The FE analysis with the VPSC polycrystal simulations successfully predicted the inhomogeneous texture development through the thickness direction in the hot-rolled Al-5wt%Mg alloy sheets.

Fabrication of High Mg Containing Al-Mg Alloy Sheets by Twin Roll Strip Casting

Al-Mg alloy sheets with high Mg contents (3~10wt%Mg) were fabricated by twin roll strip casting. The optimum process conditions to get a good surface quality of Al-Mg strip have been suggested in this experiment. Controlling the cooling rate of cast roll was important to improve the surface quality of the strip and a compositional homogeneity through the thickness. The size of intermetallic particle like Al-Fe compound was reduced down to 1~2μm due to a high cooling rate of Al melt during strip casting. In addition, the dendrite structure was fine and the segregation of Al8Mg5 phase between grains was remarkably reduced. Therefore, the strips with a thickness of 3mm have good workability during additional hot/warm rolling processes. The hot/warm rolled Al-5wt%Mg sheets show high strength and elongation. When the rolled sheets were annealed at 300 oC for 1hr., the tensile strength and total elongation of the sheets reached at 290 MPa and 30%, respectively.

Edge Termination Technique for SiC Power Devices

The breakdown characteristics of 4H-SiC power device with the three types of edge termination technique, field limiting ring(FLR), field plate(FP) and junction termination extension(JTE), are investigated. The n-type 4H-SiC wafer with 10μm epi-layer and a doping concentration of 5.4 10 15 /cm 3 is used to simulate the power pn diode with each type of edge termination structure. From the optimized simulation results, the breakdown voltages of pn diode with FLR, FP and JTE termination are 1810V, 1130V and 1000V, respectively. The peak electric field of the FP and JTE termination occurs at the surface and the interface between main junction and JTE region, which yields lower breakdown voltage. Conversely, the peak electric field of FLR termination at the interface between SiC and oxide is lower than that at the junction of the 2 nd ring edge. This means that the breakdown of the FLR termination occurs at the bulk region of the device, so that the breakdown voltage of the FLR termination is higher and stable. The breakdown voltage of the fabricated pn diode with FLR termination is measured 1782V, which is similar to the simulated one. Therefore, in SiC power devices, the FLR structure seems to be suitable for obtaining an ideal breakdown and a practical planar termination technique. Introduction The excellent physical and electrical properties of silicon carbide make it an important device material for high power, high temperature, and high frequency applications [1,2]. Due to its material characteristics, silicon carbide power devices exhibit superior device characteristics than that of the silicon power devices, such as high breakdown voltage, low on-state voltage drop, etc. In the high voltage devices, edge termination technique has an important role for determining the breakdown voltage. In silicon power devices, FLR, FP, and JTE techniques are mostly used to terminate the edge of the devices due to its advantages. The FP structure can be fabricated easily and compatible but hard to obtain high voltage. Even if JTE structure is usually considered as an ideal termination technology, it is virtually hard to optimize because it is so sensitive to the doping concentration of JTE region. The doping concentration is usually controlled by diffusion process in traditional silicon technology. The floating field rings can be formed without an additional processing step during the fabrication of the power pn diode and MOSFET. However, it needs large termination area to obtain high breakdown voltage. In SiC power devices, some of these disadvantages can be eliminated due to the material characteristics of the SiC. In this paper, 4H-SiC pn diode using several edge termination techniques is presented. We simulate and compare the feature of the each termination technique. Especially for the FLR edge termination technique, we simulate to optimize the structure and fabricated pn diodes. The characteristics of the structures are verified by the two-dimensional device simulator ATLAS [3]. Results and Discussion N-type 4H-SiC pn diode with 10μm epi-layer thickness and a doping concentration of 5.4 10 15 /cm 3 is used to simulate the each type of edge termination structure. Figure 1 shows the cross-sectional view of the various termination structures for the simulation. Two floating field rings are basically Materials Science Forum Online: 2004-06-15 ISSN: 1662-9752, Vols. 457-460, pp 1241-1244 doi:10.4028/www.scientific.net/MSF.457-460.1241