Yang Gon Kim

Dynamic deformation and fracture behaviors of two Zr-based amorphous alloys

Dynamic deformation and fracture behaviors of Zr-based amorphous alloys were investigated in this study. Quasi-static and dynamic compressive tests were conducted using a universal testing machine and a compressive Kolsky bar, respectively, and then the test data were analyzed in relation to microstructure and fracture mode. Quasi-static compressive test results indicated that the compressive strength of the amorphous alloy containing dendritic β phases was similar to that of the amorphous alloy, while the ductility was better. Under dynamic loading, the maximum shear stress and ductility of the amorphous alloys were considerably lower than those under quasi-static loading because of the decreased resistance to fracture. The deformation and fracture behaviors occurring under quasi-static and dynamic loading conditions were explained by fracture mechanisms observed on fractured surfaces.

Dynamic compressive properties obtained from a split Hopkinson pressure bar test of Boryeong shale

Dynamic compressive properties of a Boryeong shale were evaluated by using a split Hopkinson pressure bar, and were compared with those of a Hwangdeung granite which is a typical hard rock. The results indicated that the dynamic compressive loading reduced the resistance to fracture. The dynamic compressive strength was lower in the shale than in the granite, and was raised with increasing strain rate by microcracking effect as well as strain rate strengthening effect. Since the number of microcracked fragments increased with increasing strain rate in the shale having laminated weakness planes, the shale showed the better fragmentation performance than the granite at high strain rates. The effect of transversely isotropic plane on compressive strength decreased with increasing strain rate, which was desirable for increasing the fragmentation performance. Thus, the shale can be more reliably applied to industrial areas requiring good fragmentation performance as the striking speed of drilling or hydraulic fracturing machines increased. The present dynamic compressive test effectively evaluated the fragmentation performance as well as compressive strength and strain energy density by controlling the air pressure, and provided an important idea on which rock was more readily fragmented under dynamically processing conditions such as high-speed drilling and blasting.

Metal Powder Injection Molding Process for Manufacturing Adapter Component

Adapters are a component of the output system in the internally geared hub for a bicycle. Originally, adapter parts were produced by a machining process with low productivity and material usage rate. In this study, the metal powder injection molding (MIM) process has been applied as an alternative manufacturing method to the machining process. Microstructure analysis and mechanical property testing has been conducted in order to select the material for the adapter with changes in the nickel content. The geometrical precision of the adapter is measured by using three-dimensional scanner with various mixing ratios of the powder and the binder. The developed alternative process for the adapter results in increased productivity and material usage rate. Previously, this process was only used for small parts less than 10 mm in diameter. With this development, the MIM process may be used more widely than before.

Dynamic Deformation of Submicrocrystalline Aluminum Alloys

In this study, dynamic deformation behavior of submicrocrystalline aluminum alloy was established with respect to equal-channel angular (ECA) pressing routes such as A, B, and C. After 8-pass ECA pressings, the deformed samples, regardless of the routes applied, were consisted of ultrafine grains together with high dislocation density near the boundaries. Microstructural observation revealed that the sample deformed via route B showed more diffused diffraction pattern than those deformed via route A and C, suggesting the fact that route B was most effective for a rapid evolution in the grain boundary orientation from low-angle to high-angle characteristics. In the torsion tests, the shear stress decreased once reaching the maximum point. This maximum was the highest in the sample deformed via route B, and decreased in the order of the route C and route A. The dynamic deformation was explained based on microstructural uniformity associated with ECA pressing routes.

DYNAMIC DEFORMATION AND FRACTURE BEHAVIOR OF ULTRA-FINE-GRAINED DUAL PHASE STEELS FABRICATED BY EQUAL CHANNEL ANGULAR PRESSING

In this study, dynamic deformation and fracture behavior of ultra-fine-grained dual phase steels fabricated by equal channel angular pressing (ECAP) was compared with that of conventionally annealed ultra-fine-grained steels. In the ECAPed conventionally annealed specimen, pearlites were decomposed, and the ferrite grain size increased up to 0.5 μm. The intercritically annealed specimen consisted of dual phases of equiaxed ferrites and blocky martensites (volume fraction; 35%, size; 1 μm). The dynamic torsional test results indicated that maximum shear stress of the annealed specimens was lower than that of the as-pressed specimen, but fracture shear strain was higher. These results suggested that annealing or promoting dual phase microstructures of the ECAPed specimens could be a good way to increase the fracture resistance under dynamic loading, as it reduced or prevented the formation of adiabatic shear bands or voids. In particular, the ECAPed ultra-fine-grained dual phase steel can be more reliably used under dynamic conditions since it overcomes the shortcomings of reduced strain hardenability, ductility, and dynamic properties.

EFFECTS OF TEMPERATURE AND COMPOSITION ON DYNAMIC DEFORMATION BEHAVIOR OF ZR-BASED AMORPHOUS ALLOYS

Effects of test temperature and alloy composition on dynamic deformation behavior of Zr-based amorphous alloys were investigated in this study. Dynamic compressive tests were conducted in the temperature range from room temperature to 380°C using a compressive Kolsky bar. Dynamic compressive test results indicated that both maximum compressive stress and total strain of the amorphous alloys decreased with increasing test temperature because shear bands could propagate rapidly as the adiabatic heating effect was added at high temperatures. Maximum compressive stress and total strain of the alloy containing ductile β crystalline phases were higher than those of the monolithic amorphous alloys over the tested temperature range because β phases played a role in forming multiple shear bands. The alloys having lower Tg or ductile phases had more excellent dynamic properties than the LM1 alloy.

Dynamic Deformation and Fracture Behavior of Zr-Based Bulk Metallic Glasses

Dynamic deformation and fracture behavior of Zr-based bulk metallic glass (BMG) and BMG composite containing dendritic β phases was investigated in this study. Dynamic compressive test results indicated that both maximum compressive stress and total strain of the BMG and BMG composite decreased with increasing test temperature because shear bands could propagate rapidly as the adiabatic heating effect was added at high temperatures. Above the glass transition temperature, total strain decreased more abruptly due to crystallization of amorphous phases. Maximum compressive stress and total strain of the BMG composite were higher than those of the BMG because β phases played a role in forming multiple shear bands. The BMG composite having more excellent dynamic properties than the BMG can be more reliably applied to the structures or parts requiring dynamic properties.