Kyung Sub Lee

Electrode Characteristics of Nanostructured Mg2Ni ‐ Type Alloys Prepared by Mechanical Alloying

A mechanical alloying process is introduced to produce nanocrystalline Mg{sub 2}Ni-type metal hydride. The mechanical alloying process has recently emerged as a novel technique for producing alloy powders whose structures are nanocrystalline. Mg{sub 2}Ni alloys were prepared using planetary ball mill starting from mixtures of elemental powders. Nanocrystalline Mg{sub 2}Ni-type alloys were obtained by both the long-term ballmilling (mechanical alloying) and heat-treatment of the ballmilled powders. In comparison with the polycrystalline material, nanocrystalline Mg{sub 2}Ni showed a lower temperature for hydrogenation and a higher discharge capacity at 30 C. The partial substitution of Cu for Ni increased hydrogen absorption and desorption at 200 C and greatly improved the discharge capacity of the nanostructured Mg{sub 2}Ni electrode at 30 C. The discharge capacity of the 120 h milled Mg{sub 2}(Ni{sub 0.9}Cu{sub 0.1}) electrode was 350 mAh/g.

Influence of deformation induced ferrite transformation on grain refinement of dual phase steel

Abstract A controlled rolling process was simulated by thermomechanical simulator (Gleeble1500) to produce as-hot-rolled dual phase steel. The microstructure of as-hot-rolled dual phase steel using deformation induced ferrite (DIF) transformation could be refined to as fine as that of Intermediate Quenching (martensite as a starting microstructure). DIF transformation was influenced by austenite grain size, amount of strain and strain rate, and the grain of about 2 μm could be produced by heavy deformation of 80%. However, the ferrite growth occurred during the intercritical isothermal holding after deformation. Microalloying such as Nb, V was effective for suppressing the ferrite growth. Thus, microalloying elements that could restrain ferrite grain coarsening were required to produce fine grained dual phase steel.

Mechanism of rapid degradation of nanostructured Mg2Ni hydrogen storage alloy electrode synthesized by mechanical alloying and the effect of mechanically coating with nickel

Abstract Nanocrystalline Mg 2 Ni metal hydride was prepared by mechanical alloying. It had good activation properties and showed a large discharge capacity at room temperature. Nevertheless, it lost 80% of its maximum capacity within ten cycles. The mechanism underlying the rapid, early capacity loss of this material was investigated via various analyses (scanning electron microscopy [SEM], X-ray diffraction, X-ray photoelectron spectroscopy, Auger electron spectroscopy, inductively coupled plasma atomic emission spectroscopy [ICP]. The Mg 2 Ni phase decomposed to form a Mg(OH) 2 layer at the surface. The thickness of this layer was about 4000 A after ten cycles. Severe surface cracking during charging/discharging was not revealed by SEM. The amount of the dissolved active materials (Mg, Ni) in the electrolyte was dimunitive. The main cause of the rapid degradation of the electrode was the formation of the passive layer at the surface, which hindered the charge transfer reaction. Mechanical coating with nickel effectively reduced the degradation rate of the electrode.

The surface state of nanocrystalline and amorphous Mg2Ni alloys prepared by mechanical alloying

Amorphous (Mg1−xZrx)2Ni alloys with the composition of x=0.1 and 0.3 were synthesized by mechanical alloying (MA). A nanocrystalline Mg2Ni phase was also formed by MA without Zr addition. X-ray photoelectron spectroscopy (XPS) was used to investigate the atomic bindings of the alloying elements at the electrode surface. The analysis of the XPS of Mg 2p spectra showed that the atomic binding of Mg at the amorphous or nanocrystalline surface was lower than at the crystalline one. However, the Ni 2p spectra showed little variation in the amorphous or nanocrystalline phase compared to the crystalline. The looser binding of Mg at the surface indicates that the surface energy of the amorphous or nanocrystalline electrode was increased, and the hydrogen diffusion and charge transfer reaction enhanced. A passive layer of Mg(OH)2 and rapid degradation of the electrode were also caused by the looser binding of Mg.

A study of the microstructure of nanocrystalline Al–Ti alloys synthesized by ball milling in a hydrogen atmosphere and hot extrusion

Abstract Nanocrystalline Al–Ti alloy powders were produced by reactive ball milling (RBM) in a hydrogen atmosphere and its microstructure consisted of nano-sized Al and nano-sized TiH 2 . Thermal analysis of as-milled powders showed that the decomposition of TiH 2 and the subsequent formation of Al 3 Ti occurred at 370–480°C. The powder was consolidated by hot extrusion at 500°C. The grain size of as-extruded specimens was about 50–100 nm. The hardness of Al-5 at.%Ti specimens synthesized by RBM and subsequent hot extrusion was 25–75% higher than that of Al-8 wt.%Ti alloys produced by mechanical alloying (MA) in Ar atmosphere and hot extrusion. Room temperature and high temperature (300, 400, 500°C) tensile strength of RBM Al-5 at.%Ti alloys were superior to those of MA Al-8 wt.%Ti alloys. The strength in these alloys appeared to be related to a large extent to the very fine grain size. The ductility of RBM alloys decreased with grain refinement. It is possible that the deterioration in ductility of nanocomposite Al–Ti alloys has to be attributed to the increase of the interface area between Al and Al 3 Ti and its high energy. SEM fractograph showed that fracture progressed intergranularly.

The electrochemical hydriding properties of Mg-Ni-Zr amorphous alloy

Abstract The amorphous alloys of Mg–Ni–Zr were synthesized by mechanical alloying process, in which the effects of Zr addition were studied. A small shift of the broadening amorphous peak due to Zr additions was observed. The equilibrium hydriding pressure increased continuously with hydrogen contents in the amorphous alloys, while there was an obvious plateau pressure of 10 −4 MPa in the crystalline Mg2Ni phase. The Zr addition lowered the equilibrium hydriding pressure and increased the hydrogen solubility from 1.4 wt % for Mg–Ni to 2.2 wt % for Mg–Ni–Zr. Accompanied by amorphization, the hydrogen discharge capacity increased largely, and reached 580 mA h / g for Mg–Ni–Zr alloy. The chronopotentiometric experiments were performed to investigate the hydrogen diffusion in the alloy powder in the high over-potential range. The diffusivity of hydrogen in the Zr-contained alloy was lower than the Mg–Ni binary amorphous alloy or in Mg2Ni nanocrystalline alloy.

Effects of Zr addition on discharge properties of mechanically alloyed Mg2Ni hydrogen-storage alloy electrode

Abstract (Mg 1− x Zr x ) 2 Ni ( x =0.0, 0.1, 0.2, 0.3 and 0.4) hydrogen-storage alloy electrodes are synthesized by means of a mechanical alloying process using a planetary ball mill. After milling for 160 h, (Mg 1− x Zr x ) 2 Ni alloy with x =0.0 remained the nanocrystalline phase. By contrast, Zr addition to this alloy enhances the structural disorder and amorphization. The discharge capacity of a (Mg 1− x Zr x ) 2 Ni electrode increases with Zr content and reaches the highest capacity of 530 mAh g −1 at x =0.3, then decreases to 230 mAh g −1 at x =0.4, i.e., lower than that of pure Mg 2 Ni. Cyclic stability and rate capability vary with Zr addition.

The relationship between the fracture toughness and grain boundary character distribution in polycrystalline NiAl

Abstract This paper reports the results of experimental studies on the relationship between grain boundary character distribution and the fracture toughness in the polycrystalline NiAl. The fracture toughness has been investigated by the ring on ring disk bend test in conjunction with the acoustic emission (AE) measurement, and the grain boundary character distribution is measured by the electron back-scatter diffraction pattern (EBSD or EBSP) technique. The fracture toughness of the annealed specimen (1473 K, 20 h) is 6.42±0.88 (MPa m−1/2) and that of the forged (1473 K) and annealed (1473 K, 20 h) specimen is 5.08±0.36 (MPa m−1/2). The strong boundary and weak boundary were decided by comparing the distribution of grain boundary characteristics at the intergranular cracks to the distribution in general population. Σ1, Σ3 and Σ5 boundaries have strong and the good crack resistance, but Σ7, Σ11, Σ13, Σ21 and Σ23 boundaries are relatively weak boundary in the polycrystalline NiAl. Consequently, the annealing process makes the fraction of Σ1 and Σ5 boundaries higher than forging and annealing process. The high fraction of Σ1 and Σ5 boundaries that have the good crack resistance seem to affect the increase of fracture toughness in the annealed specimen.

Microstructure and mechanical properties of nanocrystalline Al–Ti alloys consolidated by plasma activated sintering

Abstract Nanocrystalline Al–Ti alloys were produced by reactive ball milling and subsequently plasma activated sintered (PAS). The consolidation behavior and microstructure of the nanocrystalline alloys were studied as a function of sintering pressure and temperature with constant holding time of 60 s. This showed that the pressure and temperature had large effects on the final density and grain size. The PAS consolidated Al–Ti alloys exhibited full density (99% of theoretical density) while retaining a grain size of the order of 50–100 nm at a temperature of 500°C with a pressure of 75 MPa. This alloy showed a much more uniform grain distribution and finer grain size compared with Al–Ti alloys manufactured by conventional consolidation methods. At room temperature, the compressive yield strength (about 692 MPa) of PAS alloy was much higher than that of hot pressed Al–Ti alloys. The higher yield strength is considered to be due to the effect of grain refinement strengthening. At all temperatures above 300°C, the PAS alloys exhibited plastic strain beyond 60%.

Investigation on the oxidation characteristics of copper-added modified Zircaloy-4 alloys in pressurized water at 360°C

Abstract In order to assess the influence of copper addition on the oxidation behavior of modified Zircaloy-4 (Zry-4) alloys as well as Zr–Cu binary alloys, weight gain changes in a pressurized autoclave at 360°C were investigated. The copper content varied from 0.1 to 0.8 wt% for the binary alloys and changed from 0 to 0.5 wt% for the modified Zry-4 alloys. The optimum copper content for improved oxidation resistance turned out to be in the range 0.1–0.2 wt% from weight gain measurements; in the binary alloy, the weight gain generally decreased with decreasing Cu content from 0.8 to 0.2 wt% although 0.1 wt% Cu specimen showed higher weight gain than 0.2 wt% Cu specimen. This tendency was observed in the modified Zry-4 alloy showing the lowest weight gain at 0.1 wt% Cu content. To understand the microstructural influence on oxidation of the copper-added zirconium alloys, the effects of copper additions on the crystal structure of the oxide, second phase precipitates and microstructure of the metal matrix were studied.