Papers by Keyword: Pinning Effect

Abstract: This study was presented due to the increasing demand of High Strength Low Alloy (HSLA) steel, such as demand for thinner-walled and large diameter pipes in oil and gas industries. In order to meet the imposed economic restrictions, the high standard of all kinds of steel properties is required and can be achieved by controlling the steel microstructure. The austenite grain size influences the microstructure and properties of steel significantly, in which fine austenite grain size leads to higher strength, better ductility, and higher toughness. Studying the behavior of steel grain growth during the reheating process is still being a fascinating subject. P.R. Rios and D Zollner [1] mentioned that grain growth is the most important unresolved issue that has been a topic of research for many years. In this research, the behavior of austenite grain growth at a high niobium-low carbon (High Nb-low C) and low Nb-high C HSLA steel was evaluated, and the result was compared with other investigation. The results found that the austenite grain growth at high Nb-high C steel was slower than the growth at a low Nb-low C steel. The activation energy of austenite grain growth and both constant A and exponent n ware determined close agreement was obtained between the prediction of the model and the experimental grain size value.

Abstract: To predict austenite grain growth behavior in the heat-affected zone (HAZ) in low alloy steels, a new calculation model is proposed herein. This model mainly considers the solute-drag effect and pinning effect, which restrain the austenite grain growth. To calculate the solute-drag effect, the grain boundary concentration of each element is obtained by Hillert’s Law. Calculations are performed by simulating the HAZ with a temperature gradient using the phase field method for two dimensions. This calculation demonstrates the possibility of quantitatively predicting the pinning force for welding heat inputs.

Abstract: Microstructural and mechanical properties of powder metallurgy (PM) with carbon nanotube (CNTs) dispersed copper (Cu) composites were investigated in detail. Pure copper powder was coated with un-bundled CNTs by using the zwitterionic surfactant solution containing CNTs. The powder rolling process was applied to increase the powder surface area to be coated with CNTs. The total rolling reduction of Cu-CNT composite powder by 5 steps rolling was about 75%. With increasing the number of rolling steps, the content of CNTs coated on the Cu powder surface increased because of the increment of the flat surface area of flaky Cu rolled powder. As a result, the CNT content was 0.67mass% after 5 steps powder rolling. It was about twice as that of as-coated Cu-CNT composite powder without rolling. The grain size of PM extruded Cu-CNT composite was about one fifth of that of the extruded monolithic Cu material without CNT. Yield stress of the extruded Cu-CNT composite via the rolling process was 192 MPa, which is about twice that of the extruded monolithic Cu material (88 MPa). CNTs distributed at primary particle boundaries were effective to prevent the grain coarsening by their pinning effects, and this grain refinement was the main strengthening factor of the Cu-CNT composite via rolling process.

Abstract: Titanium has high specific strength, low elastic modulus, and good corrosion resistance. Especially, beta titanium alloys are used for jet engine, turbine blade in automobile and aerospace industries because of its good formability. Among the beta titanium alloys, LCB (Low-Cost Beta) titanium alloys were developed to make economical and mechanical advantages by not using high-cost beta stabilizer like Nb, Zr, Ta but using low-cost beta stabilizer like Mo, Fe, Cr, etc. In LCB titanium alloys, adding a small amount of boron makes grain refinement in cast ingot. This study has analyzed the changes of microstructure which can change mechanical properties after heat treatment and the plastic deformation in case of adding a small amount of boron.

Abstract: The applications of laser scanning confocal microscopy (LSCM) system were introduced to the track the phase transformation of steel. For the low and high carbon steel, the microstructural change along various thermal cycles was directly observed with LSCM system. The nature of non-themoelastic martensite was directly observed. Furthermore, direct checking of the nucleation site of ferrite phase was carried out for aluminum killed weld metal and titanium killed-steel. It was carried out concerning to acicular ferrite formation in high strength and low alloy steel. The assessment of the pinning effect of alumiunu nitride on the austenite phase at high temperature was also carried out by using LSCM system.

Abstract: Lead-free ceramics with the composition of Mn-doped Ba(Zr_0.06Ti_0.94)O₃ – xmol%MnO₂ (BZT6-xMn, x=0, 1 and 2) have been synthesized by a mixed oxide process. Structure and micrograph characterization were examined using X-ray diffraction (XRD) and scanning electron microscopy (SEM), which revealed that all samples have the orthorhombic phase with dense and the uniform microstructure. The grain size of Mn-doped BZT6 ceramics decreased with increasing MnO₂ doping. The effect of MnO₂ doping on the dielectric and ferroelectric properties was also investigated. The maximum dielectric permittivity decreased dramatically with increasing MnO₂ doping in the BZT6 ceramics. A peculiar double-hysteresis-like loop was observed. Mn ions as acceptor doping occupy the B site of ABO₃ in the form of Mn²⁺ and Mn³⁺, which brought an increase in the oxygen vacancies concentration. The defect dipole model was explained the pinning mechanism.

Abstract: Study researches durable factors of the grain size in 18CrNiMo7-6 by heating and cooling method. Results show that alloying elements Nb and Al have a strong pining effect on the grain boundaries, Al/N ratio between 2.0-2.5 can promote the grain not to grow up for a long time, and refine grains significantly with furnace heating and after carburizing-quenching once, and can inhibit the abnormal growth of individual grains.

Abstract: We report a fabrication of high-density nanodots by photodoping in overdoped Bi2Sr2CaCu2O8+d thin film (Tc = 80 K). A scanning near-field optical microscope probe is used to locally excite carrier, and photodoped region is associated with lower Tc phase (Tc = 75 K) via overdoping. Nanoscale characterizations with optical reflectivity reveal that nanodots (30-nm diameter) are regularly distributed in 50-nm step. The resultant films with photoinduced nanodots enhance Jc, a situation being similar to strong pinning effects observed in films modified by either ion irradiation or sputtered nanoparticles. These results suggest that photoinduced nanodots with lower Tc act as effective pinning centers.

Abstract: The effect of Al2O3 addition on sinterability of Tetragonal Zirconia Polycrystal powders including 3mol% Y2O3 (3Y-TZP) was investigated. Each 3Y-TZP powder dispersed with 0.3, 0.6, 0.9 and 1.2wt.% of Al2O3 was prepared by the spray-drying method. The prepared powders were pressed into a disk type and sintered at 1350
, 1400
, 1450
and 1500
for 2 hours in the air. Resultant microstructures and mechanical properties of specimens were investigated by using Vickers/Micro hardness Tester, FE-SEM and XRD. Most of the specimens showed high relative density over 99% and a higher fracture toughness than pure 3Y-TZP. Al2O3 particles dispersed in 3Y-TZP microstructure depressed grain growth of 3Y-TZP by the pinning effect. Increase in fracture toughness of 3Y-TZP was explained by the crack deflection due to dispersed Al2O3 particles.