Effect of Ni and vacancy concentration on initial formation of Cu precipitate in Fe–Cu–Ni ternary alloys by molecular dynamics simulation

In the present work, the effects of Ni atoms and vacancy concentrations （0.1%, 0.5%, 1.0%） on the formation process of Cu solute clusters are investigated for Fe-1.24%Cu-0.62%Ni alloys by molecular dynamics （MD） simulations. The presence of Ni is beneficial to the nucleation of Cu precipitates and has little effect on coarsening rate in the later stage of aging. This result is caused by reducing the diffusion coefficient of Cu clusters and the dynamic migration of Ni atoms. Additionally, there are little effects of Ni on Cu precipitates as the vacancy concentration reaches up to 1.0%, thereby explaining the embrittlement for reactor pressure vessel （RPV） steel. As a result, the findings can hopefully provide the important information about the essential mechanism of Cu cluster formation and a better understanding of ageing phenomenon of RPV steel. Furthermore, these original results are analyzed with a simple model of Cu diffusion, which suggests that the same behavior could be observed in Cu-containing alloys.

Modulating oxygen vacancy concentration for selective growth of semiconducting single-walled carbon nanotubes with narrow diameters

Semiconducting single-walled carbon nanotubes(s-SWCNTs)with narrow diameters are promising for future applications in many fields,especially in nanoelectronics and optoelectronics.In this study,the oxygen vacancy concentration modulating strategy was utilized for growing narrow diameters s-SWCNTs by the chemical vapor deposition(CVD)method.The Fe_(0.01)Mg_(0.99)O solid solution based catalyst was syn-thesized to anchor the Fe particles and inhibit aggregation for growing SWCNTs with uniform diameters.CeO_(2)was introduced into the catalyst to provide oxygen vacancies through H_(2)prereduction.These oxygen vacancies could form an oxidative environment during the growth of SWCNTs,and the chemically active metallic carbon nanotube caps are selectively etched away under this environment.The Fe/Ce molar ratio and H2 prereduction time were optimized to modulate the oxygen vacancy concentration.Ultimately,us-ing the Fe_(0.01)Mg_(0.99)O/CeO_(2)(3)catalyst with 10 min of H_(2)prereduction time,high purity s-SWCNTs with diameters ranging from 1.41 to 1.71 nm and a content of 95.1%were obtained with high selectivity and carbon yield(1.33 wt%).The mechanism behind this phenomenon was elucidated through experimental characterizations and first-principle simulations,further expanding the understanding of the growth of s-SWCNTs through the modulation of oxygen vacancy concentration.

Effect of vanadium on the room temperature ferromagnetism of V-doped 6H-SiC powder

This study focuses on the effect of V-doping on the ferromagnetism（FM） of 6H-SiC powder.The X-ray diffraction results indicate that V is inserted into the 6H-SiC lattice.The Raman spectra reveal that with a V concentration of 25 ppm,the crystalline quality and carrier concentration of 6H-SiC are hardly varied.It is found that after the V-doping process,the saturation magnetization（Ms） and the vacancy concentration of 6H-SiC are both increased.From these results,it is deduced that the effect of V might contribute mainly to the increase of vacancy concentration,thus resulting in the increase of Ms of V-doped 6H-SiC.

Thermal Wave Effect and Sintering Activation Energy at the Initial Stage of Field Assisted Sintering Process for Non-conductive Al_2O_3 Powders

The effect of thermal wave at the initial stage for non-conductive Al_2 O_3 powders compact in field assisted sintering technique(FAST) was investigated. The Lord and Shulman type generalized thermoselastic theory was introduced to describe the influence of thermal-mechanical interaction, as well as the heat transport and thermal focusing caused by thermal wave propagation. The expression of vacancy concentration difference of the particles was deduced by considering transient thermal stress. Subsequently, the relationship between activation energy and vacancy concentration difference was obtained. The mechanism of surface diffusion, volume diffusion, simultaneous surface and volume diffusion was analyzed. The numerical simulations indicate that low sintering temperature can obtain high local temperature by the superposition effect of thermal wave. Vacancy concentration differences were improved during FAST compared with hot-pressure and pressureless sintering, thereby decreasing the sintering time. By contrast, the activation energy declined with the decrease of vacancy concentration difference in the neck growth process.

STABILIZATION OF MARTENSITE AND PARENT PHASE ORDERING IN A Cu-Zn-Al ALLOY

The reverse transformation has never occurred in a quenched and aged specimen upon heating from room temperature to 320℃ at which the martensite disappears thoroughly.Both B2 9R and DO_318R coexist in step-quenched and short-time isothermally treated specimen. As prolonging the isothermal holding,the B29R disappears and DO_318R intensifies. B29R occurs only in specimen quenched and up-quenched immediately to 100—150℃. Rapid quenching from high temperatures will depress the B2→DO_3 ordering but not the A2 →B2 one.A plenty enough concentration of vacancy is a necessary condition of occurrence of B2→DO_3 ordering.The possible mechanism for the stabilization of martensite in a Cu-Zn-Al alloy was also discussed.