Characteristics and formation mechanisms of defect bands in vacuum-assisted high-pressure die casting AE44 alloy

The microstructure in vacuum-assisted high-pressure die casting(HPDC) Mg-4Al-4RE(AE44) alloy was studied. Special attention was paid to the characteristics of defect bands and their formation mechanisms. Since double defect bands are commonly observed, the cross section of die cast samples is divided into five parts with different grain morphologies and size distributions. The inner defect band is much wider than the outer one. Both the defect bands are solute segregation bands, resulting in a higher area fraction of Al;RE;phase than that in the adjacent regions. No obvious aggregation of porosities is observed in the defect bands of AE44 alloy. This may be due to a narrow solidification temperature range of AE44 alloy and a large amount of latent heat released during the precipitation of intermetallic phases. The formation of the defect bands is related to the shear stress acting upon the partially solidified alloy, which can lead to collapse of the grain network. However, the generation mechanisms of shear stress in the outer and inner defect bands are quite different.

Defect band formation in high pressure die casting AE44 magnesium alloy

The characteristics of defect bands in the microstructure of high pressure die casting(HPDC)AE44 magnesium alloy were investigated.Special attention was paid to the effects of process parameters during the HPDC process and casting structure on the distribution of defect bands.Results show that the defect bands are solute segregation bands with the enrichment of Al,Ce and La elements,which are basically in the form of Al_(11)RE_(3) phase.There is no obvious aggregation of porosities in the defect bands.The width of the inner defect band is 4-8 times larger than that of the outer one.The variation trends of the distribution of the inner and outer defect bands are not consistent under different process parameters and at different locations of castings.This is due to the discrepancy between the formation mechanisms of double defect bands.The filling and solidification behavior of the melt near the chilling layer is very complicated,which finally leads to a fluctuation of the width and location of the outer defect band.By affecting the content and aggregation degree of externally solidified crystals(ESCs)in the cross section of die castings,the process parameters and casting structure have a great influence on the distribution of the inner defect band.

Design of Novel Compact Electromagnetic Bandgap Structures with Enhanced Bandwidth

Two kinds of compact electromagnetic band gap （EBG） structures are designed. A two layer compact EBG structure configured with cross spiral shape line inductors and interdigital capacitors is first presented. Because of its significantly enlarged equivalent inductor and capacitance, the period of the lattice is approximately 4.5% of the free space wavelength. By insetting several narrow slits in the ground plane, the bandwidth of the main bandgap is enhanced by nearly 19%. Further effort has been made for designing a three layer compact EBG structure. Simulation results show that its period is reduced by about 26% compared to that of proposed two layer EBG structure, and the bandwidth of the main bandgap is about 3 times as that of the proposed two layer EBG structure. The detailed designs including a two layer compact 3×7 EBG array with and without defect ground plane and the three layer EBG array are given and simulation results are presented.

Effect of electron traps on long afterglow behavior of Sr_3Al_2O_6:Eu_(0.01)^(2+),Dy_(0.02-x)^(3+),Ho_x^(3+)

The long persistent phosphors Sr3Al2O6：Eu0.012＋,Dy0.02-x3＋,Hox3＋ （x=0, 0.01, 0.02） were prepared by a high temperature solid state reaction. All samples showed a broad band emission peaking at 510 nm, which could be ascribed to Eu2＋ transition between 4f65d1 and 4f7 electron configurations. With the increase of substitution of Ho3＋ ions for the Dy3＋ ions in the as-prepared phosphors Sr3Al2O6：Eu0.012＋,Dy0.02-x3＋,Hox3＋ （x=0, 0.01, 0.02）, the initial intensity of the afterglow obviously decreased. From the thermoluminescence （TL） curves of the samples, we concluded that codoped Ho3＋ ions led to a decline of the trap depth and redistribution of the trap. This may be responsible for the change of afterglow of Sr3Al2O6：Eu0.012＋,Dy0.02-x3＋,Hox3＋ （x=0, 0.01, 0.02）.