Microstructural characterization of cobalt treated by laser surface melting under different powers

The effect of laser power outputs on microstructural evolution of cobalt(Co) treated by laser surface melting(LSM) are systematically characterized by electron channeling contrast and electron backscatter diffraction techniques equipped in scanning electron microscope. Results shows three distinctly different microstructural characteristics from surface to substrate in Co samples treated by LSM: solidification substructure(cell and cellular-dendritic) and phase transformation structure(ε martensitic plates) in the elongated columnar grains for melting zone(MZ), ε martensitic plates in insufficient recrystallization grains for heat affected zone(HAZ),and fully recrystallization microstructure for base metal(BM). Although the similarity of general evolution profiles for LSM treatment on Co under different laser power outputs, the trend of the penetration and width depth for MZ is obviously increased with increase of laser power outputs, and cellular substructure is more pronounced in the specimens treated by the higher power. The metallurgical mechanisms for LSM treatment of Co, which are closely related to the interaction between heat sources and metal, are systematically discussed in this paper.     

Inhomogeneous deformation and recrystallization behavior of through-thickness tantalum sheet under one-cycle clock-rolling

Effects of low clock rolling passes on through-thickness deformation and recrystallization behavior of the Ta sheet have been investigated. The clock-rolled samples with one cycle (8 passes) were systematically examined under different thickness layers. X-ray diffraction (XRD) and X-ray diffraction line profile analysis (XLPA) showed that the extremely inhomogeneous texture evolution generated and the bulk stored energy existed in the through-thickness clock-rolled Ta sheet after 8 passes. The electron backscatter diffraction (EBSD) results revealed the misorientation of the grains in the deformed samples, indicating that grain subdivision in the surface and center layer was more serious than that in the quarter layer. High intensities misorientation indicates the presence of microshear bands in the surface and center layer. During annealing, the difference of the stored energy and the fragmentation of deformation microstructure along the thickness led to a heterogeneous driving force for the nucleation of the grains, resulting in subsequent different recrystallization rate in the different regions of the sample.     

Twinning,grain orientation,and texture variations in Mg alloy processed by pre-rolling

A pre-rolling(PR) process with a small pre-strain was applied along the transverse direction(TD) and the extrusion direction(ED) of non-basal and basal textured AZ61 Mg alloys at room temperature to investigate the induced twinning behavior. The microstructural evolution of the alloys was used to evaluate the grain-boundary effect in terms of the texture variation. The results demonstrated that■ extension twinning was introduced by the PR process and that the volume fraction of twins increased with increasing thickness reduction. The subdivision of grains via twinning induced grain reorientation that generated a prominent basal texture with the c-axis parallel to the normal direction(ND) after PR. The textured Mg alloy with this c-axis//TD feature can promote twinning activity. The twinning performance was critical to initiate plastic deformation, therefore to determine deformation behavior at room temperature. The deformation mechanism was also addressed related to the extension twin lamellae.     

Simulation of dynamic recrystallization for an Al-Zn-Mg-Cu alloy using cellular automaton

A cellular automata (CA) model has been developed to predict and control the microstructure evolution during hot deformation on 7085 aluminum alloy. The initial microstructure and thermal-mechanical parameters were used as the input data of the CA model. To link microstructure evolution with macroscopic flow stress, dislocation density was set as an important internal state variable. The hot deformation behavior of 7085 aluminum alloy was studied by isothermal compression tests under a deformation temperature range of 623–723 K and a strain rate range of 0.001-1s^?1 up to true strains of 0.53–1.20. Electron backscattered diffraction technique and the CA model were utilized to systematically investigate the effects of strain, strain rate and deformation temperature on the microstructure evolution, and further to predict the average grain diameter and the recrystallization fraction after deformation. The simulated results were validated by the experimental data to demonstrate the feasibility and predictability of the CA model.     

冷却速率对AZ91D镁合金半固态组织制备的影响

：研究了不同冷却速率(金属型、湿砂型和干砂型)对AZ91D镁合金半固态组织制备的影响．结果表明：增加冷却速率有利于初始凝固组织中存在的非平衡组织的分散细化．原始组织中的非平衡共晶组织在加热过程中大部分扩散溶解而溶入基体中,剩余部分在加热过程中首先熔化．冷却速率越大的试样在熔化过程中更容易发生二次枝晶臂之间的合并．提出了半固态熔化过程可分为成分均匀化、共晶熔化和部分初生相的熔化和球化三个阶段,不同熔化阶段时,控制性因素不同．熔化后的半固态组织中固态颗粒的尺寸和形貌主要与初始组织的形貌、加热过程中非平衡组织的溶解速度及加热速度有关。     

下扎深度对AZ91镁合金回填式搅拌摩擦点焊接头组织和断裂行为的影响

We used refill friction stir spot welding (RFSSW) to join 2-mm-thick AZ91D-H24 magnesium alloy sheets, and we investigated in detail the effect of tool plunge depth on the microstructure and fracture behavior of the joints. A sound joint surface can be obtained using plunge depths of 2.0 and 2.5 mm. Plunge depth was found to significantly affect the height of the hook, with greater plunge depths corresponding to more severe upward bending of the hook, which compromised the tensile-shear properties of the joints. The hardness reached a minimum at the thermo-mechanically affected zone due to the precipitation phases of this zone as it dissolved into the α-matrix during the welding process. The fracture modes of RFSSW joints can be divided into three types: shear fracture, plug fracture, and shear–plug fracture. Of these, the joint with a shear–plug fracture exhibited the best tensile-shear load of 6400 N.     

Photo-improved hydrogen evolution reaction activity of the Pt/CdS electrocatalyst

The effect of CdS content on the photo-induced improvement of hydrogen evolution reaction activity of the Pt/CdS electrocatalyst was investigated. Although the electrons transferred from CdS to Pt in the Pt/CdS electrocatalyst increased with the CdS content under illumination, the electrochemical active surface area of the Pt/CdS electrocatalyst decreased with the CdS content, resulting that the photo-induced improvement of hydrogen evolution reaction activity of the Pt/CdS electrocatalyst decreased with the CdS content.     

Macrosegregation and the underlying mechanism in Ti-6.5Al-1.0Cr-0.5Fe-6.0Mo-3.0Sn-4.0Zr alloy

A novel method for the analysis of composition distribution of titanium alloys over a large area(64 mm × 72 mm) was investigated by exploring the original position statistic distribution based on spark spectrum(OPA-SS) in Ti-6.5 Al-1.0 Cr-0.5 Fe-6.0 Mo-3.0-Sn-4.0 Zr titanium alloy. The results showed that OPA-SS could characterize the distribution of elements in different positions on the titanium alloy. The macrosegregation of Sn was the most pronounced, with a statistic segregation degree higher than 18%; the macrosegregation of Mo followed with a statistic segregation degree of 10%; the macrosegregation of Al and Fe was relatively milder,lower than 8%. The main reason for the macrosegregation state of the as-cast Ti-6.5 Al-1.0 Cr-0.5 Fe-6.0 Mo-3.0 Sn-4.0 Zr alloy can be the solute redistribution during liquid solidification and the diffusion rate of each element in the solid phase.     

Sintering mechanism of Cu-9Al alloy prepared from elemental powders

The sintering behavior of Cu-9Al alloys prepared from die pressing of elemental powders was investigated. The experimental results and kinetic analysis showed the formation of three consecutive layers of Al₂Cu, Al₄Cu₉, and AlCu phases, with Al₂Cu appearing first in the initial solid phase sintering stage. A liquid phase formed in the intermediate stage, resulting from the eutectic reaction between Al and Al₂Cu phases at 500 °C, which is 47 °C lower than the equilibrium reaction temperature. Swelling occurred when the liquid phase infiltrated the gaps between the copper particles, leaving pores at the original sites of Al particles and Al₂Cu. In the final stage of sintering, the Al-rich phases (Al₂Cu and AlCu) transformed to Al-poor phases (Al₄Cu₉ and α-Cu) in the temperature range of 500–565 °C. Al₄Cu₉ and α-Cu then transformed to AlCu₃ (β) above the eutectoid reaction temperature (565 °C), whereas AlCu₃ transformed to α-Cu and eutectoid phases (α-Cu + Al₄Cu₉) during cooling. The pure copper transformed to AlCu₃, and the pore volume decreased at 1000 °C. The microstructure study helps manipulate precisely the sintering process of Cu-Al alloys and optimize the microstructure with a high dimensional accuracy.     

Deformation and damage features of a Re/Ru-containing single crystal nickel base superalloy during creep at elevated temperature

The deformation and damage features of a 4.5%Re/3.0%Ru-containing single crystal nickel-based superalloy during the creep in the temperature range of 1040–1070 °C and stress range of 137–180 MPa was investigated by means of creep properties measurement and contrast analysis of dislocation configuration. The results showed that the alloy exhibited a better creep resistance in the range of the testing temperatures and stresses, the deformation mechanism of the alloy during steady state creep was dislocations climbing over the rafted γ′ phase.In the latter period of creep, the deformation mechanism of the alloy was dislocations shearing into the rafted γ′phase. It is believed that the dislocations shearing into γ′ phase may cross-slip from {111} to {100} planes for forming the K-W locks to restrain the slipping and cross-slipping of dislocations on {111} plane. As the creep goes on, the alternate slipping of dislocations results in the twisted of the rafted γ′ phase to promote the initiation and propagation of cracks along the γ/γ′ interfaces up to creep fracture, which is considered to be the damage and fracture feature of alloy during creep at high temperature.     

Radiation-induced precipitation of transmutation elements rhenium/osmium and their effects on hydrogen behavior in tungsten

Here,we review recent computational findings focused on the behavior of transmutation elements (TEs) in Tungsten (W).W is chosen as one of the most promising materials for the divertor armor and plasma facing components in future fusion reactors.However,W in exposure of fusion neutrons can cause the transmutation reaction and produce TEs,such as rhenium and osmium.The presence of TEs can change chemical component of materials,and therefore inevitably influence the microstructure and properties of W materials.Multiscale simulation methods have been employed to investigate the dissolution,diffusion and radiation induced precipitation of TEs,as well as their effects on the behaviors of hydrogen isotopes in W.This systematic review can provide an insightful understanding to estimate the influence of TEs on the properties and performance of Wbased materials.     

Dielectric properties of porous SiC/Si_3N_4 ceramics by polysilazane immersion-pyrolysis

The SiC nanotubes were synthesized on porous Si_3N_4 matrix by polysilazane immersion-pyrolysis. The β-SiC and free C contents increased with the increasing dipping times, which cause some clear vibration and loss peaks of the dielectric constant patterns emerged at around 12.9 GHz and 14.7 GHz in the samples of 20 wt% and 30 wt%benzoic acid, respectively. The reflectivity of the obtained ceramics were -7 dB ～-10 dB from the frequency of 8 GHz–18 GHz, which means that the amount of 80%–90% electromagnetic wave could be attenuated. The ceramics containing 40 wt% benzoic acid showed the high loss, low reflectivity and a wide absorbing band,indicating the high absorbing efficiency and good dielectric properties.     

Room-temperature facile synthesis of MnO2on carbon film via UV-photolysis for supercapacitor

A facile UV-photolysis method has been developed to synthesize birnessite-type MnO_2 nanosheets on carbon film at room temperature for supercapacitor electrode materials. The results clearly demonstrated that the specific capacitance of carbon/MnO_2 composite electrodes was controlled by the adjustable UV-photolysis reaction time in KMnO_4 solution. For the optimized composite electrode obtained after 2 h reaction, a superior specific capacitance of 277 Fg~(-1) was achieved at current density of 0.2 Ag~(-1). This finding suggests that UV-photolysis is a potential method to fabricate MnO_2 on carbon film for supercapacitor.