Effects of anode material on the evolution of anode plasma and characteristics of intense electron beam diode

In this paper,three kinds of materials including graphite,titanium(Ti)and molybdenum(Mo)are used as anodes to figure out the influence factors of anode material on the characteristics of the intense electron beam diode.The results show that the characteristics of diode are mainly determined by the cathode plasma motion under a 15 mm diode gap,in which the typical electron beam parameters are 280 kV,3.5 kA.When the diode gap is reduced to 5 mm,the voltage of the electron beam reduces to about 200 kV,and its current increases to more than 8.2 kA.It is calculated that the surface temperatures of Ti and Mo anodes are higher than their melting points.The diode plasma luminescence images show that Ti and Mo anodes produce plasmas soon after the bombardment of electron beams.Ti and Mo lines are respectively found in the plasma composition of Ti and Mo anode diodes.Surface melting traces are also observed on Ti and Mo anodes by comparing the micromorphologies before and after bombardment of the electron beam.These results suggest that the time of anode plasma generation is closely related to the anode material.Compared with graphite,metal Ti and Mo anodes are more likely to produce large amounts of plasma due to their more significant temperature rise effect.According to the moment that anode plasma begins to generate,the average expansion velocities of cathode and anode plasma are estimated by fitting the improved space-charge limited flow model.This reveals that generation and motion of the anode plasma significantly affect the characteristics of intense electron beam diode.

Achieving high strength and ductility in high-entropy alloys via spinodal decomposition-induced compositional heterogeneity

The compositional heterogeneity in high-entropy alloys(HEAs)has been reported to be an inherent en-tity,which significantly alters the mechanical properties of materials by tuning the variation of lattice resistance for dislocation motion.However,since the body-centered cubic(BCC)structure is not close-packed,the change of lattice resistance is less sensitive to the normal concentration wave compared to that in face-centered cubic(FCC)structured materials.In this work,we selected a refractory bcc HEAs TiZrNbTa for the matrix and added a small amount of Al to facilitate the special spinodal decomposition structure.In particular,(TiZrNbTa)98.5 Al 1.5 displayed a typical basket-weave fabric morphology of spinodal decomposition structure with a characteristic periodicity of∼8 nm and had an optimal combination of strength and ductility(the yield strength of 1123±9 MPa and ductility of∼20.7%±0.6%).It was de-termined that by doing in situ TEM mechanical testing,the plastic deformation was dominated by the formation and operation of dislocation loops which provided both edge and screw components of dislo-cations.The synergetic effect of the remarkable chemical heterogeneity created by the spinodal decompo-sition and the spreading lattice distortion in high entropy alloys is quite effective in tuning the mobility of different types of dislocations and facilitates dislocation interactions,enabling the combination of high strength and ductility.

相溶解协同提升高熵合金的强度和塑性

热处理是金属材料热机械加工的常用手段.随着热处理温度的升高而湮灭的缺陷通常导致材料的塑性提升而强度降低.本研究中,我们通过提高热处理温度促进相溶解而协同提升了TiZrNbTa高熵合金的强度和塑性.当热处理温度从800提升至1250°C,合金的拉伸屈服强度提高了40%,达到1003±16 MPa.同时,合金的伸长率增加了近一倍,达到16.79%±1.03%.热处理温度提升引起的相溶解加剧了晶格畸变,从而增强了晶格摩擦应力并提升了屈服强度.相溶解也降低了界面失配并缓解了应力集中.此外,1250°C热处理合金中的局部化学有序结构促进了位错共平面滑移和位错增殖.两种机制共同提升了合金的塑性.该研究不仅扩展了关于金属材料中热处理和相溶解的理解,而且也为合金的强韧化设计提供了思路.