Theoretical Study On Thermodynamic And Interfacial Properties Of Tungsten And Tungsten Alloys For Fusion Reactors

Nuclear fusion energy is considered to be the most promising ultimate energy to solve human energy problems,which has numerous advantages,such as superior economic performance,safety and non-environmental pollution,etc..Tungsten(W)and tungsten alloys have been considered as potential plasma-facing materials(PFMs)in fusion reactor due to their high melting point and excellent mechanical properties.However,the extreme service environment poses great challenges to their application,especially the problem of tungsten brittleness and hydrogen isotope retention behavior of the material.There are still some problems in the research,for example,experiment researches only obtained tungsten toughening mechanism at the finite temperature,the effects of typical alloying elements on the behavior of hydrogen in tungsten have not been systematically studied,there are few studies on the interface between hydrogen escape to tungsten and other materials,as well as the theoretical mechanism to strengthen interface cohesion.By means of highly accurate first principles calculation based on density functional theory,the present work is dedicated to carry out theoretical research according to the above areas,and the main conclusions are as follows:(1)The thermodynamic properties of W and WRe alloys were obtained by first principles calculation,quasi harmonic Deby model and thermo electron excitation,as well as the variation of elastic properties and G/B values in the temperature range from 0 to 2000K.The derived results demonstrate that the alloy element Re reduces the mechanical properties and ideal strength of tungsten.The study of ideal strength shows that the failure plane of W and WRe alloy is(100)plane.The mechanical properties of W and WRe alloys decrease with the increase of temperature,while the brittleness increases with the increase of temperature.At a certain temperature,the G/B value of WRe is smaller than that of pure W,which indicates that Re can improve the brittleness of tungsten.By further studying the changes of elastic modulus,bulk modulus and G/B value with the temperature rising,it is found that the increase of temperature reduces the improvement effect of Re on tungsten toughness.(2)The occupation of hydrogen in W,WRe and WMo alloys was researched by theoretical calculation.According to research findings,hydrogen at the tetrahedral(T)site can hold on to their BCC structure,and hydrogen at the octahedral(O)site eventually relaxes into the BCT structure.Meanwhile,the WH,WRe H and WMo H phases can exist stably from the mechanical aspect,and the toughening effect of Re on WH phase is more obvious than that of Mo.In addition,this article also studied the solubility and diffusion coefficient of hydrogen in W,WRe and WMo alloys.The results show that the addition of alloying elements Re and Mo has an opposite tendency for the dissolution of hydrogen in tungsten,i.e.,Re reduces the dissolution and Mo promotes the dissolution.However,the addition of alloy elements Re and Mo can decrease the diffusion barrier of hydrogen in tungsten and thus promote the diffusion coefficient of hydrogen.Considering the effect of hydrogen solubility and diffusion coefficient,it is shown that the addition of Re to W decreases the permeability of hydrogen,while the addition of Mo increases the permeability of hydrogen.(3)By comparing the binding energies of hydrogen between WFe bulk and W/Fe interface,it is found that there is a negative binding energy at the interface,which indicates that the formation of the interface promotes the stability of hydrogen.Moreover,the solubility of hydrogen at the interface in the temperature range of600-1600K is obtained using the Sievert's law.It is noted that the solubility of hydrogen at the interface is much higher than that of metallic W and Fe,which indicates that hydrogen facilitates to accumulate and form bubbles at the interface.At the same time,the interface hydrogen solubility decreases with the increase of temperature,while the hydrogen solubility in WFe bulk is opposite,which means that the effect of the interface will gradually decrease with the increasing temperature.In addition,the effect of hydrogen on the interface cohesion was also obtained in this paper.We can draw a conclusion that hydrogen at the interface O1,O6 and T5 sites promoted the interfacial binding properties,while hydrogen at the interface T1,T2,O2 and O4 sites had an opposite effect.(4)The structural stability and cohesion properties of alloy elements Re and Cr at the interface are studied,and the results show that Re and Cr atoms are more inclined to replace Fe atoms in the interfacial layer.The substitution of Re atom for Fe atom can improve the interface strength and stability,and increase the fracture toughness of the interface.However,the interface cohesion can be enhanced only when Cr replace Fe atom in the interfacial layer.In addition,in order to comprehensively consider the interface strength and interfacial fracture toughness,and to reduce the physical performance gap between W and Fe,a W₄/W₂Fe₂/Fe₃W₁/Fe₄ four-layer graded design is thus proposed.Finally,this paper provides a deep understanding of the conclusions from the perspective of electronic structure and charge transfer.In this paper,the thermodynamics and interfacial properties of tungsten and tungsten alloy are systematically discussed,the influence mechanism of temperature on the strength and toughness of W and WRe alloy is revealed,and the effects of typical alloying elements on the behavior of hydrogen in tungsten and tungsten interface were studied.The results show that the alloying element Re can improve the toughness of tungsten,promote the interface cohesion and inhibit the retention of hydrogen in tungsten.The addition of Re in tungsten has better performance than that of Mo and Cr.Furthermore,the mechanism of hydrogen bubble formation at the W/Fe interface is discussed,and the scheme of strengthening the W/Fe interface cohesion is also explored,which provides a reference and theoretical basis for the design and selection of materials used in fusion reactors.