| Controlled thermonuclear fusion reaction energy is one of the most promising future energy source. However, there are many technical issues to be solved before we effectively using the fusion energy. The selection and preparation of the first wall materials in fusion reactor is one of the key problems. W and 316 L stainless steel are considered as ideal materials for manufacturing nuclear fusion reactor. W has advantages of good thermal stability, low coefficient of thermal expansion, good thermal shock resistance, high strength at elevated temperatures, low sputtering yield, high sputtering threshold and low tritium retention. However, due to its brittleness at low temperatures, tungsten must be supported by other structural materials with low temperature toughness in order to make it works. 316 L stainless steel is one of the most ideal supporting materials for the first wall structure. Therefore, the combination of tungsten and stainless steel and the preparation of their materials has become an important topic in the study of the first wall materials.So far, the joining of W and 316 L was mainly achieved by brazing or solid diffusion bonding, which may bring about undesirable third phase or severe thermal stress. In this research, we fabricated the 316L-(316L+50W)-W FGMs under the condition of 1050?*45.5MPa*3mins and 316L/316L-20W/316L-50W/316L-80W/W FGMs with conditions of 1050?*45.5MPa*3mins by mechanical alloying(MA) and spark plasma sintering(SPS) technology. The 316L/W MMC were also prepared by traditional casting process for comparative study. The following related tests and analysis were also completed to accumulate some theoretical and experimental basis for the development of the first wall materials.The morphology, phase and elemental composition of processed powder, interface of composites or joints were characterized by OM, XRD, SEM and EDS. The results show that with the increase of milling time, the mechanically activated W powder particles become thinner and smoother, with some broken fragments aggregated or inserted into the severely deformed 316 L particles. For 316L-(316L+50W)-W gradient composites, the microhardness of transition layer was always between that of 316 L and W plate. Interface with short milling time appears hardness gradient; interface with long milling time appears hardness mutation; moderate pre-alloying technology is more favorable for sintering integral, poreless, smooth transitive gradient composite.A distinguishable gray belt surrounding the retained W particles were formed in 316L/316L-20W/316L-50W/316L-80W/W FGMs. Such a belt, which has a increasing width depending on increasing milling time and mainly contains Fe7W6, Fe3W3 C and Fe2 W phases, is bound to be a transitional region between the retained W particles and the316 L matrix. The composite materials with high interfacial bonding strength, low sintering defects and gradient changes can be obtained by controlling the milling time.For casting 316L/W composite, structure evolution mainly resulted from particle pushing and diffusion reaction on the solid/liquid interface, and residual porosity were always existed. Short holding time resulted in weaker interface bonding for insufficient diffusion reaction. When holding time extended, W particles undergo serous dissolving, diffusing, precipitating and clustering. Although W particles improve the friction coefficient of the 316 L matrix and change their friction mechanism, but composites' microstructure were difficult to control for W particles undergoing dynamic dissolution, diffusion and segregation. So, it is not easy to obtain regular or integral reinforcing particles for casting 316L/W composite.Compared to casting process, SPS and MA process are more convent to regulate the component, microstructure and properties of 316L/W MMCs. Such as proportion and uniform dispersal of W content, density and porosity of composite, composition gradient or interface diffusion reaction. |
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