Study On The Effect Of Tungsten Content On The Microstructure And Tensile Properties Of Ni-xW-6Cr Alloys

Molten salt reactor(MSR) has been envisioned as one of the six promising Generation IV reactors in the fourth International symposium, in respect of its safety, economical and environmental performance. However, due to its unique design where fission fuel is dissolved in the molten salt, the structural material is directly subjected to fission fuel and high temperature molten salt, making the structural material suffer from a complex operating condition of high temperature, serious fluoride salt corrosion and strong neutron irradiation. Hastelloy N alloy has been the most successful structural material for molten salt reactor because of its excellent performance. But Hastelloy N is limited to use in applications to a maximum allowance temperature of 725℃. As high efficiency of energy-convertion applications such as hydrogen production should be operated above 800℃, the structural material has become a significant bottleneck for the development of molten salt reactor, so there is a need for structural alloy designed to possess good performance at higher temperature.The National Centre for Scientific Research(CNRS, France) has focused efforts on the development of the EM-721(Ni-26W-6Cr)alloy, which bases on the Hastelloy N alloy and use W as a substitute for Mo in Hastelloy N. Their preliminary results indicate that EM-721 alloy performs well on tensile and creep properties at 850℃. But there is still a lack of mechanical study on low Cr content Ni-W-Cr alloys and the root of the problem lies in the uncertainty of strengthening mechanism of W. So systematic study on the effect of W on the mechanical properties of Ni-x W-6Cr alloys seems necessary for composition optimization.In this study, the microstructure and tensile deformation behavior of Ni-(10-35 wt. %)W-6Cr alloys have been systematically investigated. This study relies on the characterization methods of SEM, TEM, XRD, etc., combining with phase diagram calculation and relevant alloy design theory, to study the strengthening mechanism of Ni-x W-6Cr alloys and the optimum W content. The main results of this study are as follows:1. Ni-x W-6Cr alloys with W content ranging from 10 wt. % to 30 wt. % show a typical equiaxed grain structure with an average grain size of about 210 μm. In the case of Ni-35W-6Cr alloy, there are lots of precipitation of α-W phase uniformly distributed in the matrix and the average grain size is about 20 μm.2. W atoms expand the lattice of Ni-x W-6Cr alloys, and 5wt. % W leads to 0.22-0.31% lattice expansion.3. Mechanical properties of Ni-x W-6Cr alloys at room temperature and intermediate temperature: When tested at room temperature, the YS and UTS increase monotonously with increasing W content, and it is worth noting that both YS and UTS increase sharply when W content increases from 30 wt. % to 35 wt. %. Similarly, with increasing W content, EL also slightly increases when W content is less than 30 wt. %, but decreases when the W content is 35 wt. %, which indicates that the tensile properties of the alloys are highly sensitive to the W content. Fracture morphologies of the alloys after tensile test at room temperature indicate that the alloys exhibit a transgranular ductile fracture behavior. When tested at 650℃, the YS and UTS increase monotonously with increasing W content, and they also increase sharply when the W content increases from 30 wt. % to 35 wt. %. The EL is relatively low and does not show a clear trend, but it increases when W content increases from 30 wt. % to 35 wt. %. Fracture morphologies of the alloys after tensile test at 650 ℃ indicate that the alloys exhibit a intergranular brittle fracture behavior.4. The strengthening mechanism of Ni-x W-6Cr alloys includes solid solution strengthening, twin strengthening(decrease of stack fault energy), fine grain strengthening and second phase strengthening. Intermediate temperature brittleness generally exists in Ni-x W-6Cr alloys and it is mainly caused by the uncoordinated deformation of grains and grain boundaries.5. The optimum W content of Ni-x W-6Cr alloys designed for MSR is 27.7-34 wt. %.