Preparation And Properties Of Vanadium Alloys By MA-SPS

Vanadium alloys are the preferred structural material candidate for fusion blanket application because of their good formability,low activation,paramagnetic,good compatibility with liquid lithium.But V-4Cr-4Ti alloy prepared by smelting process exhibits a low tensile strength due to the coarse grains.What's more,the Ti(CNO)precipitates decomposes at heat treatment temperatures higher than 1273K.In order to improve the stability of precipitates that can refine grains and increase the materials'strength,a proper amount of YH2 was added to vanadium alloys by mechanical alloying(MA).And a series of powders were prepared including V-4Cr-4Ti-1.7YH2,V-4Cr-4Ti-0.3YH2,V-4Cr-0.3YH2,V4Cr-2Zr-0.3YH2,to the formation of a high-density nano-structured precipitates in vanadium alloys,which is stable at high temperature.The effects of mechanical alloying process parameters such as speed,ball-to-material ratio and grinding ball diameter,were analyzed and it was found that the best MA process is rotation speed 200rpm,ball material ratio 10:1(single diameter)and milling time 50 h.The MA powders were consolidated by the spark plasma sintering(SPS).According to the analysis of alloy densities under different sintering conditions,the best sintering condition is 1250? for 5min.The microstructures of vanadium alloys with different contents of YH2 prepared by MA-SPS were analyzed.There are irregular blocks about 100 nm and near-spherical nano-scale precipitates along grain boundaries.Deceasing the YH2 contents,the block precipitates are not found in vanadium alloys with the same sintering conditions.And the nano-structured precipitates density decreased.On the basis of the further analysis of.microstructural differences and mechanical properties of V-4Cr-4Ti-0.3YH2,V-4Cr-0.3YH2 and V-4Cr-2Zr0.3YH2,the following conclusions were obtained:(1)The majority precipitates of V-4Cr-4Ti-0.3YH2 are Y2O3 with hexagonal crystal structure,which is spherical with the size of about 10-30 nm and is irregular shape with the size above 30nm.The mean grain size is about 291nm and the strength increases owing to the precipitates pinning effect.However,the plasticity decreases because of the formation of precipitates distribution along grain boundaries.(2)The bimodal grain distribution of V-4Cr-0.3YH2 prepared by SPS was characterized by EBSD.And the average grain sizes are 17.5 ?m and 328 nm respectively.The precipitates are spherical Y2O3 with a size distribution of 10-50 nm.The alloy exhibits excellent comprehensive mechanical properties.The compressive strength at room temperature is 3569MPa and the uniform elongation is 22.1%.(3)The nano-scale precipitates are formed in the Zr containing vanadium alloy.The polygon particles(50-100 nm)is Zr-O-rich.But the near-spherical precipitates(10-50 nm)is Y-Zr-O-rich,which is consisted with Y4Zr3O12.The vanadium alloy is obviously refined by the addition of Zr but still is bimodal.The average grain sizes are 3.6 ?m and 410 nm respectively.The yield strength is increased by 100 MPa.But the coarse Zr-O-rich particles make the grain boundary bonding weaken,which reduced the plasticity of V-4Cr-2Zr0.3YH2.