| The process flow of made V-4Cr-4Ti alloys by VAR was explored.The structure, Vickers hardness and surface oxidation of as-melt and annealing alloys were studied by OM (optical microscopy),SEM(sacnning electron microscopy),TEM(transmission electron microscopy),XRD(X-ray diffraction) and XPS(X-ray photoelectron spectrum).The main conclutions are reported as following:Through the process flow of pure metal powder→mix powder→cold isotropic press electrodes→weld self-consumable electrodes→VAR,the nominal V-4Cr-4Ti alloy(～400 g) was successfully fabricated.The real chemical composition of the alloy is V-3.SCr-3.6Ti,and the impurities concentration of C,O and N are rather high.The size of equiaxed grains is not uniform,ranging from 10μm to 500μm.The average Vickers hardness is 282.8.A V-3.7Cr-3.6Ti alloy,about 900 g in weight,was also prepared by improved process parameters.The concentrations of O,N are decreased dramatically.The sizes of equiaxed and column grains are also not uniform.The Hv of the alloy is 217.3.The two alloys are typically multiphase structures,which contains many of second phases,dislocation,and so on.The dominant second phase is the titanium riched,and chromium,vanadium and zirconium-alloyed oxycarbonitride,Ti-(CNO),with FCC structure and lattice parameter of 0.423nm～0.428 nm.The Ti-(CNO) phase has a shape of plates with the thickness up to 100 nm～150 nm and the other two dimentions up to 1μm～3μm,arranged as precipitation chains in the bulk of grains and form continuous films at the grain boundaries.The first alloy with higher impurity content also contains some globular precipitations with diameter of 10 nm～40 nm.At 1×10-2Pa,The V-3.7Cr-3.6Ti alloy were annealed 1 hr at 1000℃,1100℃and 1400℃,and annealed 1 hr and 3 hr at 1200℃and 1300℃,respectively.With temperature going up and time increase,the oxidations of the grain boundaries are more serious but the grain growths are not observable.There are two type of new precipitations appearing,globular phase with diameter of 30 nm～50 nm and 5 nm～10 nm and more finer lamellar phase,while the primary "plate" Ti-(CNO) became coarsening.The lattice parameter of vanadium solution goes down with temperature increasing and the hardness changes correspondingly. The dislocation density decreases but the stacking fault density increases after annealing.The annealing twins are also observed.The bend fractures of the alloy changes from typically brittleness to the brittleness and ductile slowly with increasing annealing temperature.The surface oxidation of V-3.7Cr-3.6Ti alloy after annealed at 1×10-2Pa,1100℃/1hr and 1300℃/1hr were studied by XPS.The results show that under these two conditions there are clear but very thin oxide layers formations.The thickness and component of oxide layer are different from annealing parameters,while the structure and elements along depth direction are similar:the outer of oxide layer are the high valence oxide of V and Ti;the inner are low valence oxide mixed with carbides and nitrides of V and Ti.Within the whole oxide layer,the Cr is absence and Ti is segregated at the surface.The external oxide is the dominate way. |
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