| Molybdenum-niobium alloy single crystal is an important material for preparing the emitter and receiver of advanced space equipment power supply system.At present,the commonly used preparation method is the electron beam floating zone melting,the technological parameters and niobium content in the single crystal growth process have significant influence on the microstructure performance of Mo-Nb alloy single crystal.Although excellent high temperature performance of Mo-Nb alloy single crystal,but expensive and sensitive,few reference is available.Only some of the researches mainly summary the common preparation methods,most of researchers are not clear about the organization performance and deformation mechanism of the materials,and current use of this material is very limited.If there is a deep insight into the performance of the Mo-Nb monocrystal,it will have practical implications for further broadening its applications.Based on previous work,the high purity Mo-Nb alloy single crystal is prepared by the electron beam floating zone melting method.The different growth parameters,niobium content and high temperature annealing,which effect on the properties of Mo-Nb alloy single crystal are studied,the microstructure after high temperature deformation and nanoscale mechanical behavior of Mo-Nb single crystal are investigated by isothermal compression and nanoindentation.The main conclusions are summarized as follows:1.Niobium plays an important role in the solid solution which can strengthening of Mo-Nb alloy single crystal.As content of niobium increase,the maximum strength of Mo-Nb alloy single crystal rises and the plasticity declines.When content of niobium is in the range of 03wt%,the yield strength drops sharply with increase of niobium content,and descends gradually at content of niobium is in the range of 36wt%.2.The dislocation density of Mo-Nb single crystal grown by the electron beam floating zone melting is on the order of 106 cm-2.The growth rate is closely related to the dislocation density and the single crystal dislocation density of Mo-Nb single crystal grown at 2.5 mm/min,2.0 mm/min and 1.5 mm/min were 3.37×106 cm-2,3.02×106 cm-2 and 1.42×106 cm-2,respectivley.At lower growth rates,the dislocation density is relatively low due to a sufficient time polyganization of the dislocations.The radial residual stress of Mo-Nb single crystal increase because the high growth rate resulting in a larger temperature gradient.The radial residual stress of Mo-Nb single crystal,which grew at 2.5 mm/min,is-770 MPa,much larger than which grown at 1.5mm/min?-379 MPa?and 2.0 mm/min?-413 MPa?.3.In Mo-Nb alloy single crystal,there is a small amount of precipitation phase in elliptic or definite crystal shape.On the basis of diffraction spots analysis of the precipitated phase,and in combination with EDS spectrum.It is found that the precipitated phase of diffraction spots come from the Nb2C[233?]direction of diffraction spots,indicate that the presence of carbide precipitation phase.4.With the increase of annealing temperature,the yield strength and tensile strength of Mo-Nb single crystal are decreasing.With 1800?/1 h annealing condition,the mechanical properties of Mo-Nb alloy single crystal tend to stabilize.At 900?/1h,1100?/1 h and 1300?/1 h annealing conditions,elongation rates are about 15%.After annealing at 1600?/1 h,the extension rate is 26%.After annealing at 1800?/1h,elongation rate is more than 30%.The extension rate of specimen after annealing at2000?/1h is about 13%.After the high temperature annealing,the specimen presents the different degree of substructure which still maintains the monocrystal structure and the original crystal orientation.As the annealing temperature increases,the bulk structure inside the crystal becomes larger,the amorphous size becomes larger,the number of subcrystals decreases,and the subcrystal boundary is relatively wide.After1800?/1 h annealing condition,the block structure has a tendency to refine and the number of subcrystals increases after annealing at 2000?/1 h.5.Isothermal compression of the Mo-Nb single crystal is conducted on a Gleeble-3500 thermal simulator at the deformation temperature range of 1100-1300?with strain rates ranging from 0.001 to 10 s-1 and height reduction of 50%,true strain is 0.7.The results show that the deformation temperature and strain rates affected the flow stress during the thermal deformation of Mo-Nb single crystals significantly.The true stree-strain curves exhibit a peak flow stress,flow softening and steady flow behavior.The constents of Mo-Nb single crystals,such as the activation energy Q,the stress exponent n,are calculated.The constitutive equation is estabilished based on a hyperbolic-sine equations of Arrhenius type.6.The hot deformation behavior of Mo-Nb single crystal has been characterized through stress-strain curves and processing map in the temperature range of11001300?and strain rates range of 0.00110 s-1.The results indicate that the flow stress is affected significantly by deformation temperature and strain rate during the hot deformation of Mo-Nb single crystal.Base on the processing map,the optimum condition for hot deformation of Mo-Nb single crystal is in the region of high temperatures and high strain rates.The microstructure and EBSD result shows that the specimen deformed at 1300?/10 s-1 retains single crystal structure,indicating that the specimen does not undergone instability of flow during hot deformation at the deformation condition of 1300?/10 s-1.7.The load-displacement curves,elastic modulus,indent morphology,nanohardness and elastic recovery of high purity Mo-Nb single crystal are investigated by nanoindentation and scanning probe microscopy.The nanohardness of Mo-Nb?110?,?111?and?112?single crystal is 4.88 GPa,4.01 GPa and 4.36 GPa respectively and the elastic modulus is respectively 240.5 GPa,210.9 GPa and 232.8 GPa by the continuous stiffness method?CSM?.A size effect on the nanohardness and elastic modulus of tungsten single crystal is observed,which decrease with the increase of indentation depth.8.The Nix-Gao model is employed to analyze the nanoindentation mechanical characteristics of Mo-Nb single crystal,and the intrinsic hardness in the limit of infinite depth are 3.96 GPa,2.61 GPa and 3.47 GPa,respectively;the size effect index is calculated to be 0.18,0.16 and 0.18,respectively.The characteristic length of?110?,?111?,?112?crystal plane are 1196 nm,2753 nm and 1559 nm,respectively.Mo-Nb single crystal has a significant size effect when the indentation depth is below the characteristic length,and the size effect becomes more insignificant when indentation depth is deeper than the characteristic length.The nanoindentation size effect will disappear as the impression are deeper than 4106 nm,5645 nm and 4693 nm,respectively.9.Based on the preparation of Mo-Nb alloy monocrystalline materials,a W-Nb alloy single crystal rod with diameter 22 mm and length 150 mm was prepared.The misorientation angle of the single crystal,impurity content,microstructure,chemical composition are analyzed,and the measured misorientation angle is less than 8?,impurity element content is less than 200 ppm.The straightness of the rod is very good,the surface is bright and free of cracks. |
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