Calculation And Experimental Study Of Mechanical Properties Of V-Ta Alloys

In this thesis, mechanical properties of V alloys were studied by combining computation and experiment method, through which V alloys with high mechanical performance, especially at high temperature, could be designed and developed.Phase diagram of binary alloy systems V-Ta, V-W, V-Ti were optimized through calculation of phase diagram method (Calphad).(1) The bcc/liquid phase boundary of V-Ta system fits not very well with experiment data. The phase area of C14didn’t show up, probably due to the unclear nature of V2Ta.(2) The optimized results of V-W and V-Ti are well obtained and the calculated phase diagram of V-Ti agrees well with experiment.First-principles calculations were carried out to obtain the elastic constants of V-Ta alloys. The calculated tensile and shear strength of alloy with composition close to V-10wt.%Ta are much higher than that of pure V metal, indicating high strength. And the values of B/G and Cauchy pressure C’ assessing the ductility are very close to that of V, which could result into its good ductility. According to the calculations results, alloys with composition around V-10wt.%Ta should show best mechanical properties at0K. The calculated ideal tensile and shear strength also illustrate the high strength of V-10wt.%Ta, while the prediction of ductility from κ was not well agreed with reality. The calculations of density of states indicate that Ta and V atoms could form certain level of covalent bond, resulting into strengthening effect on alloy.Standard tensile test at room temperature shows that the strength of V-Ta alloys increases a lot when alloying with Ta, while the ductility behavior decreases. The best mechanical properties with high strength and good ductility at room temperature were obtained with Ta amount to be about10wt.%. The maximum tensile strength is478.6MPa, much higher than the308.0MPa of traditional V based alloy V-5Cr-5Ti that was made and processed under our experimental conditions. The total elongation and reduction in area were measured to be about30.72%and60.76%, respectively. V-10wt.%Ta shows the best mechanical properties among those alloys considered in the paper, indicating good agreement with calculations.The high temperature tensile tests on V-10wt.%Ta and V-15wt.%Ta binary alloys at1100℃show that the maximum tensile strength grows up with the increment of Ta amount and the ductility of alloys increases at high temperature. V-15wt.%Ta has the highest strength of about122.6MPa. The ductility of V-10wt.%Ta seems very good with the total elongation and reduction in area to be40.6%and67.3%. However, its maximum tensile strength is only 83.4MPa, even lower that of V-5Cr-5Ti.Experiments show that binary V-Ta alloys with Ta up to15wt.%display obvious brittle behavior at room temperature. Based on the V-Ta system, in order to achieve alloys with density close to steel, the amount of Ta need to be about30wt.%, which would definitely result in obvious brittle behavior. The mechanical properties of alloy could be improved by adding other elements into the system. Elastic constants of several V-30wt.%Ta-X (X=Fe, Ti, Nb, Zn, Al, Cu, Y and Mg) alloys were calculated through first-principles to assess their mechanical behavior. Our results show that when alloying with Fe, V-30wt.%Ta-Fe alloy shows the highest ideal tensile (17GPa) and shear strength (7G Pa), much higher than that of V and any other alloy system considered. However, if the emphasis is put on the ductility of alloy, which is assessed by the Cauchy pressure and BIG, then the mechanical properties should be better by adding Cu. Therefore, we designed out two alloy compositions V-30wt.%Ta-Fe and V-30wt.%Ta-Cu that may show good mechanical properties and close density to that of steel from total calculations.