High Temperature Oxidation And Compressive Deformation Behaviors Of Nb-W Refractory Alloys Fabricated By Powder Metallurgy

In the present research, Nb-xW（x=10,20,30wt.%）, Nb-28.5W-5Cr,Nb-27W-10Cr and Nb-27W-10Si refractory alloys were prepared by mechanicalalloying and uniaxial hot pressing, and a model of leading edge were fabricated bysinter-forging short process. The effects of ball-milling parameters on the qulity ofNb-W powders were quantitatively investigated using orthogonal test of threefactors at three levels. Meanwhile, the hot-pressed sintering processes of Nb-W,Nb-W-Cr and Nb-W-Si alloys were drawn up, the microstructure and mechanicalproperties at room temperature of hot pressed materials were studied. Moreover,the high temperature oxidation behaviors of Nb-W alloy and the effect of Cr and Sion the oxidation behaviors of Nb-W alloy have been studied. The high temperaturecompressive deformation behaviors of Nb-W alloys were investigated and theprocessing maps were drawn.Results of the mechanical alloying indicated that the particle size of milledpowders was mainly dominated by the solid-to-liquid ratio. The content ofimpurity Fe was mainly determined by the ball-to-powder ratio and the millingtime. The refining process and morphology evolution of the powder were affectedsynthetically by all three parameters. The results of mechanical alloying showNb（W）ss and W（Fe）ss phases in the coarse powders transformed to Nb（W, Fe）ssand W（Fe）ss, and eventually to the W（Nb, Fe）ss and Fe2Nb in the refined powders.According to the analysis above, the best paramters were choosed to be as follows,the milling time of24h, ball-to-powder ratio of10:1and solid-to-liquid ratio of1:0.2.Nb-W, Nb-W-Si and Nb-W-Cr alloys were hot pressed at1800℃,1700℃and1600℃for90min with pressure of30MPa, respectively. Microstructures ofthree alloys consist of single phase Nbss, Nbss+Wss+Nb₅Si₃+Nb₃Si andNbss+NbCr₂, respectively. The vicker hardness and flexural strength of Nb-Walloys increased and the room temperature fracture toughness decreased with theincreasing of W content. Based on mechanical test, Nb-W alloy with30wt.%Wobtained the best mechanical properties at room temperature. Additions of Cr andSi increased the vicker hardness and reduced the flexural strength and roomtemperature fracture toughness of Nb-30W alloys, which were caused by the hardand brittle NbCr₂in Nb-W-Cr, Nb₅Si₃and Nb₃Si in Nb-W-Si alloys, respectively.Moreover, a model of Nb-30W leading edge was successfully fabricated bysinter-forging short process at1800℃. The model displays a good surface qualityand high relative density. And the model contained the similar mechanical properties as well as the hot-pressed Nb-30W alloy.Nb-W alloys were exposed at1000℃, the scale on Nb-10W alloy spalledfrom the base metal and the―pest‖phenomenon appeared the scales on Nb-20Wand Nb-30W alloys. The results at1000℃indicating the poor oxidation resistance.In addition, the results of isothermal oxidation at1300℃indicated that the masschange of Nb-W alloys reduced, the oxidation resistance was improved withincreasing of W contents. The scales of three Nb-W alloys spalled from the basemetal show the poor oxidation resistance. However, the additions of10wt%Cr and10wt%Si into Nb-30W alloy result in the formation of the protective phaseCrNbO₄and SiO₂. The―pest‖phenomenon disappeared at1000℃. The relativedense scales combined well with the alloy’s matrix at1300℃. The results aboveindicate that addition of10wt%Cr and10wt%Si into Nb-30W alloy provide thegood oxidation resistance. Meanwhile, after5wt%Cr was added into Nb-30Walloy, the―pest‖phenomenon also disappeared at1000℃and showed the goodoxidation resistance. But the oxidation resistance still descreased since two phasesNbss+NbCr₂were transformed to single phase Nbss at1300℃.Nb-W alloys were compressed at1000℃,1300℃,1400℃,1500℃and1600℃, respectively. The deformation behaviors were studied in the differentstrain rates of0.001s-1,0.01s-1and0.1s-1. Meanwhile, the stress-strain relations ofhot deformation were established. The processing maps （=0.3） of hot compressingof Nb-20W alloy were drawn. The results show the good hot workability in therange of1300～1550℃at the strain rate of10^-3～10^-1s^-1 for Nb-20W alloy.