High Temperature Deformation Behavior Of Nb-Si Based Ultrahigh Temperature Alloy

Nb-Si based ultrahigh temperature alloys are promising high-temperature structural materials to be applied in the fields of aviation and aerospace.Hot working（hot extruding,forging and rolling）has been extensively employed to shape metals and alloys.But the hot workability of Nb-Si based ultrahigh temperature alloys has rarely been investigated up till now,which limits applications of these alloys to some extent.An insight into the flow behaviors and microstructure evolutions during hot deformation of Nb-Si based ultrahigh temperature alloys is prerequisite for proper design of their hot working process and accurate control of the deformed microstructures.Aiming at developing hot working techniques for Nb-Si based ultrahigh temperature alloys,hot deformation behaviors of a Nb-Si-Ti-Cr-Al-Hf alloy with different initial microstructures have been investigated in this dissertation.An ingot of Nb-Si based ultrahigh temperature alloy with a nominal composition of Nb-15Si-22Ti-5Cr-3Al-2.5Hf（at.%）was prepared by vacuum non-consumable arc melting at first and then vacuum consumable arc melting.The heat-treated alloy was prepared by heating a part of the arc-melted ingot at 1350℃ for 24 h and then at1450℃ for 50 h.The directionally solidified rod was prepared at a temperature of2050℃ and a withdrawal rate of 100μm/s.The arc-melted,heat-treated and directionally solidified samples were hot compressed on a Gleeble-3500 thermo simulator.The deformed microstructures were characterized by scanning electron microscopy（SEM）,electron backscattered diffraction（EBSD）and transmission electron microscopy（TEM）.A constitutive model has been built for both work hardening and flow softening stages during hot deformation of arc-melted alloy,and the stress-strain curves could be accurately reproduced by the established model.A hot processing map has also been established for the arc-melted alloy,and the favorable conditions for hot working of the arc-melted alloy are determined as 1380～1440℃/0.001～0.01 s^-1.The mechanisms for flow instability are the cracking of silicides and the debonding of Nbss/Nb₅Si₃interfaces at lower temperatures of 1200～1270℃ or at a higher strain rate of 0.1 s^-1,and Nbss failure at 1500℃.In addition,flow localization is mainly responsible for the instable flow at 1200℃/0.05～0.1 s^-1.The flow softening of arc-melted alloy is firstly caused by the cracking of large primary Nb₅Si₃ blocks shortly after the peak stress,and then is dominated by continuous dynamic recrystallization（CDRX）of Nbss.The CDRX in Nb₅Si₃ has been obviously activated with height reduction increasing from 30%to 60%,leading to an increase in the apparent activation energy of deformation（Q）.When the deformation temperature rises from 1250 to 1350℃ and the strain rate deceases from 0.1 to 0.001 s^-1,Nb₅Si₃cracking has been substantially alleviated,while Nb₅Si₃ CDRX has been significantly promoted.The evolution of Nbss/Nb₅Si₃ eutectics in deformed arc-melted alloy has been characterized by two process:the evolution of interdendritic Nb₅Si₃ flakes to smaller silicide particles controlled by boundary splitting mechanism,and the elimination of Ti and Cr segregations.Silicides have been refined with height reduction increasing,which is ascribed to the cracking of primary Nb₅Si₃ and the splitting of Nb₅Si₃ flakes via the penetration of Nbss along Nb₅Si₃/Nb₅Si₃ boundaries.The eutectic evolution has been promoted by an increase in deformation temperature from 1250 to 1350～1410℃ at a lower strain rate of 0.001 s^-1,but has been retarded at a higher deformation temperature of 1470～1500℃ due to lower distortion energy and also restrained at higher strain rates of 0.01～0.1 s^-1 due to the short deformation time.When the deformation temperature decreases from 1350～1410℃ to 1200～1300℃ at 0.001 s^-1or the strain rate increases from 0.001 to 0.1 s^-1,the strain rate sensitivity（m）of this alloy obviously decreases due to the lower recrystallization degree in both Nbss and Nb₅Si₃,exacerbated Nb₅Si₃ cracking and more remnant Nb₅Si₃ flakes.The m value also decreases when the deformation temperature increases from 1410 to 1500℃,due to a combination of the development of larger Nbss grains,more remaining Nb₅Si₃ flakes and the formation of cavities in Ti and Cr-rich zones.The flow stress of heat-treated alloy is lower than that of arc-melted alloy at the early stage of hot deformation,so that the change in morphology and size of silicide has been restrained in heat-treated alloy compared with arc-melted one.At 1410℃/0.001 s^-1,the formation of recrystallized grains in both Nbss and Nb₅Si₃ has been retarded in arc-melted alloy compared with heat-treated alloy,because of the evolution of eutectic morphology with smaller height reductions in the former.The hot deformation behaviors of directionally solidified alloy compressed along its growth direction have been investigated.The maximum stress of the directionally solidified alloy is approximately three times as high as that of the arc-melted one.Nb₅Si₃ has kinked in the compressed directionally solidified alloy,which initiates the formation of a macro kink band in the middle of the compressed sample,but the Nb₅Si₃kinking has been suppressed in the central area of the kink band where the long axes of Nb₅Si₃ have obviously deviated from the compressive axis.The plastic strain of the compressed directionally solidified alloy is mainly localized in the macro kink band,and also the macro kink band has extended and rotated to accommodate strain with increasing the height reducton.The maximum softening rate of the directionally solidified alloy is ten times higher than that of the arc-melted alloy,which is ascribed to the rotation of Nb₅Si₃ accompanied by the formation of the kink band,the CDRX of Nbss and Nb₅Si₃,the cracking of Nb₅Si₃ in kink band,and the micro-cracking between eutectic cells in the straight zone（outside the kink band and enduring little plastic deformation）.A second stress peak occurs with height reduction increasing from 30%to 60%,which is caused by the formation of secondary kink band in the region between the primary kink band and the straight zone,and also the the obviously increased plastic strain in the remaining straight zone.