Thermal Deformation Behavior and Processing Map of Ti-6Al-4V-0.5Nb-0.5Ni Alloy for Oil Well Pipe
Corrosion-resistant elements of Ni and Nb can be added into widely used Ti-6Al-4V to form Ti-Al-V-Ni-Nb alloys with excellent mechanical properties and corrosion resistance,and it can be used as a material for oil well pipes.However,the rheological stress and microstructure evolution during the thermal processing of the alloy after composition optimization are not clear and need to be studied.The rheological stress-strain changes and intrinsic equations of titanium alloys during high-temperature deformation are the focus of research on the processing and preparation of titanium alloy materials at home and abroad.The shape and microstructure also change significantly during high-temperature deformation.This process mainly involves softening behavior such as dynamic recrystallisation and spheroidization of the lamellar.The hot deformation behavior of Ti-6Al-4V-0.5Nb-0.5Ni alloy was investigated by using Gleeble-3800 thermal simulation test machine on condition of the deformation temperature at 1113,1143,1173,1203 K and strain rates at 0.01,0.1,1,10 s-1.The variation of flow stress with temperature and strain rate in high-temperature deformation was studied by the true stress-strain curves.The constitutive equation was obtained through analysis of true stress and strain,and the hot deformation mechanism of the alloy was studied through the establishment of the processing map and microstructural observation.The effect of temperature and strain rate on the organization had been investigated in conjunction with thermal processing diagrams and microscopic analysis methods.Zeiss Axiovert 200 Mat microscope was used for the observation of metallographic organization and morphology.The samples were observed using a Tecnai G2 F20 transmission electron microscope(TEM).The high temperature thermal deformation behavior of Ti-6Al-4V-0.5Nb-0.5Ni alloy was studied,and the effect of temperature and strain rate on the rheological stresses evolution during deformation in the two-phase region were mainly investigated.The results showed that:(1)Ti-6Al-4V-0.5Nb-0.5Ni alloy deformed in the two-phase region with a peak stress about 50 MPa lower than that of Ti-6Al-4V in the same state in the lower part of the two-phase zone.In the lower part of the two-phase zone,there was a smaller drop in flow stress and a lower temperature sensitivity to flow stress.In the upper part,however,there was a greater drop in flow stress and the rheological stress was temperature sensitive.Faster temperature changed at higher strain rates and temperatures.Dynamic recovery and dynamic recrystallisation were substantially enhanced and the flow stress were significantly reduced.At lower strain rates,the density of movable dislocations was low and work hardening was not significant.In addition,the deformation time was long and dynamic recovery/recrystallisation could take place adequately at both high and low temperatures.Therefore,the flow stresses did not vary much with temperature.(2)According to the processing map,peak efficiency of power dissipation of 50％was obtained in the temperature range of 1200～1230 K and strain rates of 0.01～0.05 s-1,where the deformation mechanism was dynamic recovery.The plastic flow instability occurring with temperatures of 1143～1203 K/1190～1200 K and strain rates of 0.01～0.1 s-1/2.7～7.4 s-1 showed the plastic flow localization.In addition,the suitable thermal processing region was at temperatures of 1200～1230 K and strain rates of 0.01～0.05 s-1.At 1113 K,α plates were compressed due to the large amount of compressive deformation(60％),and the original βgrain size became smaller at the same time.The lamellar kinking and spheroidization became more severe with increasing temperature.The thinning of the original β grain boundary was since the α phase on the grain boundary was also involved in the deformation.The hot work diagram corresponded to a destabilized zone with microcracks and non-uniform organization.(3)Ti-6Al-4V-0.5Nb-0.5Ni e alloy exhibited two dynamic spheroidization mechanisms during isothermal compression.The dynamic spheroidization mechanism included both shear spheroidization mechanism and grain boundary separation mechanism.And Ti-6Al-4V-0.5Nb-0.5Ni alloy had a lower deformation resistance and wider processing interval than Ti-6Al-4V.
Ti-6Al-4V-0.5Nb-0.5Ni alloysisothermal hot compressionthermal processing mapdeformation mechanism
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