| The difficulty in processing is the main factor restricting the mass application of TiAl based alloys.In this study,three TiAl based alloys prepared by different processes were researched.The factors that affect the flow stress and deformation activation energy were represented through analyzing the deformation curve,material constants and its corresponding evolution of the microstructure.Effective ways of enhancing deformation property of TiAl based alloys were put forward also.The study includes:(1)High Nb-TiAl based alloy ingot with nominal composition of Ti-45Al-7Nb-0.4W-0.15B(at.%)was prepared by cold crucible Induction Suspension Melting(ISM)technology.By using methods of thermal simulation test,OM,SEM,TEM and X-ray diffraction,the relationship of flow stress and strain,strain rate and temperature was researched.Dynamic recrystallization behavior of the alloy in high temperature plastic deformation was studied as well.The high temperature deformation material constants were obtained through regress analysis.Also,this research tested the heat forming technology under normal press working condition and analyzed the structural evolution mechanism of the as-forged alloy through subsequent heat treatment.(2)Ti-46Al-2Cr-2Nb-0.2W-0.15B(at.%)alloy was prepared through ceracon technology using TiAl alloy powder.The high temperature deformation behavior,as well as microstructure evolution and deformation micro-mechanism,were studied.And the high temperature deformation constants were worked out simultaneously.(3) Ti-45Al-7Nb-0.4W(at.%)alloy was prepared by controlled sintering elemental powder metallurgy(PM)method.Reacted synthesis and densification mechanism,along with high temperature deformation behavior were studied,and the high temperature deformation constants were obtained.Results show:1)As-casted Ti-45Al-7Nb-0.4W-0.15B(at.%)alloy has fine hot working properties within the range of temperature≥1000℃and strain rate≤1×10-1s-1.This alloy is a negative temperature sensitive material and normal strain rate sensitive material.Rise and fall and stabilization of flow-stress curve are corresponding with alternate controlling and dynamic balancing in work hardening and strain softening process.Zener—Hollomon parametric method is used to calculate the thermal activation energy(Q=384kJ.mol-1),and the result is close to the references.Regress analysis shows that relation among high temperature flow stress and strain rate,deformation temperature meets the Arrhenius relation corrected by hyperbolic sine function.High temperature deformation mechanism of this alloy mainly refers to the kinking,bending,spheroidization and recrystallization of the lamellar grains.The existence of B2 phase plays a positive role in high temperature deformation.Microstructure analysis indicates that twinning deformation is an important high temperature deformation mechanism of TiAl based alloys.2)By using canned forging technology,Ti-45Al-7Nb-0.4W-0.15B(at.%) alloy can be preformed severe deformation(about 80%)under conventional press-working condition.Research on subsequent heat treatment indicates that as being heat treated in two-phase region, grains grew up gradually and coarse rapidly as temperature increasing from about 1230℃.Removal of remained B2 phase is controlled by diffusion rate of the segregated elements.Experiment proves that fine and homogeneous microstructure without B2 phase can be obtained as heat treated at 1230℃for 4 hours.3)PM Ti-46Al-2Cr-2Nb-0.2W-0.15B(at.%)alloy was prepared by ceracon technology;this alloy is close to full dense with a fine nearγmicrostructure.This alloy has better hot working ability,which ensures deformation without crack at temperature≥1050℃and strain rate≤1×10-2s-1.Compared with as-cast Ti-45Al-7Nb-0.4W-0.15B(at. %)alloy,PM Ti-46Al-2Cr-2Nb-0.2W-0.15B(at.%)alloy proposes much lower flow stress and deformation activation energy,and its thermal activation energy(Q =319kJ.mol-1)is close to TiAl based alloy with fine nearγor duplex structure in references.This alloy's high temperature compressing deformation is mainly controlled by grain boundary sliding and dynamic recrystallization,etc.4)A large billet with nominal composition of Ti-45Al-7Nb-0.4W(at.%) is prepared by controlled sintering elemental PM method,and the density is over 98%.During controlled sintering,rearrangement of broken particles and increasing of densification driving force caused by pressure accelerate the alloy's densification.Ti-45Al-7Nb-0.4W(at. %)alloy has relatively worse hot working ability and narrower processing range(T≥1100℃andε≤1×10-2s-1).Its deformation activation energy is Q=420kJ.mol-1.Analysis of deformed microstructure shows that high temperature deformation mechanism of Ti-45Al-7Nb-0.4W(at.%)alloy is similar to that of lamellar structure in cast alloy.5)High temperature flow stress and activation energy of TiAl based alloy are affected by preparation method,microstructure and composition,etc.Compared with TiAl based alloys with coarse lamellar microstructure which prepared by casting process and EPM method,the high temperature flow stress and activation energy of the alloy prepared using alloy powder is much lower because of its fine microstructure.High oxygen content of TiAl based alloy prepared by elemental powder causes high stability of lamella,which can leads to alloy's high flow stress and thermal activation energy;Also,addition of Nb element increases the flow stress and thermal activation energy; Methods like refining original grain size and improving microstructure uniformity,adding appropriate alloy elements and strictly controlling elements content,reducing original material internal defect etc.,can largely enhance the high temperature deformation properties.
|
PDF Full Text Request