Hot Deformation Behavior and Microstructure & Properties Control of Ni-based Alloy GH4720LI

Author:Wan Zhi Peng

Supervisor:hu lian xi sun yu


Degree Year:2019





GH4720LI alloy is a precipitation strengthened Ni-based superalloy which is able to meet the requirements of critical components in aeroengine for high temperature and high strength application in recent years.Hence,GH4720 LI alloy was widely applied in high performance applications such as disks and blades of either aircraft engines or land-based gas turbines attributing to its excellent properties including resistance to creep and fatigue,together with corrosion,fracture and microstructural stability for the intended applications.High content of alloying elements and strengthening phases can improve the service temperature and high-temperature strength of the alloy,while the hot workability of the alloy was inevitably decreased as a result.Therefore,an optimized parameter during hot working plays a very important role in obtaining uniform microstructure and avoiding the flow instability.While,few investigations had been focused on the workability and microstructure evolution of as-cast and fine-grained GH4720 LI alloy during hot deformation,included: precipitates evolution and grain refinement mechanism of homogenized as-cast GH4720 LI ingot during cogging process is not given;hot deformation behavior,optimization of hot working parameters as well as dynamic softening mechanism of the as-forged fine-grained alloy during hot deformation has not been clarified;the effect of heat treatment parameters on mechanical properties and the fracture mechanism under various tensile conditions still needed to be discussed.Based on the issues above,following investigations were conducted in present study.The experiments of the static dissolution behavior and hot compression tests on homogenized as-cast ingot show that compared with the static dissolution behavior,the dissolution process of γ’ precipitates can be promoted by the interaction between dislocation and precipitates during hot deformation.The true stress-strain curves under different temperatures and strain rates present typically dynamic recrystallization softening feature.Dispersive distributedγ’ precipitates can promote the occurrence of DRX,which is the main reason for the rapid grain refinement for the original coarse ascast microstructure when deformed in γ+γ’ dual-phase region.While for the γ single phase region,the dynamic recrystallization grains are mainly nucleated at the residual dendritic and the original grain boundaries,and the DRX grains are mainly formed by discontinuous dynamic recrystallization process.The high-temperature deformation behavior of a as-forged GH4720 LI alloy was investigated by using isothermal compression tests.The Arrhenius-type model,considering the dissolution of γ’ precipitates,was established using the friction and adiabatic heating corrected flow stress data.The processing map was established based on the dynamic material model.According to the detailed deformed microstructure analysis,the optimal hot deformation processing conditions were determined and the dominant flow instability mechanism is characterized by adiabatic shear bands,slip localization and crack formation based on the grain boundary cavitation.At low temperatures of 1060℃~1080℃,the fine DRX grains are formed by particle-induced continuous dynamic recrystallization(PI-cDRX).While,the boundary bulging and nucleation of dDRX grains were restrained as a result of decrease of dislocation substructures and subgrain boundaries density consumed by continuous original boundary migration(COBM)in deformed grains at low strain rates and high temperatures,and then the occurrence of dDRX was suppressed.The grain growth behavior and dissolution behavior of γ’ precipitates during various isothermal treatment parameters are studied.The effects of the solution treatment temperatures on the microstructure evolution and mechanical properties are analyzed.Microstructure observations show that high solution treatment temperature leads to the coarsening of grain size and secondary gamma prime precipitates,and thus results in the decrease of low temperature strength and ductility.A combination of ductile and brittle mode characterized by dimples and quasi-cleavage facets feature was observed for 1130℃ and 1140℃ solution treatment.Much coarser secondary γ′ enables the classical(111)<110> slip,and improves the ductile of tensile test at 750℃ as a result for the samples at a solution temperature of 1150℃.