Abstract
Large 2219 Al-Cu alloy aerospace integral components suffer from long-term stress relaxation aging(SRA)due to complex temperature and stress loads during aging treatment/forming and service process,which makes it difficult to ensure their appropriate residual stress and excellent mechanical and service prop-erties.However,the research is limited to a thorough understanding of macroscopic and microscopic features and underlying mechanisms of the long-term SRA under multivariable aging conditions.There-fore,this study investigated macroscopic and microscopic features of long-term SRA under different tem-peratures(120 ℃ to 190 ℃),initial stress levels(100 MPa to 250 MPa)and durations(0 h to 50 h)through stress relaxation curves,metallographic traits,Vickers hardness,tensile performance,disloca-tions and phases of precipitation.On the basis of experimental outcomes,the comprehensive mecha-nisms beneath SRA were unraveled through dislocation theory,multiphase strengthening mechanisms and thermodynamics,where the interplays of stress relaxation behavior with age-hardening response were taken into consideration.The results showed elevations in the rate of stress reduction as the tem-perature and initial stress rose.At an initial stress greater than the yield stress of alloy,a marked in-crease in stress relaxation was found,and the mechanisms transform from the intragranular motion of dislocations and diffusion of grain boundaries to the intragranular and intergranular motion of disloca-tions and migration of grain boundaries.The stress reduction rate rose sharply when the temperature exceeded 175 ℃,and the dislocation movement mechanisms transform from gliding to climbing of dislo-cations.Stress relaxation is in nature progressive transformation of strain from elastic into a permanently inelastic state via the motion of dislocations,leading to the decrease of movable dislocations and the increase of immovable dislocations with more stable configurations.The age hardening is mainly deter-mined by precipitation strengthening,supplementarily by dislocation strengthening,and obvious stress orientation effect(SOE)of G.P.zones and θ"phases degenerates strengthening effect.The interplay be-tween stress relaxation behavior and age-hardening response influences the thermal-mechanical coupling SRA of 2219 Al-Cu alloy,which depends fundamentally on the motion of dislocations and their interplay with precipitated phases.This is a thermal activation process concerning the interplay between internal(age-hardening resistance)stress and external(initial)stress.The initial energy of elastic strain offers Gibbs free energy as the SRA driver,and a steady state of stress relaxation is attained with the lowest energy of elastic strain.These findings provide valuable insights into exploring innovative aging treat-ment/forming for optimizing residual stress,mechanical performance and service property in a synergistic manner.
基金项目
国家自然科学基金(52005018)
北京市自然科学基金(3232022)
中央高校基本科研业务费专项(FRF-TP-22-035A1)
国防基础科研项目(JCKY2020601C004)
NETL
NSTL
万方数据