回归时间对RRA处理超高强铝合金力学性能的影响

Effect of retrogression time on mechanical properties of super-high strength Al slloy in RRA state

研究了回归时间对RRA处理后高Zn超高强度铝合金的组织和性能的影响.结果表明,以100℃/24 h为预时效和再时效参数,合金在200℃回归7 min的RRA(回归再时效)状态综合性能最好.其室温力学性能达到:σ_b=795 MPa,σ_(0.2)=767 MPa,δ_5=9.1%;抗应力腐蚀性能达到:在3.0%NaCl+0.5%H_2O_2腐蚀液环境及210 MPa拉伸应力下,断裂时间大于720 h.在200℃/7 min回归的RRA状态,其晶内的弥散粒子和再时效期间变窄的晶界无析出带保证了合金的超高强度.随着回归时间的延长,晶内粒子的弥散度下降,回归时形成的较宽的晶界无析出带不能再时效粒子填充而使强度大幅度降低.同时,处于该状态的合金具有优良的抗应力腐蚀性能,得益于晶界大尺寸、大间距粒子具有捕获氢原子、逸出氢气泡的作用,以及回归降低了晶界附近的位错密度、切断了氢原子向晶界迁移的通道.

After retrogressed at 200 ℃ for 7 min the sample pretreated and re-aged at 100 T3 for 24 h has the best mechanical properties： σb=795 MPa,σ（0.2）=767 MPa,δ5=9.1%, and the fracture time during the test of stress corrosion resistance is longer than 720 h when employing a load stress of 210 MPa and 3.0%NaCl＋0.5%H2O2 standard corrosion environment. It is suggested that after retrogressing at 200℃ for 7 min the RRA treated sample shows a super-high strength is because dispersed particles are formed in grain interior and the PFZ at grain boundaries formed when retrogressing is narrowed by filling of re-precipitates during re-ageing. When retrogression time is prolonged the strength of RRA treated samples will be greatly reduced because particles in grain interior grow up and the PFZ of a greater width compared to 7min retrogression can not be narrowed during re-ageing. Furthermore, the sample in RRA tempered condition is of good resistance to stress corrosion is due to two reasons, one is the particles of large size and interspacing at grain boundary play a role of un-reversible trap to catch H atom and release H molecules , the other is the decrease in dislocation density in the vicinity of grain boundary during retrogression results in reduction of H atom migration from grain interior to grain boundary, both of them reduce H atom content in grain boundaries.