铝合金回归再时效状态的超峰时效强度行为分析

ANALYSIS ON THE BEHAVIOR OF EXCEEDING PEAK AGING STRENGTH OF ALUMINUM ALLOY AT CONDITION OF RETROGRESSION AND REAGING

3种Al—Zn—Mg—Cu合金经过100℃或者120℃预时效24 h,并在200℃分别回归一定时间后再时效,均显示出超过该合金预时效状态的强度,甚至高于相同温度单级时效的峰值强度,最高达到795 MPa,且显示出较好的延伸率．显微组织分析及微区成分分析表明,较短时间的回归处理所引起的晶界无析出带经过再时效后变窄甚至消失。对应的晶内组织也非常弥散．而峰时效状态下虽然晶内组织弥散,但都显示出明显的晶界无析出带组织特征．铝合金回归再时效(RRA)状态的超峰值时效强度的原因可以归结为,再时效使回归时产生的晶界无析出带变窄或消失,使晶界强度提高;与此同时,晶内仍然保持和获得更加弥散的组织状态及更高的析出强化效应．

The tensile test indicated that after preaging at 100 or 120 ℃ for 24 h and retrogressing at 200 ℃ for various times and reaging treatment, the strengths of the three Al-Zn-Mg-Cu alloys are greater than those at preaging condition, some of them even exceed the peak strength of single-step artificial aging （T6）, reach up to 795 MPa. The alloys exhibit a desired combination of tensile strength and elongation after retrogression and reaging （RRA） treatment. TEM and EDS analyses show that the precipitate free zones （PFZ） formed during retrogressing in short period became narrowed and even disappeared after reaging treatment, while the second phase distributed dispersely in grain interior. Although the second phase in the alloys treated at T6 temper dispersed in grain interior, the PFZ in alloys still existed obviously. It is suggested that the exceeding peak aging strength in the RRA condition is ascribed to narrowing and even disappearing of PFZ, which reinforced the grain broundary, while the precipitates in the matrix still maintain a more dispersed distribution.