材料期刊网

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 behavior exceeding peak-aging strength in the RRA condition of three different aluminum alloys has been studied.The results of hardness and tensile strength test indicate that after presaging at 100℃ or 120℃ and retrogressing at 200℃ for various time and reaging treatment,the hardness and strength of studied alloys are all above the value of presaging condition,some of them even exceed the value of peak aging(T6) condition .And the alloys exhibit a desired combination of strength and elongation after RRA treatment.TEM observation shown that the PFZs formed during retrogressing in short time become narrowed and even disappeared after reaging treatment.In this condition ,the precipiatates in the matrix are maily G.P zones and a few η’precipitates.However, the PFZs formed during retrogressing for a long time can not be narrowed after reaging treatment. In this condition ,the precipiatates in the matrix are coarsen.In T6 condition the precipitates has a uniform distribution with obvious characteristic of PFZs and the volume fraction of η’phase is somewhat increased.Corresponding EDS analysis indicates that the PFZs formed during retrogression are the depletion area of solute atoms.On one hand ,This area can be reduced and even eliminated after short time retrogression and reaging treatment.On the other hand ,this area can not be narrowed when retrogressed for long time and reaged. It is suggested that the behavior exceeding peak-aging strength in the RRA condition ascribes to the narrowing and even disappearing of PFZs formed during retrogression in the process of reaging treatment, which reinforces the grain-broundary, while the precipitates in the matrix of the alloys still keep or even exhibit a more dispersed distribution, and obtain a higher effect of precipitation strengthening.