回归制度对7A20铝合金RRA过程中组织性能的影响

系统研究了RRA处理过程回归制度对7A20铝合金组织性能的影响,测试了力学性能和电导率,利用OM和TEM(HRTEM)表征了晶界和晶内的微观组织结构。结果表明:在160 ℃的回归温度下,随回归时间的延长,抗拉强度由630 MPa降低到525 MPa;经120 ℃保温12 h的再时效处理后,抗拉强度由645 MPa升高到710 MPa;回归时间越长,再时效引起的强度增量越大,80 min时达到最大值185 MPa。在200 ℃的回归温度下,回归时间为10 min时,便达到峰值强度715 MPa。随回归温度提高,导电率先升高后降低,与回归时间呈正相关关系。经160、200和240 ℃下峰值强度对应的回归处理后,基体晶粒尺寸分别为55、65和70 μm,相差不大。200 ℃×10 min的回归处理+再时效处理后,晶界处存在不连续的MgZn₂相,尺寸在10~20 nm,与基体呈非共格关系;晶内存在弥散分布的纳米级η′强化析出相,与基体呈共格或者半共格关系;晶界处存在45~55 nm宽度的PFZ(Precipitation free zone)区。

Influence of retrogression treatment on microstructure and mechanical properties of 7 A20 aluminum alloy during RRA treatment

Influence of retrogression treatment on microstructure and mechanical properties of the 7A20 aluminum alloy during RRA treatment was investigated. The mechanical properties and conductivity were tested. The microstructure at grain boundary and inner grain was characterized by using OM and TEM (HRTEM). The results indicat that the tensile strength decreases from 630 MPa to 525 MPa under 160 ℃ retrogression treatment with the increase of time. While tensile strength increases from 645 MPa to 710 MPa after reaging treatment at 120 ℃ for 12 h. The longer the retrogression time, the greater the increase of intensity caused by reaging, which reaches 185 MPa after 80 min. At a retrogression temperature of 200 ℃ and a retrogression time of 10 min, the peak strength is 715 MPa. As the retrogression temperature increases, the conductivity first increases and then decreases, which has a positive correlation with retrogression time. After retrogression at 160, 200 and 240 ℃ corresponding to the peak strength, the grain sizes of matrix are 55, 65 and 70 μm, respectively with a little difference. After retrogression treating at 200 ℃ for 10 min and reaging treatment, there is discontinuous MgZn₂ phase precipitated at grain boundaries with the size of 10-20 nm, which is incoherent with the matrix. There is dispersive nano-sized η′ strengthening precipitates inner the grains, which is coherent or semi-coherent with the matrix. There is a PFZ region with a width of 45-55 nm at the grain boundary.