采用单轴蠕变拉伸和无应力人工时效实验对比,系统研究了蠕变时效对欠时效态7075合金力学性能的影响,并通过EBSD、SEM、TEM观察,表征位错和析出相等组织随蠕变时效时间的演变规律,定量研究了力学性能与微观组织演变之间的关系。结果表明,欠时效态7075铝合金经过蠕变时效后,保持高强度的同时塑性明显提升。合金的力学性能对蠕变应力比较敏感,在260 MPa、426 K下蠕变时效6 h的试样屈服强度最高,达到537.9 MPa,与人工时效样品相比,蠕变时效样品韧窝分布更为密集,晶粒更偏向于高Schimid因子取向,相对于人工时效样品伸长率提升15%。TEM结果表明,晶内主要为η′相,并且随着蠕变时效时间的延长,晶内析出相的尺寸由2 h的3.04 nm增加到6 h的4.27 nm,体积分数从0.22%增加到0.46%,晶界析出相尺寸增加,发生由连续向间断的转变。EBSD结果表明,所有样品中的再结晶、亚晶比例没有明显变化,平均晶粒尺寸保持在80 μm左右,几何必须位错(GND)的分布随着蠕变时效时间的延长先减少再增多。通过屈服强度贡献模型计算发现,晶界强化贡献基本保持在17 MPa左右,位错强化和析出强化的耦合作用是强度提高的主要原因。

The 7075 alloy is widely used in the manufacture of aerospace components, such as aircraft wings and fuselage plates, owing to its high strength and light weight. Moreover, it is well-suited for manufacturing these massive aviation components using creep aging forming (CAF) technology. In this present study, the effect of creep aging on the mechanical properties of under-aged 7075 alloy was systematically studied in detail by means of a uniaxial creep tensile test and a stress-free artificial aging test. EBSD, SEM, and TEM observations were used to characterize the evolution of dislocations and precipitates with creep aging time. A quantitative analysis was performed on the relationship between mechanical properties and microstructure evolution. The results show that creep aging greatly improves the plasticity of the under-aged 7075 aluminum alloy while maintaining its high strength. The mechanical properties of the alloy are sensitive to creep stress. The sample aged for 6 h under 260 MPa and 426 K has the maximum yield strength, reaching 537.9 MPa. In comparison to the artificial aging sample, the dimple distribution of the creep aging sample is denser and the grain is more inclined toward a high Schmid factor orientation, which is 15% higher than the artificial aging sample. TEM results show that the primary phase in the crystal is η′ phase. The size of the precipitated phase in the crystal grows with increasing creep aging time from 3.04 nm for 2 h to 4.27 nm for 6 h and the volume fraction increases from 0.22% to 0.46%. The size of the grain boundary precipitates increases and the transition from continuous to discontinuous occurs. The EBSD results show that no significant change in the recrystallization and subgrain ratio occurred in any of the samples, and the average grain size remains approximately 80 μm. The distribution of geometrically necessary dislocations (GND) decreases first and subsequently increases with the extension of creep aging time. The contribution of grain boundary strengthening to the yield strength contribution model is shown to be essentially constant at about 17 MPa, and the coupling effect of dislocation and precipitation strengthening is the primary reason for the increase in strength.