时效制度对2A97铝锂合金疲劳裂纹扩展速率的影响

Effect of Aging Conditions on the Fatigue Crack Propagation Rate of 2A97 Aluminum-Lithium Alloy

针对不同时效制度下2A97铝锂合金的疲劳裂纹扩展速率进行了研究;并通过光学显微镜、透射电镜和扫描电镜对合金的微观组织和疲劳断口进行了观察分析。结果表明:2A97合金薄板(T3+135℃/12 h)时效态的裂纹扩展速率最低,其次为T3,(T3+135℃/60 h)时效态的扩展速率略低于(T3+135℃/120 h)时效态,T6态的裂纹扩展速率最高。时效制度在较低的ΔK水平下对合金的裂纹扩展速率影响较大,而在较高ΔK水平下影响较小。δ′相及细小的T1相可以促进共面滑移,增加滑移可逆性,降低裂尖应力集中程度及塑性累积水平,同时使裂纹发生偏折,促进裂纹闭合效应,提高裂纹扩展抗力;粗大、密集的T1相及半共格的θ′相抑制共面滑移,易于造成应力集中,降低裂纹扩展抗力;晶界处的无沉淀析出带(PFZ)及平衡相急剧降低晶界强度,极易造成沿晶开裂,使合金裂纹扩展速率迅速提高。

The fatigue crack propagation（FCP） behaviors of 2A97 aluminum-lithium alloy under different aging conditions were investigated. The microstructures and fracture morphologies were analyzed by TEM, SEM and OM. The results indicate that（T3＋135 °C/12 h） aging treated alloy has the minimum FCP rate, followed by T3 treated alloy;（T3＋135 °C/60 h） aging treatment has a slightly slower FCP rate than（T3＋135 °C/120 h）; T6 temper has the maximum FCP rate. Aging conditions have a significant influence on the FCP rate in low ΔK level, with less effect in high ΔK level. The δ′ and fine T1 phases enable slip planarity so as to enhance the reversibility of cyclic slip and to reduce damage accumulation and stress intensity around the crack tip; slip planarity can also lead to the deflection of crack paths and promote the crack closure effect. As a result, crack propagation resistance improves. Coarse and intensive T1 phase and semi-coherent θ′ phase restrain slip planarity and easily induce stress concentration which can reduce crack propagation resistance. The precipitation free zone（PFZ） and equilibrium phases at grain boundaries dramatically reduce the strength of grain boundaries and cause intergranular fracture, accelerating the fatigue crack propagation rate greatly.