测定应力比(R)为0.6、0.05和?1的 T6峰时效和 T7951二次时效7055铝合金的裂纹扩展速率，并利用透射电镜(TEM)和扫描电镜(SEM)观察材料的显微组织和断口形貌。结果显示，裂纹扩展速率受应力比、微观组织(如基体沉淀相、晶界析出相和无沉淀析出带)分布、尺寸和体积分数的影响。两种热处理状态材料在正应力比条件下的裂纹扩展速率随应力比增大而增大，当应力比 R=?1时，裂纹扩展速率与 R=0.06时的接近。在较小的应力强度因子变化范围内(?K)，两种热处理状态材料的裂纹扩展速率差别不大，当?K 超过一定值时，开始出现差异。断口分析表明，二次时效7055-T7951铝合金 Paris 扩展区疲劳条带间距比峰时效的小，表现出更好的断裂韧性。
The crack propagation rates of T6 peak aging and T7951 secondary aging 7055 aluminium alloys were tested under stress ratios (R) of 0.6, 0.05 and ?1, respectively. The microstructures and fracture surfaces were analyzed by TEM and SEM. The results reveal that the crack propagation rate is affected by the stress ratio and microstructure such as the distribution, dimension and volume fraction of matrix precipitates, grain boundary precipitates and precipitate free zone. For both heat-treated specimens, crack propagation rate increases with the improvement of R when it is a positive value while crack propagation rate at R=?1 is much similar to that at R=0.06. The crack growth rates exhibit no obvious difference in lower stress intensity factor range (ΔK), while the difference starts to be obvious when ΔK exceeds certain value. The fracture analysis testifies a better fracture toughness for 7055-T7951 with a smaller striation space in Paris region.
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