Fatigue Properties of High Strength Al Alloys under Ultrasonic Fatigue

为了对比研究Al 6061-T6和Al 7075-T6两种高强铝合金在空气中和室温下的超声疲劳和旋转弯曲疲劳性能，结果表明，Al 6061-T6和Al 7075-T6的疲劳寿命接近10 9次。研究表明：(1)两种合金在超声疲劳下的疲劳强度都有所提高，且在Al-7075合金的短寿命区和Al-6061合金的长寿命区，超声频率对疲劳强度的影响显著，说明材料的特性在超声疲劳研究中起着重要的作用。但在超声疲劳条件下，裂纹萌生和早期扩展受到抑制。在超声疲劳的情况下，裂纹路径从最初的近垂直方向转变为相对于应力轴的约60度，大部分疲劳寿命都花在小于^-100微米的小裂纹的扩展过程中。(3)在超声疲劳试件的断口表面很少发现条纹，而是伴随着脆性解理观察到很好的空洞，这意味着断裂模式从拉伸模式向剪切模式转变。此外，为了更好地了解超声频率对具有不同晶体结构的金属材料强度的影响，还研究了一些高强度合金，如镍基高温合金和马氏体时效钢的超声疲劳行为。

The fatigue properties of two high strength Al alloys, Al 6061-T6 and Al 7075-T6, in the life range toward 10^9 cycles were investigated in air and at room temperature under ultrasonic fatigue as well as rotary bending fatigue for the purpose of comparison. Our research shows that(1) Fatigue strength increases in both alloys under ultrasonic fatigue, and the influence of ultrasonic frequency is remarkable in the short life region for Al 7075 and in the long life for Al 6061, respectively, meaning that the characteristic of materials plays an important role in the ultrasonic fatigue study.(2) Fatigue cracks initiate from slip bands and no influence of frequency on the cracking mechanism in the two alloys is recognized. But the crack initiation and its early propagation are suppressed under ultrasonic fatigue. In case of ultrasonic fatigue, crack path changes from the initial nearly vertical direction into approximately 60 degree with respect to the stress axle, and most of fatigue life is spent in the growth of small cracks less than ^-100 micron.(3) Striations are rarely found on the fracture surfaces of ultrasonically fatigued specimens, instead, well developed voids are observed along with brittle cleavages, implying that fracture mode transits from tensile mode to shear mode.Furthermore, ultrasonic fatigue behavior in some high strength alloys such as nickel-base superalloy and marageing steels, is also being investigated in order to better understand the effect of ultrasonic frequency on the strength of metallic materials that have different crystal structures.