The Production of nanocrystal materials with high fatigue damage resistance based on clarification of non-scale fatigue damage mechanism

采用超细晶粉末冶金铝合金CT试件进行了疲劳裂纹扩展试验，其晶粒度为200~500 nm。研究了恒幅和重复两步变幅载荷序列下的疲劳裂纹扩展行为。结果表明，粗糙度引起的裂纹闭合对材料的闭合行为有重要影响。虽然断口表面粗糙度比其他常规铝合金小，但由于裂纹尖端张开位移(CTOD)相对于粗糙度较小，导致裂纹闭合是由粗糙度引起的。在重复两步加载下，裂纹张开载荷低于相同K值的恒幅加载下的高水平载荷，与其他铝合金相比，加载历史对裂纹扩展的抑制作用并不明显。认为变幅载荷作用下的裂纹张开载荷为…更多的是由粗糙度引起的裂纹闭合。塑性诱发裂纹闭合的小贡献是由于超细晶晶界对塑性应变的约束。采用200~500 nm的超细晶粉末冶金铝合金，研究了高温下微疲劳裂纹的萌生和扩展行为。在室温(R.T.)、200℃(473K)和250℃(523K)三种不同温度下进行了反平面弯曲疲劳试验，并利用复制技术对裂纹萌生和小裂纹扩展进行了详细研究。疲劳强度随试验温度的升高而降低。无论试验温度如何，裂纹都是从粉末之间的边界开始的，总疲劳寿命主要由裂纹萌生寿命决定。疲劳裂纹的萌生和扩展行为与试验温度几乎无关。疲劳裂纹扩展速率da/dN在固定的Kmax下随试验温度的升高而略有加快，而用Kmax/E(E：弹性模数)来估算的da/dN符合得很好。疲劳强度和裂纹扩展速率对试验温度的依赖性远小于常规铝合金，表明超细晶粉末冶金铝合金具有较高的耐热性。较少

Fatigue crack growth tests were carried out using CT specimens of ultra-fine grained P/M (Powder Metallurgy) aluminum alloy of which grain size was from 200 to 500 nm. Fatigue crack growth behavior was investigated under constant amplitude and repeated two-step variable amplitude load sequences. It was found that roughness induced crack closure played an important role in closure behavior of this material. Although the roughness of fracture surface was smaller than the other conventional aluminum alloys, the roughness induced crack closure was dominant because of the small crack tip opening displacement (CTOD) with respect to the roughness level. Under repeated two-step loading, crack opening load was lower than that under constant amplitude loading with the same K value as the high level load, and the crack growth retardation caused by load history was not prominent comparing with other aluminum alloys. It is considered that the crack opening load under variable amplitude loading was … More dominated by the roughness induced crack closure. The small contribution of the plasticity induced crack closure was due to the constraint of plastic strain by the grain boundaries of ultra-fine grains.Small fatigue crack initiation and growth behavior at elevated temperatures were investigated using ultra-fine grained P/M (Powder Metallurgy) aluminum alloy of which grain size was from 200 to 500 nm. Reversed plane bending fatigue tests were conducted at three different temperatures of room temperature (R.T.), 200℃ (473K) and 250℃ (523K), and crack initiation and small crack growth were studied in detail by means of replication technique. The fatigue strength decreased as test temperature increased. The cracks initiated at the boundary between powders regardless of test temperatures, and total fatigue life was dominated by crack initiation life. The fatigue crack initiation and growth behavior was almost irrelevant to the test temperatures. Fatigue crack growth rates, da/dn, were slightly accelerated at a fixed Kmax with increasing test temperature, while da/dn evaluated in terms of Kmax/E (E : Elastic modulus) agreed well. The dependence of fatigue strength and crack growth rate on test temperature was much smaller than that of conventional aluminum alloys, indicating the high heat resistance of ultra-fine grained P/M aluminum alloy. Less