Discussion of “Fatigue Crack Initiation Mechanism for Cast 319-T7 Al Alloy”
Discussion of “Fatigue Crack Initiation Mechanism for Cast 319-T7 Al Alloy”
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“铸造319-T7铝合金疲劳裂纹萌生机理”的探讨
DOI：
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发表时间：
2009
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影响因子：
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通讯作者：
J. Campbell
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文献类型：
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作者：
J. Campbell
Jang和同事提出了一个有趣的结果，从重力砂型铸造铝合金缸体铸件疲劳试验。他们发现了广泛的失效模式，包括零失效（疲劳终止），从孔隙开始，以及在断裂表面上观察到广泛的粗糙穿晶刻面。他们的结论是，穿晶面是滑移面，在这些平面上观察到的氧化物是裂纹扩展过程中微动损伤的结果。我想提请作者注意这样一个事实，即拉伸和疲劳试样的断裂表面上的小平面在许多种铸造合金上是常见的，包括铝合金、铸铁和镍基高温合金。[1]其机理很简单，即在凝固过程中，通过枝晶的前进来拉直氧化物双膜（在铸造过程中由表面湍流引起的必然的双氧化物膜）。这种枝晶推动过程产生大的平面区域，其被两个膜之间的平面未结合界面分开，从而形成广泛的穿晶裂纹，有时是晶间裂纹。穿晶裂纹表现出精确的平面晶体学定义的枝晶，而双膜推入晶间平面表现出典型的弯曲表面的晶界。关键的是，考克斯和同事[2]只观察到了镍基高温合金断裂表面上的小平面，这些合金已经过铸造。那些用精心设计的填充系统铸造的，避免了从液体表面夹带氧化物的，没有刻面。作者观察到的氧化物在一些补丁的方面，但不是在其他的是典型的氧化物膜夹带到散装金属从液体表面。这种薄膜是不规则的和凌乱的，厚的地方，但往往是如此之薄（也许20纳米），难以检测到传统的微量分析。如果刻面是滑移带机制的结果，
Jang and co-workers present interesting results for fatigue tests taken from a gravity sand cast aluminum alloy cylinder block casting. They find a wide spectrum of failure modes, including zero failure (fatigue runouts), initiation from pores, and observations of extensive coarse transgranular facets on the fracture surfaces. They conclude that the transgranular facets are slip planes, and the oxides observed in places on these planes are the result of fretting damage during crack propagation. I would like to draw the attention of the authors to the fact that facets on fracture surfaces of tensile and fatigue specimens are common on many kinds of cast alloys, including Al alloys, cast irons, and Ni-base superalloys. [1] The mechanism is simply that of straightening oxide bifilms (the necessarily double oxide films caused by surface turbulence during casting) by the advance of the dendrites during solidification. This dendrite pushing process creates large planar areas, which are separated by the planar unbonded interface between the two films, thus forming extensive transgranular, and sometimes intergranular, cracks. The transgranular cracks exhibit the accurately planar crystallography defined by the dendrites, whereas the bifilms pushed into intergranular planes exhibit the typical curved surfaces of grain boundaries. Critically, Cox and co-workers [2] only observed facets on the fracture surfaces of Ni-base superalloys that had been cast turbulently. Those that had been cast with carefully designed filling systems that avoided the entrainment of the oxide from the liquid surface were free from facets. The observation by the authors of oxides in some patches of the facets but not in others is typical of oxide films entrained into the bulk metal from the liquid surface. Such films are irregular and messy, thick in places, but often so thin (perhaps 20 nm) as to be difficult to detect by conventional microanalysis. If the facets had been the result of a slip band mechanism, some evidence of extrusions and intrusions