The manufacturing process of a forged 7xxx aluminum alloy generally comprises a thermo-mechanical treatment, i.e., plastic deformation at elevated temperatures, together or followed by thermal treatment. For a given grain structure (i.e., grain morphology and crystallographic texture), the relationships between thermal treatment, precipitation kinetics, and fatigue crack propagation (FCP) were topics of intensive research and are relatively well understood [7,8,9,10,11,12,13,14,15,16]; however, even though the optimization of the precipitation condition with respect to fatigue crack growth resistance is possible, the corresponding aging kinetics affect almost all other mechanical properties, often counteractively .
The forming process controls the grain structure, distribution of primary phase particles, and crystallographic texture. Much effort has been put into elucidating the relationships between crystallographic texture and FCP behavior in recent years [18,19,20,21,22,23,24,25,26]. Zhai et al. proposed a crystallographic model for short fatigue crack growth at grain boundaries . The twist and tilt angles of slip planes in adjacent grains of an Al-Li alloy were found to be the key factor controlling crack retardation, deflection, and branching. A high twist angle, for example, can fully stop the crack growth as the crack may not be able to enter the adjacent grain. Based on their findings, the authors derived a grain boundary geometry for optimum resistance to the growth of short cracks for Al-Li and Al-Cu alloys . Other studies applied this model to longer cracks in an AA7050-T7451 thick (rolled) plate and consistently found that a growing fatigue crack can hardly enter the adjacent grain if the twist angle between the slip planes is large [20,21]. Based on these studies, Liu et al. showed that Goss grains with a low Schmid factor usually have a higher twist and tilt angle to random adjacent grains and thus have a higher resistance to FCP than cube grains and brass grains [22,23].
Brittle primary phases, e.g., Fe- or Si-containing particles, have a detrimental impact on almost all mechanical properties. Such particles easily fracture under (plastic) deformation of the Al-matrix (e.g., ), and, therefore, act as fatigue crack initiation sites  or as stress concentrations ahead of a propagating fatigue crack [33,36,38,39].
SEM micrographs of fracture surfaces of var 3 (a), (c) L-T and (b), (d) T-L samples of low ΔK (top row) and higher ΔK (bottom row). The white arrow indicates the crack propagation direction (bottom to top). (b2) Inter-(sub-) granular fractured region of (b) at higher magnification.
Figure 12 schematically summarizes the evolution of the damage mechanisms. Mechanism I dominates the fracture during regime A. The plastic zone size is small compared to microstructural features (e.g., grain-/sub-grainsize); the crack grows individually from grain to grain, changing its direction and growth rate, and significant crack closure effects are observed. The active damage mechanisms change gradually in the transition regime. Mechanism II becomes dominant, initiating a comparably microstructure-insensitive, striation-forming crack growth. At this stage, clusters of broken Al7Cu2Fe particles are found on the fracture surface of T-L specimens, indicating that Al7Cu2Fe particle fracture becomes more relevant. Mechanism II is accompanied by static modes such as tearing and frequent particle fracture in regime C. Moreover, secondary cracks orientated mainly parallel to the loading direction become relevant, as pointed out in the last part of the discussion.
Free, fully featured, software for the analysis of XPS spectra written by Raymund Kwok.XPSPeak is a XPS Peak Fitting Program.The portable app creates a sandbox folder in its current location, where it stores all its settings and temporary files. Can be downloaded from the US, UK or Hong Kong.
Please download the FullProf software here andfollow the instructions to install in your computer. I am using a Windowsmachine for this tutorial. The sample data used in this tutorial is availablehere. 2b1af7f3a8