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Microstructural aspects of crack nucleation during cyclic loading

of AA7075-T651

H. Weiland a,*, J. Nardiello b, S. Zaefferer c, S. Cheong a, J. Papazian b, Dierk Raabe c

aAlcoa Inc.,100 Technical Drive, Alcoa Center, PA 15069, USAb Northrop Grumman AEW/EW Systems, 925 S. Oyster Bay Road, Bethpage, NY 11714, USAc Max-Planck Institute for Iron Research, Max Planck Strae 1, D 40237 Dsseldorf, Germany

a r t i c l e i n f o

Article history:

Received 9 January 2007

Received in revised form 24 November 2008

Accepted 26 November 2008

Available online 10 December 2008

Keywords:

Crack initiation

AA7075

3D microstructure

Fatigue

a b s t r a c t

A series of fatigue test samples made of 7075-T651 aluminum were interrupted at various

life fractions and the number of debonded, cracked particles and cracks in the metal matrix

was determined quantitatively as a function of load cycles. It was found that only cracked

constituent particles nucleate a matrix crack. The crystallography of one individual crack

and its three-dimensional shape was determined by serial sectioning in a scanning electron

microscope by applying focused ion beam (FIB) milling in combination with orientation

imaging microscopy (OIM). The limited data suggest that the initial growth direction of a

crack is influenced by the crystallographic orientation of the matrix into which the crack

is growing.

2008 Elsevier Ltd. All rights reserved.

1. Introduction

Optimization of aluminum alloys for aerospace applications requires a quantitative understanding of the various micro-

structural attributes controlling nucleation and growth of cracks in the metallic matrix. Furthermore, in monolithic parts,

where crack arrest from joints is not given, the role of the microstructure becomes increasingly of importance, requiring

a quantitative understanding of the damage evolution in complex microstructures.

The development of current aluminum alloys for aerospace applications was based on a sound understanding of the influ-

ence of the microstructure on damage related properties such as fracture toughness and fatigue [15]. However, aluminum

alloys developed in the first half of the last century, such as AA7075, were developed mainly using Edisonian approaches.

While a few studies exists on the effect of aging conditions on properties, detailed analysis on microstructural attributes con-

trolling crack nucleation and growth during monotonic or cyclic loading were not available at the time this alloy was devel-

oped. However, it was learned early on that iron-bearing second phases in the 550 lm in diameter range, commonlyreferred to as constituent phases, were the initiation sites for cracks [1]. Consequently, later alloy developments included

the reduction in iron and silicon to improve damage related properties. On the other hand, if the particle density is reduced

as with current generation alloys, other microstructural characteristics such as grain boundaries and grain orientations, will

contribute to crack nucleation and growth. The reader is referred to Refs. [15] for a detailed discussion on the effect of

microstructure on damage in commercial aluminum alloys. It has to be pointed out that extrapolation of knowledge gained

in Al-Cu systems (2xxx series alloys) cannot readily be extrapolated to AlZn (7xxx series alloys) due to differences in phases

and strengthening mechanisms.

0013-7944/$ - see front matter 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.engfracmech.2008.11.012

* Corresponding author. Tel.: +1 724 337 3133.

E-mail address: [email protected] (H. Weiland).

Engineering Fracture Mechanics 76 (2009) 709714

Contents lists available at ScienceDirect

Engineering Fracture Mechanics

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / e n g f r a c m e c h
mailto:[email protected]://www.sciencedirect.com/science/journal/00137944http://www.elsevier.com/locate/engfracmechhttp://www.elsevier.com/locate/engfracmechhttp://www.sciencedirect.com/science/journal/00137944mailto:[email protected]


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In the current study, the number fraction of debonded and cracked particles was determined quantitatively as a function

of fatigue cycles from interrupted fatigue tests. Additionally, the size of cracked particles nucleating a matrix crack and the

associated crack length was determined. The crystallography of these cracks and their three-dimensional shape was deter-

mined from serial sectioning by focused ion beam (FIB) milling in combination with orientation imaging microscopy (OIM).

The data suggest that the initial growth direction of a crack is determined by both the local stress field around the particle

and the crystallographic orientation of the matrix into which the crack is growing.

The purpose of the current work thus is to quantitatively identify the role of large second phase particles in controlling

crack nucleation during cyclic loading with the goal to enable microstructure-based life prediction of airframe parts made

from these alloys. The latter will be reported separately.

2. Experimental procedures

A 76.2 mm thick plate of 7075-T651 was produced (YS 511 MPa, UTS 557 MPa). A series of two open hole fatigue coupons

of 47.5 mm 355.6 mm 5.7 mm (Fig. 1a), with a rib pad were machined at the T/4 location, that is at about 19 mm below

the plate surface. The hole diameter was 4.8 mm. Surfaces of sample holes were electro-polished prior to testing to facilitate

microstructure analysis by scanning electron microscopy (SEM). Samples were tested in low cycle fatigue (3 Hz, R = 0) at

276 MPa (room temperature, 35% relative humidity, constant amplitude loading). Under these test conditions, the sample

typically failed close to 9000 cycles. A series of intermediate samples was produced with cycles ranging from 10 cycles to

8000 cycles (see Table 1). For each life time, match-stick shaped samples were cut out such that each hole was cut in the

middle along the loading direction (Fig. 1b), resulting in four samples for each life time. This sample geometry allowed

microstructure observation in each hole at the locations of maximum stress that is at the location were initiation of dam-age is expected.

Failed samples were analyzed for crack initiation sites as well as for secondary cracking in the hole. The match-stick sam-

ples were quantitatively analyzed for the number density of debonded and cracked particles and for cracked particles with

an associated matrix crack. All SEM analysis was performed with a SIRION SEM optimized for backscattered electron (BSE)

imaging. Twenty BSE images were collected for each test condition at a magnification of 2000. The individual image area

was 22,008 lm2, resulting in a total area analyzed of 0.44 mm2. A 30 square pixel limit excluded any particle less than

0.08 lm2. Stereological characterization of all particles detected, identification of debonded and cracked particles, and of

cracked particles nucleating a matrix crack was performed using semi-automatic image processing with a KS400 image anal-

ysis software by Zeiss. Three-dimensional microstructure analysis by serial sectioning was performed with a Zeiss Cross-

beam XB 1540 FIBSEM using a EDAX/TSL EBSD system with a DigiView camera. Sections of 20 lm 30lm were cut at

an angle of 15 relative to the sample surface and at a vertical spacing of 0.5 lm. Each section was characterized by BSE

images and (OIM). More details on this three-dimensional microstructure analysis technique can be found in [6].

Fig. 1a. Schematic of fatigue test coupon with load direction indicated.

Fig. 1b. Geometry of match-stick samples cut from test coupon. Area of analysis shaded gray.

Table 1

Damage of microstructure as a function of fatigue cycles. In average 1160 particles were analyzed at each fatigue life step.

Number of cycles 10 100 1000 3000 5000 7000

Cracks into matrix 0 0 3 8 5 10

Cracked particles 0 10 41 35 46 45

Debonded particles 0 31 93 82 115 105

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3. Results and discussion

3.1. Fracture initiation sites

All initiation sites of primary cracks were associated with individual constituent particles ( Fig. 2a and b) of sizes larger

than 5 lm in diameter. Initiation from slip localization at grain boundaries was not observed in this material. Multiple sec-

ondary cracks were observed at the surface of the holes below the primary crack (Fig. 2b and c). These secondary cracks were

associated with stringers of constituent particles. From the geometry of the stringers of particles it is concluded that the ob-served initiation sites of the primary crack make up one end of a stringer of constituents. The fracture surface showed trans-

granular fracture typical of fatigue damage in these alloys, characterized by the tortuosity of the fracture surface and the

absence of intergranular facets.

3.2. Damage progression

The series of samples with increased cycle life provide the opportunity to study the onset and progression of damage in

the microstructure. The constituent particle density was determined as 2637 particles/mm2. After 100 cycles, the first dam-

Fig. 2. (a) and (b) examples of initiation sites, (b) view on crack and hole surface, (c) hole surface.

Fig. 3. Debonding at a constituent particle.

H. Weiland et al. / Engineering Fracture Mechanics 76 (2009) 709714 711


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age in the microstructure can be observed in the form of debonded (Fig. 3) and cracked constituent particles (Fig. 4). Less

than one percent of all particles are cracked, while about three times as many show signs of debonding ( Table 1). The sizes

of the debonded particles lie on the small side of the average particle size. At this point in the sample life, no cracks in the

aluminum matrix are present. After 1000 cycles, the damage to particles has significantly increased. At this stage, 3.6% of all

constituent particles are cracked and 8% are debonded. A few cracks in the matrix are present, all associated with cracked

particles, none with debonded particles. The observation that matrix cracks are only associated with cracked and not deb-

onded particles can be understood in terms of the stress intensity factor. A curved interface debonding such as seen in Fig. 3

results in a stress intensity factor significant lower than a cracked particle imposes on the aluminum matrix. Cracks will

emanate from the microstructure feature with the highest stress intensity factor, which are cracked particles and which

are created in the microstructure at about 10% of the fatigue life.

With increasing cycles, the number of debonded particles increases; however, the number of cracked constituent parti-

cles seems to reach saturation. Furthermore, the cracks in the matrix, which always emanate from a crack within a broken

particle, reach a saturation number as well, with about 20% of all cracked constituent particles having an associated matrix

crack. Considering the relatively constant number of cracks growing into the matrix at high cycles, the overall fatigue life is

decided by the cracks initiated at low cycles.

3.3. Three-dimensional analysis of a crack initiation site

The microstructure analysis reported above characterized the surface of the hole, where cracks had formed. However, it is

very likely that the underlying microstructure contributes to the observed crack nucleation and growth. Thus serial section-

ing was performed using a FIBSEM. The sample with a loading of 1000 cycles was selected. Two locations were analyzed,

only one crack analysis will be discussed here in detail. A particle with an emanating matrix crack was selected. Ten sections

were cut with a section step of 500 nm. Each layer was characterized by OIM, determining grain morphologies and crystal-

lographic orientations of individual grains. The selected particle, P1, was about 8 lm in length (Fig. 4), and was completely

cracked with a matrix crack emanating from the particle crack. The matrix crack grew at about 45 to the long particle axis,

with the latter being in line with the fatigue loading axis. After two cuts into the material surface, a second but larger sub-

surface constituent particle, P2, began to be revealed (Fig. 5). This subsurface particle was partially cracked (black arrows in

Fig. 5) and matrix cracks were not associated with it. Particle P1, which at the surface just showed one crack, has a second

matrix crack at the opposite site of the first crack emerging. It can be seen from the relative position of the cracks in each

section analyzed that the fracture surface of both cracks extends approximately normal to the sample surface into the grain

matrix, that is the cracks do not show signs of slanted surfaces.

The crystallographic orientations of the grains containing the initiated cracks were determined by OIM (Fig. 6). The crack

visible at the hole surface (above P1 in Figs. 4 and 5) nucleated in a grain of an orientation close to 1 1 0110. The matrix

containing the subsurface crack pointing from P1

to P2

in Fig. 5 resided in a grain of an orientation close to

0 01

1 10. This

grain orientation has been reported in the literature as frequently being associated with cracks in such alloys [2]. Comparing

the crack plane with the available slip systems in each section revealed that the matrix crack visible at the hole surface is

throughout the analyzed volume in the plane of one of the available slip systems (Fig. 5 top). The alignment of the crack with

an available slip system does not imply that the fracture is of cleavage type. The subsurface crack, however, is not aligned

with any slip systems. Nor do the available slip systems form a symmetrical, 45 configuration with respect to the crack

plane such as in Mode 1 cracking.

From the alignment of the major crack with the slip system and the angle the crack forms with respect to the loading

direction, it is assumed that this crack has formed under Mode 2 conditions that is shear stresses. The secondary crack most

Fig. 4. Microstructure selected for 3D analysis at 5000 cycles. Arrow pointing to matrix crack.



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likely is also a Mode 2 crack, but due to the lack of available slip systems has not propagated far. The open question is the role

of the subsurface particle. Due to its larger size and less elongated shape, it did not fracture completely at this stage of the

fatigue life. It is clear from these observations, that a crack nucleating in the matrix adjacent to a second phase particle, the

second phase particle needs to be completely fractured. Establishing the conditions for fracturing particles requires further

study, however, it is clear that geometrical attributes, that is elongation, relative size, and geometry are critical.

3.4. Summary

The systematic study of the damage evolution in a high-strength aluminum alloy containing a large density of constituent

particles showed that the local microstructure has an affect on the nucleation of fatigue cracks. Specifically it was observed

that debonding of the particlematrix interface does not contribute to crack formation in the aluminum matrix. Thus a high

density of debonding does not result in cracks into the matrix. Cracks in the aluminum matrix were always associated with a

Fig. 5. Section 6 at 3 lm below hole surface. P1: surface particle; P2: subsurface particle. White arrows point to matrix cracks. Black arrows point to partial

cracks in subsurface particle. Vertical white lines in P2 are an imaging artifact.

Fig. 6. (a) OIM map of Fig. 5, color coded by the crystallographic direction normal to the aluminum plate. Unit cells with primary slip plane and slip

direction are plotted for grains with a matrix crack. The double arrow indicates loading direction. (b) Color code for (a). (For interpretation of the

references to color in this figure legend, the reader is referred to the web version of this article.)

H. Weiland et al. / Engineering Fracture Mechanics 76 (2009) 709714 713


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cracked constituent particle. Particles were not cracked in the as-received state and are undergoing a nucleation event dur-

ing fatigue exposure of the sample before reaching a saturation value. While not all particles crack, even for some of those

which developed a complete through-particle crack, only a small fraction initiate a crack into the aluminum matrix. The re-

sult of the three-dimensional analysis of a single crack indicate that in this case the crack nucleation in the aluminum matrix

is influenced by the availability of slip systems in the matrix adjacent to the particle as controlled by the direction of the

maximum stress. This in turn is determined by the size and geometry of the cracked particle, as well as the effect that neigh-

boring particles have on the first one.

References

[1] Staley JT. How microstructure affects fatigue and fracture of aluminum alloys. In: Peronne N, editor. Tenth symposium on naval structural mechanics,

Washington DC, 1978. University Press; 1978, ISBN 0-8139-0802-7. p. 67184.

[2] Starke Jr EA, Lutjering G. Cyclic plastic deformation and microstructure. In: Fatigue and microstructure. Metals Park: ASM; 1979. p. 2269.

[3] Magnusen PE, Bucci RJ, Hinkle AJ, Brockenbrough JR, Konish HJ. Analysis and prediction of microstructural effects on long-term fatigue performance of

an aluminum aerospace alloy. Int J Fatigue 1997;19(Suppl. 1):S27583.

[4] Patton G et al. Study of fatigue damage in 7010 aluminum alloy. Mat Sci and Engng 1998;A254:20718.

[5] Oswald LW. Effects of microstructure on high-cycle fatigue of an AlZnMgCu alloy (Al-7055). Master thesis, University of Pittsburgh; 2003.

[6] Konrad J, Zaefferer S, Raabe D. Investigation of orientation gradients around a hard Laves particle in a warm-rolled Fe3 Al-based alloy using a 3D EBSD

FIB technique. Acta Mater 2006;54(5):136980.


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