1. School of Materials
Faculty of Engineering and Physical Sciences
1. Micro-examination
• Imaging
• Serial Sectioning & Imaging of Volume of Interest (VoI):
Removal of slices of sample surface with diamond knife or
Xenon-plasma beam
• Crystallographic orientation mapping
• Chemical mapping
2. 3D Tomographic Analysis
3. Macro-examination
Contact details:
Sarah Busef
Tel.: +44 (0) 7415829300
E-Mail: sarah.busef@postgrad.manchester.ac.uk
Investigation into the relationship between grain
boundary character and the path of stress corrosion
cracks in AA7032
Sarah Busef
Supervisors: Timothy L. Burnett and Philip J. Withers
School of Materials, The University of Manchester
Objectives
X-Ray Computed Tomography (CT)
Transmission Electron Microscopy (TEM)
[1] Burnett, Timothy et al. “The role of crack branching in stress corrosion
cracking of aluminium alloys.”CorrosionReviews 33.6 (2015): 443–454.
[2] Dichtl, C. “Using 3D analysis to explore cracking ahead of the crack
tip in SCC of high strength aluminium.” MSc Dissertation, University of
Manchester (2015).
[3] Christodoulou, L., Flower, H. M. “Hydrogen embrittlement and
trapping in al- 6%-zn-3%-mg.” Acta Metallurgica, 28.4 (1980): 481–487.
Micrograph showing grain boundaries and precipitates. [2]
Hydrogen Embrittlement
• solute hydrogen gas embrittles
material ahead of the crack tip
along grain boundaries
• encouraged by coarse grains
and large grain boundary
precipitates [3]
Crack Branching
• along grain boundaries, twin
boundaries and slip bands
• lower driving force for crack
propagation, crack shielding
[1]
•
Map of crack opening displacement
(COD) showing crack shielding. [1]
8 T.L. Burnett et al.: Crack branching in stress corrosion cracking of aluminium alloys
Nonspatially resolved methods for measuring crack
length (e.g. potential-drop or back-face strain-gauge
monitoring) are open to misinterpretation when it comes
to assessing the role of crack branching in SCC. Similarly,
measurements of the crack length from the outer surface
of the specimen offer limited insight. The presence (or
not) of branching as measured at the outer surface is no
guarantee of the behaviour through the thickness of the
specimen and should be treated more as an indication.
Fracture surface analysis is limited to postmortem investi-
gations but does offer some insight to events that occurred
across the entire fracture plane. However, interpretation
is not always straightforward because, when the sample
is broken apart for analysis, parts of the surface may be
deformed, remaining bridging ligaments broken, and
failed ligaments lost from one fracture surface or both.
However, fracture face matching can add some certainty
to this analysis.
There are severe shortcomings in most methods of
assessment, as they do not allow a full appreciation of
the real 3D morphology of the crack. Abramson, Evans,
and Parkins (1985) showed some early indications of
the true 3D morphology of the stress corrosion cracks by
sectioning the cracks in different directions. Other more
recent studies (Singh et al., 2014; Zhu et al., 2014) have
begun to discover the true morphology of stress corro-
sion cracks using X-ray CT and, in particular, proving
what appear as crack jumps as observed from the outer
surface of the samples are shown to be continuous cracks
internally. Thus, it is shown that crack jumps also depend
on crack branching. We have shown using 3D analysis,
as suggested by Blain, Masounave, and Dickson (1984),
that the stress corrosion crack is composed of a “multi-
tude of closely spaced parallel intergranular cracks”. The
presence of secondary cracks is the clearest example of
this, but also, when we look at the crack front, we can
observe that it is split into multiple, closely spaced paral-
lel fingers. The crack morphology revealed by 3D analy-
sis shows features that can be easily overlooked when
only analysing the fracture surface [i.e. (1) the presence
of metal bridges and (2) the stepped topography of the
fracture surface]. The presence of metal bridges is a direct
result of crack branching and the emergence of the crack
onto different, closely spaced grain boundaries. Close to
the crack front, the metal bridges actually separate dis-
tinct crack fronts, but as the crack progresses these metal
bridges are ruptured and the cracks coalesce. Remnants
of ruptured metal bridges can be found all across the frac-
ture surface back to the crack origin (Figures 6 and 9). On
a fracture surface, it is the more substantial metal bridges
that can most easily be observed, as they are deformed
out of plane as the stress corrosion test specimen is
broken apart manually by overloading. In fact, these
regions can often be peeled back to reveal what would
have been a secondary crack path. The metal bridges that
were ruptured during SCC are more difficult to observe,
Figure 10: Crack opening displacement (COD) (in microns as shown in the legend) mapped as a cumulative total over the three primary
AA7032 cracks from the CT image.
Although the COD profile is surprisingly even across the primary cracks, branch-style variations can are evident.
solute H
void
crack propagation
AA7032 (Al-6.0Zn-2.0Mg-2.0Cu-0.2Cr,wt%), pre-cracked compact
tension specimens are exposed to ambient air. Samples are
dissected into 2 sets of 3 orientations by abrasive cutting; one for
metallurgical examination and one for serial sectioning.
which vary with heat treatment,
as shown by the stress intensity
versus crack growth rate (k-ν) for
various AA7032 tempers (right).
The aim of this project is to pinpoint the material weakening
mechanism for SCC that corresponds with grain boundary
character observed in AA7032.
Susceptibility of high-strength aluminium alloys to stress corrosion
cracking (SCC) under mechanical stress and in corrosive
environments, and a lack of its understanding on multiple scales,
has contributed to a shift from its use in aerospace and
automotive sectors. The mechanism of SCC of aluminium alloys
in aqueous environments is strongly dependent on microstructural
characteristics of:
• grain boundary chemistry
• grain size and distribution
• grain boundary precipitates
• grain boundary coherency
k-ν for AA7032. [1]
Compact tension
specimen. [2]
5 Sample preparation – Selection of the VOI
39
5 Sample preparation – Selection of the VOI
5.1 Tested specimen
The analysed alloy is aluminium 7032-T7x. It contains 6% Zinc, 2% Magnesium, 2%
Copper and 0.2% Chromium. The tested specimen (see figure 5-1) is a compact
tension specimen. The dimensions are 2 cm height, 2 cm width and 1 cm thickness.
The specimen has a machined notch and was loaded with the help of two screws to
an initial stress intensity of 20-24 MPa√ . It was stored in loaded condition in
ambient air for around eleven years. In this time, a crack with a length of about 0.5
mm was formed. A sample of the same alloy but with a different temper (TSCC),
making it more susceptible to SCC, was analysed by Burnett et al. [66]. This sample
has exactly the same geometry as the one analysed in this project and was also
stored in the same environment. The main difference is that a sharp notch was
machined by electrical discharge machining. This influences the process of crack
initiation and might result in different dwell times until crack growth starts. Therefore,
the calculated average crack growth velocity would be more inaccurate. But it does
not influence the crack propagation process.
Figure 5-1: Left image: photograph of a compact test specimen with two screws.
Right image: Schematic drawing of a compact test specimen. Sample orientation:
x: extrusion direction, y: short transverse, z: long transverse)
2D grain boundary
microstructure
examination
3D correlation
with crack tip
behaviour
Determination
of mechanism
of SCC
References
A combination of the following mechanisms related to grain
boundary character may explain SCC of aluminium alloys.
Electron Backscatter Diffraction (EBSD)
Materials
Results
Methods for Multi-Scale Analysis
Scanning Electron Microscopy (SEM)
Optical Microscopy
X-ray
3D reconstruction with
FEI Avizo software [2]
X-ray Computed Tomography (CT)
Plasma Focused Ion Beam (FIB) SEM
Serial Block Face (SBF) SEM
Figure 6-11: The four images show the surfac
orientations. Only the area analysed by mi
displayed in the surface rendering. Besides the
CT-scan (blue box) and one ortho-slice (x-y
(background) are displayed. a) Overview imag
crack c) Orthogonal view (x-y plane) against
direction. The scale bar is only accurate for t
6.2.3 Selecting volumes of interest
The X-ray CT-data was also used to select a vo
by serial sectioning. The main criterion was to fi
crack and small unconnected voids ahead of the
position of the crack tip should not change its p
when the bounding box is moved along the z
could still be captured, also when there is a sp
volume and the volume received in the sample
Active-Path Inter-granular or
Anodic Dissolution
• preferential dissolution at grain boundary
defects such as precipitates (η-MgAl2),
depleted zones, slip bands and second-phase
particles [2]
Introduction
Figure 6-4: Images of the process used for anal
precipitates. All images show the same region o
direction. Image size: 49.6 x 35.3 µm². a) Initial BS
voltage is 10 kV. b) Precipitates have been segmen
of grey scales in the initial image. The precipitate
interiors of the grains have been selected manually
of the precipitates at and near the grain boundaries.
In the first step the initial SEM images were filter
(radius 2.0 pixels) and the “Smooth” operation in
remove some white pixels from the image that are n
selected afterwards. In the second step, the imag
black and white image (binarization). This is done b
brighter grey scale than a selected threshold value.
black and all pixels that have not been selected are
As described in paragraph 6.1.2 the main precipita
BSE-SEM image. As such, this step leads to the sel
matrix and all other features are removed. In the
10µm
rotating
specimen
2D
projections
2 T.L. Burnett et al.: Crack branching in stress corrosion cracking of aluminium alloys
The propensity to SCC of high-strength aluminium
alloys, as a function of alloy temper, is often character-
ised by v-K curves, where the crack propagation rate, v,
promoted in macroscale precracked fracture mechanic-
type specimens, is plotted as a function of the applied
stress intensity factor, Kapplied
, calculated by assuming
a single crack propagating with a relatively uniform
through-thickness crack length. A typical v-K curve is
provided in Figure 1b. It shows a highly K-dependent
region of crack growth (region I) at low K’s, a relatively
K-insensitive region for intermediate K’s (region II, often
referred to as the “plateau” region), and a second K-sen-
sitive region (region III) at high K’s, typically only gen-
erated under rising load conditions. An alloy or temper
of a given alloy is deemed more SCC resistant, as the
minimum applied stress intensity factor, K1SCC
(the start
of region I), needed to generate a significant SCC growth
rate (e.g. > 10-11
m/s) increases and the region II “plateau”
velocity decreases. Typical v-K data for AA7032 (one of
the materials used in this study) as a function of alloy
temper when totally immersed in 2% NaCl solution with
a sodium dichromate inhibitor addition are shown in
Figure 1a, where the SCC propensities of the -TSCC, -T6,
-T76, and -T73 tempers would be deemed to be highly
susceptible, susceptible, relatively resistant, and highly
resistant, respectively.
The geometry and environment at the crack tip has
also been the focus of investigations at the nanoscale
(Lozano-Perez et al., 2011; Staehle, 2010). At the grain
scale, the local chemistry and crystallography can have
a strong influence on the path that the stress corrosion
cracks follows. Consequently, the susceptibility of spe-
cific grain boundaries has received quite a lot of attention,
holding the promise of grain boundary-engineered mate-
rials that are resistant to SCC (Babout, Marrow, Engelberg,
& Withers, 2006). The use of multiple different techniques
applied across a range of scales is one of the themes of this
paper and is receiving increasing attention as a method
to investigate materials science engineering challenges
(Burnett et al. 2014).
Crack branching is a mesoscale phenomenon taking
place at a scale between that of the crack tip and the test
component. When considering the SCC of aluminium and
steel, it has remained largely at the periphery of our con-
sideration when proposing mechanisms for the advance-
ment of the cracks at the nanoscale or the features of v-K
curves obtained from macroscale tests. Speidel (1971)
and Carter (1971) described the presence and the circum-
stances under which crack branching under SCC con-
ditions could occur. In this respect, the region II crack
velocity plateau was cited as being of critical importance.
The proposition was that crack branching only became
possible after critical stress intensity was achieved
(approximately double, later revised to ~1.4, the value
at which the SCC began to measurably propagate, K1SCC
)
and that the crack velocity was then constant across a
range of stress intensities. The reason that branching
could only occur once the SCC had reached a plateau in
1·E-05
a b
-TSCC
-T6
-T73
Logcrackvelocity
-T76
TSCC
Macrobranching
Microbranching Possible
Possible
T6
T76
T73
1·E-07
1·E-09
1·E-11
0 10 20 30 40
Stress intensity (KI)
KM=1.4 KISCC
KM KP
KB
I
II
III
KB=1.4 KP
KISCC
Applied stress intensity factor (MNm-3/2
)
Av.Crackgrowthrate(m/s)
Figure 1: (a) Average stress corrosion crack rate (or velocity), v, as a function of the applied stress intensity factor, K, often referred to as a
v-K curve, for AA7032 in various alloy tempers totally immersed in an acidified inhibited saline solution and (b) schematic v-K curve showing
regions I–III cracking along with suggested minimum K conditions for the types of microcracking proposed after Speidel (1971).
KM
, minimum stress intensity required for microbranching; KP
, stress intensity at the start of the region II plateau; KB
, minimum stress inten-
sity required for macrobranching.
Authenticated | timothy.burnett@manchester.ac.uk author's copy
Download Date | 9/21/15 10:26 AM