University of Groningen
In-situ observations of materials properties with Auger electron spectroscopy
van Agterveld, Dimitri
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2001
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van Agterveld, D. (2001). In-situ observations of materials properties with Auger electron spectroscopy
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Chapter 1
INTRODUCTION
The segregation of elements to grain boundaries in engineering materials is of
great technological importance, because it may have a considerable influence on
mechanical properties, such as strength and interface toughness. One of the
most illustrious examples is the catastrophe with the Royal Mail Ship ‘Titanic’
in 1912. Investigations on material, retrieved from the bottom of the ocean in
1985, revealed that the steel contained large amounts of sulfur and phosphorus,
compared to present-day steels [1]. The presence of these segregating impurity
elements, together with a low amount of manganese, leads to an increase in the
ductile-brittle transition temperature (DBTT). This means that an otherwise
ductile material can become brittle easily, once the temperature is reduced
below the DBTT.
Another classical example of the change in properties of a material upon
addition of another element is the embrittlement of copper by small amounts of
bismuth, which was first observed in 1874 [2]. Only with the advent of Auger
electron spectroscopy in the seventies of the last century it was shown that this
was caused by grain boundary segregation, i.e. the enrichment of bismuth at
the grain boundaries in copper. The accumulation of impurity atoms at grain
boundaries leads to the formation of a very narrow zone with a different
chemical composition, having a large influence on the interface. Very small bulk
concentrations of impurity atoms can already lead to significant amounts of
those atoms at the grain boundary. This may drastically change the response of
a material on mechanical loading and can eventually lead to brittle failure of an
otherwise ductile material.
Although grain boundary segregation in most engineering materials is
detrimental to the materials properties, in some cases it has a positive effect.
1
INTRODUCTION
One of the examples is the ductilizing effect of the addition of boron to Ni3Al
with a slightly Ni-rich composition [3], where boron was observed to segregate
to the grain boundaries [4].
The embrittlement or ductilization by solute segregation was explained within
a thermodynamical framework [5]. According to this theory, a segregating
solute will embrittle the material if it has a higher free energy of segregation to
a free surface, compared to the free energy of grain boundary segregation.
Therefore, a solute with a higher tendency for grain boundary segregation than
for surface segregation will increase the ductility of a material.
The objective of the research described in this thesis is to study the influences of
impurity segregation on the materials properties in polycrystalline alloys. The
alloys that are used contain a relatively low number of different elements,
which will minimize obscuring effects such as the interaction between
segregating impurities. In this way, macroscopic changes can be correlated to
the influences of the specific elements. In principle, if the influence of a
segregating element is known, it would be possible to tailor materials for
specific applications.
Through in-situ intergranular fracture in a combined scanning electron –
scanning Auger microscope, grain boundaries are exposed, which enables the
study of grain boundary segregation and its dependence on previous heat
treatments and bulk composition. A direct comparison between grain boundary
and surface segregation can be made when pores, which were internal surfaces
during materials processing, are present on the fracture surface. Because of the
high surface sensitivity of Auger electron spectroscopy and the use of a field
emission gun, segregation can be studied with a high lateral as well as depth
resolution.
However, the instrumental conditions with which segregation is studied may
influence surface segregation of several species as well. Surface oxidation leads
to surface segregation of elements with high oxygen affinities. Prolonged
exposure of a clean fracture surface to an electron beam leads to enhanced
oxidation, compared to the non-exposed areas on which only oxygen
chemisorption takes place. The phenomenon of electron beam enhanced
oxidation is found to obscure the observation of surface segregation as a
function of time. Therefore, knowledge of the influence of an electron beam on
the processes at a surface is indispensable when interpreting the acquired
segregation data. Nevertheless, it enables a study of the kinetics of oxidation at
2
INTRODUCTION
room temperature in a UHV atmosphere where residual gases cause surface
oxidation. Several oxidation models have been developed in the past for the
oxidation of pure Ni exposed to low oxygen pressures [6,7]. The validity of
these models for the oxidation of Ni3Al under the influence of an electron beam
will be examined.
The materials that are studied throughout this thesis are Al-Mg, Cu-Sb, Cu-Bi
and Ni3Al (with or without boron) alloys. Aluminum – magnesium based alloys
are suitable candidates for a wide range of applications because they are
characterized by excellent corrosion resistance, formability and weldability
[8,9]. Enhanced Mg segregation to the surface can detoriate such properties
because it leads to the formation of relatively thick, brittle oxides. Besides the
fact that strain can be released upon Mg surface segregation, oxidation of the
surface is a driving force for Mg segregation as well, because of the high oxygen
affinity of Mg.
Segregation of both Bi and Sb in Cu is studied in combination with the presence
of small amounts of sulfur. For Bi and Sb, chemically similar because they are in
the same column of the periodic table, strain release is the main driving force
for grain boundary segregation. The segregation of sulfur occurs through very
fast diffusion of sulfur-vacancy complexes and Cu2S formation at defects.
Studies of site competition between S and Sb/Bi at surfaces and grain
boundaries yield information about the influence of each of these elements on
failure phenomena.
The strongly ordered aluminide Ni3Al has attractive properties for structural
applications. The flow stress of single crystalline material increases with
increasing temperature and at high temperatures, an oxide layer is formed that
protects against corrosion. However, polycrystalline Ni3Al is extremely brittle
at room temperature. The addition of boron, which segregates to the grain
boundaries, circumvents this problem. Auger electron spectroscopy
observations of segregated boron are combined with orientation imaging
microscopy to study processes at the grain boundaries and with tensile
experiments to examine the influence on ductility and fracture behavior. The
influences of boron concentration, grain size and grain boundary character
distribution, thermomechanical treatments and the presence of a notch in the
test specimens are investigated.
In chapter 2, theories of segregation and its influence on materials properties
will be described, as well as the experimental setup. This will be illustrated with
3
INTRODUCTION
results obtained from in situ heating experiments on an Al-20 at.% Mg alloy.
The alloy decomposes in two phases, enabling the observation of Mg surface
segregation in materials with different Mg concentration. In chapter 3, the
oxidation of Ni and Ni3Al(-B) under the influence of an electron beam is
discussed. The validity of the oxidation model will be tested by varying the
incident electron flux. In chapter 4, the results on Sb and Bi segregation in Cu,
in combination with the presence of S, will be presented. Finally, chapter 5
describes the influences of boron segregation on the mechanical properties of
Ni3Al.
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