MECHANOCHEMICAL INTERACTIONS

1
MECHANOCHEMICAL INTERACTIONS Based on
presentation by W. Ke
2
Basic mechanochemical phenomena
are mechanochemical and chemomechanical
effects Mechanochemical effect is acceleration
of chemical (electrochemical) reaction (i. e., an
additional reaction flux) caused by mechanical
action. Chemomechanical effect is acceleration
of plastic deformation (i. e., an additional
dislocation flux and plastization) caused by
chemical (electrochemical) reaction. In the tip
of corrosion-mechanical crack both effects act
simultaneously and the autocatalytic mechanism of
the crack propagation appears even in an active
dissolution state without any passivating films.
3
In general, Mechanochemistry of solids covers a
whole range of interconnected phenomena taking
place in the case of mechanical action on solids
or their separate parts participating in chemical
reactions with other substances or with each
other. We are sure that only Mechanochemical
reactivity of metals can explain many features of
stress corrosion.
Now we demonstrate theoretical background and
some practical applications of mechanochemical
phenomena that are important for the life
time prediction and security of many structures
in different industries.
4
Numerous displays of these phenomena were
experimentally observed in stress corrosion
investigations. E. g., as it has been shown B.
Yan, G. C. Farrington, C. Laird (1985),
corrosion fatigue in solutions prevented the
formation of oxide films is governed by selective
dissolution of slip bands due higher energy state
of the material in such local places on the
surface. Of course, it is necessary to mean that
applying deformation can change different surface
states (films, micro-geometry, roughness etc.)
and to mask the pure mechanochemical effects.
Moreover, in some cases the selective
dissolution of the crack tip (including that
caused by mechanochemical effect) can blunt the
sharp end of a crack, lower concentration of
stress and stop growth of a crack Bruce D.
McLaughlin (1971). Also it is necessary to
consider distribution of local and not local
processes on surfaces including significant
inhomogeneity of chemical and electrochemical
conditions within stress corrosion cracks.
5
Thus, Mechanochemical Interactions in Stress
Corrosion under static and cycling loadings are
main phenomena causing corrosion crack nucleation
and propagation. Also, residual stresses can lead
to developing Mechanochemical Interactions and
cracking. But, first of all we must coordinate
the terminology, i. e. correct working language.
6
Corrosion-deformation interaction or
mechanochemical interactions?
Corrosion-deformation interaction (CDI) seems to
be not convenient term for following
reasons. Corrosion is a result of "the
destructive attack of a metal by chemical or
electrochemical reaction with its environment"
Uhlig (1963), but it is not a phenomenon.
Plastic deformation (a permanent change of
geometrical sizes Dieter (1988)) is a result
of plastic flow and also it is not phenomenon.
While, chemical reactions and plastic flow
indeed are chemical and mechanical phenomena,
respectively. Obviously, any interaction can
arise only between physical and chemical
phenomena, but not between their results.
Therefore, more correct expression here seems
to be "chemical-mechanical interactions" denoting
synergistic interaction between chemical
reactions and plastic flow. However, because
commonly Mechanochemistry uses terms
"mechanochemical effect", "chemomechanical
effect" and in general "mechanochemical
phenomena", it seems to be more convenient to use
in corrosion practice the term "mechanochemical
interactions" instead of CDI.
7
What is Mechanochemistry In chemical physics and
physical chemistry, various scientific branches
are subdivided in accordance with the type of
energy used for initiating and stimulating
chemical reactions. Correspondingly, scientific
directions have been named, for example,
radiochemistry, electrochemistry,
thermochemistry, photochemistry, etc. According
to this classification, the area of science
concerned with the effect of mechanical energy on
chemical reactions should be termed
mechanochemistry. Therefore, the term
mechanochemistry was introduced by W. Ostwald
at the beginning of the XX century. The effect of
mechanical deformation of the solid on the course
of chemical reactions is one of the oldest
empirically observed facts in the history of
mankind, for instance in procuring fire.
However, mechanochemistry was developed into
exact science only in the last threefour decades
and now it is rapidly developing area. The
Mechanochemistry development is the result of the
requirement of industry to solve tasks of using
or preventing chemical reactions developed or
accelerated by mechanical activation.
8
Thermochemical Metallurgy

• It is remains a mystery why the mechanochemical
  preparation of mercury from its sulfide was
  forgotten during the Middle Ages.
• Indeed, Alchemists considered fire the general
  means of purification , and rubbing was not
  considered a possible alternative.
• May be, due to bad Alchemists many years
  Metallurgy existed only as Thermochemical
  Metallurgy. Today we know Hydrometallurgy,
  Electrochemical Metallurgy, etc.

9
Mechanochemistry is not Thermochemistry In the
processes of dissipation of mechanical energy,
some part of this energy is transformed to heat
and, consequently, temperature in some especially
excited point of the material can increase.
However, although according to the well-known
Arrhenius equation, this can accelerate the
reaction, in the majority of mechanochemical
reactions the temperature change plays no
significant role. To confirm this, there are many
facts.
For example, in heating mercury chloride Hg2Cl2
sublimates without dissociation. But it can be
easily decomposed to its components by abrasion
in a mortar Hg2Cl2
2Hg Cl2 Since in heating this substance
sublimates without dissociation, local heating is
excluded as the reason for the initiation of this
mechanochemical reaction.
abrasion
For the second example, pure Ag cannot be obtain
from AgCl by heating because silver halides melt
without dissociation to components. But under the
effect of low shear stresses silver halides
partially decompose even at ambient temperature.
This is accompanied by the formation of a pure
metal AgCl
Ag Cl Thus, although thermodynamics says
that reducing noble metals from oxidizing state
cannot be achieved any known thermochemical
methods, mechnochemistry does it !!! May be
unfortunate Alchemists did not know it ?....
shear
10
Mechanochemical synthesis in solid - solid and
to less extend solid gas and solid-liquid
systems is probably the most explored area in
mechanochemistry. The possibilities here are from
synthesis of inorganic compounds to composite
materials which are difficult or not possible by
other means.
Common Thermochemistry In general, solid-state
reactions involve the formation of a product
phase at the interfaces of the reactants. Further
growth of the product phase involves diffusion of
atoms of the reactant phase through the product
phase, which constitutes a barrier layer
preventing further diffusion. Hence high
temperatures are required for the reaction to
occur at reasonable speeds.
Mechanochemistry In opposite, Mechanical
activation by milling can provide the means to
increase the reaction kinetics rate even under
ambient temperatures due to the generation of
clean and fresh surfaces (a result of
fracturing), increased defect density, and
reduction of particle sizes. The key point is
an excess of the Gibbs free energy produced by a
stress in solid reactants.
Therefore, Mechanochemistry can give a
possibility to produce a new compound which could
not be obtained earlier by traditional
thermo-chemical ways.
11

• Thus, the extensive possibilities of
  mechanochemistry as the chemistry of non-thermal
  low-temperature reactions are indicated by the
  fact that in experiments in the solid phase
  without dissolution or melting of reagents it was
  possible to synthesize
• - refractory compounds and intermetallic
  compounds (mechanical alloying)
• - inorganic and organic compounds, molecular
  complexes
• - polymers and pharmaceutical preparations
• composite materials and materials in the
  nano-crystalline and amorphous states.
• Unfortunately, Mechanochemical phenomena
  accelerate chemical reactions not only in
  synthesis of new materials but also in
  degradation of existed materials , as in stress
  corrosion of metals.
• This degradation reveals in the accelerated
  dissolution of metals and minerals under
  mechanical stress.

12
Mechanochemical System and Phenomena
MECHANICAL ACTION well defined stress conditions
or comminution
Adsorption Dispersion Aggregation
Chemical/ Electrochemical reactions
Appearance of new surfaces
Enhancing rate of reactions (Mechanochemical effe
ct)
SOLID
Enhanced Plasticity (Rehbinder Effect)
Cross and Conjugate Synergistic Effects (MCE,
CME, RE, HE)
Enhanced Plasticity (Chemomechanical Effect)
Mechanochemical Technologies
13
Quantitative description of mechanochemical
effect
Contribution of the energy of structural defects
to the Gibbs free energy of non-perfect crystal
Formation of defects in the solid structure for
some reasons (plastic deformation, crystal
growth, phase transitions with non-coherent phase
boundaries, etc.) always leads to the appearance
of excess energy of elastic deformation of the
solid around structural defects (dislocations).
Thus, the energy of elastic lattice distortions
resulting from plastic deformation of a solid, is
equivalent to the increase in the enthalpy of a
solid. In case of dislocation formation, when the
entropy component may be neglected, it is
equivalent to the increase in Gibbs function, as
well. This is one of the most important points
of our background.
14
Mechanochemical activity of metal with structural
defects
The electrochemical activity of metal ion in
electrolyte and mechanoelectrochemical activity
of atoms in solid metal are, respectively

,
where ared and
aox are ordinary activities of the species in
reduced and oxidized forms, respectively j is
the electrode potential z is a valency F is
Faraday number R is the gas constant T is the
temperature R' is a special constant (R' kNmax
, where k is the Boltzmann constant and Nmax is
the maximum possible dislocation density) is
the coefficient proportionality in the linear
dependence of dislocation density on plastic
strain Dt is the mechanical stress increase due
to strain hardening n is the number of
dislocations in the pile-up.
Heterogeneous mechanical action - mechanical
stress Dt is extended only to solid electrode and
ions in electrolyte are not under stress.
15
The stages of plastic deformation (t0 is the
critical shear stress tI, tII and tIII are the
stresses corresponding to the deformation stages
I, II and III g is the shear strain).
It can be generalized that plastic deformation of
a single crystal occurs through three consecutive
stages easy slip (1), strain hardening (II) and
dynamic recovery (III). In stage II,
slip occurs in more than one set of planes with
the formation of a large number of barriers. The
theory assumes the chief strain-hardening
mechanism to be piled-up groups of dislocations
(flat dislocation clusters). The last
stage III is associated with the failure of
pile-ups, regrouping of dislocations by cross
slip, straightening of these dislocations into
polygonal sub-boundaries and cellular tangles
with mutual weakening of the elastic dislocation
fields.
t
These processes decrease the strain energy stored
in the material and lead to partial mutual
annihilation of the dislocation. The hardening
rate (coefficient ds/de) in this stage decreases
to zero with increase strain, as observed on the
stress- strain curves. Hardening of
polycrystalline solids with the cubic crystal
system during plastic deformation is similar to
that of single crystals, by the same hardening
law.
This is explained by the existence
of several non-parallel slip systems ensuring
sufficiently ductility and insensitivity of the
ductility properties, for example, of fcc metals
to grain size. In polycrystalline metals with an
hexagonal structure in which slip takes place
mainly on the basal closely packed planes the
slip path depends on the grain size.
16
Example Dislocation networks in die-cast AM50
alloy (a) and dislocation pile-ups (b) after 6.8
deformation (TEM observation)
Pile-ups
Tangles
(a)
(b)
17
Dependence of mechanochemical effect on plastic
strain
The plastic strain on the stage of strain
hardening is dependent on the stress
Dt and
consequently the anodic current density on
deformed electrode is
where corresponds to
the onset of strain hardening with dislocation
density N0 ia0 is the anodic current density for
non-deformed metal. As it is known from the
physical metallurgy, strain hardening increases
with logarithm of the strain rate (for a fixed e
value or for N, which is the same). Then
dependence of mechanochemical effect (the
increment of anodic current density) on the
strain rate is i. e., mechanochemical
effect on the stage of strain hardening is
linearly dependent on the strain rate for fixed
strain values. These predictions were confirmed
experimentally.
18
Current summary
Summing up the theoretical analysis of the
influence of plastic deformation growth on the
mechanoelectrochemical activity of metals, we
should point out three stages corresponding to
three stages of strain hardening curve a) with
growing deformation, new dislocations appear at
the easy glide stage (stage I) without planar
pile-up formation (n 1). Therefore, the
mechanochemical effect does not increase
essentially and may even decrease at the expense
of the relaxation of previously existing
pile-ups, if this stage was preceded by a certain
hardening of thin surface layers due to their
earlier plastic flow below the macroscopic yield
strength b) under intense strain hardening
(stage II), stresses grow, and planar dislocation
pile-ups appear. According to equations above
this leads to a sharp increase in mechanochemical
effect c) at the final stage of dynamic
recovery (stage III), planar pile-ups are
destroyed due to dislocation cross slip and
partial annihilation. This results in
sub-boundaries (forming cell walls) and
substructures with high-density dislocation
tangles. Thus, the magnitude of n is sharply
decreased, and strain hardening is weakened. This
leads to the decrease in the mechanochemical
effect value, which, thus, should pass over a
maximum in the process of plastic deformation.
The results of numerous experiments confirm
this regularity. Even in passive state and
inhibited solutions at different temperatures
these regularities are shown to the full.
19
Quantitative description of chemomechanical
effect
Mechanochemical systems behave as open
non-equilibrium systems. The structure of all
mechanochemical systems is of hierarchic,
heterogeneous self-correlated type and is
controlled by the minimum entropy generation
principle. Therefore, the analysis of
mechanochemical phenomena requires the
application of methods relevant to thermodynamics
of irreversible processes. In particular, it
resulted in the discovery of the effect named
chemomechanical effect Gutman (1967). It
consists in the change of physico-mechanical
properties and fine plasticization of a solid
under the influence of chemical (electrochemical)
reactions which proceed on its surface causing
additional dislocation flux. Further, conjugate
mechanochemical phenomena and crossing effects
were revealed. A synergistic interaction between
mechanochemical and chemomechanical effects was
demonstrated and used in some new mechanochemical
technologies.
20
Thermodynamics of irreversible processes of
plastic deformation and electrochemical
dissolution
The process of dislocation generation and motion
under plastic flow of solids is essentially
irreversible and leads to entropy generation in
the system. The entropy production is expressed
through a sum of the products of generalized
fluxes and generalized forces which may be
represented by reaction fluxes and affinities.
To describe the process of plastic deformation
in the frames of irreversible thermodynamics the
chemical potential of dislocations and affinity
of the reaction of dislocation generation and
motion were proposed as new notions. The affinity
of plastic deformation process is
and corresponding generalized flux is the
dislocation flow dN/dt. So, the contributions to
entropy production simultaneously given by
plastic deformation and by anodic electrochemical
reaction are, respectively
and

where the electrochemical affinity
is presented as . Under
simultaneous plastic deformation and
electrochemical processes, total entropy
production is
21
Physical explanation of chemomechanical effect
From this equation we can conclude that the
hardness decrease and the plasticity increase
should be proportional to the logarithm of the
anodic current density J (i.e., to the logarithm
of the dissolution rate). This conclusion has
been tested experimentally in order to use it for
mechanochemical surface treatment and deep
drawing. The physical meaning of chemomechanical
effect may be interpreted as follows. Up-to-date
concepts of the dissolution mechanism of a
crystalline solid (e. g., anodic dissolution of
metals) are based on the idea of initial
monoatomic pit formation (bidimensional
dissolution nucleus) and successive etching of
atomic layers along the crystallographic plane by
shifting the monoatomic step with successive
repeating of layer dissolution process. Since
monoatomic surface steps may serve as sources of
new dislocations, we can come to the conclusion
that the appearance of additional dislocation
flux due to surface atom dissolution by anodic
current (chemomechanical effect) is caused by
heterogeneous nucleation and by the action of new
surface sources of dislocations resulting from
heterogeneous surface dissolution with monoatomic
step formation.
22
Direct observations of chemomechanical effect in
metals and minerals
Microscopic examination of the Fe surface in the
region of diamond pyramid indentation with the
load of 0.4 N did not reveal slip lines exit. The
action of 1 water solution of H2SO4 resulted in
dislocation exit to the metal surface slip lines
appeared on the faces of the pyramid imprint
resulting residual stresses relaxation due to
chemomechanical effect.
23
Twinning of marble under hydrostatic pressure in
electrolyte
a saturated by 0.1 acetic acid with the
inhibitor (3 g/l of oil acid) b saturated by
acid solution without inhibitor
b
a
While in the presence of inhibitor (as in case of
a dry specimen) separate grains were deformed
only along the planes favorable for mechanical
twinning, in the absence of inhibitors, twinning
proceeded, as well, along the planes of hindered
deformation (late twins). These late twins are
intersected within the same grain by twin layers
which appeared earlier, and stop growing in
length. Due to the combined action of
mechanochemical and chemomechanical effects, they
begin growing in width, dividing the grain into
smaller subgrains. The increase in the
compressive load enhanced the observed effects.
24
Mechanochemical interactions during low cycling
corrosion fatigue
The evolution of the strain current of
Fe-26Cr-1Mo in 1M H2SO4 solution at various
strain amplitudes during low cycle fatigue shows
the currents drop at the transition from cyclic
hardening to saturation region at total strain
amplitude of 4?10-3, 8?10-3, 1?10-2. At 1.2?10-2,
the current increased sharply before the rapid
fracture. At the hardening stage, the structure
was more characteristic of pile-up, planar
arrays. The dislocation cell at the saturation
region is characteristic of the softening stage.
At low strain amplitude, 4?10-3, the dislocation
cell with low energy took longer time to develop.
The current first increased with the strain
hardening and decreased during the softening
stage. At high strain amplitude, 1.2?10-2, the
saturated dislocation structure with low energy
did not have enough time to develop before the
sample fractured so the strain current increased
sharply.
Effect of strain amplitude on the strain current
of Fe-26Cr-1Mo in 1M H2SO4 solution at 0.1Hz
during low cycle fatigue (at ??t 4?10-3, ?
8?10-3, ? 1?10-2, ? 1.2?10-2)
25
The low-alloy steel (1.6 Ni 0.3 Mo) with the
structure of tempered sorbite was tested under
the loading realized according to a rigid scheme
with a specified strain amplitude of 0.37 in the
surface layer. Tests under corrosive medium
action (3 NaCl) were conducted under similar
conditions. Crystal lattice microdistortions were
estimated using the method of moments in X-ray
diffraction patterns of steel surface. Local
electrode potentials of the same specimen areas
were measured by means of capillary
silver-chloride microelectrode in a special cell.
Changes in crystal lattice microdistortions Dd/d
and electrode potential js of low-alloy steel due
to low-cycle fatigue under cyclic loading in the
air and in electrolyte. The potential has been
measured after loading in the electrolyte.
26
During testing in the air, early loading stages
resulted in the growth of crystal lattice
microdistortions. This was caused by the
appearance of a large amount of lattice defects.
With increasing number of loading cycles, the
amount of dislocations grew, and they started
interacting, annihilating and generating
vacancies. This led to microdistortions decrease.
However, later on, dislocation density grew with
loading cycles again at the expense of involving
new slip planes, and microdistortions increased
again. After achieving a certain critical
dislocation density, the multiple slip effect was
gradually exhausted, and microdistortions
decreased as a result of recurrent dislocation
interaction. After that, an intense growth of
microdistortions values was observed.
Sub-microcracks developed into micro-cracks, the
latter becoming macro-cracks and causing failure.
Under high strain and stress levels, dislocation
density grew in a very intense way. The dynamical
recovery processes could not prevent a
sufficiently fast accumulation of the ultimate
lattice distortion leading to fatigue failure.
The change in the electrode potential of the
specimens agreed with the change in crystal
lattice microdistortions. This fact confirms the
described kinetics of microdamage accumulation
from the view-point of mechanoelectrochemical
activity.
27
Another situation was observed at specimen
loading in presence of corrosive-active medium.
At the early deformation stages, corrosive
action of the medium led to an intense growth of
lattice defects at the expense of chemomechanical
effect. (Under such conditions, cooling action of
the medium on the fatigue was not essential,
which was confirmed by intense microdamage
accumulation.) This led to chemical potential
growth and mechanochemical effect development
confirmed by the electrode potential
disennobling. After that, micro-stress relaxation
took place accompanied by intense dislocation
discharge on the surface dissolving at a high
rate. Dislocation flux caused by chemomechanical
effect was enhanced by the mechanochemical effect
according to autocatalytic mechanism. Stress
relaxation was accompanied by the electrode
potential change towards more positive values
(after 2,000 loading cycles). Number of cycles
increase up to 8,000 resulted in insignificant
microdistortions growth (much lower than in
absence of a medium) with a respective
disennobling of the electrode potential.
Pre-failure stage, just as in case of testing in
the air, was characterized by microdistortions
value drop and by the electrode potential growth.
However, in presence of corrosion active medium,
this stage was longer, and microdistortions value
was much lower. Such a difference was a result
of chemomechanical effect development
facilitating the lattice stressed state at the
expense of the relaxation of dissolving metal
microdistortions. This led to an unusual result
metal durability at testing in corrosive medium
increased 1.5-fold in comparison with the
durability in the air. Thus, it has been
established by the present experiments that at a
corrosion fatigue at high strain and stress
levels, chemomechanical effect causes an
inversion of the corrosive action of the medium.
Instead of decreasing, such a medium increases
metal durability. A decrease in strain and stress
levels weakens the development of the mentioned
effects and leads to conventional mechanisms of
corrosion fatigue with a respective decrease in
metal durability in a corrosive medium.
28
Autocatalytic mechanism of mechanochemical
failure in the crack tip
Thus, in case of corrosion under stress,
specific conditions for active development of
chemomechanical effect arise in the crack tip.
Further propagation is determined by the
properties of one crystal (transcrystalline
failure) or two adjacent crystals
(intercrystalline failure). Then the
chemomechanical effect, contributing to an
increase in chemical potential of surface atoms
(dislocations exit), promotes mechanochemical
effect. The latter, in its turn, promotes exit of
dislocations. On the basis of such a synergistic
interaction, we proposed the autocatalytic
mechanism of mechanochemical failure in the crack
tip during stress corrosion cracking and
corrosion fatigue. In fact, in a number of papers
a considerable increase in the growth rate of
corrosion-mechanical crack with time was
observed. According to a well-known hypothesis
of a periodic electrochemical-mechanical growth
mechanism, the corrosion process in the crack tip
decreases the metal strength and facilitates the
subsequent mechanical failure. However, it does
not explain the process of metal weakening.
Moreover, if, in fact, selective corrosion of a
smooth surface leads to the formation of stress
concentrator, then the anodic dissolution of the
concentrator at the crack tip decreases stress
concentration. The idea of a possible
autocatalytic process at the crack tip allows to
eliminate the contradiction between the
hypotheses of periodic electrochemical-mechanical
and continuous electrochemical mechanisms of
crack growth.
29
Thus, chemomechanical autocatalytic mechanism
can be presented to describe a nucleation and
development of corrosion cracks through following
steps The steps are 1. Mechanochemical
effect localized corrosion takes place on slip
planes due to mechanically enhanced anodic
dissolution of metal. It causes a crack
initiation. 2. Chemomechanical effect
additional dislocation flux and localized
enhanced plasticity occur due to anodic
dissolution (e. g., corrosion creep). This will
induce the formation of pile-ups by obstacles and
the local stresses will increase promoting
mechanochemical effect. The latter, in its turn,
promotes additional plasticity due to
chemomechanical effect (synergistic
mechanochemical interactions appear). 3. The
local critical stress intensity factor K1C will
be reached near enough strong obstacle and a
cracks embryo will form by a Stroh-like
mechanism at the obstacle. 4. The normal stress
to micro-facets can be sufficient to open the
crack (cleavage) along the slip plane. 5. Due to
such a synergistic interaction the autocatalytic
mechanism of the failure during stress corrosion
cracking and corrosion fatigue is developing.
30
Conclusions
1. Mechanochemical interactions play the major
role in stress corrosion irrespective of type and
conditions of stress corrosion because
mechanochemical phenomena define the enhanced
chemical reactivity and enhanced plasticity of
metal in tip of a corrosion crack. 2. These
effects can be revealed by direct observation in
a various measure depending on conditions, but
their role in nucleation and propagation of a
corrosion crack does not depend on these
conditions, because these phenomena are
inevitable consequences of non-equilibrium
thermodynamics and physics of solids. 3. The
further researches of mechanochemical behavior of
metals open new ways of a prediction and
prevention of stress corrosion.