Independent Slip Systems in Crystals

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    Independent slip systems in crystalsG. W. Groves

    a& A. Kelly

    a

    aDepartment of Metallurgy, University of Cambridge

    Available online: 20 Aug 2006

    To cite this article:G. W. Groves & A. Kelly (1963): Independent slip systems in crystals,Philosophical Magazine, 8:89, 877-887

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    [ 877

    1

    Independent Slip Systems in Crystals

    By

    G .

    W.

    GROVES nd

    A .

    KELLY

    Department of Metallurgy, University of Cambridge

    [Received

    4

    April 19631

    ABSTRACT

    The slip systems observed in a number of crystal structures common

    amongst meta ls and simple ceramic mater ials are examined t o me whether they

    allow the crystal to undergo an arbitrary strain without change of volume.

    For

    most materials, other than f.c.0. and b.c.c. metals, there are insufficient

    independent slip systems. The condition under which cross slip can give

    rise to extra independent systems is stated. The results explain in a natural

    way recent experimental Gndings on the ductility of polycrystals with the

    sodium chloride structure.

    1.

    INTRODUCTION

    von Mises 1928) first pointed out tha t for a crystal to be able to undergo

    a general homogeneous strain by slip, five independent slip systems are

    necessary. This result is often quoted in the literature and it has been

    used in studies of face-centred cubic metals in connection with the number

    of slip systems found to operate near grain boundaries (Livingston and

    Chalmers 1957, Kocks 1958, Hauser and Chalmers 1961) and in theoretical

    attempts to deduce the polycrystalline stress-strain curve from that

    of

    a

    single crystal (Taylor

    1938, 1956,

    Bishop and Hill

    1951

    a,

    b,

    Bishop

    1953).

    von Mises also gave a simple method of determining whether or not slip

    systems are independent. Nowhere in the literature

    is

    there a n account of

    the application of this t o crystals with structures other than those of the

    f.c.c. metals.

    In view of the growing volume of work on plasticity of materials with

    crystal structures and slip systems quite different from those of f.c.c.

    metals i t seems worth while to describe von Mises method and the results

    of testing a number of simple crystal structures to see hoy many indepen-

    dent slip systems they usually possess. Further, the number of slip

    systems observed in a given crystal structure can alter with temperature.

    This can lead to a sudden change in the mechanical properties which

    finds a ready explanation in terms of von Mises result. I n the course

    of

    this work we have also found that the results for f.c.c. metals have a simple

    geometrical interpretation.

    2. VON

    MISES

    RESULT

    Slip or glide leads on a macroscopic scale (i.e. if one considers

    a

    volume

    containing many slip bands) to the translation of one part of a crystal

    relative to another by a motion corresponding to a simple shear. A

    single simple shear determines the value

    of

    one

    of

    the independent com-

    ponents of the strain tensor. Since plastic flow usudly occurs without a

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    G. W. Groves and A. Kelly on

    change in volume, the six independent components of the general strain

    tensor are reduced to five because of the condition ;

    Ex

    +

    c y + c 2 =

    0.

    von Mises

    (1928)

    noted th at since the operation of one slip system produces

    only one independent component of the strain tensor, then five independent

    slip systems are needed to produce a general, small, homogeneous strain

    without change in volume.

    To determine whether a given crystal possesses sufficient slip systems we

    proceed as follows (von Mises 1928, Bishop 1953) . Write down the com-

    ponents of the strain tensor produced by an arbitrary amount of glide on a

    Do

    the same thing for four other slip systems, referring the strain tensor to the

    same set of axes as before. Finally, since the three tensile strains are not

    independent of one another, form the five by five determinant of the

    If

    this determinant has a value other than zero, then the five chosen slip

    systems are independent of one another, since the determinant will equal

    zero if any row can be expressed as a linear combination of other rows.

    Provided then that the value of the determinant is other th an zero, one has

    chosen five slip systems which are independent of one another in t he sense

    that the operation of one of them produces components of the strain tensor

    which cannot be expressed as linear combinations of the components

    produced by the operation of the other slip systems.

    Physically, the above amounts to saying that a slip system is independent

    of others provided its operation produces a change in shape of a crystal

    which cannot be produced by a suitable combination of amounts of slip

    on those other systems.

    Since five independent slip systems suffice to produce an arbitrary strain

    without change in volume, it is apparent that a crystal cannot possess more

    than five independent systems.

    given slip system. Call these components c Z , ey e Z ,

    eXy

    ,

    eX2

    ,

    quantities ex' ey' Z1 , cX y1 , eZBI c y z l ; e x a

    -

    yU

    E ~

    yl ' *

    3.

    EXPRESSIONOR THE

    STRAIN

    OMPONENTS

    We need a simple method for writing down the components of the strain

    tensor produced by glide on

    a

    given slip system. The following method is

    convenient (Bishop

    1953,

    Livingston and Chalmers

    1957).

    We define a

    glide system by a unit vector n normal to th e glide plane, with components

    nx,ny,n, parallel to an orthogonal set of axes x, , z , and by a unit vector

    p in the glide direction with corresponding components p x , ,, pa The

    components of the strain tensor are then:

    ey=

    an,

    ;

    x

    =m X p x

    c, = olnzpz;

    a a

    a

    2

    Exy =

    2By

    + n,Bx) ;

    E x z

    =

    (nxB2

    + Px)

    and

    E V E =

    - (n,Bz

    +

    nsB, *

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    Using these relations we can write down immediately the components of

    the strain tensor produced by a simple shear of magnitude t a n a by the

    operation of the glide system defined by n and p. Since the relations are

    symmetrical in

    n

    and

    p

    it is immediately clear that the strain produced by

    slip on the plane n in the direction

    p

    is the same as tha t produced by slip

    on the plane p in the direction n.

    The directions

    of

    n and

    p

    are usually given in terms of Miller indices.

    These must be referred to orthogonal axes of equal measure when the above

    relations are used in non-cubic crystals. When the directions of n and p

    are given in terms of Miller indices

    (or

    Miller-Bravais indices) we call a

    family of slip systems all those combinations of slip plane and slip direction

    which must arise from the point group symmetry of the crystal if one slip

    plane and one slip direction are given.

    4.

    RESULTS

    4.1. F.C.C. Metals

    The usual slip systems are the family

    {

    1 1 1}( 110 .

    There are

    12

    physically different slip systems. We evaluate

    the components of the strain tensor referred to axes parallel to the conven-

    tional cell edges. For the slip system

    ( l l l ) [ l IO]

    we have for the com-

    ponents of

    n

    and

    p :

    Consider a f.c.c. metal crystal.

    Using the relations of $ 3

    61

    E y Z =

    2/6

    s y = o E x , = --

    22/6

    Proceeding similarly one can write down the components of the strain

    tensor produced by operation

    of

    any

    of

    the physically different slip systems

    in the { 11 1}( 110) family.

    Only five of these are independent. The

    selection of five independent ones can be made in a number of ways. This

    selection can be illustrated in terms of Thompson s tetrahedron, which is a

    regular tetrahedron with vertices A, B, C, D and

    a, 8

    y 6 the midpoints

    of the opposite faces respectively. A slip direction corresponds to an edge

    of the tetrahedron and a slip plane normal can be designatsd by one of the

    letters 01, /3,

    y

    6. Thus (AB), corresponds to slip in the

    [Oll]

    direction on

    the ( i i l )plane. A possible set of five independent slip systems is (AC),,

    (AB),, (AD),, (AC),, (AB ),. I n this crystal structure each slip direction lies

    in two slip planes

    ;

    we might designate these the primary and the cross-

    slip plane.

    P . M . 3

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    880

    G.

    W. Groves

    and

    A. Kelly

    on

    Any slip system can be expressed in terms of two others with either the

    same primary slip plane or with the same cross-slip plane. Thus, for

    example, we can write :

    (DB),+(BC),+(CD),=O

    .

    (4.1a)

    and

    (DB),+(BC),+(CD),=O. . . . (4.1b)

    These equations mean that equal shears on the three slip systems produce

    zero net distortion of the crystal. It can be visualized that the operation

    of the first set of shears (4-1

    a)

    produces no net displacement at any point,

    while the operation of the second set

    (4-1 )

    merely rotates the crystal about

    the direction ALY,he normal t o the plane containing the slip vectors.

    There is a third group of dependent sets, similar to those of (4.1

    b ) ,

    for

    example

    Again the slip vectors are co-planar and sum to zero, and their slip planes

    are equally inclined to the plane containing the slip vectors. The appli-

    cation of this set merely rotates the crystal about the normal t o this plane,

    i.e. about a (100).

    In choosing five independent slip systems the above sets, and com-

    binations of them, must be avoided. There are 384 different ways of doing

    this (BishopandHill 1951b).

    (CD),

    +

    (DC),

    +

    (AB),

    +

    (BA),

    =

    0.

    . . .

    (4 . lc )

    4.2. B.C.C. Metals

    Since the components of the strain tensor produced by slip on a plane

    of normal

    n

    n a directionp are the same as those produced by slip on a plane

    of normal p in a direction n, von Mises analysis for a f.c.c. crystal applies

    equally well to a b.c.c. metal crystal which possesses the family of slip

    systems {110) ( 1 11 ) and a general strain can be produced by slip. These

    slip systems can also be represented in terms of a modified tetrahedron

    shown in the figure. The letters

    LY3

    y , 6 now represent slip directions and

    the pairs of letters, e.g. AB, now represent slip planes, e.g. the plane AOB

    in the figure.

    I n this system

    then

    a

    corresponds

    to

    slip in the

    [ T T l ]

    direction on the

    (011)

    plane.

    A possible set of five independent slip systems is then a BAD PAC,

    yAB,

    nd again there are 384 different ways of choosing five of these from

    the

    12

    physically distinct slip systems. Again combinations involving any

    of the relations analogous to 4.1) must be avoided.

    Each slip direction lies in three slip planes.

    4.3. NaCl Structure

    Crystals with the sodium chloride structure possess the family of slip

    There are six physically distinct slip systems.

    Consider first ( loi)[ ioi] .

    systems {110}(

    1 T O ) .

    However, there are only two independent ones.

    Following the procedure in

    3,

    and taking axes parallel to (001 ),

    nz=

    1 / 4 2 ,

    n y =O

    ns= 1 / 4 2 , Pz= 1 / 4 2 , = O , jgB= 1 / 4 2 .

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    All other components of the strain tensor

    Since

    01

    is an arbitrary constant defining th e amount of slip we

    Whence

    e z=

    -

    12 e z = 0 ~ 1 2 .

    are zero.

    C

    Figure illustrating slip planes and directions

    for

    { l l O ) t l T l >

    slip in a cubic

    crystal.

    Slip

    planes are planes such as AOC, COD, etc. Slip directions are directions

    Aa, Bp,

    etc., the normals to the faces

    of

    the tetrahedron. If

    (110)

    n o )

    slip is considered, the slip planes are the same but the slip directions are

    the edges of the tetrahedron,

    AB,

    BC, etc.

    can put i t equal to 2. Proceeding similarly for the systems

    ( o l l ) [ o i i ]

    and

    ( ~ l o ) [ i l o ] e find for the respective components of the strain tensor :

    0 is the centroid of the reguIar tetrahedron ABCD.

    It

    is clear th at only two of these are independent and thus the change of

    shape produced by slip on any one of these systems can be produced by an

    appropriate amount of slip on the other two. Since the three remaining

    physically distinct slip systems merely correepond to the interchange of

    slip plane and direction with those we have just considered, there are in all

    only two independent systems. A general deformation is thus not possible

    by slip. I n particular the off-diagonal terms of the strain tensor are always

    zero,

    so

    a deformation tending to change the angle between the crystal axes

    cannot occur, e.g. a crystal slipping only on these systems cannot be twisted

    about a (001 ) axis, nor extended along (1 11 ).

    3 N Z

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    G . W . Groves

    and

    A. Kelly

    on

    The equivalence of slip on the various systems which reduces the number

    of independent slip systems can again be seen from the figure, where now

    the slip directions are the edges of the tetrahedron and the slip planes are

    planes such as AOB, COD. There is only one slip direction in each slip

    plane. Thus a symbol such as (AB),, represents slip in the direction AB

    on the plane AOB, of which the normal is CD.

    It

    is immediately verified by

    substituting in the relations of 3 that

    and also that

    (4.3)

    is the analogue in this structure of

    (4 . lb )

    for f.c.c. metals. The

    remaining relation, which with (4.2)and (4.3) ensures that no more than

    two independent systems can be chosen, is :

    (4.4) is obtained by interchanging slip plane normal and slip direction in

    (4.3).

    There are

    12

    different pairs of independent slip systems in the NaCl

    (AB),,=(CD),,, . . (4.2)

    (DB),,+(BC),D+(CD),,=O. , . . . . (4.3)

    (AC),,+(AD),,+(,4B),,=O.

    . .

    (4.4)

    structure.

    4.4. CsCl Structure

    The family of slip systems is

    {100}(010)

    (Rachinger and Cottrell

    1956).

    A general deformation is impossi-

    I n particular extensions parallel to the crystal axes cannot be pro-

    There are eight different ways of choosing three independent slip

    These yield three independent systems.

    ble.

    duced.

    systems.

    4.5.

    C a p Structure

    Two families of slip systems have been observed, viz. (001}( 110) and at

    higher temperatures

    { 1

    lo}( 110) (Roy 1962). The first family yields three

    independent slip systems and cannot produce a general deformation. I n

    particular extensions parallel to the crystal axis are not possible. There

    are

    16

    different ways of choosing three independent systems. The second

    family

    is

    that found in NaCl and permits only extensions parallel to the

    crystal axes. It yields two independent systems which are independent of

    those in the first family. A general strain can be produced if, and only

    if, both families of slip systems operate simultaneously and independently.

    Five independent systems can be chosen in 192 different ways.

    4.6. Rutile T iO ,) Structure

    Two families of slip systems are reported

    by Ashbee (1962). These are

    { l O l } l O T )

    and (110}(001). The first

    yields four independent slip systems. This is different from the sodium

    chloride structure in which the family of slip systems has the same indices.

    The difference arises because rutile is tetragonal and hence slip on (101)

    in the direction

    [

    1011 does not produce the same shear as slip on (101)in the

    The structure is tetragonal.

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    Independent Sli p Systems i n Crystals 883

    [ l o l l

    direction. The slip systems in the family

    {110}(001)

    produce

    nothing new. All the components of the strain tensor produced by shear

    on the two planes of this family can be produced by a linear combination

    of members of the

    { l O l } l O T )

    family. There are in all four independent

    slip systems. The component of the strain tensor eZy (with the z axis

    parallel to the tetrad axis) is always zero. A general deformation is not

    possible.

    4.7.

    Hexagonal Materials

    A common family of slip systems is

    {0001}( 1120).

    There are only two

    independent slip systems and three different ways of choosing these.

    A

    crystal cannot be extended parallel to the conventional crystallographic

    axes, nor can the angle between the axes lying normal to the hexagonal axis

    be altered. This family

    of

    slip systems is the only one possessed by graphite

    (Freise 1962). Other hexagonal materials often show slip on other families

    of slip systems.

    Zirconium (Rapperport and Hartley 1960), and tellurium (Stokes et

    al.

    1961)exhibit slip on the {lOTO}( 1120) family. There are only two indepen-

    dent slip systems and again these can be chosen in three different ways.

    These slip systems allow extensions parallel to the two crystallographic

    axes lying in the basal plane and alteration of the angle between these. A

    crystal which possesses the two families of slip systems, {0001}(

    1120)

    and

    { l O Y O }

    1120), possesses therefore four independent slip systems, subject

    to the proviso in 5 . An example is magnesium a t

    a

    temperature greater

    than 18O c (Flynn

    et al.

    1961) and aluminium oxide a t high temperature

    (Scheuplein and Gibbs 1962).

    In addition many hexagonal metals show pyramidal slip on the family

    {lOil}( l l20), e.g. zinc, cadmium (Schmid and Boas 1935) and titanium

    (Churchman 1954). There are six physically different slip systems of this

    family. They produce four independent systems, which can be chosen in

    nine different ways. The changes of shape which can be produced by

    {lOil} 1120)

    slip are precisely the same as those which can be produced

    by the simultaneous and independent operation of both the

    {0001}( 1120)

    and (lOiO}(ll20) families. In no case is an extension parallel to the

    hexagonal axis possible.

    I n zinc (Bell and Cahn

    1957)

    and in cadmium (Price

    1961)

    the operation

    of the family of slip systems {1122}( 1121) has been reported. There are

    six physically different members of this family and they provide five

    independent slip systems, so that a general deformation is possible. The

    operation of one member of the

    {1122}(1121)

    family allows a crystal to

    extend parallel to the hexagonal axis.

    5.

    CROSS

    SLIPOR PENCIL

    LIDE

    At low temperatures slip usually occurs in crystals upon well-defined

    A s

    the temperature is raised slip planes often become

    lip planes.

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    G. W. Groves and A. Kelly on

    ill-defined and slip appears to be taking place upon any plane of which

    the slip direction is zone axis. We shall call such a situation pencil glide.

    Then it is easily seen by substitution in the relations given in

    3

    that glide

    in any given slip direction will produce two independent components of the

    strain tensor. There is, however, an important physical stipulation which

    must be obeyed if two truly independent components of the strain tensor

    are to be produced.

    Suppose

    pencil glide occurs in a crystal with the NaCl structure. Consider the slip

    direction

    [ O l i ] .

    At low temperatures this possesses the slip plane (011)

    and the components of the strain tensor produced are, for

    a

    strength of

    0 0 0

    eformation a

    This is best illustrated by an example.

    [:

    ;

    :I

    hen pencil glide occurs we can resolve the shears on t o

    (01

    1 ) and any other

    plane in the zone with axis [OlT]. Suppose we take the second plane to

    be

    (100). Then the components of the strain tensor due to the second shear

    will be :

    0

    a1 /2 -a /2

    all

    0

    I a l p

    0 0 1

    These two slip systems produce then two independent components of the

    strain tensor, if and only if there is no necessary connection between a and

    a .

    Whether

    or

    not there is a connection between

    a

    and

    a

    depends upon

    the physical process by which pencil glide is produced. This may be

    important for instance in deciding whether prismatic glide and basal glide

    are independent in, for instance, magnesium.

    There is a good deal of evidence from electron microscope and etch-pit

    observations of the occurrence of a microscopic form of cross slip involving

    essentially the motion of a screw dislocation out of its primary slip plane

    for a short distance upon another slip plane, of indeterminate indices but

    with zone axis along the screw, and subsequent cross slip of the screw

    back onto the primary slip plane. Gilman and Johnston

    (1960)

    first

    discovered this and called it multiple cross glide.

    As

    far as one can tell

    at

    present the amounts of slip on primary and cross-slip plane are not indepen-

    dent of one another and hence there is a connection between a and 01 .

    Such a process does not appear to provide two slip systems corresponding

    to each slip direction.

    When pencil glide occurs then a crystal need only possess, in general,

    three non-coplanar slip directions in order for a general strain to be

    possible.

    6.

    DISCUSSION

    It

    may still

    be applied to the large strains produced in plastic deformation by viewing

    The analysis in

    2

    is

    valid for infinitesimally small strains.

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    the components of the strain tensor as strain rates multiplied by an infini-

    tesimal length of time (von Mises

    1928).

    This device avoids comparing a n

    instantaneous configuration with an initial one and merely compares the

    instantaneous configuration with one neighbouring in time. This is

    usually done in theories of plasticity to avoid the mathematical difficulties

    associated with

    a

    treatment of large strains.

    The most obvious application of these results is to the deformation of

    polycrystalline specimens. It is usually assumed, following Taylor, that

    the deformation of a polycrystalline specimen can only proceed in general

    without the production of voids provided the grains can undergo a general

    strain. If this change in shape is to be produced by slip then five indepen-

    dent slip systems are necessary.

    A s

    a very general rule it appears to be

    found that when five slip systems are not available voids are formed during

    plastic deformation of a polycrystalline aggregate.

    For

    example, a t low

    temperature magnesium slips mainly on OOOl} in a 1120) direction with

    some lOiO}

    1120)

    slip. Voids are formed a t very small strains (Hauser

    et al 1955).

    The most convincing experimental evidence that five independent slip

    systems are necessary for polycrystalline ductility comes from experiments

    on crystals with the NaCl structure. At low temperatures where only

    1 lo}

    110)

    slip operates, polycrystals and even bicrystals are found to be

    very brittle (Stokes and Li

    1963).

    In

    a

    compressed bicrystal there are in

    general five conditions to be satisfied by the strains within the grains if they

    are to remain in contact whilst deforming homogeneously : the two tensile

    strains and the shear strain in the plane of the boundary must match in each

    grain, and the strains along the compression axis must equal the imposed

    strain (Livingston and Chalmers

    1957).

    Since only four independent slip

    systems are available, two from each grain, a general bicrystal cannot

    deform so as to maintain contact at the grain boundary. In practice,

    both bicrystals of MgO (Johnston

    e t al. 1962,

    Westwood

    1961)

    and poly-

    crystals (Stokes and Li

    1963,

    private communication) are observed to

    fracture from a crack which forms at a grain boundary where a slip band

    runs into it . The deformation of a general boundary produced by a slip

    band in one grain cannot be accommodated by {llO} liO) slip in the

    neighbouring grain, since the three strain components in the boundary

    plane, which must be matched, cannot, in general, be produced by the two

    independent slip systems which are available. Stokes and Li

    1963)

    lso

    show clearly that in silver chloride, sodium chloride and magnesium oxide,

    a t low temperatures, the rate of strain hardening after the yieldpoint (which

    corresponds to plastic flow occurring in some of the grains) is extremely

    rapid. This can be understood since the total plastic strain obtainable

    before fracture will only be of the same order as the elastic strain. Similar

    behaviour is found for LiF (Budworth and Pask

    1963).

    Stokes and Li

    1962)

    ave also shown that when large amounts of pencil glide are observed

    in silver chloride and sodium chloride then appreciable plastic extensions

    of a polycrystal are obtained.

    Allhe same is true of LiF (loc. cit.).

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    886

    G. W. Groves

    nd

    A. Kelly

    on

    these results receive

    a

    completely general explanation from the above

    analysis. A t temperatures below those a t which pencil glide can occur the

    operation of the six physically distinct slip systems of the

    { l l O } l T O )

    family will lead to no change in shape which cannot be produced by the

    operation of only two of these, e.g.

    l l O ) [ l T O ]

    and l O l ) [ l O i ] . The onset

    of pencil glide, having three non-coplanar slip vectors, produces five

    independent slip systems and a general strain is then possible. Poly-

    crystals then become ductile.

    It must be noted that the requirement of five independent slip systems

    to allow an arbitrary strain is

    so

    general that by itself it does not allow

    particular predictions to be made as to what will happen if sufficient systems

    are not available. Fracture may occur in a polycrystal as soon as slip occurs

    within a few grains and the slip bands intersect the boundary, as in

    MgO.

    Alternatively, pencil glide might be forced to occur in regions of very high

    stress, or slip could occur on planes not normally functioning as slip planes ;

    this last seems to occur in LiF at room temperature (Budworth and Pask

    1963 . Fracture does not

    necessarily

    occur if five independent slip systems

    are not available. Whether

    or

    not fracture occurs will depend on whether

    in a particular stress state cracks are opened at lower stresses than are

    required to produce slip on new slip systems.

    4 show that polycrystals of materials of

    CsCl type, graphite, and TiO, type will not deform without the opening of

    voids unless other slip systems operate.

    Polycrystals of CaF, (orUO, will

    not do

    so

    unless both the observed families of slip systems operate.

    The other results listed in

    ACKNOWLEDGMENTS

    We would like to thank Professor A. H. Cottrell, F.R.S., and

    Dr. F.

    J. P.

    Clarke, for stimulating this work, and for useful discussions. A special

    debt of gratitude is due to

    Dr.

    J. D. Eshelby for pointing out to us the

    relations used in 3.

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