MSE 201 S21 Lecture 13 - Module 2 - Imperfections & Point Defects

00:09:28
https://www.youtube.com/watch?v=_RnG1jn5tnM

Sintesi

TLDRCe module aborde le concept d'imperfections dans les solides, cruciales dans l'étude des matériaux. Après avoir étudié les cristaux parfaits en cristallographie, l'accent est mis sur les cristaux réels qui présentent toujours des imperfections. Les imperfections influencent diverses propriétés matérielles telles que la déformation plastique, essentielle pour les propriétés mécaniques, la catalyse et les échecs structurels. Les défauts sont classés selon leur dimension : zéro dimensionnel (vacances, atomes interstitiels), un dimensionnel (dislocations), deux dimensionnel (limites de grain, surfaces libres) et trois dimensionnel (vides, espaces poreux). Chaque type de défaut a un impact sur la structure et les propriétés des matériaux, avec des exemples détaillés comme les vacances qui créent des distorsions dans le réseau ou les interstitiels qui causent une distorsion compressive.

Punti di forza

  • 💎 Les imperfections sont présentes dans tous les cristaux réels.
  • 🔍 Importance des imperfections dans les propriétés matérielles.
  • 🧩 Classification des défauts selon leur dimension.
  • 🔧 Impact des dislocations sur la déformation plastique.
  • 📏 Défauts zéro dimensionnels : vacances et interstitiels.
  • 🔗 Défauts unidimensionnels : dislocations.
  • 🌐 Défauts bidimensionnels : frontières de grain.
  • 🚪 Défauts tridimensionnels : vides.
  • 🚀 Rôle des imperfections dans la catalyse.
  • 💥 Importance pour comprendre les échecs des matériaux.

Linea temporale

  • 00:00:00 - 00:09:28

    Dans ce module, nous allons entamer un nouveau chapitre sur les imperfections dans les solides. Les conférences précédentes portaient sur la cristallographie et les structures, principalement à partir de cristaux parfaits. Cependant, les cristaux réels sont toujours imparfaits et contiennent des imperfections. Ces imperfections influencent de nombreuses propriétés telles que le mouvement des atomes et la déformation plastique, qui est une modification permanente de la forme d'un matériau sous une contrainte. Les imperfections jouent également un rôle crucial dans des phénomènes comme la catalyse et les défaillances mécaniques, essentielles pour le traitement et la fabrication des matériaux. Pour comprendre ces aspects, il est essentiel de classer et de définir les imperfections par dimensionnalité : zéro dimensionnelle (ex : vacance, atome interstitiel), unidimensionnelle (défauts linéaires ou dislocations), bidimensionnelle (structures de surface) et tridimensionnelle (défauts volumineux comme des vides dans le matériau). Le chapitre commence par une exploration des défauts ponctuels.

Mappa mentale

Video Domande e Risposte

  • Pourquoi les imperfections dans les cristaux sont-elles importantes à étudier ?

    Les imperfections influencent les propriétés mécaniques, chimiques et les processus d'échec des matériaux.

  • Quels types d'imperfections sont mentionnés dans la vidéo ?

    La vidéo mentionne les défauts ponctuels, linéaires, surfaciques et volumétriques.

  • Qu'est-ce qu'un défaut ponctuel ?

    Un défaut ponctuel est une imperfection localisée à un point dans le réseau cristallin, comme une vacance ou un atome interstitiel.

  • Comment les imperfections affectent-elles la déformation plastique ?

    Les dislocations, qui sont des imperfections linéaires, jouent un rôle clé dans la déformation plastique des matériaux.

  • Qu'est-ce qu'un atome interstitiel ?

    Un atome interstitiel est un atome ajouté dans un site normalement inoccupé du réseau, causant une distorsion compressive.

  • Qu'est-ce qu'une vacance dans un cristal ?

    Une vacance est l'absence d'un atome à un site où il serait normalement présent, provoquant une distorsion du réseau.

  • Quels sont les exemples de défauts volumétriques ?

    Les vides et les espaces poreux dans le matériau sont des exemples de défauts volumétriques.

  • Comment les dislocations sont-elles liées à la déformation plastique ?

    Les dislocations sont des types de défauts linéaires qui permettent le glissement nécessaire à la déformation plastique.

  • Qu'est-ce qu'un cristal parfait selon la vidéo ?

    Un cristal parfait est un réseau cristallin sans défauts, avec tous les atomes aux sites corrects et une extension infinie.

  • Comment les imperfections peuvent-elles indiquer des différences dimensionnelles ?

    Les imperfections sont classées par dimension : zéro pour les point defectuels, une pour les lignes, deux pour les surfaces, et trois pour les volumes.

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Sottotitoli
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Scorrimento automatico:
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    all right in this module we're going to
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    start on a new chapter
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    uh on imperfections and solids so we've
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    just finished a
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    quite extensive uh set of lectures set
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    of modules
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    on crystallography and structures
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    and most of that had to do with uh what
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    we
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    would call perfect crystals and so now
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    we're going to look at what happens
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    when those crystals are no longer
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    perfect because real crystals
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    are uh always imperfect and have
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    imperfections
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    all right so let's look at these types
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    of imperfections and first of all let's
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    look at why we study
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    imperfections so you might think they're
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    not
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    important and so forth but they actually
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    are
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    responsible for a lot of different um
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    types of properties that we see um the
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    next chapter that we're going to
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    discuss discuss is going to be on atomic
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    motion so looking at atoms moving around
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    in a solid so these imperfections are
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    very important for that
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    and then we're also going to look at a
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    phenomenon in
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    deformation called plastic deformation
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    so this will be
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    related to a mechanical properties of
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    materials
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    then a certain type of imperfections key
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    in this
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    aspect so you know
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    plastic deformation basically means
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    permanent so if we deform something into
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    a shape
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    that is a plastic deformation catalysis
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    that is uh useful for reducing
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    toxic gases and speeding up reactions
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    uh this occurs um on other types of
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    imperfections and requires them uh for
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    catalysis to work
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    um and also uh failure we're gonna talk
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    about failure
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    um in this um semester as well
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    and processing uh flaws and
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    imperfections
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    can result in those failures so we want
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    to know about imperfections
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    for these reasons but also others
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    all right so let's sort of review let's
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    go back and look at
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    a perfect crystal so again we just
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    talked about structures
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    um and so a perfect crystal we're going
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    to define as something
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    uh you know with a crystalline structure
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    that has all of its atoms
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    and the correct sites as we define
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    uh in our um crystal structures and then
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    there's nothing extra
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    as well so everything is located at the
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    site it's supposed to be
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    and we have nothing uh extra nothing
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    missing
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    and so that's important um to
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    uh to keep in mind and also the last
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    part of this for a perfect crystal
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    is that this uh this crystal would
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    extend out
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    indefinitely infinite infinitely
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    right and so obviously this cannot occur
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    so these uh
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    things are not possible for the most
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    part and so
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    in that case we're going to have
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    imperfect crystals
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    so knowing that we're going to have
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    these imperfect crystals
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    what we do is we define and classify
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    the different imperfections that we can
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    observe
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    based on their dimension and so what i
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    mean by that
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    is we can talk about three-dimensional
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    uh two-dimensional one-dimensional
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    zero-dimensional so zero-dimensional um
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    when we think of zero
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    uh dimensional we're talking about a
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    specific point
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    so there's an imperfection at a specific
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    point in the lattice
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    and so these types of defects are
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    vacancies interstitial atoms
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    substitutional and so we're going to
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    talk about those
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    types here's an example real quick but
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    we we're going to go more into detail
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    on those next we have one dimensional so
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    1d
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    you're thinking about a line so these
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    are line defects
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    and the line defects are just another
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    word for
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    dislocations and dislocations
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    are key for plastic deformation that we
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    talked about
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    and so we're not going to talk as much
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    about that in this chapter
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    but we will in later chapters when we
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    talk about plastic
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    deformation two-dimensional
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    this is where we start getting into an
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    area or a plane
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    right and so these are the grain
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    boundaries that we mentioned
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    uh briefly when we talked about
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    polycrystalline materials
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    and also free surfaces right since a
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    since a a crystal has to be infinitely
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    extending out any type of surface
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    therefore
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    is considered an imperfection and so a
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    surface has dangling bonds and so forth
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    and so that would be considered an
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    imperfection and since it's an area
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    we call it a two-dimensional and then
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    the last three-dimensional
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    these are volume defects and so
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    these get to the more processing defects
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    so we have things like
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    voids in materials where they're these
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    black
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    uh what look like particles
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    they can either be other phases they can
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    be
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    just pores empty space but they're
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    a volume of material that is uh
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    imperfect
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    to that crystal so all of these again
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    are just defined by the dimensionality
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    of the different types of imperfections
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    and so what we're going to do now is
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    move on and just talk about
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    point defects to start with and then
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    we'll move on and talk about the others
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    uh briefly uh but point defects like you
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    see here
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    cover vacancies interstitial atoms and
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    substitutional
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    so let's talk about what a vacancy is so
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    this one's from its name is fairly
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    evident what this is right we have a
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    structure
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    where everything is where it should be
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    but a vacancy
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    is something missing that would normally
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    be occupied
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    in that lattice site so this is a
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    two-dimensional model
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    of a simple structure and we see that
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    we have atoms uh next to each other and
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    on top of each other
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    but here in this spot right here there's
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    nothing
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    there's no atom and you actually see
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    that the atoms around it are kind of
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    uh deformed into that spot and so
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    there's actually distortion
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    because there's a vacancy or an empty
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    site in this lattice
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    so this is a vacancy so this can occur
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    at
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    atomic sites in metals and so forth but
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    it can also
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    happen in ionic materials as well where
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    we're talking about ceramics and so
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    forth
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    so we don't have to just limit it to
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    atoms it could be ions
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    as well and so this causes a distortion
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    and this type of distortion is actually
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    a tensile distortion
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    based on the um the lack of that atom
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    here that causes the tensile stress uh
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    in this vicinity so that has an effect
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    on
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    the area around the vacancy
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    all right so the opposite of that then
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    would be having an
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    extra atom or ion in a normally
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    unoccupied site so we talked about those
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    interstitial sites
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    in a structure right if you look over
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    here there's an empty site
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    between these four atoms that's an
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    interstitial
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    so if we place an extra atom so if this
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    is
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    a material like copper or iron or some
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    metal
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    that is just composed of one element if
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    we have an extra atom
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    we call that a self interstitial so if
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    it's just the same material placed in an
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    interstitial
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    that's a self interstitial there's also
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    other types of interstitials that you
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    can add in right so if this is iron and
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    then we place into this
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    interstitial instead of iron if we place
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    in another atom
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    say carbon that's actually what makes
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    steel
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    and that makes an alloy so it doesn't
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    have to just be a self-interstitial
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    but that's one case that we can have so
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    and again and again
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    we talk about atoms but it could also be
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    ions
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    as well in these structures
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    and if you think about the structure
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    here um
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    this is for example iron
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    well iron is in the structure and then
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    the empty spots
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    well if it was if that empty spot was
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    big enough for an iron structure
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    or an iron atom the structure would
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    probably be different right so all of
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    these interstitial sites
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    are actually quite a bit smaller than
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    the atoms
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    so if we place an extra atom into that
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    structure
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    in a spot nor not normally occupied
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    that's again going to cause
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    distortion and uh so you can kind of see
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    that
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    all the atoms around it are pushed out
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    and so that pushing out
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    is an exam example of a compressive
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    distortion
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    on the lattice the opposite of the
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    vacancy so you can always kind of
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    remember it that way
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    this is compressing everything around it
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    because we're putting extra stuff
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    into this lattice so
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    so far we've talked about vacancies at
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    specific points
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    and interstitials at certain points and
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    therefore we have point
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    different point defects so in the next
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    section
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    we're going to talk about why this
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    occurs and
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    what it means for the overall number of
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    these
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    um vacancies or interstitials that we
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    see
Tag
  • imperfections
  • cristaux
  • déformation plastique
  • défauts ponctuels
  • dislocations
  • vacances
  • atomes interstitiels
  • défauts linéaires
  • matériaux
  • propriétés mécaniques