METALURGI FISIK
Di susun oleh:
Nama : Fernando Desfriandi Saragih
NPM : G1C019046
DosenPengampu : A. Sofwan FA, S.T.,M.Tech.,Ph.D
Nurbaiti S.T., M.T
PRODI TEKNIK MESIN
FAKULTAS TEKNIK
UNIVERSITAS BENGKULU
2020
Synopsis 2.11
1.Metals are crystalline
materials. Atoms in a crystal structure occupy positions in an arrangement,
which is characterized by periodicity. The atomic arrangement is systematically
repeated in three-dimensional space.
2. Atomic arrangement
can be described with the aid of the unit cell. Its repetition in three
directions generates the crystal structure.
3. Atoms in metals are arranged in
close-packed structures. The most important crystal structures of metals are
base-centered cubic (BCC), face-centered cubic (FCC) and hexagonal close-packed
(HCP). Structure of metals 39
4. The crystal structure influences several
phenomena in physical metallurgy such as the potential of a metal to form
alloys, the plastic deformation of metals and diffusion, which is the transport
of atoms through the lattice.
5. Metals form two types of solid solutions:
interstitial and substitutional solid solutions. The size of interstitial sites
determines the maximum solid solubility in interstitial solid solutions. The
formation of substitutional solid solutions is controlled by the Hume-Rothery
rules. Most alloy systems do not fulfill these rules and, therefore, solid
solutions exhibit a limited solid solubility.
6. Intermetallic compounds form in most alloy
systems. These compounds are either stoichiometric or exhibit a homogeneity
composition range. The factors governing the formation, composition and crystal
structure of intermetallic compounds include electronic structure,
electronegativity, atomic radii of components and chemical bonding.
7. The microstructure
of metals consists of the different phases (solid solutions and intermetallic
compounds) and crystal imperfections including point defects (vacancies and
interstitials), line defects (dislocations), grain boundaries and interfaces.
The microstructure of a metal influences its properties. Microstructure can be
altered through processing with the activation of phase transformations.
8. Thermodynamics
defines the potential for a reaction or a transformation to take place while
kinetics defines the rate of the reaction or transformation.
9. A system is said to
be at a state of thermodynamic equilibrium when it is at mechanical, thermal
and chemical equilibrium simultaneously. The criterion for thermodynamic
equilibrium is the minimization of the Gibbs free energy. Any spontaneous
transformation decreases the free energy of the system.
10. The reduction in
free energy accompanying a transformation is the thermodynamic driving force of
the transformation. The energy barrier, that should be surmounted in order for
the transformation to take place, is the activation energy of the
transformation.
11. The activation
energy is defined by the Maxwell-Boltzmann energy distribution. The rate of a
transformation is then exponentially dependent on activation energy and
temperature via the Arrhenius law.
12. In a reaction or
transformation consisting of several steps, the rate of the overall reaction is
controlled by the slowest step, which exhibits the highest activation energy.
This step is called the rate-limiting step.
Synopsis 3.5
1.Structural
imperfections have a significant influence on physical and mechanical
properties of metals. They can be classified as (a) point defects, such as
vacancies and interstitials, (b) linear imperfections, such as edge and screw
dislocations, (c) surface imperfections, such as grain boundaries and
interfaces and (d) threedimensional defects, such as voids and inclusions.
2. At each temperature
there exists a certain concentration of point defects at thermodynamic
equilibrium in the crystal. The concentration of vacancies and interstitials
increases exponentially with temperature, following Arrhenius-type temperature
dependence. The concentration of interstitials is several orders of magnitude
Structural imperfections 85 lower than the concentration of vacancies at the
same temperature.
3. Vacancies play a key
role in the diffusion of atoms, especially the substitutional diffusion.
4. Plastic deformation
takes place by dislocation glide in specific slip planes (closepacked planes)
and slip directions (close-packed directions), which constitute the slip
systems of a metal.
5. A slip system
becomes operational by the application of a certain stress on the slip plane,
called the critical resolved shear stress (CRSS), which is directly related to
the mechanical strength of the metal.
6. The glide of an edge dislocation takes
place in a direction parallel to the applied shear stress while the glide of a
screw dislocation takes place in a direction perpendicular to the applied
stress. In both cases the glide causes plastic deformation of the crystal.
7. At every position
during glide, the dislocation line is the boundary between the part of the
crystal that has slipped and the part which has not.
8. The stress required
for dislocation glide is much lower than the ideal crystal strength. This
explains the difference between the actual and ideal strength of a metal.
9. The direction and magnitude of slip, caused
by a dislocation, is expressed by the Burgers vector~b of the dislocation line.
The Burgers vector is determined by the Burgers circuit.
10. The Burgers vector
of an edge dislocation is perpendicular to the dislocation line. When this is
valid for the entire dislocation line, then this is a pure edge dislocation.
The Burgers vector of a screw dislocation is parallel to the dislocation line.
When this is valid for the entire dislocation line, then it is a pure screw
dislocation.
11. When a dislocation
is not a pure edge neither a pure screw, it is a mixed dislocation. In this
case the dislocation line forms a random angle with the Burgers vector.
12. Dislocations cannot
terminate inside the crystal. They can terminate either at the crystal surface,
at grain boundaries or on themselves, forming dislocation loops. The expansion
of a dislocation loop under the action of a shear stress causes plastic
deformation of the crystal.
13. The elastic strain energy of a dislocation
is associated with strains caused by the 86 Physical Metallurgy: Principles and
Design displacement of atoms away from their equilibrium positions around the
core of the dislocation line. The elastic strain energy is proportional to the
square of the Burgers vector.
14. In several cases,
the dissociation of a perfect dislocation into partial dislocations is
energetically favored. The glide of these partials has the same effect as the
glide of the perfect dislocation.
15. The dissociation of
a perfect dislocation in FCC crystals into partial dislocations is accompanied
by the formation of a stacking fault, characterized by the stacking fault energy
(SFE). Despite the fact that the dissociation decreases the elastic strain
energy of the crystal, if the Frank’s rule is obeyed, the dissociation will
take place only if it decreases the total energy of the crystal.
16. In FCC metals with
low SFE, slip takes place by the glide of partial dislocations, while in FCC
metals with high SFE, slip takes place by the glide of perfect dislocations.
17. The change of slip
plane by a screw dislocation is called cross slip. Cross slip of screw
dislocations is important for the plastic deformation of metals, because it
allows the dislocations to continue their glide in the crystal and produce
plastic deformation.
18. The change of slip
plane by an edge dislocation is called climb and requires the diffusion of
vacancies or interstitials to the dislocation.
19. Jogs and kinks are steps on the
dislocation line. Jogs transfer a segment of the dislocation line to a
different slip plane while kinks are steps on the same slip plane with the rest
of the dislocation line. Kinks and jogs on an edge dislocation do not impede
the glide of the dislocation. However jogs on a screw dislocation, impede its
glide, since climb is required in order for the jogs to move with the
dislocation.
20. The plastic strain
rate resulting from dislocation glide is proportional to the mobile dislocation
density and the average dislocation velocity.
21. The stress field
around a screw dislocation is pure shear. The stress field around an edge
dislocation involves shear on the slip plane as well as tension and compression
below and above the slip plane respectively.
22. The energy of a
dislocation is proportional to b 2 . The long-range stress field allows the
dislocation to interact with point defects, solute atoms and other dislocations
at long distances, up to 100b, from the dislocation core.
23. Regarding forces
acting on dislocations: the glide force acts perpendicular to the Structural
imperfections 87 dislocation line. The line tension acts to reduce the
dislocation length. The bending force acts to bend the dislocation between
obstacles. The stress required in order for a dislocation to bypass obstacles
on the slip plane, in other words the resistance of the obstacles to
dislocation glide, is inversely proportional to the distance between obstacles.
24. The major effects
of dislocations are related to plastic deformation, strengthening and phase
transformations. In the last case, dislocations enhance both the nucleation and
growth of a new phase, by providing sites for the heterogeneous nucleation and
by serving as high-diffusivity paths for the diffusional growth of the new
phase.
25. The primary interfaces in metals are the
free surfaces, grain boundaries, interphase boundaries and stacking faults. All
interfaces are characterized by an interfacial energy γ.
26. The surface energy
of free surfaces is highly anisotropic. The equilibrium shape of a single
crystal is the one that minimizes the total surface energy.
27. A grain boundary
separates two regions of a crystal with the same crystal structure but with
different lattice orientation. Grain boundaries are distinguished in tilt and
twist boundaries as well as low-angle and high-angle boundaries.
28. Interphase boundaries separate two
different phases, which may have different crystal structure or chemical
composition. Depending on whether there is full, partial or no coincidence of
the two crystal lattices at the boundary, interphase boundaries are
distinguished in coherent, semicoherent and incoherent boundaries.
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