Materials Science

80 flashcards by MentisLibrary

Why steel hardens when you quench it, why aluminum never truly has a fatigue limit, and what a phase diagram is really telling you. Crystal structures, alloys, polymers, and the ways materials fail.

What holds a metallic bond together?
A lattice of positive ion cores sharing a delocalized 'sea' of valence electrons
How does an ionic bond form?
By transfer of electrons from a metal to a nonmetal, producing oppositely charged ions held by electrostatic attraction
What distinguishes a covalent bond from an ionic bond?
Covalent bonding shares electron pairs between atoms of similar electronegativity rather than transferring them
What are van der Waals bonds and why do they matter?
Weak secondary bonds from permanent or induced dipole attraction between molecules
What is a unit cell?
The smallest repeating volume of a crystal that, translated in three dimensions, reproduces the entire lattice
How many atoms belong to an FCC unit cell, and what is its packing factor?
4 atoms per cell, with an atomic packing factor of 0.74
How many atoms belong to a BCC unit cell, and what is its packing factor?
2 atoms per cell, with an atomic packing factor of 0.68
Describe the HCP structure.
A hexagonal cell with an ABAB stacking sequence, 6 atoms per cell, coordination number 12, and a packing factor of 0.74
What is the atomic packing factor and how is it computed?
The fraction of unit cell volume occupied by atoms, treated as hard spheres
What is polymorphism (allotropy)?
The ability of a solid to exist in more than one crystal structure depending on temperature and pressure
How are Miller indices for a crystallographic plane determined?
Take the plane's intercepts on the three axes in lattice parameters, take reciprocals, clear fractions to smallest integers, and enclose in parentheses (hkl)
How does a direction index differ from a plane index?
A direction [uvw] is the vector's projections on the axes reduced to smallest integers, with no reciprocals taken
What is a vacancy and how does its concentration vary with temperature?
A vacancy is a missing atom at a lattice site, and its equilibrium number rises exponentially with temperature
What is the difference between a substitutional and an interstitial solid solution?
Substitutional solutes replace host atoms on lattice sites; interstitial solutes squeeze into the voids between them
What do the Hume-Rothery rules predict?
The conditions for extensive substitutional solid solubility between two metals
What is a dislocation?
A linear (one-dimensional) crystal defect around which atomic planes are misaligned
How do edge and screw dislocations differ?
For an edge dislocation the Burgers vector is perpendicular to the dislocation line, for a screw dislocation it is parallel
What are the two-dimensional defects in a crystal?
Interfacial defects: external surfaces, grain boundaries, twin boundaries, stacking faults, and phase boundaries
What is a slip system?
The combination of a slip plane and a slip direction lying in it, along which dislocation motion occurs
State Schmid's law.
The resolved shear stress on a slip system is $\tau_R = \sigma \cos\phi \cos\lambda$
State Fick's first law.
Diffusion flux is proportional to the negative concentration gradient: $J = -D\frac{dC}{dx}$
State Fick's second law and what it describes.
$\frac{\partial C}{\partial t} = D\frac{\partial^2 C}{\partial x^2}$, which describes non-steady-state diffusion where composition changes with time
How does the diffusion coefficient depend on temperature?
It follows an Arrhenius relation: $D = D_0 \exp\left(-\frac{Q_d}{RT}\right)$
What do the liquidus, solidus, and solvus lines mark on a phase diagram?
Liquidus: the temperature above which everything is liquid. Solidus: the temperature below which everything is solid. Solvus: the solid solubility limit between a single-phase and two-phase solid region
How do you use the lever rule?
In a two-phase field, the fraction of a phase equals the length of the tie line on the opposite side of the overall composition, divided by the total tie line length
What happens at a eutectic point?
On cooling, a single liquid transforms directly into two solid phases at one fixed temperature and composition: $L \rightarrow \alpha + \beta$
How does a eutectoid reaction differ from a eutectic one?
A eutectoid starts from a solid: one solid phase transforms into two other solid phases on cooling
What are the four key temperatures and compositions of the Fe-Fe3C diagram?
Eutectoid at 0.76 wt percent C and 727 C; eutectic at 4.30 wt percent C and 1147 C; peritectic at about 0.17 wt percent C and 1493 C; cementite fixed at 6.70 wt percent C
What is ferrite?
Alpha iron, the BCC solid solution of carbon in iron that is stable below 912 C
What is austenite and why does it matter for heat treatment?
Gamma iron, the FCC solid solution of carbon in iron, stable between 912 C and 1394 C in the pure metal