Lecture 5 Flashcards
In a simple cubic, where are the atoms touching? a = ?
along the cube’s edges
a = 2r
in a BCC, where are the atoms touching? a = ?
along the body diagonal
a = 4/sqrt(3) * r
in FCC, where are the atoms touching? a = ?
the face diagonals
a = 2sqrt(2) * r
in HCP, where are the atoms touching? a = ?
a = 2r
how do small and large atoms stack together?
small atoms fit in the interstices (voids) between larger atoms
What is the tetrahedral stie
interstitial atom fills tetra site (CN = 4)
what is an octahedral site
CN = 6
What are the options for interstitial structures for CCP/FCC
- tetrahedral site
- octahedral site
what are the options for interstitial structures for SC
cubic site
what are the options for interstitial structures for BCC
- tetrahedral site
- octahedral site
can there be interstitial sites in a SH?
no
can there be interstitial sites in a HCP?
- tetrahedral sites
- octahedral sites
how do you express the radii of atoms that can fit into interstices
radius ratio rx/rA, where X= interstitial atom and A = larger atom
what does an octahedral interstitial look like
how many octahedral interstitial sites are there in ccp
13
how many tetrahedral interstitial sites are there in ccp
8
What is the maximum radii rx, of the atom that can fit inside the tetrahedral site of ccp
0.225rA
what is the maximum radii rX of the atom that can fit inside octahedral site of ccp
0.4142rA
For what alloy are only half the ccp tetrahedral sites occupied?
TiH, only HALF are occupied by H
how do you find the rx/ra ratio of ccp octahedral
how do you find the rx/ra ratio of ccp tetrahedral
how do you find the rx/ra ratio of sc
what is the rx/ra ratio of sc
0.732
What is an important property for the particles in stable ceramics
size
how do you make a ceramic more stable
maximize the number of nearest oppositely charged neighbors
how is CN related to cations and anions? why important?
its the number of anions nearest neighbors for a cation
there’s a critical min ratio of cation-anion radius
what is the range for CN 4 for the cation to anion rato
0.255 - 0.414,
rx/ra for ccp tetra – rx/ra for ccp octa
are the cation or anion the big atom
anion
what is the rx/ra ratio range for CN 6
0.414 - 0.732
ccp octo - SC
what is the rx/ra ratio range for CN 8
0.732-1
as CN increases, does the r of cation increase of decrease
increase, so interstitial atoms get bigger
what is an example of an alloy with CN 4
ZnS
what is an example of an alloy with CN 6
NaCl
what is an example of an alloy with CN 8
CsCl
how do you find the rx/ra ratios in interstitial octo and tetra bcc
are there compounds that use interstitial bcc packing
no
how do bcc metals from interstitial compounds
they are arranged in an fcc pattern, a SH, or dif. more complicated patterns
what are three bcc metals that form interstitial compounds
vanadium, tungsten, or iron (low temp form)
what is it called when interstitial elements occupy site in the bcc structure to a limited extent
interstitial solid solutions
what is an example of an interstitial solid solution BCC vs. its FCC counterpart
Ferrite: rC/rFe , BCC lattice distorted (so hard)
Austenite: carbon atoms occupy the interstitial sites in FCC iron, less of a hardening effect.
where are the interstitial sites in bcc octahedral
where are the interstitial sites in bcc tetrahedral
where are the interstitial sites in simple hexagonal
in which simple hexagonal are half of the interstitial sites filled
WC
where are the interstices in simple hexagonal
what does the HCP octahedral look like? how many interstitial places and where?
6 different interstitial atoms allowable. they are 3 on each side, 3 in between the bottom and middle and 3 in between the middle and top
what does the HCP tetrahedral look like?
there are 2 atoms in the middle of the column on the outside and then 3 on the bottom and three on the top (like in the octa)
what are four things that form interstitial compounds
hydrides, nitrides, borides, and carbindes
after adding interstitials in ccp, is it more close-packed or not?
it’s no longer close-packed bc the interstitials “push away” the other atoms.
are ccp and fcc the same?
usually, but ccp as a “special type” of fcc where the atoms are in contact along the face diagonals
what are stacking faults?
planar defects that affect the stacking sequence of
what are four instances where stacking faults can occur?
crystallization, phase transformations, crystal growth, and deformation
what can stacking faults affect?
the mechanical properties of the material
what are extrinsic stacking faults
something is added
ex: ABCABC B ABC…
what are intrinsic stacking faults?
as if something is removed
ABCAB_CACBACB
what do extrinsic stacking faults look like in franck notation
what do ccp sequences look like in frank notation
what do hcp sequences look like in frank notation
what do intrinsic stacking faults look like in franck notation
what is a twinned crystal
where the crystal on one side of the twin plane mirror is a reflection of the other side
if you have to use the quadratic formula to find rx/rA, what do you do?
you treat rA like a constant, and solve for rx = quadratic formula (but keep the rA w their coefficients)
solve it down and then choose the + - answer that is < 1
