4. Metals and Alloys Flashcards
STRUCTURE VRS PROPERTIES
Most metals are ____ with slight differences in tint
____ and ____ are non-white
Properties of elements on periodic table change gradually from metallic to non-metallic with semi-conductors in between (____ and ____)
white Au Cu Si Ge
Periodic Table of the Elements
Common noble metals: ____, ____ and ____; silver is not a noble metal, but it is a ____ metal (economic term)
Very important category of metals we want to know is the noble metals that are within this box here (in the ductile section, the box containing elements ____ and ____)
platinum
gold
paladium
precious
44-47
76-79
Characteristics of Metals \_\_\_\_ \_\_\_\_ ring Strength and \_\_\_\_ Ductility and \_\_\_\_ \_\_\_\_ and \_\_\_\_ conductors Crystalline solids at RT except \_\_\_\_ and \_\_\_\_
luster metallic density malleability thermal electrical Hg Ga
METALLIC BONDS
Valence electrons form a ____ electron cloud around ____ charged ions arranged in a space lattice.
Properties determined by ____ and ____
Free electrons act as conductors of ____ and ____
mobile positively space lattice electron gas heat electricity
Metallic Bonds
Electron ____
Electron Gas around ____ charges ions
sharing
positively
Structure Vrs Properties
Properties vs valence electron configuration
- –Formation of ____ ions in solution
- –Leads to ____ and ____
- –Mobility of electrons responsible for conduction of ____ and ____
positive luster malleability heat electricity
Solidification of Metals
Pure metals have a ____ melting temperature - ____ temperature
Supercooling may occur before ____ begins
Importance of fusion temperature in ____
fixed
fusion
crystallization
casting
Cooling curves
Pure metal fixed temperature
Start with molten metal, and cool until you reach the melting temperature and it remains ____ as it is solidified, then cools again once completely transformed into solid
constant
Embryo Formation ____- atoms aggregate
____ - the embryo increases in size
____- tree-like formation
____
Dendrites grow until they contact other dendrites around it.
The completed dendrite is grain or crystal
embryo
nucleus
dendrites
grain or crystal
Crystal Growth
Stages in solidification of a molten metal
Homogenous vs heterogenous nucleation
____nucleation: pure metal that is cooled, and the nuclei are forming only from the pure metal
____ nucleation: impurities are present within the metal, and the impurities act as nucleating sites for atoms to aggregate around them (____ for this to accomplish rather than homogenous)
homogenous
heterogenous
easier
Grains and Grain Boundaries
Gold casting with grains and grain boundaries
Each grain is surrounded by ____
grain boundaries
Grain Boundary
Grain boundary
—The ____ between grains or crystals
____ leads to stress resulting in high energy at grain boundaries. This is due to ____ bonds
Unsatisfied bonds present: ____ energy location of the material
junction
contact mismatch
unsatisfied
high
Grain Size
____ grains - equal in size in all
dimensions
____ size of grains in microstructure
____ grain size gives better mechanical properties
equiaxed
average
small
Controlling Grain Sizes
Can be altered by: Rapid \_\_\_\_ Degree of \_\_\_\_ \_\_\_\_ design \_\_\_\_ during solidification High \_\_\_\_ between mold wall and alloy Use of \_\_\_\_
Nucleating agents are other metals that have a ____ melting temperature than the metal that you are casting (can thus act as a ____) > fine grain sizes
cooling supercooling mold vibration thermal differential nucleating agent
higher
nucleating agent
Control of Grain Size
Practical way of controlling grain size
Use nucleating agents eg: ____, Rubidium, ____
silicide, ____ and iridium
Average grain size of
Cast pure gold = ____ diameter
Gold alloy = ____
Finer grain size means you have a ____ material
ruthenium
calcium
cobalt
1.0 mm
350 um
stronger
Deformation of Metals
Two types of deformation - ____ and ____
Plastic deformation occurs by ____ of one layer of atoms over another
Large stresses are needed to cause slip in a ____
elastic
plastic
“slip”
perfect lattice
Mechanism of Deformation
Elastic deformation is deformation that is ____ when the stress/force is removed.
We have one layer of atoms on top of a second layer of atoms; it’s shown as two planes in this block of material.
Put stress on the top layer of atoms. We can keep increasing the stress on the top layer of atoms, and the atoms will roll until they become exactly ____ right on top of the bottom layer of atoms. (shown in D)
If you remove the stress, the atoms will fall back to their original position, and you get ____ deformation.
On the other hand, if we continue increasing the stress, we eventually displace all of the atoms (one unit) and we get ____ deformation.
recovered
oriented
elastic
plastic
Lattice Imperfections Point defects (Now we’re talking about plastic deformation)
Point Defects
____ in a crystal lattice are not ideal
Some lattice positions are left ____
Others are overcrowded with atoms positioned out of line with lattice planes ____
atomic rearrangements
vacant (vacancies)
interstitials
Point Defects
Vacancy: A missing \_\_\_\_ in the lattice Divancy: \_\_\_\_ atoms missing and \_\_\_\_ atom replacing them, creating a defect in the metal Interstitial atom: \_\_\_\_ located within the lattice
atom
two
one
foreign atom
Dislocations
Instead of a perfect array of atomic planes, one plane of atoms may be discontinuous, forming a ____
The atomic arrangement next to a dislocation line is ____ ie. of ____
dislocation line
high energy
Line Defects - Edge Dislocation
Instead of having a perfect arrangement of atoms, we can have one plane of atoms that ____ in the middle of the material. This is referred to as a dislocation.
The atomic arrangement next to a dislocation line is ____ and it has high energy.
What happens when you put force on a metal going up and also force going to the right?
o Instead of all of the planes breaking to create a deformation, you can have just ____ break and then the dislocation line keeps moving towards the direction of the ____.
If we continue pushing, the line will keep moving by breaking one plane at a time until it gets to the end of the material; we’ve achieved ____ deformation of the material without having to break all of the planes at
the same time.
shearend
strained
one plane
force
plastic
Dislocation Movement
This is what we have after the atomic planes are broken down one at a time to get to the end of the material; we have a unit step of ____. We call this a dislocation line because it’s a line that goes through the
material; this line is moving; it’s also called ____.
Dislocation moves until it causes a unit step of ____ at the end of the material. This is how plastic deformation occurs in metal.
Why should we appreciate this?
o If you have a foreign atom, as in if you have an alloy (so, not all of the atoms are the same), the presence of the second type of atom is going to make it more ____ for the dislocations to move.
o Whenever you have a situation where dislocations don’t move easily, you have a ____ material.
slip
slip
slip/permanent deformation
difficult
stronger
Dislocation Movement
Plastic deformation involves a movement of the edge dislocation along a ____ (____ plane at a time) until the dislocation reaches the ____ of the crystal.
slip plane
one
edge
Slip bands on a metal surface
Cold worked gold with ____
This is showing a piece of gold that has been
deformed to show ____ (bands that are
due to ____ deformation on the surface).
slip bands
slip bands
plastic
Strain Hardening
____ deformation hardens and strengthens a metal
(Defects in a metal interact and inhibit further ____ movement)
Work hardening or strain hardening eg. Bending of a ____, compaction of ____, drawing of ____ wire
Once dislocation line moves through a material, the material becomes ____
plastic dislocation clasp arm gold foil orthodontic stronger
Strain Hardening
Effects of strain hardening
- – Increase in ____
- –____
- –____ limit
- –____ in ductility and corrosion resistance
•No change in ____
hardness strength proportional decrease modulus of elasticity
Properties of Cast Vrs Wrought Metals
Wrought:
T/S: ____
Ductility: ____
Corrosion resistance: ____
Cast:
T/S: ____
Ductility: ____
Corrosion resistance: ____
Already been ____ deformed > lost some of its ductility (wrought metal)
high
low
low
low
high
high
plastically
Annealing
The effects of cold work can be reversed by heating the metal to \_\_\_\_ its fusion temperature - annealing Three stages of annealing: \_\_\_\_ \_\_\_\_ \_\_\_\_
Can ____ the work hardening of a metal
half recovery recrystallization grain growth reverse
Annealing
Stages of annealing for a coldworked metal
This shows what is happening during the heating process to recover the properties of the work hardened metal.
The internal stresses are gradually ____ .
Remember: this graph is various properties vs. time.
We are heating the metal at a ____ .
After work hardening, the ____ and ____ are higher, but the ductility is ____ .
As we gradually heat it over time, the strength of the wrought metal ____ to that of the cast metal, the hardness ____ to that of the cast metal, and the
ductility increases ____ to that of the cast metal.
This is what is happening in the grains of the metal. This (recovery) shows elongated grains that have deformed in tensile formation. As we continue to heat it, we begin
to cause ____ (new crystals are formed from the elongated grains to become more ____ ). We have grain growth to the point where the properties of the metal become ____ to what they were when they were first cast.
We go from a stronger metal after plastic deformation to a weaker metal, high hardness to low hardness, and low ductility to high ductility.
decreased fixed temp hardness strength lower decreases decreased gradually
recrystallization
equiaxed
identical
Annealing
____ grains disappear and new ____ grains are formed
Wrought structure is converted to a ____ structure
Practical significance: Bending of ____ or ____ lead to cold work
strained
strain-free
cast
orthodontic wires
partial denture clasp arms
Annealing of Metals
____ changes during annealing
microstructure
Annealing
____ or ____ of orthodontic wires
If wires are overheated, the ____ changes
____ strength of appliance
soldering
welding
microstructure
reduced
