Metals and Alloys Flashcards
- Define metal, metalloid and alloy: METAL
any of a class of chemical elements … generally characterized by ductility, luster, conductivity of heat and electricity
- Recognize the relevance of the relationship between structure and properties
see pic
- List the methods of forming and shaping metals
Casting Machining Cold Working Powder Metallurgy
- Describe metallic bonding
- Primary/Strong bonding - Metallic Bonding
- Positive ions + surrounded by ‘cloud’ of electrons
- Electrons can move in the cloud
- List the consequences of metallic bonding (4)
High Conductivity(thermal and electrical) -movement of electrons
- Opacity to light -electrons absorb electromagnetic and photon energy
- Easy ability to form alloys
- High plasticity
- Define crystallinity
shows a long-range regular arrangement of atoms
–Contrast this to an amorphous –glassy-material which has no such regularity. All metals are crystalline
- Define unit cell
smallest repeating unit in a crystal (consider 3 out of a possible 14)
- List the various types of crystalline imperfection
–point defects
–line defects
–[plane defects]
- Identify bcc, fcc and cph (or hcp) unit cells
see pic
- Describe the mechanism of plastic strain of metals in terms of movement of dislocations
Plastic strain (permanent) -slip of layers of atoms over each other. A metal is soft if the dislocations can be easily moved through the crystal structure while applying force to the material.
- Identify point, line and planar defects
- Draw a typical temperature-time curve for the cooling of a metal
see slides
- Draw and describe grain structures of the following types: equiaxed coarse grain, equiaxed fine grain and fibrous structure
fine grain leads to harder, stronger more dense.
not really sure what else i was supposed to take away from this
- Illustrate the structure of an interstitial solid solution, and quote an example of this
Steel C atoms in Fe crystal structure
- Describe the phenomena that occur in the solidification of an alloy where there is partial solid solubility
On cooling further under specific conditions, ‘precipitation’ of a second and different solid phase may occur
- Describe the effect of cold working, stress relief anneal, recrystallization and grain growth on the structure of a metal
fibrous grain
- List the dental applications of alloys
Metals are strong and tough. Polymers are not so strong and ceramics are too brittle. Metals in mouth must be biocompatible - resist corrosion and no release of metallic ions.
- Define metal, metalloid and alloy: METALLOID
exhibit both metallic and non-metallic behavior (Si, As, B)
- Define metal, metalloid and alloy: ALLOY
a mixture of metals - almost everything we use in dentistry is an alloy, we dony use metals
- List the methods of forming and shaping metals: CASTING
Pouring liquid metal into mold
- List the methods of forming and shaping metals: MACHINING
Cutting metal: drilling, turning, …
- List the methods of forming and shaping metals: COLD WORKING
Deforming metal in solid state: forging, extruding
- List the three principal factors that determine the hardness of an alloy
see slides
- Describe the effect of chemical composition, mechanical history and thermal history on the hardness of an alloy
see slides
List the four possibilities when a molten alloy is solidified
Solid Solution - Interstitial (1)
List the four possibilities when a molten alloy is solidified
Complete Solid Insolubility (2)
List the four possibilities when a molten alloy is solidified
Partial Solid Insolubility (3)
Once the initial solid solution is formed, on further cooling the alloy under specific conditions, precipitation of a second phase (same metals mixed in different quantities and with different crystal unit cell) occurs. The resulting alloy has grains of both phases. The alloy is stronger because the second phase pins the movement of the dislocations
List the four possibilities when a molten alloy is solidified (4)
Intermetallic Compound formation
- Identify point, line and planar defects: POINT
see pic
- Identify point, line and planar defects: LINE
see pic
- Identify point, line and planar defects: PLANAR
see pic
Properties of metal
Usually strong (hard) and stiff
Tough - not brittle
Lustrous - shine
Dense
Good conductors of electricity
Point defects
instead of metal A atom in one position, metal B atom is there - substitiutional
element b smaller than metal a, put in a cavity in between metal A’s with ease - interstitial
vacancies - missing atom
line defects
dislocation in a crystal - see pic
plane defects
as you cool the metal, orientation of the crystals is different and have different orientations leading to discontinuity in the boundaries
Distinguish between elastic and plastic properties - metals
ELASTIC
Elastic strain (recoverable) - stretch metal and interatomic bonds, after you remove force, returns to its structure
Elasticity of metals depends mainly on the resistance of the metallic bond to be stretched. The metallic bond acts as a spring
Modulus of elasticity mainly depends on the _____ nature of each metal
chemical
distinguish between elastic and plastic properties of metals
PLASTIC (slip of layers of atoms over each other)
Plasticity, i.e. permanent deformation of metals is due to the formation and movement of dislocations.
The easier the dislocations move with no impediments in the crystal structure, the softer the metal is.
In order words, to strengthen a metal the movement of the dislocations should be made difficult.
Metals are very tough because they can develop significant plasticity, i.e., metals can form and move dislocations. If dislocations are difficult to move,
the material will be more brittle, less tough. Ceramics are brittle because they can not form dislocations, they break bonds instead.
Polymers have permanent deformation because they are viscoelastic, they have viscous component (disentanglementof molecules). Metals are not viscoelastic, ___
heir plasticity does not depend on the time you apply a load, but on the magnitude of the load.
Pure metals are often too ___ and ___for dental use
soft and ductile
Dental Significance of Metals
Point #1 - Pure, untreated metals are usually too ____
soft/weak - gold, alumin, titanium, iron
Dental Significance of Metals
Point #2 -
The strength of an alloy mostly depends on: (3 things)
composition, mechanical history and thermal history
Dental Significance of Metals
Point #3 The elasticity of a metal mostly depends on
the composition of the unalloyed metal
Principles for selection of alloys
Metallic Alloys used in dentistry (3)
Applications of Alloys in Dentistry
