Metals Flashcards
What preparation is needed before the microstructure of a metal can be examined using optical microscopy?
Polishing and chemical etching
What are the 3 types of crystalline unit cell?
- Face centered cubic
- Body centered cubic
- Hexagonal close-packed
What is the definition of strength in metals|?
The ability of the material to resist plastic deformation (measured in how much tensile stress can be applied before the UTS is reached).
How do impurities strengthen metals (theory and quantifiable result) and what is this technique called?
- Strain field around dislocation.
- Also strain field around impurity.
- When impurity atom and dislocation meet, the strain can be cancelled: the overall strain energy is reduced.
- The solute atoms therefore tie the dislocation and make it more difficult to move further, making plastic deformation more difficult.
- Technique is called SOlid Solution strengthening - can acheive upto 20% increase in yield strength.
What is the only metal which doesn’t strengthen steel when it is dissolved into steel and why?
Chromium as the atomic size of chromium and iron are very similar.
What effect does grain size have on the strength of a metal and why?
Smaller grain size –> higher strength and toughness
- Stress applied to metal
- Dislocation takes palce across a slip plane, which has a fixed direction
- When the dislocation reaches a grain boundary, the slip plane has to change direction which takes energy.
- Dislocation piles up at grain boundary to overcome this energy barrier, instead of propagating.
- Smaller grains = more grain boundaries = less propagation of dislocations.
What is the Hall-Petch equation?
σy = σ0 + ky d-1/2
- σy = yield strength
- σ0 = intrinsic strength of iron (starting stress of dislocation)
- ky = strengthening coefficient
- d = avg. grain diameter
How is grain size controlled?
Control during processing:
casting, rolling, extrusion, heat treatment
What is phase balance control?
Adding metals to other metals past the solubility limit results in alloys which consist of more than one phase, e.g. zinc is soluble in copper up to 70% Cu and 30% Zn, which is called single phase brass (α-brass). When you keep adding zinc to get a 60% Cu/40% Zn split, you get two phase (α + β) brass, which has superior mechanical properties.
What is precipitate strengthening?
Give example
Adding elements to an alloy which do not readily go into solution but instead form discrete particles, called precipitates, which strengthen the alloy.
E.g. adding Zn and Mg to aluminium alloys forms MgZn2 precipitate particles which strengthen the alloy.
What are the two methods by which precipitate strengthening increases the strength of the alloy?
- Cut mechanism: if precipitate particle lies along line of slip plane, the dislocation must cut through the precipitate particle, breaking bonds, which requires lots of energy, so the UTS is higher.
- Bow mechanism: if the precipitate particle is too hard to cut through, the dislocation line must wrap or bow around the particle, leaving a dislocation circle around it, before the dislocation can continue. Normally happens with large, hard particles.
What is work hardening and how does it work?
Ductile materials are made stronger and harder as they are plastically deformed. As they bare plastically deformed, the dislocation density in the material increases because new dislocations are formed. The avg. separation between dislocations therefore decreases. As dislocation-dislocation strain interactions are on avg. repulsive, this means new dislocations are significantly hindered by the pre-existing dislocations, so the strength of the material increases. However, the ductility decreases.
What is the definition of steel?
What are the different grades?
Iron-based metal with the addition of carbon content.
When adding carbon, you need heat treatment to change the microstructure to get the different grades: low, medium and high-carbon steel. Also have low-alloy steel, high-alloy/stainless steel or cast iron steel, in increasing order of strength.
What are the 7 advantages of steel?
- Cheap
- High strength/stiffness
- High toughness
- Excellent formability
- Easy to join and weld
- Versatile
- Recyclable
What are the 2 disadvantages of steel?
- Very dense
- Poor corrosion resistance
What are the 4 types and properties of cast iron?
- Gray iron
- Graphite flakes
- weak and brittle in tension
- fracture surface grey as graphite-iron boundary is weak point
- stronger in compression
- excellent vibrational dampening
- wear resistant
- Ductile iron
- add Mg and/or Ce to gray iron
- graphite as nodules (spherical) not flakes, increases tensile strength
- matrix often pearlite - stronger but less ductile compared to ferrite matrix
- White iron
- < 1 wt% Si, no graphite
- pearlite matrix with cementite phase formed within
- very hard and brittle
- fracture surface white due to cementite
- Malleable iron
- heat treat white iron at 800 - 900 degC
- cemetite reverts to graphite but forms rosette structures
- reasonably strong and ductile
What are the 8 microstructure phases in ferrous materials?
- Austenite
- Ferrite
- Cementite
- Graphite
- Pearlite
- Bainite
- Martensite
- Tempered martensite
What are the properties of austenite?
- Stable phase at higher temp (above 912 degC for pure iron)
- FCC, easy to deform
- Appears in pure iron and austenitic stainless steel (nickel stabilises autenite phase which is why it can form at room temp. in stainless steel with a high nickel content)
