Microstructural Development of Metals Flashcards
What are the two types of metal microstructure?
Body Centred Cubic(bcc) - 8 atoms at each corner which are also common
with another 8 cells
1 atom at the centre
Total atoms per unit cell=>(8x1/8)+1=2
Face Centred Cubic(fcc) - 8 atoms at each corner which are also common with another 8 cells 6 atom at the centre of each face common to 2 cells. Total atoms per unit cell=>(8x1/8)+(6x1/2)=4
Hexagonal close packed hcp
What is a polycrystalline metal made up of?
Grain boundaries and Gradin that have different orientations.
ie stainless steel
What angle do twin boundaries interact at?
60 degrees
What is carbon steel made up of?
Ferrite and pearlite/carbide which can have alternating platelets of ferrite and carbide
What microstructure is fcc on iron/carbide phase diagram?
Austenite
What microstructure is bcc on iron/carbide phase diagram?
Ferrite
What phases does pure iron go through with increasing temperature and in what order?
Alpha ferrite (bcc) -> Austenite (fcc)-> delta ferrite (bcc)-> liquid
What phases does pure iron go through with continuous cooling and in what order?
Ferrite -> Pearlite -> Bainite -> Martensite
Slow cooling - goes back to original iron carbide state
Medium cooling - Bainite to austentie
Fast Cooling - like a basket weave goes to martensite which is VERY hard
What is the Eutectoid temperature?
Minimum temperature of material in a single phase
What phase is produce at a welding temperature greater than the eutectoid temperature?
Pearlite
What phase is produce at a welding temperature lesser than the eutectoid temperature?
Recrystallisation and grain growth (increasing in size)
What is the weld bead grain structure?
Large columnar grain growing from the bead interface to the centre of the bead.
What is recrystallisation and grain growth?
The grains are growing in size with a gain in energy but this can weaken the material.
Recrystallisation is where there are previous dislocations in the grains with added heating can have the grains move and reorganise (hardening)
What is spheroidization of carbide?
Where there are pearlite- alternating platelets of ferrite and carbide, the platelets break up and being to spherodise.
What happens to the material structure with an increase in gaussian beam energy density?
The structure become more deformed with a wider area of grain growth and recrystallisation.
What are 4 observation techniques for welds?
Optical microscopy, scanning electron microscopy (SEM), Electron Backscatter Diffraction (EBSD), Energy Dispersive X-ray Spectroscopy
What is Scanning Electron Microscopy (SEM)?
There is an incoming electron beam which can emit from the material Backscattered Electrons(E=E0), Secondary Electrons (E«_space;E0)and Characteristic X-rays containing information.
What is energy dispersive X-ray spectroscopy?
Electrons in the inner shell are ejected by the electron beam. Electrons in higher orbits drop down to fill the gap, emitting their excessive energy as an Xray photon.
This is detected by an Li doped Si detector as a series of current pulses which are amplified to display as a graph of intensity vs energy.
What is the size of the energy dispersive x ray spectroscopy?
The beam spreads withing the specimen for approx 1microM
The size of the beam is immaterial, the controlling factor is the scattering inside the specimen.
How does electron backscatter diffraction work?
The electron beam from the SEM is focused onto the sample surface and it is diffracted from the different crystalline structure into a phosphorous screen to be received by a low light camera.
Why is electron backscatter diffraction so important? 6 reasons
Measure the orientation of a given crystal, measure the overall orientation of the material or crystallographic texture, measure the misorientation between two points in a crystal or across grains, identify unknown crystal phases, identify multiphase material crystals,
use with EDX to obtain a full description of the material.
What is a holographic optical element(HOE)?
Computer generated hologram, reconstruct the beam intensity into any custom designed profile (shape/intensity) with an efficiency >90%
It can adapt the microstructure to give grain refinement, more equiaxed grains and increase in low energy twin boundaries.
What is the phase prediction graph for nickel chromium called?
Schaeffler Diagram
