Terms in this set (20)
Are materials homogeneous? What are they composed of? What dictates their physical behaviour?
No, composed of millions of crystals (grains), combined behaviour of grains dictates physical behaviour.
What are 3 common crystal structures? What are some examples of materials with these structures?
Body Centred Cubic (e.g. ferritic steel), Face Centred Cubic (Austenetic Steel), Hexagonal Close Packed (Zirconium)
Is a Crystals atomic structure Perfect? What are dislocations? How do they affect strength of the lattice?
No, they contain vacancies (radiation increases vacancies) and hence dislocations. This is where a few bonds are distorted and broken at one time (as opposed to all bonds having to break for movement to happen), requiring lower energy to deform structure
What are slip planes? Where does slip occur?
Directions along which sections of the crystal lattice can move over each other. Dislocations restrict how deformation occurs (arent actually free to move in any direction). Hence movement occurs in whatever way requires least effort. Slip occurs on the closest packed planes in the closest packed direction.
What is the difference in slip between BCC, FCC and HCP? How does this affect overall material behaviour?
FCC - slip is easy in many planes and directions, hence very ductile
BCC - No true close packed plane, complex behaviour and temp dependence
HCP - Mostly limited to basel slip, slip is easy in a few planes
What is basel slip?
In hexagonal metals, slip only really occurs in one plane (Basel plane), hence yield stress varies greatly with orientation of basel plane and slip direction
What is the schmid factor?
Schmid's Law states that the critically resolved shear stress (τ) is equal to the stress applied to the material (σ) multiplied by the cosine of the angle with the glide plane (Φ) and the cosine of the angle with the glide direction (λ). Which can be expressed as
τ = σ*m
where m is known as the Schmid factor
m = cos(Φ)*cos(λ)
maximum value of 0.5
What are the differences between multi-glide and single glide materials?
Single glide - Low symmetry crystals (HCP), Few slip planes available, Parallel slip bands, very high ductility if correctly orientated, otherwise low
Multiple glide - High symmetry crystals (FCC, BCC), lots of planes with low critical shear stress so many slip at once, slip always possible but interaction of slip planes limits ductility
What does critical shear stress depend on? What will increase the yield point of a material?
How easily dislocations can move. Any barriers to dislocation movement will increase yield point.
What are the 3 main mechanisms for reducing dislocation movement?
Alloying - additional alloys in lattice create strains which oppose dislocations
Work Hardening - Dislocations find it hard to move through each other, deform to produce dislocations which hinder each other
Age Hardening - Dislocations find it difficult to move through other phases, cycle temp of material(heat/quench) to build up different phases.
What are the 2 issues with translating single crystal behaviour to polycrystals?
Most grains not optimally orientated, Grain deformations must be mutually compatible.
Not optimally orientated - grains prefer to slip in different directions
Stress and Strain compatibility - aggregates of crystals cannot behave like simple combinations of slip. If each grain free to distort, gaps would occur, but dislocations have to match
What is the difference between the number of slip planes in FCC and BCC metals, and HCP metals?
FCC + BCC have many slip planes (12+) - High ductility
HCP have few (about 3) - low ductility
How does the gradual transition from elastic to plastic material behaviour occur at a microscopic level? Is deformation homogeneous across material?
in elastic region, early yielding of well orientated grains
approaching plastic, yielding of less well-orientated grains
plastic - all grains reach critical shear stress and full plasticity occurs
Deformation is inhomogeneous
Does an increase in grain size lead to an increase or decrease in ductility?
More boundaries - More brittleness, smaller grains, more boundaries.
Hence, no, decreasing grain size decreases ductility.
what is the difference in isotropicness between crystals and aggregate materials?
Crystals anisotropic (usually). Large material has random orientations of crystals, hence isotropic.
Describe 3 methods of viewing grain boundaries
Scanning Electron Microscope
X ray diffraction
X-ray tomography (material rotates and builds up 3D picture)
How does fatigue in metals appear in the crystal/grain structure?
Initiation - First cyclic stress causes dislocation movement, then a persistent slip band develops within a grain, which becomes a crack, which is then arrested at grain boundaries
Propagation - Above Fatigue limit crack breaks through grain boundary and continues to grow
What is stage 1 and stage 2 fatigue crack growth? When does the transition occur?
Stage 1 is crystallographic (takes place in grains), Stage 2 is non-crystallographic (bigger than grains). Transition occurs when crack tip plastic zone is approximately grain size.
What is fatigue?
The nucleation and propagation of cracks caused by cyclic loading.
What is (Delta)CTOD
Change in crack tip opening displacement range. The range of a crack opening under cyclic loading. This increases with stress amplitude and crack length in stage 1, and mean stress in stage 2.