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Lucy D.
3rd year Materials Engineering Student at the University of Manchester
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Material Science
TutorMe
Question:

Name the different crystal structures of Yttria-stabilised Zirconia (YSZ), and how this helps it resist crack propagation.

Lucy D.
Answer:

YSZ has different stable structures at different temperatures. When it solidifies from the melt it is cubic, then shifts to tetragonal, and finally monoclinic from 1200 C to room temperature. It is possible to cool the structure very fast with yttria, resulting in a metastable state that will change to tetragonal and monoclinic when energy is provided. When a crack propagates, the energy in the plastic zone at the crack tip causes the YSZ to change crystal structure to the tetragonal then monoclinic state, both of which have greater crystal cell volume. This increase in volume around the crack causes the crack to shut, and it no longer propagates.

Materials Science
TutorMe
Question:

Describe the main features of a Stress/Strain graph, naming the elastic region, plastic region, main stress features, and how to calculate the Young's Modulus.

Lucy D.
Answer:

The stress/strain graph starts with a straight line of linear proportionality, where the Young's modulus is calculated from the gradient. This is the elastic region, where energy absorbed will not cause permanent damage to the material. The top of this linear line is the yield stress (sigma y). Past that, there is the plastic region, where energy absorbed causes permanent damage. The line curves up to a point, the ultimate tensile stress (sigma uts), then descends briefly resulting in the failure stress (sigma f).

Physics
TutorMe
Question:

Name the three types of radiation, what they are, how they are stopped, and where they would be used.

Lucy D.
Answer:

Alpha radiation is a helium nucleus (two protons, two neutrons), it is stopped by 5 cm of air or a thick sheet of paper, and it is used in smoke detectors. Beta radiation is a high energy electron, it is stopped by thin aluminium, and it is used in carbon dating. Gamma radiation is a high energy and frequency electromagnetic wave, it is stopped by thick lead or several metres of concrete, and it is used to sterilise hospital equipment.

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