2018/2019 Flashcards
Define yield stress in context of material deformation
Stress applied which causes a transition from linear elastic performance to non linear plastic deformation
Define true and engineering stress, differences?
True stress accounts for reduced CSA during deformation. Reduction is significant at point of necking. Necking is from localised deformation in tensile specimen after max load. With necking, engineering stress decreases but true stress continues to rise.
Define alloy:
Mixture of metal with one or more other metals or non metals
Define component:
An element in an alloy
Define phase:
A region of material having uniform physical and chemical characteristics
Define phase diagram:
The equilibrium constitution of all combinations of temperature and composition
What is complete solid solubility?
The solute atoms are completely soluble in solvent atoms for any composition of solute and solvent. Thus, a second phase doesn’t form in this case.
What is limited solid solubility?
Not all solute atoms are soluble and they will precipitate out as a solid
Describe pearlite structure
Eutectoid. An equilibrium structure of ferrite and cementite layers or lamellar. A eutectoid alloy will transform to pearlite when cooled from Y phase under equilibrium conditions.
How does cold working/strain gardening a material affect it’s mechanical properties and how does this change come about?
It increases its tensile strength BUT at the cost of reduced ductility. This is from mechanical deformation which increases dislocation density. Dislocations get closer together and interact making further deformation harder.
Describe precipitation hardening:
1st treatment
First a solution heat treatment where solute atoms are dissolved to form a single phase solid solution. Then rapidly cooled to a 2 phase region of the phase diagram but rapid cooling means diffusion can’t occur and 2nd phase isn’t achieved.
Thus non equilibrium structure of A with supersaturated B is achieved.
Weak and soft material.
Describe precipitation hardening:
2nd treatment
Precipitation heat treatment:
Supersaturated solid heated to 2 phase region. This accelerates diffusion rate allowing B phase to separate and form precipitates. Temp is held to grow B phase, size defines properties.
How does solid solution hardening strengthen a material?
Alloying a metal with an impurity introduces a lattice strain which hinders the movement of dislocations.
Why are stress/strain properties of ceramics not done by tensile tests?
1) hard to prepare samples of right shape
2) hard to grip materials without fracturing
3) need to be aligned perfectly since they fail after ≈0.1% strain
What is crystallisation in context of glasses?
Process whereby glass material is caused to transform into a crystalline solid, usually by heat treatment.
