Light Alloys Flashcards
What is plastic deformation occurring within an alloy?
Plastic deformation is the result of slip occurring in the same metal. Slip is the movement of one part of the crystal relative to another. Slip always occurs on the same crystallographic planes. The deformation is permanent.
What is the critical shear stress for different slip systems?
How can it be altered?
Critical revolved shear stress (CRSS): The stress required to initiate slip in a pure and perfect single crystal.
Increasing temperatures reduces the CRSS value.
What are the two types of deformation? What are the differences between them?
Slip plane and twinning deformation.
Slip plane includes deformations greater than the atomic spacing whereas twinning can include deformations less than the atomic spacing.
The orientation of the atoms remains the same for slip plane whereas twinning results in a change in orientation
What is the effect known as hot tearing?
Where does it occur and how?
What effect does it have on a cast alloy?
Hot tearing is a common casting failure involving the formation of a crack within a cast part.
It occurs within the mushy zone of an alloy due to the miss match between the shrinkage of the alloy on solidification and the compensation by additional liquid.
It causes breakages within the cast piece.
What is the rule of mixtures equation used for?
Volume fractions of specific samples can be calculated using the materials Young’s modulus.
Ec = (1-Vp)Em + VpEp
What equation is used to determine values involving cracks in a structure?
Kic = Yσ(πα)
Fracture only occurs when K > Kic
How do the crystalline structures of α and β alloys of Ti differ?
What can cause a structure to convert from an alpha phase to a beta phase?
α alloys display a HCP structure and β alloys show a BCC. A transition temperature of 882°C induces this change.
What are α/β Ti alloys? How can they form?
α/β alloys contain both alpha and beta phases. They can form if alloyed with beta stabilisers or alpha stabilisers after a certain concentration and temperature.
What is the strengthening mechanism within meta-stable alloys?
Aging metastable β phase to produce some α phase.
(Ageing causes precipitation of fine α phase (dispersion strengthening))
How are stable β alloys formed and what property has changed to induce this?
Increasing the content of β stabilising elements causes the beta transus temperature to decrease below room temperature.
After cooling to room temperature, β phase remains stable.
What are the three heat treatments applied to Al-Cu alloy and how does it improve the strength of the alloy?
- Solution treatment (solutionising).
Al and Cu atoms diffuse randomly into a uniform substitutional solid solution. It is held at 515°C for a period of time to enable solutions. - Quenching in water, to room temperature or below.
All solute is trapped, and it creates a supersaturated solid solution (ssss) of Cu in Al.
The alloy in this condition is unstable and strives to form more stable (metastable) phases to lower the overall energy of the system. (The driving force is the high energy of unstable ssss). - Artificially age.
Most alloys need slightly elevated temperatures to encourage substantial precipitation to improve strength levels in reasonable times.
This is referred to as artificial aging and the temperature required depends on the a lot. In Al-4%Cu, the appropriate range is ~130 - 190°C.
Which phase forms in Al-3Li alloys and how does it improve the strength of the material?
Al3Li forms rapidly as a spherical, coherent precipitate in a binary Al-Li matrix. These precipitates are homogeneously distributed and impede movement of dislocations during plastic deformation, thereby strengthening the alloy.
