Magnesium Alloys General Questions Flashcards
How does magnesium’s density compare with other structural metals?
The most outstanding characteristic of magnesium (Mg) is its density, (1.74 gcm-3). This is the lowest of all the “structural” metals, (lithium (Li) 0.534 gcm-3 is the lowest of all metals).
How can the high temperature strength of magnesium be increased?
High temperature strength in Mg alloys can be improved by the use of ceramic particles or fibres, eg. SiC, to produce Metal - Matrix – Composites (MMC). The solubility of many elements, in magnesium, increases with temperature (e.g. Al in Mg), therefore, dispersion
strengthening (age [precipitation] hardening) is possible.
What is plastic deformation occurring within an alloy?
Plastic deformation is the result of slip occurring in the 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 and for different slip systems how can it be altered?
CRSS: The stress required to initiate slip in a pure and perfect single crystal. On increase of temperature the value decreases.
What are 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 spacings. 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 default occurring 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 rate of shrinkage of the alloy on solidification and the compensation by additional liquid. It causes breakages within the cast piece.
What are the phases present within Mg alloy AZ71 at 500 oC? At 250oC? At 200oC?
- At 500 oC phases present are Pure Mg (α) and 9wt% Al
- At 250 oC phases present are 95.5 wt % Pure Mg (α) and Mg17Al12
(β) (42 wt % Al) - At 200 oC phases present are 97 wt % Pure Mg (α) and Mg17Al12
(β) (42 wt % Al)