Magnesium Alloys Flashcards
What are the general properties of magnesium alloys?
➢ 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).
➢ Mg alloys is 34% lighter than Al alloys by volume and
50% lighter than Ti alloys.
➢ Mg alloys have the highest known damping coefficient.
➢ Sea water is the most common source of Magnesium.
➢ 1m3 of seawater contain 1.3kg of Mg.
➢ Magnesium alloys are fully recyclable and recycling only requires
5% of the energy required to manufacture new magnesium.
What are applications of magnesium alloys?
➢Phone cases and bodies
➢Aerospace
➢Automotive
What elements is magnesium usually alloyed with?
➢Mg is usually alloyed with elements such as Al, Zn (Zinc), Zr and the
rare earth metals, such as cerium (Ce), to produce alloys with high
strength – weight ratios.
What are characteristics of magnesium alloys?
➢Low young’s modulus (45GPa).
➢Poor resistance to fatigue, creep and wear.
➢Mg readily combines with O2 and burns, this can cause problems
with processes such as casting and machining.
➢HCP in and is less ductile than aluminium. Some deformation can
be done at room temperature but generally fabrication is done by
casting or “hot working” (200 – 350˚C).
➢Mg alloys are very easy to machine, which permits higher feed
rates and deeper cuts – time saving.
➢Improved die life when used for die casting, due to lower
operating temperatures and less of a reaction with the tool
steel. Up to 4x tool life.
➢Better dimensional stability than other engineering alloys, due
to less residual stress from casting process.
➢The solubility of many elements, in magnesium, increases with
temperature (eg. Al in Mg), therefore, dispersion strengthening
(age [precipitation] hardening) is possible.
➢ High temperature strength in Mg alloys can be improved by the
use of ceramic particles or fibres.
➢ Advanced Mg alloys contain low levels of unwanted impurities.
➢ Often alloyed with 5% of “rare earth metals”, eg. cerium (Ce).
➢ These form a protective MgO film that improves corrosion
resistance.
What is slip in magnesium alloys?
➢ Recall: 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.
➢ A slip system is made up of a slip plane and a slip direction, which
lies on the slip plane.
➢ Generally one slip system will dominate plastic deformation ,
however others can occur at elevated temperatures or applied
loads.
➢ For slip to occur the resolved shear stress on this plane must
increase to activate the dislocation movement.
➢ Magnesium alloys have an hexagonal close packed (HCP) crystal
structure and hence the only active slip plane at room
temperature is the basal slip plane.
➢ The activation of non-basal slip planes at elevated temperatures
quickly improve formability.
➢ Orientation of atoms remains the same.
What is twinning?
➢ Twinning occurs when there is limited present slip systems.
➢ Typically forms at low temperatures and high loading rates (shock
loading).
➢ Describes a small number of plastic deformations
compared to slip deformation.
➢ Reorientation of atomic direction across twin plane.
What are characteristics of magnesium alloys containing Zi, AL and Mn?
➢ Mg-Al Alloys:
❑ Plus small amounts of zinc results in higher tensile strength.
❑ Small amounts of manganese results in improved corrosion
resistance.
❑ Addition of Al improves cast-ability.
Describe hot tearing in magnesium alloys.
➢ Hot tearing is a casting defect as a result of the solidification
process, occurring in the mushy zone (a+L) during casting.
➢ Alloys which have a large a large freezing range are susceptible to
hot tearing.
➢ Hot tearing is thought to occur due to inadequate feeding by
liquid of the volumes created by shrinkage during solidification.
How can hot tearing be prevented?
➢Hot tearing can be prevented by eliminating large stress or
temperature gradients in a casting
➢The temperatures can be controlled by lowering pouring
temperature and increasing mould temperature by preheat
➢Modifying the casting design to eliminate any sharp geometric
features
Describe the mushy zone in more detail.
The mushy zone is a mixture of both solid dendrites and liquid flowing around the solids. Is the diffuse interface between the dendrites and the liquid. Dendrites can be very small (nanometre), or large (millimetre)
