“Influence of Si, Sb and Sr Additions on the Microstructure ...

Chapter 4: Results <strong>and</strong> Discussion
solid solution strengthening to precipitation hardening. Then prolonged exposure
leads to the coarsening <strong>of</strong> precipitates <strong>and</strong> it looses its ability to pin both the
dislocations <strong>and</strong> boundary. As it is clear that the Mg17Al12 precipitates has low
melting point (43 7°C) <strong>and</strong> aluminum has higher diffusivity in magnesium, coarsening
takes place very easily <strong>and</strong> faster. Moreover the decomposition <strong>of</strong> Mg;7Al|; at grain
boundary during elevated temperature exposure increases the aluminum content in
local areas. Subsequently this reduces the solidus temperature <strong>and</strong> hence increases the
homologous temperature [3l6]. This represents the tertiary stage <strong>of</strong> creep. As long as
the hardening mechanism dominates the creep rate decreases. Whereas, when
s<strong>of</strong>tening mechanisms become dominant, the creep rate increases. The minimum
creep rate measured is the point <strong>of</strong> balance between hardening <strong>and</strong> s<strong>of</strong>tening
mechanisms, which happens during secondary stage <strong>of</strong> creep.
Apart from the coarsening effect, the massive Mg|-,Al12 particles are also
involved in cavity formation. Figure 4.63 also shows most <strong>of</strong> the Mg]-;Al1;> particles
suffer from sever cracking <strong>and</strong> cavity formation at the matrix — precipitate interface.
These cavities develop along the grain boundary since more coarse discontinuous
precipitates are present in the grain boundary <strong>and</strong> finally introduce matrix cracking.
One <strong>of</strong> the major reasons for the formation cavity is the weak interface between
matrix <strong>and</strong> Mg17Al12 intermetallic. It is well known that the crystallographic lattices
<strong>of</strong> Mg matrix <strong>and</strong> Mg17Al|2 are different: the magnesium matrix has an hcp lattice,
while the |3-MgnAl12 precipitates have a cubic lattice. It is also known that
dislocations cannot pass as easily through hard precipitates as through the matrix. ln
such cases pileups <strong>of</strong> dislocations near the precipitates lead to increase in local stress
concentration <strong>and</strong> resulted in cracking. Moreover cavity may forms at the grain
boundary triple points. One such cavity at triple point in creep ruptured AZ91 alloy at
150°C is shown in the Figure 4.65. According to Regev [29], the intergranular
cavitation <strong>and</strong> cracking also indirectly contributes to the s<strong>of</strong>tening process during
creep. From dislocation theory, it is well known that cavitation enables dislocations to
leave the bulk <strong>of</strong> the grain to the free surface without being blocked by obstacles. This
prevents the activity <strong>of</strong> dislocation sources <strong>and</strong> hence, the number <strong>of</strong> pileups is
reduced [29].
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