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

W g_ Chapter 4: Results <strong>and</strong> Discussion
Table 4.3: Hardness <strong>of</strong> various alloys at different tempering conditions
.T¢mp@ring condition e
up g g H _ 8 llllll
\ Alloy A As cast, Solution Peak aged, Time to reach
BHN treated, BHN BHN peak “,g°i“g’
up AZ91 63.9 5_7.8 85.8 ~900
‘_gAz91+0.s% <strong>Si</strong> 68.1‘ 66.2 99.2 ~900
Az91+o.5% <strong>Sb</strong> 273.5 i“T6s.s 95.4T ~s4o
T AZ91+0.5% <strong>Sr</strong> 72.2 69.5" 89.54 ~600
it AZ9l+0.5%
<strong>Si</strong>+0.2%<strong>Sb</strong> 76 W 102 K ~7so
Presence <strong>of</strong> hard intermetallies also leads to formation <strong>of</strong> large number <strong>of</strong>
continuous precipitates during ageing by changing the precipitation process (as
explained in section 4.3.3) <strong>and</strong> hence increases the peak aged hardness. The possible
reason for the poor ageing behavior <strong>of</strong> <strong>Sr</strong> added alloy is attributed to the amount <strong>of</strong> Al
available for the precipitation process during ageing. The microstmcture <strong>of</strong> solution
treated alloy shows the presence <strong>of</strong> Al4<strong>Sr</strong> <strong>and</strong> some undissolved Mg11Al|2 massive
intermetallics. These intermetallics retain the aluminum <strong>and</strong> hence only part <strong>of</strong> the
total aluminum then dissolves into the matrix during solution treatment <strong>and</strong> involves
in the precipitation process during subsequent ageing. <strong>Si</strong>nce high hardness is
associated with fine <strong>and</strong> evenly distributed precipitates occurred during ageing
process (in this case, continuous Mg;-;Al;2) compared to the massive phases occurred
during solidification, <strong>Sr</strong> added alloy, despite it contain retained Mg]-,Al1; <strong>and</strong> Al4<strong>Sr</strong>
intermetallics, does not exhibits higher hardness compared to other alloys at peak
aged condition.
4.3.3 Aged Microstructures
The microstructural development <strong>of</strong> AZ9l during ageing at a range <strong>of</strong> ageing
temperatures is investigated <strong>and</strong> well documented in literature [l35, 153, 288]. During
ageing process the aluminum in supersaturated matrix precipitates out as Mg17Al1;; in
two forms: coarse discontinuous (cellular) <strong>and</strong> fine continuous (transrgranular)
precipitates. Discontinuous precipitates are a cellular growth <strong>of</strong> altemating layers <strong>of</strong>
Mg17Al12 phase <strong>and</strong> near equilibrium magnesium matrix at high angle grain
boundaries. Growth <strong>of</strong> the discontinuous precipitation regions ceases relatively early
113