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

4.5 CREEP BEHAIVOR
i g Chapter 4: Results <strong>and</strong>Discussion
The role <strong>of</strong> different alloying additions to AZ91 alloy on its creep behavior is
presented in this section. The creep behavior <strong>of</strong> alloys was investigated at 150 <strong>and</strong>
200°C with an initial stress <strong>of</strong> 50 MPa. Two type <strong>of</strong> creep testing was carried out in
the present study: creep testing for 500 h (referred as short term test) <strong>and</strong> creep testing
till fracture (referred as long term test). It is found that the base alloy’s creep life at
150°C is around 6000 h, which means the creep testing is time consuming at 150°C.
Hence, all the alloys were initially creep tested at 150°C for 500 h. Based on the result
<strong>of</strong> 500 h test, few samples were selected <strong>and</strong> creep test till fracture was carried out.
But in the present investigation, it is found that, in most <strong>of</strong> the cases, the steady stage
creep is not reached even at 500 h. Variation in minimum creep rate is observed
between 500 h test <strong>and</strong> long term testing. For example, the minimum creep rate <strong>of</strong>
base alloy AZ91 at 150°C in short term test (500 h test) is 9.348 x 10*‘ % h" whereas
during long term test (till fracture) it is 5.9239 X 104 % h". <strong>Si</strong>milarly, the minimum
creep rate <strong>of</strong> AZ9l+0.5% <strong>Si</strong> alloy in short term testing is 5.0825 X 10'4 % h" compare
to the value <strong>of</strong> 3.5803 X 10"“ % h" during long term testing. This indicates that
secondary steady state creep reaches only after 500 h. However, it is interesting to
note that the creep behavior <strong>of</strong> any two alloys, when compared to each other, exhibits
similar trend in both testing i.e., the alloy showed lesser creep resistant in 500 h (in
terms <strong>of</strong> minimum creep rate) compared to the other alloy, also showed lesser creep
resistance in long time testing too. Moreover, most <strong>of</strong> the literature suggests that 100
h creep behavior could be taken as a comparative tool while comparing the creep
behavior <strong>of</strong> different alloys [22], 313, 314]. Hence, in the present study, 500 h test
was considered as a tool to select samples for long term creep testing. This procedure
minimized the total testing time. Alloys, which exhibit good creep resistance at
150°C, were further subjected to creep testing at 200°C.
4.5.1 AZ91 Alloy
Figure 4.52 (a) show the creep curve <strong>of</strong> AZ91 base alloy obtained at 150°C
with an initial stress <strong>of</strong> 50 MPa. It resembles a typical creep curve, which has a short
primary <strong>and</strong> long secondary region followed by an extended tertiary creep region. The
creep rate continuously decreases during primary stage to reach a constant creep rate
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