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

1.3 CREEP BEHAVIOR OF AZ91 ALLOY
Chapter 1: Introduction
Creep is a special kind <strong>of</strong> plastic deformation which takes place when the
material is loaded at high temperature for a prolonged time. The load is very much
low compared to its yield strength. So, the deformation is not mainly due to the
applied load but temperature plays a major role. Based on applied stresses <strong>and</strong>
operating temperatures, the creep deformation mechanisms for most engineering
materials classified into main groups: the dislocation creep <strong>and</strong> the diffusion creep.
They can also be further subdivided into dislocation glide, dislocation climb, grain
boundary sliding, boundary diffusion (Nebrro~Herror creep) <strong>and</strong> bulk diffusion (Coble
creep) [16].
Very little is known about the high temperature properties <strong>of</strong> AZ91 alloy.
Only few elaborative creep studies were conducted on die cast AZ9l [17-27]. Most <strong>of</strong>
the literature suggests that the dislocation climb is the major creep mechanism in the
temperature range <strong>of</strong> 100-300°C with an initial load 20-100 MPa. However, some
literature proposed a grain boundary sliding mechanism at the low stress levels <strong>of</strong> 20­
40 MPa [28]. Dynamic discontinuous precipitates occur at the grain boundary during
creep is said to be responsible for such sliding. Detailed creep studies on gravity cast
AZ91 alloy is lack in literature. lt is said that dislocation creep is the only dominant
mechanism at all temperatures <strong>and</strong> stress ranges in ingot gravity casting [29].
Structural instability is observed during creep exposures in both die cast <strong>and</strong>
gravity cast AZ91 alloy [29-31]. However, this effect is more pronounced in die cast
alloy where the amount <strong>of</strong> Al supersaturated Mg matrix is large due to the faster
solidification. All the literature supports the formation <strong>of</strong> dynamic precipitation
during creep but it is still inconclusive about its influence on the creep behavior.
Occurrence <strong>of</strong> more dynamic discontinuous precipitates at the grain boundary in the
die cast AZ91 alloy <strong>and</strong> grain boundary sliding due to the precipitate formation is
reported [28]. In contrast, it is also observed that the formation <strong>of</strong> continuous
precipitates in die cast AZ91 provides precipitation hardening [29]. Strengthening due
continuous precipitates in ingot cast AZ91 is also reported [29].
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