Creep is a time-dependent deformation of materials under stress at elevated temperatures.
The phenomenon of creep allows materials to plastically deform gradually over time, even
at stress levels below its yield point or below its transformation temperature. The issues
involving creep are especially significant for magnesium alloys, since they are susceptible
to creep deformation from temperatures as low as 100 ºC, which inhibits their potential
application in areas such as automotive engines.
The University of Canterbury has developed a significant level of experience and
infrastructure in the field of Electron Backscatter Diffraction (EBSD). EBSD allows
microstructures to be characterized by imaging the crystal structure and its crystallographic
orientation at a given point on a specimen surface, whereby the process can be automated
to construct a crystallographic “orientation map” of a specimen surface. In light of this, the
creep of magnesium and its alloys was studied using a novel technique, in which a
conventional tensile creep test was interrupted at periodic intervals, and the EBSD was
used to acquire the crystallographic orientation maps repeatedly on a same surface location
at each interruption stages. This technique allows simultaneous measurement of the rate of
creep deformation and the evolution of the specimen microstructure at various stages of
creep, bringing further insight into the deformation mechanisms involved.
This thesis summarizes the study of the microstructural and crystallographic texture
evolution during creep of pure magnesium and a creep resistant magnesium alloy Mg-
8.5Al-1Ca-0.3Sr. Pure magnesium exhibit a conventional “power-law” type creep, and
although its creep properties are well established in the past literatures, there has been little
in terms of reconciliation between the observed creep rates and the underlying deformation
mechanisms. The alloy Mg-8.5Al-1Ca-0.3Sr, on the other hand, is a modern die casting
alloy used in the automotive industry for engine and gearbox applications, and despite its
superior creep resistance, little is known about the microstructural contributions to its creep
properties.
This research was conducted to provide a link between the creep properties, observed
microstructures, and theories of creep deformation by the use of advanced microscopy
techniques. For the first time, the detailed, sequential microstructural development of
magnesium and its alloys during creep has been revealed.
| Identifer | oai:union.ndltd.org:canterbury.ac.nz/oai:ir.canterbury.ac.nz:10092/1576 |
| Date | January 2008 |
| Creators | Sato, Takanori |
| Publisher | University of Canterbury. Mechanical Engineering |
| Source Sets | University of Canterbury |
| Language | English |
| Detected Language | English |
| Type | Electronic thesis or dissertation, Text |
| Rights | Copyright Takanori Sato, http://library.canterbury.ac.nz/thesis/etheses_copyright.shtml |
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