Investigations on post service material showed that Alloy 800H pigtails from methanol producer Methanex have service lives ranging from 3 to 18 years. Because of this variability in service life, Alloy 800H creep performance was assessed and a new criterion for its procurement developed. The current criterion recommends an ASTM grain size of 5 (72µm) or coarser with no consideration given to grain size distribution, grain boundary types, or grain boundary network topology. Results from the investigation showed that this current criterion may produce variations in steady state creep rates of an order of magnitude between ASTM grain size 1 and 5, and a 2.5 times variation in creep ductility.
The ability to accurately reveal grain boundaries and assess grain boundary types is fundamental to the identification and quantification of coherent twin boundaries, and the measurement of average grain size and grain size distribution. EBSD mapping has the ability to distinguish grain boundary types using crystal orientation measurement. Grain size measurement from optical micrographs relies on morphological indicators to identify coherent twins. However, it is shown that many of the boundaries observed as straight line morphology on 2D sections did not possess {111} (coherent) interfaces.
3D reconstructions of Alloy 800H revealed the deficiencies in classifying geometry from two-dimensional (2D) sections. Σ3 Crystal volumes can be categorized as lamellar or edge structures. Lamellar structures are characterized by the appearance of parallel Σ3 boundary planes while an edge structure contains a single Σ3 interface. Sectioning plane location alters the perception of morphology. For simple twin structures, the tradition 2D classifications of morphology (complete parallel, incomplete parallel and corner Σ3) may all appear on a section plane from a single lamellar structure.
| Identifer | oai:union.ndltd.org:canterbury.ac.nz/oai:ir.canterbury.ac.nz:10092/9949 |
| Date | January 2014 |
| Creators | Gardiner, Benjamin Robert |
| Publisher | University of Canterbury. Mechanical |
| Source Sets | University of Canterbury |
| Language | English |
| Detected Language | English |
| Type | Electronic thesis or dissertation, Text |
| Rights | Copyright Benjamin Robert Gardiner, http://library.canterbury.ac.nz/thesis/etheses_copyright.shtml |
| Relation | NZCU |
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