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Investigation of the reversible hysteresis effect in hexagonal metal single crystals and the max phases
Dissertation   Open access

Investigation of the reversible hysteresis effect in hexagonal metal single crystals and the max phases

Justin P. Griggs
Doctor of Philosophy (Ph.D.), Drexel University
Dec 2015
DOI:
https://doi.org/10.17918/etd-7427
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Abstract

Metal crystals Heat resistant materials Materials Science
Hexagonal close packed (hcp) materials are abundant in nature, and are of great technological importance since they are used in many applications. When cyclically loaded some hcp solids outline fully and spontaneously reversible stress-strain hysteresis loops. To date, the micromechanical origin of these loops is unknown. To shed light on the subject, a spherical nanoindenter was repeatedly indented - up to 50 times in the same location - into Mg, Zn and Ti₃SiC₂ single crystals of various orientations, followed by select, post-indentation transmission electron microscope (TEM) cross sectional analysis. Based on the totality of the results, the energy dissipated per unit volume per cycle in the hexagonal metals can be related to the bowing out - and back - of geometrically necessary dislocations - in most cases in the form of low angle kink boundaries (LAKBs) - through statistically stored ones. Kinks were observed after indentations normal to the basal planes in Mg and Zn and also when indented normal to the (101̄1) and (101̄2) planes in Zn. When indented parallel to the basal planes in Zn, if hysteresis loops formed at all, they were insignificant in area. When (101̄0) planes in Mg were indented, tensile twins formed. The most probable explanation for the energy dissipated in this direction is the growth and contraction of these twins. In the case of Ti₃SiC₂, hysteresis loops were observed even in absence of kink boundaries. No direct evidence for twins or non-basal slip was found nor has been reported in literature. Evidence presented in this study supports the existence of a new type of defect in bulk layered solids known as ripplocations - which combine features of dislocations and surface ripples - that are able to explain the phenomena observed in this study on Ti₃SiC₂ in ways conventional dislocations cannot explain. It is the migration of these ripplocations that are believed to cause energy dissipation in Ti₃SiC₂. The energy dissipation due to ripplocations was found to be higher than the energy dissipation due to dislocations, which may offer a possible signature to distinguish between the two. However, the simplest method to distinguish between ripplocations and dislocations is to load the basal planes edge-on under a spherical indenter as carried out here. The formation of cracks, normal to the basal planes, are the unequivocal signature of ripplocations.

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