Dissemination of IT for the Promotion of Materials Science (DoITPoMS)

DoITPoMS Teaching & Learning Packages Slip in Single Crystals Slip in HCP metals 4: observing slip in cadmium
Slip in Single Crystals AimsBefore you startIntroductionSlip geometry: the critical resolved shear stressGeometry during slipSlip in HCP metals 1: slip systemsSlip in HCP metals 2: application of Schmid's LawSlip in HCP metals 3: calculation of forcesSlip in HCP metals 4: observing slip in cadmiumVideo clips of slip in a single cadmium crystalExercise: Determination of the critical resolved shear stress for slip in cadmiumSlip in CCP metalsSummaryQuestionsGoing further
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Slip in HCP metals 4: observing slip in cadmium

Cadmium is a hexagonal close-packed metal (with a non-ideal axial ratio c/a: more ). We have seen that it slips on <100>{001}.

Cadmium single crystals can be grown in the form of long cylinders. The crystal orientation of each crystal is random - i.e. no particular orientation is favoured. The specimen can be deformed under extension control, where the crystal is extended by a given amount and the load required to achieve that extension is recorded.

Typical load-extension curves obtained from such an experiment are shown below. Geometric softening may or may not be observed, depending on the geometry of the crystal.

Graph of load against extension

Schematic load-extension curves for tensile deformation of two cylindrical cadmium crystals with the same initial diameter but different crystallographic orientations. After an initial period of elastic deformation, both begin to plastically deform. The solid line shows the behaviour of a crystal showing geometric softening, where the load decreases over part of the extension. The broken line shows a sample where no geometric softening occurs. The two crystals yield at different loads because the Schmid factors for the primary slip systems are different but τc is constant for the material. Both crystals have the same Young's modulus.

As the crystal deforms, the geometry of the crystal changes according to the relationships given in Geometry during slip. The intersection of the slip planes with the surface of the crystal gives rise to steps and bands on the surface, which can be seen in the optical and scanning electron microscopes.

Scanning electron micrograph of undeformed cadmium single crystal

Scanning electron micrograph of undeformed cadmium single crystal. (Click on image to view larger version.)

Scanning electron micrograph of cadmium single crystal after deformation to 100% strain

Cadmium crystal after deformation to 100 % strain. (Click on image to view larger version.)

Scanning electron micrograph of cadmium single crystal after deformation to 200% strain

High magnification image of slip steps in cadmium crystal after deformation to 200 % strain. The tensile axis runs approximately from the bottom left corner to the top right corner of the image. (Click on image to view larger version.)

Video clips of a crystal undergoing deformation in the SEM are available on the following page.

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