Liiders bands

In Sect. 4.11 the question has been raised whether Liiders bands exist in ceramics. A report specifically refers to the observation of Liiders bands in a two-phase ceramics. The material is high temperature precipitation hardening Y203-partiaUy... 

Fig. 4.79 a Optical micrograph of Liiders band in deformed specimen ((110) face) arrows show polishing scratch sheared by first Liiders band, b Scanning electron micrograph of Liiders band on (111) face [2]. With kind permission of John Wiley and Sons... 

A 10 /i. The Liiders bands seen in Fig. 4.79 are parallel to the 001 slip planes. The large yield drop corresponds to the first Liiders band formation while the formation of additional Liiders band is associated with the serration of the stress-strain curve. Many specimens tested gave the same stress-strain curves with Liiders bands formation. 

They are almost completely elastic until the upper yield strength Ren (UYS) is reached. At this stress, plastic deformation sets in rather suddenly, which is localised in so-called Liiders bands or flow lines. While the stress oscillates, these lines extend until they cover the whole specimen. The lowest stress occurring during this process is called lower yield strength i eL (lys). Why this localised plastic deformation occurs, will be explained in section 6.4.3. After the specimen has plastified completely, it behaves identical to a metal without apparent yield point. 

Although our evidence is indirect, we feel the observed periodicity is due to the formation of incipient deformation bands, or Liider s bands, in the specimen. Such bands have been observed in polystyrene (ll.>i2) in compression and in preoriented polystyrene specimens in tension and in torsion (12). Such bands have also been observed in poly(vinyl chloride) (13). In polystyrene the angles between the band axis and the axis band of deformation were 38° in compression and 52° in tension (12). (Our results are calculated arbitrarily for the intermediate value of 45°.) The measured widths in polystyrene are 1-10 pm (] 1,1 ). Our results are in reasonable agreement with this band size. 

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