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Title: Slip deformation of crystalline mercury between 77°K and 234°K
Author: Aytas, S. Isak
ISNI:       0000 0001 3587 2835
Awarding Body: University of Surrey
Current Institution: University of Surrey
Date of Award: 1971
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Various orientations of single crystals of mercury, grown by a modified Bridgman technique from triply-distilled mercury containing << 3 ppm non-gaseous impurities have been tested by four-point bending and by tension in the temperature range 77°K-234-°K. The predominant deformation mode was crystallographic {111} slip at all temperatures. Wavy slip in the close-packed > direction operates very readily at about 200°K in crystals for which the associated Schmid factor is sufficiently large. In this case slip can occur on any plane containing this direction, and the macroscopic slip plane tends to lie near the maximum resolved shear stress plane. The results of the bend-tests indicated that slip asymmetry can be pronounced for slip in the direction. Whether this effect is due to structural asymmetry influencing the cores of the dislocations involved in the slip processes, or whether it is due to opposite normal stresses across the slip planes is however difficult and perhaps impossible to ascertain. Wear the melting point temperature it was shown that non-crystallographic slip in the direction can also operate particularly for the bend-tests in which the choice of the bend-axis influences the slip systems and the macroscopic slip plane. This unexpected behaviour appears to arise because of the constraints imposed by the bending mode of deformation. In the case of wavy slip the macroscopic slip plane tends to be nearer the crystallo-graphic {111} slip plane than the m.r.s.s.p. At lower temperatures slip in irrational directions has been observed in several crystals, and the operative slip plane was determined as crystallographic {111}. The orientation dependence of this slip mode is interpreted using Schmid-factors for the most highly stressed variants of the maximum resolved shear stress direction, {111} slip system and the , m.r.s.s.p. slip system. Tensile tests at about 200°K gave critical resolved shear stresses of 0.08 +/- 0.02 Nn[-2] and 0.06 +/- 0.02 Nm[-2] for {111} and {111} slip respectively. The ratio of these values is consistent with the ratio of at least 1.9 deduced from Schmid factors, although in the bend-tests a ratio of 1.3 was obtained. The resolved shear stress for slip at yield is both temperature and orientation dependent for temperatures below about 150°K, and below 84°K a critical value for the r.s.s.s. in , and irrational directions cannot be deduced. At 77° K, the slip systems '{ }' and {111} '' are approximately four times larger than those for the system {111} . The orientation dependence of the r.s.s.s. at yield cannot be attributed either to the normal stresses across the slip planes' or to slip in more than one system. A critical resolved shear strain criterion is also considered but unfortunately cannot be fully investigated until accurately determined values of the six elastic constants for all test temperatures become available. It is concluded that the controlling factor for the thermal component of the r.s.s.s. at yield is probably some average between a c.r.s. stress and a c.r.s. strain criterion.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available