Effect of Crystal Orientation on Plastic Deformation of Magnesium Oxide

Abstract

Stress-strain data for single crystals of MgO tested in compression with (110) and (11 1) loading axes are presented for temperatures ranging from 26' to 125OOC. Stress-strain data for polycrystalline MgO are also presented over the same temperature range. Single crystals with a (111) loading axis were found to deform plastically on the {loo) (110) slip systems at temperatures above 35OOC. The total strain at fracture for polycrystalline MgO at room temperature was about 0.6%; above 600'C it was about 2%. The general inability of the { 110) (110) slip systems of this structure to satisfy the Taylor requirement, i.e., the necessity of five independent slip systems, ease of cleavage, and slip nonuniformity, limits polycrystalline ductility at low temperatures. At higher temperatures, slip can occur on { 100) (110) slip systems, thus providing the additional slip ,systems necessary to satisfy Taylor's criterion ; also, stress-induced climb and high dislocation mobility inhibit cleavage fracture.

Dislocations and Plastic Deformation in MgO Crystals: A Review

“…They are referenced in Table 1. In the study of Hulse and collaborators, the authors showed that CRSS are larger in 1 ⁄2<110>{100} than in 1 ⁄2<110>{110} slip systems for temperatures ranging from 300 K to 1500 K [5]. This study emphasizes that only four out of six 1 ⁄2<110>{110} slip systems are effective under <100> and <110> compression, with Schmid factors of about 0.5 and 0.25, respectively.…”

“…Several additional studies addressed the effect of temperature on the CRSS for the 1 ⁄2<110>{110} slip systems [18][19][20][21][22] while only few datasets focus on 1 ⁄2<110>{100} [5,8,22]. These data are shown in Figure 2.…”

“…At low temperatures, this is normally preceded by the occurrence of fracture, such that no CRSS for dislocation glide can be obtained. In Figure 2, this is evident from the lack of data without cracking below 650 K [3,5]. In recent years, a new approach in mechanical testing has emerged, which overcomes this challenge of suppressing brittle fracture at low temperatures: the down-scaling of samples to the micrometer range in the so-called micro-compression technique [191,192].…”

“…A size effect on CRSS is observed on the soft slip system, while the previously unavailable data on the hard slip system correspond well to the extrapolation from higher temperatures due to its higher intrinsic stress level. Data from references [3,5,201].…”

“…Hulse, Copley and Pask performed a set of compression tests on single crystals investigating the CRSS and deformation processes in both slip modes [5,8,14]. Temperatures investigated range from 100 K up to 1400 K for the 1 ⁄2<110>{110} slip systems.…”