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.
Stress-strain curves of single crystals of magnesia compressed in the [IOO] direction are reported at temperatures from -196' to 1200°C.; curves are also shown for different rates of loading at room temperature. The crystals show considerable ductility at all temperatures and at room temperature can be deformed plastically about 6% before fracture at stresses which are about one-quarter of reported polycrystalline fracture strengths. The macroscopic yield drops apparently exponentially from an extrapolated value of 50,000 lb. per sq. in. at absolute zero to about 4500 lb. per sq. in. at temperatures of 900°C. and higher. Heat-treatment has an appreciable effect on the yield stress. The resistance of the material to deformation increases with the number of slip systems and bands activated because of the barriers to dislocation movements which occur at slip band intersections. At about 2 to 3% strain, stress concentrations begin to be relieved by small internal cracks which are not easily propagated. This effect is extensive before final macroscopic failure of the crystal occurs. Preliminary creep tests above the macroscopic yield stress and in the temperature range 800' to IOOO'C. show large instantaneous plastic deformations followed by slow constant-rate creep.
l a 1 T=1009'c U-718 P S I j I I / I I Rate process theory was applied in a study of the creep behavior of a fireclay refractory (firebrick) after different heat treatments. The analysis was based on the assumption that, in the absence of phase changes, equivalent deformation substructures are produced for a given strain at constant stress independent of temperature. An activation energy of about 170 kcal/mole was determined for the flow process in a heattreated firebrick, which corresponds closely to the value for mullite specimens. Varying but higher values obtained under different test conditions indicated that phase changes were also occurring during the creep tests. The effect of the amount and texture of the components on the creep behavior and strength of the firebrick specimens is discussed. Useful correlations based on this approach are presented, making it possible to predict creep behavior at constant or variable temperatures and also to calculate stress distributions through a wall containing a temperature gradient after a given strain is reached in a given time.
The high-temperature compressive strength of block pyrophyllite is not significantly less than the room-temperature strength at temperatures below 1200° and it is noticeably greater at 600° and 1100°C. The major increase in strength occurs above 1050°C and is associated with the decomposition of the pyrophyllite to mullite and silica phases. This increase in ultimate strength is time dependent and may amount to as much as five times the room-temperature strength. Because pyrophyllite is also used as a thermal insulator in high-temperature high-pressure applications, a shell of this stronger material will be present around samples heated for long times above 1050°C. This may adversely affect its efficiency as a solid pressure-transmitting medium. Pyrophyllite is superior to talc for similar high-temperature applications because it can be dehydrated prior to use without increasing its strength, and it develops its high-temperature strength more slowly.
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