DISLOCATION CONFIGURATIONS AND THE INITIATION OF YIELDING IN MAGNESIUM OXIDE.
Technical rept. no. 24,
HONEYWELL RESEARCH CENTER HOPKINS MINN
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The paper distinguishes between the role of fresh, aged, grown-in and high-temperature dislocations in the initiation of plastic flow in magnesium oxide single crystals. Grown-in dislocations in commercial purity crystals cannot be moved even under very high stresses and play no role in initiating slip at room temperature. Plastic deformation originates from fresh dislocation loops injected into the crystal surface. Fresh dislocations, introduced by prestrain at room temperature, become aged at high temperatures resulting in dislocation locking and an increase in stress to reinitiate slip. Weak locking occurs above 600C and strong locking above 1200C. Dislocations are almost completely immobilized by heating to 1900C. Changes in dislocation configuration upon heating were studied by etch pit and electron transmission techniques. It is considered that the immobility of grown-in, aged and high temperature dislocations is due to a combination of impurity locking, configurational locking and the reduction of internal stresses. Author