Abstract
Surface detail and the underlying dislocation substructures have been examined in single crystals of beryllium after bend fatigue at room temperature. Crystals were oriented for preferential slip on the basal plane or on a <mml:math><mml:mrow><mml:mo>{</mml:mo><mml:mn>10</mml:mn><mml:mover><mml:mn>1</mml:mn><mml:mi>¯</mml:mi></mml:mover><mml:mn>0</mml:mn><mml:mo>}</mml:mo></mml:mrow></mml:math> prism plane. A persistent slip structure, in the form of closely spaced pits, exists below the bend surface, the depth of penetration depending upon the stress level. Discoloration of the bend surface accompanied duplex slip in crystals oriented for prism slip. Dislocation loops constitute the most common form of the fatigue substructure. The loops are classified as: long loops ≥ 1000 Å in length elongated in <mml:math><mml:mrow><mml:mo>〈</mml:mo><mml:mn>10</mml:mn><mml:mover><mml:mn>1</mml:mn><mml:mi>¯</mml:mi></mml:mover><mml:mn>0</mml:mn><mml:mo>〉</mml:mo></mml:mrow></mml:math> directions; elongated loops formed from the longer loops by a “pinching-off” process; small loops of approximately circular shape having dia. down to ∼70 Å. This form of substructure is compared to that resulting from tensile loading. Diffraction contrast effects indicate that for the elongated loops the loop vector is of the form <mml:math><mml:mrow><mml:mo>⅓</mml:mo><mml:mo>〈</mml:mo><mml:mover><mml:mn>2</mml:mn><mml:mi>¯</mml:mi></mml:mover><mml:mn>110</mml:mn><mml:mo>〉</mml:mo></mml:mrow></mml:math> and that the loop plane can rotate. The loop plane and vector of the small circular loops have not been unambiguously determined. Recovery of the bend-fatigue substructure can be promoted in thin foils by prolonged exposure in the electron microscope at high beam intensities. However, in bulk fatigued crystals, temperatures of ∼300° C are required for dislocation rearrangements.
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