Abstract
A statistical study of the structural changes during creep of Nimonic 108 has shown that under stressed conditions recrystallization and grain growth can occur at temperatures as low as 800°C, which is appreciably below the recrystallization temperature of this material in unstressed conditions. Additionally, longitudinal stringers of massive carbides present in the extruded alloy inhibit lateral grain growth and cause the grains to grow preferentially in a longitudinal direction. Metallographic evidence was found for grain-boundary sliding during creep at above 700° C. However, at above 800° C the measured sliding values were greatly in error owing to the effect of the directional grain growth taking place during creep. Usually recrystallization gives exaggerated values of grain-boundary sliding but, because of the directed grain growth occurring in these alloys during creep, values of ‘apparent grain strain’ higher than the total specimen strain were obtained. Such results could lead to the obviously anomalous postulation of a negative grain-boundary-sliding contribution under tensile conditions, unless the structural changes are fully appreciated.
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