Prestrain, Cavitation, and Creep Ductility

Abstract

Specimens of Nimonic 80A have been prestrained in tension at room temperature by various amounts up to 15% elongation. Each prestrained specimen was crept at 750° C at one of two stresses, 154 N/mm² or 460 N/mm², which gave unprestrained creep lives of 1800 and 5 h, respectively. Some of the tests were interrupted after 160 h but most specimens were fractured. At both stresses, the material suffered progressive loss of creep strength, life, and fracture ductility as prestrain increased, while quantitative optical metallography revealed a concomitant dramatic increase in the number of grain-boundary cavities. Cavity density and size measurements have established that creep ductility is reduced because of increased cavity density and not because of any increase in cavity growth rate. It is suggested that the loss in creep strength is probably due to an increase in the mobile dislocation content with cold work.