“Necklace” structure of the 4th generation powder metallurgy superalloy after hot deformation

• Main Authors: HOU Qiong, TAO Yu, JIA Jian
• Format: Article
• Language: zho
• Published: Journal of Materials Engineering 2019-03-01
• Series: Journal of Materials Engineering
• Subjects: powder metallurgy superalloy; “necklace” structure; low angle grain boundary; recrystalli-zation
• Online Access: http://jme.biam.ac.cn/CN/Y2019/V47/I3/94

The "necklace" structure of an advanced 4th generation powder metallurgy(PM)superalloy after isothermal forging was investigated by means of scanning electron microscope, transmission electron microscope and electron back scattering diffraction technique. The formation mechanism and elimination method of the "necklace" structure were studied. The results show that after multiple isothermal forging, most region of the forging are fully recrystallized, and equiaxed fine-grained microstructures are obtained. Nevertheless, a typically incomplete recrystallization microstructure, namely the "necklace" structure, is formed nearing the upper and lower end faces of the forging. A great amount of fine recrystallized grains are distributed around the non-equiaxed deformed grains. The deformed grains contain a high density of low angle grain boundaries, so it means plenty of entangled dislocations exist. The samples containing "necklace" structure are annealed over the temperature range from 1080℃ to 1180℃. With the increasing temperature, both the recrystallization volume fraction and recrystallization grain size are increased gradually. The "necklace" structure of the alloy forging is basically eliminated after recrystallization annealing at 1150℃. Therefore, the uniform fine-grained disk can be obtained, which meet the requirements of dual microstructure heat treatment.