Thermal stability of nanocrystalline surface layer of AISI 321 stainless steel. (December 2017)

• Record Type: Journal Article
• Authors: Pour-Ali, Sadegh; Kiani-Rashid, Ali-Reza; Babakhani, Abolfazl; Virtanen, Sannakaisa
• Is Part Of: Vacuum. Volume 146(2017)
• Page Start: 297
• Page End: 303
• Publication Date: 2017-12
• Subjects: Nanocrystalline -- Vacuum annealing -- Thermal stability -- TEM -- Grain growth -- Dislocation annihilation; Vacuum -- Periodicals; 621.55
• DOI: 10.1016/j.vacuum.2017.09.053
• Languages: English
• ISSNs: 0042-207X

Abstract

Abstract: A nanocrystalline layer with an average grain size of 75 nm was formed on the top surface layer of 321 stainless steel (321SS) alloy via severe shot peening (SSP). Grazing incidence X-ray diffraction (GI-XRD) and transmission electron microscopy (TEM) were applied to characterize the grain size changes, phase compositions and microstructure evolution after vacuum annealing at temperatures ranging from 200 °C to 1000 °C. The results revealed that nanograins slowly grow to ≈411 nm as the temperature increases to 600 °C. Dislocation annihilation is shown to be responsible for the changes in grain size up to 600 °C. However, an abnormal grain growth is observed at annealing temperatures of 800 °C and 1000 °C in which grains grow to ≈1267 nm and ≈2012 nm, respectively. This abnormal grain growth is attributed to the synergistic effect of dislocation annihilation in grain boundaries and triple points, re-arrangement of dislocations, and formation of middle angle (13°–17°) grain boundaries. Transformation of stress induced martensite (α′) to austenite (γ) and microhardness evolution in the nanocrystalline top surface layer during annealing are also discussed. Highlights: Nanograins boundaries start to dissociate from 200 °C by the emission of dislocations. Nanograins growth linearly from room temperature (≈75 nm) to 600 °C (≈411 nm). Sharp grain growth of nanocrystalline surface layer of 321 ASS occurs at 600 °C. Formation of middle angle (13°–17°) GBs plays a key role at 800 °C and 1000 °C.