Compositional Variants of Cu-rich Precipitate in Thermally Aged Ferritic Steel

Abstract

Atom probe tomography was utilized to investigate Cu precipitation in a high-strength low-alloy steel isothermally aged at 500°C for 1, 4, 16, and 64 h after water-quenching from 900°C. With prolonged aging time, the Curich precipitates (CRPs) increased in size and decreased in number density, and gradually evolved from spheroidal to elliptical in morphology. The small CRPs were rich in a high amount of Fe and a certain amount of Ni and Mn at their early nucleation stage. The large CRPs with increased size due to extensive aging contained less Fe and more Cu at their later growth stage. Additionally, Ni and Mn were both readily to segregate at the CRP/matrix heterophase interfaces, and Mn was higher in content than Ni in the precipitate interior especially when the CRPs were large in size. KEY WORDS: High-strength low-alloy steel; Thermal aging; Cu-rich precipitate; Atom probe tomography * 400°C [7-13]. Therefore, it is important to tailor the precipitation morphology of Cu-rich precipitates (CRPs) for attaining desired strength and toughness balance. Actually, the characteristics of CRPs in aspect of size, number density, shape, etc., that determine final mechanical properties are largely dependent of the compositional and resultant structural evolution during heat treatment process, and the segregation of Ni and Mn at the precipitate/matrix interface significantly prohibits the growth of CRPs to large size [14-21]. In additional, the compositional evolution of CRPs provides important information for composition modification, processing optimization, and property improvement of Cu-containing steels. However,

Atom Probe Tomography Study of Fe Segregation at Phase Interface in Zr–2.5Nb Alloy

“…1 Fabrication process of APT specimen by using FIB: a select an area of 10 μm × 2 μm on the surface; b isolate the volume, weld it to a micromanipulator and lift it out; c move the volume to a silicon post; d weld the volume to the silicon post; e, f cut off the volume with the micromanipulator; g milling stage; h cleaning-up to finish the fabrication of APT tip laser pulsing mode. To avoid the rupture of the APT specimen, a pulse repetition rate of 200 kHz and a laser energy of 70-100 pJ were used [18]. During the APT test, the temperature of the specimens was maintained at ~ 50 K while the standing voltage varied automatically to maintain a detection rate of 0.005 ion/pulse.…”

“…Therefore, accurate analysis of Fe distribution in Zr-Nb alloy is necessary. The atom probe tomography (APT) is known for the high atomic scale resolution and the good chemistry sensitivity in three dimensions (3D) [18]. In this work, the 3D distribution of Zr, Nb and Fe in bulk Zr-2.5Nb alloy was investigated using APT, in order to clarify the relationship between Fe segregation and β-Nb phase.…”

Low-carbon advanced nanostructured steels: Microstructure, mechanical properties, and applications

“…These elements enter the core at the early stages of precipitation and their concentration decreases with further aging [10]. Liu et al [101] reported that Mn is more readily to retain in the core as compared with Ni. Jiao et al [5] pointed out that the presence of Ni in the Cu-rich core is very important as it can encourage the Cu-rich precipitation by reducing the strain energy nucleation barrier.…”

High-resolution Transmission Electron Microscopy Characterization of the Structure of Cu Precipitate in a Thermal-aged Multicomponent Steel

“…The structural evolution of Cu precipitates in α-Fe is extremely complex. It is known that the Cu atoms initially experience a coherent precipitation to form a B2 or BCC structure in the BCC α-Fe [14], and then the metastable structures transform to an internally twinned martensitic 9R structure (the stacking sequence of the closely packed planes is ABC/BCA/CAB/A) by the repeated shear of {110} α-Fe planes with an orientation relationship (011) α-Fe // (11-4) 9R , [1][2][3][4][5][6][7][8][9][10][11] α-Fe // [-110] 9R [10]. The following possible changes in the 9R structure, including the detwinning of typical twin variants by migration of intervariant (twins) boundary, rotation of closed-packed (009) 9R basal planes to align more close to {110} α-Fe , and elimination of regular stacking faults on the (009) 9R basal planes [15], can produce a more stable 3R structure with non-cubic heavily distorted FCC structure [11].…”