Kinetics of G-phase precipitation and spinodal decomposition in very long aged ferrite of a Mo-free duplex stainless steel

Abstract

a b s t r a c tEvolution of spinodal decomposition and G-phase precipitation in ferrite of a thermally aged Mo-free duplex stainless steel was studied by Atom Probe Tomography (APT). Kinetics was compared to kinetics observed in ferrite of some Mo-bearing steels aged in similar conditions. This paper shows that formation of the G-phase particles proceeds via at least a two-step mechanism: enrichment of a/a 0 interdomains by G-former elements followed by formation of G-phase particles. As expected, G-phase precipitation is much less intense in the Mo-free steel than in Mo-bearing steels. The kinetic synergy observed in Mo-bearing steels between spinodal decomposition and G-phase precipitation is shown to also exist in Mo-free steel. Spinodal decomposition is less developed in the ferrite of the Mo-free steel investigated than in Mo-bearing steels: both the amplitude of the decomposition and the effective time exponent of the wavelength (0.06 versus 0.16) are much lower for the Mo-free steel. Neither the temperature of homogenisation nor quench effects or Ni and Mo contents could successfully explain the low time exponent of the spinodal decomposition observed in the Mo-free steel. The diffusion mechanisms could be at the origin of the different time exponents (diffusion along a/a 0 interfaces or diffusion of small clusters).

The two-step nucleation of G-phase in ferrite

“…In steels both with and without containing Mo as an alloying element, Pareige et al confirmed by using APT that the NieSieMn clusters contained the matrix elements (Fe and Cr), and that their concentration decreased with increasing aging time. Their results are consistent with the present study; however, inconsistent in that their NieSieMn clusters contained fewer Mn atoms than Si atoms [27,28]. APT analysis by Li et al on thermally-aged and irradiated CF3 cast duplex stainless steels also detected low-Mn NieSieMn clusters [29].…”

“…In their 5000-h aged sample all NieSieMn clusters were visible in DF images using the diffraction spot of the G-phase, whereas in our 5000-h aged samples no G-phase particles were detected in DF images and in electron diffraction patterns. The difference is presumably attributable to a difference in the alloy composition, specifically, the concentration of Mo; Mo is known to enhance the precipitation of the G-phase [27]. In steels both with and without containing Mo as an alloying element, Pareige et al confirmed by using APT that the NieSieMn clusters contained the matrix elements (Fe and Cr), and that their concentration decreased with increasing aging time.…”

Quantification of hardening contribution of G-Phase precipitation and spinodal decomposition in aged duplex stainless steel: APT analysis and micro-hardness measurements

“…B and G alloys were reannealed at 550 C for 2 h after their initial heat treatment ( Table 1). The microstructural characterization of A-350, B and C steels aged at 350 C were performed by Pareige et al [5,6] and Novy [16].…”

Effect of 475°C embrittlement on the low cycle fatigue behaviour of lean duplex stainless steels

“…Over the last decades, many materials characterization instruments have been vastly improved, and simultaneously, a number of new techniques have emerged. Among these techniques are Atom Probe Field Ion Microscopy and Atom Probe Tomography, which have been widely used to obtain an atomic level microstructural characterization of SD . On the other hand, scanning transmission electron microscopy (STEM) in combination with energy dispersive X‐ray spectroscopy (EDS) provides both microstructure and microchemical mapping of materials at nanometer scale.…”