G-phase Precipitation Research Articles

Irradiation-induced precipitation and inverse coarsening of G-phase in austenitic stainless steel weld metal

The irradiation-induced precipitation and inverse coarsening of G-phase in austenitic stainless steel weld metal were studied by using transmission electron microscopy and atom probe tomography. Following 0.5-dpa (displacement per atom) irradiation, Ni/Si/Mn/P-enriched G-phase was observed in delta-ferrite. By increasing the irradiation dose from 0.5 to 3 dpa, coarsening of G-phase occurred at first, followed by an obvious inverse coarsening. The segregation of Ni and Si atoms at α/α′ interface due to spinodal decomposition was directly observed, which contributed to the G-phase precipitation. The inverse coarsening of G-phase was caused by irradiation recoil and could be interpreted by the cell model.

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

Ageing of cast duplex stainless steels (DSS) is attributed to the decomposition of the ferrite: spinodal decomposition and precipitation of G-phase particles. This leads to an increase in hardness and a decrease in Charpy toughness. According to the literature, spinodal decomposition is accepted to play a major role on the hardening even if the role of G-phase precipitation on mechanical properties is still not clear. This work links microstructural characterization performed using atom probe tomography to micro-hardness of the ferrite for a wide variety of duplex steels (from cast steels with and without Mo to lean steels) aged under different conditions. An attempt to quantify the contribution of both spinodal decomposition and G-phase precipitation is made by applying linear and square superposition principle of Ardell, Orowan and a modified BKS models. The models used are shown to give an excellent estimation of the experimental values of the hardness increase of the ferrite of the cast and lean steels for a wide range of composition and temperature. This work shows that, conversely to what is said in the literature, spinodal decomposition is not systematically the main contributor to hardening.

A novel ferritic steel family hardened by intermetallic compound G-phase

In this paper, G-phase precipitation and the resulting hardening effect on Fe-20Cr-3Ni-1Mn-3Si ferritic alloys by the addition of Ti, Nb, Ta and Zr were individually studied by electron microscopy and atom probe tomography, combined with thermodynamic and first principle calculations. The high resolution scanning electron and transmission electron microscopy observations confirmed that four kinds of Ni16M6Si7 (M=Ti, Nb, Ta and Zr) G-phase particles were distributed uniformly in the matrix of the four alloys aged at 560–860 °C. The 3D-APT results revealed the formation of the four different nanoscale precipitates, with the highest number density (6.05 × 1023 m−3) and smallest radius (1.64 ± 0.45 nm) in the Ti added alloy. The nanoscale sized precipitates stability (~3 nm for Ti added alloy; ~5 nm for Nb added alloy and Zr added alloy; ~25 nm for Ta added alloy) was attributed to their particular cube-cube orientation relationships, which led to a very low interfacial energy. The effects of the G-phase on precipitation hardening and quasi steady-state deformation resistance were examined. The 560 °C-aging hardening experiments suggested a rapid precipitation process of the nano-particles. The peak hardness values of the four alloys are in the descending order: Ti ˃˃ Nb > Ta ˃˃ Zr. The 660 °C quasi steady-state deformation experiments showed threshold stresses of 110 and 140 MPa in cases of the Nb and Ti added alloys, which were higher than that of the previously reported B2-NiAl strengthened steels and commercial heat-resistant steels. By using the formation of nanoscale G-phase precipitates, a new ferritic steel family with very high strength and creep resistance has been proposed. Further alloy optimization is in progress.

Evaluation of the thermal aging of \u03b4-ferrite in austenitic stainless steel welds by electrochemical analysis

Cr-segregation by spinodal decomposition and G-phase precipitation were observed in δ-ferrite of austenitic stainless steel welds thermally aged at 400 °C for up to 20,000 h. A reversion heat treatment (R-HT) at 550 °C for 1 h dissolved the Cr-segregation in the aged welds while some intermetallic precipitates were present. The double-loop electrochemical potentiokinetic reactivation (DL-EPR) analysis showed no significant differences among them. However, after selective etching of the austenite phase, the DL-EPR values of δ-ferrite phase steadily increased with aging time due to the growth of Cr-depleted regions by spinodal decomposition. The electrochemical behavior of δ-ferrite after R-HT condition was similar to that of unaged welds, indicating the intermetallic precipitates did not affect the corrosion resistance in this case. Overall, DL-EPR analysis of δ-ferrite phase provided better correlation with spinodal decomposition.

The formation and structure of Fe-Mn-Ni-Si solute clusters and G-phase precipitates in steels

Solute clustering and G-phase precipitation cause hardening phenomena observed in some low alloy and stainless steels, respectively. Density functional theory was used to investigate the energetic driving force for the formation of these precipitates, capturing temperature effects through analysis of the system's configurational and magnetic entropies. It is shown that enrichment of Mn, Ni and Si is thermodynamically favourable compared to the dilute ferrite matrix of a typical A508 low alloy steel. We predict the ordered G-phase to form preferentially rather than a structure with B2-type ordering when the Fe content of the system falls below 10–18 at. %. The B2 → G-phase transformation is predicted to occur spontaneously when vacancies are introduced into the B2 structure in the absence of Fe.

Mechanical property degradation and microstructural evolution of cast austenitic stainless steels under short-term thermal aging

Mechanical testing and microstructural characterization were performed on short-term thermally aged cast austenitic stainless steels (CASS) to understand the severity and mechanisms of thermal-aging degradation experienced during extended operation of light water reactor (LWR) coolant systems. Four CASS materials–CF3, CF3M, CF8, and CF8M–were thermally aged for 1500 h at 290 °C, 330 °C, 360 °C, and 400 °C. All four alloys experienced insignificant change in strength and ductility properties but a significant reduction in absorbed impact energy. The primary microstructural and compositional changes during thermal aging were spinodal decomposition of the δ-ferrite into α/α′, precipitation of G-phase in the δ-ferrite, segregation of solute to the austenite/ferrite interphase boundary, and growth of M23C6 carbides on the austenite/ferrite interphase boundary. These changes were shown to be highly dependent on chemical composition, particularly the concentration of C and Mo, and aging temperature. The low C, high Mo CF3M alloys experienced the most spinodal decomposition and G-phase precipitation coinciding the largest reduction in impact properties.

Influence of Ni, Mo and Mn Content on the G-Phase Precipitation and Spinodal Decomposition of Aged Duplex Stainless Steels.

Influence of Ni, Mo and Mn Content on the G-Phase Precipitation and Spinodal Decomposition of Aged Duplex Stainless Steels.

Radiation induced segregation and precipitation behavior in self-ion irradiated Ferritic/Martensitic HT9 steel

In this study, Ferritic/Martensitic (F/M) HT9 steel was irradiated to 20 displacements per atom (dpa) at 600 nm depth at 420 and 440 °C, and to 1, 10 and 20 dpa at 600 nm depth at 470 °C using 5 MeV Fe++ ions. The characterization was conducted using ChemiSTEM and Atom Probe Tomography (APT), with a focus on radiation induced segregation and precipitation. Ni and/or Si segregation at defect sinks (grain boundaries, dislocation lines, carbide/matrix interfaces) together with Ni, Si, Mn rich G-phase precipitation were observed in self-ion irradiated HT9 except in very low dose case (1 dpa at 470 °C). Some G-phase precipitates were found to nucleate heterogeneously at defect sinks where Ni and/or Si segregated. In contrast to what was previously reported in the literature for neutron irradiated HT9, no Cr-rich α′ phase, χ-phases, η phase and voids were found in self-ion irradiated HT9. The difference of observed microstructures is probably due to the difference of irradiation dose rate between ion irradiation and neutron irradiation. In addition, the average size and number density of G-phase precipitates were found to be sensitive to both irradiation temperature and dose. With the same irradiation dose, the average size of G-phase increased whereas the number density decreased with increasing irradiation temperature. Within the same irradiation temperature, the average size increased with increasing irradiation dose.

An investigation on microstructure and mechanical property of thermally aged stainless steel weld overlay cladding

Microstructural evolution and mechanical property change of E308L stainless steel weld overlay cladding aged at 400 °C for 400, 1000 and 5000 h were investigated by transmission electron microscope (TEM) and small punch test (SPT). The results indicated that thermal aging had no obvious effect on the volume fraction of ferrite, but can cause microstructural evolution by spinodal decomposotion and G-phase precipitation in the ferrite phase. Spinodal decomposition took place after aging up to 1000 h, while G-phase formed along dislocations, and growed up to 2–11 nm after aging for 5000 h. The total energy for inducing deformation and fracture by the small punch tests decreased with the increase of thermal aging time, and this decline was associated with spinodal decomposition and G-phase precipitation. Plastic deformation of the aged ferrite proceeded via formation of curvilinear slip bands. Nucleation of microcracks occurred at the δ/γ interface along the slip bands. The hardening of the ferrite and high stress concentration on δ/γ phase interface resulted in brittle fracture and phase boundary separation after thermal aging.

Aging Degradation of Austenitic Stainless Steel Weld Probed by Electrochemical Method and Impact Toughness Evaluation

The present study discriminates the spinodal decomposition and G-phase precipitation in stainless steel welds by double loop electrochemical potentio-kinetic reactivation method and correlates it with the degradation in toughness property. The welds produced with different heat inputs were aged up to 10,000 hours at 673 K to 723 K (400 to 450 °C) and evaluated subsequently for the degree of sensitization (DOS) and impact toughness. The DOS values obtained were attributed to the spinodal decomposition and precipitation of G-phase. Study shows that the DOS correlates well with the impact toughness of the 304LN weld. Prolonged aging at 673 K and 723 K (400 °C and 450 °C) increased the DOS values while the impact toughness was decreased. The weld fabricated at 1 kJ/mm of heat input, produced higher DOS, compared to that at 3 kJ/mm. The geometrical location along the weld is shown to influence the DOS; higher values were obtained at the root than at the topside of the weld. Vermicular and columnar microstructure, in addition to the spinodal decomposition and G-phase precipitation, observed in the root side of the weld appear risky for the impact toughness.

核电站堆内构件用304不锈钢焊缝热老化行为研究

压水堆核电站堆内构件用奥氏体不锈钢焊缝在长期服役过程中会面临着热老化脆化。本文开展了核电站堆内构件用304不锈钢焊缝在325℃、365℃和400℃下15,000 h的加速热老化试验，利用TEM和HRTEM研究了热老化后焊缝微观组织演变，采用冲击实验设备和纳米力学探针分别对热老化后焊缝冲击行为和纳米硬度变化进行了测试，并使用SEM观察了冲击断口形貌。结果表明：热老化后，焊缝内铁素体相发生了调幅分解；随着热老化温度增加，焊缝的冲击韧性显著下降，冲击断口呈现解理断裂特征；焊缝中铁素体相塑形变形能力不断下降，纳米硬度快速增加，而奥氏体相纳米硬度未发生改变。并以焊缝铁素体纳米硬度作为热老化程度指标，利用阿累尼乌斯Arrhenius方程得出304不锈钢焊缝在325℃~400℃内的热老化激活能约为80.4 kJ/mol。 The stainless steel welds used for PWR internals after a long service faces thermal aging embrit-tlement. In this study, the accelerated thermal aging experiments of nuclear internals 304 stainless steel weld were performed at 325˚C, 365˚C and 400˚C for 15,000 h. Microstructure evolution of weld after aging was observed by TEM and HRTEM. Changes of the Charpy impact energy and na-nohardness were studied by Charpy impact and nano-indenter test. The results indicated that Spinodal decomposition and G-phase precipitation in the ferrite was observed after aging. The impact toughness of weld decreased with the increase of aging temperature, and cleavage feature was observed after aging. The nanohardness of ferrite increased, plastic deformation ability of ferrite phase declined, and the nanohardness of austenite hardness had no change. Taking the nanohardness as the standard of thermal aging embrittlement, the thermal aging activation energy was determined for 304 weld in the temperature range 325˚C - 400˚C based on Arrhenius rela-tionship.

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

Evolution 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 α/α′ inter-domains 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 α/α′ interfaces or diffusion of small clusters).

Effects of long term thermal aging on high temperature tensile deformation behaviours of duplex stainless steels

Tensile deformation behaviours of unaged and thermal aged duplex stainless steels (DSS) were investigated at 350°C in order to understand the effects of long term thermal aging on the high temperature deformation behaviours of DSS. After aging for 20 000 h, the strength of DSS has a slight increase, the plasticity has a considerable decline, and the tensile fracture transfers from ductile to brittle. Nanoindentation tests indicate that ferrite has a considerable increase in hardness, and austenite has only a negligible increase with aging time. Thermal aging embrittlement is primarily concerned with ferrite. After long term thermal aging, spinodal decomposition and G-phase precipitation occur in ferrite and these reactions result in the dramatic decline of the ferrite phases' deformation ability. Cleavage cracks can easily initiate and propagate in ferrite of long term thermal aged DSS.

G-phase precipitation in duplex stainless steels after long-term thermal aging: A high-resolution transmission electron microscopy study

High-resolution transmission electron microscopy was used to investigate the G-phase precipitation behavior in duplex stainless steels long-term thermally aged at 300–450°C. Rising temperature can accelerate thermal aging kinetics and shorten the incubation period of G-phase. G-phase adopts a cube-on-cube orientation relationship with the ferrite matrix. Transformation from ferritic cluster to G-phase can be accomplished through smaller atomic position readjustments. After aging at 400°C for 20,000h, G-phase grows up to 4–5nm and the interface between G-phase and the ferrite matrix still maintains semi-coherent. Disarrangement of crystal planes causes misfit dislocations and strain fields around G-phase.

Kinetics of secondary phase precipitation during spinodal decomposition in duplex stainless steels: A kinetic Monte Carlo model – Comparison with atom probe tomography experiments

This paper presents the first simulations of the kinetics of spinodal decomposition and G-phase precipitation occurring in ferrite of duplex stainless steels. The kinetics was simulated using a simple but effective atomic kinetic Monte Carlo model in a ternary alloy. The simulations reproduced the α/α′ spinodal structure with precipitates at the α/α′ interface. The comparison of simulated results with experiments shows that the simulations quantitatively reproduce the kinetics of phase transformation and the synergy observed experimentally between the spinodal decomposition and G-phase precipitation. By following the vacancy pathway, we show that the coarsening of G-phase precipitates proceeds via diffusion along the α/α′ interfaces. The simulations made it possible to explain the origin of the kinetic synergy between spinodal decomposition and G-phase precipitation.

Effects of radiation on spinodal decomposition of ferrite in duplex stainless steel

Abstract Duplex stainless steel specimens embrittled by temperature-accelerated thermal aging at 400 °C for 40,000 h were irradiated at 300 °C to 1 dpa with 6.4 MeV Fe3+ ions to study the effects of radiation on spinodal decomposition of ferrite. The microstructural change was examined by atom probe tomography, and the hardness change was measured with an ultra-micro hardness tester. Hardening of the ferrite in thermal aged specimens was reduced by irradiation, whereas the ferrite of the unaged specimen was hardened by it. The spinodal decomposition of the ferrite into Fe-rich α phase and Cr-rich α′ phase, and G-phase precipitation occurred after the thermal aging. Fluctuation of the Cr concentration based on the formation of Cr-rich α′ phase decrease by irradiation. This suggested that irradiation caused the disappearance of spinodal decomposition. The decrease in spinodal decomposition correlated with a decrease in hardness.

Microstructures and mechanical properties of cast austenite stainless steels after long-term thermal aging at low temperature

The cast austenite stainless steels were investigate in order to understand the microstructural evolution and mechanical properties in the long-term thermal aging at 400°C for up to 20,000h. Spinodal decomposition and G-phase precipitation in ferrite after long-term thermal aging lead to the degradation of mechanical properties. Ferrite hardness increases with aging time, but the austenite hardness does not change. Tensile strength is not strongly affected by aging time, but the plasticity has a significant decrease after long-term aging. Under impact with high strain rate, the ferrite phases deform by the way of deformation twinning. High stress concentration on the phase boundaries cause the phase boundary separating and the austenite’s tearing off.

THE MICROSTRUCTURE AND TENSILE FRACTURE BEHAVIOR OF LONG TERM THERMAL AGED Z3CN20-09M STAINLESS STEEL

Cast austenite stainless steels (CASS) were widely used in the primary circuit piping of pressurized water nuclear reactors (PWRs), for their excellent mechanical behavior, corrosion resistance and good weldability. CASS, however, were known to have a tendency of thermal aging embrittlement after long term service at mid-temperature (about 280—320 ). The microstructures, micro-mechanical properties and tensile fracture behaviors of Z3CN20-09M stainless steels, thermal aged at 400 for 2×10 4 h, were studied in this work. TEM was utilized to observe the microstructure. A nano-indenter was used to investigate the micro-mechanical properties. The tensile tests were completed by an in situ fatigue tester. The tensile surface morphology was observed in SEM and the fracture of thermal aged CASS was examined by an electron probe micro-analyzer (EPMA). The results indicated that the spinodal decomposition and G-phase precipitation distributed in ferrite of the aged CASS. The nano-hardness of ferrite increased and the plastic deformation ability

Effects of aging temperature on G-phase precipitation and ferrite-phase decomposition in duplex stainless steel

G-phase precipitation and ferrite-phase decomposition in a cast duplex stainless steel (DSS) aged at 623–723 K for up to 8000 h were investigated using atom probe tomography (APT). Large sample volume was observed in every APT experiment, which yielded significantly statistical results. The number density of G-phase precipitates tended to be high and their sizes were small at lower aging temperatures. G-phase precipitates grew during prolonged isothermal aging. The concentrations of nickel, silicon, manganese and molybdenum in G-phase precipitates tended to increase as the precipitates grew. Heterogeneous distributions of alloying elements within G-phase precipitates were observed. An interesting positional relationship of G-phase precipitates with dislocations was revealed. Regarding the ferrite-phase decomposition, local chromium concentrations in the ferrite phase varied fast at higher aging temperatures. Good correlation between the variation of local chromium concentrations and aging conditions was revealed, which indicates that the variation can be estimated for arbitrary aging conditions. Representative distances between chromium-enriched and chromium-diluted regions were long at higher aging temperatures. Time exponent of the representative distances of ferrite-phase decomposition as well as the size of G-phase precipitates increased with aging temperatures.

Accurate determination of the number density of G-phase precipitates in thermally aged duplex stainless steel

G-phase precipitation in the ferrite phase in thermally aged duplex stainless steel (DSS) was investigated. A needle-shaped sample of DSS aged at 673 K for 5000 h was observed by transmission electron microscopy (TEM), and subsequently by atom probe tomography (APT). The precipitates of the G-phase observed by TEM corresponded well to clustering atoms observed by APT. On the other hand, regarding the precipitates of the G-phase that formed in an earlier stage of aging, the present study suggests that not all the precipitates can be detected by TEM. A large area of DSS aged at 673 K for 5000 h containing both the ferrite and austenite phases was observed. The number density of precipitates of the G-phase in the ferrite phase was small in the vicinity of the phase boundary and increased with the distance between the phase boundary and each field of view. The number density reached an almost constant value at a distance of approximately 4 µm from the phase boundary. The suppression of G-phase precipitation in the vicinity of the phase boundary is discussed in terms of the depletion of alloying elements that comprise the G-phase.