G-phase Precipitation Research Articles

Precipitation behavior of G-phase and its effect on mechanical properties of 30Cr2Ni4MoV steel

Precipitation behavior of G-phase and its effect on mechanical properties of 30Cr2Ni4MoV steel during thermal aging at 350–600 °C for 0–10,000 h are investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), atom probe tomography (APT), hardness and impact tests. Aging at 500 °C, the martensitic matrix of 30Cr2Ni4MoV steel basically keeps stable. With the lath width almost remaining unchanged, carbides coarsen to a slight degree, but G-phase will precipitate at grain boundary (GB). The G-phase is Ni16Mn6Si7 and its “nose” temperature is around 500–550 °C. Due to relatively high enrichment level of forming elements including Ni, Mn and Si at GB, the G-phase can nucleate at triple junction or adjacent to M23C6 at GB. Subsequently, the G-phase grows along GB until contacts with M23C6, and then coarsens along the width direction, which results in alternating distribution morphology of G-phase and M23C6 at GB. The Vickers hardness results show that the hardness remains stable due to the relatively stable microstructure after aging. After aging for 10,000 h, the hardness undergoes little decrease due to the slight coarsening of M23C6 and G-phase. Interestingly, the impact energy drops obviously after the precipitation of G-phase. The reason is that the G-phase at GB promotes the initiation and propagation of cracks during the impact process, leading to intergranular fracture (IGF).

Effect of pre-strain on thermal aging of austenitic stainless steel weld metal

Austenitic stainless steel welds (ASSWs) suffer from severe pre-strain during assembly process, which threatens the long-term operation safety of pipelines in nuclear power plant. In this work, the 316L weld metals (WMs) with 0 % and 8 % pre-strain were thermally aged at 400 °C for up to 39000 h to investigate the pre-strain effect on thermal aging of ASSWs. The results showed that the pre-strain caused work hardening and further promoted the hardening of thermally aged ferrite. After thermal aging for 39000 h, the nano-hardness increment of ferrite with 8 % pre-strain was about 1.6 GPa more than that without pre-strain. By microstructure characterization, it is found that the high dislocation density induced by pre-strain promoted spinodal decomposition and G-phase precipitation. The spinodal decomposition morphology and corresponding element concentration fluctuations were more obvious in the WM with 8 % pre-strain. Moreover, the size and density of G-phase along dislocations were larger in the ferrite with 8 % pre-strain than those without pre-strain.

Microstructure evolution during oxidation of protruded NiCoCrAlYTa-Al2O3 abrasive coating produced by vacuum infiltration sintering

In this paper, protruded NiCoCrAlYTa-Al2O3 abrasive coatings were prepared by vacuum infiltration sintering, and the effect of the filler metal content on the microstructure before and after oxidation is investigated. The results show that, the Al2O3 grits are bonded to the coating matrix by a layer of Y-Al-O compound, and various types of G-phase are presented depending on the locations. Adjacent to the G-phase, larger γ' phase and γ/γ' eutectics are presented due to the expel of Al during the formation of G-phase. With the increase of the filler metal content, G-phase precipitation and the Si penetration depth into the single crystal superalloy substrate increase. After 1000 °C/100 h oxidation, the Y-Al-O intermediate layer between Al2O3 grits and the coating matrix is little changed, with an additional thermally grown oxide (TGO) layer between the Y-Al-O layer and the coating matrix presented. The selective oxidation of Al and Cr facilitates the formation of G-phase layer underneath the oxide film, and the rapid diffusion of Ti and the selective oxidation of Al yields the formation of γ' depletion region adjacent to the G-phase and on coating surface without Al2O3 grits.

Precipitate evolution and mechanical properties of Si-modified A286 alloy aged at 510 °C

Si-modified A286 alloy was designed to be compatible with liquid lead-bismuth eutectic (LBE) metal. The addition of Si resulted in a distinct microstructure compared to A286 alloy. In this study, we investigated the microstructure and mechanical properties of the experimental alloy after 510 °C isothermal aging for 5000 h. The results showed the presence of two types of G-phase in the alloy before aging: one with a core-shell morphology, the other with a chain-like morphology. After long-term aging, the Si-modified A286 alloy maintained the same refined grains as in the SAT(solid solution and aging treatment) specimen, with minimal changes observed in the two G-phases even after 5000 h of aging. However, a newly precipitated submicron spherical-like G-phase was observed on the grain boundaries after 500 h of aging, leading to a significant decrease in impact toughness. These spherical-like G-phases gradually coarsened with increasing aging time and eventually stabilized after 2000 h. The reduction in impact toughness in the Si-modified A286 alloy was attributed to the precipitation of the spherical-like G-phase.

Measurement of G-phase volume fraction and number density in duplex stainless steels using transmission electron microscopy

Duplex stainless steels (DSS) have a high toughness and strength due the presence of both austenitic and ferritic phases. These alloys have had limited use in power production applications due to thermal embrittlement caused by spinodal decomposition and development of G-phase precipitates in the ferrite. Lean grade DSS alloys (e.g., 2101, 2003) may offer improved thermal stability due to the reduction of Cr- and Ni-equivalent elements when compared to standard grade compositions (e.g., 2205, 2209). The abundance of the G-phase was measured in five duplex stainless steels, three wrought alloys (2101, 2003, 2205) and their matching filler metals (2101-w, 2209-w), after aging at 427 °C for 1000 h and 10,000 h. The G-phase volume fraction, number density, size, and precipitate spacing were found using quantitative analysis of transmission electron microscopy dark field images and the composition of G-phase precipitates on other clusters were characterized with atom probe tomography (APT). In the welded alloys, the G-phase was found to develop rapidly, relative to the wrought material. A positive correlation was found between the nickel equivalent composition of the alloy and the G-phase volume fraction. The alloys 2205, 2209, and 2101-w, which are higher in Cr and Ni, all showed significant G-phase precipitation, further strengthening the hypothesis that lean grade DSS alloys are more thermally stable against precipitation in the ferrite. Electron diffraction showed a secondary phase present in the 2101 wrought alloy at 10,000 h, but it was not crystallographically consistent with the G-phase; APT showed the presence of nanoclusters rich in both nickel and copper for this alloy. No secondary phases or clusters were found in 2003 after 10,000 h of aging, so it may be a candidate alloy for applications that require long-life times at high operating temperatures.

Effect of Thermal Aging on the Microstructure and Mechanical Properties of ER308L/Z2CND18.12N2 Dissimilar Welds.

A multi-analytical approach was used to investigate the effect of thermal aging on the microstructure and mechanical properties of ER308L/Z2CND18.12N2. The results demonstrated that fractures occurred preferentially on the ER308L side. Z2CND18.12N2 exhibited superior fracture toughness compared to ER308L regardless of thermal aging time. The ultimate tensile strength significantly increased from 564.5 MPa in the unaged condition to 592.7 MPa to MPa after thermal aging and the fracture mode changed from ductile fracture into a ductile + quasi-cleavage fracture. The fusion zone (FZ) with the chemical composition gradient was about 40 μm from the Z2CND18.12N2 to ER308L. After thermal aging, spinodal decomposition and G-phase precipitation were observed for the first time in the ferrite phase of the FZ. Moreover, the hardness presented the following trend: FZ > ER308L > Z2CND18.12N2. The hardness of the ferrite phase dramatically increased from 6.13 GPa in an unaged condition to 8.46 GPa in a 10,000 h aged condition.

Precipitation behavior of the G-phase strengthened 7Ni maraging steels

In this study, G-phase strengthened 7Ni maraging alloys were studied using a combination of thermodynamic prediction based on the TCFE-7 database and advanced experimental techniques, including micro-hardness testing, electron backscatter diffraction (EBSD), in-situ X-ray diffraction (XRD), transmission electron microscopy (TEM), and atom probe tomography (APT). The martensite reversion and phase stability of overcooled austenite were precisely determined for a series of Fe–7Ni–2Si-based alloys, validating the effectiveness of thermodynamic predictions in martensite transformation.Based on these theoretical prediction, aging hardness measurements and microstructural observations further revealed that Ni16X6Si7-G (X = Ti, Nb, Ta) precipitates are effective strengthening phases in 7Ni maraging steel, with the exception of Ni16X6Si7 (X = Mn, Zr) due to their significantly different thermal stabilities. Experimental results showed that the Ni16X6Si7-G (X = Ti, Nb, Ta) precipitates remained stable and densely distributed within the martensitic matrix after aging at 500 °C, resulting in high aging hardness values ranging from 350 to 550 HV. Among the studied alloys, the 1Ti alloy strengthened by the Ni16Ti6Si7-G phase exhibited the finest particle radius (estimated at 1.4 nm) and the highest number density (estimated at 1.9 × 1024/m3). Additionally, it is worth noting that the Ni16Zr6Si7-G phase was believed to form through eutectic reaction with α-Fe during solidification and the Ni16Mn6Si7-G phase was only stable at temperatures below 460 °C and was not detected experimentally.These findings enhance our comprehension of G-phase precipitation and strengthening in 7Ni maraging steel and underscore the potential for utilizing thermodynamic calculations and advanced experimental techniques to guide the design and optimization of high-strength alloys.

Interactive effect of thermal aging and proton irradiation on microstructural evolution and hardening of δ-ferrite in 308L stainless steel weld metal

In the harsh service environment of high temperature and intense neutron irradiation in water-cooled nuclear reactors, the austenitic stainless steel weld overlay cladding on the inner surface of the reactor pressure vessel suffers from thermal aging and irradiation damage simultaneously, which can induce microstructural evolution and hardening of the material. Since it is quite difficult to achieve this simultaneous process out of the pile, two kinds of combined experiments, i.e., post-irradiation thermal aging and post-aging irradiation were performed on 308 L stainless steel weld metals in this work. The interactive effect of thermal aging and proton irradiation on microstructural evolution and hardening of δ-ferrite in 308 L weld metal was investigated by combining atom probe tomography, transmission electron microscopy and nanoindentation tests. The results revealed that thermal aging could eliminate the dislocation loops induced by irradiation and affect the phase transition process by accelerating spinodal decomposition and G-phase precipitation, thus enhancing hardening of irradiated δ-ferrite. For the effect of irradiation on the microstructure and hardening of thermally aged δ-ferrite, however, intensive collision cascades can intensify G-phase precipitation and dislocation loop formation but decrease spinodal decomposition, leading to a limited effect on hardening of thermally aged δ-ferrite. Furthermore, the interaction of thermal aging and irradiation can promote G-phase precipitation. Meanwhile, the interaction can cause δ-ferrite hardening, which is mainly influenced by spinodal decomposition, followed by G-phase and dislocation loops, where spinodal decomposition and G-phase cause hardening by inducing strain fields.

Stability and strengthening effect of aging induced-nanofeatures in δ-ferrite in an austenitic stainless-steel weld

The deformation behavior of δ-ferrite in an aged austenitic stainless-steel weld was investigated to obtain insights on the contribution of aging-induced nanofeatures, such as spinodal decomposition and G-phase precipitation, to aging embrittlement. To evaluate the hardening effect of the G-phase, a reversion heat treatment was applied to the aged weld. The strengthening effect of spinodal decomposition and the G-phase was measured using nanopillar compression tests. The deformed microstructure exhibited the dissolution of G-phases but a slight change in the spinodal nanostructure, indicating a clear difference in the stability of the nanofeatures during deformation. The quantitative measurement of the G-phase and the application of the Orowan model showed that the measured strengthening effect from the G-phase was less than that of the estimation, implying that the G-phase was not as effective as an impenetrable obstacle. On the other hand, spinodal decomposition showed high stability after deformation, leading to a noticeable hardening effect.

Development of the high-strength ductile ferritic alloys via regulating the intragranular and grain boundary precipitation of G-phase

• Although the nano-dispersed precipitation of G-phase within the grain hardens the alloy remarkably, its large amount of precipitation at the grain boundary leads to serious alloy embrittlement. • For the first time, the crystal structure transition of nano-dispersed precipitates from the Fe 2 TiSi-L2 1 to the Ni 16 Ti 6 Si 7 -G phase is observed within the ferrite at the early stage of aging. • Guided by current TEM and DFT results, the interfacial energy is predicted to play a leading role in forming the cube-on-cube orientation relationship between α-Fe and G-phase. • Adopting the “cold-rolling and aging” process to effectively avoid its precipitation at grain boundaries, a class of high-performance ferroalloys with both high strength and good ductility is realized. A typical G-phase strengthened ferritic model alloy (1Ti: Fe-20Cr-3Ni-1Ti-3Si, wt.%) has been carefully studied using both advanced experimental (EBSD, TEM and APT) and theoretical (DFT) techniques. During the classic “solid solution and aging” process, the superfine (Fe, Ni) 2 TiSi-L2 1 particles densely precipitate within the ferritic grain and subsequently transform into the (Ni, Fe) 16 Ti 6 Si 7 -G phase. In the meanwhile, the elemental segregation at grain boundaries and the resulting precipitation of a large amount of the (Ni, Fe) 16 Ti 6 Si 7 -G phase are also observed. These nanoscale microstructural evolutions result in a remarkable increase in hardness (100 - 300 HV) and severe embrittlement. When the “cold rolling and aging” process is used, the brittle fracture is effectively suppressed without loss of nano-precipitation strengthening effect. Superhigh yield strength of 1700 MPa with 4% elongation at break is achieved. This key improvement in mechanical properties is mainly attributed to the pre-cold rolling process which effectively avoids the dense precipitation of the G-phase at the grain boundary. These findings could shed light on the further exploration of the precipitation site via optimal processing strategies.

Effect of microstructure evolution at the interphase boundary on the thermal ageing embrittlement of CF8M cast austenitic stainless steel

Thermal ageing embrittlement of a cast austenitic stainless steel, CF8M with 25% of δ-ferrite, was evaluated using various mechanical tests such as micro-hardness, small punch, tensile, and fracture toughness (J-R) tests. To simulate long-term service at operating temperature in light water reactors (LWRs), CF8M was thermally aged at 400 °C up to 10,000 h. After thermal ageing for 5000 h, phase separation into Fe-rich and Cr-rich phases by spinodal decomposition and G-phase precipitation were observed in δ-ferrite, resulting in significant hardening in δ-ferrite. As ageing duration increased to 10,000 h, additional changes in microstructure and micro-hardness in δ-ferrite were not significant. On the other hand, fracture properties, such as fracture toughness and small punch (SP) energy showed further reduction with ageing time, which could be related to elemental segregation at the austenite/ferrite interphase boundary. After reversion heat treatment at 550 °C for 1 h, spinodal decomposition was mostly disappeared, but G-phase precipitate in δ-ferrite and elemental segregation at the austenite/ferrite interphase boundary were not recovered, which could be responsible for the incomplete recovery of mechanical properties.

Morphological instability of the G-phase induced a different contribution to hardening of ferrite phase in a duplex stainless steel

G-phase plays an indispensable role in the hardening of ferrite during thermal aging process of the duplex stainless steel. Although there are many studies on quantifying the contribution of spinodal decomposition and G-phase precipitation to the hardening of ferrite, details regarding the influence of G-phase morphology transformation on hardening are lacking. These knowledge gaps hinder a deep understanding of the correlation between G-phase and ferrite hardening, and hence hinder the exploration of new techniques for repairing performance degradation. In this study, after aging at 400 °C and 475 °C, driven by the minimization of interfacial energy, the morphology of G-phase changes from sphere to cube, and consequently the contribution of G-phase to hardening increases significantly. After annealing at 550 °C, the G-phase does not dissolve, but coarsens, resulting in a further increase in the contribution to hardening. The difference from annealing is that after pulsed electric field treatment at 460 °C, the unstable spherical G-phase precipitates are almost completely dissolved, while the stable cubic G-phase is spheroidized. The behaviors of dissolution and spheroidization are driven by the intervention of the electric free energy, and as a result, the hardening caused by G-phase is eliminated or relieved due to the dissolution and refinement of G-phase. It is conceivable that this work is a decisive step to completely repair the deteriorated performance of duplex stainless steel after long-term operation.

Evolution of the role of molybdenum in duplex stainless steels during thermal aging: From enhancing spinodal decomposition to forming heterogeneous precipitates

Thermal aging of ferrite in duplex stainless steels leads to a complex microstructural evolution that can give way to large deleterious changes in mechanical properties. The microstructural evolution mechanisms that happen concurrently are strongly linked to minor changes in composition. By characterizing duplex stainless steels with various compositions over a range of aging conditions, it was revealed that adding a modest amount of Mo to cast austenitic stainless steel (such as changing from grade CF3 to grade CF3M) not only increases the kinetics of spinodal decomposition and Ni-Si-Mn-rich G-phase precipitation, but it also forms a separate Mo-rich phase. At shorter equivalent aging times, Mo segregates from Fe and towards Cr accelerating spinodal decomposition. At longer aging times, Mo enriches in the Ni-Si-Mn clusters before phase separating into an adjoined Mo-rich cluster. These phase changes and precipitation events result in a rather steep drop in fracture and impact toughness. These results will inform the design of similar alloys for use at high temperatures.

Effect of Ni, Mo and Mn content on spinodal decomposition kinetics and G-phase precipitation of aged model cast austenitic stainless steels

• Effect of Ni, Mo and Mn content on ferrite decomposition. • Thermal ageing of cast austenitic stainless steels (CASS) and model alloy. • Spinodal decomposition kinetics and G-phase precipitation. During ageing, the ferritic phase of Cast Austenitic Stainless Steels (CASS) undergoes decomposition by spinodal decomposition and G-phase precipitation, which leads to increasing hardness and decreasing impact toughness. Mo-bearing steels are more prone to ageing than Mo-free steels. To study the effects of Ni, Mo, and Mn, duplex model alloys were thermally aged at 350 °C and 400 °C. The microstructure evolution during ageing was characterized by atom probe tomography. Only Ni additions seem to affect the spinodal decomposition kinetics. Ni accelerates the spinodal decomposition kinetics regardless of the presence of Mo or Mn. Additions of Mo and Mn promote the precipitation of G-phase.

Tensile properties on dissimilar welds between 11Cr-ferritic/martensitic steel and 316 stainless steel after thermal aging

The aim of this study was to evaluate the tensile properties and microstructures of dissimilar welds between 11Cr-ferritic/martensitic steel and 316 stainless steel after thermal aging at temperatures between 400 and 600 °C up to 30,000 h. Characterization of microstructure was carried out by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Microstructural analysis showed that the microstructure in the weld metals (WMs) consisted of lath martensite containing a small amount of residual austenite. Thermal aging hardening of WMs occurred at 400 and 450 °C due to the effects of both α-α’ phase separation and G-phase precipitation. However, there was no significant change in the total elongation, and fracture surfaces indicated that very fine dimpled rupture was predominant rather than the cleavage rupture. It was suggested that lath martensite phases enhanced the tensile strength due to phase separation, while residual austenite played a role in keeping elongation as a soft phase.

Ultra-high strength steel made from AISI 304L using a novel thermo-mechanical processing technique

Presented here is a technique of producing an ultra-high strength steel starting with an annealed low strength AISI 304L stainless steel. Annealed AISI 304L stainless steel was severely rolled at room temperature to a thickness reduction of 93%. Microstructure after rolling of SS304L showed subgrain formation with high dislocation density. The resulting strength of this steel after rolling was 1.7 GPa. Subsequent aging of this at 400 °C for 120 h further increased the strength to 2.2 GPa. The ductility of the as-rolled and subsequently aged conditions was similar and in the range of 5 to 7%. Electron back scatter diffraction analysis and vibrating-sample magnetometer showed that in the as-rolled condition the fraction of strain induced martensite was ≈ 72% with the remaining being untransformed austenite in the deformed state. The median spacing between boundaries (low as well as high angle) varied between 700 nm for the as-rolled condition to 900 nm after rolling and subsequent aging at 400 °C for 120 h. Atom probe tomography along with small angle neutron scattering showed that the process of aging resulted in a spinodal phase separation of Cr rich regions with 7 nm wavelength and precipitation of the G-phase having 1.75 nm radius. It was estimated that G-phase precipitation, rather than spinodally decomposed Cr-rich regions, is the dominant hardening phase.

Phase-field modelling of spinodal decomposition with G-phase precipitation during ageing

Phase-field modelling of spinodal decomposition with G-phase precipitation during ageing

Integrated effect of thermal ageing and low flux irradiation on microstructural evolution of the ferrite of welded austenitic stainless steels

With the purpose to quantify microstructural changes with respect to ageing degradation, the microstructure of aged type 308 stainless steel welds with a ferrite content of 5-7% has been analysed using atom probe tomography. The weld metal of the core barrel of a decommissioned light water reactor, irradiated during operation of the reactor to 0.1 dpa, 1 dpa and 2 dpa at 280-285°C (231,000 h), are compared to two similar thermally aged welds. In the ferrite of the irradiated welds, there is spinodal decomposition into Cr-rich α’ and Fe-rich α, with a similar degree of decomposition for all investigated doses, amplitudes of 21-26% and wavelengths between 6 and 9 nm. The ferrite of the thermally aged material showed evidence of decomposition when aged at 325°C (an amplitude of 13-14% and wavelength of 5 nm), but not when aged at 291°C, thus the irradiation significantly increases the rate of spinodal decomposition. There is G-phase (Ni16Si7Mn6) precipitation in the ferrite of all the weld metals except the one that was thermally aged at the lowest temperature. After irradiation to 1 and 2 dpa, the G-phase is considerably more well developed than after 0.1 dpa or thermal ageing.

“Effect of proton irradiation on δ-ferrite in the thermally aged austenitic stainless steel weld: Precipitation of G-phase and additional hardening”

An austenitic stainless steel weld was thermally aged at 400 °C for 20,000 h, and subsequently proton irradiated up to 10 displacement per atom (dpa) at 360 °C. After thermal ageing, only spinodal decomposition was observed without G-phase precipitates in δ-ferrite. However, ~ 1023 m−3 of fine (~ 3 nm in diameter) G-phase precipitates were observed after proton irradiation. Also, wavelength of spinodal decomposition was increased with proton irradiation. The additional hardening in δ-ferrite after proton irradiation was attributed to enhanced spinodal decomposition and G-phase precipitation. After irradiation, the contribution to addition hardening was greater for G-phase than enhanced spinodal decomposition.

Effect of multiple heat treatments on fracture property of centrifugal casting stainless steels Z3CN20.09M with long-term thermal aging degradation

It is of great significance to investigate effect of multiple heat treatments on fracture property of centrifugal casting stainless steels Z3CN20.09M cut from pump casing with long-term thermal aging degradation for nuclear power plants to consider actual operation of nuclear power plants. Both multiple heat treatments and accelerated thermal aging experiment at the same temperature of 400 °C for different time were successively carried out on centrifugal casting stainless steels Z3CN20.09M in order to examine the metallographic modification and impact properties. Finally, an additional investigation on the related fracture properties was carried out, in which the critical initial fracture toughness Ji was determined by stretch zone width and 0.2 mm offset line methods. These results indicated that the multiple heat treatments led to the dispersed distribution of ferrite phases in austenite matrix and thus microhardness increased, but impact energy exhibited a decreasing tendency significantly. After long-term aging, the metallographic structure remained almost unchanged, but the size of ferrite phases showed a slight increasing trend because of spinodal decomposition in ferrite phases and G-phase precipitation. In addition, centrifugal casting stainless steels Z3CN20.09M with multiple heat treatments exhibited the higher microhardness, Charpy impact toughness, critical initial fracture toughness JIC (J-integral determined by 0.2 mm offset line method), and JSZW (J-integral determined by stretch zone width method) than those with primary heat treatment, while the specific number of the heat treatment had a low influence on fracture toughness.