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Precipitation of the secondary phase is a key factor that influences the strengthening effect of nickel-based superalloy. The precipitation behaviours of the M23C6 carbide and γ′-Ni3(Ti, Al) phase in Inconel 617B alloy aged at 700°C for different times were studied by scanning electron microscopy, transmission electron microscopy and atom probe tomography. Plate-like carbide nucleated in the interior of grain and irregular shape carbide precipitated at grain boundary respectively during the early stage of ageing. γ′ particle co-precipitated around carbide is a common microstructure feature during ageing for 5–160 h. After ageing for 160 h, γ′ phase is the main secondary phase. The carbide only distributes on the grain boundary, high density γ′ particle homogeneously distributes in the matrix after a longer ageing time. Elemental partitioning to the secondary phases and composition profiles across different phases was quantitatively analysed. Enrichments of Ti and Al at the carbide/γ interface are directly observed, which act as the nucleation site of γ′ particles during ageing. Enrichments of B, Si and Co at different interfaces are also detected. Based on the experimental results, the precipitation mechanisms of γ′ particles and carbide were discussed. [ABSTRACT FROM AUTHOR]

ABSTRACTThe post-heat treatment of laser powder bed fusion (LPBF) Sc-modified Al alloy is a remarkably necessary method to improve mechanical properties. In this work, the LPBF Al-Mn-Sc alloys with a high yield strength of 586 ± 4 MPa and excellent elongation of 14.5% were obtained by a novel and efficient non-isothermal aging (NIA) treatment at the heating rate of 2 °C/min from 30°C to 350°C. The precipitation behaviour and mechanical properties of the LPBF Al-Mn-Sc alloys after NIA treatment were investigated by differential scanning calorimetry (DSC), X-ray diffraction (XRD), transmission electron microscopy (TEM), hardness test as well as tensile test. The activation energy and Avrami exponent were calculated through DSC results with various heating rates. A non-isothermal aging kinetic model was applied to evaluate the NIA process referring to isothermal aging. The excellent mechanical property was analysed by revealing the strengthening mechanism of LPBF Al-Mn-Sc alloys after NIA.

The influence mechanisms of Ce addition on the precipitation behaviour of σ phase and nitrides were systematically investigated. The results demonstrated that the addition of Ce significantly promoted the nucleation and slightly inhibited the growth of intergranular and intragranular σ phases. Meanwhile, Ce addition also accelerated the precipitations of π phase and Cr2N. Ce promoted the nucleation of intergranular σ phase mainly through: (i) leading to a linear increase in the driving force for σ phase and Cr activity; (ii) resulting in apparent grain refinement and providing more favorable nucleation sites. Besides increasing driving force and Cr activity, Ce addition also promoted the nucleation of intragranular σ phase by: (i) leading to lattice distortion, which not only caused local element concentration, but also reduced σ/γ interface energy, jointly increasing nucleation sites; (ii) providing other nucleation sites (Ce-bearing inclusions) for σ phase; (iii) enhancing Cr and Mo diffusion coefficients, which accelerated the supply of elements required for nucleation. Furthermore, Ce addition enhanced the driving force for Cr2N, Cr activity as well as Cr and N diffusions, and it also promoted the formation of more favorable nitride nucleation sites around σ phases. These combined actions accelerated the precipitations of π phase and Cr2N.

In this study, we report an unconventional precipitation and martensitic transformation behaviour of directly aged Ni-rich NiTi alloys fabricated via laser-directed energy deposition (LDED). Ni4Ti3 particles precipitate uniformly under all ageing conditions and no traditional multiple-step martensitic transformations are observed. We conclude this unique behaviour to the intrinsic characteristics of the LDED technique, which are metastable microstructures and high residual stresses. On the one hand, these features make grain boundaries no longer a fevered location for precipitation and, on the other hand, significantly suppress the martensitic transformation when ageing at low temperatures (300°C/400°C). As the aging temperature increase (500°C), residual stresses release significantly, accompanied by the growth of Ni4Ti3 precipitates from several nanometres to 452 ± 181 nm with increased interparticle spacing. At the same time, reverse martensitic transformations change from two-step (B19′ → R → B2) to single-step (B19′ → B2).

In this paper, the characteristics of manganese sulphide in the continuous casting bloom of 38MnS6 steel were revealed. Manganese sulphide in low oxygen (w(O) = 7 ppm) bloom mainly exists in the form of pure manganese sulphide distributed along grain boundaries. The morphology changes from fine droplets/rods to dendrites with large size and the aspect ratio gradually increases. The thermodynamic calculation results show that manganese sulphide precipitates at the end of solidification (fs = 0.86), and its morphology is related to local supersaturation and extrusion of solid-phase boundary. The solid-phase first transforms from high-temperature ferrite to austenite, then austenite grows up, which compresses the liquid phase boundary enriched with Mn and S and finally forms a large amount of manganese sulphide at the grain boundary. Dendritic, angular and droplet/rod manganese sulphide have different growth processes. Different types of morphological growth processes of manganese sulphide are described by combining crystal growth mechanism and solidification process. [ABSTRACT FROM AUTHOR]

Electropulsing treatment processing (EPT)-induced microstructural evolution and precipitation behaviour of the β-Mg17Al12 phase in the AZ61 alloy were investigated by using scanning electron microscopy, X-ray diffraction and electron backscattering diffraction patterns. The results revealed that the EPT method is efficient in achieving a near-spherical Mg17Al12 phase (approximately 300 nm) and uniform ultrafine-grained structure (approximately 900 nm) featuring a weakened texture in AZ61 alloy plates after EPT for 40 μs-3 min. The improved structural characteristics are attributed to the rapid recrystallization and grain refinement induced by EPT at lower temperatures and in a short time. On this basis, kinetics analysis of the spheroidization and the dissolution of the β-Mg17Al12 precipitation phase and grain refinement and growth in the samples after EPT were performed. Furthermore, the mechanism of precipitation phase transformation and grain refinement during EPT was discussed.

In this research, three types of super austenitic stainless steel (SASS) with nitrogen levels of 0.2, 0.28, and 0.38 wt.% were developed. By maintaining a relatively high manganese content of approximately 1.5 wt.% and nickel content of around 18 wt.%, nitrogen was fully incorporated into the austenite, resulting in alloy samples designated as 0.2N, 0.3N and 0.4N. The hot-rolled plates of these alloys underwent a solution treatment at 1180°C for 30 min, followed by aging at 950°C for durations of 30 min, 2 h and 6 h, respectively. The influence of nitrogen content on phase precipitation behaviour was examined through various micro-structure characterisation techniques. The corrosion resistance of the samples was further evaluated using potentiodynamic polarisation experiments, Electrochemical impedance spectroscopy (EIS) and double loop-electrochemical potentiokinetic reactivation (DL-EPR) method. The results indicated that with an increase in nitrogen (N) addition, the quantity of σ phases decreased while and the formation of Cr2N phases increased during the same aging period. Furthermore, at the same aging duration, the 0.3N SASS exhibited the highest pitting resistance and the lowest degree of sensitization (DOS) value compared to all other steels, attributed to the minimal precipitates present along the grain boundaries. [ABSTRACT FROM AUTHOR]

Precipitation behaviour of commercially available pure aluminium with trace impurities throughout repeated melting has been carried out. Cold rolled alloy samples are isochronally aged for 60 minutes at different temperatures. Isothermal ageing is carried out at 250ºC temperatures of cold rolled samples for different time up to 240 minutes. Hardness values of the aged alloys have been measured to comprehend the precipitation behaviour of the alloys with trace impurities. Trace impurities causes small age hardening effect as well as they pin moving grain boundaries and inhibit grain growth. The kinetics of precipitation of metastable phase in trace added alloys are found to be controlled by the diffusion of GP zones in aluminium. The results of this study confirmed a positive influence of alloy remelting upon the material properties and structure. The SEM images of fracture surfaces show spherical “dimples” correspond to microvoids that initiate crack formation and with the increasing of impurities the dimples of the fracture surfaces ever more decrease and become shallower.

In the smelting process of large castings, the addition of deoxidiser is bound to introduce other elements, such as Al and Ti. This research investigates the influence of Al and Ti on the precipitation behaviour of the γ" phase in cast alloy 625. The results showed that Al and Ti can improve the supersaturation of Nb in inter-dendritic zones during long-term aging. Furthermore, Al and Ti atoms replace partial Nb atoms in the γ" phase and promote the precipitation and growth of γ", which lead to an increase in strength at high temperatures. [ABSTRACT FROM AUTHOR]

The following paper examined changes in the microstructure of 7055 aluminium alloy before and after high-speed cutting (HSC) were examined, for various (T6, T7, and T84) heat treatment conditions. In the T6 state, the results demonstrated that the number of η″ particles gradually reduced and there was pinning in the η″ particles. After both the T7 and T84 heat treatments, a dislocation was observed in the microstructure by transmission electron microscopy. Grain boundaries in the T6 state were continuous with no obvious non-precipitate zone before HSC. The grain boundary width in the T7 state before HSC was double the T6 value. The grain boundary width for the T84 state alloy was similar to that in the T7 state, but less clearly defined. This is part of a thematic issue on Small Scale Mechanics - EUROMAT. [ABSTRACT FROM AUTHOR]

Thermal simulated specimens with the heat inputs of 20, 50 and 80 kJ/cm were used to investigate the effects of heat input on the microstructure and mechanical properties of the Fe-2Cr-Mo-0.12C pressure-vessel steel. The results indicated that the microstructures in the coarse-grained heat affected zone of tested steels with various heat inputs were mainly consisted of lath martensite and bainite ferrite. As the heat input increased, the fraction of martensite decreased and the bainite ferrite fraction increased. The toughness (tested at −40°C) and hardness for the heat input of 50 kJ/cm were 102 J and 346 HV, respectively, which was attributed to the high-volume fraction (60%) of the high-angle grain-boundary and the fine bainite lath. This paper is part of a thematic issue on Nuclear Materials. [ABSTRACT FROM AUTHOR]

To reveal the precipitation behaviour in relation to creep deformation, microstructural investigation on the specimens creep-ruptured and creep-interrupted at various time under 600°C and 120 MPa was conducted in a 10%Cr steel forging. M23C6 carbides, Cr-rich M2X and V-rich VX carbonitrides are detected on the heat-treated specimen. Most of the precipitates are M23C6 carbides and the recovery of the martensitic lath microstructure during creep are suppressed by them. There is a good relationship between the carbides growth and the creep deformation behaviour: the beginning of the significant coarsening corresponds to the transition from the primary to acceleration creep. The creep deformation is not affected by the precipitation of Laves phase and VX carbonitrides present around the grain boundaries. This paper is part of a thematic issue on the 9th International Charles Parsons Turbine and Generator Conference. All papers have been revised and extended before publication in Materials Science and Technology. [ABSTRACT FROM AUTHOR]

To reveal the precipitation behaviour in relation to creep deformation, microstructural investigation on the specimens creep-ruptured and creep-interrupted at various time under 600°C and 120 MPa was conducted in a 10%Cr steel forging. M23C6 carbides, Cr-rich M2X and V-rich VX carbonitrides are detected on the heat-treated specimen. Most of the precipitates are M23C6 carbides and the recovery of the martensitic lath microstructure during creep are suppressed by them. There is a good relationship between the carbides growth and the creep deformation behaviour: the beginning of the significant coarsening corresponds to the transition from the primary to acceleration creep. The creep deformation is not affected by the precipitation of Laves phase and VX carbonitrides present around the grain boundaries. [ABSTRACT FROM AUTHOR]

The microstructural evolution and precipitation behaviour of Nb-V-Mo and single V containing transformation-induced plasticity-assisted steels with an acicular/bainitic ferrite matrix were investigated by a heat treatment up to the austenite formation range. It was found that during the heating stage the acicular/bainitic ferrite matrix resisted recrystallisation, while cementite and martensite were decomposed and austenite was formed in the acicular/bainitic ferrite. Both Nb-V-Mo and V containing steels after the heat treatment showed a microstructure consisting of a polygonal ferrite matrix with small islands of pearlite. During these transformations, the microscopy observations showed that 0.04 wt% Nb and 0.08 wt% Mo additions to the 0.16 wt% V microalloyed steel considerably reduced the growth-coarsening of microalloy precipitates. [ABSTRACT FROM AUTHOR]

In this study, elemental segregation during solidification and subsequent precipitation behaviour in a continuous cast Nb-V-Ti high-strength low-alloy steel was investigated by optical microscopy, scanning electron microscopy with energy dispersive X-ray spectroscopy and thermodynamic modelling. It is known that for steels with low carbon contents the pearlite that forms on slow cooling does so where the interdendritic liquid was present prior to final solidification. The alloying elements of Nb, Ti, Mn and V segregate into the interdendritic liquid during solidification, while Al preferentially segregates into the solidifying solid phase. The composition analysis on the slab samples verified the predicted element segregation behaviour, with a smaller difference in the concentrations of Mn and V in the pearlite and dendritic ferrite regions being observed compared to the Nb levels. Small (30-100nm) spherical or irregular shaped Nb-rich precipitates (Nb(C, N) and (Nb, V)(C, N)) were mainly found in the pearlite regions, while angular Al-rich (60-300nm) precipitates were found in the dendritic ferrite regions, in the form of AlN and complex AlN-V(C, N) precipitates. Small isolated ferrite regions surrounded by pearlite were observed in the microstructure and has two origins: one type is dendritic ferrite that appears as an isolated island due to a sectioning effect when observing the two-dimensional microstructure; the other is a ferrite idiomorph that forms in the interdendritic region due to the low carbon content of the steel. Accordingly, in these isolated ferrite islands two different precipitation behaviours are found; predominantly Al-rich particles in the dendritic regions or predominantly Nb-rich precipitates in the interdendritic ferrite idiomorphs. No Al-rich precipitates were observed in the interdendritic regions (pearlite or isolated ferrite idiomorphs) despite the Thermo-Calc predictions indicating a higher volume fraction of AlN in these regions compared to the dendritic regions. This is believed to be due to back diffusion of nitrogen after solidification reducing the available nitrogen, which reacts with the high Nb levels present. [ABSTRACT FROM AUTHOR]

Fe-Ni base superalloy GH4706 has been used for large size disc forgings of heavy duty gas turbines. In this paper, based on the change law of precipitation, influential effects of stabilisation time on precipitation behaviour and mechanical properties of GH4706 alloy were investigated. Results show that with prolongation of the stabilisation time, the morphologies of η phase changed from chain and rod-like to cellular, cluster and needle-shaped; meanwhile, large globular γ′ phase and rod-like γ″ phase gradually precipitated. Furthermore, the content of the η phase increased with increasing stabilisation time, and the precipitation free zone tended to be wider, owing to the reaction between the γ′ phase and the η phase. The room temperature and 650°C tensile strength and rupture life of the GH4706 alloy improved after stabilisation treatment at 845°C for 1-3 h. [ABSTRACT FROM AUTHOR]

We have investigated microstructure and orientation relationship (OR) of the?and?phases in an as-cast ingot and nodular bainite of Cu-14.9 at.% Sn alloy by using transmission electron microscopy. Proeutectoid?grains and surrounding?phase (prior?phase) possess well-defined (1?1?1)?//(1?1?0)?,OR, that is, Kurdjumov–Sach (K–S) OR, while these two phases formed by eutectoid decomposition exhibit some deviation from K–S OR. On the other hand, the?and?phases in nodular bainite exhibit (1?1?1)?//(1?1?0)?,, which is Nishiyama–Wassermann (N–W) OR, as well as K–S OR. The emergence and variations of these ORs between the?and?phases have been interpreted from the viewpoints of (i) matching of interplanar spacing of {1?1?1}?, {1?1?0}?and {6?6?0}?planes, (ii) minimum strain criteria based on the invariant line concept for K–S OR and (iii) correspondence of atomic positions in the relevant phases by employing the structural ledge concept for N–W OR. [ABSTRACT FROM AUTHOR]

In the present paper, a high titanium molten (high Ti) slag was employed as an additive to modify the titanium bearing blast furnace (Ti-BF) slag in order to investigate the influence of the addition of high Ti slag on the crystallisation behaviour of the Ti-BF slag. The variation in the Ti enriched phase and the precipitation behaviour of the Ti enriched phase in the Ti bearing slag were studied in this paper. The Ti bearing slag in this paper was composed of different proportions of high Ti slag and Ti-BF slag. A combination of X-ray powder diffraction, scanning electron microscope equipped with energy dispersive X-ray spectroscope and optical microscopy was applied to determine the crystal structure, clarify the chemical composition and observe the morphology of the Ti enriched phase. It was found that the Ti enriched phase transformed from CaTiO3 to Mg4Al2Ti9O25 with the addition of high Ti slag. When the Ti bearing slag contained 60 wt-% high Ti slags, Mg4Al2Ti9O25 became the only Ti enriched phase, and it could crystallise completely when the slag was held at 1450°C for 25 min. The formation mechanism of Mg4Al2Ti9O25 is also discussed in this paper. [ABSTRACT FROM AUTHOR]

Abstract: Stainless maraging steels have a Cr content higher than 12wt% and show a excellent combination of high strength and ductility, which make them attractive for use in machinery fields and aircraft applications. The massive increase of strength during ageing treatment of maraging steels is related to a precipitation sequence of various nm-scaled intermetallic phases. The peak hardness especially in Ti-containing maraging steels can be reached after short-time ageing at temperatures around 500°C. However, precipitation reactions in different stainless maraging steels are not fully understood, especially the evolution from clustering over growing to coarsening. In the present work a commercial maraging steel and a Ti-containing model alloy are investigated and compared to each other. The steels were isothermally heat treated at 525°C for a range of times. Special emphasis was laid on the correlation of hardness to the formation and presence of different kinds of precipitates. The isothermal aged samples were investigated by using two advanced three-dimensional energy compensated atom probes (LEAP™ and 3DAP™) both in voltage mode and in laser mode. The atom probe data were correlated to standard hardness measurements. The results show that the partial substitution of Al by Ti results in a different precipitation behaviour. While the Ti-free maraging steel exhibit only one type of precipitate, the Ti-containing grade shows a change in the type of precipitates during ageing. However, this change leads to an accelerated coarsening and thus to a faster drop in hardness. [Copyright &y& Elsevier]

The influence of prior cold work (90% reduction) on the length change and precipitation behaviour of a Cu-1.8wt%Be-0.2wt%Co alloy aged at 320°C up to 350 h has been investigated. The alloy gradually expands to a maximum and then contracts during ageing. The maximum expansion is attributable to both the expansion due to the loss of Be solute atoms and the contraction due to precipitation of the [image omitted], γI, γm and γ' phases. The subsequent contraction results from the decrease in amounts of the [image omitted] and γm phases and the increase in amount of the γ' phase. The new-found γm phase, consisting of alternate Be and Cu matrix layers parallel to the matrix {001}α, is body-centred monoclinic with a = b = 0.263 nm and c = 0.279 nm and α = 83°, and aligns with the matrix according to the Bain orientation relationship. The Guinier-Preston (GP) zone transforms continuously to the γm or γI phase via γ'' and [image omitted]. The transformation from the GP zone to γ' via γ'', [image omitted] and γI is retarded in comparison with that for the un-deformed alloy. [ABSTRACT FROM AUTHOR]

Abstract: Small-angle neutron scattering (SANS) and energy-filtered transmission electron microscopy (EFTEM) were employed to characterise the precipitation behaviour in the high-speed steel HS6-5-2 during tempering. With SANS the evolution of the size distribution of precipitates during isothermal tempering at 590°C was investigated. The SANS results are discussed assuming that three populations of precipitates can be distinguished during tempering at 590°C. The smallest population with sizes around 1nm is supposed to be carbon-rich clusters. Intermediate-sized particles between approximately 1 and 10nm might be either MC and/or M2C carbides, which cannot be distinguished by SANS. In addition, the ratio A of magnetic and nuclear scattering intensity was used to gain information on the composition of the precipitates. The A-ratio is discussed assuming a substitution of iron in the precipitates by carbide forming elements with increasing tempering time. Finally, the correlation of the results obtained by SANS with those achieved by EFTEM is presented. [Copyright &y& Elsevier]

Abstract: The Analysis of variance (ANOVA) method has been used to illustrate the implementation of adaptive numerical (AN) techniques for the prediction of the precipitation behaviour of commercial materials. Case studies involving the analysis of σ-phase precipitation kinetics in duplex stainless steel (DSS) produced by sand casting and age hardening of 2219 aluminium alloy microalloyed with Ge and/or Si are presented. For each alloy, complex datasets comprised the results obtained from heat treatment trials on a range of commercial processing conditions, so that a single and combined effect of various parameters can be determined. This enabled an estimate of the most influential parameters to be made, providing an effective means of commercial alloy development and process optimisation. [Copyright &y& Elsevier]