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The effects of heat treatment conditions on the microstructure and the deformation behavior of Haynes 282 superalloy in tension at 750 °C were investigated. The standard 2-step aging heat treatment (1010 °C/2 h + 788 °C/8 h) was compared to alternative, more economical, 1-step aging treatment (800 °C/4 h). Moreover, three different cooling rates from the solution temperature of 1135 °C were studied to represent the effective cooling rates that large-scale components may experience. Regardless of the heat treatment conditions, as much as about 20% of fine spherical intragranular γ' particles were successfully precipitated with an average size between 12 nm and 39 nm. The average γ' particles size increased as the cooling rate from the solution temperature decreased. All four heat-treated alloys exhibited good mechanical properties at the high temperature of 750 °C with a yield strength in particular well over 620 MPa. As it could be expected, the yield strength increased and the ductility decreased as the average γ' particles size decreased. The alloys exhibited a mixed mode of deformation characterized by shearing and bypassing. However, the dominant deformation mechanism depended on the γ' characteristics resulting from different heat treatment conditions: only 1-step aged specimens with the largest γ' particles of 39 nm size, which was furnace-cooled from solution temperature, exhibited a plastic behavior typical of a dislocations shearing mechanism while the other alloys showed primarily an Orowan dislocations bowing and looping mechanism behavior. The major operative deformation mechanism could be well predicted by strength increment calculations based on the precipitation strengthening model. Our results suggest that wrought Haynes 282 produced by a more economical 1-step aging treatment may be a reliable candidate for high temperature applications under advanced ultra-super-critical (A-USC) conditions.

International audience; Additive Manufacturing offers an innovative route for producing high-quality parts in various fields. A bi-material system, consisting in a NiCrAlY bond coat deposited onto a Ni-based Alloy 625 substrate, was manufactured by laser powder bed fusion (LPBF). Test samples were prepared and included SolGel ceramic ZrO2(Y2O3) top coats as well as a specific grain boundary serration (GBS) heat treatment for promoting high temperature resistance. These specimens were subjected to very severe thermal shock cycles between 950 °C and 300 °C, characterized by steep heating and air quenching rates in a state-of-the-art burner rig designed to render gas turbine conditions, and their integrity was compared. While LPBFed NiCrAlY coatings were relatively spared from degradations due to thermal shocks, ceramic top coats exhibited clear spallation. Poor bonding was particularly experienced by specimens subjected to the GBS heat treatment due to the unavoidable formation therein of surface oxides. Numerous cracks were detected within NiCrAlY bond coats, both in tested specimens as well as in pre-cycled as-built ones, which suggested a dominant role of the LPBF process known for generating residual stress. Heat treated specimens exhibited nearly no cracking. Hardness was found to significantly increase within as-built NiCrAlY bond coats as a result of heat exposure during thermal shock cycling and was attributed to precipitations. The fully recrystallized microstructure of heat treated specimens, on the other hand, was found more stable. The present study completes a series of investigations demonstrating the great potential for manufacturing excellent high temperature structural components by means of LPBF as opposed to more constraining conventional routes.

Metal Additive Manufacturing and Laser Powder Bed Fusion (LPBF), in particular, have come forth in recent years as an outstanding innovative manufacturing approach. The LPBF process is notably characterized by very high solidification and cooling rates, as well as repeated abrupt heating and cooling cycles, which generate the build-up of anisotropic microstructure and residual stresses. Post-processing stress-relieving heat treatments at elevated temperatures are often required in order to release some of these stresses. The effects of 1 h&ndash, hold heat treatments at different specific temperatures (solutionizing, annealing, stress-relieve and low-temperature stress-relieve) on residual stress levels together with microstructure characterization were therefore investigated for the popular Alloy 625 produced by LPBF. The build-up of residual stress is accommodated by the formation of dislocations that produce local crystallographic misorientation within grains. Electron backscattered diffraction (EBSD) was used to investigate local misorientation by means of orientation imaging, thereby assessing misorientation or strain levels, in turn representing residual stress levels within the material. The heavily constrained as-built material was found to experience full recrystallization of equiaxed grains after solutionizing at 1150 &deg, C, accompanied by significant drop of residual stress levels due to this grains reconfiguration. Heat treatments at lower temperatures however, even as high as the annealing temperature of 980 &deg, C, were found to be insufficient to promote recrystallization though effective to some extent to release residual stress through apparently dislocations recovery. Average misorientation data obtained by EBSD were found valuable to evaluate qualitatively residual stress levels. The effects of the different heat treatments are discussed and suggest that the peculiar microstructure of alloys produced by LPBF can possibly be transformed to suit specific applications.

The effect of Si addition on the initial deformation mechanism of Fe–30Mn–9Al–0.9C–0.5Mo cast lightweight steel has been investigated. Atomic-scale analysis by three-dimensional atom-probe tomography confirmed that Si addition accelerates the formation kinetics of the κ-carbide, and increases remarkably the partitioning coefficient of carbon by more than 2 times. Promotion of C partitioning into κ-carbides by Si addition leads to deformation localization in an aged state. This is attributed not only to the increase of the κ-carbide size associated with higher coherency strain, but also to higher frequency of Al-C bonding formation. First-principles calculations indicated that the energy required for a/2 shearing of κ-carbide in the aged 1%-Si steel is higher (561 mJ/m2) than that in the aged Si-free steel (494 mJ/m2). Therefore, Si addition leads to a significant energy difference for shearing and results in different shear bands formation. The operative deformation mechanism can be calculated for this alloy system as a function of precipitates volume fraction and size. It was determined that deformation proceeds primarily by κ-carbide shearing as long as the radius of κ-carbides is smaller than 13.4 nm.

Grain boundary serration is an effective way to increase the high temperature resistance of superalloys and steels. The popular Alloy 625 Ni-based superalloy was until now believed not to form serrated grain boundaries based on previous considerations of serrability criteria. Following the recent strain-induced serration mechanism, a special heat treatment involving continuous slow cooling between the solution and aging temperature was designed. As a result, significant serration was observed for the first time for Alloy 625 promoted by slow cooling. Grain boundary M 23C6 carbides were systematically detected from either degenerescence of solidification MC carbides or heterogeneous nucleation. Upon aging, serration amplitude increased and precipitation of the δ phase proliferated.

This study investigated the mechanical behaviour and microstructure of as-built and heat-treated Inconel 625 (IN625) samples processed by laser powder bed fusion (LPBF). This process offers freedom in design to build complex IN625 components in order to overcome extensive machining. However, post heat treatments must be performed to obtain specific mechanical properties to match industrial requirements. For this purpose, different heat treatments were performed on IN625 samples, and through hardness measurements, three different heat treatments were selected, as optimised conditions. A direct ageing, a solutioning and a solutioning followed by ageing treatments were chosen to study the effects of these specific heat treatments on the microstructure and tensile properties, comparing them to those of as-built condition. The tensile properties of as-built and selected heat-treated IN625 samples showed superior values to minimum requirements for wrought IN625 alloys, whereas the investigation on the microstructures and fracture surfaces of as-built and heat-treated IN625 contributed to an understanding of the tensile properties evolution. The high tensile strength of as-built samples essentially derived from very fine dendritic structures mainly below 1 µm with high dislocation density and nanometric MC carbides. The high tensile properties of ageing treatments performed at 700 °C for 24 h, whether directly aged or post-solutioning, were found to be primarily dependent on γ" phases (10–30 nm) and M23C6 carbides formation. By contrast, the tensile properties of solution-treated IN625 samples at 1150 °C for 2 h showed higher ductility coupled to lower strength than other conditions, due to the grain growth.

The formation of serrated grain boundaries and its mechanism correlated with straining exerted on grain boundaries were investigated in a Ni–20Cr binary model alloy. This model alloy is a full solid-solution which is deliberately free of carbon and aluminum to exclude precipitation of second phase. A special heat treatment involving slow cooling was used to promote grain boundary serration, which is known to enhance creep properties by decreasing interfacial free energy. No sign of serration could be observed for the Ni–20Cr model alloy, as expected. However, we found that a compressive 5% strain hold applied during slow cooling successfully induced grain boundary serration. The compressive 5% strain hold generated dislocations near the grain boundary, which promoted Cr enrichment near and at the grain boundary. To dismiss the effect of dislocations, straining was removed and the solutionizing temperature was increased in order to promote Cr enrichment near and at grain boundaries by means of vacancy-assisted diffusion of Cr atoms from the bulk to grain boundaries during slow cooling. Significant grain boundary serration was again observed. Atom probe tomography analyses showed that the serrated grain boundary with a wide width of ~ 10 nm was enriched with Cr atoms, as much as 87 at.%, while the γ matrix only contained 34 at.%. First-principles calculations showed that the strain energy necessary for the onset of grain boundary serration is lower than 0.97 kJ/mol.

International audience; The high temperature oxidation of fully SLM-processed NiCrAlY coated Alloy 625 parts was investigated between 800 °C and 1000 °C. For comparison, bulk NiCrAlY and bulk Alloy 625 were also fabricated using SLM. Two thin layers of NiCrAlY powder were lasered at the surface of Alloy 625 to form the bi-materials. Bulk NiCrAlY and bi-materials showed an improved oxidation behavior compared to Alloy 625. The oxidation of the NiCrAlY coating results in regions with a dense and continuous external Al2O3 layer and regions composed of a mixture of external Cr2O3 and internal Al2O3. Large EDS maps at the surface and cross-sections of the bi-material highlighted a heterogeneous distribution of constitutive elements of the NiCrAlY coating, resulting in some regions with an Al activity lower than the one required for the formation of a continuous and dense Al2O3 layer (≤10 atomic percent). Low Al activity and high Al activity regions were related to the topography of the SLMed surface and correspond to hill and valley regions, respectively. In addition, cracks, mainly occurring in high Al activity regions, were observed across NiCrAlY specimens. The fabrication of coated but small components with a brittle coating by SLM is not trivial and needs further investigations.

International audience; The popular superalloy Alloy 625 was produced by Selective Laser Melting (SLM) and post-processing heat treatments were designed to optimize the inhomogeneous and constrained as-built microstructure (AB) for high temperature structural applications. A single-step solution heat treatment (RX) was designed to promote full recrystallization and approach the conventional wrought microstructure. To enhance high temperature properties, a grain boundary serration heat treatment (GBS) was successfully designed involving higher solution temperature and time to promote recrystallization and homogeneity, and a direct slow cooling step followed by a short aging to assist solute diffusion and grain boundary motion. The resulting microstructures were characterized by fully recrystallized fine equiaxed grains and fine intra and intergranular NbC precipitates. The GBS alloy also exhibited as much as 80% of serrated grain boundaries with enhanced resistance to cracking at high temperatures. Tensile properties of all three materials were evaluated at room temperature, 500 °C, 600 °C and 700 °C and compared with their conventional solutionized wrought Alloy 625 counterpart (Wrought). While the AB material exhibited high strength and low ductility, due for the most part to the high density of tangled dislocations resulting from SLM, both RX and GBS alloys showed tensile properties comparable to the conventional wrought material, higher strength in particular. At all temperatures, all four alloys exhibited yield strength values well over 200 MPa. Due to significantly different microstructures, deformation and fracture behaviors were different. While Wrought clearly presented irregular plastic flow at elevated temperatures typically attributed to dynamic strain aging (DSA), the materials produced by SLM and moreover those subjected to post-processing heat treatments exhibited more stable plastic deformation. The results and characterization reported in the present article highlight the predominant role of microstructure and outstanding potential of SLMed Alloy 625.

Ni-based model alloys were used to study the effect of alloying elements, namely Cr, Mo, C and Zr on the occurrence of grain boundary serration. The model alloys were free of aluminum to exclude precipitation of second-phase γ′. Similarly, the carbon content was very low, when present, to prevent precipitation of carbides. A special heat treatment involving slow cooling was used to promote grain boundary serration. No significant sign of serration was observed for Ni-10Cr-10Mo, Ni-20Cr-10Mo and Ni-10Cr-10Mo-0.05C model alloys. However, substantial serration was observed for Ni-10Cr-10Mo-0.5Zr and Ni-20Cr-0.5Zr model alloys. Serrated grain boundaries were observed in the absence of either γ′ or carbides. Zirconium-rich precipitates were recognized at serrated grain boundaries though their involvement in the occurrence of serration was doubtful. A mechanism of grain boundary serration formation is proposed.

To evaluate the deformation mechanism in Fe-20Mn-11.5Al-1.2C lightweight steel, the relationship between mechanical properties and microstructural evolution during deformation at room temperature was investigated. Although the wrought alloy under investigation was only subjected to conventional solution treatment at 1050 °C for 2 h, the microstructure consisted of intragranular nano-sized κ-carbides within a γ-matrix and intergranular micro-sized κ-carbides along grain boundaries. As a result, the alloy exhibited high yield and tensile strengths of 1187 MPa and 1233 MPa, respectively, and a relatively low ductility of 15.4%. A series of microanalyses by means of electron backscattered diffraction and transmission electron microscopy revealed the deformation and fracture mechanism. During tension, initial deformation of the softer γ-matrix containing nano-sized κ-carbides led to unidirectional shearing of the κ-carbides. Deformation of intergranular κ-carbides initiated at the interface with the γ-matrix due to stress concentration, resulting in a planar glide of super-partial dislocation pairs. Cracking initiated within the harder intergranular κ-carbides upon deformation and propagated to grain boundaries or within the γ-matrix until fracture.

Additive Manufacturing defines the fabrication of objects by successive consolidation of materials, layer by layer, according to a three-dimensional design. The numerous technologies available today were recently standardized into seven categories based on the general method. Each technology has its own set of advantages and limitations. Though it very much depends on the field of application, major assets of additive manufacturing compared to conventional processing routes are the ability to readily offer complexity (in terms of intricate shape and customization) and significant reduction of waste. On the other hand, additive manufacturing often suffers of relatively low production rates. Anyhow, additive manufacturing technologies is being given outstanding attention. In particular, metal additive manufacturing emerges as of great significance in industries like aerospace, automotive and tooling. The trend progresses toward full production of high value finished products.

Titanium aluminide alloys are good candidates for structural applications thanks to their low density and good balance of properties up to relatively high temperatures. However, their application is still limited by significant oxidation. Four γ-TiAl alloys with different content of aluminum and niobium were produced by electron beam melting: Ti-45Al-2Cr-2Nb, Ti-48Al-2Cr-2Nb, Ti-45Al-2Cr-8Nb, and Ti-46Al-2Cr-8Nb. The behavior of these alloys in response to oxidation in air during constant heating up to 1000 °C and isothermal oxidation for 10 h at 850 and 950 °C were studied by thermogravimetric analysis. The mass gain due to oxidation of the low Nb-containing alloys was always at least twice that of the high Nb-containing alloys. Both low and high Nb-containing alloys exhibited on their surface oxidation products of the same nature: oxides TiO2 and Al2O3, and nitrides TiN and Ti2AlN. Niobium addition up to 8 at.% did not suppress the growth of rutile and promote the formation of a protective alumina layer. However, it efficiently reduced the formation of rutile, mainly responsible for the mass gain due to oxidation of γ-TiAl alloys and with tendency to spallation.

The solidification path of directionally solidified Ni-based superalloys is an important feature to define the heat treatments necessary after casting. Ni-based superalloy CMSX-10 was solidified at different velocities in a modified Bridgman type furnace allowing rapid quenching of the samples in water. The results of the experiments allowed to clarify the solidification of the interdendritic region, from nucleation of fine γ′ precipitates at the primary γ dendrite surface to the spatial growth and coarsening of the interdendritic colonies' morphology towards the end of solidification. A three-dimensional model is proposed and explains the different morphologies in the interdendritic region according to the observation plane. The study revealed strong evidence of a solidification in the interdendritic region of peritectic nature. Solute rejection into the liquid ahead of the growing phases, diffusion and convection effects lead to the coupled growth of the γ and γ′ phases according to the reactions L → γ and L + γ → γ′ respectively.

International audience; The present study investigated for the first time the feasibility of producing by Selective Laser Melting (SLM) a NiCrAlY bond coat material directly onto an IN625 substrate itself produced by SLM. A typical parameters optimization was conducted by varying laser power (P) and scanning speed (v). Single-line scanning tracks and two-layer coatings were carried out and analyzed for 15 different P/v conditions. Several criteria were defined for the selection of appropriate SLM parameters. The results showed significant remelting of the underlying substrate, which is a typical feature of SLM manufacturing. This led to the formation of an intermediate dilution zone characterized by substantial mixing between IN625 superalloy substrate and NiCrAlY bond coat suggesting excellent metallurgical bonding. Optimum processing conditions were found for P = 250 W and v = 800 mm/s. It produced a dense 242 µm thick bond coat including a 36% dilution zone. The SLMed system exhibited a smooth microhardness profile slightly increasing from 275 Hv in the bond coat to 305 Hv in the substrate. A progressive Al concentration distribution between the phases and low residual stress levels were found in the system. This suggested that SLM might be a valuable alternative manufacturing process for bond coat systems promoting excellent adhesion for high temperature applications.

Phase transition temperatures of γ-TiAl alloys were successfully identified by Thermo Mechanical Analysis. The derivative curve of the displacement as a function of temperature exhibits well defined peaks at the temperatures 1100 ≤ Teu ≤ 1200 °C and 1250 ≤ Tα ≤ 1350 °C which correspond to the phase transformations (α2 + γ) ↔ α and (α + γ) ↔ α respectively. The well-known effects of Al and Nb content on transition temperatures were studied for validation.

Third generation γ-TiAl alloys with a high niobium content, Ti–(47–48)Al–2Cr–8Nb, were processed by electron beam melting (EBM). This near-net-shape additive manufacturing process produces complex parts according to a CAD design. The starting powder is deposited layer by layer on the building table and selectively melted to progressively form the massive part. The EBM parameters such as layer thickness, melting temperature, scanning speed, or building strategy were set up to minimize porosity. The chemical composition of the built material is similar to the composition of the base powder despite a slight evaporation of aluminum and reveals a neglectable oxygen pick-up. The very fine equiaxed microstructure resulting after EBM can be then set up by heat treatment (HT). According to the HT temperature in particular, an equiaxed microstructure, a duplex microstructure with different lamellar ratio and a fully lamellar microstructure is obtained. Not only test bars have been produced but also complex parts such as demo low pressure turbine blades.