Sub-solvus Heat Treatment Research Articles

Effect of heat treatment on the microstructure and recrystallization of an SLM nickel-based superalloy

Heat treatment is essential for selective laser melting (SLM) of nickel-based superalloys. However, the conventional solution heat treatment method of cast superalloys usually leads to serious recrystallization in the samples. The recrystallized grain boundaries under high-temperature conditions reduce the strength of the alloy, which hinders their widespread applications in aerospace. In this work, a nickel-based superalloy with high γ′ content fabricated by SLM was heat-treated at different solution temperatures. The microstructure of the alloy in different heat-treated states was also characterised. A sub-solvus heat treatment was proposed, which can effectively reduce residual stress and inhibit recrystallization. The microstructure showed that the dislocation density was significantly reduced and recrystallized after the solution heat treatment at 1285 ℃/2 h+1290 ℃/2 h+1295 ℃/4 h and 1250 ℃/0.5 h, and the proportion of recrystallization were 60.0 % and 42.9 %, respectively. However, after sub-solvus heat treatment at 1240 °C/0.5 h, the sample dislocation density decreased and the volume fraction of recrystallization (3.2 %) was slightly higher than that of the as-SLM sample (2.5 %). The sample maintained a strong cubic texture in the build direction. After sub-solvus + aging heat treatment, the microstructure maintained a columnar grain structure. The γ/γ′ interfacial dislocation networks and dislocations pile-up were the main dislocation configurations.

Dissolution behavior and kinetics of the γ′ precipitates within a novel powder metallurgy Ni-based superalloy

Isothermal heat treatment experiments were used to studied the dissolution behavior and kinetics of the γ′ precipitates within the hot-extruded novel powder metallurgy Ni-based superalloy A1. The characteristics of the γ′ precipitates were accurately and rapidly extracted by deep learning methods. The JMAK models were established to predict the change of the area fraction of γ′ precipitates during heat treatment. The determination coefficient R2 of model is 0.99. The results indicate that the dissolution sequence, morphological evolution, and dissolution mechanisms of the three types of γ′ precipitates are different. The small-sized tertiary γ′ precipitates (γ′t) dissolve first, while the larger-sized primary γ′ precipitates (γ′p) dissolve last. After 15 min at 1120 °C, there were only a small amount of γ′ precipitates with 0.19 ± 0.03 % area fraction. And there was not significant changed over time. After 1 min at 1150 °C, the γ′ precipitates completely dissolved. During the process of low temperature subsolvus heat treatment (1060 °C, 1090 °C), the large-sized γ′s grown at the expense of consuming the small-sized γ′ precipitates, and the γ′s tend towards directional arrangement due to elastic interaction energy. During the subsolvus heat treatment process, the γ′p spilt due to elastic strain energy and the uneven diffusion of alloying elements, while neck-connected coalescence coarsening occurs due to the overlapping diffusion fields.

Overcoming intermediate-temperature brittleness via regulating trimodal γʹ structure in a novel multi-principal element alloy

Solving the intermediate-temperature brittleness (ITB) of traditional superalloys and multi-principal element alloys (MPEAs) is a serious challenge. In this paper, a novel MPEA with superior mechanical performance is prepared and successfully overcomes the ITB by simple heat treatment. The effects of different solution treatments on the microstructure and mechanical properties of the alloy are studied. This work shows that the dendrite structure with trimodal γʹ structure is preserved by the subsolvus heat treatment. The heterogeneously distributed trimodal γʹ structure with high volume fraction primary γʹ phases in the interdendritic region effectively prevents the localized slip and enhances the pinning effect of grain boundaries, which inhibits the generation of ITB. The yield strengths of the MPEA with trimodal γʹ structure are 802 ± 17 MPa at 973 K, 814 ± 20.7 MPa at 1023 K and 776 ± 9 MPa at 1073 K, respectively, which are superior than that of most cast superalloys and MPEAs. Moreover, the elongations of the MPEA at 973 K, 1023 K and 1073 K are 10.6 ± 1.3 %, 14.2 ± 1.4 % and 11.0 ± 1.5 %, respectively, which increase by 214.9 %, 510 % and 219.4 %, respectively, compared with the MPEA with uniformly distributed bimodal γʹ structure. Meanwhile, the MPEA shows the best intermediate temperature plasticity (14.2 ± 1.4 %) at 1023 K due to the microstwinning. The scheme of combining MPEAs with the trimodal γʹ structure and microtwinning opens up a new avenue for the development and manufacture of advanced high-temperature structural materials.

Structure and properties of LW 4280, a new high γ′ Ni-based superalloy fabricated by laser powder bed fusion

This study presents the successful development of a new high γ′ nickel-based superalloy, LW 4280, using laser powder bed fusion. Microstructural analysis revealed the absence of γ′ precipitates in the as-built condition. A designed sub-solvus heat treatment resulted in a bimodal distribution of γ′ precipitates. The heat-treated specimens displayed yield strength of 1038, 904, 690, 500, 333, 264, and 169 MPa, the ultimate tensile strength of 1455, 1055, 807, 621, 424, 329 and 205 MPa, and elongation of 26%, 26%, 17%, 13%, 24%, 19% and 30% at 21, 760, 871, 927, 982, 1038, and 1065 ⁰C, respectively. Stress rupture tests on heat-treated specimens displayed good rupture characteristics, with rupture times of 190, 205, and 42 h at 871, 927, and 981 °C for stresses of 276, 152, and 138 MPa, respectively. A fractographic analysis was conducted on both the tensile and stress-ruptured specimens to comprehend the failure mechanism.

Boron trapping at dislocations in an additively manufactured polycrystalline superalloy

Segregation of solutes at dislocations in a nickel-based superalloy produced by laser powder bed fusion was investigated. Transmission electron microscopy has shown a high dislocation density present in the as-built microstructure, which was significantly reduced after a subsolvus heat-treatment, but not completely removed. Atom probe tomography revealed segregation of boron at dislocations in the γ matrix in the as-built state. Such segregation was observed for dislocations both in the interdendritic and dendritic core region, but also for dislocations close to grain boundaries. Molybdenum, carbon and tungsten were also found to segregate at dislocations in the γ matrix. After the subsolvus heat treatment, the segregation behaviour of dislocations remaining in the γ matrix is not altered. Boron, molybdenum and tungsten were found to segregate at dislocations but at higher amounts compared to those in the as-built state. Ramifications of boron trapping at dislocations in additively manufactured superalloys are briefly discussed.

Microstructure evolution in GH4742 superalloy by combining hot deformation and heat treatment

The microstructure evolution of GH4742 superalloy under the coupling action of hot deformation and heat treatment was studied by isothermal compression and then solvus heat treatment (SHT), with particular attention focused on the interaction mechanism between γ′ precipitates and dynamic recrystallization (DRX) grains. The deformation of material below the γ′ solvus temperature (1020–1050 °C) by DRX was retarded because of the strong pinning effect of γ′ precipitates. At a strain of 0.6 and all strain rates (0.01–1 s−1), a necklace-like microstructure formed with fine DRXed grains (∼2 μm) and a small DRXed fraction (<20%). The γ′ precipitates dissolved with the migration of DRX grain boundaries (GBs), and there were no secondary γ′ precipitates within DRXed grains. During sub-solvus heat treatment (sub-SHT) of pre-deformed samples below the γ′ solvus temperature, the nucleation rate of post dynamic recrystallization (PDRX) increased significantly (30–50%), and the recrystallized grains were small (∼10 μm) due to a large amount of energy stored after thermal deformation. Rod-shaped γ′ precipitates appeared on the DRXed GBs, while coarse γ′ precipitates (3–5 μm) appeared on the PDRXed GBs and formed an approximate γ+γ′ duplex structure. When the strain was 0.6, regardless of changes in the strain rate (0.01–1 s−1) and temperature (1020–1140 °C) of pre-deformed samples, the microstructure was comprised of large, equiaxed grains with straight GBs and precipitates (20 nm) formed by cooling during super-solvus treatment (super-SHT).

A new class of alumina-forming superalloy for 3D printing

A new class of crack-resistant nickel-based superalloy containing high γ′ fraction is studied for the laser-powder bed fusion (L-PBF) process. The effects of the (Nb+Ta)/Al ratio is emphasized, a strategy that is shown to confer excellent low-temperature strength whilst maintaining oxidation resistance at high temperatures via stable alumina scale formation. The processability of the new alloys is characterized with respect to defect assessment by micro-focus X-ray computed tomography; use is made of a prototype turbine blade geometry and the heritage alloy CM247LC as a benchmark. In all cases, some processing-related porosity is present in thin wall sections such as the trailing edge, but this can be avoided by judicious processing. The cracking seen in CM247LC – in solid-state, liquation and solidification forms – is avoided. A novel sub-solvus heat treatment strategy is proposed which takes advantage of AM not requiring solutioning; super-solvus heat treatment is inappropriate since it embrittles the material by deterioration of the texture and coarsening of grain boundary carbides. The tensile strength of the new superalloy is greatest when the Nb+Ta content is highest and exceeds that of CM247LC up to ∼900°C. The oxidation resistance is best when Al content is highest, and oxidation-assisted cracking resistance maximized when the (Nb+Ta)/Al ratio is balanced. In all cases these are equivalent or superior to that of CM247LC. Nevertheless, the creep resistance of the new alloys is somewhat inferior to that of CM247LC for which the γ′, C, and B contents are higher; this implies a processing/property trade-off which requires further clarification.

Laser powder bed fusion of a new high gamma prime Ni-based superalloy with improved weldability

This study reports the crack-free fabrication of a new high gamma prime precipitation hardened Ni-based superalloy by Laser Powder Bed Fusion (LPBF) process. In the as-built condition, scanning electron microscopy and electron backscattered diffraction analysis indicated the presence of elongated columnar grains with an orientation along 〈1 0 0〉 direction. Following the sub-solvus heat treatment cycle determined using Thermo-Calc software, crack-free microstructure with columnar grains was observed along 〈1 0 0〉 orientation. High-resolution scanning electron microscope analysis revealed a bimodal distribution of gamma prime precipitates and a uniform distribution of carbide precipitates in the heat-treated samples. As a result of heat treatment, hardness increased from 365 HV to 508 HV. The heat-treated samples also displayed excellent yield strength, ultimate tensile strength, and elongation of 939 MPa, 1368 MPa, and 23% respectively at room temperature. Mechanical testing at elevated temperatures confirmed the yield strength, tensile strength, and elongation reached 80 MPa, 106 MPa, and 34% respectively at 1121 °C. The stress rupture test of the heat-treated samples revealed good creep properties with rupture times of 559 h and 75 h, respectively, at 927 °C and 982 °C, for stresses of 152 MPa and 138 MPa.

Effect of heat treatment on the microstructure and elevated temperature tensile properties of Rene 41 alloy produced by laser powder bed fusion

The microstructure and elevated temperature mechanical properties of a precipitation hardenable Nickel based superalloy, Rene 41, fabricated by laser powder bed fusion followed by two different heat treatment regimes were studied. The as-built (AB) microstructure consists of γ columnar grains aligned in <100> direction. No γ′ precipitates were observed in the AB condition. Following to a sub-solvus solutionizing and aging heat treatment, the AB grain morphology was maintained. The development of fine γ′ precipitates within the grains along with the discrete carbide particles on the grain boundaries occurred. Heat treatment above the solvus temperature of γ’ resulted in the formation of a random equiaxed grain morphology. The γ′ and carbide precipitation was also observed in this heat treatment regime, but their distribution and morphology were different. Uniaxial tensile tests were conducted at 760 °C. Average yield strength values for AB, sub-solvus and super-solvus heat treated alloys were 879, 824 and 855 MPa respectively. The three tested conditions showed similar strength values that are comparable with a wrought alloy at the testing temperature. However, the elongation and deformation behaviors were different for each condition. Sub-solvus heat treatment lead to the highest elongation at break with 22% whereas super-solvus heat treatment resulted in the highest work hardening rate during deformation.

Spinodal decomposition versus classical γ′ nucleation in a nickel-base superalloy powder: An in-situ neutron diffraction and atomic-scale analysis

Contemporary powder-based polycrystalline nickel-base superalloys inherit microstructures and properties that are heavily determined by their thermo-mechanical treatments during processing. Here, the influence of a thermal exposure to an alloy powder is studied to elucidate the controlling formation mechanisms of the strengthening precipitates using a combination of atom probe tomography and in-situ neutron diffraction. The initial powder comprised a single-phase supersaturated γ only; from this, the evolution of γ′ volume fraction and lattice misfit was assessed. The initial powder notably possessed elemental segregation of Cr and Co and elemental repulsion between Ni, Al and Ti with Cr; here proposed to be a precursor for subsequent γ to γ′ phase transformations. Subsolvus heat treatments yielded a unimodal γ′ distribution, formed during heating, with evidence supporting its formation to be via spinodal decomposition. A supersolvus heat treatment led to the formation of this same γ′ population during heating, but dissolves as the temperature increases further. The γ′ then reprecipitates as a multimodal population during cooling, here forming by classical nucleation and growth. Atom probe characterisation provided intriguing precipitate characteristics, including clear differences in chemistry and microstructure, depending on whether the γ′ formed during heating or cooling.

Nucleation mechanism of hetero-epitaxial recrystallization in wrought nickel-based superalloys

The mechanism of inverse precipitation of γ-like shells forming around primary γ' leading to heteroepitaxial recrystallization after thermomechanical processing was studied. The wrought nickel-based AD730™ alloy was subjected to a sub-solvus heat treatment at 1050°C and cooled at 5°C/min in order to form heteroepitaxial γ-like shells on primary γ'. Electron backscattered diffraction analysis confirmed no measurable crystal misorientation between the γ-like shells and primary γ'. Atom probe tomography revealed segregation of Cr, Co and Fe at dislocations within the primary γ', while the γ-like shell was found enriched in these solutes compared to the γ matrix. A mechanism rationalizing the inverse precipitation of γ-like shells is proposed aiming to better understand the nucleation process of heteroepitaxial recrystallization in polycrystalline Ni-based superalloys.

Nucleation Mechanism of Hetero-Epitaxial Recrystallization in Wrought Nickel-Based Superalloys

The mechanism of inverse precipitation of γ-like shells forming around primary γ' leading to heteroepitaxial recrystallization after thermomechanical processing was studied. The wrought nickel-based AD730™ alloy was subjected to a sub-solvus heat treatment at 1050 o C and cooled at 5 o C/min in order to form heteroepitaxial γ-like shells on primary γ'. Electron backscattered diffraction analysis confirmed no measurable crystal misorientation between the γ-like shells and primary γ'. Atom probe tomography revealed segregation of Cr, Co and Fe at dislocations within the primary γ', while the γ-like shell was found enriched in these solutes compared to the γ matrix. A mechanism rationalizing the inverse precipitation of γ-like shells is proposed aiming to better understand the nucleation process of heteroepitaxial recrystallization in polycrystalline Ni-based superalloys.

Microstructural characterization of three different size of gamma prime precipitates in Rene 65

Cast and wrought Ni-based superalloy Rene 65 was developed for turbine engine components applications. In the as-extruded condition the alloy presents trimodal distribution of gamma prime (γ′) precipitates. A detailed microstructural and chemical characterization of the precipitates is presented at a near atomic-scale before and after a softening sub-solvus heat treatment. Scanning and Transmission Electron Microscopy (SEM and TEM) as well as Atom Probe Tomography (APT) were employed to identify the differences in compositions. In the as received condition differences in Cr, Ti and Al content were found between the three precipitate sizes. After a softening sub-solvus heat treatment, the interface between secondary and grown tertiary γ′ with the matrix had become chemically homogenized. Lattice mismatch of secondary and tertiary γ′ were empirically calculated based on the APT compositions.

Spinodal Decomposition Versus Classical Gamma-Prime Nucleation in a Nickel-Base Superalloy Powder: An In-Situ Neutron Diffraction and Atomic-Scale Analysis

Contemporary powder-based polycrystalline nickel-base superalloys inherit microstructures and properties that are heavily determined by the thermo-mechanical treatments during processing. Here, the influence of a thermal exposure alone to an alloy powder is studied to elucidate the controlling formation mechanisms of the strengthening precipitates using a combination of atom probe tomography and in-situ neutron diffraction. The initial powder comprised a single-phase supersaturated γ only; from this, the evolution of γ-prime volume fraction and lattice misfit was assessed. The initial powder notably possessed elemental segregation of Cr and Co and elemental repulsion between Ni, Al and Ti with Cr; here proposed to be a precursor for subsequent γ to γ-prime phase transformations. Subsolvus heat treatments yielded a unimodal γ-prime distribution, formed during heating, with evidence supporting its formation to be via spinodal decomposition. A supersolvus heat treatment led to the formation of this same γ-prime population during heating, but dissolves as the temperature increases further. The γ-prime then reprecipitates as a multimodal population during cooling, here forming by classical nucleation and growth. Atom probe characterisation provided intriguing precipitate characteristics, including clear differences in chemistry and microstructure, depending on whether the γ-prime formed during heating or cooling.

On the Evolution of Primary Gamma Prime Precipitates During High Temperature and High Strain Rate Deformation and Subsequent Heat Treatment in the Ni-Based Superalloy, RR1000

The microstructural evolution following compression and subsequent sub-solvus and super-solvus heat treatment was studied in the Ni-based superalloy, RR1000, typically used for rotor disc applications in aero-engines. For a low strain rate of 0.1 s−1 at close to solvus temperature, 1413 K (1140 °C), the flow stress is constant. For larger strain rates of 1 and 10 s−1 at sub-solvus temperature, 1373 K (1100 °C) dynamic re-crystallization (DRX) of γ grains occurs during forging with accompanying stress decay. Incoherent primary γ′ precipitates form mainly via meta-dynamic re-crystallization (MDRX) at 1 s−1 and are as intergranular. For 10 s−1, the coherently nucleated or existing precipitates present in the initial as-HIP condition become incoherent when the grain boundary sweeps past them during DRX and subsequent grain growth. The incoherent primary γ′ precipitates are mainly intragranular. During sub-solvus heat treatment at 1373 K (1100 °C), dissolution of the incoherent precipitates occurs through coarsening of the coherent intragranular population with only sporadic incoherent precipitates remaining. The prior induced deformation (strain and strain rate) influences the evolution of precipitate morphologies during cooling following heating to super-solvus temperature. Using numerical simulations, a quantitative calculation of the different precipitate morphologies was carried out during slow cooling from super-solvus temperature, 1443 K to 1373 K (1170 °C to 1100 °C).

Overgrown grains appearing during sub-solvus heat treatment in a polycrystalline γ-γ’ Nickel-based superalloy

The present article focuses on establishing the nature of excessive grain growth in a new γ-γ’ superalloy, by investigating the underlying driving forces. Excessive grain growth upon annealing after deformation is a common phenomenon among Nickel-based superalloys. It shares common features with abnormal grain growth, but is fundamentally different, since not driven by capillarity forces. It consists in the selective and exaggerated growth of some grains, leading to the formation of heterogeneous microstructures and dramatically decreased mechanical properties. The present study demonstrates that the stored energy in the deformed matrix is the key driving force for exaggerated grain growth. The self-impingement of those grains is the main phenomenon limiting their growth, as Zener pinning by second-phase particles plays a minor role in the process. The absence of stored energy in the overgrown grains, the kinetics of their development and the existence of an incubation time before their appearance in the microstructures lead to the conclusion that excessive grain growth is a case of “critical” static recrystallization, involving a limited amount of nuclei. By evaluating the magnitude of the driving forces in presence before annealing, the microstructure evolutions during further annealing can be predicted, independently from the processing conditions.

Effect of hot extrusion and heat treatment on microstructure of nickel-base superalloy

Effects of hot extrusion (HEX) and heat treatment on prior particle boundary (PPB), MC carbides, γ′ precipitates and grain size of nickel-base FGH96 superalloy were studied. The results show that PPB consists of large γ′, MC carbides enriched with Ti, Nb and a modicum of oxides. Thereafter, it can efficaciously tune γ′ precipitate size from micrometer down to nanometer region and simultaneously results in the annihilation of PPB by HEX process. The activation energy for grain growth of as-HEXed FGH96 superalloy was measured to be 402.6 kJ/mol, indicating that γ′ precipitate serves the critical role in inhibiting grain growth under sub-solvus heat treatment. Moreover, the results reveal that grain growth is primarily restrained by MC carbide in the case of super-solvus temperature.

Microstructure and Mechanical Properties of an Advanced Nickel-Based Superalloy in the as-HIP Form

This study evaluates the suitability of as-hot isostatically pressed (HIP) RR1000 for non-critical applications in aero-engine components. RR1000, an advanced powder nickel-based superalloy, was developed for disc rotor components in aero-engines. For these critical applications, the consolidated alloy powder particles are extruded to break down carbide and oxide networks, known as prior particle boundaries (PPBs), and to refine the structure into a fine grain size for isothermal forging. In this study, hot isostatically pressed compacts, made from two different powder particle size fractions have been assessed following heat treatments below and above the gamma prime solvus temperature. A microstructural evaluation shows a greater degree of PPB decoration occurs in the finer powder particle size fraction. Following a super-solvus heat treatment these PPBs pin grain boundaries of the fine powder particle size compacts, whilst the reduction of PPB decoration in coarse powder particle compacts allows significant grain growth. Tensile test results of as-HIP RR1000 show, good yield strengths, ultimate tensile strengths and ductility, which are comparable with extruded and isothermal forged RR1000 disc material. Dwell crack propagation tests show that finer powder particle size compacts, which have received a sub-solvus heat treatment, give the highest crack growth rates; whilst the remaining material conditions show markedly improved crack growth resistance. In conclusion, as-HIP RR1000 demonstrates clear potential for use in non-critical applications, employing either powder particle size fraction used in this study subject to the appropriate solution heat treatment.

Mykrolis Corp, USA

The microstructure and elevated temperature mechanical properties of a precipitation hardenable Nickel based superalloy, Rene 41, fabricated by laser powder bed fusion followed by two different heat treatment regimes were studied. The as-built (AB) microstructure consists of γ columnar grains aligned in <100> direction. No γ′ precipitates were observed in the AB condition. Following to a sub-solvus solutionizing and aging heat treatment, the AB grain morphology was maintained. The development of fine γ′ precipitates within the grains along with the discrete carbide particles on the grain boundaries occurred. Heat treatment above the solvus temperature of γ’ resulted in the formation of a random equiaxed grain morphology. The γ′ and carbide precipitation was also observed in this heat treatment regime, but their distribution and morphology were different. Uniaxial tensile tests were conducted at 760 °C. Average yield strength values for AB, sub-solvus and super-solvus heat treated alloys were 879, 824 and 855 MPa respectively. The three tested conditions showed similar strength values that are comparable with a wrought alloy at the testing temperature. However, the elongation and deformation behaviors were different for each condition. Sub-solvus heat treatment lead to the highest elongation at break with 22% whereas super-solvus heat treatment resulted in the highest work hardening rate during deformation.