High-temperature Solution Treatment Research Articles

Achieving Homogeneous Microstructure and Superior Properties in High-N Austenitic Stainless Steel via a Novel Atmosphere-Switching Method

Powder metallurgy is widely used to fabricate high-nitrogen, nickel-free austenitic stainless steel. However, after sintering and nitriding, additional solution treatment is typically required to achieve uniform nitrogen distribution and a homogeneous austenite phase. This work proposes a novel method to eliminate the need for lengthy and high-temperature solution treatment by switching the nitrogen atmosphere to argon during the cooling process. The effects of different N2-Ar atmosphere-switching temperatures (750–1320 °C) on the phase composition, element distribution, microstructure, mechanical properties, and corrosion resistance of the studied steels were systematically investigated. Results show that cooling in the N2 atmosphere initially transforms the matrix to a fully austenitic structure enriched with nitrogen. Excessive nitrogen infiltration leads to Cr2N precipitation, inducing partial austenite decomposition and forming a multiphase structure comprising austenite, α-Fe, and Cr2N. Strategic switching from N2 to Ar reverses this reaction, yielding a high-nitrogen, chemically uniform austenitic structure. Specifically, switching at 1150 °C, the steel exhibits excellent mechanical properties and corrosion resistance, with a yield strength of 749 MPa, an ultimate tensile strength of 1030 MPa, an elongation of 38.7%, and a corrosion current of 0.06 mA/cm2, outperforming the steels cooled solely in N2 and subsequently solution-treated. This novel method offers advantages in cost reduction, energy saving, and operational effectiveness, highlighting its potential for broad industrial application.

Optimization of the grain boundary network and tensile properties of powder metallurgy-hot isostatic pressed Ni-based alloy Inconel 625

Powder metallurgy-hot isostatic pressing (PM-HIPing) is an advanced manufacturing technology for large-sized and complex-shaped components for nuclear applications. In the present study, a novel PM-HIPing strategy was proposed to optimize the grain boundary network and tensile properties of Ni-based alloy Inconel 625, and it was verified by a series of experiments. Samples with 69 % and 74 % of coincidence site lattice (CSL) grain boundaries were obtained when the narrow-size ranged powder was consolidated at 1140 °C and 1180 °C, respectively. The fraction of CSL grain boundaries was further increased to ∼78.5 % after a high temperature solution treatment. Room temperature yield strength and ultimate tensile strength of PM-HIPed alloy 625 were comparable to that of the wrought specimens, elongation of the specimens was ∼56–60 %, the samples all featured with dimple ductile dominant fracture mode.

Uniaxial deformation behaviors and mechanism of Ti–6Al–4V alloy with bi-oriented grains

It is of great importance to investigate the deformation and damage mechanisms of polycrystalline materials with soft crystal orientations, considering the soft grains are usually the fatigue crack initiation sites. In this study, a Ti–6Al–4V alloy fully consisted of <01 1‾ 0> and <1‾ 21‾ 0> α (soft) grains was prepared and its uniaxial deformation behavior was investigated. Interestingly, no significant asymmetry on the tension-compression yield strength was found in this alloy with bi-grain orientations, though mechanical asymmetry was generally found in the traditional Ti alloys with strong texture. After high temperature solution treatment, the preferred bi-orientations were retained and the asymmetry degree of yield strength did not change, showing an excellent stability in texture and mechanical performance. Deformation mechanisms during tension and compression were studied and discussed. This work provides a new understanding on the deformation and damage mechanisms in the grains with soft crystal orientations, and may guide the microstructural design of Ti alloy for pursuing high mechanical and fatigue performances in the future.

Effect of solution treatment on the microstructure, phase transformation behavior and functional properties of NiTiNb ternary shape memory alloys fabricated via laser powder bed fusion in-situ alloying

Post-heat treatment is commonly employed to improve the microstructural homogeneity and enhance the mechanical performances of the additively manufactured metallic materials. In this work, a ternary (NiTi)91Nb9 (at.%) shape memory alloy was produced by laser powder bed fusion (L-PBF) using pre-alloyed NiTi and elemental Nb powders. The effect of solution treatment on the microstructure, phase transformation behavior and mechanical/functional performances was investigated. The in-situ alloyed (NiTi)91Nb9 alloy exhibits a submicron cellular-dendritic structure surrounding the supersaturated B2-NiTi matrix. Upon high-temperature (1273 K) solution treatment, Nb-rich precipitates were precipitated from the supersaturated matrix. The fragmentation and spheroidization of the NiTi/Nb eutectics occurred during solution treatment, leading to a morphological transition from mesh-like into rod-like and sphere-like. Coarsening of the β-Nb phases occurred with increasing holding time. The martensite transformation temperature increases after solution treatment, mainly attributed to: (i) reduced lattice distortion due to the Nb expulsion from the supersaturated B2-NiTi, and (ii) the Ti expulsion from the β-Nb phases that lowers the ratio Ni/Ti in the B2-NiTi matrix, which resulted from the microstructure changes from non-equilibrium to equilibrium state. The thermal hysteresis of the solutionized alloys is around 145 K after 20% pre-deformation, which is comparable to the conventional NiTiNb alloys. A short-term solution treatment (i.e. at 1 273 K for 30 min) enhances the ductility and strength of the as-printed specimen, with the increase of fracture stress from (613 ± 19) MPa to (781 ± 20) MPa and the increase of fracture strain from (7.6 ± 0.1)% to (9.5 ± 0.4)%. Both the as-printed and solutionized samples exhibit good tensile shape memory effects with recovery rates >90%. This work suggests that post-process heat treatment is essential to optimize the microstructure and improve the mechanical performances of the L-PBF in-situ alloyed parts.

Kinetic Monte Carlo simulations of solute clustering during quenching and aging of Al–Mg–Zn alloys

The physical mechanisms behind cluster formation during quenching and aging of age-hardening metallic alloys are poorly understood based on classical nucleation and growth theories, especially in multicomponent alloys with supersaturated vacancies and fast-diffusing solute atoms. Here, solute clustering in an Al–Mg–Zn alloy during quenching immediately after high-temperature (800 K) solution treatment is modeled with a newly developed kinetic Monte Carlo (kMC) framework. This includes accurate surrogate models trained using first-principles-calculated data to predict vacancy migration energetics on the fly as functions of local lattice occupations. First, kMC simulations at constant aging temperatures revealed that temporal evolution of the number, sizes, and compositions of solute clusters change with aging temperature. Such changes are consistent with our analyses based on classical nucleation theory (CNT) and Monte Carlo simulations. Second, the kMC algorithm was revised for simulating quenching processes by iteratively updating the kMC temperature based on simulated cooling rate profiles. These quenching simulations show that rapid solute clustering mainly occurs from ∼600 to ∼400 K during cooling. Below ∼400 K, the clustering is suppressed to a steady state because vacancies are trapped by stable clusters with sizes of ∼2 nm and high magnitudes of formation energies. The configurations of these steady-state clusters closely resemble the solute clusters we observed experimentally in 7XXX series Al–Mg–Zn-based alloy samples immediately after quenching. A two-stage vacancy trapping mechanism revealed in our simulations can provide physical guidance to tune precipitation kinetics during natural and artificial aging.

Microstructure evolution and its effect on the impact toughness of the Ti-Al-V-Mo-Zr alloy tube

Titanium alloys are widely used in the petrochemical industry owing to their excellent overall performance. In this study, a high-temperature solution treatment in the two-phase region was proposed, with a slow cooling rate resulting in the successful partial globularization of lamellar α microstructure, achieving an extruded Ti-Al-V-Mo-Zr alloy tube elongation increase of 69% and impact toughness increase of 51%. The kinking of lamellar α microstructure promoted its continuous globularization by increasing misorientation. Consequently, the elongation and impact toughness of Ti-Al-V-Mo-Zr alloy tubes could be greatly improved by tuning the microstructure to a mix of coarse equiaxed and lamellar α microstructures, the improved strength being attributed to the secondary α phase (αs) obtained using the proposed method and aging. The results of this study can be expected to provide guidance for the fabrication of highly ductile titanium-alloy oil-drilling tubes.Graphical abstract

Effect of Aging Parameters on Inconel 718 Fabricated by Laser Directed Energy Deposition

Inconel 718 is a nickel-based superalloy of high interest in high temperature applications such as turbine parts. To be used in such applications, heat treatments are commonly applied to dissolute Laves phase and to achieve γ′′ phase. However, conventional heat treatment methods for wrought Inconel 718 may not be suitable for Inconel 718 fabricated by laser directed energy deposition (LDED) due to its unique microstructure formed during the rapid solidification process. There has been a lack of investigation in heat treatments for Inconel 718 fabricated by this process, specifically around the impact of aging parameters on this alloy. In this study, the effects of aging parameters were studied by performing seven different heat treatments, including solutionising and aging treatments. Our results indicate that for LDED Inconel 718, a high temperature solution treatment of 1100 ℃ for 1 h followed by single aging at 650 ℃ for 20 h achieved a tensile strength and elongation of 1247 MPa and 23%, respectively. Further, results indicated that even with a shorter aging time of 10 h, γ′′ phase was found to be of comparable size to the standard double aged treatment.

Precipitation and Transformation of Carbides during Tempering of 7Cr14 Martensitic Stainless Steel

The precipitation and transformation of carbides in steel are gaining more interest since they are closely related to the properties of steel. Herein, the precipitation and transformation of carbides during tempering of the 7Cr14 martensitic stainless steel subjected to high‐temperature solution treatment (HTST) are investigated. The results indicate that the precipitation of M3C (300–333 °C) and M23C6 (612–645 °C) carbides occurs during tempering of the HTST steel. The effective precipitation activation energies of M3C and M23C6 carbides are 120 ± 7 and 272 ± 13 kJ mol−1, respectively. The precipitation processes of M3C and M23C6 carbides are dominated by C and Cr atomic diffusion, respectively. Moreover, it is found that two transformation processes occur during tempering with increasing temperature, namely, M3C carbides dissolve in the matrix and then M23C6 carbides precipitate along the boundaries of martensitic laths, and the plate‐like M3C carbides decompose into several globular or rod‐like M23C6 carbides that arrange in a chain‐like morphology.

Synergetic regulation of hierarchical microstructure to improve the 704 ℃ tensile property in laser directed energy deposition IN718Plus superalloy

IN718Plus superalloy with hierarchical microstructure has been successfully manufactured by laser directed energy deposition (LDED). Along building direction, microstructure varies from composing of epitaxial columnar grains oriented along <001>γ direction to equiaxed grains in flower like configuration. Striped Laves phase and fine γ'/γ'' precipitates form in the columnar grains, and would transform into fine Laves phase particles and nanoscale γ' precipitates at equiaxed layer. To improve the compositional homogeneity and microstructural stability, high temperature solution treatment has been explored on the superalloy. The treatment would not affect the morphology and orientation of columnar grains, but would redissolve Laves phase precipitates. Double aging treatment has also been performed to obtain homogeneous distributed γ' precipitates, and the synergistic regulation of microstructure enhances the mechanical property significantly, as causing the room temperature yield strength and tensile strength increased from 0.88 GPa and 1.14 – 1.04 GPa and 1.28 GPa, respectively. Moreover, the excellent mechanical property would be maintained upon 704 ℃, with yield strength of 0.93 GPa, tensile strength of 1.04 GPa and high elongation of 27.3%.

Introduction of a New Method for Regulating Laves Phases in Inconel 718 Superalloy during a Laser-Repairing Process

• An in situ laser heat treatment method has been proposed and verified. • In situ regulation of Laves phase can be achieved during LAM process. • A flow diagram has been made based on the in situ laser heat treatment method. The morphology, size, and distribution of Laves phases have important influences on the mechanical properties of laser-repaired Inconel 718 (IN718) superalloy. Due to the deterioration of the substrate zone, the Laves phase in the laser cladding zone of IN718 superalloy cannot be optimized by a high-temperature solution treatment. In this study, an in situ laser heat-treatment method was proposed to regulate the morphology and size of the Laves phase in the laser cladding zone of IN718 superalloy without impacting the substrate zone. In the in situ laser heat-treatment process, a laser was used to heat previously deposited layers with optimized manufacturing parameters. A thermocouple and an infrared camera were used to analyze thermal cycles and real-time temperature fields, respectively. Microstructures and micro-segregations were observed by optical microscopy, scanning electron microscopy, electron probe microanalysis. It was found that the in situ laser heat treatment effectively changed the morphology and size of the Laves phase, which was transformed from a continuous strip-like shape to a discrete granular shape. The effective temperature range and duration were the two main factors influencing the Laves phase during the in situ laser heat-treatment process. The effective temperature range was determined by the laser linear energy density, and the peak temperature increased with the increase of the linear energy density. In addition, the temperature amplitude could be reduced by simultaneously increasing the laser power and the scanning velocity. Finally, a flow diagram was developed based on the in situ laser heat-treatment process, and the deposition of a single-walled sample with fine and granular Laves phases was detected.

The effect of fine W particles in matrix phase on mechanical properties of tungsten heavy alloys

Tungsten heavy alloys 90W-7Ni-3Co (WNC) and 89.6W-6.2Ni-1.8Fe-2.4Co (WNFC), produced using liquid phase sintering followed by thermo-mechanical processing, were investigated for microstructure and mechanical properties. 90W-7Ni-3Co alloy processed following a cyclic heat treatment showed fine tungsten particles in the matrix. The alloy, when heat treated at a temperature 850°C (the lower temperature of cyclic treatment), formed a W-rich intermetallic phase in the matrix that subsequently dissolved during high temperature (1150°C) solution treatment leaving behind fine W particles. On the other hand, the matrix phase of the alloy, 89.6W-6.2Ni-1.8Fe-2.4Co showed a clean structure devoid of any such precipitates. Both the alloys were subjected to a thermomechanical treatment that included two stage swaging with intermediate heat treatments. 90W-7Ni-3Co alloy exhibited superior strength, but lower elongation to failure and impact toughness as compared to 89.6W-6.2Ni-1.8Fe-2.4Co alloy. Microstructure-property correlation was undertaken in order to elucidate the effect of enhanced W dissolution and fine W precipitates (in the matrix) on mechanical behaviour of the alloys investigated.

On as-built microstructure and necessity of solution treatment in additively manufactured Inconel 939

ABSTRACT Increased adoption of additively manufactured superalloys has led to the consideration of revised heat treatment approaches for these materials. The rapid cooling during additive manufacturing processes has been seen to suppress gamma prime (γ′) precipitation, which has raised the possibilities for omitting the high-temperature solution treatment step that usually precedes ageing heat treatment for these alloys. In this work, the as-built microstructure of a high gamma prime fraction superalloy Inconel 939 is presented, where the absence of any γ′ precipitation is notable. However, transmission electron microscopy shows the presence of nano-sized Eta (η) phase. It is shown that the omission of solution treatment leads to the growth of the deleterious η phase upon ageing, which results in embrittlement in tensile loading. It is concluded that at least for this particular alloy the solution treatment plays a critical role in the establishment of the required microstructure and hence cannot be omitted from the heat treatment.

Role of Direct Aging and Solution Treatment on Hardness, Microstructure and Residual Stress of the A357 (AlSi7Mg0.6) Alloy Produced by Powder Bed Fusion

Applying additive manufacturing (AM) technologies to the fabrication of aluminum automotive components, with an optimized design, may result in improved vehicle light weighting. However, the post-process heat treatment of such alloys has to be customized for the particular AM microstructure. The present study is aimed at investigating the effect of different heat treatments on the microstructure, hardness and residual stress of the A357 (AlSi7Mg0.6) heat-treatable alloy produced by laser-based powder bed fusion (LPBF, also known as selective laser melting). There are two major issues to be addressed: (1) relieving the internal residual stress resulting from the process and (2) strengthening the alloy with a customized heat treatment. Therefore, stress-relief annealing treatment, direct aging of the as-built alloy and a redesigned T6 treatment (consisting of a shortened high-temperature solution treatment followed by artificial aging) were examined. Comparable hardness values were reached in the LPBF alloy with optimized direct aging and T6 treatments, but complete relief of the residual stress was obtained only with T6. Microstructural analyses also suggested that, because of the supersaturated solid solution, different phenomena were involved in direct aging and T6 treatment.

Coupling Effect of Grain Structures and Residual Secondary Phases on Fatigue Crack Propagation Behavior in an Al-Cu-Mg Alloy

The effect of solution treatments at high temperatures on the microstructure and fatigue performance of an Al-Cu-Mg alloy was investigated. Microstructural observation revealed that large secondary phases could be dissolved sufficiently by a strengthening solution at 507 °C for 20 min and shortening the solution time at the high temperature would bring about a significantly grain refinement. The fatigue test showed that the sample treated by the solution at the high temperature for a short time (507 °C/6 min) possessed the lowest fatigue crack propagation (FCP) rates. This was mainly attributed to the combined effect of the reduced large residual secondary phases and the conspicuous grain refinement. Reducing the large secondary phases remaining by the high temperature solution treatment was beneficial to limit the bridging of cracks, and refining the grains with Goss orientation could induce more crack deflection, consequently enhancing the fatigue resistance.

Improving room-temperature stretch formability of a high-alloyed Mg–Al–Ca–Mn alloy sheet by a high-temperature solution-treatment

Various solution-treatments were applied on a rapidly solidified and subsequently rolled Mg-7.50Al-1.07Ca-0.17Mn (mass%) alloy sheet, and a room-temperature stretch formability, tensile properties, microstructures, and textures of the solution-treated sheets have been investigated. Although the solution-treatment at 400 °C gives high strengths, the sheet shows poor room-temperature stretch formability with the Index Erichsen value of 4.8 mm due to densely dispersed second phase particles. Also, the particles tend to distribute to the rolling direction, leading to limited elongation to failure of 11.0% along the transverse direction. The solution-treatment at higher temperature leads to the sparse and relatively uniform dispersion of the particles. This results in the significant improvement in the room-temperature stretch formability and ductility. The Index Erichsen increases to 6.8 mm by the solution-treatment at 450 °C for 4 h, and the sheet shows large elongation to failure of 19.1% and 17.1% along the rolling and transverse directions, respectively. The sheet also exhibits high strengths and in-plane isotropic properties, tensile strength over 320 MPa and 0.2% proof stress of ~180 MPa could be obtained due to fine grain structure with the average grain size of 9.5 μm.

Grain boundary transition associated intergranular failure analysis at TMAZ/SZ interface of dissimilar AA7475-AA2198 joints by friction stir welding

Post-weld heat-treatment (PWHT) of dissimilar AA7475-AA2198 friction stir welds (FSW) at 560 °C followed by water-quenching since cracks were observed at the TMAZ/SZ interface in retreating AA7475 upon quenching. Microstructural studies revealed that composition difference at high-temperature solution-treatment was the driving force for Cu diffusion from advancing AA2198 through grain boundary liquid-metal thin-films, and caused intergranular segregation of a sufficient amount of Cu-Zn phase. Weld residual stresses, weak interfacial strength, grain boundary transition (premelting), and wetting along with thermal expansion coefficient discrepancies were found to be the associated phenomena in intergranular failure.

Improvement of mechanical properties of extruded AZX912 magnesium alloy using high-temperature solution treatment

Abstract

Temperature- and Time-Dependent Mechanical Behavior of Post-Treated IN625 Alloy Processed by Laser Powder Bed Fusion

The microstructure and mechanical properties of IN625 alloy processed by laser powder bed fusion (LPBF) and then subjected to stress relief annealing, high temperature solution treatment, and hot isostatic pressing were studied. Tensile testing to failure was carried out in the 25–871 °C temperature range. Creep testing was conducted at 760 °C under 0.5–0.9 yield stress conditions. The results of the present study provided valuable insights into the static and creep properties of LPBF IN625 alloy, as compared to a wrought annealed alloy of similar composition. It was shown that at temperatures below 538 °C, the mechanical resistance and elongation to failure of the LPBF alloy were similar to those of its wrought counterpart, whereas at higher temperatures, the elongation to failure of the LPBF alloy became significantly lower than that of the wrought alloy. The solution-treated LPBF alloy exhibited significantly improved creep properties at 760 °C as compared to the wrought annealed alloy, especially under intermediate and low levels of stress.

Study on Fracture of Aluminum Alloy Joints Welded by Friction Stir Welding

The precisely controlled heat treatment was studied in order to meet the special requirements of aerospace industry for materials of aluminum alloy. 2219 aluminum alloy welded plate was prepared by friction stir welding, its microstructure and mechanical properties were studied, and the reasons for strength reduction of welded joints were analyzed and summarized correspondingly. The results shows that the plasticity of joints of 2219 aluminium alloy is poor after FSW, the elongation is reduced about 50% to that of base material. With the process of general post-weld heat treatment technology, the plasticity is further reduced, the elongation is reduced about 50% to that of welded joints. But after high temperature solution treatment, the average elongation of the joints increase to corresponding level of as-welded joints, which is about 12.6%. In this precisely controlled heat treatment, the fine grains can be maintained, the plasticity of aluminum alloy can be restored, which means that the softening problem can be eliminated and the mechanical strength can be improved.

Microstructural Evolution and Mechanical Properties of Ti-22Al-25Nb (At.%) Orthorhombic Alloy with Three Typical Microstructures

Microstructural evolution, tensile and creep behavior of Ti-22Al-25Nb (at.%) orthorhombic alloy with three typical microstructures were investigated. The three typical microstructures were obtained by different solution and age treatment temperatures and analyzed by the BSE technique. The tensile strengths of the alloy at room temperature and 650 °C were investigated. The creep behaviors of the three typical microstructures were also studied at 650 °C/150 MPa for 100 h in air. The phase transformation mechanisms in creep deformation were also found. The experimental results showed that the formations of the three typical microstructures were decided by the isothermal forging and heat treatment. It was supposed that the high-temperature solution treatment might be dominant for the volume fraction and diameter of the equiaxed particles. While the double age treatment would lead to lamellar O phases. Due to grain refinement strengthening, the equiaxed microstructure presented the best tensile strength and ductility. The fully lamellar microstructure had the best creep resistance than that of other microstructures. In this paper, the phenomenon of creep-induced α 2 phase decomposition was occurred during creep deformation of the equiaxed microstructure.