Heat Treatment Schedule Research Articles

Role of the γ′′ precipitation at the cell boundaries in enhancing the creep resistance of additively manufactured Inconel 718 alloy using the laser powder bed fusion technique

Laser powder bed fusion (LPBF) manufactured Inconel 718 superalloy (IN718) parts exhibit inferior creep properties compared to their conventionally manufactured (CM) counterparts due to the unique microstructural features associated with them. In this study, two different heat treatment schedules were employed, to critically examine role of distinct microstructural features associated with LPBF on the creep performance of LPBF IN718. Experimental results reveal that the creep performance is controlled by the Laves and γ′′ phases, as well as the cellular structure. Specially, the effective dissolution of Laves phases that are distributed along the grain and cellular boundaries in the as-fabricated state during solution treatment reduces the cavity nucleation sites and hence, extends the creep lifetime. The release of Nb into the matrix contributes to the precipitation of primary strengthening γ′′ phase during subsequent aging, enhancing the creep resistance. More importantly, the formation of a unique cellular structure with densely distributed γ′′ phase along the cellular boundaries, which exhibits high impediment of dislocation movement and inhibition of crack propagation, results in superior creep performance. The findings of the current study provide an effective heat treatment strategy for achieving high creep performance in LPBF nickel-based superalloys.

Evidence to support phytosanitary policies–the minimum effective heat treatment parameters for pathogens associated with forest products

Research on reducing the movement of pests on wood products has led to several options for safer trade including heat treatment of wood to mitigate pests. In this study, pathogenic organisms commonly regulated in the trade of forest products were tested to determine the minimum heat dose (temperature and time) required to cause mortality. The mycelial stage of tree pathogens, Heterobasidion occidentale, Grosmannia clavigera, Bretziella fagacearum, Phytophthora cinnamomi, P. lateralis, P. ramorum and P. xmultiformis, which may be found in untreated wood products, were tested in vitro using the Humble water bath with parameters simulating the rate of heat applied to wood in a commercial kiln. RNA detection using reverse transcription real-time PCR was used to validate pathogen mortality following treatment for: P. ramorum, P. lateralis, P. cinnamomi, P. xmultiformis and G. clavigera. The lethal temperature for all pathogens ranged from 44 to 50°C for a 30-min treatment duration. Using this method to evaluate heat treatment for other forest product pests is recommended to accurately identify the minimum dose required to support phytosanitary trade. With more data potentially lower heat treatment applications may be recommended under specific conditions to produce more efficient and economical heat treatment schedules and reduce environmental impacts.

Effect of adaptive control of cooling rate on microstructure and mechanical properties of laser additively manufactured Inconel 718 thin walls

Ni-alloy Inconel 718 (IN718) is finding ever-increasing applications in various industries like aviation and aerospace. IN718 is a precipitation-strengthened alloy which derives its strength mainly from the intermetallic phases, γ” (Ni3Nb) along with the γ’ (Ni3(Al, Ti)) phase which can be precipitated using standard heat treatment schedules. However, the consummation of Nb in Laves phase formation during solidification tends to deteriorate the strength of the material. During laser additive manufacturing (LAM) by directed energy deposition (DED), the cooling rate variation along the build direction causes variations in the microstructure, properties and Laves phase distribution. The study aims to minimize the heterogeneities along the build direction by monitoring and controlling the cooling rate during DED of IN718. The feedback control system involves the use of two pyrometers in tandem with a leading pyrometer measuring the molten pool temperature and a trailing pyrometer measuring the temperature at a distance behind the molten pool. Two sets of thin walls of 120 layers were fabricated with one set being of constant process parameters without feedback control and the other set being of adaptive-controlled process parameters. The adaptive control was aimed at maintaining the cooling rates above a desired limit by changing the process parameters, namely the laser scan speed and the powder feed rate. The Laves phase morphology, which affects the strength of the deposited part deleteriously was finer and more discrete in the adaptive-controlled walls which led to their easier dissolution during heat treatment. This in turn helped in the precipitation of more amount of strengthening phases during heat treatment which led to better mechanical properties. The characterizations of the two sets of walls reflected the superior part quality of the adaptive-controlled wall in terms of homogeneity of microstructure, enhanced microhardness and tensile strength, thereby validating the importance of adaptive control of cooling rate in ensuring better quality in LAM parts.

Influence of cooling rate during the heat treatment process on the precipitates and ductility behavior of inconel 718 superalloy fabricated by selective laser melting

IN718 superalloy by elective laser melting (SLM) has shown great application prospects in the aerospace engine key parts. Different phases are precipitated during the SLM forming process. Although the Laves phase could be dissolved in dendrites by heat treatment, the specific evolution of the second phase particles is still unclear. In this work, the influences of solution and aging heat treatment, as well as cooling rate on the precipitates and ductility behavior of IN718 superalloy fabricated by SLM were systematically investigated. The medium temperature (980 °C) solution treatment regimes included three different colling rates and the control group only underwent aging treatment were studied. The results indicated that high temperature solid solution treatment effectively improved compositional segregation and significantly enhanced homogeneity of microstructure. The reduce of cooling rate at 980 °C was beneficial to the extension time and increased the number of δ phase precipitation. An appropriate number of δ phase improves the notch sensitivity and nails grain boundaries. In addition, the complex dislocations and second phase particles retards the migration of slip dislocations during ductility deformation process, which enhances the strength of SLM-ed IN718. The yield strength of specimens increases from 680 MPa to 1205 MPa. The microhardness is greatly improved as compared with as-deposited IN718, and the highest microhardness reaches 485 HV. The results provide a technical support and reference for formulating the heat treatment schedule of IN718 prepared by SLM.

Influence of gradual stress relieving on the microstructure, texture development, grain structure and orientation of the hot-rolled Ti6Al4V–5Cu alloy

In order to investigate the influence of gradual stress relieving treatment on the microstructure, texture evolution, grain structure, and misorientation distribution of the hot-rolled Ti6Al4V–5Cu (α+β) alloy deformed in various thickness reduction ratios (TRR's) of 15%, 58%, and 73% to maintain the burger orientation relationship (BOR), different stress reliving treatment was specially designed for relieving the residual stresses occurred due to the hot deformation process. Using IPF maps, the majority of the α grains, specifically the elongated ones were found nearly at 45 ° with respect to the rolling and transverse direction. The study also show that the heat treatment schedule on the 58% deformed alloy significantly promote the transformation of β to α phase and a special microstructure with an equiaxed grains is produced. The local misorientation angle distribution due to stress reliving treatment has also discussed. Using EBSD analysis, the alloy processed under highest TRR's (i.e., 73%) and subsequent stress relieving treatment could result a serious internal crystalline instability with various degree of element diffusion rate, which promote a severe and ease of alloy coarsening. Thus, coarsening encourage the formation of grains with the highest softening potential, comparatively. It is obvious that the stress-relived treatment does not result a change in microstructure. However the internal residual stresses on the hot deformation were relieved, the texture intensity had increased quietly. A significant textural behavior could be achieved on the 73% hot-rolled alloys by the stress-relieving process to align the α/β fiber texture interface in parallel coincidence to maintain the BOR.

Microstructural evolution and tensile properties of direct energy deposited Ti-4Al-5Co-0.25Si alloy during heat treatment

Additive manufacturing (AM) stands as an advanced manufacturing process, successfully utilized for the fabrication of high-value titanium components. However, these components have yet to find widespread use in industrial applications due to their inadequate performance when compared to conventionally processed parts. Consequently, there is a pressing demand to develop new alloys and microstructural control strategies to overcome this limitation. Here, we have designed a cost-effective Ti-4Al-5Co-0.25Si alloy for use in laser-based direct energy deposition additive manufacturing. The selection of solute elements has resulted in both solute strengthening and grain refinement. Additionally, the implementation of a heat-treatment schedule has allowed for control over the phase constituents and their stability. The as-fabricated alloy exhibits a prior-β grain size of ∼50 μm, α plate thickness of ∼0.6 μm, and tensile strength of 1145 MPa. Subsequent heat treatment induces the formation of subgrains, discontinuous grain boundary α and metastable β phases, and ultrafine martensitic features ranging from 40 to 100 nm. Moreover, this heat treatment improved the tensile strength of the alloy to 1500 MPa while maintaining an elongation of ∼5%. Importantly, the proposed hierarchical structured alloy offers an economical alternative, outperforming commercially available titanium alloys. This study presents a new direction for further research into alloy and microstructural design, aiming at the development of low-cost, high-performance titanium alloys suitable for additive manufacturing.

Enhanced strength and ductility of laser-directed energy deposition repaired IN718 superalloy via a novel tailored heat treatment

High-quality repair of damaged Inconel 718 (IN718) superalloy components can achieve great economic benefits. However, the directly double aging (DA) treatment by industrial standards, yields an inferior ductility on the repaired component than that of the wrought base metal. In this work, wrought IN718 components were repaired by laser-directed energy deposition (LDED), a novel tailored heat treatment (THT) schedule consisting of a short-term low-temperature homogenization, and subsequent DA was subsequently conducted to strengthen the repaired IN718 alloys. The microstructure evolution and mechanical properties of the DA and THT-treated repaired alloys were comparatively investigated. The results indicated that the THT effectively dissolved most of the hard brittle Laves precipitates in the deposition region with only slight coarsening of the grains in the substrate. As compared to the DA sample, the elongation of the THT sample increased remarkably by 88% with only a slight reduction of 19.2 MPa in yield stress. Moreover, the strain distribution of the THT sample was overall more even but then destabilized in a narrow abnormal coarsened grain region caused by the static recrystallization. In general, this study breaks through the limitation of the low ductility of the DA-treated repaired IN718 alloys and provides a promising way to further improve the mechanical properties.

Impact of Different Heat Treatments on the Mechanical Properties and Microstructure of Precipitation-Hardened Stainless Steel Fabricated by Laser Powder Bed Fusion

The mechanical properties of additively manufactured metal parts are often considered inferior to those of their traditionally manufactured counterparts. These inferior mechanical properties are primarily attributed to prevalent defects inherent in additive manufacturing processes, leading to reduced performance and durability. Researchers have extensively studied processing parameters and post-processing techniques to determine optimal conditions for improving the mechanical properties of laser powder bed fusion. This study investigates the densification and microstructure characteristics of laser powder bed fusion 15-5 precipitation-hardened stainless steel. The effects of three developed post-heat treatments and three build directions are examined. The results reveal that heat treatment schedules influence material strength and hardness at the cost of reduced ductility, while the fabrication build direction impacts surface porosity. Prolonged heat treatment procedures resulted in the highest hardness values due to enhanced homogenization.

Effect of heat treatment on microstructure evolution, strengthening mechanisms and mechanical properties of Zn modified Al–Mg alloys with Sc and Zr additions

In this work, a new Al–Mg–Zn alloy was designed based on thermodynamic calculations combined with the concept of cooperative strengthening and toughening. The influence of different aging processes on microstructure and mechanical properties of the designed Al–Mg–Zn alloy was studied using experimental techniques including hardness testing, room-temperature tensile testing, EBSD, and HRTEM. The results indicate that after four different aging heat treatments, fine and uniformly distributed nano T-Mg32(Al, Zn)49 phases have precipitated within the grains. Compared to single-stage aging (140 °C/36 h, 180 °C/7 h), two-stage aging (90 °C/24 h + 140 °C/18 h, 90 °C/24 h + 180 °C/5 h) resulted in a smaller size and an increased number density of precipitated phases within the grains. After the aging process of 90 °C/24 h + 140 °C/18 h, the grains exhibited the smallest precipitated phases, being of 2.77 nm in size and a number density of 1.06 × 1023 #/m³. Additionally, TEM diffraction spot analysis identified the presence of Al3(Sc, Zr) precipitates within α-Al matrix. Under the aging process of 90 °C/24 h + 140 °C/18 h, the alloy exhibited superior mechanical properties with a tensile strength of 566 MPa, yield strength of 477 MPa, and elongation of 16.5%. The primary strengthening mechanisms of the presently investigated Al–Mg–Zn alloy include precipitation strengthening, solid solution strengthening, and grain boundary strengthening. A yield strength model considering various strengthening mechanisms is used to predict the yield strength based on thermodynamic calculations and experimental data. The research findings will offer theoretical insights guiding the design of compositions and heat treatment schedule for aluminum alloys.

Innovative pathways to high-performance glass ceramics: Harnessing nature's treasures with chromium and zirconium nucleation catalysts

This study explores the novel production of high-performance glass ceramics (GCs) by harnessing readily available and cost-effective raw materials schist, dolomite, and magnesite for the first time. The nominal composition for the base glass, G(30), was designed based on the eutectic composition of the diopside-anorthite ratio with 30% enstatite content within the quaternary CaO-MgO-Al2O3-SiO2 system. To enhance nucleation, chromium (Cr2O3) and zirconium (ZrO2) oxide were separately introduced into the base glass batch in two ratios (0.5% and 1%), producing four additional batches-Cr(0.5), Cr(1), Zr(0.5), and Zr(1), orderly. All these batches were melted at temperatures ranging from 1450 °C to 1550 °C then cast into glass discs and rods. Double-stage heat treatment schedule, 750 °C /2 h followed by 950 °C and 1050 °C/1 h separately, was employed to induce the transformation of prepared glass into GCs. X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) techniques were applied to orderly investigate the developed crystalline phases and the prevailing micro-structures of the prepared GCs. The glass ceramic with the highest Cr2O3 ratio, Cy(1) which was treated at 1050 °C/1 h, displayed the most adequate microstructure and minimal pore formation in comparison with other GCs. As well, the favorable physicochemical properties, Vickers microhardness (6.43 GPa), low thermal expansion coefficient (0.171 ×10 −6 °C−1) at 29–100 °C, and the exceptional resistance for acid and alkali corrosion (95%), nominate such GS sample, Cy(1), for several architectural applications.

A novel method to evaluate the mechanical properties of resistance spot welds under dynamic loading

The mechanical properties of resistance spot welds are integral in dictating joint performance in service. However, previous studies have evaluated the joint response to loading under the quasi-static condition which is not a true indication of the loading condition in service. This work has developed a novel method to evaluate spot welds under dynamic loading using two test geometries (opening and shear mode). The testing technique was used to compare the mechanical properties of a conventional and in-situ post-weld heat treatment (PWHT) schedule. Our developed method can expand the testing capability of single-spot welds under dynamic loading.

Preparation and characterization of SiO2–Al2O3–Na2O–LaF3 upconversion glass-ceramic and its enhancing effect on the performance of photocatalyst

SiO2–Al2O3–Na2O–LaF3 glass-ceramic was prepared by high-temperature melting method and two-step heat treatment schedule. The upconversion performance was obtained under the effect of Yb and Tm. The optimal doping concentration ratio and the photoluminescence mechanism was analyzed. By introducing K+ and Li+ into the system, further optimization of upconversion performance was achieved. To investigate its impact on the efficiency of photocatalysts, glass-ceramic was mixed with photocatalysts to conduct photocatalytic degradation tests. The degradation efficiency of the mixture increased by 22 % compared to the group with only photocatalysts under near-infrared light, and this rate increased by 5.7 % under full spectrum. The first-order kinetic equation was used to fit the degradation rate for kinetic analysis, compared with the k values under near-infrared light, photocatalyst/glass-ceramic (k = 0.0035) > photocatalyst (k = 0.0006), and photocatalyst/glass-ceramic (k = 0.0135) > photocatalyst (k = 0.0124) under full spectrum.

Prediction of Mechanical Properties in the Sub-Critical Heat Affected Zone of AHSS Spot Welds Using Gleeble Thermal Simulator and Hollomon-Jaffe Model

Measuring the mechanical properties of weld Heat Affected Zone (HAZ) remains one of the main challenges in the failure analysis of spot-welded components. Due to the small size of the HAZ and variation in the temperature history, different peak temperatures and cooling rates impose a range of phase transformations across the resistance spot weld. Among the HAZ sub-regions, the sub-critical HAZ (SCHAZ), which experiences temperatures below AC1 (350–650 °C), usually shows a reduction in the hardness in most of the modern AHSS grades due to the martensite tempering phenomenon. SCHAZ softening may lead to strain localization during loading. Therefore, it is important to characterize the local properties of the SCHAZ region to accurately predict RSW failure. However, it is not feasible to extract standard mechanical test specimens out of the SCHAZ of the spot-welded structure due to its small size. In this work, the SCHAZ of the spot weld for two AHSS, 3G-980 and PHS-1500, was simulated using a Gleeble® (Dynamic Systems Inc., 323 NY-355, Poestenkill, NY 12140, USA) 3500 thermo-mechanical simulator. An in-situ high-speed IR thermal camera was used to measure the entire temperature field during the Gleeble heat-treatment process, which allowed for the visualization of the temperature distribution in the gauge area. The temperature and hardness data were fit to a Hollomon-Jaffe (HJ) model, which enables hardness prediction in the SCHAZ at any given temperature and time. Using the HJ model, a heat treatment schedule for each material was chosen to produce samples with hardness and microstructure matching the SCHAZ within actual spot weld coupons. Tensile specimens were machined from the coupons heat treated using simulated heat treatment schedules, and mechanical testing was performed. The results showed that the 3G-980 SCHAZ has a slight increase in yield strength and tensile strength, compared to the base metal, due to the formation of fine carbides within the microstructure. In contrast, the SCHAZ of PHS-1500 showed a significant reduction in the yield and tensile strength with yield point elongation behavior due to the reduction of the martensite phase and an increase in carbide formation due to the tempering process.

A non-rare-earth-doped self-activated blue fluorescent material of transparent glass-ceramics containing single-phase NaY(MoO4)2: The controllable synthesis, crystallization behavior and luminescent properties

The glass with components of 50SiO2-20B2O3-20Na2O-6ZnO-3MoO3-1Y2O3 (mol%) was synthesized by the melt-quenching method, and the corresponding glass-ceramics were obtained after heat treatment. The crystallization behavior of the glass was characterized by the DSC test. By carefully examining the structure and grain size of the NaY(MoO4)2 crystals with scheelite structure using XRD and SEM, it can be confirmed that the crystal size steadily increases after increasing the crystallization temperature or extending the crystallization time, which is in the range of 27–45 nm. The transmittance of glass-ceramics somewhat declines compared to the precursor glass, although it remains high (>80 %). Under the excitation of 365 nm, the glass-ceramic shows a broad emission band, which is related to the charge transfer of Mo6+→O2- in the [MoO4]2- unit. The glass-ceramic is heat-treated at 590 °C for 3 h as the optimal heat treatment schedule, and its CIE chromaticity coordinate is located in the blue-light region, indicating the potential application of the non-rare-earth-doped self-activated light-emitting glass-ceramics containing single-phase NaY(MoO4)2 in the blue light-emitting field.

Tailoring the mechanical properties and thermal stability of additive manufactured micro-alloyed Al-Cu alloy via multi-stage heat treatment

In this study, a novel Sc/Zr/Ti/Er-modified 2219 aluminum alloy was designed and wall-shaped components were then fabricated by wire-arc directed energy deposition (DED). Due to the heterogeneous nucleation promoted by primary Al3X dispersoids, the as-deposited alloy exhibits a highly equiaxed fine grain structure. Two heat treatment schedules were conducted to tailor the strength and thermal stability of the alloy. Compared with the classical T6 heat treatment, the additional homogenization step before the T6 heat treatment (HT6) prefabricated more Al3X dispersoids, which indirectly promoted the transition from θ″ precipitates to θ′ precipitates, resulting in a slight reduction in strength. However, this homogenization step helps to avoid the aggregation of pores by promoting pores ripening and significantly alleviates the anisotropy of ductility. In addition, the better thermal stability of the HT6 heat treated alloy is due to more Al3X dispersoids and finer θ′ precipitates with high coarsening resistance caused by the segregation of Sc and Mn at the θ′/Al interface during thermal exposure. In general, the wire-arc DEDed micro-alloyed 2219 Al alloy exhibits a promising combination of strength and ductility after heat treatment and its mechanical properties are superior to those of the wrought micro-alloyed Al-Cu alloys.

Effects of Cr3+ content on the structure and fluorescence properties of SiO2–Na2O–Y2O3–P2O5 glasses and glass-ceramics with Na3YSi2O7 crystal phase

In this work, the Cr2O3-doped SiO2–Na2O–Y2O3–P2O5 precursor glasses were prepared by the melt-quenching method. A two-step heat treatment was used to obtain Cr2O3-doped glass ceramics with oxyapatite-silicate-type Na3YSi2O7 crystal phase. The optimum heat treatment schedule was determined to be heat-treated 660 °C/1 h + 760 °C/20 min by XRD and fluorescence spectra. Compared with precursor glasses, the luminescent properties of the Cr2O3-doped glass-ceramics containing Na3YSi2O7 crystals have been greatly improved. The optimum Cr2O3-doping amount was determined to be 0.4 mol%. Under the excitation of 490 nm, the glass-ceramics have a broad emission peak centered at 980 nm in the near-infrared region, which belongs to the 4T2→4A2 transition of Cr3+ ions. This work shows that the Cr2O3-doped glass-ceramic materials have the potential to be used as near-infrared luminescent materials in the fields of illumination display, bio-fluorescence imaging, solar cells, etc.

Evolution of microstructure and its influence on the mechanical behavior of LPBF Inconel 625 upon direct aging

Additive manufacturing is known for its many benefits over traditional manufacturing, however, it produces very different microstructure compared to wrought materials. Laser powder bed fusion (LPBF) produces heterogeneous microstructure that requires different heat treatment schedule than typical treatments that are applied to wrought material and it requires extensive research to understand the response of a material to different heat treatments. In this study, an approach has been made to understand the evolution of LPBF Inconel 625(IN625) microstructure upon one step direct aging at different temperatures until it gets close to full recrystallization. Microhardness after these treatments has also been reported and explained in the light of microstructure obtained by each heat treatment. Microstructure and mechanical behavior of LPBF IN625 are compared to as-received wrought IN625. Precipitates found at each temperature are confirmed by either electron diffraction pattern or energy dispersive spectroscopy. Strengthening γ′′ precipitate is found in as-printed as well as heat treated at 700 °C LPBF IN625. Detrimental δ phase is identified after heat treatment at 900 °C and carbides are found after 1050 °C. Hardness of as-printed LPBF IN625 is higher than that of wrought IN625. Similar trend is observed for both 700 °C and 900 °C heat treatment where hardness first decreases after 2 h, then increases after 5 and 10 h. For 1050 °C, hardness first decreases after 2 h and gets closest to hardness of wrought IN625, then increases after 5 h before decreasing slightly again after 10 h.

Heat Transfer Analysis of MgB2 Coil in Heat Treatment Process for Future Fusion Reactor

State of the art MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> is reviewed as a potential material for the poloidal field (PF) coils of the future fusion reactor due to its high critical temperature and low material cost. The heat treatment process is a crucial step in the development of MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> magnets. The temperature lag in heat treatment of large magnets can lead to insufficient thermal reaction time. It may be infeasible to control the temperature of a magnet according to the heat treatment scheme recommended for the MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> wire. Hence, the heat treatment process of a large magnet needs to be evaluated. Therefore, the dynamic temperature distribution of a MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> PF coil is obtained by simulating the heat transfer in heat treatment process. A suitable heat treatment schedule for a large magnet is proposed and the experimental results of a sub-size Cable-In-Conduit Conductor manufactured with MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> strand confirmed the feasibility of the newly proposed heat treatment process. The results provide a reference for the heat treatment method of a future larger MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> coil.

Microstructural engineering by heat treatments of multi-principal element alloys via spinodal mediated phase transformation pathways

Nanoscale multi-phase microstructures observed in multi-principal element alloys (MPEAs) such as AlMo0.5NbTa0.5TiZr, Al0.5NbTa0.8Ti1.5V0.2Zr, TiZrNbTa, AlCoCrFeNi and Fe15Co15Ni20Mn20Cu30 that exhibit promising mechanical or functional properties may have evolved through spinodal-mediated phase transformation pathways (PTPs). The microstructures in such MPEA systems could be further engineered for targeted applications by appropriately designing the alloy composition and heat-treatment schedule. In this study, we investigate systematically how different heat treatment schedules such as single-step isothermal aging, two-step isothermal aging and continuous cooling alter the interplay among the various factors associated with alloy composition, such as volume fraction of individual phases, lattice misfit and modulus mismatch between the co-existing phases. We have determined the degree to which these factors influence significantly the spinodal-mediated PTPs and the corresponding microstructures by use of high-throughput phase-field simulations. In particular, we demonstrate that the microstructural topology (i.e., which phase forms the continuous matrix and which phase forms discrete precipitates) in the same MPEA having an asymmetric miscibility gap could be inverted simply by a continuous cooling heat treatment. Further, we reveal a rich variety of novel hierarchical microstructures that could be designed using two-step isothermal aging heat treatments in MPEA systems with symmetric or asymmetric miscibility gaps. These simulation results may shed light on novel microstructure design and engineering for the above-mentioned MPEA systems.

Effect of Heat Treatment Schedules on Creep Performance of Ni-Based Superalloy Mar-M247 at 871 °C and 250 Mpa

The effects of heat treatment (H1 and H2) on the creep behavior and microstructures of Mar-M247 at 871 °C/250 MPa are studied. The results show that the as-cast microstructure is composed of eutectics, γ phase, γ’ phase, MC and M23C6 carbides, while new M6C appears in heat-treated microstructures, indicating the transformation of carbides after heat treatments. The Mar-M247 is excellent, with over 1500 h of creep life, and H1 is 48% higher than H2. The addition of post-brazing and diffusion heat treatment in H2 is detrimental to creep resistance; the two steps promoted the transformation of MC into M23C6 in advance. The increase and coarsening of M23C6 would consume more γ-phase-forming elements, weakening the solution strengthening at grain boundaries. As a result, the resistance of the grain boundary and γ/γ’ interface to dislocation motion is significantly reduced, leading to the cracks’ initiation and propagation along the grain boundaries.