Multi-step Heat Research Articles

Fabrication of hierarchical layered structure in Zr/Ti composite via multi-step heat treatments and its effect on the mechanical properties

The strength and ductility of metallic structural materials are two pivotal properties generally considered mutually contradictory. In the past few decades, numerous heterogeneous materials have been designed and fabricated to overcome this trade-off. Biomimetic-designed hierarchical layered (HL) materials represent a promising approach. In the present study, HL Zr/Ti materials with three hierarchical levels were fabricated via multi-step heat treatments. Scanning Electron Microscopy (SEM), Transmission electron microscope (TEM), Electron Backscatter Diffraction (EBSD), Transmission Kikuchi Diffraction (TKD), and Energy Dispersive X-ray Spectroscopy (EDS) were employed to observe the evolution of the HL microstructure. The mechanical properties of the HL Zr/Ti materials were assessed through hardness and tensile tests, analyzing the influence of microstructural changes in the diffusion layer on the overall mechanical performance. Our results demonstrated that the HL structure could introduce extra strength beyond the rule of mixtures (ROM) due to multiple-scaled hetero-deformation induced (HDI) strengthening. Additionally, the refinement of the second-level layer thickness effectively inhibited crack propagation. This research provides new insights into developing multi-level HL materials with enhanced mechanical properties through cost-effective and scalable manufacturing processes.

New strategy to simultaneously improve strength-toughness balance for low-carbon ultrastrong steel by multi-step heat treatment process

The quenching-partitioning-tempering (QPT) multi-step heat treatment process was employed to enhance the strength-toughness balance of low-carbon ultrastrong steel based on the controllable transformation-induced plasticity (TRIP) effect and co-precipitation strengthening. The aging embrittlement of ultrastrong steel, with an impact toughness of only 5.8 J at peak strength, has been effectively addressed through the QPT process. The sufficient reversed austenite (RA) can be obtained by controlling the partition temperature to 650 °C, followed by aging at 550 °C to fully re-precipitate nanoparticles. The volume fraction of RA in QPT650 steel reached 16.4 %, and the number density of nanoparticles increased dramatically, resulting in a high yield strength of 1233 MPa and excellent impact toughness of 60.4 J. The coarsening and re-precipitation of nanoparticles were clarified, and the strengthening increments of QPT650 steel reaches 584.6 MPa, and the substantial enhancement of Orowan strengthening is responsible for the high yield strength. The excellent impact toughness of QPT650 steel is attributed to the obvious TRIP effect of RA that consumes a large amount of crack propagation energy, and a small number of sheared nanoparticles provide limited micro-cracks, resulting in a strong crack resistance.

Austenite formation and solute redistribution during double soaking of a 7Mn-steel

In this work, the influence of secondary soaking parameters on microstructural evolution in medium manganese steels is evaluated with a specific focus on the mechanisms driving austenite formation and solute partitioning at an intermediate secondary soaking temperature of 750 °C. The secondary soaking heat treatment represents the latter step of the double soaking heat treatment where it is used, after an initial intercritical annealing heat treatment, to form additional, solute-lean austenite. Here, data are obtained on a 0.14C-7.17 Mn steel for secondary soaking temperatures between 700 °C and 800 °C, and for times up to 1200 s. Austenite formation during secondary soaking is characterized using dilatometry and XRD, while Mn redistribution is experimentally observed using STEM-EDS. Manganese partitioning during secondary soaking, as well as calculated changes in carbon partitioning, are further explored through a DICTRATM model. Experimental and simulation results suggest that the ferrite-to-austenite transformation during the secondary soaking step may proceed under a partitioning, diffusional transformation at the 750 °C secondary soaking temperature. Manganese partitioning was demonstrated to primarily occur from ferrite to newly formed secondary austenite, while carbon diffusion principally occurred from primary to secondary austenite. Transformation of secondary austenite to martensite upon quenching along with some fraction of primary austenite, whose transformation was attributed to a reduction in C concentration, was shown demonstrated. The results of this study are expected to be of interest to those seeking to design multi-step heat treatments which produce austenite of variable solute concentrations and stabilities.

Accelerated multistep thermal stabilization of polyacrylonitrile fibers using an ethylenediamine pretreatment

The polyacrylonitrile multifilament yarn underwent a multistep heat treatment process including stabilization times ranging from 5 to 75 min following impregnation with a 30% ethylenediamine (EDA) aqueous solution. A series of measurements were employed to determine the structure and properties of thermally stabilized PAN samples. These included fiber thickness, fiber density, flame testing, tensile testing, X-ray diffraction, thermal analysis (differential scanning calorimetry and thermogravimetric analysis), and infrared spectroscopy. The results from XRD and IR spectroscopy indicated that the rate of aromatization reactions increased with longer stabilization times. A detailed examination of the XRD curves obtained through curve-fitting procedures suggested a rapid transformation of the original structure into a disordered amorphous phase containing pre-graphitic domains, evidenced by a gradual reduction in the degree of apparent crystallinity of the original PAN sample. The integration of EDA before the thermal stabilization stage significantly reduced the cyclization time of nitrile groups in the PAN polymer structure, thereby accelerating the stabilization reactions. This chemical pretreatment also improved the thermal stability of the samples by promoting oxidative cross-linking of the PAN polymer chains. After a 75 min multistep stabilization, the carbon yield at 1000 °C was 70.5%. Conversion index values, calculated using IR, XRD, and DSC methods, were 98.3%, 94.8%, and 89.5% respectively for the 75 min sample. These findings highlight the importance of EDA in accelerating the formation of an aromatic structure, which is critical for withstanding the high temperatures of subsequent carbonization stages.Graphical abstract

A systematic through-process rolling and extrusion study of four experimental high-strength Al-Mg-Si alloys

Al-Mg-Si alloys are an important class of Al wrought alloys. Further increasing their strength has significant economic potential. Previous approaches were focusing on increasing the amount of the main hardening elements Mg and Si, adding Cu and Zn for additional hardening potential, and adding dispersoid-forming elements such as Cr and Zr. In terms of processing, multi-step heat treatments and interrupted quenching have been pursued. Despite recent advances in the field, data points from the literature are difficult to compare due to widely varying alloy compositions and processing conditions. Here we present a comprehensive through-process rolling and extrusion study on four experimental high-strength Al-Mg-Si alloys (named A1–A4) with a particular focus on comparability of the results between our alloys and with the literature. Mechanical testing after artificial aging and in-depth microstructural investigations at various steps of the processing chain were employed, including scanning and transmission electron microscopy. We found that Cr addition leads to a strong increase in dispersoids and thus has a strong influence on hot workability, whereas the effect of increasing Mg and Si levels was negligible within the studied composition range. Al3Zr dispersoids were found to coexist with Al(Fe,Mn,Cr)Si dispersoids. Overall, the strength of the rolled sheets was slightly higher than that of the profiles. After 8 h of artificial aging at 180 °C, 2 mm thick sheet made of alloy A4, falling within the specification of AA6086, showed a notable yield strength of 393 MPa, due to nanoscale hardening phases that were smaller and appeared more numerous than in a commercial alloy (AA6082-T6).

Martensite decomposition kinetics in additively manufactured Ti-6Al-4V alloy: In-situ characterisation and phase-field modelling

Additive manufacturing of Ti-6Al-4V alloy via laser powder-bed fusion leads to non-equilibrium α′ martensitic microstructures, with high strength but poor ductility and toughness. These properties may be modified by heat treatments, whereby the α′ phase decomposes into equilibrium α+β structures, while possibly conserving microstructural features and length scales of the α′ lath structure. Here, we combine experimental and computational methods to explore the kinetics of martensite decomposition. Experiments rely on in-situ characterisation (electron microscopy and diffraction) during multi-step heat treatment from 400∘C up to the alloy β-transus temperature (995∘C). Computational simulations rely on an experimentally-informed computationally-efficient phase-field model. Experiments confirmed that as-built microstructures were fully composed of martensitic α′ laths. During martensite decomposition, nucleation of the β phase occurs primarily along α′ lath boundaries, with traces of β nucleation along crystalline defects. Phase-field results, using electron backscatter diffraction maps of as-built microstructures as initial conditions, are compared directly with in-situ characterisation data. Experiments and simulations confirmed that, while full decomposition into stable α+β phases may be complete at 650∘C provided sufficient annealing time, visible morphological evolution of the microstructure was only observed for T≥700∘C, without modification of the prior-β grain structure.

Studying the Effect of Small Additives of Sc and Zr on the Microstructure of Al–Mg–Si Alloy with Excess Silicon during Multi-Step Heat Treatment

Studying the Effect of Small Additives of Sc and Zr on the Microstructure of Al–Mg–Si Alloy with Excess Silicon during Multi-Step Heat Treatment

Solar-driven gas turbine power plants

A general model for an irreversible solar-driven Brayton multi-step heat engine is presented. The model incorporates a regenerator, an arbitrary number of turbines (Nt) and compressors (Nc) and the corresponding reheating and intercooling pro cesses; thus the solar-driven Ericsson cycle is a particular case where NtNc . For the so lar collector we assume linear heat losses and for the Brayton multi-step cycle irreversibilities arising from the non-ideal behavior of turbines and com pressors, pressure drops in the heat input and heat release, heat leakage through the plant to the sur roundings, and non-ideal couplings of the working uid with the external heat reservoirs. We obtain the collector temperatures at which maximum over all e ciency max is reached as a function of the thermal plant pressure ratio and a detailed com parison for several plant con gurations is given.

Optimization of Sulfide Annealing Conditions for Ag8SnS6 Thin Films.

Ag8SnS6 (ATS) has been reported to have a band gap of 1.33 eV and is expected to be a suitable material for the light-absorbing layers of compound thin-film solar cells. However, studies on solar cells that use ATS are currently lacking. The objective of this study is to obtain high-quality ATS thin films for the realization of compound thin-film solar cells using vacuum deposition and sulfide annealing. First, glass/SnS/Ag stacked precursors are prepared by vacuum deposition. Subsequently, they are converted to the ATS phase via sulfide annealing, and various process conditions, namely, annealing time, annealing temperature, and number of steps, are studied. By setting the heat treatment temperature at 550 °C and the heat treatment time at 60 min, a high-quality ATS thin film could be obtained. Multi-step heat treatment also produces thin films with nearly no segregation or voids.

Improvement in low-temperature toughness of Fe-6.5Mn-0.08C Medium-Mn steel by multi-step heat treatment

In this study, a novel multi-step heat treatment method, including intercritical annealing and tempering, is proposed to improve the low-temperature toughness of Fe-6.5Mn-0.08C medium-manganese (Mn) steel. The effects of the subsequent tempering treatment on the microstructural evolution and low-temperature toughness of medium-Mn steel were investigated and compared with those of a single intercritical annealing treatment. Medium-Mn steel subjected to intercritical annealing exhibited a duplex microstructure composed of martensite and nanolaminate-retained austenite. The subsequent tempering treatment after intercritical annealing generated a more homogenous distribution of retained austenite, together with an increase in the volume fraction of the retained austenite. The uniformly distributed retained austenite prevented Mn segregation at prior austenite grain boundaries, and the high fraction of retained austenite effectively inhibited crack propagation, leading to an improvement in the low-temperature toughness. In addition, the subsequent tempered medium-Mn steel contained a considerable amount of retained austenite, which could transform into martensite during the impact test, resulting in an impact fracture morphology that changed from cleavage facets to dimples at low temperatures.

Strength and stability through variable micro segregation behaviour of Ta and Zr solutes at intermetallic interfaces in Al-Cu alloys

A small amount of Zr and Ta are added to aluminium-copper (Al-Cu) alloys, and the evolution of strengthening phases with multi-step heat-treatment and their stability after prolonged thermal exposure at 250 °C has been studied. The L12 ordered (Al,Cu)3(Zr,Ta) precipitates were formed in the Al-matrix, followed by preferential nucleation of Cu-rich θ"/θ' plates on the L12/matrix interfaces. Transmission electron microscopy and atom probe tomography studies established the segregation of Zr and Ta at θ' plate/matrix in the aged alloys. It is shown that Zr segregates at coherent broad interfaces of θ' plate and inhibits the growth of these interfaces. Segregation of Ta primarily occurs at the growing θ' plate interface in the lengthening direction (semi-coherent interface) along with Zr and restricts the lengthening of the plate. The restricted growth triggers sympathetic nucleation of newer plates at the plate edges to sustain the growth process. A yield strength of ∼475 MPa at room temperature is recorded due to the presence of ordered precipitates and the growth restriction of θ' plate. Segregation of slow diffusing elements (Zr and Ta) also enhances the thermal stability of the microstructure.

Enhanced thermal conductivity of Si3N4 ceramics through pyrolytic carbon prepared by gel casting using DMAA

To reduce the lattice oxygen content of Si3N4 green bodies, samples were prepared using an N,N-dimethylacrylamide (DMAA) system. The endothermic and exothermic peaks are related to the decomposition of organic matter and the reaction between pyrolytic carbon and impurity oxides in the sintering process. Multi-step heat preservation during the thermal degreasing process reduced the carbon content and defects. The α→β phase transformation and aspect ratio of grains changed from 89.2% to 4.84:1 at 1640 °C to 96.3% and 7.80:1 at 1700 °C, respectively. The relative density, hardness, fracture toughness, and thermal conductivity of the Si3N4 samples reached 98.5%, 1353 HV, 8.51 MPa·m1/2, and 97.1 W·m−1·K−1, respectively. Pyrolytic carbon can react with SiO2, which reduces the content of the glass phase in the green body and affects the dissolution and precipitation process and lattice oxygen content. In the structure of the Si3N4 sample, carbon particles with sizes of 100–500 nm and near-spherical shapes were observed. Intergranular-phase YMgSi2O5N was formed during the sintering process, which was conducive to improving the thermal conductivity. This article uses lower temperature and no pressure conditions to prepare Si3N4 ceramics, which can provide reference for batch and low-cost production.

Effect of pre-formed martensite quenching plus inverted multi-step heat treatment on medium-carbon alloy steel

Effect of pre-formed martensite quenching plus inverted multi-step heat treatment on medium-carbon alloy steel

Mesophase pitch production from fluorine-pretreated FCC decant oil

• Effect of fluorine treatment over FCC-DO for mesophase formation firstly suggested. • Reaction mechanism of mesophase formation by fluorination was suggested. • Fluorine promoted radical polymerization of FCC-DO to form heavy mesophase nuclei. • The amount of hydrogen donating species also increased after the fluorination. • Pitch from fluorinated DO had higher mesophase yield than pitch from pristine DO. Petroleum residue oil-based mesophase pitch production suffers from its low yield of mesophase. There have been continuous efforts, such as catalyst-assisted reactions and multistep heat treatments, to produce high-purity mesophase pitch. However, most of these techniques have difficulties with the processability. In this study, we suggest a novel fluorine pretreatment of fluid catalytic cracking (FCC) decant oil for mesophase pitch (MP) production. The effects of fluorine pretreatment on FCC decant oil were evaluated with mass spectrometry (MS) and their hydrogen donating ability. Subsequently, the MPs from pristine pitches with fluorine pretreatment were intensively compared in terms of structural, thermal, and chemical characteristics. From these results, it was found that fluorine pretreatment helps mesophase formation by catalytic radical polymerization but does not cause coking and does not require posttreatment. It was found that the fluorination method can also provide an effective method for generating MP by increasing the hydrogen donating ability (HDA) fraction. We believe that the results of this work could inspire new insights into MP production.

Numerical modelling and fire testing of gypsum plasterboard sheathed cold-formed steel walls

Gypsum plasterboards are used in Light-gauged Steel Framed (LSF) walls as the primary fire-resistant material. In addition to thermal protection, they provide restraints to the cold-formed steel studs at the screw locations and improve the load-bearing capacity. In this study, three full-scale standard fire tests were conducted first to investigate the thermal and structural behaviour of LSF walls in fire. Close examination of the plasterboard joint opening up and plasterboard fall-off phenomena showed that most of the plasterboard joint compound fell-off after 17 min of fire exposure and the joint gap gradually widened afterwards. A multi-step heat transfer finite element (FE) model was developed incorporating the physical changes observed during the fire tests and validated using the test results. The important time–temperature profiles obtained from this study and past literature were compared, and idealised time–temperature profiles of wall studs were developed for use in structural FE models. Past studies involving elevated temperature structural FE models considered mainly the in-plane restraints provided by plasterboard sheathing. Hence the effects of their out-of-plane restraints were investigated using structural FE models and fire tests, and suitable out-of-plane restraint values were proposed for numerical analysis. This study has shown that out-of-plane restraints significantly reduced the lateral deflections of LSF walls and improved their fire resistance levels (FRL) when double layers of plasterboards were used. However, excessive out-of-plane restraints could adversely affect the FRL. Overall, this research has used the fire test results to enhance the understanding of the thermal and structural behaviour of LSF walls and provided useful data and recommendations for more accurate thermal and structural modelling of LSF walls.

Boosting polysulfide capture and redox conversion by functional separator combined with porous hosts for advanced Li-S batteries

Large-scale applications of lithium-sulfur (Li-S) batteries are hampered by the notorious ‘shuttle effect’ of lithium polysulfides (LiPS) and their sluggish redox kinetics. Herein, advanced sulfur host materials and multi-functional separator are proposed to collaboratively address these conundrums. The phosphorus-doped porous carbon (PPC) with a specific surface of 2131.5 m2/g was prepared as a sulfur host material in combination with a multi-step heat treatment. The PPC with high concentration of phosphorus doping, large surface area and reasonable pore sizes can improve the chemical adsorption of soluble polysulfides. The three-dimensional TiN/polyimide (PI) composite separator (TiN/PI) was prepared via electrospinning and stepwise imidization. The high affinity of the polar imide group in PI greatly improves the wettability of electrolytes, while the highly porous interconnected fibrous structure facilitates the fast transportation of Li+. Besides, the TiN nanoparticles can effectively inhibit the LiPS shuttle and stimulate the LiPS conversion. Consequently, the Li-S battery combining a PPC/S cathode and a TiN/PI separator offers a satisfactory performance of 630 mAh/g at 1.0C after 140cycles. This work provides a low-cost, facile and controllable method to design functional components for Li-S batteries.

High-efficiency hybrid supercapacitor based on three-dimensional interconnected nitrogen-rich caCTF-1 @pC-C3N4 network and NiCoTe2

The caCTF-1 derived from CTF-1 by carbonization is introduced into the multi-step heat treatment carbon repaired pC-C3N4 to form a 3D interconnected nitrogen-rich carbon structure ([email protected]). Through carbon repairing process, the obtained pC-C3N4 has an extended π-conjugate plane and it is closely interconnected with the nitrogen-rich caCTF-1, thereby accelerating charge transfer, improving energy storage, and exhibiting high specific capacitance (535.8 F·g−1 under 1 A·g−1) and good rate performance. In addition, by employing MOF with diphenyl semicarbazide structural unit as a precursor, NiCoTe2 is prepared by a simple hydrothermal process, which exhibits high redox activity with a high specific capacitance of 2006.0 F·g−1 at 1 A·g−1 and good cycle stability. The hybrid supercapacitor NiCoTe2//[email protected] HSC is assembled with [email protected] as the negative electrode and NiCoTe2 as the positive electrode. When the power density is 790.05 W·kg−1, it exhibits a high energy density of 50.84 Wh·kg−1 and high cycle stability. After 25,000 cycles, the capacitance retention rate is 87.88%.

An investigation on the effect of different multi-step heat treatments on the microstructure, texture and mechanical properties of the DED-produced Ti-6Al-4V alloy

This work deals with the effect of different heat treatments on directed energy deposition (DED)-produced Ti-6Al-4V samples. Annealing treatments at 1050 °C followed by different cooling rates were conducted to allow a complete recrystallization of the microstructure and remove the columnar prior-β grains, thus increasing the overall isotropy of the material. An agine treatment at 540 °C was also performed for further microstructural stabilization. The microstructures, textures and mechanical properties were then assessed. Due to the heat treatments, greatly differing microstructures were achieved in an equiaxed grain morphology. However, a “grain memory” effect was detected which resulted in the grains size increasing along the height of the samples. This effect was correlated to the intrinsic prior-β grain width variation along Z on the as-printed specimens, typical of the DED technology. Electron backscatter diffraction analyses proved that the intensity of the preferential directions increased after the heat treatments, likely due to the crystallographic variant selection mechanisms taking place when the samples cool down from the annealing temperature. This effect is also influenced by the significant difference in terms of prior-β grains sizes between the heat-treated and the as-printed specimens. To sum up, a complete homogenization of the material via heat treatment above the β-transus temperature proved to be challenging. In fact, the data suggest that the intrinsic texture-related anisotropy granted by the manufacturing process is very difficult to be eliminated.

Phase transformation pathway and microstructural refinement by feathery transformation of Ru-containing γ-TiAl alloy

The phase transformation sequence and transition temperatures of Ti–48Al–4Nb–2Cr-0.3Ru alloy are studied by using in-situ synchrotron radiation high energy X-ray diffraction, which was confirmed as α → α+γ → α/α2+β/B2+γ. The α transus and β/B2 solvus temperatures are located at 1390 °C and 1300 °C, respectively. Significantly, the acceleration of Ru element on the precipitation of metastable massive and feathery microstructures was found for the first time. Based on that, a multi-step heat treatment consisting of homogenization treatment, cyclic air-cooling heat treatment, γ equiaxed treatment and lamella reprecipitation treatment was designed to optimize the Ti–48Al–4Nb–2Cr-0.3Ru alloy. The coarse microstructure after homogenization was refined by feathery microstructure during cyclic air-cooling heat treatment. Finally, a fine duplex microstructure with γ grain and lamellar colony sized approximately 25 μm and 40 μm, respectively, was achieved. First-principles calculation revealed that the interfacial energies can be decreased when Ru atom occupies the Al sublattice position of true twin and Σ5 coincidence site lattice boundaries, which promote the occurrence of metastable transformations. Comparing with the initial microstructure, the tensile properties of the duplex microstructure were highly improved both at room temperature and 800 °C.

Effect of Intercritical Annealing Time on Microstructure Evolution and Mechanical Properties of Low Carbon Medium Manganese Steel Subjected to Multi-Step Heat Treatment Process.

A novel multi-step heat treatment process was performed for 0.2C–5Mn steel, and the effect of intercritical annealing (IA) durations on the microstructure evolution and mechanical properties was studied. The results showed that the content of primary reversed austenite (PRA) hardly changed as the IA time increased from 6 h to 50 h, but only less than 10% of PRA remained after being tempered at 200 °C due to the appearance of secondary martensite (SM). The final microstructure contained SM, the primary martensite (PM), and RA, which was protected by the SM so that the transformation-induced plasticity (TRIP) effect was unlikely to occur. Meanwhile, the (Ti, V, Mo)C particle sizes were 14.27, 14.68 and 15.65 nm for the intermediate processes of IA-6 h, IA-12 h, and IA-50 h, respectively. As the IA time increased from 6 h to 50 h, both the dislocation and precipitation strengthening increment decreased. As a result, the best mechanical properties were obtained from the intermediate process of IA-12 h, with a yield strength of 1115.5 MPa, tensile strength of 1573.5 MPa, and −20 °C impact energy of 30.4 J.