Heat Treatment Schedule Research Articles

Heterogeneous precipitate microstructure in titanium alloys for simultaneous improvement of strength and ductility

• We propose a step-aging method to improve the comprehensive mechanical properties of titanium alloys by designing heterogeneous microstructures through simulations and experiments. • The formation of heterogeneous microstructure can be attributed to the integration of conventional nucleation and growth, and non-conventional pseudospinodal decomposition mechanisms. • Ti-1023 (Ti-10V-2Fe-3Al) alloy with heterogeneous microstructure has shown enhanced ductility by ∼50% and strength by ∼10% comparing with that of homogeneous microstructure. • Both deformation twin and dislocation slip for heterogeneous microstructure have been activated and contributed to the enhanced mechanical properties. The design of alloys with simultaneous high strength and high ductility is still a difficult challenge. Here, we propose a new approach to designing multi-phase alloys with a synergistic combination of strength and ductility by engineering heterogeneous precipitate microstructures through the activation of different transformation mechanisms. Using a two-phase titanium alloy as an example, phase field simulations are carried out firstly to design heat treatment schedules that involve both conventional nucleation and growth and non-conventional pseudospinodal decomposition mechanisms, and the calculated microstructures have been evaluated by crystal plasticity finite element modeling. According to simulations, we then set a two-step heat treatment to produce bimodal α + β microstructure in Ti-10V-2Fe-3Al. Further mechanical testing shows that the ductility of the alloy is increased by ∼50% and the strength is increased by ∼10% as compared to its unimodal counterpart. Our work may provide a general way to improve the mechanical properties of alloys through multiscale microstructure design.

Effect of ZrO2 addition on the structure and the crystallization delay in lithium disilicate based glasses

Lithium disilicate (LD) glasses in system (99-X)(1Li2O:2SiO2)-(1K2O)-XZrO2 (X ​= ​0, 1 and 2 ​mol %) have been synthesized by melt quenching technique and converted into glass-ceramics by three and four-stage heat treatment schedules. The sequence of phase formation in glass-ceramics with ZrO2 addition has been discussed. The activation energy of crystallization increases with the addition of ZrO2. Rietveld refinement and micro structural analysis show an increase in the amorphous content with the addition of ZrO2. 29Si MAS-NMR, Raman and XPS spectroscopy studies for the glasses reveal that the ZrO2 addition to LD based glasses causes network polymerization and changes the role of network modifying Li+ and K+ species into charge compensators. The increased connectivity of [ZrO6]2- units in the glass network restricts the mobility of Q2 [Si2O6]4-, Q3 [Si4O10]4- and Q4 [SiO2] units, which leads to an increase in activation energies of crystallization and hampers the devitrification ability of glasses.

Vapor-phase assisted sintering of lutetium oxide

Conventional methods of green-body formation of powder compacts, along with temperature/pressure processing methods, were optimized to sinter and post-HIP Lu2O3 compacts to optical transparency (99.1 % of theoretical transparency at 1100 nm). Fissures between remnants of spray-dried granules, yielding opaque spheroids around them, were eliminated by lower temperature heat treatment schedules which minimized fissure coarsening via preferential intragranular densification. A key development was the use of LiF vapor produced adjacent to the compact in the furnace environment, which physisorbed onto particle surfaces of the pre-sintered compact. This acted as a grain growth inhibitor and hence sintering aid.

Effects of Cobalt on solidification characteristics and as-cast microstructure of an advanced nickel-based single crystal superalloys

Containing Re and Ru, an advanced Ni-based single crystal superalloys with different cobalt (Co) content (5.9 wt.%, 9.0 wt.% and 12.0 wt.%, respectively) were investigated to study the effects of Co on solidification characteristics and as-cast microstructure. In the aspect of solidification characteristics, the solidus, especially the precipitation temperature of γ′ phase of the alloys decreases gradually with the increase of Co. Furthermore, the dendritic segregation of refractory elements, such as Re, W and Cr, is restrained. With regard to as-cast microstructure, the results show that primary dendrite spacing shorten significantly since Co increased from 5.9 to 9.0 wt.%, and the content of γ/γ′ eutectic is lowest in the alloy with 9.0 wt.% Co. The addition of Co decreases the size of γ′ and homogenizes the size of γ′ between the dendrite core and the interdendritic region. A customized heat treatment schedule is implemented for the three alloys based on the research.

The influence of DED process parameters and heat-treatment cycle on the microstructure and hardness of AISI D2 tool steel

The impact of both the laser directed energy deposition (DED) process parameters and post-deposition heat-treatment cycles on the microstructural characteristics and hardness are evaluated for cladding AISI D2 tool steel onto annealed AISI D2 substrates. The influences of powder feed rate, and laser scanning speed were assessed. After laser deposition, the DED samples were subjected to various tempering heat-treatment cycles. The microstructural and compositional characteristics of both the ‘as-printed’ and heat-treated DED processed samples were investigated using scanning electron microscopy with energy dispersive X-ray spectroscopy, confocal laser scanning microscopy, and X-ray diffraction. The indentation and scratch hardness responses were assessed to evaluate the effects of various heat-treatment schedules, with comparison made to wrought D2. It was demonstrated that the microstructure of the DED processed samples had a dendritic morphology with columnar grains. Moreover, the hardness values of the DED fabricated parts were found to be higher than an annealed wrought D2, but lower than the quenched D2. Subsequent tempering of the DED printed parts resulted in a final hardness, essentially equivalent to the as-quenched level.

Construction and investigation of graphitic carbon nitride/expanded perlite composite photocatalyst with floating ability

Using melamine as the precursor of g-C3N4 and expanded perlite (EP) as the carrier, a kind of floating g-C3N4/expanded perlite composites (CN/EP-x, where x was denoted as 1, 2, 4 and 6, respectively, corresponding to the mass ratio of melamine to EP 1:2, 2:1, 4:1 and 6:1) were synthesized by cooperating wet-chemistry method with a unique heat treatment schedule, which aimed to enhance the g-C3N4 loading ratio and construct a tightly interface between g-C3N4 and EP. The phase composition, microstructure, element composition, optical property, etc. of the g-C3N4/EP composites were investigated, and their photocatalytic performances vs. the mass ratio of melamine to EP (loading ratio of g-C3N4) were evaluated through photo-degrading Rhodamine B (RhB) aqueous solution. It is found that g-C3N4 immobilized on the surface or embedded into the open pores of EP, which were beneficial to the interface bonding strength between g-C3N4 and EP. With the increasing of the mass ratio of melamine, the loading ratio of g-C3N4 in CN/EP-x composite increased gradually, and the specific surface area of the composite increased step by step but then dropped for CN/EP-6 composite owing to the intensified agglomeration of g-C3N4 particles. The photocatalytic performance of CN/EP-x composites also improved gradually and achieved the optimum for the CN/EP-4 composite, then reduced for the CN/EP-6 composite. CN/EP-4 composite (0.35 g) degraded 94 % of RhB molecules (10 mg/L, 100 mL) after visible light illumination for 120 min. The reduced band gap, improved separation efficiency of holes and electrons, enhanced photo-current response, reduced electron transfer resistance, as well as the enhanced specific surface area were responsible for the optimum photocatalytic performance of CN/EP-4 composite. Moreover, CN/EP-4 composite exhibited excellent floating performance, stability and reusability. After four degradation cycles, the removal rate of RhB by CN/EP-4 composite still achieved 83 %. This work will provide a new insight into synthesizing floating composite photocatalysts for the future practical application.

A critical review on the microstructure and mechanical properties correlation of additively manufactured nickel-based superalloys

Inconel group of alloys, particularly IN 625 and IN 718 are the most widely recognized commercially available Ni -based superalloys employed for high temperature applications. These superalloys are known to retain their extraordinarily high strength at elevated temperatures either by solid solution or precipitation strengthening mechanisms. Consequently, they find crucial applications in the field of aerospace, automobile, and energy industries involving higher temperatures. Recently, attempts are being made to fabricate and repair the complicated superalloy components using metal additive manufacturing techniques. The ability of the additive manufacturing process to produce intricate geometry parts with greater dimensional accuracy as compared to various conventional techniques appear to be beneficial and cost-effective. However, this unique manufacturing method influences the microstructural evolution and thereby affects the performance of the IN 625 and IN 718 alloys significantly. Considering various research efforts in this direction, this article pursues a thorough review and presents a wide overview of the microstructure and mechanical properties correlation for these two additively manufactured Ni-based superalloys. • IN 625 and IN 718 alloys are the most extensively used Ni -based superalloys for various high temperature applications. • Additive manufacturing is a viable alternative to traditional methods for manufacturing these superalloy components. • The repeated heating and cooling cycles of the process alters the microstructure and mechanical performance of superalloys. • Various heat treatment schedules are designed to achieve homogenized microstructures that influences properties as well. Microstructure – property correlations for the additively manufactured IN 625 and IN 718 superalloys are explored.

Effect of galvannealing on the microstructural and mechanical properties of a Si and Al alloyed medium-Mn quenching and partitioning steels

The effect of a subsequent galvannealing (GA) treatment in the temperature range of 480–560 °C after a quenching and partitioning (Q&P) process was investigated via two medium-Mn steels 0.2C–4Mn-1.5Si and 0.2C–4Mn-1.5Al, focusing on the microstructural evolution and mechanical behavior. Thereby, the influence of a replacement of the alloy element Si by Al on the microstructure, especially on the retained austenite (RA), was analyzed via scanning electron microscopy, electron backscatter diffraction, and X-ray diffraction. Heat treatment schedule and alloy steel turned out to be the main influencing factors on the amount of RA and its morphology. By traditional Q&P, Si-steel was more efficient in stabilizing high amounts of lath like and globular RA, compared to Al. However, an additional GA-treatment led to a drastic decomposition of RA into pearlite and fresh martensite with increasing temperature for the Si-steel. In contrast, the GA-treatment showed no significant reduction at the RA content in the Al-alloyed steel. The different decomposition behavior of RA during GA was also reflected in the mechanical behavior measured by means of tensile testing. Hereby, it could be shown that Al, compared to Si ensured high elongations, which are reached due to a sufficient amount of stable RA (good TRIP-behavior) even after high temperature galvannealing.

Effect of Copper Content and Heat Treatment Schedules on Mechanical Properties, Cold Resistance, and Microstructural Constituent Morphology in Precipitation-Hardened Steel

Effect of Copper Content and Heat Treatment Schedules on Mechanical Properties, Cold Resistance, and Microstructural Constituent Morphology in Precipitation-Hardened Steel

Study of Lithium Disilicate Based Nano Glass Ceramics Containing P2O5

Multi component lithium disilicate based glasses containing P2O5 have been synthesized by conventional melt quenching technique. The replacement of (Li2O + SiO2) by P2O5 and its nucleating effect has been discussed. Structural features of glasses were evaluated by differential thermal analysis (DTA), Fourier transform infrared (FTIR) and Raman spectroscopy. The glass samples have been converted into glass ceramics by following three stage heat treatment schedules. X-ray diffraction analysis (XRD), field emission scanning electron microscopy (FESEM), high angle annular dark field imaging (HAADF), energy dispersive X-ray analysis (EDX) and high resolution transmission electron microscopy (HR-TEM) has been carried out for the glass ceramics. Vickers microhardness (Hv) and Vickers indentation fracture toughness (KIC) of all the glass ceramics have been measured. UV–Visible spectroscopy study has been carried out for glass ceramics to investigate the optical properties. The glass ceramic with 1.5 mol % P2O5 has the highest transmittance. Glass ceramics with P2O5 ≥ 1 mol % having Li2Si2O5 (LS2) as main phase exhibit high Hv about 6.71–6.82 GPa which is suitable for dental and armor applications.

Effects of microstructural heterogeneity and structural defects on the mechanical behaviour of wire + arc additively manufactured Inconel 718 components

This study investigates the extent to which the build orientation and heat treatment schedule affect the microstructure and mechanical properties for thin-walled and additively manufactured IN718 components produced with the cold metal transfer process (CMT-WAAM). Uniaxial tensile tests using digital image correlation (DIC), microhardness analysis and fractography were used to characterise the mechanical behaviour, both in the as-deposited condition and after heat treatments used in the aerospace and oil and gas industries. Wrought material was also tested to benchmark the measured properties. The solution treatment of 1040 °C for 1 h in the oilfield specification reduced the area fraction of Laves phases significantly (∼80%) and promoted higher homogenisation of ageing constituents. However, grain growth near interlayer boundaries resulted in localised low hardness (∼50 HV0.2 below the average) after age hardening. In the as-deposited condition, the yield strength was ∼10% lower along the build direction and changed to being 8–13% higher along the same direction in heat-treated samples, while the elastic modulus relative to deposition orientation was unaffected. Furthermore, solidification defects, such as porosity and hot cracking, caused strain localisation during tensile testing and substantial scatter in macroscopic strain. The ductility was improved with oil and gas heat treatment, but it was substantially lower along the build direction due to the longer axis of defects being perpendicular to loading direction. This study highlights the importance of optimising process parameters to minimise defects and tailoring heat treatments to achieve a higher ductility in IN718 processed by WAAM.

Enhanced defect emission of TiO2-doped transparent glass-ceramics embedding ZnO quantum dots with optimized heat-treatment schedule

This work mainly concentrated on the structural and optical properties of TiO 2 -doped transparent glass-ceramics embedding ZnO quantum dots (QDs). The results show that the concentration of non-bridging oxygen and related defects in the glasses increases and thermal parameters decreases by the introduction of TiO 2 . The surface defect concentration of ZnO QDs is greatly improved and the size of ZnO QDs is reduced by heat treatment below the T g and the introduction of TiO 2 . The green emission of glasses is mainly derived from oxygen vacancy defects and the yellow emission of glass-ceramics primarily originated from oxygen interstitial defects which show a tendency of enhancing first and then reducing with the increase of TiO 2 content. Among them, the optimal content for the strongest emission intensity of TiO 2 -doped glasses and glass-ceramics was determined to be 1.0 mol%, and the relative exceeds 10 times and 5.4 times that of TiO 2 -free glasses and glass-ceramics, respectively. The average size of ZnO QDs (∼2.1 nm) remarkably close to its exciton Bohr radius is successfully prepared by the optimal heat treatment condition (630 °C/8 h, below the T g ). The results show that these glass-ceramics embedding ZnO QDs are good yellow emission materials and have application prospects in WLEDs.

Influence of microstructural parameters on nanohardness of various dual-phase steels: Experiment, Finite Element simulation and Statistical analysis

This study aims to achieve insight related to the role of various microstructural parameters on nanohardness of different phases in ferrite-martensite (FM), ferrite-bainite (FB) and ferrite-pearlite (FP) steels. Low-carbon microalloyed steel has been subjected to suitable heat treatment schedules to develop FM, FB and FP dual-phase (DP) microstructures with nearly the same amount of ferrite. Macro, micro, and nanohardness values and tensile properties of the developed DP steels have been measured while their microstructures have been characterized. It has been found that the characteristics of hard phase strongly affect the nanohardness of the ferrite phase. Load-depth profiles of diverse phases obtained from the nanoindentation tests have been simulated by the finite element (FE) technique. The dislocation-based model has been modified to estimate the flow behavior of constituent phases which have been provided as input to the FE simulations. The predicted nanoindentation load-depth profiles are found to be in good agreement with the experiment ones. Subsequently, FE simulations have been performed by incorporating the microstructural parameters to predict the nanohardness of individual phases in the developed DP microstructures. Finally, statistical analyses following the Taguchi method and ANOVA have been conducted to demonstrate the influence of different microstructural parameters on nanohardness in various DP steels. It has been revealed that the nanohardness values of constituting phases are significantly influenced by the percentage of carbon and the hardness ratio of two phases in FM, FB and FP DP steels except the pearlite for which the interlamellar spacing is the most contributing factor.

Optimization of Heat Treatment Scheduling for Hot Press Forging Using Data-Driven Models

Scheduling heat treatment jobs in a hot press forging factory involves forming batches of multiple workpieces for the given furnaces, determining the start time of heating each batch, and sorting out the order of cooling the heated workpieces. Among these, forming batches is particularly difficult because of the various constraints that must be satisfied. This paper proposes an optimization method based on an evolutionary algorithm to search for a heat treatment schedule of maximum productivity with minimum energy cost, satisfying various constraints imposed on the batches. Our method encodes a candidate solution as a permutation of heat treatment jobs and decodes it such that the jobs are grouped into batches satisfying all constraints. Each candidate schedule is evaluated by simulating the heating and cooling processes using cost models for processing time and energy consumption, which are learned from historical process data. Simulation experiments reveal that the schedules built using the proposed method achieve higher productivity with lower energy costs than those built by human experts.

PbS quantum dots and BaF2:Tm3+ nanocrystals co-doped glass for ultra-broadband near-infrared emission [Invited

With the rapid growth of optical communications traffic, the demand for broadband optical amplifiers continues to increase. It is necessary to develop a gain medium that covers more optical communication bands. We precipitated PbS quantum dots (QDs) and BaF2:Tm3+ nanocrystals (NCs) in the same glass to form two independent emission centers. The BaF2 NCs in the glass can provide a crystal field environment with low phonon energy for rare earth (RE) ions and prevent the energy transfer between RE ions and PbS QDs. By adjusting the heat treatment schedule, the emission of the two luminescence centers from PbS QDs and Tm3+ ions perfectly splices and covers the ultra-broadband near-infrared emission from 1200 nm to 2000 nm with bandwidth over 430 nm. Therefore, it is expected to be a promising broadband gain medium for fiber amplifiers.

Microstructure homogeneity and mechanical property improvement of Inconel 718 alloy fabricated by high-deposition-rate laser directed energy deposition

The nonuniform microstructure of the as-built Inconel 718 superalloy fabricated by high-deposition-rate laser directed energy deposition (HDR-LDED) brings about heterogeneous room temperature tensile properties along the building direction (BD). Thus, it is imperative to explore suitable post heat treatment processes for homogeneous and better performances of HDR-LDED built Inconel 718 superalloy. In this work, three kinds of heat treatment schedules were employed to investigate the microstructure evolution and room temperature tensile properties. The results showed that the microstructure uniformities along the BD after three heat treatment processes were all significantly improved. In addition, the three heat treated samples present different microstructural characteristics which were reflected by the morphology, size and distribution of the Laves, δ and γ" phases. However, all the samples exhibited similar microhardness, indicating that the difference in the characteristic parameters of the second phases was not enough to influence the microhardness. Furthermore, the heterogeneity in room temperature tensile property along the BD was improved when compared with the as-built sample. And the S-30 sample demonstrated the best combination of strength and ductility (yield strength: 1013 ± 13 MPa, ultimate tensile strength: 1192 ± 11 MPa, elongation: 16 ± 6%). The fragmentation and the debonding of the “Laves+δ” phase contributed to the fracture in all the samples. The initial dislocation density inside the bulk sample may become one of the factors that have to be considered in the future when optimizing the heat treatment processes in HDR-LDED process.

Effect of heat treatment on the microstructure and mechanical properties of a multidirectionally forged Mg-Gd-Y-Zn-Zr-Ag alloy

Multidirectional forging (MDF) was successfully applied to fabricate large-size Mg-Gd-Y-Zn-Zr-Ag alloy in this work and effects of T4, T5 and T6 treatments on the microstructure and mechanical properties of the as-forged alloy were analyzed. Results show that dynamic recrystallization (DRX) occurs and second phase particles precipitate along the grain boundary during the MDF process. After annealing treatment (T4), the volume fraction and size of dynamic precipitates slightly increase at a lower temperature (430 °C) compared with those of MDFed sample, while they are dissolved into the α-Mg matrix at a higher temperature (450 °C). At the meantime, short plate-shaped long-period stacking ordered (LPSO) phases are observed in the DRX grains of the MDFed sample and then dissolved into the α-Mg matrix during annealing at both temperatures. Typical basal texture is identified in the MDFed sample, but the basal pole tilts away from final forging direction and rare-earth texture component with <112¯1> orientation parallel to penultimate forging direction becomes visible after annealing. The T6 sample annealing at 430 °C for 4 h and ageing at 200 °C for 34 h exhibits the superior strength and ductility in this study. The ultimate tensile strength, tensile yield strength and elongation to failure, which is 455 MPa, 308 MPa and 7.7%, respectively, are overall improved compared with the directly-aged (T5) sample. This paper provides a superior heat treatment schedule to manufacture high-performance large-scale Mg-Gd-Y-Zn-Zr-Ag components for industrial production.

Cascading of the as-built microstructure through heat treatment and its role on the tensile properties of laser powder bed fused Inconel 718

The microstructures and mechanical properties of the Ni based superalloy, Inconel 718 (IN718), which was additively manufactured using the laser beam powder bed fusion (LB-PBF) technique in the as-built (AB) and heat treated (HT) conditions were investigated, with emphasis on the microstructural evolution from the AB to HT conditions and, in turn, on the tensile properties. Optimized LB-PBF parameters in two different processing instruments led to distinct textures in the AB alloys, which, in turn, impart diverging combinations of strength-ductility properties. These variations are rationalized by recourse to an analysis of the effective mean free path for dislocations within the solidification cell structures. Amongst the five different heat treatment schedules examined, one set led to the retention of as-built grain morphology and texture with the formation of the δ phase that is oriented at either ±45° to build direction in the alloy with 〈100〉 texture or at 0/ ± 90 ° in the alloy with 〈110〉 texture. The second set of heat treatments led to complete recrystallization, and hence loss of as-built microstructural signatures, with no δ phase. The critical role of δ phase in determining the grain growth kinetics and subsequently the anisotropy in mechanical properties of heat treated IN718 was elucidated. The absence of δ phase was found to enhance both strength and ductility, while its occurrence in the alloy with 〈110〉 texture along the build direction reduces ductility markedly. Implications of these results in terms of developing and designing the processing strategies of LB-PBF Inconel 718 with tailored microstructures and good mechanical properties are discussed.

Effect of heat treatments on the microstructure and mechanical properties of an ultra-high strength martensitic steel fabricated via laser powder bed fusion additive manufacturing

A newly developed ultra-high strength martensitic steel, AF9628, has generated academic and industrial attention due to its combination of high yield strength and ductility at a low cost. Several studies have demonstrated that this steel can be successfully additively manufactured (AM) using laser powder bed fusion (LPBF). However, as-printed parts have displayed microstructural inhomogeneity, anisotropy, and a discrepancy in mechanical properties as compared to the traditionally processed material. There is currently no work detailing the effects of post processing heat treatments on the microstructure or mechanical properties of as-printed AF9628. In the interest of producing robust near-net-shape parts with complex geometries, this work explores the effects of LPBF and post processing heat treatments on the microstructure and mechanical properties of AF9628. Parts fabricated by LPBF were subjected to three different quench and temper heat treatment schedules. Quench and temper treatments displayed refined equiaxed prior-austenitic grain structures, reducing grain size by up to 45%. Significant variability was observed in Vickers hardness measurements of as-printed samples due to tempered and untempered bands of martensite alternating throughout the microstructure caused by thermal cycling during printing. Heat treatments were observed to increase the overall hardness of the material and reduce variability. As-printed samples displayed a tensile strength of 1.41 GPa, 10% elongation, and a Charpy impact toughness of 28 J. Quench and temper heat treatments were observed to increase tensile strengths to 1.66 GPa, but reduce the elongation to 7.6% and impact toughness to 24 J. Understanding the effects of these post-processing treatments is expected to allow for property optimization of AF9628 additively manufactured parts.

Effects of Powder Atomization Route and Post‐Processing Thermal Treatments on the Mechanical Properties and Fatigue Resistance of Additively Manufactured 18Ni300 Maraging Steel

In this work the effect of microstructural homogenization and solution annealing prior to ageing on mechanical properties and fatigue resistance of 18Ni300 maraging steel parts processed by laser powder bed fusion (L‐PBF), using two different feedstock powders with different N and O contents is investigated. The static mechanical properties are not significantly affected by the selection of the different feedstock powders. This is ascribed to the extremely fine size of the nonmetallic inclusions, characteristic of L‐PBF process, even by using feedstock powders containing higher O and N contents. The tensile properties in both “directly aged,” and “homogenized, solution annealed and aged” samples are comparable. The tensile strength and ductility are slightly improved compared with the vacuum melted wrought counterparts. The dynamic mechanical properties are affected by the O and N contents of the powders. Heat treatment schedule imposed a significant effect on the fatigue properties. The obtained fatigue strength in directly aged samples is already comparable to the wrought counterparts. Fatigue resistance is improved by using feedstock powders with lower oxygen and nitrogen levels. Proper homogenization heat treatments prior to ageing led to a further improvement of fatigue strength because of the elimination of intercellular microsegregation.