Non-isothermal Heat Treatment Research Articles

Phase-field modeling of stored-energy-driven grain growth with intra-granular variation in dislocation density

We present a phase-field (PF) model to simulate the microstructure evolution occurring in polycrystalline materials with a variation in the intra-granular dislocation density. The model accounts for two mechanisms that lead to the grain boundary migration: the driving force due to capillarity and that due to the stored energy arising from a spatially varying dislocation density. In addition to the order parameters that distinguish regions occupied by different grains, we introduce dislocation density fields that describe spatial variation of the dislocation density. We assume that the dislocation density decays as a function of the distance the grain boundary has migrated. To demonstrate and parameterize the model, we simulate microstructure evolution in two dimensions, for which the initial microstructure is based on real-time experimental data. Additionally, we applied the model to study the effect of a cyclic heat treatment (CHT) on the microstructure evolution. Specifically, we simulated stored-energy-driven grain growth during three thermal cycles, as well as grain growth without stored energy that serves as a baseline for comparison. We showed that the microstructure evolution proceeded much faster when the stored energy was considered. A non-self-similar evolution was observed in this case, while a nearly self-similar evolution was found when the microstructure evolution is driven solely by capillarity. These results suggest a possible mechanism for the initiation of abnormal grain growth during CHT. Finally, we demonstrate an integrated experimental-computational workflow that utilizes the experimental measurements to inform the PF model and its parameterization, which provides a foundation for the development of future simulation tools capable of quantitative prediction of microstructure evolution during non-isothermal heat treatment.

Modeling of heterogeneous site energy distributions in precipitate nucleation

The simulation of heat changes resulting from phase transitions can help to interpret differential scanning calorimetry (DSC) measurements, e.g. of metallic alloy systems in which multiple reactions overlap during non-isothermal heat treatments. So far, simulated DSC curves mostly exhibit sharp reaction peaks as commonly just one mean energy value for a certain type of nucleation site is assumed. This work proposes an efficient model for treating heterogeneous nucleation site energy variations within the framework of classical nucleation theory (CNT). The site energies are assumed to vary according to a Rayleigh distribution and a scaling function. The effect on the nucleation behavior of precipitates is studied. A consideration of the distribution of heterogeneous site energies has the potential to significantly smoothen the numerical treatment of precipitation processes compared to the non-distributed case. The comparison to previously published simulations of DSC curves during the cooling of an AA6005 aluminum alloy demonstrates the advantages of this extension, especially for slow cooling rates.

Accelerated coarsening behavior of the γ´-phase in CMSX-4 during non-isothermal heat treatment

In literature, microstructural investigations can be found comparing the evolution of the well-known γ/γ ´ -phase morphology of nickel-based superalloys for long-term heat treatments with and without mechanical load, as well as for isothermal and non-isothermal creep tests. However, data of experiments conducted under non-isothermal conditions without an applied load are hard to find. To close this gap, isothermal and non-isothermal heat treatments without a mechanical load were conducted. Conductive heating was applied to samples that consisted of segments with different cross sections and allowed the simultaneous testing at different temperatures. The microstructural changes were examined by scanning electron microscopy after 500 h and 1000 h. Several established methods for the quantification of the γ ´ -phase coarsening were evaluated and employed for comparison of the two heating conditions studied. The coarsening was more distinct in samples heat-treated under non-isothermal conditions at all temperatures investigated. This was also verified by calculating the connectivity number NA, which was found to be the most useful parameter for quantification of microstructural changes observed. The γ ´ -precipitates became more connected during the heat-treatment, but no preferred direction of coarsening was observed like during rafting under an applied load. Therefore a more general particle agglomeration mechanism is assumed to be responsible for coarsening of the γ ´ -phase.

Precipitation behavior of an Al7SiMg alloy processed by laser powder bed fusion during non-isothermal and isothermal heat treatments

Laser powder bed fusion (LPBF) is drawing great interest in industry due to its ability of producing complex-shaped components with superior mechanical properties and unique microstructures. Al7SiMg alloy is one of the most used aluminum alloys in LPBF due to its easy processing window. Direct T5 aging is often applied to LPBF-processed Al7SiMg alloy to obtain better mechanical performances over T6 heat treatment. However, after T5 heat treatment, a comprehensive understanding of the precipitation sequence of the strengthening phase in LPBF-processed Al7SiMg alloy is still lacking. In the present study, a high-resolution TEM examination of the precipitation behavior of Al7SiMg alloy processed by LPBF during heating without and with isothermal holding was conducted to further understand the precipitation sequence of Si precipitates and the associated microstructure evolution. During the heating process, irregular and needle-shaped pure Si precipitates were observed which shared the same [011] zone axis with the aluminum matrix. After analyzing the phase transformation process during heating, the following precipitation sequence is proposed: Si in supersaturated solid solution → Si-rich clusters → Si-rich intermediate phases → pure Si precipitates. During isothermal holding at 160 ℃ (T5), nano-scale Si-rich clusters were observed at the peak aging condition. When the holding time extended to 24 h, larger Si-rich clusters with reduced coherence were found. Strain fields caused by these Si-rich clusters are believed to strengthen the LPBF-processed Al7SiMg alloy after T5.

Clustering and precipitation in Al-Mg-Si alloys during linear heating

Differential scanning calorimetry (DSC) is a technique extensively applied to analyse precipitation phenomena in Al-Mg-Si alloys, yet the processes occurring during non-isothermal DSC heating, in particular the formation of clusters in the early stage and their evolution at higher temperatures, remain obscure. Here, we carry out experiments not only to measure heat (via DSC) but also to measure hardness, positron lifetime, electrical resistivity and microstructure of an Al-Mg-Si alloy heated at 3 different rates. Electrical resistivity is measured in situ, the other properties after interrupting the heating process. It is demonstrated that the precipitation process during heating can be divided into various stages, with transition temperatures depending on the heating rate, but the relative behavior of the various measured quantities is connected in the same way. Quenched-in excess vacancies are found to play an important role in cluster formation at lower temperatures, which explains the seeming peculiarity that linear heating at lower temperatures can lead to stronger clustering than isothermal ageing at a higher temperature. These trends are well simulated using a recently developed precipitation model. New aspects about the evolution of these clusters at higher temperatures are revealed by correlating the different measured properties. The methodology applied here could also be extended to investigating more complex non-isothermal heat treatments.

Quantitative characterization of lamellar α precipitation behavior of IMI834 Ti-alloy in isothermal and non-isothermal heat treatments

To reveal the affecting mechanism of cooling rate on lamellar α precipitation, the precipitationbehaviors of lamellar αphase in IMI834 titanium alloy during isothermal and non-isothermal heat treatments were quantitatively characterized using experimental analysis. Critical precipitation temperatures at various cooling rates were obtained using thermal dilatation testing. Using metallographic microscopy, electron microprobe analysis, and data fitting methods, the quantitative evolution models of average width, volume fraction, and solute concentration in the α and β phases were built for different temperatures or cooling rates. A comparison between the two precipitation behaviors showed that the average width and volume fraction of lamellar αphase under non-isothermal conditions were smaller than those under isothermal conditions. With increasing cooling rate, the average width and volume fraction were decreased significantly, and the critical precipitation temperatureswere reduced. This phenomenon is mainly attributed to the decreased diffusion velocity of solutes Al, Mo, and Nb with increasing cooling rate.

Mechanical and Corrosion Properties of 7N01 Aluminum Alloy After Non-isothermal Heat Treatment under Different Quenching Conditions

Mechanical and Corrosion Properties of 7N01 Aluminum Alloy After Non-isothermal Heat Treatment under Different Quenching Conditions

Study of the β"→ β' Transition in an Al-Mg-Si Alloy

A detailed experimental study of the processes involved in the transition between the β” and β’ metastable phases in an Al-Mg-Si is presented. Measurements were performed by means of differential scanning calorimetry (DSC) and Vickers microhardness (Hv). A combination of isothermal and non-isothermal heat treatments, aimed to study the transition to β’ with different degrees of transformed fraction of β”, were applied. Due to the proximity and overlapping of the events, the techniques of DSC and Hv for non-isothermal treatments were combined in order to obtain a better description. The obtained results were also related to the evolution observed for isothermal treatments, making the results independent of the heating rate. The results showed the presence of a dissolution in the transition β”→ β’, that this dissolution corresponds to the (at least partial) dissolution of the β” phase and that it is always present in the transition.

Combined athermal and isothermal martensite to austenite reversion kinetics, experiment and modelling

A novel laser heat treatment setup is presented and used to characterize the reverse transformation of martensite to austenite resulting from highly dynamic laser heat treatments of stainless steel. During laser heat treatments the irradiated spot and its surroundings can experience completely different thermal loads, yet both experience reverse transformation. The experiments are conducted such to reflect these diverse conditions. Next to experiments, a new kinetic model is reported which combines both athermal and isothermal transformation mechanisms to cope with the diversity in conditions in a unified framework. The experimental results show that reverse transformation can proceed extremely fast, yet saturates at intermediate temperatures. Additionally, it is shown that there is good agreement between experiment and model and it is essential to embed both the athermal and isothermal transformation mechanism in the model for achieving this performance. Initial steps towards model validation are performed showing good predictability of a non-isothermal heat treatment with conditions realistic and relevant for industrial laser heat treatments.

Structure and crystallization characterization of chemically developed bioactive glass based on ICIE16

Structure and crystallization of new chemically modified bioactive glasses based on the composition of ICIE16 were investigated. Solid-state NMR analysis was used to investigate the glass structure. For crystallization, non-isothermal heat treatment was conducted to calculate the crystallization activation energy (Ec) using Kissinger method. The structural analysis results indicate that BP1 glass has the most complicated structure among the synthesized glasses. NMR findings tell that phosphate groups are in QP0 and QP1 units while boron is bonded in B[3] and B[4] units. Moreover, BP1 also exhibited high tendency to volume crystallization (n ​= ​2.6; Ec: 390.4 ​kJ/mol) in comparison to ICIE16 glass which showed mostly surface crystallization (n ​= ​1.4; Ec: 275.7 ​kJ/mol). The phases that formed after crystallization are Na4Ca4Si6O18, Ca2SiO4 and Ca5(PO2)4 (SiO4)6. The experimental data indicate that BP1 glass has promising features of processing which may ease bone scaffolds (grafts) fabrication with valuable strength.

Multiphase modelling of the growth kinetics of precipitates in Al-Cu alloys during artificial aging

ABSTRACT In the aluminum industry, the needs of predictability of the kinetics of precipitation during the artificial aging processes increase as the targeted applications require the maximisation of properties at the lowest costs possible. In this regard, kinetics modelling can be helpful to design the heat treatment processes. Despite using many fitting parameters, available models show a lack of fitting with experimental data, especially for the apparent heat capacity measured at high temperatures by a differential scanning calorimeter (DSC). To address this issue, a mixed-mode model was recently developed for isothermal heat treatment, whereas non-isothermal heat treatment must be considered to compare the calculated results with those measured by isochronal heating in a DSC. In this contribution, the model is extended to non-isothermal heat treatments. To this end, the growth kinetics pathway and sequence of precipitation in a binary Al-Cu alloy have been simulated, optimising the pre-exponential factor and the activation energy of the interfacial mobility of the secondary phases. This calibration of the interfacial mobilities allowed a very good reproduction of the evolution of the apparent heat capacity with temperature. The model and calibrated interfacial mobilities were then used to compute the size evolution of θ′ precipitates in an Al-4 wt%Cu. The isothermal growth rates calculated at 4 temperatures were in good agreement with those measured and reported by independent researchers. The good predictability of the model indicates that the assumptions made were suitable and well funded, especially regarding the growth rates of embryos emerging from the subcritical growth regime.

Nanosheets-nanorods transformation during the non-isothermal decomposition of gadolinium acetate

Gadolinium oxy-carbonate and oxide nanostructures have been successfully obtained via simple non-isothermal heat treatment of gadolinium acetate precursor. Nanosheets of (GdO)2CO3 were developed as a consequence of heating the acetate precursor over the 400–500 °C range and these were stripped into thin lamellar nanorods of cubic Gd2O3 when the heat treatment was applied at the 600–800 °C range. The analysis of N2 adsorption isotherms revealed the microporosity of both phases and the BET calculated surface areas showed a gradual decrease with the pretreatment temperature of Gd2O3. Surface inspection using XPS indicated the presence of Gd in 3+ valence states at the surfaces of (GdO)2CO3 and Gd2O3. H2-TPR results revealed the refractory nature of Gd2O3 nanoparticles against reduction. Moreover, a possible reaction of H2 and CO2 was suggested. The electrical conductivity has been measured during the decomposition of the gadolinium precursor and for the 500–800 °C pre-treated samples.

Simulation of recrystallization and martensite revision in 304L austenitic stainless steel after multi-pass rolling processes

In this work, non-isothermal annealing of cold-rolled 304L austenitic stainless steel was studied at temperatures ranging between 400 and 800 °C. In the first place, a dual-phase structure containing work-hardened austenite and strain-induced martensite was produced by means of multi-pass plate rolling, and then, the deformed steel was subjected to annealing heat treatments. To determine the distribution of stored strain energy in the rolling operations, an elastic–plastic finite-element modeling was conducted. Afterwards, an artificial neural network technique was coupled with the thermal-cellular automata model for prediction of martensite reversion, temperature history, and recrystallization progress during subsequent non-isothermal heat treatments. Experimental methods including X-ray diffraction, optical metallography, and Feritscope measurements were carried out to assess the material constants as well as to verify the predictions. Comparison between the predicted results and experimental data showed a good agreement indicating the capability of the model under practical conditions. It was found that that martensite revision started at about 550 °C, while the onset of recrystallization occurred around 670 °C. The activation energies for nucleation and growth during static recrystallization were determined as 180 kJ/mole and 240 kJ/mole, respectively. In addition, the homogenous nucleation mechanism was found to be operative in the rolled steel subjected to total reduction of 35% or higher that resulted in a uniform fine-grained structure.

Fabrication and characterization of 1-dimensional neodymium-iron-boron fibers using new reduction-diffusion process

A novel method for the synthesis of neodymium-iron-boron (Nd-Fe-B) fibers is demonstrated in this study by adopting three continuous processes: electrospinning, step heat treatment, and a new process comprising facile reduction-diffusion. The structural and crystalline studies revealed that as-spun precursor fibers were transformed into FeNdO3, Fe2O3, and NdBO3 phases by non-isothermal heat treatment; further, the oxide mixture fibers were reduced to a single Nd2Fe14B phase by the facile reduction-diffusion process. A facile reduction-diffusion process in this paper can obtain Nd2Fe14B fibers without brokenness; this is because it leads to the formation of a thin salt layer on Nd2Fe14B fiber surface when reacting to by-product CaO granules. The BHmax of Nd2Fe14B fibers was exhibited by 3.34 MGOe. To our knowledge, this is the first attempt to fabricate 1-D Nd2Fe14B fibers successfully using a new reduction-diffusion process. This work provides a simple way to prepare 1-D hard magnetic Nd-Fe-B fibers with excellent shape anisotropy attributed to a high aspect ratio.

Thermal cycling induced stress–assisted sigma phase formation in super duplex stainless steel

Thermal cycling induced the phase transformation originated from the physical simulation of power plant on/off operation. The effect of thermal cycling on duplex stainless steel was studied to understand its phase transformation comprehensively. Cyclic thermal processing is a nonisothermal heat treatment method known to accelerate the kinetics of phase transformation. In this study, when the number of thermal cycles increased, the ferrite grains gradually contracted from the grain boundaries and finally disappeared due to the eutectoid reaction. However, the austenite phase demonstrated a parabolic increase and the intermetallic sigma (σ) phase, formed at the grain boundaries, exhibited a similar trend. The KAM maps reveal the highest degree of misorientation (up to 2°) may have either been at the γ′ or σ phase at 40 and 80cycles. In DSS, the misorientations (>15°) among grains also affect σ formation. A high crystallographic misorientation between the austenite and ferrite phases favored σ precipitation. The calculated thermal stress of 422MPa assisted the σ formation by reducing the activation energy and accelerating the alloying element diffusion. The σ phase formation caused by thermal cycling increased the yield strength and ultimate strength but degraded ductility. The formation of tensile cracks was related to the σ phase at the interface or the σ matrix.

Influence of cyclic non-isothermal heat treatment on microstructure, mechanical property and corrosion behavior of Al–Zn–Mg alloy

The influence of cyclic non-isothermal heat treatment times on microstructure, mechanical properties and corrosion behavior of Al–Zn–Mg alloy is investigated. The results show that Mg segregation at grain boundary becomes severer with the increase of non-isothermal heat treatment times, and the tensile strength and elongation of the alloy decrease gradually. The increase of Mg segregation at grain boundary can be interpreted by non-equilibrium segregation theory. The size and quantity of MgZn2 precipitation decrease after one time of non-isothermal heat treatment, but increase significantly after two and three times of non-isothermal heat treatment. The change of MgZn2 precipitation results in slight improvement of corrosion resistance of Al–Zn–Mg alloy after one time of non-isothermal heat treatment, and serious deterioration of corrosion resistance of Al–Zn–Mg alloy after two and three times of non-isothermal heat treatment.

Quenching Sensitivity of Al-Zn-Mg Alloy after Non-Isothermal Heat Treatment.

Type Al-Zn-Mg alloy has a wide ranges of application in vehicles, but corrosion resistance and mechanical properties of this alloy after heat treatment or heat straightening limits its utilization. This paper investigates the effect of quenching condition during non-isothermal heat treatment on corrosion behavior and mechanical properties of Al-Zn-Mg alloy. The corrosion resistance of Al-Zn-Mg alloy decreases after non-isothermal heat treatment and the sample after air quenching has the lowest corrosion resistance, the specimens can get better corrosion resistance when suffered 5 min air cooling followed by water quenching process. The evolution of mechanical properties and corrosion resistance of heat-treated specimens is caused by the modification of distribution of precipitates at grain boundary and the microchemistry of precipitates at grain boundary, precipitate-free zones and the matrix.

Microstructural development during heat treatment of a commercially available dental-grade lithium disilicate glass-ceramic

ObjectiveTo elucidate the microstructural evolution of a commercial dental-grade lithium disilicate glass-ceramic using a wide battery of in-situ and ex-situ characterization techniques. MethodsIn-situ X-ray thermo-diffractometry experiments were conducted on a commercially available dental-grade lithium disilicate glass-ceramic under both non-isothermal and isothermal heat treatments in air. These analyses were complemented by experiments of ex-situ X-ray diffractometry, field-emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, differential scanning calorimetry, and field-emission scanning electron thermo-microscopy. ResultsIt was found that the non-fired blue block consists of ∼40 vol % crystals embedded in a glass matrix. The crystals are mainly lithium metasilicate (Li2SiO3) along with small amounts of lithium orthophosphate (Li3PO4) and lithium disilicate (Li2Si2O5). Upon heating, the glassy matrix in the as-received block first crystallizes partially as SiO2 (i.e., cristobalite) at ∼660 °C. Then, the SiO2 crystals react with the original Li2SiO3 crystals at ∼735 °C, forming the desired Li2Si2O5 crystals by a solid-state reaction in equimolar concentration (SiO2 + Li2SiO3 → Li2Si2O5). Precipitation of added colourant Ce ions in the form of CeO2 appears at ∼775 °C. These events result in a glass-ceramic material with the aesthetic quality and mechanical integrity required for dental restorations. It also has a microstructure consisting essentially of elongated Li2Si2O5 grains in a glassy matrix plus small cubic CeO2 grains at the outermost part of the surface. SignificanceIt was found that by judiciously controlling the heat treatment parameters, it is possible to tailor the microstructure of the resulting glass-ceramics and thus optimizing their performance and lifespan as dental restorations.

Crystallization kinetics study of silver-doped germanium glasses

A series of germanium glasses in the systems (100-x)[40GeO2-30Bi2O3-20PbO-10TiO2] – (x) AgCl with x = 0, 1, 3 and 5 (mol%) was prepared using the melt-quenching method and then characterized taking into account their crystallization kinetics mechanisms and microstructural analysis. Through Differential Scanning Calorimetry (DSC) performed in different linear heating rate programs, the experimental data obtained were based on non-isothermal models of phase transformation, since their versatility allows us to calculate kinetic parameters, for example, activation energy for crystallization (Ec) and Avrami index n. Thus, it is possible to obtain information on the crystallization mechanism of these glasses. Independent of the model, the values of the Avrami index for each composition are the same, considering the uncertainty attributed to them. It is safe to affirm that bulk and surface nucleation contribute to the crystallization of compositions containing 0 and 1 mol% of AgCl, whereas internal and surface crystallization with one-dimensional growth occur in samples with 3 and 5 mol% of AgCl, respectively. These results were supported by a microstructural analysis from bulk samples that underwent non-isothermal heat treatment at different temperatures according to DSC crystallization peak.

Modelling and experimental validation of microstructure evolution during the cooling stage of homogenization heat treatment of Al–Mg–Si alloys

A CALPHAD-coupled multi-component multi-phase Kampmann–Wagner Numerical modelling framework has been adapted and coupled with a homogenization model to predict the competitive nucleation and growth of multi-sized metastable/stable phase particles during the cooling stage of a homogenization heat treatment. The reported model is the continuation of our previous work on the homogenization soaking modelling (Du et al., 2013). It takes the experimentally verified soaking modelling prediction results as the initial microstructural status and predicts the microstructural evolution during the very last step of the homogenization treatment, i.e. the final cooling process. The model has been applied to two Al–Mg–Si alloys: AA6061 and AA6082. The simulation shows a multi-modal MgSi particle size distribution forms due to multiple nucleation events. For the AA6082 alloy the multiple nucleation events occur mainly due to the local micro-chemistry variations along a dendrite while for AA6061 it is due to the two opposite contributions to supersaturation: the positive contribution by the cooling and the negative contribution by the particles' growth. The interaction of intergranular and intragranular MgSi containing particles have been captured, and the partitioning of Mg and Si solutes among the two different sized MgSi particles has been predicted. The model predictions are in reasonable agreement with the microstructure characterization results obtained with Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Electrical Resistivity Measurement on the quenched samples from some dedicated laboratory-scale homogenization heat treatment experiments. It is concluded that the extended KWN modelling framework is applicable to predict the microstructure evolution during the non-isothermal heat treatment of multicomponent aluminium alloys.