Effect Of Thermal History Research Articles

Microstructural evolution, crystallographic texture, grain morphology, and mechanical integrity of wire arc additively manufactured Inconel 625 alloy

The material in wire arc additive manufacturing (WAAM) undergoes complex material flow and multiple thermal heating and cooling cycles, forming highly heterogeneous microstructures in terms of size, crystallographic orientations, and mechanical properties. The inhomogeneity also depends on the dislocation density and phases, which are influenced by the thermal history of the process. In this study, the Cold Metal Transfer (CMT) process was used to deposit a 60-layer build of Inconel 625 alloy. Detailed variations in the microstructural size, orientations, and phases along the building direction were studied using optical microscopy, electron backscatter diffraction (EBSD), and X-ray diffraction (XRD). Microstructural observations reveal dendrites, equiaxed crystals, cellular, and columnar structures with primary and secondary dendrites. Dynamic recrystallization (DRX) followed by abnormal grain growth was found in the build. The average grain size varies with deposited height, with a grain size of around 13 ± 1 μm near the substrate, 45 ± 1 μm in the middle region, and 18 ± 1 μm at the top. The top region exhibited a strong intensity of recrystallized Cube, Cube-ND, and Cube-RD textures, with weaker intensities of copper and brass textures. The middle and bottom regions show strong intensities of Goss, copper, F, S, and E textures, respectively. The highest dislocation density of 5.122 × 10−4 nm−2 was found in the top region, while the lowest (4.14 × 10−4 nm−2) was observed in the bottom region. The ultimate tensile strength of the build ranged from 603 ± 05 MPa to 699 ± 10 MPa, while the yield strength varied from 313 ± 07 MPa to 365 ± 08 MPa along different orientations. Vickers hardness results showed a slight variation, from 240 ± 5 to 260 ± 2 HV, from bottom to the top of the deposited build. The findings from this study provide valuable insights into the microstructural evolution mechanism and mechanical behavior of WAAM-fabricated Inconel 625, which can guide other researchers in optimizing process parameters, enhancing material properties, and understanding the effects of thermal history on additive manufacturing of high-performance alloys.

Stepwise Multi-Temperature Thermogravimetric Analysis (SMT-TGA) for Rapid Alloy Development

A stepwise multi-temperature thermogravimetric analysis (SMT-TGA) method is a rapid and time- and material-efficient measurement procedure for oxidation kinetics over a wide range of temperatures. It is suitable for alloy design and material selection procedures. It involves subjecting a sample to a series of steps at increasing temperatures, followed by steps at decreasing temperatures to identify possible effects on the evolution of oxide layer microstructures on oxidation kinetics. This method has been tested for a wide range of metallic alloys in the present work, allowing for the mapping of possible ranges of parabolic oxidation kinetics of industrial alloys between 600 and 1300°C. Two examples of effects of thermal history have also been described in this publication.

Effect of processing parameters and thermal history on microstructure evolution and functional properties in laser powder bed fusion of 316L

In this paper, we explain and quantify the causal effect of processing parameters and part-scale thermal history on the evolution of microstructure and mechanical properties in the laser powder bed fusion additive manufacturing of Stainless Steel 316L components. While previous works have correlated the processing parameters to flaw formation, microstructures evolved, and properties, a missing link is the understanding of the effect of thermal history. Accordingly, tensile test coupons were manufactured under varying processing conditions, and their microstructure-related attributes, e.g., grain morphology, size and texture; porosity; and microhardness were characterized. Additionally, the yield and tensile strengths of the samples were measured using digital image correlation. An experimentally validated computational model was used to predict the thermal history of each sample. The temperature gradients and sub-surface cooling rates ascertained from the model predictions were correlated with the experimentally characterized microstructure and mechanical properties. By elucidating the fundamental process-thermal-structure–property relationship, this work establishes the foundation for future physics-based prediction of microstructure and functional properties in laser powder bed fusion.

Crystallization Kinetics of Phosphonium Ionic Liquids: Effect of Cation Alkyl Chain Length and Thermal History.

The effects of alkyl chain length on the crystallization kinetics and ion mobility of tetraalkylphosphonium, [P666,n][TFSI], (n = 2, 6, 8, and 12) ionic liquids were studied by differential scanning calorimetry (DSC) and broadband dielectric spectroscopy (BDS) over a wide temperature range. The liquid-glass transition temperature (Tg) and ion dynamics examined over a broad T range were almost insensitive to structural modifications of the phosphonium cation. In contrast, the crystallization kinetics were strongly affected by the length of the fourth alkyl chain. Furthermore, the thermal history of the sample (cold vs melt crystallization) significantly impacted the crystallization rate. It has been found that the nature of crystallization phenomena is the same across the homologous series, while the kinetic aspect differs. Finally, electric conductivity in supercooled liquid and crystalline solid phases was measured for all samples, revealing significant ionic conductivity, largely independent of the cation structure.

Influence of the thermal shrinkage-induced volume contraction on the yielding behavior of waxy oil

The present article investigates the effect of thermal shrinkage on the yielding behavior of waxy oil gel. The paper compares constant gap and constant normal force measurement protocols to examine the effect of measurement protocol on the rheological behavior of thermoreversible gel. The findings of this study reveal that the extent of stress overshoot and yield stress in a constant shear-rate start-up flow shows a lower magnitude in a constant gap protocol compared to a constant normal force protocol. The decrease in gel strength in the former protocol is mainly attributed to the formation of voids. These voids cause localized fractures within the crystal network. In contrast, in the constant normal force measurement protocol, gel contraction is compensated by utilizing a variable gap setting. Variable gaps compensate for the lower specific volume of the gel after crystallization, minimizing void formation and subsequent rupture of the crystal network. Hence, the gel network formed using the constant normal force protocol is more homogeneous, eliminating the uncertainties in yield stress measurement. Finally, the effect of thermal history, wax content, and aging period on the yield stress values highlights notable findings. Contrary to the conventionally accepted results, the aging period is found to impact the yield stress negatively, and a nonmonotonic relationship between the cooling rate and yield stress is noticed under the constant gap protocol. Thus, the results obtained under the constant normal force protocol are more reliable and can help in developing a fundamental understanding of the yielding behavior.

The Effects of Thermal History on Toughness of Ni-Based Corrosion Resistant Alloys during In Situ Hydrogen Charging

The Effects of Thermal History on Toughness of Ni-Based Corrosion Resistant Alloys during In Situ Hydrogen Charging

A thermal state‐dependent bounding surface model for volume change of clay

AbstractThermally induced volumetric strain of clay is crucial for geotechnical applications involving thermal loading. The volumetric response of clay shows a ratcheting pattern during thermal cycles until reaching a thermal stabilized state. It is also affected by the stress history of soil, including over‐consolidation ratio (OCR) and recent stress history (RSH). This paper introduces a new bounding surface model to capture effects of OCR and RSH on thermo‐mechanical behaviour of soil. A thermal state parameter is proposed to characterize the effects of stress history and thermal history. Based on the new thermal state parameter, the commonly recognised thermal softening mechanism is modified and incorporated with a bounding surface. The newly proposed model, with only 10 parameters, can provide an elegant approach to predict the volumetric response under thermal cycles coupled with different stress histories well.

Effect of Thermal History During Device Fabrication on Quantum-Dot Light-Emitting Diode Characteristics

Effect of Thermal History During Device Fabrication on Quantum-Dot Light-Emitting Diode Characteristics

Comparison of indoor thermal environments and human thermal responses in Northern and Southern China during winter

Human thermal adaptability has a significant impact on energy savings and emission reduction in buildings. To investigate the effect of long-term thermal history on thermal adaptability, this study explored the characteristics of indoor thermal environments and the differences in human thermal responses during winter in northern and southern China. Furthermore, the effects of the region (heating patterns), building types and payment patterns on the thermal responses were clarified. Finally, the thermal neutral temperatures and 80 % thermal acceptable ranges were determined. The results indicate that there are significant differences in the indoor thermal environments and human thermal responses in different types of buildings during winter (P < 0.00). However, the effect size of the thermal responses (d ≥ 0.3) is lower than that of the environmental parameters (d ≥ 0.7). This demonstrates that individuals have strong thermal adaptability. Due to the higher indoor air temperature of district heating in northern China, the northerners have a higher thermal neutral temperature and lower clothing insulation. They are stricter in the requirements for room temperature and weaker in thermal adaptability. Nevertheless, the southerners can actively adapt to lower indoor temperatures by clothing adjustments, with a wider acceptable range of indoor temperatures and lower thermal neutral temperatures. Charging for the actual heating usage can lower the thermal neutral temperature and boost the thermal satisfaction rate, which is conducive to saving energy and reducing emissions. Thus, these findings can contribute to the promotion of the comfortable and sustainable development of built environments.

The effect of thermal history on the additively manufactured AlSi10Mg alloy response to ion irradiation

Aluminum alloys, mainly from the 5xxx and 6xxx families, are used extensively in research reactors due to their suitable properties. However, alloys common in nuclear applications are not yet suitable for Additive Manufacturing (AM). The alloy AlSi10Mg has good engineering properties and is the most used and investigated AM Al-alloy. In this work, stress relieved (SR) samples of AM AlSi10Mg have been compared to samples that underwent a subsequent heat treatment at 500°C (SHT), producing two distinctly different microstructures. The SHT samples contain larger grains with a cleaner matrix and coarser silicon particles compared to the SR samples. Samples with the two microstructures were irradiated using silicon ions up to damage of ∼20 dpa at a depth of about 1 µm. To compare the response of both conditions to irradiation, properties were measured using nano-indentation and transient grating spectroscopy (TGS). Microstructure was characterized using TEM. Results show that the SHT condition is much more sensitive to irradiation, compared to SR. Irradiation caused thermal diffusivity to decrease by ∼5% in the SR condition and by more than 50% in the SHT condition. Hardness increased by 12 and 24% in the SR and SHT conditions, respectively. Also, the void number density was significantly larger in the SHT condition. No change was observed in young's modulus. The lower sensitivity of the SR condition is attributed to a higher sink density present in the initial microstructure, resulting in overall less irradiation damage retained in the material after irradiation. The explanation offered for the major drop in thermal diffusivity observed only in the SHT condition is the large size of the silicon particles, which increases their instability during irradiation, causing them to partly dissolve and release atomic silicon into the matrix around them. These silicon atoms are effective electron and phonon scatter centers. The findings of this work show that the pre-irradiated microstructure of AM AlSi10Mg has a crucial role in the material's performance in nuclear reactor core conditions. Careful thought and care need to be implemented to the heat treatment applied to an as-built part.

AgeSpectraAnalyst: A MATLAB based package to model zircon age distributions in silicic magmatic systems

In the last decade, improvements in the analytical precision achievable by zircon U-Pb geochronological techniques have allowed to resolve complexities of zircon crystallization histories in magmatic rocks to an unprecedented level. A number of studies have strived to link resolvable dispersion in zircon age spectra of samples from fossil magmatic systems to the physical parameters of their parent magma bodies. However, the methodologies developed have so far been limited to reproduce the effect of simple thermal histories on the final distribution of zircon ages. In this work we take a more nuanced approach, fine-tuning a thermodynamics-based zircon saturation model to predict the relative distribution of zircon ages in samples from silicic magma reservoirs experiencing open-system processes (e.g. heat/mass addition, mechanical mixing). Employing the MATLAB package (AgeSpectraAnalyst) presented in this contribution:•Users can forward model the effect that diverse thermal histories and mechanical mixing processes characteristic of silicic magma bodies have on zircon age distributions as measured by high-precision, chemical abrasion thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb geochronology.•Zircon CA-ID-TIMS datasets from silicic magmatic systems can be easily compared with model output to gain semi-quantitative information on thermo-mechanical history of the system of interest.•We demonstrated (Tavazzani et al., in press) that distribution of high-precision zircon ages in crystallized remnants of shallow (∼ 250 MPa), silicic magma reservoirs can discriminate between systems that experienced catastrophic, caldera-forming eruptions and systems that underwent monotonic cooling histories.

Adaptability law of central heating environment under the effect of thermal history: A continuous field survey

Creating an optimal indoor thermal environment that meets the needs of diverse populations is a crucial aspect of building design. Given the escalating frequency of population migration and the widespread adoption of centralized heating systems, it is important to establish the effects of various prior thermal histories on human adaptability to centralized heating environments. In this study, 15 subjects with no prior experience of heating in hot summer and cold winter regions (HSCW) and 13 subjects with prior experience of heating in cold regions (C) were selected for a 40-day follow-up study at a university in Qingdao. In total, 992 valid responses were obtained from the questionnaires. The results indicated that prior thermal history had a notable influence on thermal perception in centrally heated environments, with those lacking heating experience reflecting a better initial perception of heat. Specifically, in a classroom environment, the differences between the two groups, in terms of thermal sensation, thermal acceptability (percentage of acceptance), and thermal expectation (percentage of desiring “warmer”) decreased from 0.26, 12%, and 19% in the first cycle to 0.03, 2%, and 3% in the fourth cycle (25–32 days), respectively. The difference in thermal comfort also reduced from 0.23 in the second cycle (9–16 days) to 0.04. Similar trends were observed in the dormitory environment, indicating that the duration of the impact of heat experience on adaptation to the centrally heated environment was 32 days. The results of this study are useful for describing the dynamic thermal acclimatization of different populations in unfamiliar thermal environments and for providing indoor thermal environments that match the thermal preferences of residents with different thermal experiences.

Effect of short-term thermal history on thermal comfort and physiological responses: A pilot study

The purpose of this study is to examine the effect of outdoor short-term thermal history on the thermal comfort and physiological responses of human in the indoor environment. We conducted a field study in 4 split air-conditioned dormitory buildings. A total of 345 valid datasets were collected from 62 participants. We investigated thermal comfort, physiological responses and adaptation behaviours. According to the indoor staying time, we clarified the participants into two groups, namely STN30 (indoor staying time less than 30 min) and ST30 (indoor staying time more than 30 min). We compared the thermal comfort responses and physiological responses between two indoor staying time groups. We determined the relationship between thermal sensation and physiological responses. Subjects had a higher percentage of warmer thermal sensation, thermal discomfort and cooler preference in STN30 than in ST30. The upper extremity skin temperature was highly correlated to operative temperature and was consistent in both ST30 and STN 30 groups under the same operative temperature. Participants with shorter indoor staying time had lower neutral temperature (1.1 °C) and lower preferred temperature. The upper extremity skin temperatures were significantly correlated to thermal sensation.

Simulations of the effect of smectite‐to‐illite transition in shales on permeability and overpressures using a stochastic approach, a Norwegian margin case study

AbstractThe smectite‐illite transition in shales due to subsidence, temperature changes and diagenesis influences many processes in a sedimentary basin that can contribute to overpressure build up like reducing the shale permeability. The smectite‐rich layers can form sealing barriers to fluid flows that will influence pore pressure prognosis for drilling campaigns, contribute to sealing caprocks for possible CO2 storage and to sealing of plugging and abandonment wells. In this work, we have included the diagenetic smectite‐illite transition into a three‐dimensional pressure simulation model to simulate its effect on pressure build‐up due to reduced shale permeabilities over geological time scale. We have also tested effect of thermal history and potassium concentration on the process of smectite‐illite transition and the associated smectite‐illite correction on permeability. A new smectite‐illite correction has been introduced, to mimic how shale permeability will vary dependent on the smectite‐illite transition. Stochastic Monte Carlo simulations have been carried out to test the sensitivity of the new correction parameters. Finally, a 3D Monte Carlo pore pressure simulation with 1000 drawings has been carried out on a case study covering Skarv Field, and Dønna Terrace offshore Mid‐Norway. The simulated mean overpressures are in range with observed overpressures from exploration wells in the area for the Cretaceous sandy Lysing Formation and for the two Cretaceous Intra Lange Formation sandstones. The simulated smectite content versus depth is in line with published XRD dataset from wells. The corresponding modelled present‐day permeabilities for the shales including the smectite‐illite transition are two magnitudes higher than measured permeabilities on small samples in the laboratory using transient decay method. The measured permeabilities are in the range of 2.66·10−18 to 3.94·10−22 m2 (2695 to 0.39 nD) for the North Sea database and represent the end members for shales‐permeabilities with the lowest values, since the small samples are selected with no or minor natural fractures. This work shows that by upscaling shale permeabilities from mm‐scale to km scale, natural fractures and sedimentary heterogeneities will increase the shale permeabilities with a factor of two and that by including permeability correction controlled by the smectite fraction, pressure ramp can be simulated due to diagenesis effect in shales.

Effect of interfacial thermal history on bonding mechanism of laser assisted joining of QP980-CFRTP with adjustable flat-top rectangular laser beam

Carbon fiber reinforced thermoplastic composite (CFRTP) - metal hybrid structures, which have been increasingly used in the industry, can significantly reduce structure weight and energy consumption. In this paper, an adjustable flat-top rectangular diode laser beam was used as a heat source to join the quenched and portioned QP980 steel and CFRTP in a lap joint configuration. Compared to conventional laser beam scanning process in the literatures, the rectangular laser beam provides a relatively uniform temperature distribution at interface. The relationship between interfacial thermal history and the joint performance was explored. Experimental results showed that thermal history had a significant impact on interfacial chemical compositions and the formation of joint defects via affecting the resin state, resulting in change of the joint fracture behaviors. Furthermore, higher interfacial temperature and sufficient heating/holding time contributed to the complete melting and diffusing of resin matrix on QP980 surface, filling the micro pores of the interface, and promoting the chemical bonding between QP980 and CFRTP. On this basis, a novel strategy to achieve excellent interfacial bonding via adjusting the interfacial thermal history was proposed to produce a high-quality joint with the peak load higher than 10 kN and the shear strength of 22 MPa.

Insights into the gradient microstructure and mechanical properties of Ti-6.5Al–2Zr–Mo–V alloy manufactured by electron beam freeform fabrication

Herein, the Ti-6.5Al–2Zr–Mo–V (TA15) alloy with gradient characteristics is fabricated via electron beam freeform fabrication (EBF3) technology. The gradient microstructure and mechanical properties are investigated in detail. Specifically, the microstructural evolution containing the morphology and size along the building direction and its impact on microhardness and tensile properties are systematically illustrated. Besides, the numerical simulation is developed for the establishment of the EBF3 temperature field thermal cycling profile to further elaborate the effect of temperature and thermal history on the microstructural evolution. The results reveal that the columnar to equiaxed transition and coarsening of the prior β grains, as well as successive increase and decrease of thickness of α lamellae along the building direction. The formation of fine lamellar α colony within the layer band is associated with the survived α phase when the peak temperature is just below Tβ. The gradient microstructure endows the TA15 deposit with different mechanical properties. Specifically, the microstructure with fine α lamellae manifests marginally higher microhardness in the layer-band-free region. The elemental dilution, loss and partitioning result in relatively low microhardness in the region with fairly fine α lamellae immediately adjacent to the substrate. The top region of the TA15 deposit, characterized by fine basket-weave microstructure, is imparted with better comprehensive properties relative to the middle and bottom regions, i.e., 1007.4 ± 30.8 MPa, 915.31 ± 28.7 MPa, and 12.08 ± 1.96% for tensile strength, yield strength and elongation, respectively. The integrated effect of coarsened prior β grains and refined α lamellae allow the middle and bottom regions to exhibit similar tensile properties.

When is warmer better? Disentangling within‐ and between‐generation effects of thermal history on early survival

Abstract Understanding the fitness consequences of thermal history is necessary to predict organismal responses to global warming. This is especially challenging for ectotherms with complex life cycles, where distinct life stages can differ in thermal sensitivity, acclimate to different thermal environments and accrue responses to acclimation within and between generations. Although acclimation is often hypothesized to benefit organisms by helping them (or their offspring) to compensate for negative impacts of environmental change, mixed support for this hypothesis highlights the need to assess alternatives. Assessments that explicitly dissect responses across life stages and generations, however, remain limited. We assess alternative hypotheses of acclimation (none, beneficial, colder‐is‐better and warmer‐is‐better) within and between generations of a marine tubeworm whose vulnerability to warming rests on survival at early planktonic stages (gametes, embryos and larvae). First, we acclimate parents, gametes and embryos to ambient and projected warmer temperatures (17°C and 22°C) factorially by life stage. Next, we rear offspring with differing acclimation histories to the end of larval development at test temperatures from 10°C to 28°C (lower and upper survival limits respectively). Last, we estimate thermal survival curves for development, and compare them among thermal histories. We show that survival curves are most responsive to parental acclimation followed by acclimation at embryogenesis, but are buffered against acclimation at fertilization. Moreover, curves respond independently to acclimation within and between generations, and respond largely as predicted by the warmer‐is‐better hypothesis, despite the semblance of beneficial acclimation after successive doses of warmer temperature. Our study demonstrates the varied nature of thermal acclimation and the importance of considering how responses aggregate across complex life cycles when predicting vulnerability to warming. Read the free Plain Language Summary for this article on the Journal blog.

Effects of thermal history of in-situ thermal management on as-built property heterogeneity of plasma arc additively manufactured Inconel 625

In-situ thermal management (ISTM) is one of the most prospective approaches to alleviating heat accumulation and achieving progressive forming for arc-based additive manufacturing. The aim of this study was to investigate the influences of thermal history, i.e., the cooling rate and the intrinsic heat treatment (IHT), of ISTM on as-built property heterogeneity of plasma arc additively manufactured Inconel 625 thin-wall deposit. The thermal history is obtained from numerical simulation. Results show that the variation of the thermal history of ISTM results in the bottom-up varied cooling rate and IHT, which together lead to the location-heterogeneous primary dendrite arm spacing, interdendritic phases and grain size. IHT of each layer is quantified by temperature-time integration (TTI), and Gaussian distributed TTI (above 600 °C) indicates the middle region of the deposit with ISTM is subjected to the most intense IHT. Compared with the counterpart of the conventional interlayer cooling strategy, the intensified high-temperature IHT of ISTM contributes to significant mitigation of micro-segregation, i.e., the brittle-hard interdendritic phases reduce 61.9%. Furthermore, a weighted value of cooling rate and TTI is innovatively introduced to depict the heterogeneity of mechanical property, which presents an exponential growth of ultimate tensile strength with the weighted value. Moreover, the layer-by-layer decreased heat input and the high-temperature preheating between layers of ISTM contribute to a certain residual stress release effect along the building direction.

Thermal hysteresis phenomena in aqueous xanthan gum solutions

The anionic hydrocolloid polysaccharide xanthan gum is widely used in the food and petroleum industries (among others) as a viscosity enhancement polymer due to its high viscosity at low concentrations and moderate temperatures. The physical properties of microbial polysaccharide xanthan gum aqueous solutions were investigated using temperature dependent viscosity measurements. Specifically, the effect of thermal history on the solution viscosity was investigated. Heating and cooling cycles were assessed in two ways, by using a “sawtooth” and “triangle” pattern, which essentially differed in the rates of cooling. The sawtooth method used a cooling rate of 2.0 °C min−1 whereas the triangle pattern had a cooling rate of 0.20 °C min−1. The sawtooth cooling rate was controlled by the speed at which the Peltier device could cool the sample, and the triangle rate was governed by the time required to measure the viscosity at each temperature on return to the initial value. Cycles measured using the sawtooth pattern for 16 mg/kg xanthan gum in water showed an 8–10% overall decrease in the viscosity over four complete cycles. Comparatively, at 320 mg/kg the xanthan gum solution showed a 25% decrease in viscosity over four cycles. The observed temperature dependent viscosity variation suggested minor modifications in the physical network structure of xanthan gum. When using a triangle heating/cooling pattern, the overall decrease in the xanthan gum solution viscosity was 5–7% for 16 mg/kg and only 10% change for 320 mg/kg solutions. The activation energy of viscous flow for the aqueous xanthan gum solutions by either method was ∼15.0 kJ/mol under all conditions. The data showed that temperature and heating cycles influence xanthan gum viscosity and thermal history, which depends more strongly on xanthan gum concentration than solution temperature.

Thermal History Effects on Decomposition Behavior and Pyrolysis Mechanism of Cellulose Nitrate

Nitrocellulose is an important kind of energetic material produced by replacing hydroxyl of cellulose molecule to nitro, which has a wide application range in social life. During transportation and storage, inevitably the quality of the nitrocellulose will be affected due to external ambient heating. In this study, two kinds of NC samples, original and heated ones, were used as research objects and taken into DSC experiments under several constant heating rates to explore thermal history effects on its decomposition and combustion behavior. A series of calculation methods based on model fitting were main ways for research, so were model free methods. Numerical results by model fitting method showed that decomposition reaction of NC follows n-th reaction model. The comparison between experimental results of two kinds of samples claimed that thermal history had positive influence on heat flow, and increased the reaction order of decomposition process, and decreased the characteristic temperatures. So the thermal history made the decomposition reaction more difficult to take place and more stable. This study is obviously meaningful for the research of thermal pyrolysis process of NC after thermal history.