Time Temperature Transformation Diagram Research Articles

Investigation of the effect of P2O5 and CaF2 on the crystallization behavior of La2O3-bearing calcium–silicate–aluminum slags using the single hot thermocouple technique

The effects of P2O5 and CaF2 components of rare earth (RE)-bearing slags on their crystallization behavior were investigated using single hot-thermocouple technique (SHTT). The time temperature transformation (TTT) diagram exhibited single-nose curves within the examined slag composition range, in agreement with the individual CaLa4(SiO4)3O-type phase detected by an isothermal crystallization experiment in a furnace. Additionally, an increase in the P2O5 content shortened the incubation time and increased the nucleation rate of the columnar CaLa4(SiO4)3O-type phase in the 1350–1150 °C temperature range, while an increase in the CaF2 content had the opposite effects on the incubation time and nucleation rate. The sizes of the CaLa4(SiO4)3O-type phases in the slags with CaF2 were larger than those in the slags with P2O5, as verified by both in situ SHTT images and the crystallization experiment. The continuous cooling transformation (CCT) diagram suggested that an increase in the P2O5 content increased the initial crystallization temperature, whereas an increase in the CaF2 content decreased it. An analysis of the isothermal crystallization kinetics revealed that the columnar CaLa4(SiO4)3O-type phase exhibited phase diffusion-controlled growth. An increase in the P2O5 content tended to decrease the crystallization activation energy, while an increase in the CaF2 content increased the activation energy.

Tailored Time–Temperature Transformation Diagram for IN718 Alloy Obtained via Powder Bed Fusion Additive Manufacturing: Phase Behavior and Precipitation Dynamic

Tailored Time–Temperature Transformation Diagram for IN718 Alloy Obtained via Powder Bed Fusion Additive Manufacturing: Phase Behavior and Precipitation Dynamic

Aging behavior of a 1.5 mol% yttria doped zirconia exhibiting optimized toughness and strength

The aging behavior of a 1.5Y-TZP with 0.25 wt% of alumina exhibiting high mechanical properties was investigated in both water vapor and in air with in situ XRD analysis. The results showed that the material exhibited a high resistance to aging in water vapor at 134 °C compared to 3Y-TZP, which is explained by the addition of alumina and a lower sintering temperature. The aging kinetics were fitted by the Mehl–Avrami–Johnson law, three stages were identified corresponding to predominance of nucleation mechanism for temperatures below 180 °C and above 340 °C and to growth mechanism for intermediate temperatures. It was shown that for the temperature range investigated in water vapor and air, the mechanism was independent on the environment. A strong influence of the cooling rate on the tetragonal to monoclinic phase transformation was observed and discussed using a time–temperature-transformation diagram derived from the aging results in air.

Understanding the Effect of Nucleation in Amorphous Solid Dispersions through Time-Temperature Transformation.

In an earlier investigation, the critical cooling rate to prevent drug crystallization (CRcrit) during the preparation of nifedipine (NIF) amorphous solid dispersions (ASDs) was determined through a time-temperature transformation (TTT) diagram (Lalge et al. Mol. Pharmaceutics 2023, 20 (3), 1806-1817). The current study aims to use the TTT diagram to determine the critical cooling rate to prevent drug nucleation (CRcritN) during the preparation of ASDs. ASDs were prepared with each polyvinylpyrrolidone (PVP) and hydroxypropyl methylcellulose acetate succinate (HPMCAS). The dispersions were first stored under conditions promoting nucleation and then heated to the temperature that favors crystallization. The crystallization onset time (tC) was determined by differential scanning calorimetry and synchrotron X-ray diffractometry. TTT diagrams for nucleation were generated, which provided the critical nucleation temperature (50 °C) and the critical cooling rate to avoid nucleation (CRcritN). The strength of the drug-polymer interactions as well as the polymer concentration affected the CRcritN, with PVP having a stronger interaction than HPMCAS. The CRcrit of amorphous NIF was ∼17.5 °C/min. The addition of a 20% w/w polymer resulted in CRcrit of ∼0.05 and 0.2 °C/min and CRcritN of ∼4.1 and 8.1 °C/min for the dispersions prepared with PVP and HPMCAS, respectively.

Controlling the Thermal Stability of a Bainitic Structure by Alloy Design and Isothermal Heat Treatment.

The aim of this work was to develop a novel bainitic steel that will be specifically dedicated to achieving a high degree of refinement (nano- or submicron scale) along with increased thermal stability of the structure at elevated temperatures. The material was characterized by improved in-use properties, expressed as the thermal stability of the structure, compared to nanocrystalline bainitic steels with a limited fraction of carbide precipitations. Assumed criteria for the expected low martensite start temperature, bainitic hardenability level, and thermal stability are specified. The steel design process and complete characteristics of the novel steel including continuous cooling transformation and time-temperature-transformation diagrams based on dilatometry are presented. Moreover, the influence of bainite transformation temperature on the degree of structure refinement and dimensions of austenite blocks was also determined. It was assessed whether, in medium-carbon steels, it is possible to achieve a nanoscale bainitic structure. Finally, the effectiveness of the applied strategy for enhancing thermal stability at elevated temperatures was analyzed.

Machine learning study on time–temperature–transformation diagram of carbon and low-alloy steel

Machine learning study on time–temperature–transformation diagram of carbon and low-alloy steel

Influence of Preheating Temperature on Microstructure Evolution and Hardness of High‐Speed Steel AISI M50 Processed by Laser Powder Bed Fusion

The laser powder bed fusion (LPBF) technology has been involved in the tooling industry to produce tools with complex geometry and integrated functions. However, tool steels with high carbon content tend to crack due to the thermal stresses during the LPBF process. One solution is increasing the powder bed temperature to avoid large thermal gradients. In the present study, the influence of the preheating temperature on microstructure and corresponding hardness is systematically investigated. With the help of time–temperature–transformation diagram, the phase evolution during the LPBF process is systematically explained. AISI M50 samples are produced by LPBF from room temperature to a preheating temperature of 650 °C. Higher preheating temperatures shift the optimal laser parameter window to lower volume energy densities. A cellular/dendritic microstructure formed during the rapid solidification with retained austenite is located at the interdendritic regions. Moreover, a high preheating temperature reduces the retained austenite fraction, specifically from 39% without preheating to 7.6% at 650 °C preheating temperature.

Phase formation kinetics during heat treatments of Inconel 718 deposits based on cold metal transfer-based wire arc additive manufacturing

In this study, various heat treatments were conducted on Inconel 718 (IN718) products deposited by wire arc additive manufacturing (WAAM) to investigate the formation kinetics of phases at microstructural locations. The formation kinetics of the secondary phases precipitated by heat-treating of WAAM products differ from those of wrought or cast products because of the heat that accumulates during layer-by-layer deposition. Therefore, the secondary phases in the heat-treated WAAM products differ from those in conventional products according to the microstructural locations, e.g., the dendritic region, interdendritic region, and grain boundaries. To investigate such phase differences, a 100-layer IN718 wall was deposited using cold metal transfer (CMT)-based WAAM and then heat-treated under various conditions. In each heat-treated specimen, the δ, γ’, and γ” phases were identified by microstructural analyses. Based on the results, time–temperature–transformation diagrams were constructed for each microstructural location, which confirmed that the secondary phases were precipitated faster in the interdendritic region and at grain boundaries than those in the dendritic region. In addition, the applicability of single-aging, which can be used to satisfy the desired mechanical properties in less time with less processing, was evaluated by the hardness and tensile tests in the WAAM products.

Material extrusion thermal model mapped across polyetheretherketone isothermal and continuous cooling transformation charts

This study provides an insight into the polyetheretherketone (PEEK) crystallinity progression throughout the material extrusion (MEX) additive manufacturing process as a function of time and temperature, comparing it with the isothermal and continuous cooling transformation charts created over a wide range of isothermal crystallisation temperatures and constant cooling rates. The isothermal and non-isothermal crystallisation kinetics were explored using Differential Scanning Calorimetry (DSC) and Fast Scanning Calorimetry (FSC). The half-time, onset and ending of crystallisation were obtained for isothermal crystallisation temperatures between 150 °C and 330 °C, while the crystallisation under constant cooling was obtained using rates between − 0.5 K s −1 and − 45 K s −1 . The results were used to draw the Continuous Cooling Transformation (CCT) and the Time-Temperature Transformation (TTT) diagrams and calculate the Avrami numbers using the parallel Avrami model. These results were then compared to the degree of crystallinity as a function of time and temperature for the MEX process. To evaluate the crystallisation within the MEX process a 1D transient transfer heat model was used to obtain the printing thermal profile, which was replicated using the FSC technique. The results showed that for the MEX printing process, the crystallisation usually is a product of a combination of rapid cooling and heating processes followed by periods of greater thermal stability which, depending on the nature of the process, can approach a quasi-isothermal crystallisation process. By superimposing the process thermal profile on the TTT and CCT diagrams and comparing the crystallinity values measured from each point in the thermal profile, it was possible to understand the crystallinity evolution and the remelting of the build surface promoted by the subsequent printed layers. • Temperature fluctuations in the MEX process are a combination of ultra-fast dynamic cooling steps with heating and remelting. • The FSC and standard DSC allowed the replication of the modelled thermal profile. • The crystallinity varies as a function of local temperature and the fluctuations caused by the multi-layer printing process. • Higher chamber temperature increased the crystalline phase remelting, resulting in greater mobility of polymeric chains. • Shorter return times improved overall crystallinity levels due to more frequently heat input in the building surface.

Influence of Partial Replacement of Co with Cu on Isothermal Transformation Kinetics in Ferritic/Martensitic Heat‐Resistant Steel

This study examines the influence of partial replacement of Co with Cu on isothermal transformation kinetics in 10Cr‐based heat‐resistant steels. Two kinds of heat‐resistant steels, having similar chemical compositions except for Co and Cu contents, are prepared and isothermally heat‐treated at 710–790 °C up to 240 h after austenitization. During isothermal heat treatment, austenite is decomposed via two different reactions, γ → α + M23C6 and γ → α; the cellular precipitation (γ → α + M23C6) nucleates at prior austenite grain boundaries or the interior of prior austenite grains and grows toward the interior of austenite, followed by γ → α transformation originating from C depletion in the remaining austenite. A time–temperature–transformation diagram exhibits that the partial replacement of Co with Cu not only decelerates the isothermal transformation kinetics but also lowers the nose temperature of transformation. The calculation of the driving forces for the successive reactions reveals that the increase of Cu content predominantly reduces the driving force for both γ → α + M23C6 and γ → α reactions, contributing to the lower nose temperature and sluggish transformation kinetics as Co is partially replaced with Cu.

Precipitation kinetics of secondary phases induced by heat accumulation in the deposit of Inconel 718

The precipitation kinetics were investigated of the secondary phases including γ'', γ', and δ phases formed by the heat accumulation produced in Inconel 718 (IN718) deposition of cold metal transfer (CMT)-based wire arc additive manufacturing (WAAM) process. Heat accumulation from the layer-by-layer deposition causes a difference in the holding time within the aging temperature range for each layer, resulting in a difference in precipitation of secondary phases that enhances hardness. In order to investigate the precipitation kinetics, which lead to establish the WAAM-related heat treatment standard specification unlike that of the wrought or cast IN718, the thermal history was measured during the deposition process and microstructural analysis was performed according to the height of the deposit. In addition, continuous cooling transformation (CCT) and time-temperature transformation (TTT) diagrams were prepared for dendritic and inter-dendritic regions at each height of the deposit. Based on the thermal measurements, the holding time within each precipitation temperature bands for the δ phase and for the strengthening phases were correlated with the ratio of δ phase fraction and hardness depending on the height, respectively. In addition, using the thermal history, Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation was applied to calculate the hardness of the IN718 deposit which was compared with the measured hardness depending on the holding time.

Kinetic study of the β → α + γ transformation reaction in a CuZnAl alloy

Abstract In the present work we study the phase transformation reaction of a Cu-20Zn-7Al (wt.%) alloy by isothermal and nonisothermal in situ X-ray diffraction. An effective activation energy for each phase is obtained for the range between 275 and 395 °C. The experimental data are also adjusted with the JMA equations. Finally, we present the TTT (time – temperature – transformation) diagram corresponding to the precipitation of alpha and gamma for the lower temperatures. Below 340 °C, we found for this alloy that the gamma phase precedes the formation of alpha.

Non-classical nucleation in vapor\u2013liquid\u2013solid growth of monolayer WS2 revealed by in-situ monitoring chemical vapor deposition

The very early nucleation stage of a transition metal dichalcogenide (TMD) was directly observed with in-situ monitoring of chemical vapor deposition and automated image analysis. Unique nucleation dynamics, such as very large critical nuclei and slow to rapid growth transitions, were observed during the vapor–liquid–solid (VLS) growth of monolayer tungsten disulfide (WS2). This can be explained by two-step nucleation, also known as non-classical nucleation, in which metastable clusters are formed through the aggregation of droplets. Subsequently, nucleation of solid WS2 takes place inside the metastable cluster. Furthermore, the detailed nucleation dynamics was systematically investigated from a thermodynamic point of view, revealing that the incubation time of metastable cluster formation follows the traditional time–temperature transformation diagram. Quantitative phase field simulation, combined with Bayesian inference, was conducted to extract quantitative information on the growth dynamics and crystal anisotropy from in-situ images. A clear transition in growth dynamics and crystal anisotropy between the slow and rapid growth phases was quantitatively verified. This observation supports the existence of two-step nucleation in the VLS growth of WS2. Such detailed understanding of TMD nucleation dynamics can be useful for achieving perfect structure control of TMDs.

A novel physics-based and data-supported microstructure model for part-scale simulation of laser powder bed fusion of Ti-6Al-4V

The elasto-plastic material behavior, material strength and failure modes of metals fabricated by additive manufacturing technologies are significantly determined by the underlying process-specific microstructure evolution. In this work a novel physics-based and data-supported phenomenological microstructure model for Ti-6Al-4V is proposed that is suitable for the part-scale simulation of laser powder bed fusion processes. The model predicts spatially homogenized phase fractions of the most relevant microstructural species, namely the stable beta -phase, the stable alpha _{text {s}}-phase as well as the metastable Martensite alpha _{text {m}}-phase, in a physically consistent manner. In particular, the modeled microstructure evolution, in form of diffusion-based and non-diffusional transformations, is a pure consequence of energy and mobility competitions among the different species, without the need for heuristic transformation criteria as often applied in existing models. The mathematically consistent formulation of the evolution equations in rate form renders the model suitable for the practically relevant scenario of temperature- or time-dependent diffusion coefficients, arbitrary temperature profiles, and multiple coexisting phases. Due to its physically motivated foundation, the proposed model requires only a minimal number of free parameters, which are determined in an inverse identification process considering a broad experimental data basis in form of time-temperature transformation diagrams. Subsequently, the predictive ability of the model is demonstrated by means of continuous cooling transformation diagrams, showing that experimentally observed characteristics such as critical cooling rates emerge naturally from the proposed microstructure model, instead of being enforced as heuristic transformation criteria. Eventually, the proposed model is exploited to predict the microstructure evolution for a realistic selective laser melting application scenario and for the cooling/quenching process of a Ti-6Al-4V cube of practically relevant size. Numerical results confirm experimental observations that Martensite is the dominating microstructure species in regimes of high cooling rates, e.g., due to highly localized heat sources or in near-surface domains, while a proper manipulation of the temperature field, e.g., by preheating the base-plate in selective laser melting, can suppress the formation of this metastable phase.

Evolution of nanoscale precipitates during common Alloy 718 ageing treatments

Improving mechanical properties of the Ni-based superalloy Alloy 718, commonly used in turbine discs, is of vital importance to commercial aviation. The need for higher efficiency will push materials to the limit. Modifications in the processing routes of this alloy can be more easily implemented in industry than changes in the alloying elements. Variations in ageing treatment or cooling rate from hot forging can produce a wide variety of mechanical properties. These are mostly caused by changes in the nanoscale precipitates responsible for strengthening. The effects of different common ageing treatments such as conventional and direct ageing on clustering, size, volume fraction, morphology, configuration, and chemical composition of γ′-and γ′′-precipitates are still largely unknown. We investigate these aspects of the precipitates with atom probe microscopy and correlate them with hardness. The flux of the main chemical elements in the formation of co-precipitates such as duplets and triplets is remarkably different in conventionally and directly aged materials. Dislocation rich structures also inflict in unexpected rapid formation of γ′′-precipitates at temperatures more than 90 °C higher than predicted by time–temperature-transformation diagrams. Water quenching from forging impedes precipitates formed along dislocations to coarsen during ageing, considerably impacting hardness, in comparison to air cooling.

Effect of sulfur on the glass-forming ability, phase transformation, and thermal stability of Cu-Zr-Al bulk metallic glass

The effects of minor sulfur addition on the glass-forming ability (GFA), phase formation, thermal stability, and mechanical properties of the Cu47.5Zr45.1Al7.4 bulk metallic glass (BMG) were studied by conventional X-ray diffraction (XRD), in-situ high-energy synchrotron XRD, calorimetry, thermomechanical analysis, and scanning electron microscopy. Results show that sulfur has positive and negative effects on the GFA and thermal stability of Cu-Zr-Al alloys. On the one hand, sulfur can slow down the liquid dynamics, thus inhibiting the precipitation of eutectic phases during casting from the melt and heating from the as-cast glass, and hence favoring the glass formation and thermal stability of the sulfur-bearing alloys. On the other hand, the addition of sulfur promotes the formation of an Al-rich τ3 phase, which easily precipitates and is therefore catastrophic for the GFA and melt stability. The two factors compete with each other and ultimately lead to an optimal sulfur content of 0.75 at.% for the investigated Cu-Zr-Al alloys. Compared with the base alloy, this novel sulfur-containing BMG possesses improved GFA, enlarged supercooled liquid region, and comparable mechanical strength and toughness, making it a promising candidate for industrial applications as structural material. Furthermore, the schematics of the change in time–temperature–transformation diagrams with increasing sulfur concentration were proposed on the basis of experimental results.

Effect of nitrogen on precipitation behavior of hyper duplex stainless steel S32707

Thermodynamic calculations, time–temperature–transformation diagram construction, microstructural characterization and precipitation kinetics analyses were conducted to investigate the effect of N content on the precipitation behavior of hyper duplex stainless steel S32707. The results demonstrate that the nose time of 3% σ phase precipitation gradually increases with increasing N content. Increasing the N content substantially reduces the quantity of the σ phase at δ/γ interfaces. Within the first 5 min of aging, the growth rate of the σ phase decreased in the order N0.35, N0.45 and N0.56. Between 5 min and 48 h, the growth rates were essentially the same. The mechanism by which N inhibits the σ phase is attributed to three aspects: (i) N reduces the quantity of favorable nucleation sites by increasing the fraction of δ/γ interfaces with low Kurdjumov–Sachs orientation relationship deviation angles; (ii) N diminishes the tendency of Cr and Mo to diffuse from the δ to γ phase by decreasing the natural partitioning coefficients; and (iii) N decreases the σ phase nucleation driving force and Cr activity. Raising the N content promotes Cr2N precipitation only after long-term aging. The role of N in promoting Cr2N is generalized in two aspects: first, N increases the diffusion rate of Cr and N in austenite based on its increased diffusion coefficient with increasing N content; second, N increases the Cr2N nucleation driving force and N activity.

Laser additive manufacturing of metallic glasses: issues in vitrification and mechanical properties

Abstract Bulk metallic glasses (BMGs), on account of their attractive properties, have now begun to witness a few commercial applications, e.g. in coatings and micro-gears. Additive manufacturing (AM) or 3D printing, although established for crystalline alloys, has only recently been used for synthesizing BMG components. The issues arising in 3D printing of BMGs are of current relevance, and this review focuses on the key scientific aspects, namely vitrification (or crystallization) during printing, mechanical properties of printed glassy alloys and the use of AM in identifying newer BMGs. Available data on crystallization during printing of a variety of BMGs are analysed in terms of schematic time–temperature–transformation diagrams and the complex interplay between thermal cycles, the presence of quenched-in nuclei in the glass and oxygen contamination in a way that is hoped to be broadly applicable to most alloy systems. Also reviewed are three key factors influencing mechanical properties of printed BMGs, i.e. porosity, crystallinity and oxygen contamination and thereby potential strategies for improvement are suggested. The review concludes with a discussion on the use of AM for combinatorial alloy development aimed at identifying better glass-forming compositions, which may in turn facilitate greater use of AM in manufacturing glassy components with desired properties.

Fatigue Fracture of Pie Channel Weld of Sinter Screen: A Metallurgical Insight

Sintering is an agglomeration process to convert iron ore fines into a larger mass suitable to be charged directly into the blast furnaces. A screen separates the incoming iron ore raw materials based on their size fraction. These screens are supported by pie channels. One of these channels failed during operation that led to unwarranted interruption in production. Owing to the criticality of the matter, detailed failure investigation was conducted. It involved visual observations, micro- and macro-fractography to determine the mode of fracture, chemical analysis and microstructure examination using optical and scanning electron microscopes aided with energy-dispersive x-ray spectroscopy and electron probe microanalyzer. The analysis revealed incompatible austenitic stainless-steel filler metal was used for welding mild steel material along with poor welding workmanship as suggested by underfilling and presence of thick layer of welding slag. There was formation of un-tempered martensite in dilution zone (the interface layer between heat-affected zone/fusion zone). The formation of undesirable martensite in the dilution zone is correlated with the time–temperature transformation diagram where the nose of the C-curve got significantly shifted toward the right due to enrichment with chromium and nickel as revealed by electron probe microanalysis. It was concluded that the failure occurred in fatigue mode due to deficiencies in welding and recommendations to prevent failures are also provided.

Kinetics of Isothermal Phase Transformations in Premium and Standard Rail Steels

Recently published articles have enhanced the importance of understanding phase transformations in rail eutectoid steels in an attempt to improve their manufacturing and welding processes. Herein, the kinetics of isothermal phase transformations of three eutectoid steels used in railways around the world are evaluated. Computer simulations are performed to evaluate the effect of steel chemical compositions on phase transformations considering thermodynamic equilibrium conditions. Austenitizing temperatures under continuous heating and the time intervals of isothermal austenite decomposition are measured by dilatometry. The time–temperature–transformation diagrams are determined. Qualitative and quantitative microscopy techniques are applied to characterize the phase transformation products. It is shown that by applying a well‐planned heat treatment, it is possible to obtain microstructural characteristics for a standard steel as refined as those achieved for premium steels. Empirical equations proposed in the literature to predict the Bs temperatures are applied, and the experimental values are compared. Using the dilatometry data, it is possible to determine the global activation energy for austenite–bainite transformation. Results show that premium steels have higher overall activation energy for austenite–bainite decomposition than what occurs in standard. This article presents unpublished data useful for steel researchers as well for rail manufacturers and welders.