Isothermal Hold Time Research Articles

Microstructures and hot cracking formed at the interface of Zircaloy-4/stainless-steel-304L junctions

ABSTRACT Dissimilar junctions between the zirconium alloy (Zy-4) and 304L stainless steel (SS) are prepared via two bonding processes: solid-phase diffusion at T1 = 1223 K and in transient liquid phase (TLP) at T2 = 1293 K and T3 = 1323 K for an identical isothermal hold time of 2700 s. The formed layers and phases at the interface were characterised by the combination of EDS-EBSD-ESEM, EPMA and X-RD and impact fracturing tests. The Zy-4/SS interface is formed of three layers containing Cr-rich α-Fe and Zr(Fe,Cr)2 and Zr2(Fe,Ni) intermetallic compounds. We establish the relationship between the interface microstructures and hot cracking formation which are of thermo-mechanical and solidification origin. The knowledge of the solidification path of the TLP obtained in LIII layer at T2 and T3 makes it possible to explain the formation of micro-pores. The fracture is for both bonding processes, takes place in the brittle mode, of the intergranular type by cleavage and is in the LIII layer of the junction.

A boundary-based approach to the multiscale microstructural characterization of a W-Ni-Fe tungsten heavy alloy

A combination electron backscattered diffraction and transmission electron microscopy based approach has been implemented to study the effects of purposefully introduced anisotropy in a tungsten heavy alloy (WHA) through hot-rolling. Particular attention has been paid to changes in number and proportion of various boundary types from a quantitative standpoint; incorporating qualitative behavioral observations from prior analyses to generate experimentally-validated bases for the examination and application of a microstructure which exhibits an optimal balance of strength and ductility. It is asserted that a combination of increased temperatures during rolling and additional isothermal hold time for the post-rolling annealing steps may lead to a reduction in unfavorable textural components due to rolling in the W-phase and a decrease in premature fracture due to W-W microcracking respectively. This is expected to further increase the proportion of interphase boundaries and improve the ductility of these rolled structures, producing a superior rolled WHA microstructure.

Hot‐pressing platelet alumina to transparency

AbstractAlumina powder with a platelet morphology was hot‐pressed to transparency with preload pressures of 0‐8 MPa, maximum temperatures of 1750‐1825°C, maximum pressures of 2.5‐80 MPa, and isothermal hold times of 1‐7 hours. Optical transmission (in‐line and total), as well as optical losses (backward/forward scattering and absorption), of the hot‐pressed samples were measured and related to the microstructure. Higher hot‐pressing temperatures increase the in‐line transmission. A gray discoloration of the samples (indicative of high absorption) was minimized by heat treating the powder in air prior to hot pressing and reducing the preload pressure. Maximum pressures above/below 10 MPa increased porosity, which decreased in‐line transmission and increased backward/forward scattering. Lower densities at higher pressures are attributed to a pore‐swelling phenomenon. Increasing isothermal hold time decreased porosity, which increased in‐line transmission and reduced backward/forward scattering. Best optical properties with an in‐line transmission of 65.3% at 645 nm (0.8 mm thick) were achieved by hot‐pressing heat‐treated platelet alumina powder with a preload pressure of 0 MPa, maximum temperature of 1800°C, maximum pressure of 10 MPa, and an isothermal hold time of 7 hours. This high in‐line transmission, despite its large grain size (65 µm), is attributed to crystallographic orientation of the platelets during hot pressing.

Early Time Evolution of Selective Oxidation in a CMnSi AHSS

The influence of dew point temperature (DPT) and oxidation time on the selective oxidation of a Fe-0.09C-2.02Mn-0.91Si (wt pct) advanced high-strength steel during the early stages of oxidation was investigated in this study. Samples were annealed at 1123 K (850 °C) in a high-temperature confocal scanning laser microscope for 0, 30, 60, 90, and 120 seconds in an N2 + 5 pct H2 atmosphere with 243 K (− 30 °C) or 273 K (0 °C) DPT. Oxide morphology and chemistry were examined with respect to time and atmosphere DPT using scanning electron microscopy, scanning transmission electron microscopy, focused ion beam (FIB) milling, FIB serial sectioning, and energy dispersive spectroscopy techniques. Significant internal and external oxidation for all samples, including those with a 0 second isothermal hold, was observed. This indicated significant oxidation occurred upon heating. Samples oxidized in the 273 K (0 °C) DPT atmosphere exhibited intra- and intergranular internal oxidation and surfaces covered in discrete iron-rich nodules. These were attributed to stress generation during oxidation. Samples oxidized in the 243 K (− 30 °C) DPT atmosphere had shallow internal oxidation, sparse intragranular oxides, and surfaces rich in silicon and manganese containing oxide. Internal oxides for both atmospheres and all isothermal hold times were comprised of a silicon-rich core and manganese-rich outer shell was frequently observed. This core/shell structure was related to evolving oxide stability during the high-temperature exposure.

Differential scanning calorimetry isothermal hold times can impact interpretations of drug-polymer dispersability in amorphous solid dispersions

Differential scanning calorimetry (DSC) is a commonly employed analytical technique for the analysis and characterization of amorphous solid dispersions. However, steps typical of standard temperature programs can alter the material in situ. Data for two active pharmaceutical ingredients are detailed, wherein isothermal hold times, traditionally employed to remove thermal history and/or residual solvent, were observed to impact the observed dispersability of the compounds in polyvinylpyrrolidone vinyl-acetate copolymer (PVPva). Re-crystallized tolbutamide was observed to re-dissolve in PVPva, while terfenadine was observed to crystallize during the isothermal hold period. Exposing co-solidified drug-polymer mixtures to temperature changes and experimental hold times can potentially confound correct categorization of dispersability, particularly when DSC is used as the lone characterization technique. This work illustrates the importance of using a combination of techniques to improve the certainty of conclusions made with respect to the true, initial physical state of a co-solidified mixture.

Staged thermal fractionation for segregation of lignin and cellulose pyrolysis products: An experimental study of residence time and temperature effects

Thermal conversion technologies may be the most efficient means of production of transportation fuels from lignocellulosic biomass. In order to increase the viability and improve the carbon emissions profile of pyrolysis biofuels, improvements must be made to the required catalytic upgrading to increase both hydrogen utilization efficiency and final liquid carbon yields. However, no current single catalytic valorization strategy can be optimized to convert the complex mixture of compounds produced upon fast pyrolysis of biomass. Staged thermal fractionation, which entails a series of sequentially increasing temperature steps to decompose biomass, has been proposed as a simple means to create vapor product streams of enhanced purity as compared to fast pyrolysis. In this work, we use analytical pyrolysis to investigate the effects of time and temperature on a thermal step designed to segregate the lignin and cellulose pyrolysis products of a biomass which has been pre-torrefied to remove hemicellulose. At process conditions of 380°C and 180s isothermal hold time, a stream containing less than 20% phenolics (carbon basis) was produced, and upon subsequent fast pyrolysis of the residual solid a stream of 81.5% levoglucosan (carbon basis) was produced. The thermal segregation comes at the expense of vapor product carbon yield, but the improvement in catalytic performance may offset these losses.

Grain Growth Characteristics of Hydrothermally Prepared Yttria Stabilized Zirconia Nanocrystals during Calcination

Yttria stabilized zirconia (YSZ) nanocrystals were prepared by hydrothermal synthesis and were calcinated at different temperatures and with different isothermal hold time. X-ray diffraction via whole powder pattern modeling approach was used to study the grain growth as well as the evolution of grain size distribution of YSZ nanocrystals. Results show that YSZ nanocrystals start to grow along with the grain size distribution broadening at about 300 °C. The grain growth rate and the grain size distribution of YSZ nanocrystals are dependent on the calcination temperature and the isothermal hold time. The grain growth exponent and the active energy of grain growth were calculated. Grain rotation induced grain coalescence is suggested as the predominant way of grain growth of YSZ nanocrystals.

Wetting and Mechanical Performance of Zirconia Brazed with Silver/Copper Oxide and Silver/Vanadium Oxide Alloys

The wetting behavior and mechanical strength of silver/copper oxide and silver/vanadium oxide braze alloys were investigated for both magnesia‐stabilized and yttria‐stabilized (Mg‐PSZ and Y‐TZP) transformation toughened zirconia substrates. The temperatures investigated were 1000–1100 °C, with oxide additions of 1–10 wt% V2O5 or CuO, and hold times of 0.9–3.6 ks. Increasing either the isothermal hold temperature or time had a distinctly negative effect on the joint strength. The maximum strengths for both braze alloys were obtained for 5 wt% oxide additions at 1050 °C with a hold time of 0.9 ks. The Mg‐PSZ/Ag‐CuO system exhibited a average fracture strength of 255 MPa (45% of the reported monolithic strength), and the Y‐TZP/Ag‐CuO system had an average fracture strength of 540 MPa (35% of the reported monolithic strength). The fracture strengths were lower for the Ag‐V2O5 braze alloys, with fracture strengths of approximately 180 MPa (30% of the monolithic strength) for Mg‐PSZ versus approximately 160 MPa (10% of the monolithic strength) for Y‐TZP. The fracture always occurred at the braze–ceramic interface. No interfacial products were observed in low magnification SEM analysis for the brazing alloys containing V2O5 additions, while there were interfacial products present for brazes prepared with CuO additions in the braze alloy.

An Experimental Study of Transient Liquid Phase Bonding of the Ternary Ag-Au-Cu System Using Differential Scanning Calorimetry

An experimental approach using differential scanning calorimetry (DSC) has been applied to quantify the solid/liquid interface kinetics during the isothermal solidification stage of transient liquid phase (TLP) bonding in an Ag-Au-Cu ternary alloy solid/liquid diffusion couple. Eutectic Ag-Au-Cu foil interlayers were coupled with pure Ag base metal to study the effects of two solutes on interface motion. Experimental effects involving baseline shift and primary solidification contribute to a systematic underestimation of the fraction of liquid remaining. A temperature program has been used to quantify and correct these effects. The experimental results show a linear relationship between the interface position and the square root of the isothermal hold time. The shifting tie line composition at the interface has been shown to affect the DSC results; however, the impact on the calculated interface kinetics has been shown to be minimal in this case. This work has increased the knowledge of isothermal solidification in ternary alloy systems and developed accurate experimental methods to characterize these processes, which is valuable for designing TLP bonding schedules.

Thermal cycling induced δ to α phase transformation via an intermediate β phase on a δ-stabilized Pu–Ga alloy

The thermal cycling induced δ to α phase transformation in a δ-stabilised Pu–Ga alloy has been investigated by differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The alloy specimen was thermally cycled from 25°C to 180°C within the DSC furnace enabling coincident collection of phase transformation information. The cycling was carried out with varied isothermal hold periods at 25°C between each DSC run. The α phase was detected in the original well-homogenised alloy and the amount of α phase was found to increase with both thermal cycling number and isothermal hold time. Nearly 8% α phase was detected after 210 cycles, with no sign that α phase in-growth had ceased or slowed. Possible mechanisms, by which thermal cycling facilitates the nucleation and in-growth of the increasing α phase content are discussed. It is hypothesised that the δ to α phase transformation proceeds via an intermediate β phase at elevated temperature. Based on this investigation, it is also suggested that thermal cycling may be used as a novel experimental method to study the meta-stable nature of δ-stabilized Pu alloys.

Grain Growth in Dilute Tungsten Heavy Alloys during Liquid-Phase Sintering under Microgravity Conditions

Tungsten heavy alloys with compositions ranging from 35 to 93 wt pct tungsten were liquid-phase sintered at 1500 °C under microgravity conditions for isothermal hold times ranging from 1 to 600 minutes. The solid-volume fraction, grain size, grain size distribution, connectivity, and contiguity of the sintered microstructures were quantitatively measured. From these data, grain-growth-rate constants are determined for solid-volume fractions ranging from 0.048 to 0.858 and are compared to the predictions of several grain-coarsening models. The measured grain size distributions are shown to be self-similar and are fit to a Weibull distribution. Three-dimensional (3-D) grain size distributions from several coarsening models are transformed into grain size distributions for two-dimensional (2-D) cross sections, for comparison with the experimental data. Chi-squared tests and G-tests show that a coalescence model for grain growth fits the experimental observations better than solution-reprecipitation models, even for dilute tungsten heavy alloys.

Effects of Hot Water Extraction on Physical and Chemical Characteristics of Oriented Strand Board (OSB) Wood Flakes

AbstractResearch has already shown that extraction of valuable hemicellulose‐rich streams is a viable option for revenue generation in the pulp and paper industries. Applying this value prior to pulping concept to the composite panel industry is a natural extension. If an extraction of hemicellulose is accomplished under the right conditions, a non‐trivial amount of chemicals can be generated while leaving the woody substrate structurally intact for production to traditional products, such as oriented strand board (OSB). This research studied the effects of hemicellulose removal by hot water extraction on softwood OSB wood flakes and focused on changes that occurred in the physical and chemical properties of the wood flakes and liquid hydrolysates during extraction. Three reaction temperatures (120, 140, 160°C) and three isothermal hold times (20, 40, 60 min) were investigated. Results indicated that the extraction of hemicellulose in quantifiable levels begins at 120°C and 40 min and cellulose extraction begins at 140°C, 40 min. The level of extraction of lignocellulosic materials, the decrease of wood flake thickness, and the acidity of the recovered hydrolysates all increase with increases in extraction severity. The most promising results in regards to industrial implementation of hemicellulose extraction occur at a temperature of 140°C.

Supporting evidence for double-C curve kinetics in the isothermal δ → α′ phase transformation in a Pu–Ga alloy

Time-temperature-transformation (TTT) diagrams for the δ → α′ transformation in a number of Pu–Ga alloys were first reported in 1975 by Orme et al. Unlike typical single-C curve kinetics observed in most isothermal martensitic transformations, the Pu–1.9 at.% Ga alloy exhibits two noses, and thus double-C curve kinetics. The authors attributed the occurrence of the double-C to a difference in mechanism: a massive transformation for the upper C and a martensitic transformation for the lower C. Since that time, the nature, and the existence of the double C have received only limited attention. The results of Deloffre et al. suggest a confirmation of this behavior, but the fundamental origin of the double C remains unknown. Here, we apply differential scanning calorimetry (DSC) as an alternative approach to acquiring the TTT data and our experimental evidence suggests a confirmation of the double-C behavior after 18 h of isothermal hold time. In addition, we report three exothermic peaks corresponding to transformations during cooling at 20 °C/min prior to the isothermal holds. These three peaks are reproducible and suggest a number of possibilities for the origin of the unique kinetics: α′ forms with different morphologies, or from different embryos in the upper and lower C curves; α′ forms directly in one C curve and forms via an intermediate phase in the other C curve; the two C curves result from α′ forming by two or more distinct mechanisms (e.g., massive and martensitic transformations).

Use of Retention Data as the First Step in the Identification of Cyclic Organic Peroxides in Temperature-Programmed Gas Chromatography

Temperature-programmed retention indices for eleven cyclic organic peroxides were determined by gas chromatography on slightly polar 5% biphenyl 95% dimethylpolysiloxane columns (DB-5 and Rtx-5MS) at three heating rates (5, 10, and 20° min−1) from 60 to 300°C, using different chromatographs. Cyclic organic diperoxides and triperoxides had nearly constant retention indices when different heating rates and a short isothermal hold time (5 min) before the programmed increase in temperature were used. The usefulness of temperature-programmed retention indices was shown by using the data to predict the retention times and structures of unknown diperoxides or triperoxides derived from ketones. This is the first step in the identification of unknown cyclic organic peroxides, a process would otherwise require the availability of reference compounds.

Thermal analysis of the compositional shift in a transient liquid phase during sintering of a ternary Cu-Sn-Bi powder mixture

In this work, differential scanning calorimetry (DSC) and microstructural analysis were used to study the transient-liquid-phase sintering (TLPS) of a Cu-Sn-Bi powder mixture. During sintering, the liquid phase shifts from a Sn-rich (i.e., ∼90 wt pct Sn) to a Bi-rich (i.e., >78 wt pct Bi) composition. In addition, the presence of Bi creates two melting events: a Sn:Bi eutectic reaction at 139 °C and a reaction involving the melting of (Bi) at 191 °C. The Sn:Bi eutectic melting event was fully transient. The melting event at 191 °C was consistent with the formation of a terminal Bi-rich liquid phase. The rate of compositional shift toward this terminal liquid phase at 260 °C was dependent on the rate of the reaction of the Sn with the Cu powder to form intermetallic phases. For mixtures made with medium and fine Cu powder, the terminal Bi-rich composition was reached after isothermal hold times of 20 and 15 minutes, respectively. This resulted in a new melting point for the mixture of 191 °C. For coarse Cu powders, the rate of the compositional shift toward a Bi-rich composition was much slower. The liquid phase remained at a hypoeutectic Sn-Bi composition estimated at 80 wt pct Sn, while the mixture maintained a melting point of 139 °C.

Quantifying metallurgical interactions in solid/liquid diffusion couples using differential scanning calorimetry

A novel method using differential scanning calorimetry (DSC) has been developed to quantify the interface kinetics in a solid/liquid diffusion couple. The Ag–Cu binary eutectic system is investigated by heating an assembly of Ag base metal and Ag–Cu eutectic foil to 800 °C and holding. The fraction of liquid remaining after various isothermal hold periods is measured by comparing the melting endotherms with the solidification exotherms. Detailed analysis of the results show the per cent liquid remaining is inversely proportional to the square root of isothermal hold time; however, effects of the base metal cause an apparent loss of 25% of the liquid immediately after heating. The fundamental understanding of the effects of bi-phase diffusion couple geometry is advanced to manifest the mechanisms resulting in the error. Further carefully devised experiments reveal that primary solidification during cooling is not included in the enthalpy of solidification measured by the DSC. Furthermore, baseline shift across the melting endotherm increases the measured melting enthalpy. These effects combine to systematically underestimate the fraction of liquid remaining. Development of a modified temperature program and application of an appropriate correction can remedy these effects. The experimental results compare well with a prediction generated by an analytical model. Successful quantification of these phenomena has broadened the knowledge of DSC operational characteristics in the solid/liquid diffusion couple treatment, which can now be applied to other material systems.

Grain boundary phase in AlN ceramics fired under reducing N2 atmosphere with carbon

Abstract An AlN ceramic was prepared with a dopant Y 2 O 3 under a reducing nitrogen atmosphere with carbon at 1900 °C for 20 h. The AlN ceramic had thermal conductivity, 220 W/m°C, which contained crystalline Y 2 O 3 and an amorphous intergranular film. The intergranular phase decreased during the isothermal hold period by the migration of a liquid phase that consisted of Y 2 O 3 , Al 2 O 3 , and AlN. The liquid phase composition was maintained during the firing process. Comparison of the microstructures of the ceramics prepared with different isothermal hold times revealed that the lower the quantity of intergranular phase, the higher the thermal conductivity attained.

A Pre-Normative Study on the Cyclic Oxidation Behaviour of PM Chromium: The Effect of Experimental Parameters

In this study the importance of experimental parameters for the cyclic oxidation behaviour of chromium is discussed. In particular, the effect of different batches, sample geometry, maximum temperature during cyclic oxidation tests, and the effect of isothermal hold-time in relation to the oxidation behaviour are investigated in more detail. It is shown that small differences in the experimental method or material properties could already significantly influence the oxidation kinetics of the material under investigation. Consequently, poorly chosen and/or characterised experimental conditions can cause misleading results and even wrong conclusions.

Improved Model to Predict Properties of Aluminum Alloy Products after Continuous Cooling

Previous models of quench sensitivity of age-hardening alloys have been extended to include loss of toughness as well as loss of yield strength upon postquench aging. Loss of toughness on slow quenching was modeled by the loss of solute to grain-boundary precipitates that promote intergranular fracture. The phenomena are modeled using differential equations, and the model includes temperature-dependent values of the minimum toughness and strength expected after extended isothermal hold times. Time-temperature-property (TTP) curves for the postaging yield strength and toughness were used to provide empirical kinetic and property data for fitting the proposed relationship. The model was tested against experimental data, both nominally isothermal and truly continuous cooling, for an Al-Cu-Li alloy plate. For nominally isothermally cooling, the model proved to be capable of accurately describing the loss of toughness and the loss of strength to a much larger loss in strength than previous models. The model also successfully predicted the loss of strength on continuous cooling but provided a conservative overestimate of the loss of toughness under the same continuous-cooling conditions. It is suggested that this bias arises from the lack of consideration of differences in the microstructure of the precipitates formed during isothermal treatments and those formed during continuous cooling.

The temperature factor in nitriding titanium alloys in a low-density dynamic nitrogen atmosphere

A study has been made on the effects of isothermal hold time during nitriding of titanium alloys in a low-density dynamic nitrogen atmosphere on the nitride formation and gas saturation. To obtain a good hardened while retaining satisfactory strength and plasticity characteristics, the nitriding should be done in the temperature range corresponding to passive nitride formation.