High Isothermal Temperature Research Articles

Microstructure and texture evolution of 2A12 aluminum alloy semi-solid billet prepared by SIMA process

In order to make up for the lack of research on the preparation process of semi-solid billet of 2A12 aluminum alloy, this paper adopts strain-induced melting activation (SIMA) process to prepare semi-solid billet of 2A12 aluminum alloy, and studies the effect of different SIMA process parameters on the microstructure and texture evolution. The results indicated a notable increase in both the average grain size (AGS) and shape factor (SF) with prolonged isothermal time under the condition of constant isothermal temperature. When the isothermal time is the same, the AGS is larger at low isothermal temperature, the SF is higher at high isothermal temperature, and the grain spheroidization is more obvious; The influence of the uneven strain distribution of the compression sample on the microstructure evolution during the subsequent semi-solid isothermal treatment (SSIT) process was studied by finite element method; At an isothermal temperature of 590°C and an isothermal time of 15 minutes, the recrystallization is basically completed, and the elongated grains are completely transformed into spherical isaemic crystals. The main texture types of the original extruded sample and the compression sample are <110> ‖CD and <010> ‖CD texture, respectively. The SSIT process will make the grain orientation approach to random orientation; The strong segregation of Cu and mild segregation of Mg and Zn were observed at the grain boundary during SSIT process; A suitable coarsening kinetic equation was determined, and the influence of isothermal temperature on the coarsening rate constant was studied.

Tailoring the crystallization and melting behavior of 3D protein-polymer biocomposite materials: A study of pressure-induced silk fibroin/poly(L-lactic acid) foams

Crystallization and melting behavior of 3-D composite materials is critical in the field of biopolymer materials, because the crystal structure and crystallization kinetics of the materials will affect the performance of the biocomposite materials and their fabrication process. In this study, thermal analysis combined with structural analysis such as X-ray diffraction (XRD) were used to study the melting behavior of silk fibroin/polylactic acid (SF/PLA) composite foam materials prepared under a pressure of 0–6 MPa, and the mechanism of their isothermal crystallization kinetics was discussed. The results showed that both α and α'-type crystals exist in the composite material, and their contents can be tuned by isothermal crystallization temperature. When the isothermal crystallization temperature is around 105 °C, the crystallization time is the shortest and the crystallization rate is the fastest. Besides, silk fibroin can shorten the crystallization half-life of the composite foam and improve the crystallinity of the material. However, the high isothermal temperature will facilitate the interaction between SF and PLA segments to slow down the diffusion, folding and crystallization of the PLA segments. Using Avrami analysis, it is found that the crystal growth in 3-D composites follows a non-three-dimensional truncated spherical direction. This study provides a deeper understanding of the crystallization behavior of protein-synthetic polymer biocomposite materials and offers experimental and theoretical guideline for the regulation and utilization of their structure and properties for the design of new biocomposite materials.

Effect of rapid manufacturing on the performance of carbon fibre epoxy polymers

The effect of cure rate on the mechanical and fracture properties of both bulk epoxy polymers and carbon fibre-reinforced polymers (CFRP) containing reaction-induced phase-separating liquid rubber was investigated in the current work. The fracture energy of the bulk epoxy polymers increased from 229 to a maximum value of 3434 J/m2. This was achieved by introducing 9 wt% phase separating rubber while keeping a curing isothermal temperature of 45 °C. A reduction in the maximum measured toughness was noted when these polymers were cured at higher isothermal temperatures and hence higher rates. The mode I propagation fracture energy for composite laminates cured using the modified bulk polymers as matrices increased from 633 to only 934 J/m2. The low rise in toughness for the composite laminates is explained due to the inefficient transfer of fracture toughness from bulk polymers to fibre-reinforced polymers for toughened resins. The measured properties of the bulk polymer together with fractography studies demonstrate that this occurs due to the geometrical obstruction of the fibres preventing an unbounded fracture process zone from forming ahead of the crack tip. Increasing the weight fraction of toughening nano-modifiers in CFRP beyond a small amount does not result in further toughness gains. The results of this work underline the importance of considering material processing history when designing composite structures.

Establish time-temperature-transformation diagram based on dilatometry results and microstructural evolutions in an AISI 1010 steel

A time-temperature-transformation (TTT) diagram has been constructed using dilation results. The phase transformations have been confirmed through microstructural changes of AISI 1010 steel. The strength of this steel has been improved for various applications using heat treatments. Experiments for a series of steel samples have been conducted between 310-730°C isothermal temperature range using a thermo-mechanical simulator Gleeble®3800. The austenitised microstructure of the steel has been transformed into a combination of ferrite, pearlite, widmanstätten ferrite, massive ferrite, upper and lower bainite, and martensite, etc., when held at different isothermal temperatures. Ferrite along with pearlite have been observed at higher isothermal temperatures while bainite with martensite have been observed at lower isothermal temperatures. Two C-curves have been observed in pearlite and bainite transformation regions.

Achievement of High Ductility and Ultra-high Strength of V-Nb Microalloyed Spring Steel by Austempered Multiphase Microstructure

A V-Nb microalloyed spring steel was investigated through a series of austempering heat treatments. Optimal properties were obtained after austempering at 240 °C for 2 hours because of the good coordination of multiphase martensitic/bainite, filmy retained austenite and fine carbides. The tensile strength was 2239 MPa and the total elongation was 16.1 pct. With an increase in temperature, the matrix changed from martensite to bainite. The volume fraction of retained austenite decreased from 13.7 pct at 240 °C to 9.6 pct at 280 °C and then increased to a maximum of 19.2 pct at 360 °C. The tensile strength remained above 1500 MPa and the impact energy increased to 30 J. MC carbides formed in martensite lathes and ɛ- or θ-type carbides with acicular nanoscale were precipitated in bainite lathes at a low temperature. MC- and M3C-type precipitated in bainite lathes at high temperature. Samples that were subjected to a low isothermal temperature had a high strain-hardening rate before necking, whereas samples at a high isothermal temperature could exert a TRIP effect for a long time. After stretching to fracture, a hierarchical nanotwinned structure and many shear cells formed, which enhanced the austempered sample ductility and toughness at 360 °C.

Transformation Stasis Phenomenon of Bainite Formation in Low-Carbon, Multicomponent Alloyed Steel

The transformation stasis phenomenon of bainite formation in low-carbon steel was detected using a high-resolution dilatometer. The phenomenon occurred at different stages for different isothermal temperatures. In combination with microstructural observation, the calculated overall activation energy of transformation and interface migration velocity shed new light on the cause of formation of the stasis phenomenon. The temporary stasis formed at the initial stage of phase transformation for high isothermal temperature was attributed to the drag effect of substitutional atoms, which leads to low-interface migration velocity and large overall activation energy.

Crystallographic orientation variation of isothermal pearlite under high magnetic field

Crystallographic orientation (CO) variation of magnetic-induced pearlite (MIP) during its microstructure evolution in 19.8T was investigated by electron back-scatter diffraction (EBSD). It is closely related to the isothermal temperatures (ITs) and the applied magnetic time (MT) during the process of MIP formation. The <100> easy magnetization direction in MIP colonies is strengthened with the MT within the certain transformed fraction of MIP (fMIP) at the relatively lower IT (983K) above the eutectoid temperature but below the magnetically shifted upward eutectoid temperature, while this special CO tends to be weakened at a relatively higher IT (995K). For the same MT, the higher the IT, the relatively larger is the proportion in <100> orientation for MIP colonies at the early growth stage. These results have demonstrated that the change of <100> orientation of MIP is closely related to the growth rate of pearlite ferrite (PF), and strengthened mainly at early transformation stage. When fMIP reaches some value, the growth rate of MIP at other COs, such as <110>, even at the hard magnetization direction, turns to present speed-up.

The emulsion crystallization of hydrogenated castor oil into long thin fibers

The present study discusses the optimal crystal growth conditions required for the emulsion crystallization of hydrogenated castor oil (HCO) into several crystal morphologies. The best possible crystal shape is furthermore identified in case high rheology modifying performance is required. HCO was crystallized in a meso- and micro-continuous process which allowed for a controlled and fast screening of several crystal morphologies at different crystallization conditions. Applying high isothermal temperatures (above 55°C) resulted in a combination of rosettes, thick fibers and thin fibers. At lower isothermal temperatures (below 55°C) the shape gradually evolved to a combination of short needles, spherically shaped and/or irregularly shaped crystals. Crystals with the highest aspect ratio were obtained when crystals were grown between 30°C and 45°C and were subsequently reheated above 63°C, forming high amounts of large thin fibrous crystals. When diluted to 0.25wt% these fibrous crystals increased the low shear viscosity far better compared to the other crystal shapes.

Growth of phosphorus concentrating phase in modified steelmaking slags

Recycling steel slag to metallurgical process requires removing phosphorus from slag. In this paper, solidification experiments of converter slag modified by SiO2 at three isothermal temperatures with several durations were performed at air conditions from superheating temperature (1520°C). Grain size evolution of P concentrating phase (C2S, dicalcium silicate) was investigated. Results show that crystal size distributions of C2S can be fitted by lognormal distributions, the average size of C2S increases with increasing dwelling time and grows faster at higher holding temperature, and the relationship between average size of C2S and holding time can be best described by equation D = ktm. It is concluded that the growth mechanism of C2S is diffusion controlled, and higher isothermal temperature promotes the growth of C2S.Le recyclage de la scorie de l’acier par un procédé métallurgique nécessite l’enlèvement du phosphore de la scorie. Dans cet article, on discute d’expériences de solidification de scorie de convertisseur modifiée par du SiO4 à trois températures isothermes avec durées variées, effectuées à l’air à partir d’une température de surchauffage (1520°C). On a examiné l’évolution de la taille de grain de la phase de concentration du P (C2S, silicate dicalcique). Les résultats montrent que l’on peut ajuster les distributions de la taille de cristal (CSD) du C2S à une distribution log-normale, que la taille moyenne du C2S augmente avec l’augmentation du temps de résidence et croît plus vite à une température plus élevée de rétention et que la relation entre la taille moyenne du C2S et la durée de rétention est bien décrite par l’équation D = ktm. On conclut que le mécanisme de croissance du C2S est contrôlé par la diffusion et que la température isotherme la plus élevée favorise la croissance du C2S.

Influence of alkyl chain length on the gelation mechanism of thermoreversible gels of regioregular poly(3‐alkyl thiophenes) in xylene

AbstractThe gelation rates of poly(3‐alkyl thiophenes) (P3AT) are strongly dependent on the pendent alkyl chain length. Poly(3‐hexyl thiophene (P3HT) gels at higher isothermal temperatures than that of poly(3‐dodecyl thiophene), (P3DDT). Gelation rate has been expressed as a combination of a temperature dependent function f(T) and a concentration dependent function (φn); φ being the reduced overlapping concentration of P3AT. From the slope of double logarithmic plot of gelation rate and φ, n values are calculated to be 0.52 and 0.60 for P3HT and P3DDT, respectively. These values are close to the value of percolation exponent β (0.45) for three‐dimensional lattice; so both the gels may approximately obey three dimensional percolation model. The f(T) of gelation rate consists of two steps (i) coil‐to‐rod transformation followed by (ii) fibrillar crystallization. The coil‐to‐rod transition is accompanied by a red shift in the π–π* transition band in UV–vis spectra. Flory and Weaver theory is used to understand the coil‐to‐rod transformation process. The theory of fibrillar crystallization extended to dilute solution has been employed to understand the crystallization process. The results clearly indicate that the activation energy of conformational change (ΔF) is higher for P3DDT than that of P3HT. But the free energy change for the formation of critical size nucleus (Δ G) is lower for P3DDT than that of P3HT. A comparison of ΔF and Δ G values indicate conformational ordering is the rate‐determining step for P3AT with dodecyl substituent whereas crystallization is the rate‐determining step for P3AT with hexyl substituent. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 2528–2537, 2007

Synthesis of sodium zeolites from a natural halloysite

The kinetics of hydrothermal crystallisation of sodium zeolites from a natural mixture of halloysite and amorphous silica with Si/Al ≈ 4 was investigated. The sample collected at Scarpara (Tuscania, Italy) is the final product of an intense hydrothermal alteration process on the pre-existing leucitic tufites. In order to enhance its reactivity in the NaOH solution, the sample was thermally activated at 600 °C for 1 h. The hydrothermal crystallisation sequence of zeolites formed in the range 90–150 °C has been followed using real-time synchrotron powder diffraction. The reaction kinetics of Na-X, Na-P and analcime were analysed using a model developed for the study of the kinetic data from X-ray diffraction experiments. Na-X and Na-P cocrystallize with an autocatalytic nucleation at lower isothermal temperatures and with a heterogeneous nucleation at higher isothermal temperatures. Na-X tends to dissolve before Na-P, which in turn transforms into analcime. This work is part of a general project on the kinetics of formation of zeolites from clay precursors which is important for either engineering and production of valuable industrial materials and for the interpretation of poorly understood processes of formation of zeolites in natural hydrothermal environments.

Aspects and applications of non-aqueous high temperature packed capillary liquid chromatography

The effect of using high isothermal temperatures and temperature programming above ambient in liquid chromatography has been investigated by the use of relatively long packed reversed phase capillary columns and non-aqueous mobile phases. Efficiency measurements have been performed in the temperature interval 50– 175 °C, indicating that the best efficiency was achieved at 100 °C, with a corresponding optimum linear velocity of approximately 0.07 cm s–1. The efficiency measurements revealed that the linear velocity had to be slightly increased in order to operate at the optimum reduced plate height at elevated temperatures. The contribution from extra-column volumes increased the plate height at temperatures above 100 °C where the solute had low retention. This effect was closely examined by introducing larger pre-column dead volumes, demonstrating the need to minimize dead volumes. Temperature programming has successfully been used for partial characterization and for purity testing of different polymer additives. The within and between day precision of retention times for the temperature-programmed separations gave a relative standard deviation of 0.3–2.9%.

Phase behaviour in a thermotropic polyether involving rod-like mesogenic groups based on conformational isomerism

A thermotropic polyether (MBPE-5) which was synthesized by coupling of 1-(4-hydroxyphenyl)-2-(2-methyl-4-hydroxyphenyl)ethane α, ω-dibromopentane shows complicated phase transition behaviour. Our thermal analysis, polarized light microscopy and wide angle X-ray diffraction results lead to a recognition of a liquid crystalline phase in this polymer, and it is monotropic. In addition, crystals with two different metastabilities are identified. The more stable crystal grows from its melt in a high isothermal temperature region, characterized by a normal spherulitic texture with banding pattern. The less stable crystal develops from its parent liquid crystalline phase in a low temperature region. The schlieren texture is still kept after this crystallization. Isothermally, in an intermediate temperature region, mixed crystals, formed by both different stabilities, can be observed.

Degradation of dinitrated-poly( n-vinylcarbazole) by thermogravimetry

The techniques of isothermal and dynamic thermogravimetry have been used to study the degradation of six nitrated homodisperse fractions of poly( N-vinylcarbazole). The effects of nitrogen content and molecular weight of the samples on kinetic parameters are reported. Increasing the heating rate or using higher isothermal temperatures causes a sudden mass-loss and an increasing sample temperature was observed. An induction period in which −NO 2 scission took place may be the cause of this sudden mass-loss.

The kinetics of the oxidattve degradation of poly(aryl-ether-ether-ketone) (PEEK)

The kinetic parameters of the thermal decomposition of poly(aryl-ether-ether-ketone) (PEEK) in air have been investigated using an isothermal weight-loss method. The results from these studies indicate that the decomposition process is a complex one which proceeds in several overlapping stages. Calculation of the kinetic parameters by a variety of models suggests that at the lower isothermal temperatures the initial weight-loss process occurs with an apparent activation energy of about 100 kJ mol −1. At higher isothermal temperatures, however, the rates appear to be controlled by diffusion factors and higher apparent activation energies are noted of around 185 kJ mol −1 for the earlier weight-loss fractions and of about 220 kJ mol −1 for the later weight-loss data. The analyses presented demonstrate the difficulties which can arise in determining meaningful mechanistic information from standard weight-loss data even when isothermal heating conditions are employed.

The solubility of boron in iron

area of the second peak at the expense of the first. While the suppression of the first peak at high isothermal temperatures may be apparent, the complete decimation of the first peak at high isothermal temperatures indicated by the figure is difficult to prove via this method and may in fact not be the case. The indication that the first peak depletes to zero occurs in the temperature regime where the SAS curve fits are the poorest. We, therefore, must use microscopic observation to confirm the predictions we have drawn from this DSC technique. In previous work, 5'6 we were unable to generate superimposed peaks with the same amorphous alloy using a DuPont 1090 DSC.* We believe this to be a result of dif-