Study Of Transformation Kinetics Research Articles

The study of transformation kinetics for overcooled austenite of the new high-strength steel with increased cold resistance

The study of transformation kinetics for overcooled austenite of the new high-strength steel with increased cold resistance

Back-transformation of high-pressure minerals in shocked chondrites: Low-pressure mineral evidence for strong shock

Post-shock annealing of meteorites can destroy their shock-induced features, particularly high-pressure minerals, and complicate the estimation of impact pressure–temperature conditions. However, distinguishing post-shock annealing features from thermal metamorphism effects can be practically difficult. Here we report results from Mbale, a highly shocked L chondrite, to investigate the mechanisms, kinetics and identification criteria for post-shock annealing of high-pressure signatures. Olivine fragments within shock-melt veins in Mbale occur as chemically heterogeneous nanocrystalline aggregates that contain trace wadsleyite and ringwoodite. Their strong variation in fayalite content provides evidence of iron partitioning during transformation of olivine to wadsleyite, followed by back-transformation to olivine after decompression. Experimental studies of transformation kinetics show that wadsleyite transforms to olivine in seconds at temperatures above ∼1200K and in hours at temperatures between 900 and 1200K. Thermal models of shock-melt cooling show that shock veins in Mbale cooled to 1200K in 1s. The shock pulse must have been shorter than ∼1s to provide the high temperature conditions for post-shock back-transformation of wadsleyite. Many highly shocked L chondrites, which have abundant high-pressure minerals, must have experienced relatively long shock durations combined with rapid cooling of shock-melt regions to preserve high-pressure phases. The most highly shocked samples, such as impact melt breccias, lack high-pressure phases because of post-shock back-transformations.

Characterization of solid phases and study of transformation kinetics in m-chlorofluorobenzene by 35Cl nuclear quadrupole resonance

Polymorphism is of widespread occurrence in the world of molecular crystals. In this work we present experimental results showing the existence of four solid phases in m-chlorofluorobenzene. A glass structure is achieved by quenching the liquid phase at 77K. This glassy state crystallizes in a disordered phase at T∼143K, which in turn transforms to the high-temperature stable phase (phase I) at T∼153K. Depending on the thermal history of the sample, a different ordered phase (phase III) can be obtained. The disorder is attributed to a molecular orientational disorder. There is no evidence of molecular reorientation in any phase. A study of the disorder-order phase transformation kinetics, using nuclear quadrupole resonance, is presented. The results are analyzed following Cahn's theory. Nucleation seems to take place at grain boundaries. Growth rates for different temperatures have been determined.

DLTS Studies of Carbon Related Complexes in Irradiated N- and P-Silicon

DLTS studies of transformation kinetics of different carbon–related complexes in electron irradiated n- and p-type silicon have been performed. It has been found that silicon self-interstitials have very low mobility even at room temperature in p-Si, but become extremely mobile under elec-tron injection. It is shown that upon annealing of interstitial carbon in p-Si a metastable state for interstitial carbon-interstitial oxygen complex is formed. This state has an energy level of about Еv+0.36 eV. The formation of the stable and metastable states takes place concurrently. The observed features of the carbon-related complexes formation are likely related to the existence of different crystallographic orientation of the equiprobable pathways through which the interstitial carbon and oxygen atoms can approach each other.

The crystallisation and stability of a polymorphic salt, ethylene diammonium dinitrobenzoate

The crystallisation of the polymorphic salt, ethylene diammonium dinitrobenzoate, is reported. In particular it is noted that in this monotropic system it is possible at certain temperature, pH and supersaturation conditions, for the stable form to crystallise directly rather than via the metastable triclinic polymorph. The underlying reason for this is sought via studies of transformation kinetics and speciation of the salt components. The ideas developed are extended to demonstrate, for the first time, the selection of additives for the control of selective polymorph crystallisation of a molecular salt.

Ageing Kinetics of Al.-4.7 % Cu Alloy. Dilatometric and DTA Studies

Studies of transformation kinetics during ageing of Al-4.7 % Cu were performed with use of dilatometric and DTA methods. The dilatometric method was used for studies of isothermal ageing, DTA for studies of ageing during isochronal heating. In order to determine isothermal transformation kinetics the JMA (Johnson – Mehl – Avrami) equation was applied, for isochronal heating transformation kinetics Kissinger method was used. Ageing of Al- Cu alloys consist of precipitation processes and dissolution processes that overlap each other. These processes, during isothermal ageing were distinguished with help of the time exponent n of JMA equation that is very sensitive to change of the transformation mechanism. During isochronal ageing, precipitation processes produce heat, dissolution processes consume heat. That helps to distinguish them. Results obtained during isothermal studies were in agreement with isochronal heating studies. Activation energies for precipitation processes 324-530 K temperature range varied from about 50 kJ/mol to about 100 kJ/mole and complied with the values obtained before. For precipitation processes at 650- 735 K temperature range, activation energies varied from 226 to 300 kJ/mole. The results obtained were discussed with literature data.

Mechanical alloying of Fe–B powders

The nature of the solid-state amorphization induced by mechanical alloying has been studied. The XRD and Mössbauer spectroscopy at room temperature of samples mechanically alloyed for different times indicate that the mechanical alloying induces the amorphization reaction in the compositions Fe 60B 40 and Fe 70B 30. A detailed study of transformation kinetics suggests that amorphization occurs via one-dimensional diffusion-controlled growth.

DSC study of transformation kinetics of liquid-crystallines based on 1,4-phenylene 4-n-methoxybenzoate-4-allyloxybenzoate

The transformation kinetics of liquid-crystalline 1,4-phenylene 4-n-methoxybenzoate-4-allyl-oxybenzoate (PMOBAOB) and a liquid-crystalline poly(mesogen/methyl) siloxane (PMMS) was studied by means of a non-isothermal method using differential scanning calorimetry. This determination led to an apparent activation energy of transformation of 639.0 and 306.5 kJ mol−1, respectively. The values of the Avrami exponentn were 1.8 and 1.1. The results show that the process of transformation of PMOBAOB involves a constant number of nuclei growing in two dimensions at a constant rate, while the process of transformation of PMMS-PMOBAOB involves a constant number of nuclei growing in one dimension at a constant rate.

Computer Simulation of Martensitic Transformation Under Stress

This paper reports a computer simulation of martensitic transformation in both Cu-Zn-Al and Fe-Ni-C alloys by a 2-dimensional model. The influence of the imposed stress on the mechanical behaviour, the transformation kinetics, and the microstructure are illustrated for Cu-Zn-Al alloy. The study of transformation kinetics shows the following features: (1) in case of stress-free transformation, both alloys transform in burst, the transformation will go directly to near completion once a band can overcome the energy barrier. (2) the transformation temperature range spreads monotonically as the load is increased from 0 to 500 MPa. (3) M f is nearly the same for the different tested loading conditions. Even if the assumption of elastic accommodation is not realistic for Fe-Ni-C alloy, we can analyse the influence of the volume change associated with the transformation on the local stress level and on the behaviour of the alloy. This volume change has no notable influence at the earlier stage of the transformation. It becomes larger as the transformed fraction increases.

Amorphization in Fe-Si multilayers by solid-state reaction.

Solid-state reaction in Fe-Si multilayers occurring at 573 and 658 K has been studied. Iron is the predominantly diffusing species at lower temperatures. It is found that a large asymmetry in the mobilities of the two constituents is a basic requirement for amorphization to occur. A detailed study of transformation kinetics suggests that amorphization occurs via one-dimensional diffusion-controlled growth. Information about the possible mechanism of amorphization has been obtained.

The influence of transformation progress in the activation energy changes during low - temperature tempering of quenched steel

Studies of transformation kinetics during tempering of quenched values Q/sub/A/ - 64-104 KJ/Mol/. The activation energy steels were performed mainly in the years 1950-1965. On the basis of these works it is known that during low temperature tempering the progress of a phase transformation depends on the carbon diffusion in alpha iron because of activation energy was calculated mostly from data determined for one selected fraction transformed /e.g. 0.5/ however, the dependence of the activation energy of the fraction transformed was noticed. The present work is concerned with the study of activation energy changes during the progress of transformation of steel tempered at low temperatures.

Study of transformation kinetics in the Fe40Ni38Mo4B18 metallic glass by positron annihilation methods

We report on transformations induced in the metallic glass Fe 40 Ni 38 Mo 4 B 18 by thermal treatments from the amorphous to the crystallized state, following the variation of the positron lifetime and the Doppler broadening of the annihilation line. The behaviour of the annihilation lineshape shows reversible and non-reversible contributions. We suggest the reversible part to be related to the changes induced by thermal expansion. The non-reversible variation chiefly shows phase transitions. Crystallization of the sample appears as a three-stage process. The results support the idea that positrons are trapped in the as-received metallic glass Etude experimentale des transformations de l'etat amorphe a l'etat cristallin induites par des traitements thermiques. Le comportement de la forme de la raie d'annihilation de positons montre des contributions reversible et irreversible. La partie reversible semble due aux changements induits par dilatation thermique; la partie irreversible correspond a la transformation de phases

The study of transformation kinetics of the amorphous PdSi alloys

The kinetics of transformation behavior of roller-quenched amorphous Pd 83Si 17 and Pd 80Si 20 alloys after linear and isothermal heating is reported. The transformation was examined with electron microscopy, electron diffraction and electrical resistivity measurement. The crystallization of amorphous alloys with 17 at% Si begins with metastable ordered fcc solid solution. The ordered fcc solution is transformed to a ordered metastable phase with, probably, orthorhombic structure. The crystallization of amorphous alloys with 20 at% Si begins by formation of spherulites with lamellar structure. Using electron diffraction we found that spherulites consist of two phases - orthorhombic Pd 3Si silicide and Pd-rich silicide. From resistivity measurements, activation energies of 28.5 kcal/mol for Pd 83Si 17 and 80 kcal/mol for Pd 80Si 20, respectively, were calculated.