Investigation Of Phase Transformations Research Articles

Phase Transformation and Domain Switching Behaviors during Superplastic Deformation in 3Y-TZP

Test pieces of a tetragonal zirconia polycrystal containing 3mol% yttria were deformed in tension under superplastic conditions in air to pre-determined nominal strains ranging from 0.0 to 200%. The X-ray diffraction experiments were conducted on surface regions of gauge sections of the deformed test pieces to investigate phase transformation and domain switching behaviors that might occur during the superplastic deformation. The measured X-ray patterns were analyzed using a Rietveld refinement method. As a result: (i) Reflections from monoclinic (m) phase were not observed in the X-ray patterns, indicating that tetragonal (t) to m phase transformation might not occur during superplastic deformation under the above condition. Meanwhile, the fraction of cubic (c) phase seemed to decrease with increasing strain. (ii) Traces of ferroelastic domain switching were observed in the X-ray patterns of the deformed specimens. The switching amount was found to depend on flow stress and deformation amount. The tetragonal axis ratio (c/a) along the scattering vector used in this study was found to decrease with increasing the switching amount.

Advances in hydride phase growth: Automatic high precision calorimeter-volumetric devices, for thermodynamic and kinetics analyses

A new automatic apparatus has been specifically developed for investigating phase transformations in hydrogen absorbing intermetallic compounds, providing accurate characterizations of the thermodynamic properties as well as of the dynamic aspects of the hydride phase growth over a wide range of pressures 0–4 MPa and temperatures 250–800 K. It consists essentially of a heat flow calorimeter coupled with high precision volumetric devices. The ensemble constitutes of a closed system in which high purity hydrogen gas within the system is transferred between hydrides reservoirs and reactors with high thermal transfer capacity. The excellent stability of the signal of the calorimeter, ±4 nV over a long period of time (>10 days), allows one direct measurement of the heat of H2 absorption or desorption during a scan of an hysteresis loop with an average accuracy of 1%. To maintain quasi-isothermal conditions during the transformation, a reliable control of the temperature inside the sample is insured by optimizing the hydrogen gas flow rate. Simultaneously the heat flux, pressure, temperature, composition data collected have been used to obtain kinetic parameters through two different and complementary techniques. The first one is based on an analysis of the measured heat flux evolved during the reaction which gives the true rate law at the sample level by deconvolution of the measured signal. It is shown that only overall information can be expected by this method. The results of the numerical treatment raise the problem of the location of the heater used for calibration of the calorimeter. The second technique takes into account the component volumes of the system. Here, the time variation of hydrogen mass flow (excitation of the system) and hydrogen pressure in the reactor (response of the system) is analyzed in the frequency domain which requires knowledge of the experimental transfer function of the volumetric equipment. In the complex plane, the shape of the transfer function appears as a signature of a proposed mechanism. ZrNi–H2 and LaNi5–H2 systems have been used to demonstrate the detailed analysis.

Kinetics of the hydrogen-induced diffusive phase transformation in Nd–Fe–B industrial alloy

The kinetics of the hydrogen-induced diffusive phase transformation in industrial alloy Nd–Fe–B has been investigated. The isothermal kinetic curves were received for temperatures from 750 to 620°C at the hydrogen pressure 0.1 MPa. An isothermal kinetics diagram was obtained. This diagram is similar to the ones of the transformations in steels during heating. It is shown that the investigated phase transformation is a diffusion-controlled one with the mechanism of nucleation and growth.

Ex situ characterization of phase transformations and associated microstructures in polycrystalline thin films

This article presents new ex situ techniques, as well as novel applications of conventional methods, to investigate phase transformations and associated microstructures in polycrystalline thin films. Specifically, we review the results of a dark-field transmission electron microscopy study of the disorder-order transformation thermodynamics and kinetics in CoPt and FePt thin films and, in addition, discuss the applications of differential scanning calorimetry to the interrogation of solid-state reactions in Nb/Al, Ni/Al, and Ti/Al systems. These investigations have naturally prompted the development of methods to analyze and better understand our experimental data. Thus, we also outline a new, automated image processing algorithm for transmission electron micrographs and discuss its implementation to interpret images of thin Al film microstructures. Further, as computer simulation has been employed to highlight important nucleation and growth parameters, we briefly summarize the results of these investigations.

Real time neutron transmission investigation of the austenite-bainite transformation in grey iron

The first successful application of a new method to investigate phase transformations in real time, like the decomposition of austenite into bainite in grey iron, is described. During the ongoing transformation, transmission spectra of thermal neutrons, which contain Bragg edges corresponding to the crystal structure of the transforming phases, are recorded. By evaluating the height of these Bragg edges, which is a measure of the volume fraction of the phase, at different transformation times, the transformation can be followed in-situ in a time resolved manner. The method is compared to other previously used methods (micrographs, dilatometry, diffraction techniques); also a summary and an outlook are given.

高圧未知相構造解明への高圧ラマン散乱の応用

In this review, we outline the application of Raman spectroscopic measurements to high pressure researches. Raman spectroscopic measurements provides a useful means of investigating phase transformations because its spectral structure is closely related to the atomic arrangement in the crystal. Recently, Raman spectral measuring systems with high sensitivity are developed by using liquid nitrogen cooled CCD camera. Combining these Raman measuring systems with diamond anvil cell, Raman spectroscopic measurement under high pressure provides a useful means to investigate the crystal structure of unknown high pressure phase. The Raman spectroscopic studies for high pressure phases of TiO2 and ZrO2 are reviewed.

高温秀過型電子顕微鏡によるＣａ‐ｐｏｏｒ輝石の相転移のその場観察

A new system has been constructed for high temperature transmission electron microscopy (HTTEM) to investigate phase transformation of silicate minerals. This HTTEM system, available up to 1200-1300°C, was successfully applied to high_??_low phase transitions of Ca-poor clinopyroxenes. This phase transition was in-situ observed for the first time under HTTEM and was revealed to be a thermoelastic type of martensitic transformation. Formation process of antiphase boundaries in pigeonite was also in-situ observed during the phase transition under HTTEM.

HREM Studies of Structural Changes Induced by Phase Transformations in Zirconia Powders

The use of microcrystalline (submicron) and nanocrystalline (≤10nm) particles as powder materials for sintering of advanced ceramics like oxide-based ceramics or high-Tc superconductors , requires new techniques for characterization. Thus classical optical microscopy or even scanning electron microscopy are no longer effective for examination of 1-10 nm particles. In previous work we developed quantitative analysis of transmission electron microscopy (TEM) observations at medium resolution (0.4-1 nm, typically) to study morphology, size distribution, agglomeration and/or aggregation state of elementary powder particles characterized by bright- and dark - field imaging as well as diffraction TEM techniques.Here the high resolution TEM (HREM) techniques (at 200 kV) have been used to investigate phase transformations occuring in ultrafine zirconia powders processed by calcination of an amorphous hydrated zirconium oxide prepared at low temperature. X-ray examination showed the sequence : amorphous , cubic (C) or tetragonal (T) , monoclinic (M), at increasing temperatures.

Study of the reversible martensitic transformation in CuZnAl alloy by positron annihilation

AbstractThe positron lifetime measurement to investigate phase transformations: martensitic phase (MP) ⇔︁ parent phase (PP) in CuZnAl alloy has been used. We have observed the generation of new defects by MP → PP transformation; these defects are liquidated by martensitic transformation. This effect is only observed in case of high concentration vacancies in alloy. In order to explain the nature of these defects we take into consideration the supervacancies idea.

Phase transformation during the rapid heating of titanium (?+?)-alloys and their effect on mechanical properties

The authors investigate phase transformations occurring in a variety of titanium base alloys--VT6, VT14, VT15, VT16, VT22, and VT23--during various heat treatment regimes and the effects of these transformations on mechanical properties as well as the effects of temperature and different contents of molybdenum and aluminium on grain boundary growth.

The use of dta for determination of the equilibrium diagram for 9-0-2+Si high-speed steel

DTA was used to investigate phase transformations taking place during Melting and crystallization of 9-0-2 high-speed steels containing 0.3–5% Si and to develop pseudobinary phase equilibrium diagrams for steels with variable Si contents.

Apparatus for investigating phase transformations and properties of metals and alloys with programmed electromechanical treatment

An apparatus was developed for investigations of phase transformations and mechanical properties under programmed thermal and mechanical conditions, which makes it possible to study electrothermal treatment processes under any desired conditions, high-temperature thermomechanical treatment, lowtemperature thermomechanical treatment, machining, and treatment to obtain superplasticity.

Current state of the theory of age-hardening of metallic alloys

In recent years we have produced new types of age-hardening alloys and new hardening treatments for which the combination of several methods of hardening is typical. We should note first of all the high-nickel and chromium-nickel maraging steels and various methods of thermo-mechanical treatment for age-hardening alloys. In all cases the decomposition processes involve a high density of structural defects. Explanation of the decomposition mechanism in connection with specific types of defects is an urgent problem, the solution of which is already possible with present techniques of electron microscopy (crystallographic analysis of structural defects and new formations is possible with the use of goniometric devices in transmission electron microscopy). Further development of mathematical methods is needed for investigating phase transformations in alloys and refining the quantitative theory of the processes of precipitation and coalescence used for present-day multicomponent alloys and combined treatment. The major problem in this area of metal science is to obtain more complete characteristics of the structure of the material in the state of maximum hardness and establish quantitative relationships between the structural and strength characteristics.