The roles of thermodynamics and transformation kinetics on phase selection in the non-equilibrium processing of materials (original) (raw)
Related papers
Crystallisation kinetics and microstructure development in metallic systems
Progress in Materials Science, 2002
The primary crystallisation of a highly undercooled/supersaturated liquid is considered, and the application to nanocrystallisation by heat treatment of metallic glasses is studied from the thermodynamic, kinetic and microstructural point of view. The thermodynamic evolution is modelled assuming transformation rates low enough to ensure thermal equilibrium to be almost achieved. A mean field approximation is used, which allows us to determine the time evolution of the kinetic variables governing the transformation. The interplay between interface and diffusion controlled growth rate is studied, and both nucleation and crystal growth changes within the transformation are considered as soft mechanisms. The kinetics of the transformation is described in the framework of the Kolmogorov, Johnson and Mehl and Avrami (KJMA) model, which is adequately generalized for primary transformations. The microstructural evolution is described by a populational model, also based on KJMA. The predicted kinetic evolution results are compared to the experimental data on the primary nanocrystallisation of a FINEMET alloy. #
Nature Communications, 2021
A combination of complementary high-energy X-ray diffraction, containerless solidification during electromagnetic levitation and transmission electron microscopy is used to map in situ the phase evolution in a prototype Cu-Zr-Al glass during flash-annealing imposed at a rate ranging from 10 2 to 10 3 K s −1 and during cooling from the liquid state. Such a combination of experimental techniques provides hitherto inaccessible insight into the phase-transformation mechanism and its kinetics with high temporal resolution over the entire temperature range of the existence of the supercooled liquid. On flash-annealing, most of the formed phases represent transient (metastable) statesthey crystallographically conform to their equilibrium phases but the compositions, revealed by atom probe tomography, are different. It is only the B2 CuZr phase which is represented by its equilibrium composition, and its growth is facilitated by a kinetic mechanism of Al partitioning; Al-rich precipitates of less than 10 nm in a diameter are revealed. In this work, the kinetic and chemical conditions of the high propensity of the glass for the B2 phase formation are formulated, and the multi-technique approach can be applied to map phase transformations in other metallic-glass-forming systems.
Phase change materials: From structures to kinetics
Journal of Materials Research, 2007
Phase change materials possess a unique combination of properties, which includes a pronounced property contrast between the amorphous and crystalline state, i.e., high electrical and optical contrast. In particular, the latter observation is indicative of a considerable structural difference between the amorphous and crystalline state, which furthermore is characterized by a very high vacancy concentration unknown from common semiconductors. Through the use of ab initio calculations, this work shows how the electric and optical contrast is correlated with structural differences between the crystalline and the amorphous state and how the vacancy concentration controls the optical properties. Furthermore, crystal nucleation rates and crystal growth velocities of various phase change materials have been determined by atomic force microscopy and differential thermal analysis. In particular, the observation of different recrystallization mechanisms upon laser heating of amorphous marks is explained by the relative difference of just three basic parameters among these alloys, namely, the melt-crystalline interfacial energy, the entropy of fusion, and the glass transition temperature.
NATURE COMMUNICATIONS, 2021
A combination of complementary high-energy X-ray diffraction, containerless solidification during electromagnetic levitation and transmission electron microscopy is used to map in situ the phase evolution in a prototype Cu-Zr-Al glass during flash-annealing imposed at a rate ranging from 102 to 103 K s−1 and during cooling from the liquid state. Such a combination of experimental techniques provides hitherto inaccessible insight into the phase-transformation mechanism and its kinetics with high temporal resolution over the entire temperature range of the existence of the supercooled liquid. On flash-annealing, most of the formed phases represent transient (metastable) states – they crystallographically conform to their equili- brium phases but the compositions, revealed by atom probe tomography, are different. It is only the B2 CuZr phase which is represented by its equilibrium composition, and its growth is facilitated by a kinetic mechanism of Al partitioning; Al-rich precipitates of less than 10 nm in a diameter are revealed. In this work, the kinetic and chemical conditions of the high pro- pensity of the glass for the B2 phase formation are formulated, and the multi-technique approach can be applied to map phase transformations in other metallic-glass-forming systems.
Phase transformation kinetics and the assessment of equilibrium and metastable states
Journal of Phase Equilibria, 1993
The general characteristics of phase transformation kinetics during coofing and heating are introduced. It is demonstrated that the cooling process always depresses the phase boundaries away from equifibrium and towards lower temperatures; similarly, the heating process always shifts the phase boundaries to higher temperatures. Moreovei; the larger the cooling or heating rate, the larger the discrepancy. According to this observation, the reliabifity ofcoofing and heating data (irrespective of the methods used to determine them: electrical resistance, dilatometry, etc.) for phase diagram assessments is discussed. The principle for correct assessment of metastable phase information is also briefly introduced. It is pointed out that each kind of metastable phase has its own transformation. start temperature, Ts (Ms, etc.), during coofing. More specifically, each kind ofmartensite has its own Ms. Examples are shown for Fe-Ni, Fe-Mn, and Ti-Cr alloys.
Amorphization and alloy metastability in undercooled systems
In systems with larger undercoolings, crystal nucleation and growth limitations can expose alloy metastability due either to the suppression of an equilibrium phase or else by the formation of a kinetically favored metastable phase. Under nucleation control, crystallization may be bypassed in bulk volumes as the liquid is uniformly undercooled below the glass transition. Alternatively, during interface reactions, nucleation can be suppressed at early times by larger concentration gradients that can expose several forms of metastability and increase the probability of amorphization. For amorphous phase formation during melt processing the kinetic control may be analyzed in terms of nucleation limitations or growth restrictions. Many metallic glasses require quenching for vitri®cation and often do not have a resolved glass transition upon reheating. The marginal glass formation is related mainly to growth limitations. How- ever, this same kinetic control also provides the foundation for the development of a high density (10 22 m ÿ3 ) of na- nometer sized (20 nm) crystals during primary crystallization. With alternate synthesis routes based upon solid state alloying resulting from deformation, the kinetic pathways to glass formation can be altered to avoid nanocrystallization reactions in marginal glass-forming alloys. These developments present new opportunities for controlling crystallization in multicomponent glasses.
Phase Change Materials: from crystal structures to kinetics
Phase change materials are characterized by a structural transition that is accompanied by a pronounced change of properties. We have recently focused our efforts to understand and identify suitable phase change materials onto the identification of suitable structures and the detailed study of crystallization kinetics. For a large number of samples it could be shown that only those samples with a particular group of structures enabled phase change recording. All materials that showed the required optical contrast between the amorphous and crystalline state had cubic or near-cubic crystal structures, while materials based upon tetrahedral crystal structures showed insufficient contrast. The different behavior of these two groups of materials could be explained in part by density functional theory which has been employed to determine the density of states, the band structure and the total energy for different structures of ternary alloys containing Cu, Ag, Au, Ga, In, Ge, Sn, As, Sb and Te. While these results help to understand which stoichiometries are suitable for phase change recording and why certain structures are frequently encountered, the question still remains to be solved what characterizes the kinetics of crystallization for the different classes of phase change alloys. Ex-situ atomic force microscopy in combination with a high-precision furnace has been employed for a systematic study of crystallization kinetics of sputtered amorphous Ag 0.055 In 0.065 Sb 0.59 Te 0.29 , Ge 4 Sb 1 Te 5 , and Ge 2 Sb 2 Te 5 thin films used for optical data storage. Direct observation of crystals enabled us to establish the temperature dependence of the crystal nucleation rate and crystal growth velocity around 150° C. While these alloys exhibited similar crystal growth characteristics, the crystal nucleation behavior of Ag 0.055 In 0.065 Sb 0.59 Te 0.29 differed significantly from that of Ge 4 Sb 1 Te 5 and Ge 2 Sb 2 Te 5 . These observations provide an explanation for the different re-crystallization mechanisms observed upon laser-heating of amorphous marks.
Phase transformations in engineering materials
SPIE Proceedings, 1997
Los Alarnos National Laboratory, an affirmative action/equal opportunity employer, is operated by the University of California for the US. Department of Energy under contract W-7405-ENG-36. By acceptance of this article, the publisher recognizes that the US. Government retains a nonexclusive, royalty-free license to wblish or reDToduce the wblished form of this contribution. or to allow others to do so, for U S. Government purposes. The Los Alamos National LabOratOry ;quests t h i the publisher identify this article as work performed under the auspices of the U.S. Department of Energy. FOnn No. 636 A5 ST 2629 101'91 ~~I B U T t O N OF THIS DOCUMENT I S UNLIMITED z c DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document.