Recent TEM studies of precipitate growth mechanisms

In this review recent results of transmission electron microscopy studies of precipitation in a number of model binary alloy systems are presented. Emphasis is placed on identifying (i) the nature of the structural change occurring during the transformations, and (ii) the crystal lattice defects and deformation modes required to accommodate the growing precipitate in the parent phase. Precipitation reactions are classified in terms of their transformation strains, and simple predictive models for the precipitate morphology are developed for each case. Brief discussions are also given on the nature of a general plate precipitate interface and the effect of crystal symmetry on morphological distributions.Des exemples récents d'étude par microscopie électronique en transmission sont analysés dans cette revue. L'accent est mis sur l'identification (i) de la nature des modifications de structure intervenant lors des transformations, (ii) des défauts cristallins et des modes de déformation nécessaires à la croissance du précipité dans la phase mère. Un classement des réactions de précipitation est effectué selon les mécanismes de déformation mis en jeu. Des modèles simples permettant de prévoir la morphologie des précipités sont précisés dans chaque cas. Des éléments de discussion concernant la nature de l'interface entre la matrice et un précipité quelconque en plaquette sont également fournis. Les effets de la symétrie cristalline sur les distributions de forme sont brièvement abordés

A STUDY OF PRECIPITATION IN INTERSTITIAL ALLOYS. I. PRECIPITATION SEQUENCE IN Ta-C ALLOYS

A systematic transmission electron microscopy study of carbide precipitation in quenched-aged tantalum-carbon alloys has clarified the mechanism of precipitation in refractory BCC metal-carbon alloys. Diffraction contrast analysis shows that the precipitate platelets lie on {l_brace}310{r_brace} planes of the matrix, are interstitial in nature, and fully coherent before they thicken further and lose coherency. The precipitation sequence is continuous and involves no renucleation during the formation of the non-coherent phase. Thus, the final orientation relationship of the precipitate with the matrix already is found at the earliest stage at which it is possible to detect it. The interpretation of the results indicates that, as in FCC alloys, vacancies play an important role in the precipitation process during the nucleation and early growth stages and permit the formation of the hexagonal equilibrium M{sub 2}C structure early in the sequence. The model proposed to explain the observations is also consistent with the multistage hardening observed in quenched-aged refractory metal interstitial alloys

Interatomic potentials for atomistic simulations of the Ti-Al system

Semi-empirical interatomic potentials have been developed for Al, alpha-Ti, and gamma-TiAl within the embedded atomic method (EAM) by fitting to a large database of experimental as well as ab-initio data. The ab-initio calculations were performed by the linear augmented plane wave (LAPW) method within the density functional theory to obtain the equations of state for a number of crystal structures of the Ti-Al system. Some of the calculated LAPW energies were used for fitting the potentials while others for examining their quality. The potentials correctly predict the equilibrium crystal structures of the phases and accurately reproduce their basic lattice properties. The potentials are applied to calculate the energies of point defects, surfaces, planar faults in the equilibrium structures. Unlike earlier EAM potentials for the Ti-Al system, the proposed potentials provide reasonable description of the lattice thermal expansion, demonstrating their usefulness in the molecular dynamics or Monte Carlo studies at high temperatures. The energy along the tetragonal deformation path (Bain transformation) in gamma-TiAl calculated with the EAM potential is in a fairly good agreement with LAPW calculations. Equilibrium point defect concentrations in gamma-TiAl are studied using the EAM potential. It is found that antisite defects strongly dominate over vacancies at all compositions around stoichiometry, indicating that gamm-TiAl is an antisite disorder compound in agreement with experimental data.Comment: 46 pages, 6 figures (Physical Review B, in press

ARGONNE NATIONAL LABORATORY HVEM SUMMER INSTITUTE LECTURE - KINETICS II

The purpose of this contribution is to outline some of the ways in which the High Voltage Electron Microscope can be used to study the kinetics of secondary defect shrinkage and precipitate particle growth and dissolution. In many cases, good agreement between the predictions of theory and the experimental observations are found and this provides not only insights into the mechanisms underlying the processes, but also quantitative measurement of parameters of metallurgical importance. To summarize, some of the advantages of the HVEM in kinetic studies include: (1) unlike bulk studies where only average behavior is measured, precise measurements on individual particles may be made; (2) in some systems it is useful for rapidly establishing phase boundaries, (3) anomolous behavior such as may occur by interactions with lattice defects and boundaries may be studied (4) growth and dissolution mechanisms may be established, (5) phase stability in an irradiation environment may be studied directly