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The classical theory of solid-state transformation based on nucleation and growth processes, developed by Kolmogorov, Johnson and Mehl, and Avrami (KJMA theory), is widely used in many different fields of research. In KJMA theory, two parameters (the frequency factor and, particularly, the Avrami exponent) can supply information about the mechanisms involved in the transformation. Despite its apparent simplicity, on the one hand, the results derived from this theory can be strongly affected by the indetermination of experimental data (e.g., onset of the transformation). On the other hand, KJMA theory is developed for isothermal polymorphic transformations in which randomly distributed nuclei grow in convex shapes. However, several procedures have extended KJMA theory to nonisothermal regimes and to many different processes deviating from those premises. Herein, the requirements of KJMA theory and the expected deviations for these approximations are briefly discussed. In addition, some strategies are proposed for recovering physical meaning from the effective parameters deduced in several transformations including nanocrystallization and martensitic transformations for which results can be interpreted under the approximation of instantaneous growth.

In this study, the non-isothermal kinetics of the martensitic transition from 14M modulated martensite to austenite phase in Ni55Fe19Ga26 ribbons obtained by melt-spinning has been analyzed. The proximity of the martensitic transition to room temperature makes it very sensitive to pressure and subtle differences for different pieces of the ribbon (ascribed to stresses stored in the ribbon during its rapid solidification process). Despite the dispersion in the characteristic parameters of the transition, a general behavior is observed with a decreasing activation energy as the heating rate increases due to the nucleation driven character of the transition. It has been shown that a first-order autocatalysis can describe the temperature evolution of the austenite fraction using only two experimental temperatures. Predicted curves are in good agreement with experimental data

Devitrification of mechanically alloyed amorphous FeZr at. % compound consists on a two-step process: amorphous → amorphous + bcc Fe + FeZr → FeZr + FeZr. This sequence is inferred from the evolution of the Mössbauer spectra, the thermomagnetic experiments and the X-ray diffraction (XRD) patterns. Hyperfine parameters for both intermetallics have been obtained from Mössbauer spectroscopy in correlation with the phase identification from XRD results. The broadening of the stable compositional range of FeZr intermetallic above 1000 K is responsible for a strong dependence of the phase fractions on heating and cooling rates. Despite the overlapping of the two processes involved in the devitrification, the individual Avrami exponents of each one have been estimated.

Ribbons of a Ni55Fe19Ga26 Heusler alloy were prepared by melt spinning technique. The effect of pressure on magnetostructural transitions was studied comparing as-spun ribbons to ribbons previously submitted to an axial pressure and to powder obtained from manually grinded as-spun ribbons. The martensitic transformation present in the as-spun ribbon almost vanishes after pulverization. The structural change driven by mechanical treatment is a stress-induced intermartensitic transformation from a 14 M modulated to a non-modulated tetragonal (NM) structure. There is a progressive change from the structure of as-spun samples to that of the pulverized one in samples submitted to axial pressures as it increases.

The presence of a distribution of transition temperatures (DTT) is ubiquitous in materials science. It is common to ascribe deviations from theoretical pure-phase behavior to this fact. To adapt the di erent pure phase models to systems with a DTT, the parameters of such distribution must be known or at least estimated. In this review, the di erent sources for the existence of such distributions and their e ects on magnetothermal properties are summarized. In addition, di erent models proposed to extract the parameters of the corresponding DTT are discussed and extended, starting from Weiss model, to account for other phenomenologies. Experimental results on amorphous Fe-Nb-B and intermetallic MnCo(Fe)Ge systems are also reported

The kinetics of amorphization during ball milling is generally analyzed using two different approaches: the classical Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory and Delogu and Cocco’s model for which a region deterministically transforms after it reaches a certain number of collisions. The application of JMAK analysis to the latter model predicts Avrami exponents to be higher than the experimental ones (typically close to one). We develop simulations based on the probabilistic character of the nucleation phenomenon and concave growth of the amorphous phase in the core of a nanocrystal. The predictions of our simulations are in good agreement with the low Avrami exponents and with the size evolution of the remaining crystallites found experimentally. From these values, the parameters involved in the simulated model (growth rate and probability of nucleation) can be estimated.