Surface morphology and phase transformations of femtosecond laser-processed saphire

The morphological and structural modifications induced in sapphire by surface treatment with femtosecond laser radiation were studied. Single-crystal sapphire wafers cut parallel to the (0 1 2) planes were treated with 560 fs, 1030 nm wavelength laser radiation using wide ranges of pulse energy and repetition rate. Self-ordered periodic structures with an average spatial periodicity of similar to 300 nm were observed for fluences slightly higher than the ablation threshold. For higher fluences the interaction was more disruptive and extensive fracture, exfoliation, and ejection of ablation debris occurred. Four types of particles were found in the ablation debris: (a) spherical nanoparticles about 50 nm in diameter; (b) composite particles between 150 and 400 nm in size; (c) rounded resolidified particles about 100-500 nm in size; and (d) angular particles presenting a lamellar structure and deformation twins. The study of those particles by selected area electron diffraction showed that the spherical nanoparticles and the composite particles are amorphous, while the resolidified droplets and the angular particles, present a crystalline a-alumina structure, the same of the original material. Taking into consideration the existing ablation theories, it is proposed that the spherical nanoparticles are directly emitted from the surface in the ablation plume, while resolidified droplets are emitted as a result of the ablation process, in the liquid phase, in the low intensity regime, and by exfoliation, in the high intensity regime. Nanoparticle clusters are formed by nanoparticle coalescence in the cooling ablation plume. (C) 2013 Elsevier B.V. All rights reserved.

Influence of the Al content on the phase transformations in Cu-Al-Ag alloys

The influence of the Al content on the phase transformations in Cu-Al-Ag alloys was studied by classical differential thermal analysis (DTA), optical microscopy (OM) and X-ray diffractometry (XRD). The results indicated that the increase in the Al content and the presence of Ag decrease the rate of the b1 phase decomposition reaction and contribute for the raise of this transition temperature, thus decreasing the stability range of the perlitic phase resulted from the b1 decomposition reaction.

Numerical model of solid phase transformations governed by nucleation and growth: microstructure development during isothermal crystallization

A simple numerical model which calculates the kinetics of crystallization involving randomly distributed nucleation and isotropic growth is presented. The model can be applied to different thermal histories and no restrictions are imposed on the time and the temperature dependences of the nucleation and growth rates. We also develop an algorithm which evaluates the corresponding emerging grain-size distribution. The algorithm is easy to implement and particularly flexible, making it possible to simulate several experimental conditions. Its simplicity and minimal computer requirements allow high accuracy for two- and three-dimensional growth simulations. The algorithm is applied to explore the grain morphology development during isothermal treatments for several nucleation regimes. In particular, thermal nucleation, preexisting nuclei, and the combination of both nucleation mechanisms are analyzed. For the first two cases, the universal grain-size distribution is obtained. The high accuracy of the model is stated from its comparison to analytical predictions. Finally, the validity of the Kolmogorov-Johnson-Mehl-Avrami model SSSR, is verified for all the cases studied

Evidence of distinct phase transformations in lithium phosphate glass Li2O-P2O5

The purpose of this work is to study the Li2O-P2O5 glass using the differential scanning calorimetry (DSC) and x-ray diffraction (XRD) techniques to understand the crystallization process in this glass matrix. To study the glass by DSC, screened samples with different particle sizes to resolve the crystallization peaks were used. Both crystallization peaks were attributed to Li6P6O18 and LiPO3 phases. This evidence was corroborated by XRD analysis on glasses annealed at different temperatures in order to crystallize these phases.

Influence of the Al content on the phase transformations in Cu-Al-Ag alloys

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

beta Phase transformations in the Cu-11mass%Al alloy with Ag additions

In the Cu-Al system, due to the sluggishness of the beta a dagger" (alpha + gamma(1)) eutectoid reaction, the beta phase can be retained metastably. During quenching, metastable beta alloys undergo a martensitic transformation to a beta' phase at Al low content. The ordering reaction beta a dagger" beta(1) precedes the martensitic transformation. The influence of Ag additions on the reactions containing the beta phase in the Cu-11mass%Al alloy was studied using differential scanning calorimetry and in situ X-ray diffractometry. The results indicated that, on cooling, two reactions are occurring in the same temperature range, the beta -> (alpha + gamma(1)) decomposition reaction and the beta -> beta(1) reaction, with different reaction mechanisms (diffusive for the former and ordering for the latter) and, consequently, with different reaction rates. For lower cooling rates, the dominant is the decomposition reaction and for higher cooling rates the ordering reaction prevails. on heating, the (alpha + gamma(1)) -> beta reverse eutectoid reaction occurs with a resulting beta phase saturated with alpha. The increase of Ag concentration retards the beta -> (alpha + gamma(1)) decomposition reaction and the beta -> beta(1) ordering reaction, which occurs in the same temperature range, becomes the predominant process.

Phase transformations in the Cu-Al alloy with Ag addition

The effect of Ag addition on the phase transformations that occur in the Cu-10% Al alloy was studied using differential thermal analysis, scanning electron and optical microscopies and energy dispersive X-ray analysis. The results indicated that Ag addition is responsible for the separation of the reverse martensitic transformation in two stages, and for the refinement of the α-phase grains. The relative amount of the β1 martensitic phase, retained on slow cooling (above 2 K min-1 of cooling rate), and the relative fraction of phase α2 are increased. The solubility limit of Ag in the matrix is close to 6 mass% and at this concentration the maximum stability of the β-phase is reached. © 2005 Akadémiai Kiadó, Budapest.

Atomistic origins of the phase transition mechanism in Ge(2)Sb(2)Te(5)

The fast and reversible phase transition mechanism between crystalline and amorphous phases of Ge(2)Sb(2)Te(5) has been in debate for several years. Through employing first-principles density functional theory calculations, we identify a direct structural link between the metastable crystalline and amorphous phases. The phase transition is driven by the displacement of Ge atoms along the rocksalt [111] direction from stable octahedron to high energy unstable tetrahedron sites close to the intrinsic vacancy regions, which generates a high energy intermediate phase between metastable and amorphous phases. Due to the instability of Ge at the tetrahedron sites, the Ge atoms naturally shift away from those sites, giving rise to the formation of local-ordered fourfold motifs and the long-range structural disorder. Intrinsic vacancies, which originate from Sb(2)Te(3), lower the energy barrier for Ge displacements, and hence, their distribution plays an important role in the phase transition. The high energy intermediate configuration can be obtained experimentally by applying an intense laser beam, which overcomes the thermodynamic barrier from the octahedron to tetrahedron sites. The high figure of merit of Ge(2)Sb(2)Te(5) is achieved from the optimal combination of intrinsic vacancies provided by Sb(2)Te(3) and the instability of the tetrahedron sites provided by GeTe.

Diffusionless first-order phase transitions in systems with frozen configurational degrees of freedom

We consider systems that can be described in terms of two kinds of degree of freedom. The corresponding ordering modes may, under certain conditions, be coupled to each other. We may thus assume that the primary ordering mode gives rise to a diffusionless first-order phase transition. The change of its thermodynamic properties as a function of the secondary-ordering-mode state is then analyzed. Two specific examples are discussed. First, we study a three-state Potts model in a binary system. Using mean-field techniques, we obtain the phase diagram and different properties of the system as a function of the distribution of atoms on the different lattice sites. In the second case, the properties of a displacive structural phase transition of martensitic type in a binary alloy are studied as a function of atomic order. Because of the directional character of the martensitic-transition mechanism, we find only a very weak dependence of the entropy on atomic order. Experimental results are found to be in quite good agreement with theoretical predictions.

Entropy change of martensitic transformations in Cu-based shape-memory alloys

We have investigated the different contributions to the entropy change at the martensitic transition of different families of Cu-based shape-memory alloys. The total entropy change has been obtained through calorimetric measurements. By measuring the evolution of the magnetic susceptibility with temperature, the entropy change associated with conduction electrons has been evaluated. The contribution of the anharmonic vibrations of the lattice has also been estimated using various parameters associated with the anharmonic behavior of these alloys, collected from the literature. The results found in the present work have been compared to values published for the martensitic transition of group-IV metals. For Cu-based alloys, both electron and anharmonic contributions have been shown to be much smaller than the overall entropy change. This finding demonstrates that the harmonic vibrations of the lattice are the most relevant contribution to the stability of the bcc phase in Cu-based alloys.

Effects of domain morphology in phase-separation dynamics at low temperature

We present numerical results of the deterministic Ginzburg-Landau equation with a concentration-dependent diffusion coefficient, for different values of the volume fraction phi of the minority component. The morphology of the domains affects the dynamics of phase separation. The effective growth exponents, but not the scaled functions, are found to be temperature dependent.

Avalanches in a fluctuationless first-order phase transition in a random-bond Ising model

We study the behavior of the random-bond Ising model at zero temperature by numerical simulations for a variable amount of disorder. The model is an example of systems exhibiting a fluctuationless first-order phase transition similar to some field-induced phase transitions in ferromagnetic systems and the martensitic phase transition appearing in a number of metallic alloys. We focus on the study of the hysteresis cycles appearing when the external field is swept from positive to negative values. By using a finite-size scaling hypothesis, we analyze the disorder-induced phase transition between the phase exhibiting a discontinuity in the hysteresis cycle and the phase with the continuous hysteresis cycle. Critical exponents characterizing the transition are obtained. We also analyze the size and duration distributions of the magnetization jumps (avalanches).