Influencia de la adición de estaño en el proceso de precipitación en una aleación de Cu-Ni-Zn

ABSTRACT: Influence of tin additions on the precipitation processes in a Cu-Ni-Zn alloys. The influence of 1.1 wt% tin additions on the precipitation hardening of Cu-11 wt% Ni-20 wt% Zn alloy was studied by Differential Scanning Calorimetry (DSC), microhardeness measurements and High Resolution Transmission Electron Microscopy (HRTEM). The calorimetric curves, in the range of temperatures analyzed, show the presence of two exothermic reactions in the ternary alloy, associated to the short-range-order development assisted by migration of excess vacancies. On the other hand, one exothermic and one endothermic reaction are observed in the quaternary alloy, associated to the formation and dissolution of Cu2NiZn precipitates, respectively. It has been show that an addition of 1.1% tin plays an important role in the formation of Cu2NiZn precipitates, responsible for the precipitation hardening of the ternary alloy.Peer Reviewe

Applications of sample-controlled thermal analysis (SCTA) to kinetic analysis and synthesis of materials

The advantages of the sample-controlled thermal analysis (SCTA) for both the kinetic analysis of solid-state reactions and the synthesis of materials are reviewed. This method implies an intelligent control of the temperature by the solid-state reaction under study in such a way that the reaction rate as a function of the time fits a profile previously defined by the user. It has been shown that SCTA has important advantages for discriminating the kinetic model of solid-state reactions as compared with conventional rising temperature methods. Moreover, the advantages of SCTA methods for synthesising materials with controlled texture and structure are analysed.Peer Reviewe

Development of a universal sample controlled thermal analysis (SCTA) system for being used with any thermoanalytical instrument

Peer Reviewe

Kinetic model for thermal dehydrochlorination of poly(vinyl chloride)

In this paper, a novel method for calculating degradation kinetics is presented. The method has been applied to the thermal dehydrochlorination of two different samples of PVC. It has been observed that this dehydrochlorination is complex and involves two different processes. A model that accounts for the entire dehydrochlorination is proposed. This model involves nucleation and growth and diffusion controlled mechanisms. The kinetic parameters are obtained from linear heating rate, isothermal and sample controlled thermal analysis experiments. Kinetic results obtained from the macroscopic thermal analysis measurements demonstrate the correlation between the kinetics of the thermal dehydrochlorination of PVC and the structure of this macromolecule. © 2010 Elsevier Ltd.Peer Reviewe

Mechanochemical preparation of BaTiO3-Ni nanocomposites with high dielectric constant

A mechanochemical procedure is proposed for an easy preparation of a BaTiO3-Ni composite in a single step. BaTiO3 and Ni powders available in the market are mixed by dry ball milling producing a decrease of particle size and an evenly distribution of both phases. In the sintered pellets the nickel particles are homogeneously distributed into the BaTiO3 matrix and isolated from others Ni particles. The dielectric constant of the composite is considerably higher than that of the barium titanate. Moreover, the temperature of the ferroelectric ↔ paraelectric transition of the BaTiO3-Ni composite here prepared is much lower than the one of the pure BaTiO3 single phase. © 2009 Elsevier Ltd.Peer Reviewe

Non isothermal calorimetric study of the precipitation processes in a Cu-10% Ni-3% Al alloy

8 páginas, 6 figuras, 2 tablas.[ES] Mediante calorimetría diferencial de barrido (DSC), se estudió el proceso de precipitación de átomos de níquel y aluminio a partir de una solución sólida de Cu-Ni-Al. El análisis de las curvas calorimétricas muestra la presencia de dos reacciones exotérmicas (etapas 1 y 2), que se interpretan como la formación de dos tipos de precipitados. La primera etapa corresponde a la formación de la fase de equilibrio b (NiAl), en tanto que la etapa 2 corresponde a la formación del precipitado g’ de composición estequiométrica Ni3Al. Además, se observa que las curvas se desplazan a temperaturas más bajas a medida que la velocidad de calentamiento lineal decrece. Las energías de activación de las reacciones fueron evaluadas a partir de un método de Kissinger modificado. Los parámetros cinéticos se calcularon mediante el formalismo de Johnson- Mehl-Avrami. Los valores de n obtenidos son indicativos de un proceso de nucleación de partículas a partir de núcleos pre existentes. Las medidas de microdureza y las micrografías obtenidas por microscopía electrónica de transmisión ayudaron a confirmar la formación de las fases antes indicadas.[EN] Using differential scanning calorimetry (DSC), the precipitation process of nickel and aluminium atoms from a solid solution of Cu-Ni-Al were studied. Analysis of calorimetric traces displayed shows the presence of two exothermic reactions (stage 1 and 2), which were interpreted as the formation of two types of precipitates. The first stage corresponds to the formation of an equilibrium b (NiAl) phase, whilst stage 2 corresponds to the formation of g’ precipitate with Ni3Al stoichiometric composition. It is observed the shift of the traces to lower temperatures while linear heating rate decrease. The activation reaction energies were evaluated from modified Kissinger’s method. Kinetic parameters were calculated by means Johnson-Mehl-Avrami formalism. The values obtained for parameter n were indicative of a particle nucleation process from pre-existent nucleus. Microhardness measurements and micrographies obtained by transmission electron microscopy were helpful to confirm the formation and the dissolution of the mentioned phases.Los Autores desean agradecer al Fondo Nacional de Desarrollo Científico y Tecnológico (FONDECYT), Proyecto N° 1061114, por el apoyo financiero.Peer reviewe

Non-isothermal Characterization of the Precipitation Hardening of a Cu-11Ni-19Zn-1Sn Alloy

The precipitation hardening of a Cu-11Ni-19Zn-1Sn alloy has been studied by means of Differential Scanning Calorimetry (DSC), High-Resolution Transmission Electron Microscopy (HRTEM), and hardness measurements. The calorimetric curves, in the range of temperatures analyzed, show the presence of one exothermic reaction followed by an endothermic one. The exothermic DSC peak is due to the segregation of Cu2NiZn precipitates and it is associated to a noticeable improvement of the mechanical properties of the alloy. The endothermic effect is associated to the dissolution of the Cu2NiZn precipitates into the copper matrix for restoring the starting Cu-11Ni-19Zn-1Sn homogeneous solid solution. The reaction mechanisms of these processes have been proposed from the kinetic analysis of the exothermic and endothermic DSC signals. The results obtained point out that tin plays a decisive role on the precipitation hardening of the alloy, because age hardening is not observed in the case of a Cu-Ni-Zn ternary alloy of similar composition.The authors would like to acknowledge the Fondo Nacional de Desarrollo Científico y Tecnoloógico (FONDECYT) for financial support, Project No. 1140782

Study by DSC and HRTEM of the aging strengthening of Cu-Ni-Zn-Al alloys

The structural changes of a Cu-12wt.% Ni-17wt.% Zn-1.7wt.% Al alloy as a function of the aging temperature have been studied by means of Differential Scanning Calorimetry (DSC), high resolution transmission electron microscopy (HRTEM) and hardness measurements. It has been proposed a hardening mechanism that implies the crystallization of a Ll CuNiZn phase coherent with the matrix α phase followed, firstly, by its transformation into a Ll coherent phase and, secondly, by the precipitation of this phase. It has been shown that aluminum play an important role in the precipitation hardening process because CuNiZn precipitates are not formed by aging a ternary Cu-Ni-Zn alloy of similar composition. It has been shown by the first time that DSC could be a powerful tool for discriminating the whole set of phase transformations undergone by alloys as a function of the annealing temperature from a single heating run.The authors would like to acknowledge the Fondo Nacional de Desarrollo Científico y Tecnológico (FONDECYT) for financial support, Project N° 1140782

The calorimetric analysis as a tool for studying the aging hardening mechanism of a Cu-10wt%Ni-5.5wt%Sn alloy

The transformations of a Cu-10wt%Ni-5.5wt%Sn alloy as a function of the aging time in the range from room temperature up to 600 C have been followed by Differential Scanning Calorimetry (DSC). The results obtained have shown that this alloy undergone two overlapping exothermic phase transitions with DSC peaks at 208 C and 305 C, respectively, followed by an endothermic phase transformation with a DSC peak at 526 C. The structural analysis by TEM, ED, EDX and XRD of the intermediates phases previously discriminated by DSC suggests that the first exothermic peak is associated to the spinodal decomposition of the sample, while the second one is associated to the segregation of a DO22 (Cux-Ni1x)3Sn tetragonal phase coherent with the a-Cu structure of the starting alloy. The endothermic peak has been associated to the precipitation of cubic DO3 nanocrystals from the DO22 phase previously formed. The microhardness measurements carried out in combination with the structural characterization demonstrate that the aging hardening of the alloy under study is exclusively due to the formation of the coherent DO22 phase. The DO22/ DO3 transition leads to a dramatic drop of the hardness of the alloy

Microcalorimetry: A powerful tool for quantitative analysis of aging hardening response of Cu-Ni-Sn alloys

The method for the deconvolution of overlapping DSC peaks here proposed has been used by the first time for the quantitative determination of the enthalpies associated to the phase transitions undergone during the aging of an alloy. They have been determined the enthalpies evolved along the first and the second overlapping DSC traces of Cu-10 wt%Ni-5.5 wt%Sn alloy, which are associated, respectively, to the spinodal decomposition of the alloy and the segregation of a DO (CuNi)Sn tetragonal phase. The fraction of the DO phase (responsible of the aging hardening of this alloy) has been successfully determined from DSC as a function of the annealing treatment, while TEM and XRD failed for this purpose. It has been demonstrated that a threshold higher than 50% of crystallization of the DO phase is required for achieving a significant increase of the hardness as a function of the crystallization percentage. These results suggest that microcalorimetric measurement can be a powerful tool to establish quantitative relationships between the mechanical, electrical or functional properties of alloys and their structural changes undergone by aging.The authors would like to acknowledge the facilities provided by the Instituto de Ciencia de Materiales de Sevilla (Spain) and the Departamento de Ciencia de Materiales of the University of Chile (Chile). One of us (A. Perejón) also thank VPPI-US for the AP current contract. Financial support by projects CTQ2014-52763-C1–R (Ministerio de Economia y Competitividad, Spanish government-FEDER) and TEP-7858 (Junta de Andalucia-FEDER) is acknowledged