En este trabajo se presenta el estudio del efecto de la adición de 3% en peso de V 2 O 5 , Na 2 O, CaO y FeTiO 3 en la estabilización térmica del Titanato de Aluminio (Al 2 TiO 5 ). El procesamiento de las muestras fue mediante la molienda en seco de los polvos de alúmina y titania, mezcla vía húmeda con alcohol isopropílico y sinterización a 1400ºC de los compactos prensados uniaxialmente a 300 MPa. Una vez sinterizadas las muestras fueron tratadas térmicamente a 1000ºC durante 12 y 20 horas para evaluar la descomposición del Al 2 TiO 5 . La microestructura de las probetas se analizó mediante Microscopía Electrónica de Barrido (MEB) y las fases presentes se determinaron mediante Difracción de Rayos X (DRX) y Espectroscopía de Rayos X por dispersión de Energía (EDX). Esta caracterización evidenció el efecto beneficioso de la adición de FeTiO 3 (Ilmenita) en la formación y estabilización térmica del Al 2 TiO 5 . Palabras clave: titanato de aluminio,sinterización reactiva, estabilización térmicaAdditives effect on the formation and thermal stability of aluminum titanate at low temperature This work presents the study of the effect of 3 wt% V 2 O 5 , Na 2 O, CaO and FeTiO 3 on the stabilization of Aluminum Titanate (Al 2 TiO 5 ). Samples were prepared by the conventional processing route of dry ball milling followed by wet mixing and final uniaxial pressing at 300 MPa before sintering at 1400ºC. Specimens were heat treated at 1000ºC during 12 and 20 hours in order to evaluate Al 2 TiO 5 decomposition. The microstructure was observed by Scanning Electron Microscopy (SEM) and phases present were determined by X Ray Diffraction (XRD) and Energy Dispersive Spectroscopy (EDS). It has been elucidated the effect of FeTiO 3 (Ilmenite) on the improvement of the formation and thermal stabilization of Al 2 TiO 5 .Keywords: aluminum titanate, reactive sintering, thermal stabilization INTRODUCCIÓNEl Titanato de Aluminio (Al 2 TiO 5 ) se ha convertido en un material cerámico de gran importancia a nivel ingenieril debido a su bajo coeficiente de expansión térmica (0,8 -1,5x10 -6 ºC -1 ), alta resistencia al choque térmico (500 W/m), baja conductividad térmica (1,5 W/mK) y su alto punto de fusión (1850ºC), otorgándole un gran potencial en aplicaciones estructurales. Este material es producido por la sinterización reactiva de la mezcla equimolar de polvos de alúmina (Al 2 O 3 ) y titania (TiO 2 ) por encima de 1300ºC. El Al 2 TiO 5 presenta una transformación de fase eutectoide alrededor de 1280ºC[1]. La formación de microgrietas en materiales monofásicos de titanato de aluminio se atribuye a la fuerte anisotropía en su expansión térmica, α a = -2.9; α b = 10.3; α c = 20.1x10 -6 ºC -1 , que hace que se generen tensiones térmicas durante el enfriamiento que dan lugar a las mencionadas microgrietas. Las diferencias entre los coeficientes de dilatación de los diferentes ejes de un monocristal de titanato son mayores que entre la pequeña fracción de alumina sin reaccionar y el propio titanato. La aplicación exitosa del material ha depe...
El propósito del presente trabajo fue determinar la correlación entre las microestructuras y las propiedades mecánicas resultantes en la unión soldada por fricción-agitación de la aleación de magnesio AZ31B, como una función de lasvariables principales del proceso implementado a partir de una maquina fresadora universal acondicionada. Se lograron soldaduras sanas, sin defectos, para diferentes niveles de velocidad de rotación y velocidad de avance, con valores correspondientes de resistencia mecánica de la unión soldada cercanos a la del metal base, con una eficiencia mecánica máxima del 93%. La caracterización microestructural mostró granos equiaxiales homogéneos con tamaño promedio de 10 μm, en la zona agitada. La falla de las probetas durante el ensayo de tensión se presentó generalmente en el lado de avance, con una morfología superficial de tipo dúctil pero con baja deformación plástica aproximada del 5%. Los resultados de calidad y de propiedades mecánicas de las uniones soldadas por fricción-agitación dealeaciones de magnesio AZ31B, permiten asegurar la conveniencia de ser usadas para aplicaciones estructurales teniendo en cuenta su elevada y conveniente relación resistencia mecánica/peso.
The Fe-Ni-Ag system is of particular interest for its potential applications as soft magnetic granular alloys formed by small magnetic grains embedded in a non-magnetic metal matrix. It is formed by two binary immiscible systems under equilibrium conditions : Fe-Ag and Ni-Ag and one binary system Fe-Ni. These materials are particular important for magnetoresistive properties [1]. The properties of these alloys are closely related to their microstructure, therefore a detailed study of the transformations occurring during milling was undertaken considering materials pre-alloyed with several concentrations of Fe and Ni and then further alloyed with different Ag content. Heat treatments of the mechanically alloyed powders were performed and structural characterization of the sintered powders was carried out.Elemental high purity (99.99%) Fe, Ni and Ag powders with an average particle size of 4.5, 1.2 and 1.3 µm respectively, were blended in a WAV turbula for 2h. The compositions were Fe 30, 50 and 70 at.%, Ni 70, 50 and 30 at% and Ag in balancing concentration to give alloys compositions of (Fe x -Ni 100-x ) y -Ag 100-2y (x= 30, 50, 70 and y=5,20,60). The powders of Fe and Ni were mechanically alloyed under nitrogen atmosphere in a high energy mill SPEX 8000, using hardened steel vials and balls with a ball-to-powder weight ratio of 8:1 and milling periods of 1, 5, 10, 25 to followed the formation of the Fe x Ni y compound. And then further milled with Ag was carried out for 10 h. The powders from the different milling periods and compositions were pressed at 350 MPa and sintered in a graphite crucible at 900ºC for 40 min in argon atmosphere. The products were characterized by optical microscopy, x-ray diffraction (XRD) using a Siemens 5005 diffractometer with Cu-K α radiation (Ni filter) operating at 40 keV and 20 mA. Scanning electron microscopy (SEM) analysis was performed in a Phillips XL30 attached with an EDX DX4 and a EBSD system. Transmission electron microscopy (TEM) was carried out in a Phillips CM10. Samples for TEM were cut by ultramicrotomy techniques and collected in a holey carbon grid.The Fe 30 Ni 70, Fe 50 Ni 50, are very similar showing the formation of the ordered FeNi compound. In the Fe 70 Ni 30 composition the formation of a mixture of phases was observed, the ordered FeNi and the disordered Fe(Ni) compounds. The mixture of these systems with Ag showed the metal compounds surrounded by Ag forming islands of Fe x Ni y -Ag, There was also evidence of Ag diffusing into the FeNi resulting in the formation of a FeNiAg phase. Backscattered images together with EDX analysis confirmed this result as seen in Fig. 1. For low Ag content (5%), silver seems to joint the grains and help sintering. For high Ag content 60%, islands of FeNi surrounded by Ag are observed. Sintering is always improved with the Ag content. Backscattered electron diffraction patterns were taken on the different Fe x Ni y phases showing a cubic symmetry for all the cases studied. References[1] N.
Mechanical Alloying is an interesting method to obtain compounds of nanometric dimensions and amorphous alloys, at room temperature. The formation of amorphous or crystalline phases of metallic compounds starting from a mixture of powders of the pure elements can be induced by interdiffusion in the solid state produced by the mechanical alloying process (MA). High energy ball milling induces a high density of lattice imperfections which affect the energy barriers leading to different reactions far from thermodynamic equilibrium, like alloying, order-disorder transformations, amorphization and other phase changes. An enhancement in the diffusion coefficients during severe plastic deformation of various elements above the thermal value has been reported [1] which can affect the reaction path that different elements choose for alloying.. For the TiAl system interesting structural transformations during milling are expected due to the different crystal structures of both metals (Ti -hexagonal and Al-face centered cubic) their limited solubility and the capability of crystal structure transformations with mechanical deformation (fcc hcp).In the present work the study of the phase transformations taking place during mechanical alloying of Al-Ti in equiatomic concentration was carried out , considering morphology, structural evolution and formation mechanisms for the different milling periods and after sintering.Elemental high purity Al and Ti powders, analytical grade, with an average particle size 12 and 29 µm respectively, were mixed in a proportion of 50 at. %, in a WAV turbule during 1 h and then mechanically alloyed in nitrogen atmosphere, using a SPEX 8000, for different milling periods: 1, 3, 5, 10, 20, 30 and 50h, using stainless steel vial and balls, and a ball-to-powder weight ratio (BPR) of 8:1. The powders were characterized from each milling interval by x-ray diffraction in a Siemens diffractrometer D-5005, scanning electron microscopy was carried in Philips XL 30 and transmission electron microscopy was performed in a JEOL 1220 operating at 120 keV. Sintering was carried out at 2GPa and 700ºC for 30 min.The XRD patterns of the Ti-50at%Al for the different milling periods showed the intensity of the Al reflections decreasing progressively with milling until they disappear completely after 5h, indicating the formation of a Ti(Al) hexagonal solid solution with a grain size of 8 nm. After 10 h amorphization takes place and by further milling a transformation to a fcc solid solution Ti(Al) is observed, with a lattice parameter of 4.11 Å and a grain size of 5 nm. TEM bright field images for the Ti-Al system are shown in Fig.1. After 5 h of milling a microcrystalline hcp Ti(Al) solid solution was formed with a grain size in the range of 2 nm but some large grains of aprox. 20 nm were also observed (Fig. 1a) and after 50 h of milling a nanocrystalline Ti(Al) fcc solid solution was identified by electron diffraction (Fig. 2) with a grain size of in the range of 1.5 nm. After sintering, the crystal structure of the...
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