Transformación de fase sin difusión en aleaciones de Zr-Nb y Zr-Nb-Al
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Universidad Nacional de Cuyo (Argentina). Instituto de Física "Dr. J. A. Balseiro
Comisión Nacional de Energía Atómica (Argentina). Centro Atómico Bariloche.
Comisión Nacional de Energía Atómica (Argentina). Centro Atómico Bariloche.
Resumen
Se describe fenomenológicamente la transformación martensítica y se enuncian las teorías cristalográficas más generalizadas, como así también algunos criterios aplicados a la estabilidad de fases en aleaciones de Zirconio y efectos asociados a la transformación martensítica, como el seudoelstico y TRIP. Debido a la similitud entre el Zirconio y el Titanio respecto a estructuras, diagramas de fase, etc., se brinda un resumen de la bibliografía correspondiente a ese material, en especial el tipo de dislocaciones reportadas. Asimismo, se muestran los diagramas de fase de equilibrio de los sistemas binarios Zr-Nb y Zr-Al con una recopilación de las fases metaestables para el Zr-Nb. Para el desarrollo experimental de este trabajo, se construyeron equipos como para la fundición de las muestras, horno de tramiento térmico y dispositivo de deformación sobre la platina del microscopio óptico. Otros ensayos y observaciones se realizaron con equipamiento existente, como máquinas de deformación, microscopías óptica y electrónicas de transmisión y de barrido. Distintos estudios relativos a la estabilidad de la fase B retenida por templado en la aleación Zr-10%Nb, muestran la dificultad para modificarla por tratamiento termomecánico. En el sistema ternario Zr-Nb-Al, a pesar de formarse precipitados u> luego del templado, es posible mediante deformación plástica inducir una transformación de fase martensítica. Variando levemente el contenido de Nb-alrededor del 7% en peso- es posible obtener por templado martensita espontánea a' o una martensita que por sus características morfológicas denominamos combada a^,. Incrementando del orden de 0,5% de Nb en peso, se retiene fase 0 a partir de la cual se puede obtener martensita inducida por deformación a^. Se presentan resultados experimentales de deformación a distintas temperaturas entre -193 y 100°C, con un análisis de las morfologías y cantidad de placas formadas a cada temperatura, como así también observaciones de la zona de fractura y efecto TRIP. De un análisis de múltiples superficies sobre proyecciones estereográficas, resultan un plano de hábito tipo (334)^ para a' y a^, un plano de macla tipo (1011) y una subestructura fina dentro de las maclas formada por dislocaciones de hélice con vector de Burgers tipo 1/3 <1210>. También se da para cada caso las relaciones de orientación entre placas y placa-matriz, verificándose las llamadas relaciones de Burgers (0001)a ■ | | (011)^ y [1120]a , | |[111]^ . De la medición del módulo de deformación macroscópica S (0.114) comparado con el resultante por cálculo, S(0,188), se indica que la subestructura interna es debida a deformación de la placa para compensar el S y no a la transformación. Ese resultado, junto con un análisis de módulos elásticos, explican la alta histéresis en temperatura y deformación observadas, indicando a las aleaciones de Zirconio como un caso intermedio entre latones y aceros. Cálculos cristalográficos con la teoría WLR, muestran dislocaciones tipo <c+a> activas para la transformación y no a las del tipo <a>. El sistema {1011} [2113] ajusta al plano de hábito medido.
A phenomenological description of the martensitic transformation is presented, including a description of the more generalized crystallographic theories, of the criteria applied to the phase stability in Zirconium alloys and of other effects associated with the martensitic transformation as pseudoelasticity and TRIP. Because of the similarities between Zirconium and Titanium, with respect to their structures, phase diagrams, etc., a resume of the literature is given referring to these materials and including the dislocation types reported. Also, the equilibrium phase diagrams of the binary systems Zr-Nb and Zr-Al are shown, with a compilation of the Zr-Nb metastable phases. In the experimental part we describe the equiproent developed for this work, consisting of an are furnace for melting samples, a furnace for thermal treatment and a tensión device for mounting on the optical microscope slide. Other tests and observations were performed with existing equipment, such as deformation machines, optical microscopes and transmission and scanning electrón microscopes. Various studies concerning the stability of the B phase which was retained after quenching in the Zr~107o Nb alloy, show the difficulty in modifying it by thermomechanical treatments. In the ternary system Zr-Nb-Al it is possible to induce a martensitic phase transformation by plástic deformation, in spite of the formation of w precipitates after quenching. Changing lightly the Nb content around 7 wt% it is feasible to obtain spontaneous martensite a' by quenching. Also a martensite is observed which is called "bended" because of its morphological characteristics. Increasing Nb approximately by 0,5 wt%, the B phase is retained on quenching, and subsequently a martensite can be stress induced. Experimental results on deformation which have been performed at different temperatures between -193 and 100°C are presented, including an analysis of the morphology and amount of plates formed at each temperature, and also observations of the fractured zone and the TRIP effect. From multisurface trace analysis for both a' and we found a (334)^ type habit plañe, a (lOll) type twinning plane and a fine substructure inside the twins formed by screw dislocations with Burgers vector type 1/3<1210>. Also, in each case the orientation relationship between the plates and the matrix are given, by the so called Burgers relations (00 01 )a ,
(0 1 1 ) and [ 1 1 2 0 ]a ,
[ 1 1 1 ]fl. By comparing the measured macroscopic shear modulus S = 0,114 with the calculated one, S = 0.188 it is indicated that the internal substructure is due to plástic deformation inside the píate in order to reduce S and not to the transformation itself. This result, together with an analysis of the elastic moduli, can explain the high temperature or strain hysteresis between transformation and retransformation, placing the martensitic transformation for Zirconium alloys as an intermedíate case between those for brasses and steels. Crystallographic calculations using the WLR theory, show that the < c+a > type dislocations can describe the lattice invariant shear of the transformation but not the <a> type. The system {1011} [2113] adjusts to the measured habit plane.
A phenomenological description of the martensitic transformation is presented, including a description of the more generalized crystallographic theories, of the criteria applied to the phase stability in Zirconium alloys and of other effects associated with the martensitic transformation as pseudoelasticity and TRIP. Because of the similarities between Zirconium and Titanium, with respect to their structures, phase diagrams, etc., a resume of the literature is given referring to these materials and including the dislocation types reported. Also, the equilibrium phase diagrams of the binary systems Zr-Nb and Zr-Al are shown, with a compilation of the Zr-Nb metastable phases. In the experimental part we describe the equiproent developed for this work, consisting of an are furnace for melting samples, a furnace for thermal treatment and a tensión device for mounting on the optical microscope slide. Other tests and observations were performed with existing equipment, such as deformation machines, optical microscopes and transmission and scanning electrón microscopes. Various studies concerning the stability of the B phase which was retained after quenching in the Zr~107o Nb alloy, show the difficulty in modifying it by thermomechanical treatments. In the ternary system Zr-Nb-Al it is possible to induce a martensitic phase transformation by plástic deformation, in spite of the formation of w precipitates after quenching. Changing lightly the Nb content around 7 wt% it is feasible to obtain spontaneous martensite a' by quenching. Also a martensite is observed which is called "bended" because of its morphological characteristics. Increasing Nb approximately by 0,5 wt%, the B phase is retained on quenching, and subsequently a martensite can be stress induced. Experimental results on deformation which have been performed at different temperatures between -193 and 100°C are presented, including an analysis of the morphology and amount of plates formed at each temperature, and also observations of the fractured zone and the TRIP effect. From multisurface trace analysis for both a' and we found a (334)^ type habit plañe, a (lOll) type twinning plane and a fine substructure inside the twins formed by screw dislocations with Burgers vector type 1/3<1210>. Also, in each case the orientation relationship between the plates and the matrix are given, by the so called Burgers relations (00 01 )a ,
(0 1 1 ) and [ 1 1 2 0 ]a ,
[ 1 1 1 ]fl. By comparing the measured macroscopic shear modulus S = 0,114 with the calculated one, S = 0.188 it is indicated that the internal substructure is due to plástic deformation inside the píate in order to reduce S and not to the transformation itself. This result, together with an analysis of the elastic moduli, can explain the high temperature or strain hysteresis between transformation and retransformation, placing the martensitic transformation for Zirconium alloys as an intermedíate case between those for brasses and steels. Crystallographic calculations using the WLR theory, show that the < c+a > type dislocations can describe the lattice invariant shear of the transformation but not the <a> type. The system {1011} [2113] adjusts to the measured habit plane.