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Title:Domain structure, phase transformation, mechanical behavior and shape memory effect in a rare-earth orthoniobate lanthanum niobate
Author(s):Li, Jian
Doctoral Committee Chair(s):Wayman, C. Marvin
Department / Program:Materials Science and Engineering
Discipline:Materials Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Engineering, Materials Science
Abstract:Rare-earth orthoniobates, such as LaNbO$\sb4$, undergo a tetragonal-to-monoclinic phase transformation. Both high- and low-temperature phases possess the scheelite structure, and the low temperature monoclinic phase is heavily domained. The transformation mechanism is unknown. The domain structure in the monoclinic phase was reported as "type III" twins with highly mobile boundaries, exhibiting a rubber-like behavior in monoclinic LaNbO$\sb4$ single crystals. This behavior implies a new dimension for ductile ceramics and smart materials and clearly indicates the necessity for a thorough and systematic investigation of rare-earth orthoniobates. In the present study, using LaNbO$\sb4$ as an archetypal system, the scope of the investigation has involved several central features: (1) determining the structure of the domains in the monoclinic phase; (2) establishing the transformation mechanism; (3) verifying the rubberlike behavior and (4) exploring the shape memory effect in polycrystal ceramic LaNbO$\sb4$.
By using SEM electron back scattering technique and TEM, it has been found that the domains orient in two different orientations, separated by a boundary with an irrational Miller index between $\rm (20\bar 4)\sb{I}/(402)\sb{II}$ and $\rm (20\bar 6)\sb{I}/(602)\sb{II}$ planes, which is in good agreement with the calculated boundary index by virtue of the elasticity theory and spontaneous domain tensor. The orientation relationship between the domains has been determined as a rotation of 95.6$\sp\circ$ about the (010) axis. A model for domain boundary configuration, a diffused boundary with a transition zone, has been proposed according to TEM diffraction patterns and HRTEM images. Based on this domain boundary model, a domain switching mechanism has been suggested. The driving force for domain switching has been considered.
The mechanism of the monoclinic-to-tetragonal transformation has been discovered as a second order transformation, and confirmed by the results from X-ray diffraction, DSC, dilatometer measurement and in-situ examination by optical microscope, and by comparing the calculated magnitude of spontaneous strain with the order parameter defined in Landau theory. A crystallographic model for the transformation, which suggests that two steps: shearing and atomic shuffling, are involved in the processes of the phase transformation, has been established and the reason for domain formation as a consequence of phase transformation has been discussed based on the consideration of volume change and volume accommodation during the phase transformation.
In the aspect of mechanical behavior, yielding phenomenon of polycrystalline LaNbO$\sb4$ in compression test has been found, which has been explained in terms of the orientation state of grains with respect to the direction of the applied stress. The shape-memory-effect-like behavior has been discovered instead of the rubber-like behavior in monoclinic LaNbO$\sb4$, which is related to the domain boundary motion and new domain formation in response to the applied stress. Based on the observation of domain formation in the vicinity of the indentation, the mechanism for the shape-memory-like effect has been suggested, and compared with that for the shape memory alloys.
Issue Date:1995
Rights Information:Copyright 1995 Li, Jian
Date Available in IDEALS:2011-05-07
Identifier in Online Catalog:AAI9624416
OCLC Identifier:(UMI)AAI9624416

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