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Abstract
As a consequence of a structural phase transformation with a decrease of the symmetry between the parent austenitic phase and the martensite, a finite number of variants is generated in each grain of the parent phase. They can be counted using simple group theory argument and each of them has the same probability of nucleation. Each lost symmetry operation of the parent phase transforms one martensite variant into another one. In the case of martensitic transformation, because of the unit cell transformation, the self accommodation of the variants is necessary for minimizing the deformation but the equiprobability of nucleation of the variants will lead to an equiprobability of the production of the self-accommodated group of martensite in each grain of the austenite grains (Ni-Ti, Cu-Al-Ni alloys). The situation can be different when the martensitic transformation is constrained by some structural features.
RuNb and RuTa alloys exhibit martensitic transformations above 1000K providing them an interesting high temperature shape memory effect. If equiatomic compounds undergo two successive martensitic transformations, β (B2) à β’ (tetragonal) à β’’ (monoclinic), out of stoechiometry alloys exhibit a single transition from cubic to tetragonal. In the case of two successive martensitic transformations, we expect to have a finer microstructure of the second martensite because it is supposed to develop inside the smallest twin elements of the former one. In equiatomic Ru-based alloys, if the first martensitic transformation is “normal”, the second one gives unexpected microstructures with twins with a thickness which is larger than the smallest spacing between twin variants of the first martensite. In fact, the second martensitic transformation takes place in special conditions and it is: geometrically constrained (twin variant with a few nanometers thickness for the first martensite), elastically constrained (with a large evolution of the tetragonal martensite on cooling in the temperature range of its existence domain, the stored elastic increases and acts as a trigger for the second transformation), crystallographically constrained (twins planes of the tetragonal martensite are inherited by the monoclinic martensite).TEM and EBSD, have permitted to understand this unexpected microstructure and allowed explaining some unexpected aspects of the shape recovery behaviour of equiatomic alloys.
CV
Richard A. Portier, Engineer from Chimie-ParisTech, received Dr. Sc. in Physic from Pierre and Marie Curie University in 1976. He was CNRS researcher in Paris and in Vitry (Center of Research for Chemical Metallurgy, CNRS institute). He was Professor in 1981 in Chimie ParisTech, teaching quantum mechanics, solid state physic, Phase Transformations, Mechanical Properties and defects, Material selection and Electron Microscopy. He is Emeritus Professor since 2009. His current research areas include Bicrystallography and Action Group Theory for Phase Transformations; Reconstructive and Displacive Phase Transformations; Commensurate and Incommensurate Modulated Structures, Complex Intermetallic and Quasicrystals; Metallic Glasses; Genesis and Evolution of Microstructures of Alloys. Major contributions from his group are focussed on the investigation of many Shape Memory Alloys and on the formalism of the Dynamical Theory and of Symmetry properties in Electron Microscopy.
Chimie-Paris Tech, 11, rue P. et M. Curie, 75231 Paris, France
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