Lee Hsun Lecture Series

Topic: UNEXPECTED CONSTRAINED TWIN HIERARCHY IN RU-BASED HIGH TEMPERATURE SHAPE MEMORY ALLOYS MARTENSITE

Speaker: Richard A. Portier

　　　　 Structural metallURGY GROUP；CHIMIE-PARIS TECH, FRANCE

Time: 10:00-11:30 AM., Wed., Apr. 03, 2013

Venue: Room 403, R&D Center, IMR CAS

Abstract:

As a consequence of a structural phase transformation with a decrease of the symmetry between the parent phase and the martensite, a finite number of variants are 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 (numbering of variants, associated coset for the interfaces between variants, determination of the genetated twins – simple arguments using stereographic projection help this determination–, bicrystallographic description). 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). However, the situation can be different when the martensitic transformation is constrained by some structural or mechanical features.

RuNb and RuTa alloys exhibit martensitic transformations in a range of temperature above 1000K providing them an interesting high temperature shape memory effect and promising potential actuation applications in aeronautic industry. If equiatomic compounds undergo two successive martensitic transformations, β (B2) → β’ (tetragonal) → 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 an unexpected microstructure with twins with a thickness which is larger than the average width of the twinned variants of the first martensite in certain cases.

In fact, the second martensitic transformation takes place in special conditions which resul of a:

- geometrically constrained transformation (the very small thickness of the twin domains generated during the 1^st martensitic transformation)

- elastically constrained transformation (the stored elastic energy increases when temperature decreases in the temperature existence domain of the 1^st martensite)

- crystallographically constrained transformation (the twin planes induced during the 1^st transformation are inherited by the second martensite and the twinning property is maintained).

DSC, X–Rays and neutron diffraction (versus temperature), SEM, TEM and EBSD technic, have permitted to understand this unexpected microstructure and allowed explaining some unexpected aspects of the shape recovery behaviour of equiatomic alloys.