Stressed Microstructures in M9R-M18R Martensites

Friday, April 15, 2005 - 9:30am - 10:30am
EE/CS 3-180
Giovanni Zanzotto (Università di Padova)
Joint work with Xavier Balandraud (Laboratoire de Mécanique et Ingénieries (LaMI), Institut Français de Mécanique Avancée (IFMA), Université Blaise Pascal (UBP).

We revisit the phase transformation that produces monoclinic 'long-period stacking' M9R or M18R martensites in Cu-based shape-memory alloys, and analyze some associated microstructures, in particular a typical wedge-shaped configuration (Fig.). The basic premise is that the cubic-to-monoclinic martensitic phase change in such alloys is, geometrically, a slight modification of the well-known bcc-to-9R transformation occurring for instance in Li and Na, whose basic strain, at the micro level, is the same Bain strain as for the bcc-to-fcc transition. One then determines the 'near-Bain' microstrain variants pertaining to these elements and alloys, and analyze the long-period stacking martensite as a mesoscale 'adaptive phase.' Twins, habit planes, and also more complex microstructures, such as the CuZnAl wedge, can be analyzed in this way. Earlier conclusions that this microstructure is not kinematically compatible at zero stress are confirmed. However, one can check the wedge is `close enough' to compatibility and compute the corresponding stresses, which turn out to be low, causing only minimal plastification and damage in the crystal. This microstructure is therefore rationalized as a viable path for the transformation also in these alloys. One can moreover verify this to be true for all the known lattice parameters reported for materials exhibiting long-period M9R-M18R martensites. The general conclusion is that the observed martensitic microstructures can be stressed to various degrees also in good memory alloys; and that there seem to be no need for material tuning in order tgat such stresses be low. Indeed, the lattice-parameter relations, guaranteeing the zero-stress compatibility of special configurations, favoring the transformation and its reversibility, do not need to be strictly enforced because microstructural stresses are not very sensitive to lattice parameter values.