Unexpected Constrained Twin Hierarchy in Equiatomic Ru-Based High Temperature Shape Memory Alloy Martensite

Abstract

Among the different systems for high temperature shape memory alloys (SMA’s), equiatomic RuNb and RuTa alloys demonstrate both shape memory effect (SME) and MT temperatures above 800°C. Equiatomic compounds undergo two successive martensitic transformations, β (B2) → β’ (tetragonal) → β’’ (monoclinic), whereas 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 different unexpected microstructures with, for instance, twins with a thickness which is larger than the smallest spacing between twin variants of the first martensite. In fact, the reason for this unexpected hierarchy of the twins size is that the second martensitic transformation takes place in special conditions: geometrically, elastically and crystallographically constrained.

Constrained hierarchical twinning in Ru-based high temperature shape memory alloys

“…The room temperature microstructure of all six alloys has been observed in terms of twinning size, twinning scales, misorientation between same order twins and different order twins. The well-known {101} T compound twinning, which has been shown before [10,11,15,16], is present in all investigated alloys, but it will be completed by a second type of twinning in the equiatomic alloys, which is unique in its way of forming inside an already existing twinned microstructure.…”

Geometry of polycrystals and microstructure

“…The conclusion of Theorem 4 contrasts with observations of polycrystalline materials undergoing cubic-totetragonal phase transformations, but for which some grains are completely filled by a single double laminate. Such cases arise for the ceramic BaTiO 3 [13] and in various RuNb and RuTa shape-memory alloys [14][15][16][17]. Arlt [13] gives an interesting qualitative discussion of energetically preferred grain microstructure, drawing a distinction between the microstructures in interior and boundary grains.…”

“…Additional factors could include cooperative deformation of different grains (so that the assumption of a pure dilatation on the boundary is not a good approximation), some of which may have more complicated microstructures than a double laminate. It is interesting that third-order laminates are observed for RuNb alloys undergoing cubic-tomonoclinic transformations [16].…”

“…Such cases arise for the ceramic BaTiO 3 [13] and in various RuNb and RuTa shape-memory alloys [14][15][16][17]. Arlt [13] gives an interesting qualitative discussion of energetically preferred grain microstructure, drawing a distinction between the microstructures in interior and boundary grains.…”

Martensite crystal structure in Ru-based high temperature shape memory alloys

“…The final microstructure at room temperature exhibits a complex distribution of variants which are generated by the two successive transformations and which can be described by a three level twinning. In the smallest twin domain, a wavy contrast can be observed [4][5][6][7][8]. The microstructure has been described in detail in [9] and is shown in Figure 1 as a reminder.…”