Pascal Blanc

Experimental and numerical determinations of the initial surface of phase transformation under biaxial loading in some polycrystalline shape-memory alloys

Biaxial proportional loading such as tension (compression)–internal pressure and bi-compression tests are performed on a Cu-Zn-Al and Cu-Al-Be shape memory polycrystals. These tests lead to the experimental determination of the initial surface of phase transformation (austenite→martensite) in the principal stress space (σ1,σ2). A first “micro–macro” modeling is performed as follows. Lattice measurements of the cubic austenite and the monoclinic martensite cells are used to determine the “nature” of the phase transformation, i.e. an exact interface between the parent phase and an untwinned martensite variant. The yield surface is obtained by a simple (Sachs constant stress) averaging procedure assuming random texture. A second modeling, performed in the context of the thermodynamics of irreversible processes, consists of a phenomenological approach at the scale of the polycrystal. These two models fit the experimental phase transformation surface well.

What about the Yield Transformation Surface Determination (Austenite → Martensite) with The Measurement of Austenite and Martensite Lattice Parameters for some Shape Memory Alloys?

Like in the plasticity theory, the prediction of phase transformation yield surfaces constitutes a key point in the modeling of polycrystalline shape memory alloys thermomechanical behavior. Generally in some micro-macro integration, the nature of the interface between austenite and twinned or untwinned martensite under stress free state and the choice of correspondance variants (CV) or habit plane variant (HPV) are determining for the explicit expression of the yield criterion. If the prediction of some copper-based alloys (interface between austenite and one single variant of martensite) and the Cu-Al-Ni for cubic to orthorhombic phase transformation (interface between austenite and twinned martensite) is fairly good, the prediction is not efficient for the important case of Ti-Ni (interface between austenite and twinned martensite with stress free state). The usual hypothesis consisting in neglecting the effect of stress on the interface geometrical configuration must be revisited.