Modeling phase fraction shakedown during thermomechanical cycling of shapememory materials

Shape memory actuation typically involves thermodynamic paths traversing re gions of the stress-temperature phase diagram that support microstructures involving coexistence of austenite with multiple martensite variants. The f ractional partitioning between these different phase/variant species depend s on the previous path history. We discuss a simple model for the role that repeated stress-temperature cycling has on selecting the microstructural v ariants. The treatment is in terms of transitions between the austenite pha se {A} and two martensite variants {M +} and {M -} with equal and opposing transformation strains. Purely temperature induced transitions then involve {M -, A, M +} mixtures with equal amounts of {M -} and {M +}. The {M +} va riant is favored under positive stress, and positive stress transformation activity reflects this bias by a greater incremental gain in {M +} and/or a greater incremental loss in {M -}. Stress-temperature cycling then gives a {M -, A, M +}-partitioning that approaches a periodic limit cycle, giving rise to a limiting periodic strain response. (C) 1999 Elsevier Science S.A. All rights reserved.