Motivated by recent experimental results by Glasauer [7], a thermodynamic theory of shape memory alloys is proposed, which includes not only the high temperature - pseudoelastic - behavior but also the low temperature range of quasiplasticity. Due to the occurance of three different phases - austenite and two martensitic variants - several cases of two-phase equilibria and a three-phase equilibrium have to be taken into account. Their relevance is determined by minimization of the total free energy and subsequently illustrated by the construction of phase charts. A special point of interest is the influence of interfacial energy effects on these phase charts, resulting in phenomena like, for example, the apparent violation of Gibbs' phase rule. Furthermore, their role in the hysteretic load-displacement behavior is discussed in the light of the additional quasiplastic case.