The study of intelligent materials has instigated many applications within the various knowledge areas (from medical field to aerospace industry). The most used materials in intelligent structures are the shape memory alloys (SMA), the piezoelectric ceramics, the magnetostrictive materials and the electrorheological fluids. In the last decades, SMAs have received special attention, being mainly used as sensors or actuators. There is a number of phenomena related to these alloys that can be explored. Aiming a more precise analysis of SMA behavior, the interest on the development of mathematical models capable of describing these phenomena properly has grown, allowing to explore all their potential. The aim of this work is to propose a unidimensional constitutive model which considers four microconstituent variants (austenite, martensite induced by temperature, martensite induced by tensile loading and martensite induced by compressive loading) and different material properties for each phase. The effect of thermal strains is included in the formulation. The model considers the effect of plastic strains and the plastic-phase transformation coupling. Besides, some changes are introduced in the formulation in order to enlarge the stress-strain hysteresis loop, resulting in better agreements with experimental data. Eventually, the tensioncompression asymmetry is incorporated. The model validation is obtained through the comparison between the numerical results given by the model and experimental results found in the literature for tensile tests at different temperatures and for tension- compression asymmetry.