The present paper is devoted to the problem of describing materials capable of structural transformations. Basing on two-component model of material with nonlinear internal force, we investigate the existence of non-stable constitutive curve containing a decreasing segment. For this purpose a kinematic loading of two-component rod is considered. The main goal is to determine the influence of the relative displacement on the stress-strain dependence and to establish the expression, connecting the position of the critical point on the diagram with the parameters of the microstructure.