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The paper examines the effect of the initial structure and prestrain on the yield stress of a number of ageing steels and alloys under combined loading in the cases of like and opposite directions of preloading and reloading. The parameters of yield loci are calculated for the case of combined two-axial tension. Their predicted dependence on temperature, ageing time, and prestrain is experimentally checked. As they increase, strain hardening changes from isotropic to kinematic and then to mixed Keywords: steels and alloys, metastable and stable structure, combined uni-and biaxial loading, yield stress, Bauschinger effect, yield locusIntroduction. The issues of efficient use of metals in current technology and optimal thermomechanical conditions for the fabrication and service of their structural members necessitate establishing the physical laws governing the formation of strength anisotropy depending on the level and technique of preloading and reloading. Recently, theoretical and experimental research studies in this area have become much more intense. For example, the strain hardening/softening model for metallic materials describes the evolution of the dislocation boundaries of substructural elements as obstacles to slip upon changes in the loading direction [16]. The dissolution of dislocations in reversely deformed alloys depending on alloying and thermomechanical preloading are studied in [17,19] in connection with the Bauschinger effect. However, there seem to be few or even no attempts to use the results of such studies to predict the mechanical behavior of structural metals with different preloading and reloading paths from their structural characteristics.One of the approaches to solving this problem for structural metals with mixed (isotropic and kinematic) strain hardening [4][5][6]8] is to examine the structural dependences of the parameters of yield loci-the radius and center shift under combined loading [1,2]. This approach is based on the possibility of determining, in the first approximation, the parameters of yield loci as functions of the compressive and tensile yield stresses of pretensioned metal, or as functions of the ratio of these stresses (a measure of the Bauschinger effect) [1,11]. With such an approach, the least understood issue is the dependence of the yield stress under combined loading (say, compression after pretension) on the prestrain, which is determined by the interaction of the preloading and reloading processes.Such research is even more important for ageing structural steels and alloys widely used in modern engineering. A distinctive feature of such materials is gradual (as they age) transformation of the initial metastable single-phase states into stable multiphase states (equilibrium solid solutions of matrix phases and dispersed particles of hardening phases) [15]. In metastable states, solid solutions of matrix phases are supersaturated with intermetallic-compound-or carbide-forming alloying elements and, hence, are susceptible to strain ageing [10], which may change ...

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