Formation of the microstructure of a composite during its dynamic loading

Abstract

621.762It was shown in [1] that the use in hard alloys of a structurally unstable binding phase permits a significant improvement of their toughness and ductility while the strength level under quasi-static loading conditions is maintained. The studies make it possible to distinguish several basic features of the material's behavior under loading: in the composite, internal compressive stresses are formed, and the structure changes as a result of a phase transformation in the matrix under action of external loading; this provides for distortion in any small volume of the material and for its simultaneous strengthening.The physical meaning of the use of a structurally unstable binding phase in composites consists in lowering the structural level of plastic deformation as a result of the formation of a microcrystalline state of the binding phase in the course of inhomogeneous loading. According to [1], such materials include the alloys TiC-TiNi, WC-NiAI, WC-G13 steel, etc. Apparently, these effects will also be preserved under dynamic loading conditions, providing for excellent mechanical properties of the composite. The object of the present work was to study the macro-and microstructure of the hard alloy, WC-G12 steel, with a stable and metastable state of the matrix after dynamic loading. The loading was carried out by the impact of a ball element made from this alloy against a plate of aluminum alloy at velocities of 700-2000 m/see.In the case of a stable state of the binding phase, impact loading results in fracture of the ball element whose fragments remain in the barrier and are partially removed from it. If the matrix is in metastable state, dynamic action does not impair the continuity of the material of the striker; at different speeds of collision, only the shape of the striking element changes. From the change in the geometrical dimensions of the striker, according to [2], one can calculate the dynamic yield strengthLI-/, where p is the density of the material; V is the velocity; L 0 and Lf are the initial and final lengths of the striker; h is the depth of the plastic front. As is evident from formula (1), to determine the dynamic yield strength, it is necessary to measure the initial and final lengths of the striker and the depth of the plastic front. Since the size of the structural elements of the composite is very small (for example, the size of the carbide particles is 1-2 ttm), metallographic studies can not be used to determined the depth of the plastic front. On the other hand, since the geometrical shape of the specimen has changed, it may be assumed that the entire volume of the material will undergo plastic deformation. With this assumption, the dynamic yield strength was calculated (Fig. 1).As the deformation rate increases, so does Y0; for V = 1000 m/see it amounts to 1700 MPa, and for V = 1800 m/see, 3600 MPa, and exceeds the yield strength %.2 by a factor of 3.5, as well as the ultimate strength (Fig. 1). As the velocity increases, a marked increase in Y0 is observed, and when V = 1600 ...