No 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 ...

No abstract

No abstract

669.018.75 The behavior of homogeneous materials under high-velocity loading is a complicated process that depends on the properties of the constituents, the inner structure of the material, loading conditions, etc. Kul'kov et al. have studied [1] the shock loading of a composite material of the hard-alloy type in which the matrix could be either in a stable or in a metastable state. In the stable austenite state, the material fails without visible change in the structure of samples. If the matrix is in a metastable state, dynamic action with equal collision velocities does not lead to discontinuity of the material. In this case the material structure shows some peculiarities that are not characteristic of the stable state of the matrix: an increase in dislocation and deformation-twin density is accompanied by a martensite rearrangement of the matrix lattice and by the formation of a particular structure of mesocracks [1], which are arranged in an ordered fashion in the sample's volume.However, these fragmentary data on structural changes have not given a comprehensive idea of the behavior of materials under high-velocity loading. A more detailed analysis of structure evolution is required at various scale levels. The present work is a continuation of [1, 2] and is aimed at studying the evolution of the structure and phase composition of the material at different scale levels: a) the macrolevel, i.e., the sample as a whole or a part of it (1-10 ram); b) the mesolevel, the level of a grain (1-20 /.~m); c) the microlevel, a slip band, second-phase particles (10-7-10 -8 m) (in accordance with the classification of [3]), and their interaction.A hard alloy of tungsten carbide and high-manganese steel (30% by weight) was studied. The matrix was in the metastable austenite state formed by saltpeter hardening above 1370 K. The average size of the starting tungsten carbide grain, as determined by the secant method [4], was 2.5 #m.Samples were taken in the form of disks 60 mm in diameter and 4.5 mm in thickness. Shock loading was performed by a cylindrical steel element striking a plate of the studied alloy at a velocity of about 1200 m/sec.The macro-and microstructures of the loaded samples were studied with a NEOFOT-21 optical microscope. The changes in the phase composition of the material were recorded by the x-ray diffraction method on a DRON-UM1 setup with filtered copper radiation.The study of the loaded samples showed significant structure changes at the different scale levels. Let us discuss these sequentially.Macrolevel. The macropicture of fracture demonstrates that the disks are broken into large fragments due to propagation of radial cracks from the striker-target contact and is similar to that of WC-Co alloy fracture observed in [5]. On some of the fragments in the immediate vicinity of the puncture hole, one can see sickle-shaped cracks. On the rear side of the plate, a split with a diameter twice as large as that of the puncture hole is formed. The central part of the striker-target contact consists o...