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In the last 15–20 years significant success has been achieved in studying the structure and behavior of grain boundaries, and this has led to the appearance of concepts such as grain-boundary dislocations, grainboundary crystal geometry models, and grain-boundary splitting, sliding and migration. The inclusion/matrix boundaries have been studied very little, although the problem of these boundaries is even more complex than the grain boundary problem. The configuration of the inclusion/matrix interphase boundaries in steel and the possible changes of their initial structure in the process of mechanical and thermal treatment are amenable to description in boundary structure defect terms.

The possibility of producing strong bimetallic joints with interaction between different compositions of gray cast irons, which contain different shapes of graphite and which have a ferritic or austenitic matrix, and carbon steel is investigated. Elementary processes that take place in the vicinity of the free contacting surfaces of the metals during forging, explosion, and thermal cycling are studied. Treatment regimes are proposed for the production of bimetallic joints. 12 refs., 5 figs.

In metallic materials the process of strengthening by dispersed particles accelerated by explosive energy is a multifactor one and consists of reinforcement by channels and channel zones, amorphization and microalloying, dispersion hardening by the particles, and deformation strengthening as the result of local plastic acts of relaxation type. To a large degree the character of the latter is determined by the type of crystalline lattice of the matrix.

In dynamic interaction of powder particles and the steel matrix at the points of their retarding and stoppage, significant high-velocity plastic deformation occurs according to shear, rotational, and twinning mechanisms, which, together with local microalloying of the matrix with elements of the particles, promotes additional hardening of the steel. Processes of complete restoration of the structure in these areas are undesirable since they reduce the effect of hardening of the steel with dispersed particles.

In high-temperature deformation of steel slippage is observed along the inclusion-matrix interphase boundaries for different types of nonmetallic inclusions. Slippage along inclusion-matrix boundaries causes an increase in crack resistance of the steel at high temperatures.

In the matrix of the steels E3 and 08Kh18N10T close to nonmetallic inclusions under explosive treatment conditions, relaxation processes of the translational-rotational type occur. These are wavelike in character and lead to the formation of structures displaying complex deformation within the relaxation waves.

Most nonmetallic inclusions are brittley ruptured in very rapid deformation, or brittle laminations form on their interface with the matrix. Ductile sulfides are nonuniformly deformed. In the matrix deformation is localized around the inclusions. Besides, zones adjacent to inclusions are characterized by the development of deformation by the rotational mechanism.

The red-shortness of steel is caused by sulfide and oxysulfide nonmetallic inclusions, the types of which are determined by the composition of the steel. The melting points of the inclusions are FeS−FeO 980°C, (Fe, Mn)S−FeO 1120°C, (Fe−Mn)S−FeS 1050°C, (Fe, Mn, Cr)S−(Fe, Mn)S 1190°C, and (Fe, Mn, Cr)S−FeO 1250°C. At lower temperatures the eutectic inclusions are plastic but the development of their deformation and failure is determined by the temperature and degree of plasticity of the surrounding steel matrix, which determine the critical parameters of fracture development.

1. Nonmetallic inclusions promote strain localization under compression, and their effect depends on temperature which governs matrix and inclusion ductility. Near rigid inclusions marked strain microinhomogeneity develops (at a maximum near the 0° pole, and a minimum near the 90° pole in relation to the external compressive stress), promoting formation of microfailure in inclusions. 2. Ductile inclusions control development of strain for the matrix of low-carbon steels, which due to high ductility is adapted to inclusion deformation. The level of inhomogeneity for strain development near ductile inclusions is less than that near rigid inclusions. Cavities forming during melting of sulfide inclusions promote development of significant strains, however, their distribution is different from near rigid inclusions.

The method of deformation influences the character of microfailures at non-metallic inclusions, their critical size and critical relative degree of deformation upon reaching which voids and cracks originate, the rate of their growth, and the contribution of the nonmetallic inclusions to development of failure of the steel. In addition, in all of the investigated methods of loading the deformation temperature has a strong influence on the parameters of development of microfailures. For the plastic matrix-nonmetallic inclusion system tension is the most severe method of deformation.

1. Brittle nonmetallic impurities are a source of microcrack nucleation in steel at the early stages of deformation at all the investigated temperatures. The temperature and type of impurities determine their critical size, the critical degree of deformation at which cracks begin to nucleate in the impurities; they determine the interval of deformations in which impurities are the sole source of crack nucleation; the role of impurities in the development of fracture and the number of impurities in which cracks nucleate. 2. The mechanism of the origin of microfractures (cavities or cracks) also affects the investigated parameters. At higher deformation temperatures the role of impurities in the development of fracture of steel 08T becomes less important.

An account of the synthesis of diamond in cast iron by the use of detonation waves was published more than a quarter of a century ago [i]. Detonation synthesis of diamond in cast iron requires pressures greater than 60 GPa, depending mainly on the density and velocity of the impacting bodies (i.e., the specimen and the detonation device). The most widely used scheme for detonation treatment of cast-iron specimens is by compression with a planar detonation wave (Fig. i). Methods for calculating the detonation adiabats of the media under investigation, taking into account the elements entering into their composition, have been worked out in order to determine the basic kinetic and thermodynamic parameters. Analysis of experimental results and calculated detonation adiabats for cast iron and graphite make it possible to specify the range of pressures necessary for the synthesis of diamond as 60i00 GPa. A pressure variation in graphite inclusions from 23 to 50 GPa lies within this range, wherein the lower value represents the beginning of transformation of the graphite lattice into that of diamond, and the higher the complete transformation of graphite into diamond. Gray cast irons with flake and spheroidal graphite, alloyed with Si, Ni, and Mn, were studied (see Table i). The parameters for detonation treatment of the cast-iron specimens were as follows: pressure 90 GPa, deformation rate i00 sec -l, pressure action time i0-~-I0 -7 sec. A portion of the specimens were forged at 950°C to 50% reduction at a rate of 10-30 sec -I before detonation treatment. Furthermore, after detonation treatment (without preliminary forging) specimens were thermal cycled following the procedure: heat at 150°C/min to 900-950°C, hold for 20-30 min, furnace cool (five cycles). The cast-iron microstructures were examined using the optical microscope "Neophot-21" and the scanning electron microscope JSM-35. One of the unique features of detonation loading is heating of the specimen under impact. The temperature field in cast-iron specimens at the moment of impact and during release of the load is inhomogeneous, due to the different wave impedances of the graphite and matrix. This results in the effect that at the detonation wave front graphite is heated to higher temperatures than the surrounding matrix. The higher the pressure, the higher the graphite temperature [2]. Consequently, there is a pressure threshold above which the synthesized diamond reverts to graphite. In order to avoid this reversion a load of minimal amplitude and maximal duration is usually selected. The heat in the graphite is rejected to the matrix of the cast iron. However, the pressure decreases more rapidly than the temperature. Insufficiently rapid temperature decrease is a major obstacle to preserving the diamond polycrystals. The shaded region IV in the graphite-diamond diagram shown in Fig. 2 is that in which the phase transformation occurs. Depending upon the point (a or b) at which the transformation to diamond occurred, the residual temperature of the specimen might lie in temperature field I or II. In temperature field I the probability of retaining the 4iamond is satisfactorily high. If the final temperature of the specimen lies in field II the reverse transformation diamond + graphite will most likely occur. The detonation loading time amounts to i-2 ~sec. The transformation of graphite to diamond may occur not only by a martensitic transformation [4], but also via fluctuations within the body of the graphite inclusions [5].