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13FEATURES OF GRAIN STRUCTURE AT SHS EXTRUSION FOR MATERIAL BASED ON TiC + Co
L. S. Stelmakh*", A. M. Stolin", and P. M. Bazhin"
aMerzhanov Institute of Structural Macrokinetics and Materials Science, Russian Academy of Sciences, Chernogolovka, Moscow, 142432 Russia *e-mail: stelm@ism.ac.ru
DOI: 10.24411/9999-0014A-2019-10166
In recent years, a promising direction in the field of materials science of metal-ceramic materials for various purposes is the study of the possibilities of refinement of the grain structure under conditions of combining combustion processes with high-temperature shear deformation of combustion products, which is implemented in the conditions of SHS-extrusion [1, 2]. This method provides very wide possibilities for controlling the process characteristics by parameters and deformation modes and, accordingly, by the structure of materials. In recent years, a large number of studies have been devoted to studying the effect of high-temperature shear deformation and heating conditions on the structure and properties of metal-ceramic materials during SHS extrusion. However, theoretical models explaining the patterns of formation and subsequent evolution of the structure of materials during hot deformation are not sufficiently developed. In the present work, based on modeling the thermal conditions of SHS extrusion, theoretical and experimental studies of grinding the grain structure during SHS extrusion and studying the features of the formed structure of alloys based on titanium carbide with a cobalt bond depending on temperature and degree of deformation were carried out. A mathematical model of the thermal conditions of the SHS extrusion process was developed earlier [3]. This model allows us to explore the temperature fields in the sample material in a cylindrical mold, heat insulator and extruded rod depending on various technological parameters of the process (combustion temperature, delay time, press plunger speed, heating temperature of various equipment zones, etc.) and predict the length of extruded rods. Since grain growth depends exponentially on temperature, information on temperature fields during the synthesis, pressing, and molding of a material can be very useful in obtaining a compact quality material. The grain growth kinetics in the final product of the synthesis reaction is investigated depending on the temperature, which according to [4] is described by the following equation:
dP _ k0 exp(-E /RT)
~dt ~ P
where D = D (r, z, t) is the flowing size (diameter) of the combustion product grain, which is a function of two coordinates r and z and time t, h is a predictor, E is the activation energy of grain growth, R is the universal gas constant, Tis the temperature, h is the exponent, the largest close to unity.
It is assumed that when the material passes from the chamber to the caliber through the profiling matrix, the grain size decreases according to the law [5], depending on the degree of deformation:
P = P
Am(e/eKp )2/3
where e is the degree of deformation, e = (r02 - r12)/r02), r0 and r1 are radiuses of the sample and the rod after extrusion, respectively, eCT is the critical strain under which the nucleation of
ÏSHS2019
Moscow, Russia
recrystallization occurs (sCT = 0.1 [5]), A is the coefficient of shape of the area of the original grain boundaries (4tc/3(A(6). In numerical calculations, data for the composition of TiC (70%) + Co (30%) were used. The calculations showed that the distribution of grain size in the material located in the chamber and located above the hole of the profiling matrix is uneven in height of the mold (Fig. 1).
Fig. 1. Grain size distribution (D) in the chamber (from 0 to H) and the rod (from H to L).
Direct 1 separates the chamber and the extruded rod.
Before the hole of the profiling matrix in the chamber, the average grain size is 11 |im, and when it leaves the chamber, it is crushed: when the material passes from the chamber to the caliber through the profiling matrix, the grain size changes from 7.7 to 3 ^m. Then, for most of the core, the grain size is smaller than 3 |im, and starting from 21 mm and to the end, it is uniform both in length and radius and is less than 2 |im.
Figure 2 shows the distribution of grain sizes along the length of the extruded rod and along the radius, which is uniform throughout the volume of the material.
Fig. 2. Grain size distribution in the rod: hole diameter is 3 mm. The length of the rod is 102 mm.
This is confirmed by experimental data (Fig. 3). To study the grain sizes of titanium carbide, 3 types of samples were obtained, the microstructure of which is shown in Fig. 3: (a) press residue, (b) and (c) images taken from the middle and edge of an extruded rod 3 mm in diameter. The press residue after SHS extrusion is a material that has been solidly compacted, but not extruded. Under the action of normal stresses, the material compacted and, due to the applied external pressure, the average grain size of titanium carbide decreased to 3.8 |im. The sizes of the largest grains also decreased to 7.4 |im. The average grain size in the extruded rod decreased to 2 ^m. Due to normal and radial stresses, the material under extrusion underwent high degrees of deformation (0.99), which led to a significant refinement of grains. It should be noted that the grain size in the samples (Figs. 3b, 3c) is almost identical, which indicates their uniform distribution along the length of the rod. Moreover, the distribution of grain sizes in the rod is uniform.
Fig. 3. Characteristic microstructure of the obtained samples: (a) press residue, (b) middle of the extruded rod, (c) edge (bottom) of the extruded rod.
Conclusions:
The proposed mathematical model of thermal regimes of SHS extrusion can be used to predict the pattern of grain distribution in an extruded rod.
It is theoretically and experimentally shown that with SHS extrusion, combining the action of shear deformations and pressure, the refinement of grain is realized due to the action of tensile stresses and shear deformations.
It was established that at the exit from the chamber the grain size along the length of the rod varies from 7.7 to 3 p,m. Further, on the most part of the rod, a uniform distribution of grains over the material volume is realized, and this part of the rod has an average grain size of 2 |im.
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