Interaction between nonstoichiometric titanium carbide and Fe-C alloys
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I. INTRODUCTION
CARBIDE-FERROUS metal cermets consist of either spherical or angular ceramic grains enclosed by the metal phase. The metallic matrix completely wets the carbide particles, resulting in a product that combines high hardness with satisfactory toughness. Cermets, based on the titanium carbide phase incorporated in low-carbon or high-alloy steels, display some unique characteristics because they can be annealed and worked by normal machining techniques and subsequently hardened by the martensitic transformation that takes place in the metallic phase.[1–4] It was reported[5] that cermets in which the binder phase undergoes a martensitic transformation display fracture toughness values much higher than conventional WC-Co cermets. The phase transformation and the ability to undergo hardening strongly depend on the chemical composition of the metal matrix. The latter can be changed by metal-carbide interaction at high temperatures. Several investigations[6–9] have shown that the chemical interaction between Fe alloys and TiC plays a very significant role in determining the physical and mechanical properties of cermets. It was suggested that this interaction could be taken advantage of in order to change the chemical composition of the metal matrix and to tailor its properties. Few studies,[10,11,12] however, have dealt with the effect of the composition of the carbide phase on the chemical interaction and the resulting properties of the composite. Most investigators assume that the stoichiometry of TiC does not change in the course of the fabrication of the composite. The composition range of this carbide extends, however, from TiC0.48 to TiC0.98. It is important to realize that the interaction between titanium carbide and the metal matrix during liquid-phase sintering or free-metal infiltration may alter the composition of the phases present
in the composite. These changes, especially the changes in the carbon content of the interacting phases, can affect the properties of the resulting composite, because the subsequent heat treatment to which it must be subjected depends significantly on the carbon content of the metallic phase. Recently, the results of the thermodynamic analysis of the ternary Fe-Ti-C system at 1773 K and some experimental results concerning TiCx-Fe composites have been reported.[10] This study presents the extended results of thermodynamic analysis of the ternary Fe-Ti-C system in the 1773 to 1873 K temperature range and some microstructural characteristics of TiCx-Fe composites based on titanium carbide with a low carbon content. II. THERMODYNAMIC CONSIDERATIONS A schematic isothermal section of the Fe-rich region in the Fe-Ti-C system, in the temperature range from 1773 to 1873 K, can be constructed (Figure 1) on the basis of the known binary-phase diagrams Fe-Ti, Fe-C, and Ti-C and in accordance with the main principles of ternary-phase diagram evaluation. This section contains the single-phase region (I) of the Fe-Ti-C liquid solution (FeL), the two-phase regions corresponding to
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