Theoretical and experimental investigations of [Ti0.75(Mo1-xWx)0.25](C0.75N0.25) system

Abstract

We developed and applied both experimental and modeling methods to study complex TiC-based solid solution materials, for their sintering, microstructure, mechanical, elastic, thermodynamic and electronic behavior or properties. Although our applications were to specific compositions, [Ti0.75(Mo1-xWx)0.25](C1-yNy) system, the techniques are expected to be applied in many different material and areas. The thermo equilibrium and elastic behavior of sub-stoichiometric TiC1-x and TiN1-x were investigated at the first step. It was found that the cluster expansion method (CEM) was suitable to modeling the vacancies issues in TiC and TiN rather than other methods, especially the ordered supercell method. The thermo equilibrium results well agreed with the experimental data, where x can be in the range of 0-1/3 and no-freedom for TiC1-x and TiN1-x, respectively. The elastic properties calculations illustrated that when x locates in the range of 0-0.1, the balance between producing cost and mechanical properties can be reached in TiC1-x. The reason TiC1-x can form is due to the raising of d-d metallic bonding, which can be observed by electronic density of states (DOS). The ordered models were used to represent (Ti0.75W0.25)(C1-yNy) and (Ti0.75Mo0.25)(C1-yNy) systems. Compared to CEM results, the ordered models can produce valid and reliable results in this case. The linear response method was applied to introduce the temperature effect to lattice for the predicting of elastic properties at a finite temperature. The results showed that nitrogen can increase the elastic properties of solid solution at low temperature. But, when it increased to around working temperature, about 800-1000oC, higher nitrogen contained compounds lose the elastic properties more rapidly. This is because W and N are hardly to co-exist at high temperature, the bonds between N and Ti weakened much faster than C-Ti and C-W. Thus, the most balanced composition should be (Ti0.75W0.25)(C0.75N0.25), which can provide high elastic properties at working temperature, and more thermo stable than (Ti0.75W0.25)C. In (Ti0.75Mo0.25)(C1-yNy), the similar behavior was found. For [Ti0.75(Mo1-xWx)0.25]C and [Ti0.75(Mo1-xWx)0.25](C1-yNy) pseudo-ternary systems, CEM became computationally expensive. The ordered models were also difficult to build. In order to solve this problem, a new modeling method, combining virtual crystal approximation (VCA) and ordered structure, was introduced. The error from VCA was corrected by linear response method. This method can produce the same results as CEM did in the case of [Ti0.75(Mo1-xWx)0.25]C. Applying it to the carbonitride pseudo-ternary system [Ti0.75(Mo1-xWx)0.25](C1-yNy), it was found the higher W concentration phase is more stable at high temperature. More important. The average elastic moduli obtained from calculations well matched with the experimental data, which were measured by ultrasonic for spark plasma sintering (SPS) specimens. Thus, the new method was considered to be valid for the pseudo-ternary systems.