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Abstract

At high impurity concentrations in metal, the precipitation and phase transformations are observed. The precipitation of second phases in metals and metallic alloys is an important phenomenon that has a high influence on the mechanical properties of the material and for the improvement of existing steel grades. They are the most relevant microstructural constituents with respect to hardness and thermal stability. The precipitation growth can be coherent, semi-coherent or incoherent with the original matrix of the metal. In the present project, the molybdenum-carbon binary system is being studied thoroughly by varying carbon concentrations in the system. In a previous computational study[1], the precipitation behavior of body-centered tetragonal(bct) carbide, MoC_x at a grain boundary has been investigated. The study showed that there is a significant strain contribution due to the lattice misfit to the interface energy. The aim of this work is to quantify this strain contribution and its effect on the stability of the metastable bct phase to the interface energy by means of ab-initio calculations using density functional theory with the VASP [2] code.

In a study for bulk materials, the structural stability of various phases of the Mo-C system has been analyzed and compared with experimental results where available. The phase diagram(PD) using the description of [3] has been obtained using Thermo-Calc [4], and the stable phases agree with our predictions at T=0K as depicted in fig.1. A metastable bct phase, which is not present in the PD, has been observed experimentally by HREM as a semi-coherent precipitate [5]. We assume that it is stabilized by the precipitate interface energy as given in the table-I. Further the work of separation for different lattice parameters was obtained. With the addition of carbon to the body-centered cubic(bcc) Mo, the compression and tensile tests were performed to calculate the energy corresponding to strain variation of the system. This strain contribution has been included in the interface energy calculations for Mo(001) and MoC_x(001). The next step is to determine the semi-coherent interface energy. For this the gamma surface calculations of the coherent interface have been carried out, which in addition yields possible Burgers vectors for the misfit dislocations.

References:
[1] Rebecca Janisch, Christian Elsässer, Physical Review B, Vol.77, 094118, pp. 1-9, 2008
[2] http://cms.mpi.univie.ac.at/vasp
[3] Thermodynamic properties of Mo-C, Jan-Olof Andersson, CALPHAD, Vol. 12, No. 1, pp.1-8,1988.
[4] www.thermocalc.de
[5] J.M.Pénisson, M.Bacia, M.Biscondi, Philosophical Magazine A, Vol. 73, No.4, pp. 1-11, 1996