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Abstract

Segregation and precipitation of second phases in metals and metallic alloys are complex
phenomena with a high influence on the mechanical properties of the material. Models exist, that
describe the growth of coherent, semi- and incoherent precipitates. One of the parameters of these
models, namely the energy of the interface between matrix and precipitate, is investigated in more
detail in this project.

Various structures of the molybdenum-carbon system have been investigated by ab-initio calculations
employing the VASP ab-initio electronic structure code [1]. After careful convergence tests with respect
to k-point mesh and plane wave energy cut-off, all structures have been relaxed. The energy of
formation of these structures has been calculated and compared with experimental results where
available. From the obtained values, the stable structures have been identified. To characterize the
mechanical properties, the bulk modulus of the stable structures has been calculated. The phase
diagram using the description of [2] has been obtained using Thermo-Calc [3]. The stable phases of this
phase diagram agree with our predictions at T=0K. A metastable phase, the body-centered tetragonal
structure, which is not present in the phase diagram, has been observed experimentally by highresolution
electron microscopy as a semi-coherent precipitate [4]. We assume that it is stabilized by the
precipitate interface energy. This contribution of the energy of the interface to the body-centered
tetragonal structure is modeled with the help of ab-initio calculations. The change in the lattice
parameters and the energy of the system with varying carbon concentration have been calculated. From
the results the elastic strain was calculated as a function of lattice parameter. This has been used to
examine the coherency of the system with the molybdenum bcc matrix and its influence on the
interface energy.

References:

[1] http://cms.mpi.univie.ac.at/vasp
[2] Thermodynamic properties of Mo-C, Jan-Olof Andersson, CALPHAD, Vol. 12, No. 1, pp.1-8, 1988.
[3]www.thermocalc.de
[4] New carbides observed by high-resolution microscopy in a molybdenum bicrystal containing Σ = 5 symmetrical boundary, J.M.Penisson, M.Bacia, M.Biscondi, Philosophical Magazine A, 1996, Vol. 73, No.4, 859-869.