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Role of coherency strain in the stabilization of a metastable precipitate of the Mo-C binary system

Date: 29.03.2012
Place: DPG spring meeting, Berlin, Germany

Sankari Sampath
Rebecca Janisch
Suzana Fries
Alexander Hartmaier

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. The insight into this precipitation behavior during heat treatment paves the way for the improvement of existing steel grades. The precipitation growth can be coherent, semi-coherent or incoherent with the original matrix of the metal. Being able to predict the transition between these modes will allow us to choose the right growth model in kinetic simulations. 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, MoCx at a grain boundary in bodycentered cubic (bcc) Mo has been investigated. The study showed that there is a significant strain contribution to the interface energy of the lattice misfit. The aim of this work is to quantify this strain contribution to the interface energy by means of ab-initio density functional theory (VASP)[2] such that the stabilization of the metastable phase(bct) could be carried out.

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. To characterize the mechanical properties, the bulk modulus of the stable structures has been calculated. The phase diagram using the description of [3] has been obtained using Thermo-Calc [4]. 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 high-resolution electron microscope as a semi-coherent precipitate [5]. We assume that it is stabilized by the precipitate interface energy. By the addition of carbon atoms to the bcc Mo, the compression and tensile tests are performed. As a result, the variation of strain with carbon concentration and the energy corresponding to strain variation of the system have been estimated. Furthermore, in the interface study, the interface is constructed between Mo(001) and MoCx(001) starting with the coherent interface. This study has been extended to gamma surface calculations that yield possible Burgers vectors for misfit dislocations. The interface energy contribution to the body-centered tetragonal structure is modeled with the help of ab-initio calculations.

References:
[1] Rebecca Janisch, Christian Elsaesser, Physical Review B, 2008, Vol.77, 094118
[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.Penisson, M.Bacia, M.Biscondi, Philosophical Magazine A, 1996, Vol. 73, No.4, 859-869

Supporting information:

Abstract_DPG_Sampath.pdf
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