A significant interest has recently arisen in investigations of titanium aluminides in the form of sheets or thin film forms as thermal protection systems because of their low density, high temperature strength, reasonable oxidation resistance, and low thermal conductivity. Therefore, improving ductility of TiAl alloys by grain refinement to submicron-size levels is a crucial issue to prevent from fracturing and failure of the thermal protection systems for applications[1,2]. The amorphous phase can then be a precursor to a nanocrystalline structure.

For a better understanding of the crystallization kinetics of the amorphous phase and microstructural evolution in the thin sheet, molecular dynamics(MD) simulations have been performed to study structural relaxation within the framework of the embedded atom method(EAM). The time and space correlation of atoms are analyzed in real and reciprocal space for equilibrium liquid, supercooled liquid and amorphous state. The details of atom motion in studied three states are analyzed using three factors including mean square displacement(MSD), non-Gaussian parameter(NGP), and incoherent intermediate scattering function[3,4]. The relaxations of the three factors clearly reveal two relaxation processes involving in slow relaxation and fast relaxation of the cooled liquid TiAl sheet. With increasing undercooling, self-diffusion slows down along with an increasing in the structural relaxation time as compared with the normal liquid state. Accompanying with massive structural rearrangement at a certain supercooling and time, the local structural patterns in the amorphous state are identified by decomposing peaks of pair distribution functions (PDFs) according to the pair analysis(PA) technique[5]. Concerning the studied sheet, the obtained results reveal how quenching rates affect the liquid-amorphous transition as well as the local structure changes.

References

[1] O.N. Senkov and M. D. Uchic, Mater. Sci. Eng. A, 340(2003), 216

[2] M. Shimono and H. Onodera , MATER. TRANS. JIM,   39(1998), 147

[3] H. Pang, Z.H. Jin and K.Lu, Phys.Rev.B 67(2003), 094113

[4]W. Kob, C. Donati, S. J. Plimpton, P. H.Poole, S. C. Glotzer, Phys. Rev. Lett. ,79(1997),15

[5]J.D. Honeycutt and H.C. Andersen, J. Phys. Chem., 91(1987), 4950

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