X-ray study of nucleation and growth of precipitates of w-phase in Ti alloys

J. Šmilauerová¹, P. Harcuba¹, J. Stráský¹, M. Janeček¹, J. Pospíšil², Z. Matěj²,
J. Ilavský³ and V. Holý²

¹ Department of Material Physics, Faculty of Mathematics and Physics, Charles University in Prague, Ke Karlovu 5, 121 16 Praha, Czech Republic

² Department of Condensed Matter Physics, Faculty of Mathematics and Physics, Charles University in Prague, Ke Karlovu 5, 121 16 Praha, Czech Republic

³ Argonne National Laboratory, IL, USA

holy@mag.mff.cuni.cz

Metastable b titanium alloys are of great interest to the automotive, aerospace and biomedical industry due to their outstanding mechanical properties. Depending on the stability of b phase in metastable b titanium alloy, several metastable phases can form. Probably the most important and most frequently studied metastable phase is the w phase formed during quenching by a diffusionless displacive transformation. We have studied this process by x-ray diffraction (XRD) and small-angle x-ray scattering (SAXS) performed on single crystals of b titanium alloy (Ti-6.8Mo-4.5Fe-1.5Al in wt.%). XRD experiments have been carried out on a standard x-ray laboratory source (CuKa, 1.6 kW) using two geometries. In the low-resolution setup (polycapillary optics, parallel-plate collimator and secondary graphite monochromator) we measured pole figures in diffraction maxima of both b and w phases before and after ageing annealing at 300 °C. From the measurement a distinct topotaxy relation between the b and w lattices follows. Further, we used a middle-resolution setup (parabolic x-ray mirror on the primary side) for the measurement of coplanar reciprocal space maps around b and w diffraction maxima. From the maps we determined the size of the w particles in the b host lattice, their lattice parameters and local lattice deformation around the particles. The results of the diffraction studies are in press [1].

SAXS measurements have been carried out at APS Argonne, USA, using the photon energy of 20 keV. The experimental data clearly indicate that the w-particles are self-organized; they create a disordered cubic lattice. From the data we determine the mean particle distance and the degree of ordering as functions of the ageing time. We developed a numerical model based on a Monte-Carlo simulation of the nucleation and growth of particles that qualitatively explains the measured data.

[1] J. Šmilauerová, P. Harcuba, J. Pospíšil, Z. Matěj and V. Holý, Acta Mater. (in press).