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Microstructure characterization and imaging in titanium alloys by atomic force acoustic microscopy

: Hirsekorn, S.; Rabe, J.; Rabe, U.

European Federation for Non-Destructive Testing -EFNDT-:
10th European Conference on Non-Destructive Testing, ECNDT 2010. CD-ROM : 7.-11.06.2010, Moskau
Moskau, 2010
Vortrag 4.2.7
European Conference on Non-Destructive Testing (ECNDT) <10, 2010, Moskau>
Fraunhofer IZFP ()
titanium alloy; atomic force acoustic microscopy; ECNDT

Titanium is a strong, highly creep-resistant, light, heat-resisting, biocompatible, and corrosion resistant transition metal with a broad field of applications in industry [1]. The metal consists primarily of two different crystal structures: the hexagonal close-packed (hcp) alpha-phase and the body-centred cubic (bcc) beta-phase. The bulk mechanical properties of titanium show big differences depending on the ratio of the two phases that have been formed during heat treatment. Pure titanium cooled slowly enough to allow diffusion will contain 100% alpha-phase at room temperature. Above the beta transition temperature (BTT = 882°C for pure titanium) the bcc structure is stable. Depending on temperature and cooling rate the beta-phase may either transform to the metastable beta-phase or be replaced by stiff hcp alpha'- or soft tetragonal close-packed (tcp) alpha''-martensite structures [2]. The hcp alpha-phase shows the highest indentation modulus (~117 GPa), but exhibits a strong elastic anisotropy. The much less anisotropic bcc structure has a lower indentation modulus (~82 GPa for the metastable beta-phase) [3]. The bcc structure contains more slip planes than the hcp structure and thus is easier to deform. Therefore and also to avoid substantial grain growth forming is preferably carried out just above the beta transition temperature (BTT) [4]. Both phases can be stable even below the BTT if stabilizing elements are added to the titanium solution. Elements stabilising the alpha-phase as e.g. N, O, C, Al, Sn, and Zr (in decreasing order of alpha stabilization) increase the BTT. Elements as Fe, Bi, Mn, Cr, Co Mo, Ni, Pb, Cu, V, and Nb (in decreasing order of ? stabilization) stabilise the beta- phase and decrease the BTT [5]. The most commonly used titanium alloy is Ti-6Al-4V covering 60% of the titanium market in which 80% accounts for aerospace industry [5]. The BTT of Ti-6Al- 4V with alpha stabilising aluminium and beta stabilising vanadium, respectively, is about 1000°C [6]. While heating, the volume fraction of the beta-phase increases. Since the total amount of beta stabilising elements is fixed, the volume fraction of stabilizing elements in the beta-phase decreases with increasing temperature. Therefore, when cooling again to room temperature, the beta-phase becomes unstable and transforms into stable beta- and secondary alpha-phase if there is time enough for diffusion. If the alloy is quenched, so that no time will be for atom arrangements the beta-phase will either remain in its metastable state or transform into soft alpha''- (below ~900°C) or stiff alpha'-martensite. At room temperature, the beta-phase is only stable when it is enriched with 15 wt.% vanadium. This can be obtained by slow cooling and/or annealing below 750°C. Depending on the heat treatment the alpha- and beta-phases form different microstructures and arrangements. The structures are classified into three categories: lamellar, equiaxed, and a mixture of both [5].