Advanced Microstructure Characterization of β Ti‐Nb‐Ta‐Fe Alloys obtained by Powder Metallurgy

Abstract

In order to obtain advanced biomaterials, with low elastic modulus and acceptable mechanical strength titanium alloys with high contents of refractory materials are used. The addition of niobium, tantalum and molybdenum difficult the manufacturing processes of these alloys. One way to obtain these β‐Ti alloys is the powder metallurgy (P/M) that allow obtaining of customized materials. Although it presents intrinsic limitations, such as porosity, lack of diffusion and the increasing of grain size with sintering parameters. The objective of this work was the microstructure characterization of phases and mechanical properties of Ti35Nb10Ta alloy with Fe additions, using transmission electron microscopy (TEM) and selected area electron diffraction (SAD). The distribution of phases and grain orientation maps were determined with an Automatic Crystal Orientation Mapping (ACOM) system installed in a FEI Tecnai F20 TEM with LaB6 gun. An ASTAR NanoMegas system was used for ACOM diffraction data acquisition. The analyzed map step was 10 nm based on a rectangular grid (400 x 200 pixels). The identification of phases and orientations are obtained through image matching between experimental diffraction patterns and calculated templates. The microstructure obtained is composed mainly by β‐Ti phase (bcc) in β‐stabilizers rich areas (Nb, Ta, Fe), and α+β phase's region confirmed in TEM image (Fig. 1.a and 1.c) and SAD with orientation relationship ([0001] a // [110] β ) in β‐stabilizers poor areas (Ti rich). The a‐Ti (hcp) phase occurs mainly along the grain boundaries, growing inwards. In between it is possible to identify metastable w phase in nanometric scale, confirmed by TEM image (Fig. 1.b) and by spots with orientation relationship with β‐Ti matrix in the SAD zone axis [11‐20] w // [1‐10] β (Fig. 1.d). Fig. 2 shows ACOM image of Ti35Nb10Ta alloy sintered at 1250ºC with virtual bright field (BF) combined with Reliability of α+β region (Fig. 2.a) and (b) PhaseMap combined with Virtual‐BF image of β (red) and α (green) region (Fig. 2.b). β‐Ti phase is mainly observed, with some α+β regions (β‐stabilizer poor elements concentration) and higher β‐Ti stabilization with Fe addition and sintering temperature. Nanometric ω phase was observed inside β‐Ti phase using TEM analysis. TEM and ASTAR provide complementary information both on phase constitution and orientation distribution in nanosized α phase precipitated inside β‐stabilizer poor regions.