Microstructure and electrochemical behavior of contemporary Ti6Al4V implant alloys

Abstract

Ti6Al4V is the most common titanium alloy within the biomaterial field. While material standards for different variations of this alloy exist, there are only minimal requirements with respect to its microstructure which is directly related to the alloy's properties. Thus, a better understanding of the Ti6Al4V microstructure of common contemporary implant components and its effect on the electrochemical behavior is needed; including additively manufactured (AM) devices. Therefore, this study aimed at characterizing the microstructures of conventional and AM total joint replacement components, and to identify the effect of microstructure on the electrochemical behavior. Thus, 22 components from conventional (surgically retrieved cast and wrought implants) and AM implants (not previously implanted) were analysed to characterize microstructure by means of electron backscatter diffraction (EBSD) and energy dispersive X-Ray spectroscopy (EDS), and tested to determine its electrochemical behavior (potentiodynamic polarization and EIS). The microstructure of the conventional implants varied broadly but could be categorized into four groups as to their grain size and shape: fine equiaxed, coarse equiaxed, bimodal, and lamellar. The AM components exhibited a fifth category: lath-type. The AM components had a network of β-phase along the α-phase grain boundaries, prior β-grains, and manufacturing voids. Finally, the electrochemical study showed that the equiaxed coarse grains and lath-type grains (AM components) had inferior electrochemical behavior, whereas cast alloys had superior electrochemical behaviour; fine-grained wrought alloys likely provide the best compromise between electrochemical and mechanical properties.

Keywords: Ti6Al4V; additive manufacturing; corrosion behavior; implant alloy microstructure; total joint replacements.

Figure 1 -

5 different types of microstructure were categorized based on grain size and shape: A) fine equiaxed grains (control), B) coarse equiaxed grains (sample 6), C) bimodal (sample 13), D) lamellar dendritic grains (sample 17) and E) coarse lath-type microstructure grains (sample 20).

Figure 2 –

Comparison of the (a) grain size, (b) grain aspect ratio and (c) β phase content of A) fine equiaxed grains, B) coarse equiaxed grains, C) bimodal, D) lamellar dendritic grains and E) lath-type microstructure grains.

Figure 3 –

Sample 20: (a) phase map of α phase (red) and its grain orientation (IPF) map (b); (c) phase map of β phase (blue), its grain orientation map (d). Different colors in (b) and (d) indicated different grain orientations. While the α phase orientation appears to be homogenous, the β phase orientation map exhibits areas with the same crystal orientation as illustrated by areas with the same color. These areas correspond to the prior β grains. The resulting texture is also confirmed by the pole figures for (e) α and (f) β phase. Texture is confirmed by the symmetry of hot sports in each quadrant.

Figure 4 -

Manufacturing defects observed on additive manufactured acetabular cups (samples 20-22). The hole shapes are characteristic of lack of fusion (blue) and retained gas from the raw metal powder or during the melting process (yellow).

Figure 5 -

Microstructural difference of the acetabular cups according to the manufacturing process where (a) sample 3, (b) sample 6, (c) sample 18, (d) sample 20 and (e) sample 22. The black arrows show the prior β phase. The orientation is represented by the colors on the inverse pole figure for each phase.

Figure 6 –

Band contrast, phase map (red=α, blue=β phase), titanium map, aluminum map and vanadium map of the control alloy, sample 12 and sample 6. The phase maps illustrate different fraction sizes and distributions of beta phase. The EDS elemental maps show differences in the chemical distribution of alloy elements. Vanadium occurred predominantly within small grains in sample 12 and aluminum within the larger grains of the bimodal microstructure, whereas in the control and sample 6 vanadium is concentrated within the β phase. The network-like β phase distribution is even better illustrated within the vanadium map than in the actual phase map. The comparably low intensity of titanium within the β phase in sample 6 suggests a strong decline of titanium content in favor of vanadium.

Figure 7 –

(a) and (b) vanadium and titanium variation between α and β led to galvanic corrosion in the equiaxed coarse grains group and lath-type grains group (AM); (c) vanadium variation (ΔV) across all groups.

Figure 8 –

(a) polarization curves, (b) Nyquist Plot, (c) Bode Plot Z Module, (d) Bode Plot Z Phase and (e) modified Randles circuit.

Figure 9 –

(a) E_corr, (b) i_corr, (c) Q and (d) R_p of A: Equiaxed fine, B: Equiaxed coarse, C: Bimodal, D: Dendritic and C: Lath-type. Equiaxed coarse and lath-type groups showed poor corrosion current, polarization resistance and capacitance.