Investigation of Copper Alloying in a TNTZ-Cux Alloy

Abstract

Alloying copper into pure titanium has recently allowed the development of antibacterial alloys. The alloying of biocompatible elements (Nb, Ta and Zr) into pure titanium has also achieved higher strengths for a new alloy of Ti-1.6 wt.% Nb-10 wt.% Ta-1.7 wt.% Zr (TNTZ), where strength was closer to Ti-6Al-4V and higher than grade 4 titanium. In the present study, as a first step towards development of a novel antibacterial material with higher strength, the existing TNTZ was alloyed with copper to investigate the resultant microstructural changes and properties. The initial design and modelling of the alloy system was performed using the calculation of phase diagrams (CALPHAD) methods, to predict the phase transformations in the alloy. Following predictions, the alloys were produced using arc melting with appropriate heat treatments. The alloys were characterized using energy dispersive X-ray spectroscopy in scanning transmission electron microscopy (STEM-EDS) with transmission Kikuchi diffraction (TKD). The manufactured alloys had a three-phased crystal structure that was found in the alloys with 3 wt.% Cu and higher, in line with the modelled alloy predictions. The phases included the α-Ti (HCP-Ti) with some Ta present in the crystal, Ti₂Cu, and a bright phase with Ti, Cu and Ta in the crystal. The Ti₂Cu crystals tended to precipitate in the grain boundaries of the α-Ti phase and bright phase. The hardness of the alloys increased with increased Cu addition, as did the presence of the Ti₂Cu phase. Further studies to optimize the alloy could result in a suitable material for dental implants.

Keywords: TNTZ; biomaterial; microstructures; titanium alloy.

Figure 1

Mole fraction of phases as a function of temperature for (a) the Ti-Nb-Ta-Zr-1 wt.% Cu alloy and (b) the Ti-Nb-Ta-Zr-5 wt.% Cu alloy. Composition of the alloys can be found in Table 3.

Figure 2

X-ray diffraction on TNTZ-Cu_x alloys: (a) Diffraction for all alloys including references from Ti₂Cu (04-003-1382), HCP- (Ti-Ta) (03-065-9616) and HCP-Ti (00-044-1294). (b) X-ray diffraction pattern showing the 2θ angular ranges for the alloys from 38°–41° and 76°–78°. Note the Ti₂Cu peak at 39.5°.

Figure 3

SEM micrographs showing (a) 10 wt.% Cu alloy with Inset showing three crystal phases, (b) 1 wt.% Cu alloy, (c) 0 wt.% Cu alloy (d) 5 wt.% Cu alloy with 3 crystal phases shown (e) 3 wt.% Cu alloy with three crystal phases shown.

Figure 4

STEM-EDS maps on the crystal boundary showing 3 crystal phases for (a) 3 wt.% Cu alloy and (b) 5 wt.% Cu alloy, with associated Annular dark field detector image, Cu K series map, Ta M series map and Ti K series map.

Figure 5

3 wt.% Cu alloy studied using TKD and EDS. From the TKD study (a) displays band contrast, (b) a phase map and (c) IPF Z map of the same area with (d) associated pole figures. (e) An electron image of the area investigated with both techniques, (f–h) EDS maps of: (f) Cu K series (where X indicates Ti₂Cu), (g) Ti K series and (h) Ta M series.

Figure 6

5 wt.% Cu alloy studied using TKD and EDS. From the TKD study (a) displays band contrast, (b) a phase map and (c) IPF Z map of the same area with (d) associated pole figures. (e) An electron image of the area investigated with both techniques, (f–h) EDS maps of: (f) Cu L series (where X indicates Ti₂Cu), (g) Ti K series and (h) Ta M series.

Figure 7

Vickers hardness of the TNTZ-Cu_x alloys (“*” indicates statistical significance of p < 0.05, as per Tukey Anova test, while “NS” indicates no statistically significant difference).