Optimization of annealing treatment and comprehensive properties of Cu-containing Ti6Al4V-xCu alloys

doi:10.1016/j.jmst.2019.05.020

Abstract

Abstract:

The Ti6Al4V-Cu alloy was reported to show good antibacterial properties, which was promising to reduce the hazard of the bacterial infection problem. For the purpose of preparing Ti6Al4V-Cu alloy with satisfied comprehensive properties, it’s important to study the heat treatment and the appropriate Cu content of the alloy. In this study, high Cu content Ti6Al4V-xCu (x = 4.5, 6, 7.5 wt%) alloys were prepared, and firstly the annealing heat treatments were optimized in the α+β+Ti₂Cu triple phase region to obtain satisfied tensile mechanical properties. Then the effect of Cu content on the tribological property, corrosion resistance, antibacterial activity and cytotoxicity of the Ti6Al4V-xCu alloys were systematically studied to obtain the appropriate Cu content. The results showed that the optimal annealing temperatures for Ti6Al4V-xCu (x = 4.5, 6, 7.5 wt%) alloys were 720, 740 and 760 °C, respectively, which was resulted from the proper volume fractions of α, β and Ti₂Cu phases in the microstructure. The additions of 4.5 wt% and 6 wt% Cu into the medical Ti6Al4V alloy could enhance the wear resistance and corrosion resistance of the alloy, but the addition of 7.5 wt% Cu showed an opposite effect. With the increase of the Cu content, the antibacterial property was enhanced due to the increased volume fraction of Ti₂Cu phase in the microstructure, but when the Cu content was increased to 7.5 wt%, cytotoxicity was presented. A medium Cu content of 6 wt%, with annealing temperature of 740 °C make the alloy possesses the best comprehensive properties of tensile properties, wear resistance, corrosion resistance, antibacterial property and biocompatibility, which is promising for future medical applications.

Key words: Cu-containing Ti6Al4V-xCu alloys, Annealing treatment, High Cu content, Comprehensive properties

Cong Peng, Yang Liu, Hui Liu, Shuyuan Zhang, Chunguang Bai, Yizao Wan, Ling Ren, Ke Yang. Optimization of annealing treatment and comprehensive properties of Cu-containing Ti6Al4V-xCu alloys[J]. J. Mater. Sci. Technol., 2019, 35(10): 2121-2131.

Figures/Tables 13

Table 1 Chemical compositions of Ti6Al4V-xCu alloys (wt%).

Alloy	Al	V	Cu	Fe	C	N	O	H	Ti
Ti6Al4V-4.5Cu	5.75	3.78	4.46	0.12	0.012	0.002	0.10	0.002
Ti6Al4V-6Cu	5.70	3.78	6.02	0.1	0.011	0.002	0.09	0.002	Bal.
Ti6Al4V-7.5Cu	5.64	3.70	7.65	0.11	0.009	0.002	0.10	0.002

Fig. 1. Optical microstructures of hot-processed Ti6Al4V-xCu alloys and corresponding differential thermal analysis (DTA) graphs: (a) Ti6Al4V-4.5Cu alloy; (b) Ti6Al4V-6Cu alloy; (c) Ti6Al4V-7.5Cu alloy.

Fig. 2. SEM microstructures of Ti6Al4V-xCu alloys annealed at different temperatures: (a) 4.5Cu-720; (b) 4.5Cu-740; (c) 4.5Cu-760; (d) 6Cu-720; (e) 6Cu-740; (f) 6Cu-760; (g) 7.5Cu-720; (h) 7.5Cu-740; (i) 7.5Cu-760.

Fig. 3. (a) XRD patterns of 4.5Cu-720, 6Cu-740 and 7.5Cu-760 samples, (b) calculated volume fractions of different phases in Ti6Al4V-xCu alloys annealed at 720 °C, 740 °C and 760 °C, (c) TEM bright images and (d) selected diffraction patterns of Ti2Cu phase in 6Cu-740 sample.

Table 2 Chemical compositions of different phases in 6Cu-740 sample shown in Fig. 3(c) (wt%).

	phase	Al	V	Cu	Ti
Point 1	Ti₂Cu	0.8	0.1	40.6	Bal.
Point 2	β	2.8	13.1	8.6
Point 3	α	6.8	1.7	3.2

Fig. 4. Engineering tensile stress-strain curves of annealed (a) Ti6Al4V-4.5Cu, (b) Ti6Al4V-6Cu and (c) Ti6Al4V-7.5Cu alloys and (d) mechanical properties of different samples.

Fig. 5. Variations of friction coefficients vs. sliding time (a), average friction coefficients (b), weight loss (c), and wear area and the maximum wear depth of scars (d) for samples of Ti6Al4V, 4.5Cu-720, 6Cu-740 and 7.5Cu-760.

Fig. 6. Cross-section profiles of wear tracks on different samples: (a) Ti6Al4V; (b) 4.5Cu-720; (c) 6Cu-740; (d) 7.5Cu-760.

Fig. 7. SEM images of wear scars on (a) Ti6Al4V, (b) 4.5Cu-720, (c) 6Cu-740 and (d) 7.5Cu-760 and their corresponding magnified graphs in (a1), (b1), (c1) and (d1), respectively.

Fig. 8. (a) Typical potentiodynamic polarization curves, (b) Bode plots, (c) Nyquist diagrams and (d) equivalent electrical circuit used for fitting EIS data, while Rs, R1, Q1, R2 and Q2 are solution resistance, charge-transfer resistance, CPE of interface of electrolyte/passive film/substrate, passive layer resistance, CPE of the passive layer, respectively.

Table 3 Electrochemical parameters obtained from potentiodynamic polarization curves.

	E_corr (mV)	I_corr (nA)	E_p (V)	I_p (μA)
Ti6Al4V	-364 ± 4	168 ± 1.9	1.41 ± 0.02	1.82 ± 0.01
4.5Cu-720	-358 ± 6	33.3 ± 1.8	1.30 ± 0.02	1.83 ± 0.02
6Cu-740	-392 ± 5	55.2 ± 2.3	1.29 ± 0.01	1.83 ± 0.02
7.5Cu-760	-410 ± 4	144 ± 2.1	1.28 ± 0.02	1.84 ± 0.01

Table 4 Electrical parameters obtained by fitting EIS data.

	R_s (Ω cm²)	R₁ (Ω cm²)	R₂ (Ω cm²)	R_p (kΩ cm²)	Q₁ (Ω^-1 cm^-2)	n₁	Q₂ (Ω^-1 cm^-2)	n₂	Error of fit
Ti6Al4V	19.9	7.3 × 10³	325 × 10³	332	108 × 10^-3	0.94	22 × 10^-6	0.93	0.9 × 10^-3
4.5Cu-720	11.1	1.5 × 10⁶	15.5	1500	19 × 10^-6	0.93	10 × 10^-6	0.94	0.8 × 10^-3
6Cu-740	23.1	530 × 10³	588 × 10³	1118	45.35 × 10^-6	0.95	20.32 × 10^-6	0.93	1.4 × 10^-3
7.5Cu-760	17.2	532 × 10³	577 × 10³	1109	20 × 10^-6	0.93	96 × 10^-3	0.96	1.3 × 10^-3

Fig. 9. Typical photographs of S. aureus colonization after culturing with samples of (a) Ti6Al4V, (b) 4.5-720, (c) 6-740, (d) 8-760, (e) calculated antibacterial rate of Cu-containing alloys and (f) absorbance of cells that cultured on different alloys for 1, 3 and 7 days.

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