Effect of high pressure torsion process on the microhardness, microstructure and tribological property of Ti6Al4V alloy

doi:10.1016/j.jmst.2021.03.044

Abstract

Abstract:

In the present study, a fully lamellar Ti6Al4V alloy was severely deformed by high pressure torsion (HPT) process under a pressure of 7.5 GPa up to 10 revolutions. Experimental results revealed that the microhardness of Ti6Al4V was increased remarkably by about ~41% and saturated at about 432 Hv after the HPT process. A relatively uniform bulk nanostructured Ti6Al4V alloy with an average grain size of about 52.7 nm was obtained eventually, and no obvious formation of metastable ω phase was detected by XRD analysis. For the first time, the tribological properties of the HPT processed Ti6Al4V alloy were investigated by a ball-on-disc test at room temperature under a dry condition. It was found that HPT process had a great influence on the friction and wear behaviors of Ti6Al4V alloy. With increasing the number of HPT revolutions, both friction coefficient and specific wear rate were obviously decreased due to the reduction of abrasion and adhesion wears. After being deformed by 10 HPT revolutions, the friction coefficient was reduced from about 0.49 to 0.37, and the specific wear rate was reduced by about 48%. The observations in this study indicated that HPT processed Ti6Al4V alloys had good potential in structural applications owing to their greatly improved mechanical and tribological properties.

Key words: Severe plastic deformation, High pressure torsion, UFG microstructure, Mechanical property, Friction and wear, Ti6Al4V alloy

Guanyu Deng, Xing Zhao, Lihong Su, Peitang Wei, Liang Zhang, Lihua Zhan, Yan Chong, Hongtao Zhu, Nobuhiro Tsuji. Effect of high pressure torsion process on the microhardness, microstructure and tribological property of Ti6Al4V alloy[J]. J. Mater. Sci. Technol., 2021, 94: 183-195.

Figures/Tables 12

Fig. 1. Typical lamellar microstructures of Ti6Al4V alloy before HPT process: (a) SEM-BSE image, (b) EBSD inverse pole figure (IPF) map of α phase, and (c) bright field TEM image.

Fig. 2. Distributions of Vickers microhardness of HPT processed Ti6Al4V alloys against (a) the radial distance from disc centre and (b) the calculated shear strain; (c) plot of the natural logarithm of microhardness of the HPT processed Ti6Al4V alloys against the natural logarithm of shear strain.

Fig. 3. X-ray diffraction patterns of Ti6Al4V alloys after HPT process by 0~10 revolutions under the pressure of P = 7.5 GPa.

Fig. 4. Bright field TEM microstructures at r = 3.5 mm from the disc centre of HPT processed Ti6Al4V alloys: (a) 0.5R-Ti6Al4V, (b) magnified region A as marked in (a), (c) 1R-Ti6Al4V, (d) magnified region B as marked in (c), (e) 2R-Ti6Al4V, (f) 5R-Ti6Al4V, and (g) 10R-Ti6Al4V; (h) dark field TEM image corresponding to (g); and (i) detailed grain size distribution of 10R-Ti6Al4V. (with SAED pattern of 10R-Ti6Al4V inserted in (f)).

Fig. 5. Bright field TEM microstructures at the disc centre of HPT processed Ti6Al4V alloys and their corresponding SAED patterns: (a and a1) 1R-Ti6Al4V, (b and b1) 5R-Ti6Al4V, and (c and c1) 10R-Ti6Al4V.

Fig. 6. (a) Friction coefficient evolution histories against the number of sliding cycles for all studied Ti6Al4V alloys and (b) plot of steady-state friction coefficient against the number of HPT revolutions.

Fig. 7. Plot of steady-state friction coefficient against the Vickers microhardness (corresponding to the location of wear position centre) of all studied Ti6Al4V alloys.

Fig. 8. Three-dimensional wear track profiles of (a) 0R-Ti6Al4V alloy and (b) 10R-Ti6Al4V alloy; (c) plots of typical wear track cross-sectional profile for all Ti6Al4V samples after HPT process.

Fig. 9. Plots of specific wear rate of Ti6Al4V alloys as a function of (a) number of HPT revolutions and (b) Vickers microhardness of Ti6Al4V alloy.

Fig. 10. SEM images showing the worn surfaces of: (a1-a3) 0R-Ti6Al4V alloy with different magnifications, (b1-b3) 1R-Ti6Al4V alloy with different magnifications, (c1-c3) 5R-Ti6Al4V alloy with different magnifications, and (d1-d3) 0R-Ti6Al4V alloy with different magnifications.

Table 1 SEM-EDS point chemical composition analysis of the marked positions (P1-P12) on the worn surfaces of HPT processed Ti6Al4V alloys in Fig. 10 (wt.%).

Sample	Position	Ti	Al	V	Fe	O
0R-Ti6Al4V	P1	34.8	2.7	2.1	50.1	10.3
	P2	67.7	4.9	2.9	4.8	19.7
	P3	88.9	4.1	3.3	0	3.7
1R-Ti6Al4V	P4	74.4	5.6	3.8	5.7	10.5
	P5	73.7	5.8	3.4	1.6	15.5
	P6	55.2	3.8	2.9	19.7	18.4
5R-Ti6Al4V	P7	71.9	5.1	3.3	6.0	13.7
	P8	70.3	4.7	3.5	3.4	18.1
	P9	88.1	6.2	4.7	0.4	0.6
10R-Ti6Al4V	P10	76.2	5.7	3.9	1.9	12.3
	P11	89.5	6.1	4.1	0.3	0
	P12	85.2	3.5	3.8	3.4	4.2

Fig. 11. SEM-EDS elemental maps of the worn surface of (a) 0R-Ti6Al4V alloy and (b) 10R-Ti6Al4V alloy.

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