Macroscopic analysis of time dependent plasticity in Ti alloys

doi:10.1016/j.jmst.2021.11.081

Abstract

Abstract:

Component failure due to cold dwell fatigue of titanium and its alloys is a long-standing problem which has significant safety and economic implications to the aviation industry. This can be addressed by understanding the governing mechanisms of time dependent plasticity behaviour of Ti at low temperatures. Here, stress relaxation tests were performed at four different temperatures on three major alloy systems: commercially pure titanium (two alloys with different oxygen content), Ti-6Al-4V (two microstructures with differing β phase fractions) and Ti-6Al-2Sn-4Zr-xMo (two alloys with different Mo content x= 2 or 6, and portion of β phase). Key parameters controlling the time dependent plasticity were determined as a function of temperature. Both activation volume and energy were found to increase with temperature in all six alloys. It was found that the dwell fatigue effect is more significant by oxygen alloying but is suppressed by the addition of Mo. The presence of the β phase did not strongly affect the dwell fatigue, however, it was suppressed at high temperature due to the low strain rate and strain rate sensitivity.

Key words: Cold dwell fatigue, Ti alloys, Stress relaxation, Time dependent plasticity

Yi Xiong, Phani S. Karamched, Chi-Toan Nguyen, David M.Collins, Christopher M.Magazzeni, Edmund Tarleton, Angus J.Wilkinson. Macroscopic analysis of time dependent plasticity in Ti alloys[J]. J. Mater. Sci. Technol., 2022, 124: 135-140.

Figures/Tables 7

Table 1. Materials composition (in wt.%) and microstructure information.

Empty Cell	Al	V	Sn	Zr	Mo	Fe	O	N	Others total	Microstructure	β phase content	Grain size (μm)
CP-Ti grade 1	<0.05	0.11	<0.05	<0.05	<0.05	<0.05	0.077	0.02	0.2	Pure α	0%	30
CP-Ti grade 4	<0.05	0.15	<0.05	<0.05	<0.05	0.055	0.32	0.006	0.4	Pure α	0%	17
Ti64 α	6.74	3.98	<0.05	<0.05	<0.05	<0.05	0.218	0.001	0.2	Primary α	6%	15
Ti64 Bimodal	6.63	3.87	<0.05	<0.05	<0.05	<0.05	0.191	0.002	0.3	α+β	18%	8
Ti6242	6.21	0.28	1.27	4.21	1.86	<0.05	0.079	<0.001	0.4	Primary α	10%	12
Ti6246	5.98	0.15	0.87	4.24	5.35	<0.05	0.157	<0.001	0.3	α+β	60%	6

Fig. 1. Microstructures of the raw materials: (a) EBSD map of the CP-Ti grade 1 alloy (IPF map in normal direction of the raw material); (b) EBSD map of the CP-Ti grade 4 alloy (IPF map in normal direction of the raw material); SE images of the (c) Ti64 α alloy; (d) Ti64 Bimodal alloy; (e) Ti6242 alloy and (f) Ti6246 alloy; Pole figures of the raw material with a 10° width used for contours plotting calculation: (g) CP-Ti grade 1 alloy; (h) CP-Ti grade 4 alloy; (i) Ti64 α alloy and (j) Ti6242 alloy.

Fig. 2. (a) Geometry of the ETMT tensile test sample; (b) An example of the stress relaxation loading (Ti64 α at room temperature).

Table 2. Values of the fixed parameters in the slip law [21], [22], [23], [24], [25].

ρ	5μm^-2
b	0.295 nm
ν	10¹¹ Hz
kB	1.38×10^-23JK^-1

Fig. 3. (a) A fitted stress relaxation curve (Ti64 α at room temperature, as an example). (b) Thermal activation energy, ΔF and (c) Activation volume, ΔV (d) Critical stress, σc, as a function of temperature for the 6 alloys; (e) Contribution of the three key parameters to the form of the plastic strain rate when the critical stress is exceeded by 1 MPa (low stress condition) and (f) 50 MPa (high stress condition).

Fig. 4. (a) Fitting a straight line to the stress vs. strain rate on a log scale, the gradient represents the strain rate sensitivity exponent m (Ti64 α at room temperature as an example); (b) Strain rate sensitivity vs. Temperatures for the six different alloys; (c) Percentage of stress relaxed vs. Temperatures for the six different alloys.

Table A1. Sensitivity of the three fitted parameters (ΔV, ΔF and σc) to the change of ρ by ±40%. Using the Ti64 α samples as examples.

Empty Cell	Change in ρ	ΔV(b³)	ΔF(10^-20 J)	σ_c(MPa)
RT	ρ	11.71	10.28	871
	ρ+40%	11.86	10.42	873
	ρ-40%	11.82	10.11	868
75 ∘C	ρ	18.33	12.42	658
	ρ+40%	18.02	12.58	656
	ρ-40%	17.71	12.27	655
145 ∘C	ρ	34.30	15.62	562
	ρ+40%	34.87	15.74	564
	ρ-40%	34.50	15.46	558
250 ∘C	ρ	85.05	21.02	560
	ρ+40%	84.99	21.17	567
	ρ-40%	86.27	20.90	560

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