Grade-4 commercially pure titanium with ultrahigh strength achieved by twinning-induced grain refinement through cryogenic deformation

doi:10.1016/j.jmst.2020.04.082

Abstract

Abstract:

The yield strength of commercially pure (CP) Ti of ASTM grade 4, the strongest among all the CP-Ti grades, is too low for structural applications that require high-strength materials. Here, we demonstrate the strengthening of grade-4 CP Ti by cryogenic-temperature rolling (CTR), which enables deformation twinning in grade-4 CP Ti to achieve twinning-induced grain refinement. CTR activated $\left\{ 11\bar{2}2 \right\}$ twinning and $\left\{ 10\bar{1}2 \right\}$ twinning, which are the most common twinning systems in pure Ti, whereas room-temperature rolling (RTR) did not activate any twinning system. CTR with imposing an area reduction of just 30% significantly increased the yield strength of the CP Ti to 946 MPa, which is not achievable through typical processes performed at or above room temperature and is comparable to that of commercial Ti-6Al-4V. The significant increase in strength was due to microstructural strengthening caused by twinning-induced grain refinement, combined with dislocation accumulation. In contrast to RTR, CTR greatly increased the stress concentration at grain boundaries (GBs), which caused the unusual activation of twinning in the grade-4 CP Ti by facilitating twin nucleation at GBs. The stress concentration increased because CTR activated the <c+a> slip to a lesser extent compared to RTR, thereby reducing the strain compatibility between neighboring grains. These results will contribute to development of ultrahigh-strength CP Ti and may thereby extend its use to structural applications that require high-strength materials.

Key words: Titanium, Twinning, Cryogenic deformation, Grain refinement, Tensile properties

Seong-Woo Choi, Jae Suk Jeong, Jong Woo Won, Jae Keun Hong, Yoon Suk Choi. Grade-4 commercially pure titanium with ultrahigh strength achieved by twinning-induced grain refinement through cryogenic deformation[J]. J. Mater. Sci. Technol., 2021, 66: 193-201.

Figures/Tables 14

Fig. 1. (a) EBSD image-quality map and (b) (0001) and $\{10\bar{1}0\}$ pole figures of the initial material. (c) A schematic describing the rolling direction of RTR and CTR with reference to the initial material.

Fig. 2. EBSD image-quality map of materials rolled to 10% - 40% AR by (a) RTR and (b) CTR.

Fig. 3. Variations in total TB length per unit area and average grain size as a function of AR. Error bars indicate the standard deviation.

Fig. 4. EBSD (0001) pole figure of materials rolled to 30% AR by (a) RTR and (b) CTR.

Fig. 5. Engineering stress vs. strain curves of materials rolled to 10% - 40% AR by (a) RTR and (b) CTR. The result for the initial material is also presented for comparison.

Table 1 YS, UTS, and EL of materials rolled to 10% - 40% AR by RTR and CTR.

Area reduction [%]	RTR			CTR
Area reduction [%]	YS [MPa]	UTS [MPa]	EL [%]	YS [MPa]	UTS [MPa]	EL [%]
10	698	791	18.1	724	816	18.9
20	773	852	15.0	840	930	12.4
30	815	882	14.2	946	1024	12.5
40	833	907	14.1	960	1060	10.1

Fig. 6. Variations in YS, UTS, and EL as a function of AR: (a) RTR and (b) CTR.

Fig. 7. (a) Stress map of materials rolled to an AR of ~3% by RTR (upper) and CTR (lower). (b) Stress profile along the line in (a) in the direction indicated by the arrowhead. The map was constructed using EBSD data.

Fig. 8. XRD line-profile analysis results of the material rolled to an AR of ~3% by RTR or CTR: (a) XRD peak, (b) integral breadth vs. g2(1 + q1x + q2x2) curve, and (c) dislocation fraction. In (b), R2 indicates the coefficient of determination.

Table 2 FWHMs of the Gaussian and Lorentzian functions and the integral breadth (β), obtained by profiling XRD peaks using the TCH pseudo-Voigt function [28].

Material	Plane*	FWHM [10^-4 $\dot{\text{A}}$^-1]		β [10^-4 $\dot{\text{A}}$^-1]
		Gaussian	Lorentzian	β [10^-4 $\dot{\text{A}}$^-1]
RTR	(00.2)	2.821	7.133	11.96
	(10.1)	1.873	5.676	9.341
	(10.2)	3.501	4.421	8.566
	(2$\bar{1}$.0)	4.923	6.548	12.49
	(10.3)	4.909	5.588	11.21
	(2$\bar{1}$.2)	5.764	8.444	15.67
	(10.4)	4.999	12.83	21.48
	(20.3)	2.993	18.89	30.01
	(3$\bar{1}$.1)	4.023	14.60	23.71
CTR	(10.0)	0.986	6.009	9.554
	(00.2)	3.447	9.439	15.68
	(10.1)	1.541	6.336	10.22
	(10.2)	4.165	4.191	8.814
	(2$\bar{1}$.0)	5.499	5.725	11.88
	(10.3)	4.827	7.497	13.70
	(2$\bar{1}$.2)	4.954	10.65	18.25
	(10.4)	1.866	17.77	28.05
	(20.3)	3.880	16.23	26.15

Table 3 Values of g and x for the reflection planes and the values of Dv, α1 ${{\bar{C}}_{\text{hk}.0}}$, q1, and q2 numerically calculated in Eq. (3).

Material	Plane	g [$\dot{\text{A}}^{-1}$^-1]	x	D_v [$\dot{\text{A}}$]	${{\alpha }_{1}}{{\bar{C}}_{hk.0}}$	q₁	q₂
RTR	(00.2)	0.4276	1.6755	2267	0.0017	0.6367	-0.4040
	(10.1)	0.4461	0.3849
	(10.2)	0.5794	0.9127
	(2$\bar{1}$.0)	0.6782	0
	(10.3)	0.7508	1.2232
	(2$\bar{1}$.2)	0.8013	0.4772
	(10.4)	0.9394	1.3889
	(20.3)	1.0108	0.6748
	(3$\bar{1}$.1)	1.0570	0.0686
CTR	(10.0)	0.3921	0	1666	0.0017	-0.2234	0.3540
	(00.2)	0.4276	1.6755
	(10.1)	0.4461	0.3849
	(10.2)	0.5794	0.9127
	(2$\bar{1}$.0)	0.6782	0
	(10.3)	0.7508	1.2232
	(2$\bar{1}$.2)	0.8013	0.4772
	(10.4)	0.9394	1.3889
	(20.3)	1.0108	0.6748

Table 4 Slip systems considered in the estimation of activated slip systems based on the XRD line-profile analysis.

Dislocation type	Slip system	Burgers vector	Slip plane
Edge	Basal	$\langle 2\bar{1}\bar{1}0\rangle $	$\{0001\}$
	Prismatic	$\langle 2\bar{1}\bar{1}0\rangle $	$\{01\bar{1}0\}$
	Pyramidal	$\langle \bar{2}113\rangle $	$\{2\bar{1}\bar{1}2\}$
		$\langle \bar{2}113\rangle $	$\{10\bar{1}1\}$
Screw	Basal or prismatic	$\langle 2\bar{1}\bar{1}0\rangle $
	Pyramidal	$\langle \bar{2}113\rangle $

Table 5 Calculated contributions of grain refinement (ΔσG) and dislocation accumulation (ΔσD) to YS of the materials rolled to 30% AR by RTR and CTR.

Process	Calculated strength [MPa]				Measured YS [MPa]
Process	Δσ_G	Δσ_D	σ_IM*	Total	Measured YS [MPa]
RTR	21	248	472	741	815
CTR	199	292	472	963	946

Fig. 9. Distribution of the angle ? between the tensile loading axis (RD) and c-axis of grains in (a) the initial material and (b) the material rolled to 30% AR by CTR. The inset in (a) defines the angle ?.

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