Enhancing low-cycle fatigue life of commercially-pure Ti by deformation at cryogenic temperature

https://doi.org/10.1016/j.msea.2020.140698Get rights and content

Highlights

  • Deformations twins were considerably increased by cryogenic-temperature rolling.

  • Low-cycle fatigue (LCF) life was increased in proportion to the amount of deformation twins.

  • Deformation twins induced significant crack deflection and decelerated the crack propagation.

  • Excellent LCF life of CTR was attributed to the development of smaller dislocation cell structure.

Abstract

In this study, the low-cycle fatigue behavior of a cryogenic-rolled commercially pure titanium alloy was investigated, and compared with those of undeformed and room-temperature rolled ones. The amounts of deformation twins increased to >69.8% after cryogenic temperature rolling, and to 27.5% after room temperature rolling. Strain-controlled low-cycle fatigue tests were performed at total strain amplitudes 0.4%Δεt2 1.2%. The Coffin-Manson and hysteresis energy-based models confirmed that low-cycle fatigue resistance was remarkably improved with increasing the volume fraction of deformation twins by pre-deformation. As the proportion of deformation twins in the microstructure increased, the hysteresis loop area decreased, the striation spacing decreased, and the severity of crack-deflection behavior increased. At low Δεt2 = 0.4%, dislocation recovery was suppressed in the pre-deformed microstructure, so cyclic behavior was stable. However, at Δεt/2 ≥ 0.8%, the recovery became the predominant mechanism, so well-defined cell structures formed and cyclic softening occurred. The smaller dislocation cells formed in RTR and CTR were considered to cause more severe crack arrest than in AR.

Introduction

Commercially pure Titanium (CP–Ti) is widely used due to its high strength/weight ratio, biocompatibility, and corrosion resistance [1]. However, CP-Ti has low strength and poor tolerance of high-cycle fatigue (HCF) [2]. Therefore, it is necessary to improve its mechanical properties to meet the needs of higher performance materials. Grain refinement is one of the effective ways to improve mechanical properties of CP-Ti. Severe plastic deformation processes (SPD) such as equal channel angular pressing (ECAP) and high pressure torsion (HPT) were successful in producing ultra-fine grained CP-Ti with a very high strength level of near 1 GPa [[2], [3], [4], [5], [6]]. However, these processes are not widely used in the industry due to their low productivity and size limitation.

Alternatively, grain refinement can also be achieved by the formation of deformation twins, which is so called the dynamic Hall-Petch effect [7,8]. Deformation twins can be easily produced by conventional deformation processes. However, it is hard to achieve high strength levels over 900 MPa without cracking. Recently, authors have found [9,10] that cryogenic rolled CP-Ti consists of very high volume fraction of deformation twins with remarkable high strength values over 900 MPa. Furthermore, cryogenic rolling process provides higher ductility than conventional room temperature rolling process. Cryogenic rolling process has a lot of potential for industrial applications due to its efficiency and outstanding mechanical properties, as compared to SPD processes and conventional room temperature rolling.

Most industrial components experience elastic or plastic cyclic loading during their service time, so fatigue properties are also important. Earlier investigations have obtained contradictory results of the effect of deformation twins on low-cycle fatigue (LCF) behavior. In twinning-induced plasticity (TWIP) steels, deformation twin boundaries acted as sites of fatigue-crack initiation due to the large stress concentration that developed at the twin boundaries [11,12]. In contrast, other papers reported that deformation twins can help to deflect the direction of advancing crack during cyclic loading, so the twins increase resistance to propagation of fatigue cracks [[13], [14], [15]]. Such difference in results has also been observed in CP-Ti: deformation twins can extend fatigue life by varying the orientation of slip, and activating secondary slip and twinning systems [16], but they can also lead to accumulation of fatigue damage [17].

The aim of this work is to clarify the effect of deformation twins on the LCF life of CP-Ti. For this purpose, LCF tests were performed using three specimens that included various amounts of deformation twins: (1) un-deformed specimen, (2) specimen deformed at room temperature and (3) specimen deformed at cryogenic temperature. Special attention was paid to microstructural evolution during cyclic deformation, cyclic hardening/softening behavior, and LCF life of these three microstructures.

Section snippets

Processing

CP-Ti used in this study was supplied by ATI, USA as 5 mm thick plate with a nominal chemical composition (wt. %) 0.12 O, 0.10 Fe, 0.01C, < 0.01 N, balance Ti. The as-received plates had a twin-free equiaxed grain structure and were named as AR. To vary the amounts of deformation twins, these plates were rolled from 5 to 3.5 mm in thickness in six passes at room temperature or at cryogenic temperature. To ensure cryogenic temperature during cryogenic rolling, the plate was soaked in liquid N2

Microstructure and tensile properties

EBSD image quality (IQ) maps (Fig. 1) were obtained. AR presented a twin-free structure with average grain size d≈ 33.6 ± 2.2 μm (Fig. 1a). Both rolled specimens had twinned structure; RTR had d≈ 14.1 ± 2.5 μm (Fig. 1b), and CTR had d≈ 5.0 ± 0.5 μm (Fig. 1c). The estimated volume fractions of deformation twins were 27.5% in RTR and >69.8% in CTR. Accordingly, compared to the AR, both rolled microstructures had higher yield strength (YS) and higher ultimate tensile strength (UTS), but

Microstructure evolution during cyclic deformation

The microstructure of the three different specimens differed before cyclic deformation (Fig. 8). AR specimen had low dislocation density without dislocation tangles or deformation twins (Fig. 8a). However, RTR and CTR had deformation twins and high dislocation density with randomly distributed tangles (Fig. 8b and c).

After fatigue failure at Δεt2 = 0.4%, the microstructure of AR specimen showed a combination of planar array of dislocations and irregular cell structure (Fig. 9a). These irregular

Conclusion

In this study, LCF behavior of three CP-Ti specimens that had different microstructures: twin-free equiaxed (AR), room-temperature rolled (RTR), and cryogenic-temperature rolled (CTR) was compared. The volume fractions of deformation twins were 27.5% in RTR and >69.8% in CTR. The effect of deformation twins on LCF was investigated at various strain amplitudes (0.4%Δεt21.2%). The main conclusions are summarized below.

  • 1.

    CTR and RTR showed higher yield and tensile strength, and lower elongation to

CRediT authorship contribution statement

Geonhyeong Kim: Methodology, Investigation, Writing - original draft, Writing - review & editing. Seyed Amir Arsalan Shams: Methodology, Investigation, Writing - review & editing. Jae Nam Kim: Investigation. Jong Woo Won: Methodology. Seong Woo Choi: Methodology, Resources. Jae Keun Hong: Funding acquisition, Project administration. Chong Soo Lee: Writing - original draft, Writing - review & editing, Validation, Project administration, Supervision, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This research was funded by a grant (16-CM-MA-10) from Civil-Military Technology Cooperation Program funded by the Ministry of Trade, Industry and Energy, Korea.

References (29)

Cited by (14)

  • A novel cryogenic rolling method for commercially pure titanium sheets featuring high strength and ductility

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    Similarly, Zhao et al. [25] showed cryogenically forged CP Ti had an exceptional combination of strength and ductility at 77 K; they attributed the improvement to high-density nanotwins that were previously formed by cryogenic forging. In addition, CTR improved hardness [26] and low-cycle fatigue resistance [27] in CP Ti for a similar reason. These studies clearly indicate that CTR can be employed as a practical approach for manufacturing mechanically high-performance CP Ti.

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