Effect of temperature and cyclic loading on stress relaxation behavior of Ti–6Al–4V titanium alloy

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Abstract

The effect of temperature and cyclic loading on stress relaxation (SR) behavior of Ti–6Al–4V titanium alloy was studied by uniaxial tensile test. SR limit and rate vary with increasing the temperature and cyclic loading times. The microstructural variations were observed by scanning and transmission electron microscopes. Unstable SR behavior happens at 450–550 °C, which becomes obvious with the increase of cyclic loading times or the decrease of relaxation temperature. The stress instability maybe attributes to the activation energy improvement. Based on dislocation distribution and morphology as well as stress exponent values varying from 1.8 to 2.4, SR mechanism of the alloy is dislocation creep in the tested temperature range. The mechanical properties of the alloy at room temperature almost maintains invariable after SR process. However, the discontinuous yielding in as-received alloy changes to continuous yielding in relaxed ones.

Introduction

Compared with iron and steel materials, titanium alloys own a series of advantages such as high strength to weight ratio, outstanding high-temperature performance, and excellent corrosion resistance. They are extensively served in the aerospace, biomedicine, and other fields [1,2]. According to the existence of α and β phases, titanium alloys are generally divided into α, β and α+β types. Ti–6Al–4V alloy, a typical α+β two-phase titanium alloy, is the backbone of titanium industry because of excellent comprehensive performance in terms of its actual production and application. However, it is difficult to form at room temperature due to high elastic resilience [3]. Furthermore, in order to manufacture Ti–6Al–4V parts with high dimensional accuracy [4], the hot sizing after forming is used to decrease the springback. Therefore, the stress relaxation (SR) and creep behaviors of the alloy at high temperature become significant for determining the hot sizing process parameters [5]. Additionally, some Ti–6Al–4V parts are used at high temperature such as fastening bolts and coil springs. The fastening stress is inevitably decreased during their long-term service, which probably causes seal failure, leakage and loosening. SR is a special creep form in which the elastic strain in metals can gradually transform into creep strain. It results in continuous stress decrease.

SR behavior links to multiple factors such as chemical composition, testing temperature, grain size and initial strain. A certain number of studies have been performed to investigate SR phenomenon of titanium alloy. Cui et al. [6] calculated a constitutive model for SR of Ti–6Al–4V alloy by controlling initial strain and temperature. A novel empirical formula for predicting the relationship between stress and time was proposed, and then an explicit constitutive model for representing the SR behavior was also further obtained. Xiao et al. [7] investigated SR rate, residual stress, and SR limit of Ti–6Al–4V titanium alloy at 800–1000 °C based on modeling and simulation with ABAQUS software. It was proved that the creep-type constitutive equation is valid for the SR simulation. Wang et al. [8] comparatively discussed the SR behavior and mechanism of Ti–6Al–4V alloy produced by additive manufacturing and conventional processing at 600 °C and 700 °C using in situ neutron diffraction. The lower stress relaxation rate in additively manufactured Ti–6Al–4V may be attributed to the narrower α lath width. The SR curves can be fitted by a simple rheological viscoplastic standard linear solid model. Sinha et al. [9] carried out the closed-loop controlled isothermal constant strain SR tests of Ti–6Al–2Sn–4Zr–6Mo titanium alloy. It was found that the stress exponent for power-law creep relies on the strain. The delayed elasticity controls primary SR stage, during which the stress drops rapidly irrespective of the constraint level and the viscous flow dominates SR behavior for a long time.

There are still some issues worthy of attention. The effect of temperature on SR behavior of titanium alloy was mainly investigated by numerical simulation analysis and macroscopic tests. Nevertheless, the corresponding microstructure mechanisms lack of deep discussion. Additionally, the study on the influence of cyclic loading on SR characteristics of fastening parts made of titanium alloy is insufficient, which involves their safety design and maintenance. Therefore, this work focused on SR testing of Ti–6Al–4V titanium alloy at different temperatures together with corresponding microstructural analysis. The effect of cyclic loading on short-term SR behavior was also discussed.

Section snippets

Experimental procedure

Commercial Ti–6Al–4V titanium alloy sheet with a thickness of 0.5 mm was supplied by BaoTi Group, China. Its actual chemical composition is Ti-5.98Al-4.25V-0.23Fe-0.23Si (wt%). In general, metals with creep deformation above 0.3Tm are manufactured by hot forming, and the melting temperature of Ti–6Al–4V alloy is Tm = 1668 °C. Consequently, both SR tests ranging from 600 to 750 °C in 30 °C intervals for 60 min (Fig. 1a) and SR ones with cyclic loading (Fig. 1b) were carried out on INSTRON Legend

Macroscopic stress relaxation behavior

Fig. 3 presents the SR curves of Ti–6Al–4V titanium alloy samples together with the initial stress, SR limit and SR rate at different temperatures. On the whole, all SR curves exhibit a similar logarithmic decline with increasing relaxation time. The stress dramatically decreases in the initial stage. Then it monotonously reduces to a comparatively stable value. The similar trend of SR curves has been reported in aluminum and titanium alloys [11,12]. It ascribes to the fast dislocation movement

Conclusions

The dependence of SR phenomenon of Ti–6Al–4V titanium alloy on temperature and cyclic loading was studied by uniaxial tensile testing. The following conclusions can be obtained:

  • (1)

    SR limit gradually decreases from 10.8 MPa to 4.4 MPa when the temperature increases from 600 °C to 690 °C and then it eventually approaches to a stable value. The required time of SR rate near to zero obviously reduces at higher temperature. Based on dislocation characteristics and stress exponent values, SR mechanism

CRediT authorship contribution statement

Heli Peng: Conceptualization, Methodology, Investigation, Formal analysis, Data curation, Writing – original draft. Zhengquan Hou: Supervision, Writing – review & editing. Xu Chen: Resources, Methodology, Validation, Investigation. Tianle Li: Formal analysis, Data curation, Writing – review & editing. Jingfeng Luo: Resources, Methodology, Validation, Investigation. Xifeng Li: Writing – review & editing, Supervision, Project administration, 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 work was supported by NSFC-Liaoning Province United Foundation (No. U1908229) and the National Natural Science Foundation of China (No. 51875350). We are grateful for these financial supports.

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  • Creep deformation and strength evolution mechanisms of a Ti-6Al-4V alloy during stress relaxation at elevated temperatures from elastic to plastic loading

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    All SR curves have a logarithmic decreasing trend and can be divided into two stages: the first rapid stress decreasing stage, and the second stable stress stage. The same trend of the SR curve has been explained by the competition between dislocation propagation and dislocation recovery [6]. The stable level of stress during SR can be approximately treated as the threshold stress of the material under the tested conditions [31,32].

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