Effects of service temperature on tensile properties and microstructural evolution of CP titanium subjected to laser shock peening
Graphical abstract
Introduction
In the past three decades, titanium alloys are widely applied in various fields such as aerospace, marine, electron, automobile and medical industries. Among these alloys, the superior specific strength, high-melting temperature, excellent acid and alkali resistance, and corrosion resistance of commercial pure (CP) titanium are attracting more and more attentions [1,2]. In addition, the mechanical property of CP titanium is still steady under high temperature condition varying over a wide range of thermo mechanical treatment or heat treatments [[3], [4], [5], [6]]. Nonetheless, at high temperatures, relative weaknesses in tensile property and fatigue strength of CP titanium directly decrease the service life of components, which also limit its exhaustive application.
Surface treatment technologies, including water cavitation peening [7], high-energy shot peening [8], surface mechanical attrition treatment (SMAT) [9], and cold rolling [10], have proved to be highly productive in strengthening CP titanium. Laser shock peening (LSP) is an emerging surface strengthening technology compared with these conventional surface treatment technologies, which can remarkably improve the fatigue performance of metallic components by utilizing the mechanical effect of laser shock wave to form considerable compressive residual stress and even can refine coarse grains of metallic component surface layer [[11], [12], [13]]. In fact, massive LSP treatment is usually used to induce uniform mechanical properties across the entire surface of metallic components, which is also used to surface modification of CP titanium [[14], [15], [16], [17]]. For example, our previous work investigated the micro-hardness, grain subdivision process and microstructural response in CP titanium after multiple LSP treatment at room temperature, and after one and three LSP treatment, the micro-hardness on the surface was improved by 16% and 41%, respectively. Furthermore, a depth of ∼650 μm hardened layer in the surface layer was caused by multiple LSP treatment, and the corresponding grain refinement mechanism was also systematically presented [18].
Service temperature is also a significant factor which affects fracture mechanism and fatigue strength of metallic materials. For example, both the yield strength and ultimate tensile strength of Ti-43Al-9V-Y alloy specimen decrease with the increase of service temperature [19]. In our previous publication, the combined effects of LSP and temperature on the fractural morphologies and micro-hardness distribution of CP titanium tensile specimens at four kinds of temperatures were investigated. It was found that the micro-hardness and UTS of CP were significantly improved after LSP treatment. However, the micro-hardness and UTS were negatively affected by high temperature while the plasticity of the CP titanium was enhanced. Furthermore, the fractural morphology evolution with increasing temperature and general fracture mechanism as a function of service temperature were also presented. With increasing service temperature from 20 °C to 350 °C, the fracture pattern was transformed from brittle fracture to mixed mode fracture to ductile fracture [20].
In fact, mechanical properties of metallic materials are often the result of micro-plastic deformation accompanying the micro-structure changes, but the tensile properties as a function of microstructures of the LSPed CP titanium at four kinds of service temperatures was not discussed in previous publications. In addition, microstructural evolution at the top surface close to the fracture zone reflects the mechanical properties of tensile specimens. Up to now, there is little publication to present microstructural evolution as a function of service temperature during tensile experiment. Hence, microstructural evolution mechanism at the fracture zone of the LSPed CP titanium at different service temperatures is worthy studying.
The aim of the work discussed in this paper is to investigate the effects of service temperature on the tensile properties and microstructural evolution of LSPed tensile specimens manufactured by CP titanium. Special attention is paid to the microstructural features at the top surface close to the fracture zone of the failed LSPed specimens at four kinds of service temperatures. Finally, the influence mechanism of service temperature on the microstructural evolution at the fracture zone of the LSPed CP titanium is revealed.
Section snippets
Material and specimen preparation
CP titanium provided by Xi'an Aerospace New Materials Co., Ltd with a mean grain size of ∼15 μm was adopted in the present study, and its chemical composition was listed as below: 30 wt% Fe, 10 wt% C, 15 wt% Si, 5 wt% H, 5 wt% N, 15 wt% O, and balanced Ti. Fig. 1a presented the dimensions of “dog-bone” tensile specimens, which were cut from a same plate. Both sides of each specimen were progressively mechanically polished using SiC paper with grit numbers from 500 to 2000 (including 500#, 800#,
Tensile properties of LSPed specimens at different service temperatures
Fig. 3a shows the engineering stress-strain curves of LSPed specimens at four kinds of service temperatures. It can be found that the engineering stress of all tensile specimens reach to the yield point within a small engineering strain and then change into the plastic stage. In the plastic stage, the engineering stress increases with the increment of engineering strain. In the necking stage, the engineering stress decreases with the increment of engineering strain, and subsequently decreases
Conclusions
- (1)
Service temperature is the important influencing factor on the tensile properties of the LSPed CP titanium. Increasing temperature is easier to generate the tensile necking phenomenon on the fractural surface of the LSPed CP titanium. Furthermore, the UTS value of LSPed specimens gradually decreases, but the area reduction and elongation of LSPed specimens increase with the increment of service temperature.
- (2)
With the increment of service temperature from 20 °C to 350 °C, the number and width of
Acknowledgments
The authors are grateful for the support provided by National Key R&D Program of China (2017YFB1103603), National Natural Science Foundation of China (Nos. 51471078 and 51775250), Natural Science Foundation of Jiangsu Province in China (Nos. BE2016148 and BE2017142), Graduate Research Innovation Program of Jiangsu Province (KYCX18_2221), and Priority Academic Program Development of Jiangsu Higher Education Institutions.
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