Low cycle fatigue crack initiation in Ti-5Al-5Mo-5V-3Cr in relation to local crystallographic orientations

doi:10.1016/j.matlet.2020.128198

Materials Letters

Volume 276, 1 October 2020, 128198

https://doi.org/10.1016/j.matlet.2020.128198 Get rights and content

Highlights

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Low cycle fatigue lives were evaluated in Ti-5Al-5Mo-5V-3Cr.
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Prismatic slip bands are associated with crack formation in primary α.
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Crack initiation occurs in β grains with specific crystallographic orientations.
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Multiple lengthscales have to be considered to predict crack initiation sites.

Abstract

Low cycle fatigue crack initiation was investigated in a Ti-5Al-5Mo-5V-3Cr alloy with a bimodal microstructure. In particular, the crystallographic orientations of both α and β phases at crack initiation sites were characterized. Crack formation is shown to generally proceed along prismatic slip bands in primary α grains. A key role of the interplay between elastic and plastic anisotropies of both α and β phases was evidenced as the crystallographic orientation of the surrounding β matrix is a critical parameter as well. Different lengthscales have thus to be considered for an accurate prediction of fatigue crack initiation in near-β titanium alloys.

Introduction

High strength near-β titanium alloys are extensively employed in the aerospace industry for their outstanding strength-to-weight ratio combined with a good corrosion resistance [1]. In particular, Ti-5Al-5Mo-5V-3Cr is the most recent addition to the list of commercialized β titanium alloys for airframe applications. This alloy offers similar mechanical properties with an improved processing tolerance in comparison with the widely used Ti-10V-2Fe-3Al [2]. The fatigue performance is critical for engineering components and an intense research work was carried out in this field over the past years [3], [4], [5], [6]. However, to the best of the authors’ knowledge, no detailed investigation of the crystallographic orientations at crack initiation sites has been reported yet in spite of its importance regarding the prediction and the understanding of the fatigue durability. While the primary α phase has a key role in the crack initiation process [7], recent studies revealed a highly heterogeneous deformation behavior in relation to the crystallographic orientation of the β matrix [8]. In particular, the pronounced elastic anisotropy of the β phase [9] governs the onset and development of plastic slip. Thus, the crystallographic orientations of both α and β phases at crack initiation sites in Ti-5Al-5Mo-5V-3Cr submitted to low cycle fatigue loadings were investigated in the present study.

Section snippets

Materials and procedure

Solution treated and aged Ti-5Al-5Mo-5V-3Cr was provided by SAFRAN Landing Systems. The bimodal microstructure is composed of primary α grains with an average diameter about 2.5 µm and secondary α lamellas embedded in the β matrix. A scanning electron micrograph is shown in Fig. 1a. Forging steps such as applied during the processing route can induce crystallographic and morphologic textures of β grains. In order to consider these microstructural features, samples were machined along 3

Results and discussion

The half stress amplitude is plotted as a function of the number of cycles in Fig. 2a for C specimens tested up to failure. The evolution reveals a limited but noticeable softening. Beyond 90% of the lifetime, the stress values are suddenly dropping, suggesting a rapid long crack growth. Lifetimes are comprised between 9,000 and 30,000 cycles. No clear relationship could be evidenced between the machining direction and the fatigue behavior. Fractographic analyses revealed that crack initiation

Conclusion

The low-cycle fatigue properties of Ti-5553 with a bimodal microstructure were investigated with a special emphasis on the role of crystallographic orientations on crack formation. Crack initiation was found to generally proceed along prismatic slip bands in primary α grains and embedded in β grains with specific crystallographic orientations. Elastic and plastic anisotropies with different associated lengthscales are thus expected to play a key role in the fatigue behavior of near-β alloys.

CRediT authorship contribution statement

A. Helstroffer: Investigation, Visualization, Conceptualization, Writing - review & editing. S. Hémery: Visualization, Conceptualization, Supervision, Writing - original draft. S. Andrieu: Conceptualization, Resources, Supervision, Writing - review & editing. P. Villechaise: Conceptualization, Supervision, Writing - review & editing.

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.

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Featured LetterLow cycle fatigue crack initiation in Ti-5Al-5Mo-5V-3Cr in relation to local crystallographic orientations

Highlights

Abstract

Introduction

Section snippets

Materials and procedure

Results and discussion

Conclusion

CRediT authorship contribution statement

Declaration of Competing Interest

Mater. Sci. Eng., A

Mater. Sci. Eng., A

J. Mater. Res. Technol.

J. Alloy. Compd.

Acta Mater.

Featured Letter
Low cycle fatigue crack initiation in Ti-5Al-5Mo-5V-3Cr in relation to local crystallographic orientations