Elsevier

Materials Letters

Volume 220, 1 June 2018, Pages 205-208
Materials Letters

Effect of partial replacement of V with Nb on phase transformations and mechanical properties of Ti-5553 alloy

https://doi.org/10.1016/j.matlet.2018.03.031Get rights and content

Highlights

  • Effects of replacing V with Nb on phase transformations of Ti-5553 was evaluated.

  • Replacement of V with Nb caused no significant difference in β phase decomposition.

  • Hardenability increased in comparison to that of the original Ti-5553 alloy.

Abstract

Ti-5553 is one of the most exhaustively studied metastable β Ti alloys for load-bearing applications thanks to its excellent properties. The effect of partially replacing V with Nb on phase transformations and mechanical properties of Ti-5553 was evaluated here. The modified alloy presented similar β phase decomposition and α phase precipitation behaviors, thus generating microstructures and mechanical properties very similar to those of the original alloy. However, hardenability was significantly improved, suggesting that this new alloy is promising for the manufacture of large section parts when β phase retention during processing is important.

Introduction

Much attention has recently focused on metastable β Ti alloys for load-bearing applications, especially Ti-5553 alloy, which is employed in large section forged parts due to its enhanced processability and excellent mechanical behavior [1], [2], [3]. After undergoing solution heat treatment, the high hardenability (or “quenchability”) of Ti-5553 alloy ensures β phase retention in air-cooling conditions over a large thickness (around 150 mm). Thus, aging heat treatments can lead to controlled precipitation of α phase in the β matrix, resulting in tensile strength exceeding 1 GPa [3].

Compositional modifications have been employed as a means to improve the performance of Ti-5553 alloy [4], [5]. In previous studies, interesting results were achieved when the entire V content was replaced with Nb. Although the alloy’s hardenability increased in response to this replacement [6], its mechanical properties remained at comparable levels [7]. The partial replacement of V with Nb in the Ti-5553 alloy was investigated in this study.

Section snippets

Experimental procedure

Nb-modified Ti-5553 ingots were arc melted in an argon atmosphere. To maintain the same Mo equivalent value [8], the original Ti-5553 alloy was modified ensuring that the chemical composition (in wt%) remained at 5% Al, 5% Mo, 2.5% V, 6% Nb, 3% Cr, 0.5% Fe, <0.3% O, <0.05% N, and the balance of Ti. As-cast ingots were homogenized at 1000 °C for 720 min and water quenched (WQ). Two aging heat treatments (similar to those typically employed for the Ti-5553) were then applied: (HT1) solution

Results and discussion

Fig. 1a shows the WQ sample, which exhibits a fully β phase microstructure. Based on the XRD pattern, the β lattice parameter, aβ, was found to be 3.24 Å (Rietveld refinement using the MAUD software [9]). This is slightly higher than previous results reported by Jones et al. [1] and Sabol et al. [10], and can be explained by the larger size of the Nb atom compared to V. Upon rapid cooling, the collapse of {1 1 1} β planes along the <1 1 1> β direction is expected, producing a thermal ωath phase.

Conclusions

This study led to the following conclusions:

  • a.

    The expected β + ωath microstructure was observed in the sample quenched from the β phase field.

  • b.

    The partial substitution of V for Nb caused no significant difference in the β phase decomposition events and temperatures.

  • c.

    Hardenability increased in comparison to that of the original Ti-5553.

  • d.

    Presenting similar microstructures and mechanical properties under the same conditions, the Nb-modified Ti-5553 alloy under study is a promising material for the

Acknowledgements

The authors acknowledge the support of the Brazilian research funding agencies FAPESP (grant # 2016/24693-3) and CNPq (grant # 405054/2016-5), and the LME laboratory (LNNano/CNPEM) for access to its SEM/TEM facilities.

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