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