Characterization of fracture toughness and toughening mechanisms in Laves phase Cr2Nb based alloys

doi:10.1016/j.msea.2015.04.077

Materials Science and Engineering: A

Volume 638, 25 June 2015, Pages 340-347

https://doi.org/10.1016/j.msea.2015.04.077 Get rights and content

Abstract

The effect of Ti content on the fracture toughness of the Cr₂Nb–20/30/40Ti (in at.%) alloys has been investigated in the arc melting and heat treatment (1473 K for 48 h) conditions. The results show that the reduction of grain size and the facture toughness of the alloys increased with increasing the Ti content. After the heat treatment, the facture toughness of the Cr₂Nb–20/30Ti alloys increased by almost twice, and the crack bridging was primarily observed as the toughening mechanism, and for the Cr₂Nb–40Ti alloy, the fracture toughness increased by 77% and reached 18.80 MPa m^1/2, and the toughening mechanism transited to the crack defection. The alleviation of deformation constraint during the heat treatment was elucidated to be effective in improving the fracture toughness of the alloys.

Introduction

Laves phase Cr₂Nb, as an important intermetallics, has attracted considerable attention due to its high-temperature strength, promising creep resistance, and excellent oxidation resistance [1], [2], [3]. However, like other intermetallics, its engineering applications are remarkably restricted by low fracture toughness (∼1.4 MPa m^1/2) at room temperature [4], [5].

In order to improve the fracture toughness of Cr₂Nb, the incorporation of a toughening phase forming composites was proved to be effective. Li et al. [6] suggested that the fracture toughness of Cr₂Nb can be improved significantly by introducing the Cr solid solution in binary Cr–Nb system. In ternary Cr–Nb–Hf and Cr–Nb–Ta systems, Fujita et al. [7] found that the deformability of Cr₂Nb can be enhanced markedly due to the formations of toughening bcc phase. For the same reason, Lu et al. [8] demonstrated that the brittleness and oxidation behavior of Cr₂Nb can be improved by introducing Si element. Particularly, Chan et al. [9] showed that the addition Ti was more effective, and the fracture toughness of the composites in the ternary Cr–Nb–Ti system can reach 20 MPa m^1/2. However, the volume fraction of Laves phase was too low to maintain the excellent high-temperature properties, e.g. the yield strength [10].

For balance of the volume fraction of Laves phase and the fracture toughness of Cr–Nb–Ti alloys, three Laves phase Cr₂Nb based alloys were prepared at the same Cr/Nb ratio of 2. The increase of Ti content aims to change the relative amount of Laves phase. The variation of alloy compositions can have effects on the morphology of Laves phase and hence the propagation of crack [11], [12]. Meanwhile, the corresponding toughening mechanism would also have relation with the alloy compositions. However, to our knowledge, detailed fracture behaviors in the ternary Cr–Nb–Ti alloys have remained scarcely. In this study, detailed observations on the microstructures and fracture behaviors of Cr–Nb–Ti alloys containing the crack propagations and fracture surfaces have been presented. These results can be directly used to optimize the alloy compositions and mechanical properties of Cr–Nb–Ti alloys.

Section snippets

Experimental

The high-purity metals of 99.90 wt% Cr, 99.99 wt% Nb, and 99.95 wt% Ti were used to prepare three Cr–Nb–Ti alloys with nominal compositions of Cr₂Nb–20/30/40Ti (in at%) by vacuum non-consumable arc melting under a titanium-gettered argon atmosphere (hereafter, all compositions in this paper are given in at% unless otherwise stated). Every ingot was remelted for five times to ensure the chemical homogeneity. Subsequently, the ingots wrapped in Nb foils were homogenized at 1673 K for 48 h, and cooled

Microstructure

Fig. 1 shows the XRD patterns of the Cr₂Nb–20/30/40Ti alloys processed by arc melting (AM) and heat treatment (HT) (marked as AM20/30/40 and HT20/30/40, respectively). As revealed by the XRD patterns, all microstructures consist of Laves phase Cr₂(Nb,Ti) and solid solution phase (Ti,Nb,Cr)_ss. Fig. 2 shows the SEM micrographs of these alloys. Apparently, the microstructures of the alloys are all composed of two phases, agreeing well with the results of XRD. Further, EDS results shown in Table 1

Grain refinement in the microstructures

The grain size of the alloys is reduced with increasing the Ti content both in the arc melting and heat treatment conditions. For the AM alloys, the crystallization temperature interval decreases with the increased Ti content as predicated by Cr–Nb–Ti phase diagram [14]. Thus, with the same processing parameters of the arc melting, the undercooling degree and thereby the nucleation rate of the alloys can be increased by increasing the Ti content, resulting to the reduction of grain size as

Conclusions

(1)
The fracture toughness of the Cr₂Nb–20/30/40Ti alloys increased with increasing the Ti content both in the arc melting and heat treatment conditions. It is effective in reducing the grain size and alleviating the deformation constraint during the heat treatment, which contributes to the improvement of fracture toughness of the alloys.
(2)
After the heat treatment, the fracture toughness of the Cr₂Nb–20/30Ti alloys increased by almost twice, and the toughening mechanism was the crack bridging. For

Acknowledgments

This work was supported by the National Natural Science Foundation of China (No. 51074127), the Fundamental Research Funds for the Central Universities (3102014JCQ01021), and the SRF for ROCS, SEM.

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Citation Excerpt :
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Characterization of fracture toughness and toughening mechanisms in Laves phase Cr2Nb based alloys

Abstract

Introduction

Section snippets

Experimental

Microstructure

Grain refinement in the microstructures

Conclusions

Acknowledgments

Acta Mater.

Scr. Mater.

Intermetallics

Acta Mater.

Int. J. Refract. Met. Hard Mater.

J. Alloy. Compd.

Mater. Des.

Mater. Sci. Eng. A

Mater. Sci. Eng. A

Acta Metall.

J. Mech. Phys. Solids

Mater. Sci. Eng. A

Eng. Fract. Mech.

Intermetallics

J. Alloy. Compd.

Intermetallics

Characterization of fracture toughness and toughening mechanisms in Laves phase Cr₂Nb based alloys