Phase field and room-temperature mechanical properties of C15 Laves phase in Nb–Hf–Cr and Nb–Ta–Cr alloy systems

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Abstract

The pseudo-binary NbCr2–HfCr2 Laves phase alloys exhibited a large off-stoichiometric range while the pseudo-binary NbCr2–TaCr2 Laves phase exhibited a limited off-stoichiometric range. The fracture toughness of the ternary Nb–Hf–Cr and Nb–Ta–Cr Laves phase alloys was higher than those of the binary NbCr2, HfCr2 and TaCr2 Laves phase alloys, and decreased in the (Nb,Hf)-rich region of the C15 Laves phase. Depending on the alloying species and associated defect structures, room-temperature toughness of the C15 phase can be beneficially or deleteriously affected.

Introduction

The AB2 Laves phase alloys, which are called as a topologically close packed (TCP) compound [1], [2], [3], [4], exist as a line compound with an ideal atomic size ratio, RA/RB of 1.225 (where RA and RB are diameters of A and B atoms, respectively). There are three types of Laves phases, i.e. C15 phase with a cubic structure, C14 phase with a hexagonal structure and C36 phase with a dihexagonal structure [1], [2], [3], [4]. Among the numerous AB2 Laves phase alloys, the XCr2 Laves phase alloys such as TiCr2, ZrCr2, HfCr2, NbCr2 and TaCr2 are particularly attractive for high-temperature structural materials because of their low density, superior corrosion and oxidation resistance, and high microstructural stability.

The XCr2 Laves phase alloys are generally very brittle at low temperatures as well as other types of intermetallic compounds. However, the previous studies in the pseudo-binary NbCr2–TiCr2 Laves phase alloys showed that the off-stoichiometry extending by about 7 at.% toward the (Nb,Ti)-rich region in the center of the pseudo-binary line resulted in the improved room-temperature fracture toughness [5], [6], [7]. Also, recent studies using the pseudo-binary ZrCr2–NbCr2 [8], ZrCr2–HfCr2 [8] and ZrCr2–TaCr2 [9] Laves phase alloys showed that alloying along their pseudo-binary lines generally has the effect of enhancing the room-temperature fracture toughness, and also that the off-stoichiometry extending by about 20 at.% toward the (Zr,Nb)-rich region in the pseudo-binary ZrCr2–NbCr2 alloys [8], and by 15 at.% toward the (Zr,Ta)-rich region in the pseudo-binary ZrCr2–TaCr2 alloys [9], respectively, resulted in the improved room-temperature fracture toughness [8], [9]. These results suggest that the random substitution of atoms and the defect structures, e.g. the vacancies introduced in the C15 Laves phase are promising for improving the low-temperature deformability.

From the results of the C15 Laves phase fields previously reported for the pseudo-binary ZrCr2–XCr2 Laves phase alloys [8], [9], [10], the development of a large off-stoichiometric range is expected when the group numbers of elements Zr and X in the periodic table are different but not, when they are the same. In this study, two pseudo-binary NbCr2–HfCr2 and NbCr2–TaCr2 Laves phase alloys were chosen. It is expected that the pseudo-binary NbCr2–HfCr2 Laves phase alloys exhibit a large off-stoichiometric range while the pseudo-binary NbCr2–TaCr2 Laves phase alloys do not exhibit a large off-stoichiometric range because Nb belongs to the same group number as Ta, but to different group number from Hf. First, the C15 Laves phase fields developed in two pseudo-binary NbCr2–HfCr2 and NbCr2–TaCr2 Laves phase alloys are determined. Then, a beneficial (or harmful) effect of alloying or off-stoichiometry is shown on the room-temperature fracture toughness.

Section snippets

Experimental procedures

Alloys were made using a non-consumable arc-melting furnace with a water-sealed copper hearth in an argon gas atmosphere. Starting row materials were the purity of 99.99 wt.% Cr, 99.9 wt.% Nb, 99.9 wt.% Hf and 99.8 wt.% Ta. The button ingots were remelted more than 4 times to ensure chemical homogeneity and cast into a mould with a diameter of 30 mm. Chemical analysis of the prepared alloys was not carried out, but the mass loss during melting and annealing was very small, i.e. mostly less than 0.5 

Results

Fig. 1 shows the determined isothermal phase diagrams in the Nb–Hf–Cr and the Nb–Ta–Cr ternary alloy systems at 1573 K. No other intermediate phase was identified except for the C15 Laves phase, the Cr-rich and the (Nb,X)-rich bcc solid solution phases. The C15 Laves phase field was continuous between NbCr2 and HfCr2, and had a broad off-stoichiometric range toward the (Nb,Hf)-rich region around at one-quarter concentration ratio from NbCr2 along the pseudo-binary NbCr2–HfCr2 line. The

Discussion

The TCP compounds such AB2 Laves phase alloys are well known to be generally governed by the atomic size factor, i.e. the atomic radius ratio (RX/RCr in this case) and the electron concentration (e/a) of the constituent elements [14], [15], [16], [17]. The contour maps for the electron concentration (e/a) as well as the atomic radius ratio (RX/RCr) were calculated on the present NbCr2–XCr2 alloy systems, as have been done on the pseudo-binary ZrCr2–XCr2 Laves phase alloys [8], [9].

Conclusions

The phase field, the alloying behavior and the room-temperature fracture toughness of the C15 Laves phase in the Nb–Hf–Cr and the Nb–Ta–Cr ternary alloy systems were investigated by X-ray diffraction, density measurement and Vickers indentation. The following results were obtained from the present study.

  • (1)

    The pseudo-binary NbCr2–HfCr2 Laves phase alloys exhibited a large off-stoichiometric range while the pseudo-binary NbCr2–TaCr2 Laves phase exhibited a limited off-stoichiometric range.

  • (2)

    It was

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