Elsevier

Journal of Alloys and Compounds

Volume 682, 15 October 2016, Pages 124-131
Journal of Alloys and Compounds

Enhancement of ductility in Fe-Co based alloys by substitution of Pd

https://doi.org/10.1016/j.jallcom.2016.04.275Get rights and content

Highlights

  • Co-Fe-Pd alloys consisted of a B2-matrix and Fe-Co-Pd precipitates.

  • The B2-matrix and Fe-Co-Pd precipitates had a near N-W orientation relationship.

  • Co-Fe-Pd alloys had a high tensile strength and high total elongation over 10%.

  • The ductile Fe-Co-Pd precipitates suppressed the intergranular and cleavage fracture.

Abstract

We investigated the relationship between the microstructure and mechanical properties of the ternary Co50Fe50−xPdx alloys with different Pd content. Co50Fe45Pd5, Co50Fe40Pd10 and Co50Fe35Pd15 alloys annealed at 873 K were composed of the α’-matrix having a B2 structure with a high degree of order and two kinds of Fe-Co-Pd precipitates with L10 and L12 structures, which formed at the grain boundaries and in the grain interior. The 0.2% proof stress, tensile strength, hardness and elongation were increased by substituting Pd for Fe in Co-Fe-Pd alloy. It is noteworthy that the Co50Fe40Pd10 alloy had a high tensile strength of 1293 MPa and high total elongation of 12% on average. The significant improvement in the ductility of the Co50Fe40Pd10 alloy is attributed to the ductile Fe-Co-Pd phase that precipitates at the grain boundaries and in the α′-grain interior. The ductile precipitates suppressed the intergranular and cleavage fracture. The increase in strength of Co50Fe50−xPdx alloys is likely to come from both the precipitation hardening and the grain refinement of the α′-matrix.

Introduction

Most B2-type intermetallic compounds are poor ductilities at ambient temperatures in spite of its relatively simple crystal structure [1], [2], [3]. Near-equiatomic Fe-Co alloys having a B2 structure are extremely brittle at room temperature [4], [5], [6], [7]. According to the Fe-Co binary phase diagram, Fe-Co alloys have a face-centered cubic (fcc) structure at temperature above 1258 K and a body-centered cubic (bcc) structure at lower temperatures. The bcc structure transforms into a B2 structure at temperatures below 1003 K. It has been reported that the ductility and workability of the Fe-Co alloys can be improved by using the order-disorder transformation [4], [7], [8], [9] or by the addition of vanadium [10], [11]. We also found that the B2-type Fe-Co based alloys substituted by Ni or Pd of 10 at% have a remarkably high tensile strength and elongation at room temperature [12]. The marked increase in the ductility of the Co50Fe40Ni10 alloy would be due to the considerably ductile γ-phase (Co46Fe27Ni27) which not only decorated the grain boundaries but also finely precipitated in the grain interior in the α’-phase, and that such precipitates may have suppressed the intergranular and cleavage fracture [13]. In addition, high strengthening of its alloy arises from both the solid-solution hardening of Ni and the grain refinement of the α’-matrix. However, the origin of the drastic improvement in the strength and ductility of the Co50Fe40Pd10 alloy and the microstructure have not yet been clearly understood.

In this study, we investigated the relationship between the microstructure and the mechanical properties in the ternary Co50Fe50−xPdx alloys with different Pd content. On the basis of the transmission electron microscopy (TEM) observations of the tensile specimens, the mechanism of ductility enhancement in Co-Fe-Pd alloys is discussed.

Section snippets

Experimental procedures

Fe50Co50, Co50Fe45Pd5, Co50Fe40Pd10 and Co50Fe35Pd15 alloys were prepared from 99.99% Fe, 99.9% Co, and 99.9% Pd (mass%) by arc-melting under an argon atmosphere. Those ingots were homogenized in an argon atmosphere at 1373 K for 86.4 ks Subsequently, these ingots were hot- and cold-rolled to the thickness of 0.3 mm. Tensile specimens with gauge sizes of 30 × 5 × 0.3 mm3 were spark-cut from the strip. The samples were solution-treated in an argon atmosphere at 1173 K for 3.6 ks and then

Results and discussion

The phase changes associated with the substitution of Pd for Fe in the ternary Co50Fe50−xPdx alloys were investigated using X-ray diffraction. Fig. 1 shows the XRD patterns for Fe50Co50, Co50Fe45Pd5, Co50Fe40Pd10 and Co50Fe35Pd15 alloys. All these specimens were solution-treated, following by annealing at 873 K for 86.4 ks XRD peaks corresponding to the bcc or B2 structures are only observed in the Fe50Co50 alloy. Although the Fe50Co50 alloy consists of an α’-phase with B2 structure, the XRD

Conclusions

We investigated the relationship between the microstructure and the mechanical properties in the ternary Co50Fe50−xPdx alloys with different Pd content in this study. The mechanism of the enhancement of ductility was discussed on the basis of TEM observations of the tensile specimens. The obtained results are summarized as follows.

  • (1)

    The Co50Fe45Pd5, Co50Fe40Pd10 and Co50Fe35Pd15 alloys annealed at 873 K were composed of the α′-matrix having a B2 structure with a high degree of order and two kinds

Acknowledgments

This work was supported by JSPS KAKENHI Grant No. 26420725. The authors would like to express their sincere gratitude to Professor Y. Morizono of Kumamoto University for his support in the arc melting and for his valuable comments. The authors are most grateful to Prof. T. Waitz of University of Vienna for arrangements to complete this research.

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