Elsevier

Journal of Alloys and Compounds

Volume 586, 15 February 2014, Pages 87-93
Journal of Alloys and Compounds

Antiphase boundary-like structure of B19′ martensite via R-phase transformation in Ti–Ni–Fe alloy

https://doi.org/10.1016/j.jallcom.2013.10.015Get rights and content

Highlights

  • APB-like structures with curved and wide contrasts were observed in martensite of Ti–Ni–Fe alloy.

  • APB-like structure via R-phase exhibited facets composed of the (0 1 0), (0 0 1), and (1 0 0) planes.

  • APB-like structure reflects the atomic movement stemming from the R-phase transformation.

Abstract

The antiphase boundary (APB)-like structure of B19′ martensite via R-phase transformation in a Ti–Ni–Fe alloy was investigated by means of transmission electron microscopy. The APB-like structure exhibited shifts along the (0 1 0)B19, (0 0 1)B19, and (1 0 0)B19 planes; that is, it exhibited facets composed of those planes at the atomic level. This atomic displacement reflects the atomic movement stemming from the R-phase transformation.

Introduction

Near equiatomic Ti–Ni and Ti–Pd alloys undergo thermoelastic martensitic transformation from the B2 to B19′ (monoclinic) and B19 (orthorhombic) structures upon cooling, respectively. The Ti–Ni alloy is a technologically important material with superior shape memory effect and superelasticity [1]. The Ti–Pd alloy is a candidate for high-temperature shape memory material because its transformation temperature is approximately 800 K [2]. We recently discovered an antiphase boundary (APB)-like contrast of the martensite in Ti–Ni and Ti–Pd shape memory alloys [3], [4]. We characterized those APB-like structures as a kind of stacking fault, with an APB-like morphology that is induced by the martensitic transformation. The characterization of the APB-like structure reveals details of the atomic movements during martensitic transformation, resulting in a further understanding of the shape memory behavior.

Recently, Inamura et al. discovered an APB-like structure in α-martensite of a β-Ti shape memory alloy [5]. The displacement vector was determined as a transformation-induced APB, with an additional small displacement stemming from a specific variant of the pre-existing athermal ω-phase. As atomic displacements before a martensitic transformation from the B2 parent phase, it is well known that the athermal ω-phase transformation forms during quenching of β-Ti alloys and that R-phase transformation in Ti–Ni-based alloys occurs. Concerning the R-phase transformation, a B2–R–B19′ two-step transformation has been observed in Ti–Ni–Fe alloys and aged Ni-rich Ti–Ni alloys with Ti3Ni4 precipitates [1]. Hwang et al. [6] and Murakami and Shindo [7] analyzed the peculiar APB-like contrast in the R-phase in a Ti–Ni–Fe alloy. Their results indicate that different subvariants can grow upon cooling and then impinge against neighboring subvariants, forming boundaries [7]. However, the relationship between APB-like structures induced by martensitic transformation and R-phase transformation itself has not been clarified. The purpose of the present paper is to investigate the crystallography and morphology of the APB-like structure in B19′ martensite via R-phase transformation in a Ti–Ni–Fe alloy by conventional transmission electron microscopy (CTEM) and high-resolution transmission electron microscopy (HRTEM). Furthermore, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) is applied to analyze the interface of the APB-like structure at the atomic level, by which the relatively heavy atom positions are identified by bright contrast in the image as a result of atomic number (Z) contrast [8], [9].

Section snippets

Experimental procedure

A Ti–47.75 at.% Ni–1.5 at.% Fe alloy was prepared from 99.7% Ti, 99.7% Ni, and 99.5% Fe (mass%) by arc melting in an argon atmosphere. A Ti–50.0 at.% Ni alloy was also fabricated as a specimen for comparison. The ingots were homogenized in an argon atmosphere at 1273 K for 86.4 ks. Subsequently, the samples were solution treated in an argon atmosphere at 1273 K for 3.6 ks and then quenched in ice water. Differential scanning calorimetry (DSC) measurements were performed using a calorimeter (DSC-60,

Results and discussion

Fig. 2 shows DSC curves for the quenched Ti–Ni–Fe alloy after solution treatment at 1273 K for 3.6 ks. There are two exothermic peaks corresponding to the B2 to R and R to B19′ transformations during cooling. The Rs, Rf, Ms, and Mf temperatures are determined to be 303, 294, 269, and 215 K, respectively, where the notations of transformation temperatures are referred from the recent review of Otsuka and Ren [1]. Furthermore, there seem to be two peaks during heating, as has been previously

Conclusions

An APB-like structure of the B19′ martensite via an R-phase transformation in a Ti–Ni–Fe alloy was investigated by means of CTEM, HRTEM, and HAADF-STEM. The APB-like structure exhibited shifts along the (0 1 0)B19, (0 0 1)B19, and (1 0 0)B19 planes; that is, facets composed of those planes at the atomic level. The atomic displacement on the APB-like structure of the martensite via an R-phase transformation in the Ti–Ni–Fe alloy reflects the atomic movement stemming from the R-phase transformation

Acknowledgments

This work was supported by JSPS KAKENHI Grant No. 24656417. 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.

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