Phase Identification and Microstructure of Ni-Ti-Cu and Ni35Ti50Cu10X5 (X= Fe, Sn, Cr, Nb, Co, Mo) Shape Memory Alloys

Proceeding of The 12th International Conference On QiR (Quality in Research), Bali 4-7 July 2011

Efendi Mabruria), Bambang Sriyonoa), Bintang Adjiantoroa), DN.Adnyanab)

a)Research Center for Metallurgy-Indonesian Institute of Sciences (LIPI),

Kawasan Puspiptek Gd. 470 Serpong, Tangerang 15314,Indonesia

Email: effe004@lipi.go.id

b)Agency of Technology Application and Assesment (BPPT),

Kawasan Puspiptek Serpong, Tangerang 15314,Indonesia



ABSTRACT

This paper reports the influence of Ni/Ti ratio in Ni-Ti-Cu alloys and of the addition of fourth elements (X) in the Ni35Ti50Cu10X5 (X= Fe, Sn, Cr, Nb, Co, Mo) quarternary alloys on the phases formed and microstructure of the respective alloy. The experimetal results showed that the larger Ni/Ti ratio in Ni-Ti-Cu alloys, the larger fraction of Ti0.4Ni0.565Cu0.035 precipitate phase and the lower workability of the alloys. The fourth element added into the Ni-Ti-Cu alloys modified the room temperature phases presented in the alloys by different ways, i.e. by deppressing both B19’(NiTi) and B19’(TiNi0.8Cu0.2) phases (for X=Fe, Co, Nb), by revealing precipitate phases (for X=Sn and Cr) and by deppressing B19’ (TiNi0.8Cu0.2) phases (for X=Mo).

Keywords: shape memory allloys, nickel-titanium-copper, quarternary alloys, phase identification, microstructure.

1. INTRODUCTION

Shape memory alloys (SMA) have the ability to recover plastic strain and restore the alloys to the original shape upon heating, which corresponds to the martensitic thermoelastic transformation. This shape memory effect (SME) can be used for coupling, electrical connecting and actuation. To date, Ni-Ti SMA is found in widespread application in engineering and medical devices application [1,2]. Nickel-titanium (Ni-Ti) is the most potential shape memory alloys for industrial applications compared to Cu-based and Fe-based alloys due to higher recoverable strain, excellent corrosion resistance and stable transformation temperature. The important martensitic transformation in Ni-Ti Shape Memory Alloys is B2«B19’ where B2 phase is high temperature cubic structure and B19’ is low temperature monoclinic structure. However, intermediate structure frequently exists accompanying the B2«B19’ transformation especially in ternary alloys, e.g. B2«B19«B19’ in Ti-Ni-Cu and Ti-Ni-Pd alloys [3-5] and B2«R«B19’ in Ti-Ni-Fe, Ti-Ni-Al and Ti-Ni-Co [6,7] where B19 is intermediate orthorhombic structure and R is intermediate rhombohedral structure.

The addition of Cu into the binary Ni-Ti shape memory alloys to substitute Ni has been shown to reduce the sensitivity of the transformation temperature and to lead a narrow its transformation temperature hysteresis. Therefore, Ni-Ti-Cu alloys are the most atractive shape memory alloys for actuator application due to quick actuation response as a consequence of its small temperature hysteresis and due to its superior fatigue property. However, nowadays the reseachers are currently investigating the addition of fourth elements into the Ni-Ti-Cu alloy in order to increase the shape memory effect (SME) and to improve workability of the alloy.

Since the shape memory effect and the workability of SMA alloys strongly depend on the microstructure of the alloys, it is necessary to control the phases formed in the alloys. There have been many investigation regarding the Ti-Ni-Cu [8-11] and Ti-Ni-Cu-X [12-16] shape memory alloys, focusing on phase transformation behaviour, mechanical properties and shape memory characteristics. However, lack information are found on the role of Ni/Ti ratio on the microstructure of NI-Ti-Cu alloys. Furthermore, the investigation on Ti-Ni-Cu-X alloys are mostly focusing on the small addition of X (~2 At.%) in the alloys. Therefore, this paper presents the investigation on the influence of Ni/Ti ratio on the microstructure of Ni-Ti-Cu alloys and the influence of fourth element in Ni-Ti-Cu-X (where X= Fe, Sn, Cr, Nb, Co, Mo) alloys with respect to the phases formed in the respective alloy.