- •Microstructure and martensitic transformation and mechanical properties of cast Ni-rich NiTiCo shape memory alloys Nader El-Bagoury 1, 2
- •Introduction
- •Experimental procedures
- •Results and discussion
- •Microstructure
- •Fig. 1 Microstructure of investigated Ni51-xTi49Cox alloys.
- •Ni51Ti49Co0 alloy (b) Ni50.5Ti49Co0.5 alloy
- •Ni49.5Ti49Co1.5 alloy (d) Ni47Ti49Co4 alloy
- •Ni51Ti49Co0 alloy (b) Ni50.5Ti49Co0.5 alloy
- •Fig. 3 Ti2Ni ratio versus Co content in NiTi alloy.
- •Fig. 4 Ni3Ti2 precipitates in the microstructure of Ni51Ti49Co0 alloy.
- •Ni3Ti2 precipitates in small round shape.
- •Higher magnification of (a) Ni3Ti2 precipitates in needle like-shape.
- •Fig. 5 Colonies of NiTi phase inside Ti2Ni precipitates in the microstructure of Ni51Ti47Co4 alloy.
- •Fig. 6 edx of phases in Ni52Ti48Co0
- •Fig. 7 X ray diffraction patterns for NiTiCo shape memory alloys.
- •Phase transformation
- •Fig. 8 Martensitic and austenitic transformation temperatures of Ni50.5Ti49Co0.5 alloy.
- •Mechanical properties
- •Fig. 9 Hardness measurements versus Co content in NiTi alloy.
- •Conclusions
- •References
Microstructure and martensitic transformation and mechanical properties of cast Ni-rich NiTiCo shape memory alloys Nader El-Bagoury 1, 2
[Materials Science and Technology Volume 30, Issue 14 (November 2014), pp. 1795-1800]
Abstract
The influence of Co additions on the microstructure, second phase precipitates, phase transformation and mechanical properties of cast Ni51-xTi49Cox (x= 0, 0.5, 1.5 and 4 at %) shape memory alloys was investigated. At the expense of Ni, Co added to NiTi alloy significantly increases the martensitic transformation temperature. The matrix phase in the microstructure of Ni51Ti49Co0 alloy is the austenite phase (B2) in addition to martensite phase (B19') and precipitates of NiTi intermetallic compounds. However, the parent phase in the other three alloys, Ni50.5Ti49Co0.5, Ni49.5Ti49Co1.5 and Ni47Ti49Co4, is martensite. Ti2Ni phase was found in the microstructures of the all investigated alloys however, Ni3Ti2 phase precipitated only in the NiTi alloy with 0 at% Co. The volume fraction of Ti2Ni phase decreased by the addition of 0.5 and 1.5 at% Co, it is slightly increased with 4 at% of Co. Hardness value of NiTi alloy affected by Co additions.
Keywords: Ni-rich NiTiCo shape memory alloys, microstructure, martensitic transformation, X-ray diffraction, hardness.
1 Chemistry Department, Faculty of Science, TAIF University, P.O. Box 888, El-Haweyah, El-Taif, SAUDI ARABIA.
2 Casting Technology Lab., Manufacturing Technology Dept., CMRDI, P.O. Box 87, Helwan, Cairo, EGYPT.
Introduction
Shape memory alloys (SMA) are in the class of SMART materials, which exhibit Shape Memory Effect (SME) and Superelasticity (SE). Amongst available SMAs, near-equiatomic NiTi alloy is regarded as the most useful one because of its superior mechanical properties coupled with the SME. The SME has been ascribed to the crystalline phase transformation. Depending on the temperature, a SMA can be austenite, martensite or a mixture of them. Normally, a NiTi SMA transforms between the high temperature B2 phase (austenite, denoted by P) and the low temperature B19' phase (martensite, denoted by M) [1–3]. In some special situations, there exists an intermediate rhombohedral phase known as the R-phase [4]. It has been established that the properties of SMAs are very sensitive to the alloy composition, processing parameters, previous thermal and mechanical history, testing methods and conditions [5–7].
It has been well recognized that the transformation temperature of TiNi-based alloys varies dramatically with their composition. A slight change in Ni/Ti ratio or addition of a third element can give rise to a remarkable change in transformation temperature. In addition, altering Ni/Ti ratio and adding the third element can also influence the mechanical properties of the NiTi- based alloys [8-15].
Till now, a lot of studies have been devoted to the influences of the third element like Fe, Cu, Nb, etc., on the transformation behavior and shape memory effects and/or superelasticity [8-10, 14]. Elastic properties of TiNiCo shape memory alloy wires were evaluated by Andreasen to develop TiNi-based shape memory alloys for medical application [16]. However, phase transformation behavior and mechanical properties of TiNiCo alloy have not been reported. Therefore the present paper is aimed to fill this void by investigating the influence of Co additions on the microstructure, martensitic transformation and mechanical properties of Ni51-xTi49Cox shape memory alloys.