Effect of Al addition on properties of TiNiFe shape memory alloys

XIAO Li(肖 莉), LIU Fu-shun(刘福顺), XU Hui-bin(徐惠彬)

School of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics,

Beijing 100083, China

Received 20 April 2006; accepted 30 June 2006

Abstract: A little amount of aluminum substituting for Ni was added to Ti₅₀Ni₄₈Fe₂ and Ti₅₀Ni_47.5Fe_2.5 alloys to improve the mechanical properties, especially the yield stress of the TiNiFe alloys. The martensitic transformation temperature and mechanical properties of Ti₅₀Ni_48-xFe₂Al_x and Ti₅₀Ni_47.5-xFe_2.5Al_x (x=0, 0.5, 1) alloys were examined, and it was revealed that 0.5% and 1%(mole fraction) aluminum addition lead to about 10℃ and 60-80 ℃ martensitic transformation temperature (M_S) decrease, respectively. 1%(mole fraction) aluminum addition enhances remarkably the yield stresses of Ti₅₀Ni₄₇Fe₂Al₁ and Ti₅₀Ni_46.5Fe_2.5Al₁ to 560 and 580 MPa, respectively. The systemic microstructure analysis indicates that the second phase Ti₂Ni at the grain boundaries plays an important role in improving the mechanical properties of TiNiFe shape memory alloys.

Key words: shape memory alloy; yield strength; martensitic transformation temperature

1 Introduction

NiTi-based shape memory alloys combine good functional and structural properties, such as high strength, corrosion resistance, long fatigue life, stable configuration and elevated temperature stability[1-4]. Compared with Cu-based and Fe-based shape memory alloys, near-equiatomic TiNi alloys are the most important shape memory alloys (SMA) because of their superior shape memory effect (SME) and pseudoelasticity(PE). Up to now, NiTi based SMAs are applied in various fields as functional materials[5-7]. Extensive research was conducted on the transformation behaviors and mechanical properties of TiNi alloys. It was recognized that TiNi SMAs’ properties and transformation temperatures vary dramatically with their composition[8] as well as their thermal or mechanical experiences, such as rolling[9], aging[10] and thermal treatment[11]. During the past years, a great deal of attentions were paid to improve NiTi shape memory properties and mechanical properties by addition of ternary elements. TiNiFe[12-15] and TiNiNb[16] are the most popular ternary alloys applied for coupling or sealing, when an operating temperature is range from –50 ℃ to about 100 ℃. TiNiNb alloys with wide hysteresis are developed to avoid storing and installing at cryogenic temperature after deformation in the martensitic state

. However, the occurrence of strain-induced martensite in TiNiNb alloys could lead to softening of coupling at aircraft working ambient temperature. Since the cryogenic transformation temperature of TiNiFe alloys could be obtained and controlled readily by Fe additive, they became the first choice of pipefitting in aeroplane pipe couplings[15]. In this paper, the effects of aluminum added into TiNiFe alloys on the phase transformation temperature, the mechanical properties and microstructure were systemically discussed.

Ti₅₀Ni_48-xFe₂ and Ti₅₀Ni_47.5-xFe_2.5 (x=0, 0.5, 1) alloys were melted six times in a high frequency induction vacuum furnace with levitation melting technique. The ingots were homogenized in a vacuum furnace at 850 ℃ for 24 h, and then hot-worked into approximately 1.5 mm thickness sheets followed by annealing at 450 ℃ for 30  min. The specimens for tensile tests were spark-cut with a gauge dimension of 30 mm×2 mm×1.5 mm. Tensile tests were performed using a MTS (model 880) at ambient temperature. Electrical resistance samples were spark-cut to be 15 mm×1 mm×1 mm. Phase transformation temperatures were determined by electrical resistivity measurement through heating and cooling the specimens between liquid nitrogen or helium temperature and 50℃. TEM samples were electro-polished in an electrolyte with 75% methanol and 25%(volume fraction) nitric acid at –15 ℃ and 12 V. TEM work was carried out on JEOL-2000 EX-II transmission electron microscopy. 

3.1 Phase transformation behavior

Fig.1(a) and (b) show the phase transformation temperatures of Ti₅₀Ni_48-xFe₂Al_x and Ti₅₀Ni_47.5-xFe_2.5Al_x (x=0, 0.5, 1) alloys between liquid nitrogen temperature and 50 ℃, where the top line corresponds to cooling process and bottom line represents heating one. From Fig.1(a) and (b), the R-phase and martensitic transformation temperatures (marked with a steep increase or decrease in resistance) of Ti₅₀Ni_48-xFe₂Al_x and Ti₅₀Ni_47.5-xFe_2.5Al_x (x=0, 0.5) alloys are decided, but it is difficult to obtain exact those of Ti₅₀Ni₄₇Fe₂Al₁ and Ti₅₀Ni_46.5Fe_2.5Al₁ alloys, because the temperature of the latter two alloys are near to liquid nitrogen temperature. The electrical resistivity vs temperature curves of Ti₅₀Ni₄₇Fe₂Al₁ and Ti₅₀Ni_46.5Fe_2.5Al₁ alloys with temperature between liquid helium temperature and 25 ℃ are tested to confirm the transformation temperature, as shown in Fig.1(c).

On cooling, the martensitic transformation start temperatures of Ti₅₀Ni₄₈Fe₂ and Ti₅₀Ni_47.5Fe₂Al_0.5 and Ti₅₀Ni₄₇Fe₂Al₁  alloys are -88, -94 and -171 ℃, and those of Ti₅₀Ni_47.5Fe_2.5, Ti₅₀Ni_47-Fe_2.5Al_0.5 and Ti₅₀Ni_46.5Fe_2.5Al₁ alloys are -118, -130 and -176 ℃, respectively. Relative to Ti₅₀Ni₄₈Fe₂ and Ti₅₀Ni_47.5Fe_2.5 alloys, the M_S temperature decreases about 10 ℃ observed with 0.5% Al(mole fraction) addition, while at about 60-80 ℃ decreases with 1% Al additive. On heating, the reverse martensitic transformations begin at temperature A_s and finish at temperature A_f. It can be seen that the martensitic transformations of alloys without and with 0.5% Al(mole fraction) exhibit a large temperature hysteresis, while almost no temperature hysteresis is found in the Ti₅₀Ni₄₇Fe₂Al₁ and Ti₅₀Ni_46.5Fe_2.5Al₁ alloys as demonstrated in Fig.1(c). Furthermore, the resistance-temperature curves of Ti₅₀Ni₄₇Fe₂Al₁ and Ti₅₀Ni_46.5Fe_2.5Al₁ specimens are not similar to those of other four alloys, which go with an abrupt resistance increasing at R_s and stop decreasing at M_s temperature. The reason for these phenomena needs a further investigation.

Fig.1 Electrical resistivity vs temperature curves between liquid nitrogen temperature and 50 ℃for Ti₅₀Ni_48-xFe₂Al_x(a), Ti₅₀Ni_47.5-xFe_2.5Al_x alloys(b) (x=0, 0.5, 1) and between liquid helium temperature and 25 ℃ for Ti₅₀Ni₄₇Fe₂Al₁ and Ti₅₀Ni_46.5Fe_2.5Al₁ alloys(c)

3.2 Mechanical properties

Stress-strain curves obtained from a MTS model 880 at room temperature are shown in Fig.2. From Fig.2, adding 0.5% Al(mole fraction) substituting Ni in Ti₅₀Ni₄₈Fe₂ and Ti₅₀Ni_47.5Fe_2.5 alloys almost does not change the yield strength of these alloys, but alloy ductility decreases a little. 0.2% yield stress peaks of Ti₅₀Ni₄₈Fe₂ and Ti₅₀Ni_47.5Fe₂Al_0.5 alloys are 330 and 342 MPa, and those of Ti₅₀Ni_47.5Fe_2.5and Ti₅₀Ni₄₇Fe_2.5Al_0.5 alloys are 336 and 345 MPa, respectively. Fracture strengths of Ti₅₀Ni₄₈Fe₂ and Ti₅₀Ni_47.5Fe₂Al_0.5 alloys are both 680 MPa, and those of Ti₅₀Ni_47.5Fe_2.5and Ti₅₀Ni₄₇Fe_2.5Al_0.5 are 655 and 688 MPa, respectively. However, with increase of Al content to 1.0%, the 0.2% yield strengths of Ti₅₀Ni₄₇Fe₂Al₁ and Ti₅₀Ni_46.5Fe_2.5Al₁ alloys increase dramatically to 561 MPa and 585 MPa, respectively, and associated fracture strengths are enha-

nced to 878 and 940 MPa, respectively. As a result, the total fracture elongations of alloys decrease to 11% and 12%, respectively. The mechanical properties of TiNiFe alloys with different Al content are shown in Fig.3. Because

Fig.2 Stress—strain curves of Ti₅₀Ni_48-xFe₂Al_x(a) and  Ti₅₀Ni_47.5-xFe_2.5Al_x (x=0, 0.5, 1.0) (b) alloys tested at room temperature

aluminum melting point is 933 K, which is far lower than points of titanium, nickel and iron, it is very easy to be volatilized during the melting process. The true aluminum contents of Ti₅₀Ni_47.5Fe₂Al_0.5 and Ti₅₀Ni₄₇Fe_2.5Al_0.5 alloys are likely much lower than nominal composition 0.5%(mole fraction), so a little amount solution strengthening and martensitic transformation temperature change (as discussed in section of phase transformation behavior) contributed to Al solution into TiNi matrix are obtained. When the aluminum content increases to 1%(mole fraction), the solution strengthening is one of the main reasons for the high yield strength and low fracture elongation strain.

Fig.3 Mechanical properties of Ti₅₀Ni_48-xFe₂Al_x(a) and  Ti₅₀Ni_47.5-xFe_2.5Al_x (x=0, 0.5, 1.0) (b) alloys with different Al contents

3.3 Microstructure analysis

  In order to investigate the strengthening mechanism of alloys with different Al contents, the microstructures of Ti₅₀Ni_46.5Fe_2.5Al₁ alloy were studied. The SEM image of Ti₅₀Ni_46.5Fe_2.5Al₁ alloys is shown in Fig.4. It is found that there is a little amount of dispersed second phase in TiNi matrix. From TEM image and corresponding diffraction patterns, it is confirmed that the second phase is Ti₂Ni, which always occurs at the grain boundaries or sub-boundaries with spherical or elliptical shape as demonstrated in Fig.5. It deduces that the yield strength increasing of Ti₅₀Ni_46.5Fe_2.5Al₁ alloys, as discussed in mechanical properties section, is a result of Ti₂Ni phase strengthening matrix grain boundaries and hindering dislocation movement during plastic deformation. Furthermore, the phase without Al content in alloy indicates that Al addition entirely enters into TiNi matrix and Ti₂Ni phase and strengthened TiNiFe alloy.  In contrast to the Ni-rich alloys, the solubility limit on Ti-rich side of the binary TiNi alloy is almost vertical, precipitation of Ti₂Ni phase appears only at grain boundaries and the precipitation behavior of Ti-rich alloys depends upon heat treatment temperature. KAWAMURA et al [17] found that Ti₂Ni particles with random orientation in Ti-rich alloys will precipitate at the boundaries by heat- treatment at 720 K. In the present work, the specimens were annealed at 450 ℃ for 30 min, so it is possible to precipitate a little amount of Ti₂Ni phase at the sub-boundaries. Another reason for the appearance of Ti₂Ni is that additive Al prefers substituting Ti to Ni. As a result, the alloy with Al addition is easier to precipitate the Ti₂Ni phase than TiNiFe alloys. Fe additive has strong preference for entering into Ni-site found by NAKATA et al[18], so there is no Ti₂Ni phases were found in Ti₅₀Ni₄₇Fe₃ alloys.

Fig. 4 SEM image of Ti₅₀Ni_46.5Fe_2.5Al₁ alloy

Fig. 5 TEM image of Ti₅₀Ni_46.5Fe_2.5Al₁ alloy

In Ti-Ni and TiNiFe alloys, martensites always prefer nucleation at the grain boundaries[19]. In Ti₅₀Ni_46.5Fe_2.5Al₁ alloy, Ti₂Ni phase at grain boundaries will suppress martensitic nucleation and growth. So, it deduces that Ti₂Ni phases play an important role on temperature hysteresis and martensitic transformation temperature.

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(Edited by LI Yan-hong)

Corresponding author: XIAO Li; Tel: +86- 10-82317117; E-mail: liufs@huaa.edu.cn