Study on the Microstructure and Mechanical Properties of 60NiTi Alloy Quenched by Hot Oil

Abstract

60NiTi alloys have a tremendous potential to be used in aerospace, marine and automotive industries. There is still a need to further improve the deformability due to the high brittleness of the previously prepared 60NiTi. In this work, 200 °C hot silicone oil was selected as the quenching medium for 60NiTi for the first time to overcome its high brittleness. It is found that the unique microstructure of 60NiTi quenched by hot oil has a lamellar structure composed of a channel-like NiTi matrix and lenticular Ni₄Ti₃ phase containing a nano-lath NiTi phase. The 60NiTi exhibits a high compression fracture strain of 10% and large reversible strain of 7.5%; which originates from the superelastic behavior of the NiTi SMA constituent. Upon loading, the R phase reorientation releases the stress concentration at the initial stage; while the stress-induced martensitic transformation accommodates the large elastic deformation of the Ni₄Ti₃ phase at the later stage. This synergistic effect of the two promotes the compressive deformability.

1. Introduction

60NiTi alloy is a well-known NiTi shape memory alloy; it is commonly referred to as Ni₅₅Ti₄₅ (at.%), containing 60 wt.% Ni and 40 wt.% Ti. Owing to its high hardness, excellent corrosion resistance, low density and high wear resistance, it has been used in many applications in triboelements, bearings, gears, tools, etc. [1,2,3]. 60NiTi alloy has a B2–NiTi matrix and contains one or more hard secondary phases, including Ni₄Ti₃, Ni₃Ti₂ and Ni₃Ti intermetallics. Nanoscale Ni₄Ti₃ precipitates with a volume fraction of about 70% in the Ni-supersaturated NiTi matrix contribute the main strengthening effect [4,5]. Due to the large discrepancy of thermal expansivity between the B2–NiTi matrix and Ni₄Ti₃ phase, the 60NiTi can easily exert excessive internal residual stresses that can lead to quench cracking [6]. Moreover, the high-volume fraction precipitates, high residual internal stress and thermal cracking embrittle the 60NiTi during machining or loading; this severely limits machined forming and stable and safe service [7].

There are some works that have been performed to reduce the thermal cracking during the quenching treatment and improve the deformability of the 60NiTi alloy. Zhang and Yuan et al. have explored the effect of elements doping on the mechanical properties of the 60NiTi alloy, such as Nb, V, Hf, etc.; they found that the toughness is only slightly improved without a significant loss of hardness and wear resistance [8,9,10]. In addition, the effect of various quenchants on the mechanical properties of 60NiTi has been investigated; these include ice water, room-temperature vegetable oil, etc. [11] The results indicated that the quenchants also have little effect on the mechanical properties of 60NiTi.

Some researchers investigated the effects of aging time and temperature on the mechanical properties of 60NiTi alloys [12,13,14]. It was found that as the aging temperature increased from 400 °C to 500 °C, there was little improvement on the toughness due to the release of some residual stresses; however, it still showed embrittlement characteristics. When the aging temperature range was 600–650 °C, the transformation of the Ni₄Ti₃ phase to Ni₃Ti₂ and Ni₃Ti leads to a loss in strength, and improvement in toughness with prolonged aging time. In addition, the effect of solution temperature on ductility was also investigated [15]. It is found that at lower solution temperatures (980 °C), the presence of undissolved Ni₃Ti resulted in a reduction in the supersaturation degree of Ni content in the NiTi matrix and a decrease in the content of the Ni₄Ti₃ phase; although this contributed to a higher fracture toughness, this effect diminished with increasing solution temperature (1030 °C/1055 °C). All in all, up to now, the high brittleness of 60NiTi has not been effectively solved. Hence, it is crucial to explore the methods that could improve the toughness of the 60NiTi alloy to ensure the safe service of the components in various applications.

In this work, inspired by the method of avoiding excessive thermal stress in steel by raising the temperature of the quenching medium during quenching [16,17], hot silicone oil at 200 °C is innovatively used as a quenching medium of the 60NiTi alloy after a solution treatment without other treatments. The 60NiTi exhibits a large compression strain (~10%) and a high fracture stress (~2.2 GPa). In addition, 60NiTi shows a quasi-linear reversible elastic deformation (~8%) with only ~0.5% residual strain. The microstructural and phase transformation behavior characterization was conducted to reveal the underlying mechanisms for the superior toughness and large linear superelasticity of the 60NiTi quenched by hot silicone oil.

2. Materials and Methods

60NiTi alloys, with a nominal composition of 60Ni wt.% and 40Ti wt.%, were prepared by vacuum induction melting. The ingot was heat-treated at 980 °C for 4 h followed by silicone oil quenching at 200 °C for 5 min; and then, air-cooled to room temperature.

Cylindrical shaped specimens with a geometry of 3 mm × 6 mm (diameter × length) were cut by wire EDM from the hot oil and water-quenched samples for compression testing. The compressive tests were performed at room temperature (25 °C) using an MDT series universal compression machine (KQL Test Instrument Co., Ltd., Shenzhen, China); the strain rate was 10⁻³ s⁻¹. In the cyclic compression tests, the sample was first compressed to 6%; then loaded in 2% strain increments; and unloaded to 0 MPa to quantify the level of recovery and non-recoverable deformation. Loading at increasing strain levels continued until the failure of the sample.

The fracture morphology of the sample after compression tests was characterized by an SEM FEI Quanta 200F scanning electron microscope (SEM) (FEI Company, Hillsboro, OR, USA). The transformation behavior of the sample was characterized using a Q20-2503 differential scanning calorimeter (DSC from TA Instruments Waters Corporation, Newcastle, Delaware, USA) with a heating/cooling rate of 10 °C/min. Thermal-induced transformation behaviors were investigated by in situ X-ray diffraction (XRD, NANOPIX-WE system, Rigaku Co., Tokyo, Japan; Mo rotating anode target X-ray source with a wavelength of λ, 0.7093 Å). XRD samples were prepared by initial EDM cutting of 1 mm × 2 mm × 20 mm (thickness × width× length) thick slices, followed by mechanical grinding into 140 μm foils using sandpaper. The microstructures were observed by a FEI Tecnai G2 F20 transmission electron microscope (TEM) (FEI Company, Hillsboro, OR, USA). The TEM sample was ground; it was then electropolished using a RL-I twinning-jet thinning electropolishing device (Beijing Ruiling innovation Tech. Co., Ltd., Beijing, China) at an operating voltage of 30 V, using an electrolyte solution consisting of 33% HNO₃ in methanol maintained at −30 °C.

3. Results

Figure 1 shows the XRD pattern of 60NiTi quenched in silicone oil with a temperature of 200 °C after solution at 980 °C for 4 h. The XRD result indicated that the 60NiTi alloy consisted of the cubic B2–NiTi matrix phase and rhombohedral Ni₄Ti₃ phase. According to previous studies, the content of the B2–NiTi matrix phase in 60NiTi is usually only about 30%; and the Ni₄Ti₃ phase is about 70% [12,15,18]. The lattice parameters of B2–NiTi and Ni₄Ti₃ can be determined from the d-spacing values of the XRD results in this work [19]: B2–NiTi (a = b = c = 2.966 Å); and Ni₄Ti₃ (a = b = 11.085 Å, c = 5.224 Å). Compared with the standard PDF card, it can be seen that both the B2–NiTi matrix and the Ni₄Ti₃ phase have lattice distortion; this may be caused by the internal stress generated during the hot-oil quenching process.

Figure 2 shows the thermally induced transformation behavior of the quenched 60NiTi samples. Figure 2a shows that the DSC curves range from −160 °C to 200 °C; the inset is an enlarged view of the transformation range indicated by the dotted line. The unconspicuous endothermic peak and exothermic peak in the heating and cooling curves, respectively, correspond to the forward and reverse transformation peak. It is well-known that the phase transformation type of binary NiTi alloys can be initially determined by the magnitude of the difference between the start and finish temperatures of the forward and reverse transformation in DSC (A_f-M_s); however, the DSC transformation peaks in this work are unconspicuous. Thus, in this work, the type of thermally induced transformation is determined by the difference between the peak temperatures of the forward and reverse transformation (A_p-R_p). The small peak transformation hysteresis (as measured between the peak temperatures of the forward and reverse processes, marked by arrows in Figure 2a) indicates the sample may experience a single stage B2↔R transformation; which originates from the inhibition of the conventional B19′ martensitic transformation by the high content of the nano Ni₄Ti₃ phase and supersaturated Ni in the matrix (stress-induced B2→B19′ and R→B19′ transformations are still possible) [20,21]. In addition, the small transformation peaks may cause by only a small volume fraction of the transformable B2–NiTi matrix.

As shown in Figure 2b, in situ XRD was used to explore the micro-phase transformation behavior of the 60NiTi alloy It is found that the alloy essentially consists of the B2–NiTi and Ni₄Ti₃ phases. By local amplification of the part patterns (Figure 2c), a very weak R-phase peak ((103)_R) appears at 20 °C during cooling; it remains until −180 °C. This confirms that a small volume fraction of the B2–NiTi matrix is thermally transformed to the R phase; while it has not been reported in samples with water quenching and water quenching followed by aging treatment [7]. No B19′ phase was detected during the overall temperature range, which was fully suppressed to below −180 °C.

Figure 3a shows the TEM bright-field images of the 60NiTi. The bright and dark of the Ni₄Ti₃ and NiTi phases are distributed in long strips and interlaced networks. Figure 3b exhibits its one selected area electron diffraction (SAED) along the [111]_B2 axis; in addition to the main diffraction spots of the {110}_B2, there are extra diffraction spots of the Ni₄Ti₃ phase and weak R-phase spots at the classic 1/3 positions of {110}_B2. Figure 3c shows its microstructures in scanning TEM (STEM) mode. Based on the atomic number contrast of the STEM image, the bright area is the Ni₄Ti₃ phase with a large volume fraction; and the dark area is the transformable NiTi matrix with a small volume fraction, corresponding to Figure 3a. It is noted that some fine NiTi phases are also distributed inside the Ni₄Ti₃ phase, as shown in Figure 3a,c; in which special morphology is not found in the conventional water-quenched 60NiTi samples reported previously.

In order to identify the special fine NiTi phase in lenticular Ni₄Ti₃, high magnification images and their corresponding STEM were taken; this is shown in Figure 4a,b. The NiTi phase inside Ni₄Ti₃ is lath shape of about 15 nm. Figure 4c,d show the high-resolution TEM micrograph of a small lath and the corresponding fast Fourier transform (FFT) image. It is seen that the superlattice reflections at the 1/3 positions in the B2 diffraction spots demonstrate the small NiTi lath is R phase. No R phase was detected in the above room temperature XRD pattern; this is probably because of the nanoscale size and a small number of R phases. Therefore, the NiTi exhibit two phases in dual-scale: one is the B2 parent phase showing a large-size lamellar structure with a Ni₄Ti₃ phase; and the other is the nano-R phase distributed inside the Ni₄Ti₃ phase. The nano NiTi phases are strongly constrained and internally stressed inside the Ni₄Ti₃ phase; embodying their transformation to the small strain-scale R phase, as shown in Figure 4c.

In the present study, the special fine lath shape NiTi phase inside the Ni₄Ti₃ phase originated from the partial NiTi residual caused by insufficient precipitation of the Ni₄Ti₃ phase during the quenching process. This is because quenching under hot oil has a smaller cooling temperature gradient than water quenching. During the water quenching, the Ni₄Ti₃ phase will not grow as much; the NiTi phase still shows a large-sized channel structure and will not be part-coated by the Ni₄Ti₃ phase, as in Figure 4a,b. Quenching under hot oil at 200 °C in this study causes the 60NiTi to have unique two-phase NiTi structures in dual-scale; this is in addition to the Ni₄Ti₃ phase, which is expected to exhibit more novel performances than previously reported.

According to the experimental results of X-ray diffraction, residual stress increases with lattice strain. The Williamson–Hall equation was applied to compare the internal residual stress of the same water-quenched and hot oil-quenched samples after 4 h of solution at 980 °C [22,23]:

β cosθ/λ = 1/D + 4εsinθ/λ

(1)

where β is the peak broadening measured as the full width at half maximum (FWHM), θ is the Bragg angle, λ is the X-ray wavelength, D is the crystallite size, and ε is the lattice strain. According to the calculated results, the hot oil-quenched samples have a lower lattice strain (0.00476 < 0.00654) compared to the water-quenched samples; indicating that the 60NiTi quenched by hot oil has lower residual internal stress. Low residual internal stress may mitigate the high brittleness of 60NiTi.

Figure 5a shows the compression stress–strain curves with incremental strains until fracture. The 60NiTi shows an ultimate strength of 2.2 GPa and a fracture strain of 10%. In addition, there is a large reversible strain under the compressive superelasticity strain of 8% (only 0.5% residual strain). This may be derived from the fact that the B2–NiTi matrix undergoes uniform recoverable stress-induced martensitic phase transformation; and the Ni₄Ti₃ phase, as hard intermetallic, experiences a large elastic strain. This has been demonstrated in Ni-rich NiTi alloys with similar Ni₄Ti₃ nanoprecipitates [6,18,20]. As shown in Figure 5b, in order to characterize the toughness of 60NiTi more accurately, three direct compression fracture experiments were performed on the 60NiTi alloy. The experimental results were statistically found to be close to the cyclic compression test; (σ_fracture = 2.18 ± 0.12 GPa, ε_fracture = 10.61 ± 0.31%) were obtained. The 60NiTi exhibits a higher fracture strain compared to the water-quenched samples and aged samples [12,24]. In addition to the low residual stress as discussed above, the unique dual-scale NiTi with two phases is another key reason. The soft nanostructured R phase encapsulated in the hard phase Ni₄Ti₃ is susceptible to reorientation during the early stage of compression; which can release the high internal stress around the Ni₄Ti₃ phase and delay the local stress concentration [25]. When the deformation continues, the large-size NiTi matrix in the B2 phase can undergo stress-induced martensitic phase transformation and further release the interfacial stress; in addition, the synergistic effect of these two stages avoids premature failure [26,27].

Figure 5c shows the compressive fracture morphology of the sample, in which the yellow dotted line indicates that the crack propagation path is tortuous; indicating that crack propagations were hindered by the lamellar structure. In the higher magnification images (Figure 5d), the part fracture exhibits typical dimple features; implying that the NiTi component underwent a relatively complete phase transformation process. These two factors promoted the 60NiTi to undergo extensive ductile deformation before fracture.

Based on the existing results, this large reversible strain may come from the coupling effect of the NiTi and Ni₄Ti₃ phase. Upon loading and unloading, the NiTi phase undergoes uniform recoverable stress-induced phase transformation (B2→R→B19′); and the Ni₄Ti₃ phase, as hard intermetallic, experiences a large elastic strain. Stress-induced martensitic transformation of the B2–NiTi matrix produces a uniform lattice shear strain up to 7%; it can accommodate large elastic strains of the Ni₄Ti₃ phase and avoid the strain localization between the two-phase interfaces [28,29]. This has been demonstrated in Ni-rich NiTi alloys with similar Ni₄Ti₃ nanoprecipitates [30,31].

4. Conclusions

In this work, for the first time, silicone oil with a temperature of 200 °C was selected as the quenching medium for 60NiTi alloys. This novel quenching process resulted in a unique microstructure, constituting a lamellar structure composed of long channel-like NiTi at the B2 phase and lenticular Ni₄Ti₃ wrapped with a fine nano lath-like R phase. The low residual internal stress and double stress–release effect by the R phase and B19′ martensitic transformation during compression embody a compressive fracture strain of 10%. The superelastic strain of 7.5% originates from the strong coupling effect between the NiTi and Ni₄Ti₃ phase.