1. Introduction
Titanium–aluminum alloys based on intermetallic compounds (e.g., α-Ti3Al, γ-TiAl, δ-TiAl
3, etc.) possess high high-temperature strength, strong oxidation resistance, good creep resistance, and excellent organizational stability due to the combined effect of the metallic and covalent bonds [
1,
2,
3]. However, the development of titanium–aluminum alloys is limited by poor room-temperature brittleness. Therefore, Ti
2AlNb alloys based on ordered rhombohedral phases (O) have been developed in the past decades, which were first discovered by Banerjee in his 1988 toughening experiments of Ti
3Al-based alloys [
4]. Compared to TiAl-based alloys, Ti
2AlNb alloys possess better room-temperature plasticity, fracture toughness, as well as crack extension resistance, and possess better high-temperature strength and oxidation resistance [
5,
6]. In addition, the density of Ti
2AlNb-based alloys is reduced by about 40% compared to iron-based and nickel-based high-temperature alloys without affecting the material high-temperature properties [
7]. Ti
2AlNb is potential aviation material with broad application prospects to replace nickel-based high-temperature alloys due to its superior properties.
D The morphology and corresponding mechanical properties of Ti
2AlNb alloys are highly dependent on the fabrication process (e.g., casting, forging, powder metallurgy, etc.) and heat treatment procedures (e.g., deformation, solution treatment, recrystallization, stress relief annealing, aging, etc.). Ti
2AlNb-based alloys fabricated by conventional casting methods have large grain sizes and poor properties. The large difference between the melting points of the Al and Nb elements could induce compositional segregation and uneven microstructure during the smelting process of Ti
2AlNb alloy. Thermomechanical processing (TMP) is necessary to refine the grain size and eliminate defects of smelted Ti
2AlNb alloy, such as in hot forging and hot rolling [
8]. Ti
2AlNb is a typical difficult-to-machine material due to its excellent room- and high-temperature mechanical properties. High cutting forces and high local cutting temperatures could be induced to cause severe tool wear during the cutting process of Ti
2AlNb alloy. Therefore, it is worth researching the cutting process of Ti
2AlNb alloy. Finite element simulation is a low-cost and high-efficiency methodology for illustrating the cutting mechanism of Ti
2AlNb alloy. The prediction accuracy of finite element simulation is dependent on the established Johnson–Cook (J-C) model of Ti
2AlNb alloy [
9,
10].
Many studies on the methods of establishing the J-C model have been conducted for nickel-based alloys and titanium alloys [
11,
12,
13,
14]. These current methods could guide the establishment of the J-C model of Ti
2AlNb alloys. Hou et al. [
15] obtained stress–strain curves with the quasi-static compressive and split Hopkinson pressure bar (SHPB) experiment results of Ti-6Al-4V. The material constitutive model was modified with the function proposed to describe the coupling between temperature and strain. Tian et al. [
16] conducted similar experimental processes of GH2132 nickel-based alloy. The strain rate sensitivity coefficient was modified with a bivariate quadratic function of temperature and strain rate for the established J-C model of GH2132. Ling et al. [
14] established the J-C model considering recrystallization softening for nickel-based powder metallurgy superalloys. Lin et al. [
17] conducted experimental research on the mechanical behavior of 5A06 aluminum alloy in three different processing and heat treatment states at 25–500 °C and strain rates of 10
−4–10
−3 s
−1. Based on the J-C constitutive model, the constitutive model parameters of the materials in each state were fitted through experimental data, and the strain rate strengthening term in the Johnson–Cook constitutive model was modified. Zhou et al. [
18] conducted AZ91D compression experiments at a strain rate of 400–1000 s
−1, and the results showed that the strain rate sensitivity of AZ91D magnesium alloy increased with increasing strain rate. Hu et al. [
19] tested the dynamic mechanical behavior of V-5Cr-5Ti vanadium alloy at a strain rate of approximately 3000 s
−1 at temperatures ranging from 15–1100 °C, and the results showed that the J-C model can accurately describe the dynamic mechanical behavior of vanadium alloy. It was found that necessary modifications should be conducted for the J-C models according to the characteristic stress–strain curves of the experimental material.
In the past decade, the constitutive models for Ti2AlNb-based alloys with various TMP also have been researched by some researchers. He et al. [
20] established the Arrhenius and J-C constitutive models for the high-temperature deformation of hot-rolled Ti-22Al-25Nb (at.%) based on uniaxial tensile tests. They found that the Arrhenius model was suitable for relatively low strain rate deformation. The J-C model was more suitable for wide ranges of strain rates. The strain softening effect on the flow stress should be considered within the experimental temperature ranges of 930–990 °C. Xue et al. [
21] carried out a room-temperature compression test of Ti
2AlNb alloy with ultrasonic amplitude range of 0–31 µm and strain rate of 0.001–0.125 s
−1. The Johnson-Cook (J-C) constitutive model was established with similar methods as shown in the literature. Sim et al. [
22] conducted the isothermal uniaxial compression test of the fine-grained Ti
2AlNb-based alloy fabricated by mechanical alloying and subsequent spark plasma sintering in the deformation temperature range of 950–1070 °C and the strain rate range of 0.001–1 s
−1. He et al. [
23,
24] illustrated that the dynamic flow stress of Ti
2AlNb is sensitive to the competition of the thermal softening effect with strain rate hardening effect. Wang et al. [
25] conducted the uniaxial tension experiments of hot-rolled Ti-22Al-23Nb-2(Mo, Zr) alloys at both room (RT, 28 °C) and elevated temperatures (500, 550, and 650 °C). They found that the peak stress was significantly relevant to the test temperature. The stress softening effect should be considered in the modified J-C model. In the above research, the maximum strain rate is only 10
−3, which is an order of magnitude different from the range of cutting strain rates. At the same time, there is still a lack of corresponding research on whether there is a coupling effect in parameter components and the quantitative relationship between coupling effects.
Knowledge of the dynamic mechanical properties of Ti2AlNb based on the accurate Johnson–Cook (J-C) model could guide the accurate finite element simulation for metal cutting. In this paper, the dynamic mechanical properties of hot forged Ti2AlNb are analyzed with stress–strain curves based on quasi-static compressive and uniaxial impact tests over wide ranges of temperature (25–800 °C) and strain rate (4000–12,000 s−1). The J-C constitutive model was established and modified with consideration of the thermal and stress softening effect, etc. The relative errors between the experimental measured value and predicted values in various experimental conditions were obtained. The results verified that the modified J-C model could accurately describe the dynamic mechanical properties of Ti2AlNb at high temperatures and strain rates.