Low-Cycle Fatigue Behavior and Fracture Characteristics of Low-Cost Ti-2Fe-0.1B Alloy

Abstract

In recent decades, the effect of Fe element addition on titanium alloy has been investigated extensively due to the development of low-cost titanium alloys, as well as B microalloying, which could decrease the grain size of titanium alloys during the casting process. As a key structural material, the study of the fatigue behavior of titanium alloys is crucial and always attractive for scientists. Hence, in this paper, the low cycle fatigue (LCF) behavior and fracture characteristics of a low-cost Ti-2Fe-0.1B alloy with a lamellar structure were investigated systematically, five different strain amplitudes (Δε_t/2) in the range from 0.6% to 1.4% were selected to control the LCF process. It was found that the Ti-2Fe-0.1B alloy exhibits continuous cyclic softening behavior in the cycle as a whole at Δε_t/2 ≤ 1.2%, while at Δε_t/2 = 1.4%, it exhibits slight cyclic hardening at the initial stage of the cycle, then shows cyclic softening. Compared with pure titanium and other typical titanium alloys, the Ti-2Fe-0.1B alloy indicated maximum fatigue life under the same strain amplitude, it can be attributed to the fine grain size result from the effect of Fe element and trace B, which could hinder the dislocation movement and crack propagation.

1. Introduction

Titanium alloys are highly desirable for marine applications due to their excellent specific strength, non-magnetic properties, good corrosion resistance, and weldability. However, high cost limits their extensive application; therefore, it is important to develop low-cost titanium alloys with no loss properties [1,2,3,4]. The common method to reduce price is adding inexpensive alloying elements, in particular, Fe. Therefore, binary Ti-Fe titanium alloys have been studied extensively over the last decades [5,6,7,8]. Intermetallic compounds such as Ti₂Ni and TiFe₂ have a great influence on the properties of titanium alloys [9,10]. Binary Ti-Fe alloys are prone to produce some brittle intermetallic compounds, TiFe_2, during the melting process, which have a bad effect on the mechanical properties of such alloys. The addition of the B element can effectively reduce the brittleness of the intermediate phase TiFe₂ and improve the mechanical properties and toughness of the alloy [11]. For optimizing the comprehensive mechanical properties and good processability, a novel low cost Ti-Fe-B alloy was designed and developed by our group which was found to have a good combination of strength and ductility in previous studies [12,13] as well as low flow strength at high temperature deformation [14,15]. Additionally, a good thermal stability and corrosion behavior of an ultrafine-grained Ti-2Fe-0.1B titanium alloy have also been revealed in further investigations [13,16], but the fatigue properties of this alloy have not been studied systematically.

As a key load-bearing structure material, titanium alloy components are often subjected to cyclic loading, which is a great test and may lead to fatigue cracks and fractures in a short period of time and eventually cause accidents. Obviously, it is urgent to investigate the low cycle fatigue (LCF) properties of titanium alloys. Fatigue modeling is one of the common methods for material fatigue life prediction; for example, Li [17] established a novel probabilistic fatigue model, which was developed by combining weakest link theory with strain energy concept under notch and size effects. In addition, the low cycle fatigue properties of various typical titanium alloys have been studied gradually, such as commercial pure Ti, TA15, TC4 ELI, Ti80, and TC4 alloys [18,19,20,21]. For example, Gao [19] studied LCF behavior and properties of TA15 titanium alloy with tri-modal microstructure. Results indicated that the cyclic hardening/softening is determined by the competition (ε_ta < 0.9%) or superposition (ε_ta ≥ 0.9%) effect of the variations of back stress and friction stress at different strain amplitude (ε_ta) levels. αp increasing could delay the fatigue crack nucleation and propagation due to its positive effect for improving deformation compatibility. Wang [20] investigated LCF behavior of two typical titanium alloys for a deep-diving submersible, and it was found the Ti80 exhibits a slightly longer fatigue life than TC4 ELI when Δε_t/2 > 0.7% (plastic deformation predominates), while its fatigue life became shorter when Δε_t/2 < 0.7% (elastic deformation predominates), Both Ti80 and TC4 ELI showed a continuous cyclic softening behavior. Xu [21] studied the LCF behavior of Ti-6Al-4V alloy with a bimodal microstructure. It was found that the cyclic stress response of TC4 alloys at different strain amplitudes exhibited a slight cyclic hardening at the initial stage followed by different softening behavior, which was highly dependent on the applied strain amplitude. The higher the strain amplitude the higher the degree and rate of softening. It is revealed that the hardening/softening behavior related to the strain amplitude is determined by the dislocation behavior.

The aim of this work is to investigate the LCF properties of a low-cost Ti-2Fe-0.1B alloy under different strain amplitudes systematically; cyclic stress response behavior and fatigue life is received by analysis LCF curves and the fatigue fracture mechanism is revealed by the observation of microstructure evolution and fracture morphologies.

2. Materials and Methods

The experimental material was a hot-rolled Ti-2Fe-0.1B alloy bar with the actual chemical composition shown in

Table 1. Chemical compositions of the Ti-2Fe-0.1B alloy.

Elements	Fe	B	C	N	O	H	Ti
wt.%	1.89	0.08	0.014	0.004	0.062	0.0012	Balance

. The alloy was obtained using vacuum arc remelting (VAR), where raw alloy powder was pressed into mixed electrode blocks through a die and fed into a vacuum self-consumption electrode for two melting cycles to obtain ingots with uniform composition. The phase transformation temperature was approximately 854 °C. After that, the processes of forging and precision forging were performed at high temperature, and the Ti-2Fe-0.1B alloy bar with a diameter 20 mm was continuously hot-rolled directly.

According to Chinese Standard GB/T228.1-2010 (tensile test of metallic materials—room temperature test method) [22], the bar was taken along the rolling direction and processed into tensile samples with a diameter of 5 mm and a length of 25 mm for standard distance, and the schematic diagram is indicated in Figure 1a. The quasi-static tensile test was carried out by MTS 370 equipment, the velocity of separation of the clamps was 1 mm/min. Three sets of parallel experiments were set up to ensure the accuracy of the experimental results in order to obtain the average value. Based on the engineering tensile stress–strain curve, five strain amplitude (Δε_t/2), i.e., 0.6%, 0.8%, 1.0%, 1.2%, and 1.4%, were selected to perform in the LCF test. The strain ratio of Rε(ε_min/ε_max) is −1, the frequency is 0.5 Hz, and a triangular waveform were conducted on a servo hydraulic machines with MTS 370 controllers and three repeated experiments were tested for each strain amplitude. LCF tests were carried out on the cylindrical fatigue samples with a gauge section of 6 mm in diameter, 25 mm in length, and a total length of 125 mm. The dimension of fatigue samples is shown in Figure 1b. The fatigue samples were mechanically polished to minimize the effect of surface defects on the fatigue life before LCF tests. The experimental process was carried out in accordance with Chinese Standard GB/T 26077-2010 (axial strain control method for fatigue testing of metallic materials) [22], setting the peak cyclic stress drop at 25% when the samples fail, and the LCF test stopped when the samples fractured.

A scanning electron microscope (SEM) and a transmission electron microscope (TEM) were used to examine the initial microstructure, fracture morphology, and dislocation analysis. In order to observe the microstructure of the as-received Ti-2Fe-0.1B alloy, samples that were cut from the plate along the rolled direction were prepared by mechanical polishing using ≤3000 grit papers and chemical etching in a reagent, consisting of 10% HF, 15% HNO₃, and 75% H₂O. In addition, in order to observe dislocation development, TEM sample thin foils of 0.8 mm thickness were carefully cut from the gauge of the samples near the facture surface, oriented perpendicularly to the loading direction. Then, these foils were reduced to a thickness of 70–100 μm by mechanical polishing on both faces for TEM observation. Moreover, the fractured fatigue sample was examined for fracture surface morphology under a SEM so that the LCF crack nucleation and crack propagation mechanisms can be further investigated.

3. Results and Discussion

3.1. Microstructure

Figure 2a shows the microstructure of the Ti-2Fe-0.1B alloy by SEM, which is composed of lamellar α phase and a small amount of β phase (White arrow in the picture). The direction of the lamellar α phase is consistent with the rolling direction (Red arrow direction). The average grain size of the primary α-phase is 1.58 μm (Figure 2b). The TEM result is indicated in Figure 2c. It can be found that the microstructure is blurry due to the abundance of dislocation and an obvious lamellar α phase. In addition, the TiB phase is detected in the lamellar α phase, as is known, B is α stabilizer in titanium alloy, but it does not dissolve in both α and β lattices, and all the B added is utilized for TiB needle formation [23].

3.2. Quasi-Static Tensile Curves

The quasi-static stress–strain curves of the hot-rolled Ti-2Fe-0.1B alloy at room temperature are shown in Figure 3. The results show that ultimate tensile strength (σ_uts) of the alloy reaches 654 MPa and the yield strength (σ_y) is 423 MPa. The elongation to failure exceeds 20%. The cyclic softening/hardening of an alloy is determined by the nature of the material itself, and the cyclic deformation characteristics can usually be reasonably determined from the ratio of tensile strength to yield strength in monotonic tension. When the initial material belongs to soft state (σ_uts/σ_y ≥ 1.4), cyclic hardening occurs; however, in the hard state (σ_uts/σ_y < 1.2), cyclic softening occurs [24]. The calculated σ_uts/σ_y ratio of this alloy is 1.546 (>1.4), which implies that the Ti-2Fe-0.1B alloy is characterized by cyclic hardening behavior in addition to cyclic softening behavior during LCF. The strain amplitudes imposed in fatigue tests are selected according to the tensile stress–strain curve of the alloy, which ranges around the yield stress point of the alloy.

3.3. Cyclic Stress Response Behavior

Figure 4 reflects the stress amplitude changes of the Ti-2Fe-0.1B alloy for each cycle under the control of five different strain amplitudes. The results show that the change in strain amplitude has a large effect on the cyclic stress response of the Ti-2Fe-0.1B alloy, which characterizes cyclic softening in general. As indicated in Figure 4a, it can be seen that the stress amplitude of the Ti-2Fe-0.1B alloy decreases with the increase in cycle number when Δε_t/2 ≤ 1.2%, and the overall behavior exhibits continuous cyclic softening until the specimen is cyclically destabilized and fractured. However, when Δε_t/2 = 1.4%, the stress amplitude of this alloy increases with the increase in cycle number in the initial stage, a slight cyclic hardening phenomenon appears. It shows cyclic softening after that, then cyclic stability, and finally the cyclic stress decreases rapidly when the specimen cracks expand rapidly. The above curves were changed to the relationship curves between cyclic stress and cyclic life fraction in order to reveal the cyclic stress response behavior of Ti-2Fe-0.1B alloy more clearly, as shown in Figure 4b. It can be seen that the whole cyclic stress response process under the selected strain amplitude mainly consists of three main stages: (1) the initial cyclic softening stage which accounts for about 15% of the cyclic process; (2) cyclic smooth stage which accounts for about 80% of the cyclic process; and (3) cyclic destabilization stage. The stress amplitude decreases rapidly in this stage, which is macroscopically manifested by the rapid propagation of cracks, accounting for about 5% of the cyclic process. The greater the strain amplitude, the greater the degree and rate of cyclic softening of the titanium alloy; therefore, in order to demonstrate the cyclic softening behavior of the Ti-2Fe-0.1B alloy in fatigue experiments in more detail, the fatigue failure process was selected for a strain amplitude of 1.4% (Figure 4c). The Ti-2Fe-0.1B alloy shows initial cyclic softening followed by a steady state stage. This result also shows that the Ti-2Fe-0.1B alloy exhibits tensile–compression asymmetric behavior during LCF loading, and the ratio of tensile stress is actually somewhat in error with the strain ratio –1. However, the graph does not visualize the cyclic hardening behavior of the alloy in fatigue experiments, so a partial magnification of the cyclic hardening and softening behavior at the initial stage of cycling is required. Figure 4d shows a local enlargement of the initial stage of cycling in Figure 4b, and it can be clearly found that a slight cyclic hardening occurs in the Ti-2Fe-0.1B alloy at the beginning for strain amplitude at 1.4%, while the rest of the strain amplitudes exhibit cyclic softening from the beginning. This phenomenon is similar to the behavior of various titanium alloys in the literature [22,25,26].

The change degree of stress amplitude in a general fatigue test is called cyclic softening degree, and it is expressed by symbol η, which is defined as [19]:

η = (σ_ini − σ_i)/σ_i

(1)

where σ_ini, and σ_i are the stress amplitudes of the first cycle and its cycle after second cycle, respectively, and i = 2, 3, 4…. It should be noted that η > 0 means softening behavior and η < 0 means hardening behavior. The developments of η versus the number of cycles are shown in Figure 5a. It can be seen that when Δε_t/2 ≤ 1.2%, the softening degree of the Ti-2Fe-0.1B alloy shows a stable increasing trend with the number of cycles. When ∆ε_t/2 = 1.4%, η briefly falls below 0 during the first 10 cycles, the alloy shows a transient cyclic hardening feature similar to the study of pure titanium [27], and then the degree of softening increases with the number of cycles until it stabilizes. It can be observed that the softening rate of the Ti-2Fe-0.1B alloy shows an overall decrease with increasing strain amplitude until crack propagation and fracture of the alloy after intense softening.

In general, the fatigue crack of the alloy is ascribed to the accumulation of repeated plastic strains [28], so the smaller the plastic strain per cycle, the smaller the damage to the alloy and the longer the fatigue life. Figure 5b shows the plastic strain amplitude changes of the Ti-2Fe-0.1B alloy with the number of cycles in the LCF experiment. It can be seen that under strain control, the larger the strain amplitude, the larger the cyclic plastic strain amplitude of the alloy. However, at the same strain amplitude, the plastic strain amplitude of the alloy does not change much with the increase in the number of cycles, which indicates that the Ti-2Fe-0.1B alloy has an excellent ability to maintain plastic deformation, and the alloy can maintain good mechanical properties and fatigue life after suffering more cyclic loads.

Half-life hysteresis loops were presented based on the experimental data obtained under five strain amplitude controls, as shown in Figure 6. The area enclosed by the cyclic hysteresis line represents the plastic strain energy, which largely determines the extent of irreversible damage during the LCF deformation, such as voids and cracks. This indicates that increasing the strain amplitude leads to plastic energy dissipation and damage occurrence [29]. Obviously, as the strain amplitude increases as the hysteresis loop becomes wider and the area increases (Figure 6), which indicates that the larger the plastic strain energy during cycling, the more significant decrease the material fatigue life [30]. It also corresponds to the change in plastic strain amplitude in Figure 5b. When the strain amplitude is 0.6%, the hysteresis loop area is the smallest, the plastic strain energy is the smallest, and the fatigue life is the longest, while at a strain amplitude of 1.4%, the hysteresis loop area is the largest and the plastic strain energy is the largest, which indicates the increase in plastic strain energy and leads to more distinct plastic deformation and yields a shorter LCF life. At the same time, the half-life hysteresis loops can also reflect the asymmetric behavior of the material between tension and compression. At low-strain amplitude, the material is mainly deformed elastically, and the asymmetric behavior is not obvious, while the increase in irreversible damage due to the increase in strain amplitude makes the asymmetric behavior more and more obvious [27].

3.4. Fatigue Life Analysis

The LCF life feature is usually characterized using the Coffin–Manson (C–M) model which describes the relationship between the plastic strain amplitude (Δε_p/2) and LCF life (2N_f):

$$\frac{{∆\mathsf{\epsilon }}_{\mathrm{p}}}{2}={\mathsf{\epsilon }}_{\mathrm{f}}^{\prime }{\left(2{\mathrm{N}}_{\mathrm{f}}\right)}^{\mathrm{c}}$$

(2)

where ${\mathsf{\epsilon }}_{\mathrm{f}}^{\prime }$ and c are fatigue ductility coefficient and fatigue ductility exponent, respectively. As shown in Figure 7, the Coffin–Manson (C–M) curve of the Ti-2Fe-0.1B alloy is plotted in a log–log coordinate system. In the formula, the plastic strain amplitudes are dependent on the half-life hysteresis loops (Figure 6) for all five strain amplitudes. In this figure, the plastic strain amplitude is linearly related to the fatigue life, which is consistent with previous reports observing pure titanium [27]. It shows that the LCF behavior of the Ti-2Fe-0.1B alloy under strain amplitude control is closer to that of pure titanium.

Figure 8 shows the comparison of LCF life of the rolled Ti-2Fe-0.1B alloy compared with pure titanium [18], TA15 [19], Ti80 [20] and additively manufactured (AM) TC4 alloy [31]. It can be seen that the fatigue life of the Ti-2Fe-0.1B alloy is much higher than all the other typical alloys at the same strain amplitude; in particular, the fatigue life of this alloy increases more obviously with the decrease in strain amplitude, which demonstrates excellent LCF properties.

The microstructure characteristics, especially the grain size, have an important effect on the fatigue life of titanium alloys. Cavaliere [32] investigated the fatigue properties of pure titanium produced by equal channel angular pressing, it was found that the strength and fatigue life of pure titanium were greatly increased with a significant decrease in grain size. The reason is that fatigue limit of metals increases when the grain size decreases, and the crack initiation susceptibility decreases as the crack growth rate increases coupled with the grain refinement. It is well known that Fe and B elements play a significant role in optimization of the grain size of titanium alloys [12,33,34]. As the strongest eutectoid β-stabilizing element for titanium alloy, the Fe element has a significant higher diffusion coefficient (D_Fe = 78 × 10⁻¹⁰ cm²s), which could result in segregation in the boundaries during the casting process. The segregation effect of Fe element could inhibit the grain size growth through the reduction of boundary energy, as well as dragging the movement of grain boundaries. Element B has a very low solubility in titanium alloy, B atoms always precipitate during solidification and enrich at the solid–liquid interface, leading to an increase in subcooling of the composition, which decreases the melting point of the titanium alloy and promotes the nucleation process, increasing the nucleation rate and thus reducing the grain size [33]. In the present study, the average grain size of the Ti-2Fe-0.1B alloy reached 1.58 μm after precision forging and continuous hot rolling (Figure 2b), which made the alloy possess excellent fatigue behavior.

3.5. Fracture Characteristics

The fatigue crack mechanism is significantly influenced by the magnitude of strain amplitude. The LCF fracture morphology of the Ti-2Fe-0.1B alloy at low strain amplitude (Δε_t/2 = 0.6%) and high strain amplitude (Δε_t/2 = 1.4%) is shown in Figure 9. The overall fracture morphology of the alloy after LCF could be divided into three areas, i.e., crack initiation region, crack propagation region, and final instantaneous fracture region (Figure 9a,d). It can be seen that the fatigue crack initiation regions have a smooth and flat surface due to the repeated frictional extrusion during a high number of cycles. The nucleation of the fatigue crack forms from the specimen edge, while tear-like stripes appear near the crack nucleation point, which indicate typical morphology of plastic fracture. It is worth noting that only one crack nucleation point is observed at low strain amplitude (Figure 9b), while several crack nucleation points occurred at high strain amplitude values (Figure 9e). The crack propagation region occupied the main area in the fracture morphology, which contains voids, secondary cracks, and fatigue striations (Figure 9c,f). Among them, secondary cracks can form new interfaces implying a large amount of energy consumption, which is beneficial in reducing the rate of fatigue crack propagation [35]. Obviously, compared to low strain amplitude at 0.6%, the number of secondary cracks is much more, and the size of secondary cracks is longer and deeper for strain amplitude 1.4%. The crack propagation region is smaller, which ultimately reduced the LCF life of the alloy at higher strain amplitude [19]. Fatigue striation appears in the crack propagation region of the Ti-2Fe-0.1B alloy at low strain amplitude (∆ε_t/2 = 0.6%) (Figure 9c). Fatigue striation is always caused by residual passivation at the crack tip, which is related to plastic deformation, i.e., dislocation slip [36]. Under tensile stress, passivation occurs at the crack tip due to dislocation slip, which led to the crack extending for a distance. When the load transformed into a compression stress, the passivation effect cannot be completely eliminated, and fatigue striation formed after each cycle, although the crack closure effect reduces the passivation to some extent [19].

As mentioned above, cyclic softening/hardening behavior of the alloy happened during LCF, which is relevant to the development of back stress and friction stress during fatigue cycling [19,37]. It is well known that back stress indicates a long-range interaction with moving dislocations interaction, mainly associated with inhomogeneous deformation of microstructures with different crystal structures, and different shapes and friction stresses are short-term internal stresses that usually originate from the interaction forces between dislocations. TEM observation and analysis were performed for the Ti-2Fe-0.1B alloy after LCF at low strain amplitude (Δε_t/2 = 0.6%) and high strain amplitude (Δε_t/2 = 1.4%), as indicated in Figure 10. It can be found that the alloy produced less plastic deformation at low strain amplitude (Δε_t/2 = 0.6%), dislocations gradually proliferated to form dislocation lines and dislocation tangles during the long cycle time (Figure 10a,c), the grain size was refined, the active/launch of the dislocation slip system was promoted, and back stress softening occurred. At the same time, accompanied by dislocation accumulation at the interface of α-phase, the density of dislocation increased to a certain degree, and the stronger interaction between dislocations led to dislocation annihilation and a partial dislocation-free region (Figure 10b), which decreased the internal friction stress of the alloy and eventually led to cyclic softening [38]. At high strain amplitude (Δε_t/2 = 1.4%), it is clear that the alloy undergoes slight cyclic hardening at the initial stage and then transferred to cyclic softening (Figure 5a). It can be explained that high stress introduced a high amount of plastic deformation. The movement of dislocations was hindered by the existing dislocations generated during rolling and the phase interface during cyclic loading, which led to a large amount of dislocation accumulation at the α-phase interface, and dislocation plugging was generated (Figure 10d,f) and cyclic hardening occurred due to the synergistic effect of the combined increase in back stress and friction stress (Figure 4d). Furthermore, with strain-induced grain boundary migration, dynamic recrystallization occurred, which refined the grains to some extent and absorbed some of the energy and provided a boost for the alloy change from cyclic hardening to cyclic softening. In Figure 10e, dislocations are arranged in parallel to form a slip band. After a period of reciprocal motion, when the dislocation density within the microstructure reached saturation, dislocations were transferred to the adjacent unfavorably oriented grains by slip, and the process of dislocation transfer slowed down the dislocation plugging at the grain boundaries and phase interfaces, then the friction stress was subsequently reduced. At the same time, this process also increased the degree of homogenization of the alloy deformation so that back stress softening occurred, and the alloy changed from cyclic hardening to cyclic softening [21].

4. Conclusions

In this study, a novel low-cost Ti-2Fe-0.1B alloy with good combination of strength and ductility was designed by Fe and B microalloying. The LCF behavior and fatigue characteristics of the Ti-2Fe-0.1B alloy with lamellar structure were experimentally and systematically investigated. The results are summarized as follows:

(1)

At low strain amplitude Δε_t/2 ≤ 1.2%, the overall LCF behavior of the Ti-2Fe-0.1B alloy shows continuous cyclic softening, while at Δε_t/2 = 1.4%, slight cyclic hardening occurs in the initial stage of this alloy, then shows cyclic softening, the cyclic response depends on the evolution of dislocation structure with the number of cycles;

(2)

Different from low strain amplitude (Δε_t/2 = 0.6%), the fatigue fracture of the Ti-2Fe-0.1B alloy at high strain amplitude (Δε_t/2 = 1.4%) has more crack nucleation points and moves from the edge of the specimen to the interior, producing more and deeper secondary cracks. From the TEM results, it is due to the greater degree of plastic deformation of the alloy at high strain amplitude, which leads to faster dislocation proliferation and promotes dislocation slippage;

(3)

Compared to the pure titanium and several traditional titanium alloys, the fatigue life of the Ti-2Fe-0.1B alloy is improved dramatically, and it can be attributed to the fine grain size result from the effect of Fe element and trace B, which could hinder the dislocation movement and crack propagation.