Systematic Study of TiC Nanoparticle Effects on the Fatigue Behavior of Zn and Zn Alloys

Abstract

Zinc (Zn) is a promising material for biodegradable implants due to its moderate corrosion rate and superior biocompatibility over magnesium (Mg) and iron (Fe). However, its poor fatigue performance limits broader applications. While nanoparticles were discovered to enhance the fatigue performance of metals, the mechanisms remain underexplored. This study investigates nanoparticle-enabled mechanisms for fatigue improvement in Zn. Specifically, the microstructure, tensile, and fatigue performance of pure Zn and Zn nanocomposites containing 1 vol. % and 2 vol. % titanium carbide (TiC) nanoparticles were examined. Results demonstrated that TiC nanoparticles significantly enhanced fatigue resistance of Zn at room temperature (RT) and body temperature (BT). The improved fatigue performance was partly attributed to enhanced tensile strength. Further analysis, including Basquin’s equation fitting and fatigue fracture surfaces examination, showed that TiC nanoparticles significantly enhance fatigue resistance by refining grain structure, impeding dislocation movement, altering crack propagation pathways, and promoting energy dissipation during debonding. Moreover, nanoparticle incorporation reduced the temperature sensitivity of Zn’s fatigue behavior, ensuring more stable performance under elevated temperatures. Additionally, Zn nanocomposites like Zn-Al-Cu-TiC and Zn- Mg-TiC maintained suitable biocompatibility and corrosion rates. These findings highlight TiC nanoparticles as a powerful solution to overcome Zn’s fatigue limitations for biodegradable medical devices.

1. Introduction

Zn and its alloys have been extensively studied as promising materials for biodegradable implants. Compared to previously studied Mg and Fe, Zn exhibits an intermediate corrosion rate that is higher than Fe but lower than Mg, and thus has been reported as more suitable for biodegradable medical device applications [1–3]. Moreover, Zn demonstrates superior biocompatibility, as it does not generate harmful byproducts during degradation, unlike Mg [4–6] and Fe [7–10]. However, one serious challenge for Zn is its low strength. For biomedical applications, prospective materials must possess sufficient strength to provide adequate support to healing tissues. Another critical limitation is its poor thermal stability due to Zn’s low melting point (692.7 K). In comparison, the melting points of Mg and Fe are 923.1 K and 1811 K, respectively. Room temperature (RT, 20°C, 293 K) corresponds to only 32.5% and 16.6% of their melting temperatures, making Mg- and Fe-based materials relatively stable at RT. However, for Zn, RT is already considered “warm working temperature” as it already constitutes 43.3% of Zn’s melting point. Even when Zn materials are stored or under low loading at RT, it can lead to issues such as fatigue, creep, aging, and static recrystallization [3]. Consequently, these instability concerns present substantial challenges for the use of Zn in various applications.

To address the intrinsically low strength of pure Zn, researchers have explored alloying methods to introduce solid solution and intermetallic phases, thereby enhancing its mechanical properties. Elements including Mg [11–14], Fe [15–17], aluminum (Al) [14,18,19], copper (Cu) [12,15,20], lithium (Li) [21–23], and silver (Ag) [24,25] have been extensively studied. Although significant strength improvements were observed, certain issues were introduced or persisted in most systems. For instance, some studies reported a rapid aging phenomenon of Zn-Al-based alloys at both RT [26,27] and elevated temperature [28]. Among the alloying elements studied so far, Mg and Li were discovered to be most efficient to strengthen Zn. However, aging and significant ductility drops were observed in Zn-Mg systems at RT even after a very short period of time [29], and appreciable creep deformation in Zn-Li was found at body temperature (BT) [30,31].

Fatigue fracture is one of the most common failure mechanisms for metals. Due to repeated cyclic loadings, metals gradually weaken and eventually fracture, even when the applied stress is much lower than their yield strengths (YS). The prediction of fatigue life is especially critical for medical devices, which are typically implanted in complex physiological environments with cyclical loads. For instance, cardiovascular stents, when subjected to a heart rate of 72 beats per minute, will undergo approximately 10 million cycles during their effective working period of 3 months. Similarly, in orthopedic applications such as bone screws, repetitive activities like walking can impose cyclic loading on the device [32], potentially leading to premature fatigue failure if not properly designed and evaluated.

Several studies have investigated the fatigue issues associated with Zn systems and have explored modifications to alloy compositions and/or manufacturing methods to address these challenges. Li et al. examined the fatigue behavior of biodegradable Zn-Li binary alloys in both air and simulated body fluid (SBF) [33]. Their findings indicated that Li concentration in Zn significantly influenced fatigue performance. Another study investigated the high-cycle corrosion fatigue responses of additively manufactured Zn-Mg alloys, revealing that the incorporation of Mg and the laser powder bed fusion printing process significantly improved anti-fatigue capabilities [34]. Despite these advancements, the research process for these alloy systems remains time-consuming, and a feasible solution applicable to all Zn systems is highly desirable. Moreover, a systematic study on fatigue mechanisms for biodegradable Zn alloys is still lacking.

Composite materials, as a group, exhibit more complex fatigue mechanisms and significantly greater resistance to fatigue loading than other classes of materials [35]. Unlike conventional composites reinforced with micro-sized particles that usually exhibit poor ductility, the incorporation of nano-sized particles to form metal matrix nanocomposites can provide superior tensile properties for broader applications [36–40]. Furthermore, the long-term stability due to the pinning effect of nanoparticles is beneficial for devices implanted in complex environments at elevated temperatures. Previously, the addition of nanoparticles had been widely demonstrated to improve the fatigue life of Al alloys. For example, Al alloy AA5052 was reported to gain a 32% improvement in fatigue limits when reinforced by 3.5 vol. % TiO₂ nanoparticles [41]. In the context of Zn systems, researchers added WC nanoparticles (average size 220 nm) to Zn-Mg systems and discovered improved fatigue performance [42] and longterm stability [43].

In our previous study, we introduced TiC nanoparticles (average size 121 nm) in a Zn system for the first time [44]. TiC nanoparticles were discovered to be capable of effective phase control to improve tensile and fatigue performance of Zn-Al-Cu alloy. However, little study has yet been conducted to understand the mechanisms underlying nanoparticle’s anti-fatigue capabilities, especially at elevated temperatures. In this study, a systematic and fundamental investigation of nanoparticle effects on fatigue enhancement mechanism was conducted in pure Zn at different temperatures. Microstructure, tensile properties, and fatigue performance were characterized. Analysis of Basquin’s model and fatigue fracture surface further revealed the nanoparticle effects on Zn’s fatigue performance.

2. Materials and Methods

2.1. Fabrication process

Fig. 1 illustrates a schematic of the nanocomposite fabrication process. To make Zn-TiC master alloys, high-purity Zn ingots (99.995%, Rotometals) and Al-3.5 vol.% TiC master (121 ± 35 nm, MetaLi, LLC.) were melted at 575 °C under argon. Mixed chloride salts (molar ratio ZnCl₂: NaCl: KCl = 1:1:1) were used to react with Al and reduce the Al content from Zn. Approximately 1/3 of this mixture was added first onto the melt and manually stirred. The melt was further mechanically stirred with a graphite stirrer at 300 rpm, while the remaining mixed salt was gradually added over the next 30 minutes. After removing the salt layer, the nanocomposite was cast into a graphite mold. The pure Zn reference sample was fabricated by melting Zn ingots at 575 °C under Ar protection. All unalloyed samples were then hot rolled at 250°C.

Fig. 1.

Schematic diagram of Zn-TiC nanocomposites fabrication process: (a) melting of raw materials, (b) adding reactive mixed chloride salts, (c) mechanical stirring, (d) casting, (e) hot rolling, (e′) alloying, and (f′) hot rolling.

For the alloyed nanocomposites, Zn-TiC master was melted at 575 °C under Ar protection. Then, alloying elements were added into the melt and held for 5 minutes before casting. The alloyed reference samples were fabricated by simply melting raw materials at 575 °C under Ar protection. All alloyed samples were then hot rolled at 300 °C.

2.2. Composition characterization

Inductively coupled plasma-optical emission spectrometry (ICP-OES) on an ICPE-9000 model (Shimadzu) was used to determine the final compositions. All hot-rolled samples were cut into small chips, and aqua regia was used to extract all elements at room temperature.

2.3. Microstructure characterization

Hot-rolled samples were prepared by mechanical grinding with SiC papers up to 1200 grit, followed by surface cleaning with DI water. The microstructures of as-rolled samples and fatigue fracture surfaces were characterized by optical microscopy (OM) and scanning electron microscopy (SEM, ZEISS Supra 40VP) equipped with energy-dispersive x-ray spectrometry (EDX). Grain structures for OM were revealed using an etchant comprising 20 g CrO₃, 2 g Na₂SO₄, and 100 mL H₂O. Phase analysis was performed with X-ray diffraction (XRD, Panalytical X’Pert Pro) with Cu kα radiation at an accelerating voltage of 40 kV, a step size of 0.017°, and a scanning speed of 2°/min.

2.4. Tensile and fatigue testing

Sub-sized tensile testing specimens were prepared by wire electrical discharge machining (wire-EDM) according to ASTM E8/E8M specifications, featuring a gauge length of 7.5 mm and width of 3 mm. Static tensile tests were performed using an Instron ElectroPlus E1000 machine at a strain rate of 0.001 s⁻¹. Fatigue testing specimens were prepared according to ASTM E606/E606M-12. Ambient and body-temperature fatigue tests were performed on the same Instron machine at a frequency of 60 Hz using a sinusoidal waveform under load control. The stress ratio R was set to −1, the stress amplitude was set as a percentage of the YS. Cyclical loading was applied for 10 million cycles or until sample failure. A custom temperature control module was used to maintain the temperature at 37 ± 1 °C during body-temperature fatigue testing.

2.5. Surface roughness testing

Surface roughness of fatigue fracture surfaces was tested using a Veeco Wyco NT9300 white light interferometer with an 11.4x magnification. For each sample, 3 areas were selected in the crack propagation zone to examine the average surface roughness and evaluate the fatigue resistance of the tested materials.

3. Results

3.1. Chemical composition

ICP-OES data was used to convert elemental concentrations to weight percentages for alloying elements, and volume percentages for TiC nanoparticles (shown in Table 1). Three groups of samples were examined: reference pure Zn, Zn-TiC nanocomposite with a lower concentration of TiC nanoparticles (hereinafter Zn-1TiC), and Zn-TiC with a higher TiC concentration (hereinafter Zn-2TiC). Al concentrations were measured to ensure it had been completely removed by reacting with ZnCl₂ salt. All nanocomposites showed negligible Al concentrations and were therefore considered as zero.

Table 1.

ICP-OES analyses of pure Zn and Zn-TiC nanocomposites with two different TiC concentrations

Sample	Al (wt. %)	TiC (vol. %)	Zn (wt. %)
Pure Zn	-	-	100
Zn-1TiC	0.0390 ± 0.0011	0.9969 ± 0.0110	Bal.
Zn-2TiC	0.0612 ± 0.0011	1.8711 ± 0.0080	Bal.

3.2. Microstructure and phase characterization

Optical micrographs of rolled nanocomposites are presented in Fig. 2 (a) and (b), where the bright background corresponds to the Zn matrix, and the dark phases represent the dispersed nanoparticles. It is clear from the optical images that the Zn-2TiC sample contains higher concentration nanoparticles compared to Zn-1TiC. Fig. 2 (c) shows the magnified SEM image of TiC nanoparticles in the Zn-2TiC sample. After rolling, most nanoparticles were well-dispersed in the Zn matrix, while the rest formed pseudo clusters without sintering. The average nanoparticle size was measured as 127 ± 59 nm, showing minimal change compared to the original Al–TiC master alloy size of 121 ± 35 nm. This result further confirms that the nanoparticles remained stable during processing, with no evidence of significant clustering or agglomeration. The corresponding nanoparticle size distribution is presented in Fig. 2(d).

Fig. 2.

Optical images of (a) Zn-1TiC, and (b) Zn-2TiC, more nanoparticles (dark phases) were observed than Zn-1TiC sample; (c) SEM image of well-dispersed TiC nanoparticles in the Zn-2TiC sample; (d) TiC nanoparticle size distribution of Zn-2TiC sample; optical images after etching (e) pure Zn, (f) Zn-1TiC, and (g) Zn-2TiC. Smaller grains were observed with more loading of nanoparticles.

Grains in all samples were exposed after etching and examined using OM and SEM. As shown in Fig. 2, the existence of TiC altered the corrosion resistance of the matrix, thus the etched microstructures in nanocomposites appeared differently from those of pure Zn. Recrystallized grains were observed in all samples after rolling. Grain size was measured following the ASTM E112 Intercept Method. Pure Zn exhibited the largest grains, with an average size of 17.1 μm. With the addition of 1 vol. % TiC nanoparticles (Zn-1TiC), the grain size decreased significantly to 3.8 μm. Increasing the nanoparticle loading to about 2 vol. % TiC (Zn-2TiC) further reduced the average grain size to 2.7 μm.

The phases of all hot-rolled samples were confirmed by XRD, and the results supported the previous microstructural observations. As shown in Fig. 3, Zn peaks were detected in all three samples, and TiC peaks were detected in only the nanocomposites, with more pronounced peaks observed in Zn-2TiC samples.

Fig. 3.

Diffraction spectra of as-rolled reference pure Zn (black line), Zn-1TiC (red line), and Zn-2TiC (blue line).

3.3. Fatigue performance

High-cycle fatigue performance of all samples at RT and BT are presented in Fig. 4. For pure Zn, the fatigue limits were 16.54 MPa at RT and 12.02 MPa at BT. In contrast, Zn-1TiC exhibited a significant increase in fatigue limits, rising by 87.9% to 31.08 MPa at RT and by 123.5% to 26.87 MPa at BT. With Zn-2TiC, the higher nanoparticle loading further enhanced the fatigue limits, increasing by 135.1% to 38.88 MPa at RT and by 200.6% to 36.13 MPa at BT.

Fig. 4.

Fatigue performance of Zn systems: S-N curves of pure Zn, Zn-1TiC, and Zn-2TiC at (a) RT and (b) BT. Arrows indicate samples that reached 10 million cycles without fracture. For data points where multiple samples tested at the same stress amplitude reached 10 million cycles without fracture, n is used to indicate the number of overlapping samples. The addition of TiC nanoparticles significantly improved the fatigue limit of Zn, at both RT and BT.

3.4. Tensile properties

Fig. 5 presents the typical tensile curves and tensile properties for all three samples. As expected, both yield strength (YS) and ultimate tensile strength (UTS) increased after adding more TiC nanoparticles. The YS is 46.4 MPa, 74.1 MPa, and 92.8 MPa for pure Zn, Zn-1TiC, and Zn-2TiC, respectively. The UTS is 116.4 MPa, 132.8 MPa, and 155.7 MPa, respectively. Both YS and UTS exhibited nearly linear increments with 1 vol. % and 2 vol. % TiC additions. The enhancement of strength is a contributing factor to the improved fatigue performance. Additionally, both nanocomposites still maintained relatively high ductility levels of 46.2% and 39.6% elongation to failure, for Zn-1TiC and Zn-2TiC, respectively.

Fig. 5.

Tensile properties of pure Zn, Zn-1TiC, and Zn-2TiC: (a) representative tensile curves (n=3); (b) bar graph of tensile properties.

3.5. Fatigue Fracture Surfaces Characterization

Fig. 6 provides microstructural evidence from the nanocomposite’s fatigue fracture surfaces, supporting the finding that nanoparticles enhance fatigue performance by deflecting propagating cracks and dissipating energy. Fig. 6 (a) and (b) show magnified SEM images and EDX results of a microcrack in the propagation zone of Zn-1TiC. As the crack propagated from the left side and hit the first nanoparticle zone, it slightly deviated to the right. Upon reaching the second and third nanoparticle zones, its direction slightly deviated, proceeding in between to continue propagating along the path of least resistance. When the crack directly encountered the third nanoparticle zone, it significantly changed direction by approximately 50°. Fig. 6 (c) and (d) depict a typical feature in the nanocomposite’s overload zone. EDX mapping confirmed that the area marked by the red dashed line is a TiC-rich zone. Compared to nearby flat surfaces where the material fractured directly, the debonding of nanoparticles in the circled area caused local surface roughness, contributing to energy dissipation, thus delaying fatigue fracture.

Fig. 6.

SEM images of fatigue fracture surfaces of Zn-1TiC samples under 50% YS at RT: (a) fatigue microcrack changed its propagation directions when passing TiC nanoparticles in the material’s propagation zone, with a nanoparticle-rich area circled and the propagation direction denoted by arrows; (b) EDX mapping result on Ti signal of (a), with arrows pointing out four TiC-rich zones in the scanned area; (c) local roughness caused by nanoparticle debonding in sample’s overload zone; (d) EDX mapping result on Ti signal of (c), with the dashed red line identifying the TiC-rich zone.

Fatigue fracture surfaces of all unalloyed samples under the same test conditions were also compared to further highlight the nanoparticle effects. In a typical fatigue fracture surface, three different zones can be observed: crack initiation zone, crack propagation zone, and overload zone. The crack initiation zone is where the fatigue crack begins, usually due to stress concentration at surface defects or irregularities. In the crack propagation zone, macroscale features such as beachmarks and striations can be observed, indicating the movement and accumulation of fatigue cracks. This zone often covers a significant portion of the fracture surface and is relatively smooth compared to the overload zone. The overload zone, on the contrary, is usually very rough and resembles a static tensile fracture surface, with characteristics such as dimples and cleavages. The surface area and features of these three zones can provide insights into the fatigue mechanism within the material and provide guidelines for material design aiming at enhancing fatigue performance.

Fig. 7 shows the typical fatigue fracture surface when a cyclical stress equal to 50% of the static YS of each sample was applied at RT (23.2 MPa for pure Zn, 37.05 MPa for Zn-1TiC, and 46.4 MPa for Zn-2TiC). In each individual fracture surface, the red circle labeled A marks the crack initiation area, B indicates the crack propagation zone, and C is the final overload zone. Arrows denote the direction of crack propagation, and the dashed red line is the boundary between the propagation zone and overload zone, where cracks accumulate and transition to rapid failure during overload. Two magnified images of each sample’s propagation zone and overload zone are presented adjacent to its respective fracture surface.

Fig. 7.

SEM images of fatigue fracture surfaces at RT under 50% YS. For pure Zn: (a) overview, (b) magnified propagation zone, and (c) magnified overload zone; for Zn-1TiC: (d) overview, (e) magnified propagation zone, and (f) magnified overload zone; for Zn-1TiC: (g) overview, (h) magnified propagation zone, and (i) magnified overload zone. Red circles labeled A are the crack initiation zone, B is the propagation zone, and C is the overload zone. Red dashed lines denote the boundary of propagation and overload zones. Red arrows denote the crack propagating directions.

Typical fatigue striation morphologies were observed in the propagation zones of all samples at low magnification. Upon closer examination, they also exhibited similar morphological characteristics of slip bands. During fatigue testing, dislocations piled up as cyclic loading progressed, leading to localized plastic deformation and the eventual formation of slip bands. However, these slip bands were larger and denser in the pure Zn sample (Fig. 7b), suggesting a higher localized stress concentration. With the incorporation of well-dispersed nanoparticles, the size and density of slip bands decreased (Fig. 7e and h), especially in the Zn-2TiC samples, indicating a more uniform distribution of stress and more homogenous cyclic deformation. Possible explanations for this observation include the creation of matrix-nanoparticle interfaces and higher GB areas (due to grain refinement), which act as dislocation sinks and reduce the dislocation density in the material [45,46]. The surface morphologies in the overload zone were significantly different between pure Zn samples and nanocomposite samples. When an active crack grew to a critical size, fracture occurred similar to monotonic overloading, where the morphology should resemble a static tensile fracture. Although pure Zn is more ductile, a mixture of brittle cleavages and ductile dimples was observed in its overload zone, confirming sudden and catastrophic failure due to material fatigue. For both nanocomposite samples, a more ductile appearance with dimples was observed.

The area of the propagation zone can also provide insights into how much nanoparticles mitigate fatigue behavior in the matrix. Calculated using Image J, the percentage of propagation zone in the fracture surface is 52.2%, 71.9%, and 87.3% for pure Zn, Zn-1TiC, and Zn-2TiC, respectively. A larger propagation area usually indicates better fatigue resistance.

4. Discussion

4.1. Basquin’s equation parameters analysis

Basquin’s equation is widely used to analyze materials subjected to cyclic loading in the high-cycle fatigue regime:

$${\sigma }_a={\sigma }_f^{\prime }{\left(2N_f\right)}^b$$

Where ${\sigma }_a$ is the stress amplitude, $N_f$ is the number of cycles until failure, ${\sigma }_f^{\prime }$ is the fatigue strength coefficient, and b is the fatigue strength exponent. It is well accepted that ${\sigma }_f^{\prime }$ is positively correlated to material strength [35,47,48], meaning that a higher ${\sigma }_f^{\prime }$ typically corresponds to stronger materials and increased fatigue limits. The exponent b reflects the rate at which fatigue life deteriorates with increasing stress and is affected by various factors. Ideally, when b=0, the stress amplitude equals the fatigue strength coefficient, regardless of the number of cycles, implying no fatigue degradation over time during cyclic loading. In reality, due to the accumulation of fatigue damage, b is always negative. If b is close to 0, the material experiences less degradation in fatigue life under higher stress amplitudes, indicating a better resistance to fatigue failure [47]. In summary, higher ${\sigma }_f^{\prime }$ and less negative b values are considered beneficial for a material’s fatigue performance. By fitting Basquin’s equation and understanding a material’s fatigue properties, the selection of materials, component design, and manufacturing methods can be better informed.

Table 2 and Fig. 8 summarize the fatigue limits and Basquin’s equation parameters of all three unalloyed materials under both RT and BT conditions. The Zn-2TiC exhibited the highest fatigue limits at both RT and BT. An apparent reason for the improved fatigue limits after nanoparticle addition is the increase in strength, as mentioned in section 3.4. Benefitting from both Orowan strengthening and grain refinement, pure Zn gained higher strength after incorporating TiC nanoparticles. As a result, stronger materials better withstood cyclical stresses, leading to a higher fatigue limit. The fatigue ratio, calculated as the material’s fatigue limit divided by its UTS, was used to evaluate the influence of strength on high-cycle fatigue behavior. Based on the results, Zn-2TiC also exhibited the highest fatigue ratio at both RT and BT. This suggests that the increase in fatigue limits may not solely attributed to nanoparticle-induced strengthening mechanisms, but also to the interactions between nanoparticles and the matrix during fatigue testing.

Table 2.

Fatigue limits and constants of Basquin’s equation obtained from tested fatigue results

Sample	Fatigue limit (MPa)	Fatigue ratio ${\mathit{\sigma }}_{\mathit{a}}/{\mathit{\sigma }}_{\mathit{UTS}}$	${\mathit{\sigma }}_{\mathit{f}}^{\mathit{\prime }}$ (MPa)	b	R 2	Basquin’s Equation
Pure Zn - RT	16.54	0.142	221.4	−0.15429	0.96	${\sigma }_a={\sigma }_f^{\prime }{\left(2N_f\right)}^b$
Pure Zn - BT	12.02	0.103	198.0	−0.16665	0.95
Zn-1TiC - RT	31.08	0.234	302.9	−0.13506	0.90
Zn-1TiC - BT	26.87	0.202	218.9	−0.13817	0.92
Zn-2TiC - RT	38.88	0.250	316.1	−0.12466	0.97
Zn-2TiC - BT	36.13	0.232	231.1	−0.11039	0.91

Fig. 8.

Influences of temperature and nanoparticle addition on the (a) fatigue performance, (b) fatigue strength coefficient ${\mathit{\sigma }}_{\mathit{f}}^{\mathit{\prime }}$, and (c) fatigue strength exponent b

This hypothesis is further supported by the results of Basquin’s equation fitting. Zn-2TiC displayed the highest ${\sigma }_f^{\prime }$ at RT. However, at elevated temperatures, the material naturally becomes softer and loses strength. Due to the strong correlation between the material strength and ${\sigma }_f^{\prime }$, it was expected that ${\sigma }_f^{\prime }$ would decrease for all samples at BT. Nonetheless, the same trend of higher strength with increased nanoparticle loading persisted, indicating that nanoparticles contributed to fatigue limit improvements by boosting the material’s strength. When examining the exponent b, Zn-2TiC had the highest b at both RT and BT. As discussed earlier, a higher value of b typically corresponds to better fatigue resistance, as it reflects the material’s damage mechanisms. The higher calculated b suggests improved fatigue resistance with increasing nanoparticle content. Based on microstructure observations in section 3.5, the fatigue life enhancement could also be attributed to a few factors, including nanoparticles deflecting fatigue crack propagation, enhancing energy dissipation during debonding, and promoting uniform stress distribution by creating more grain boundaries as dislocation sinks.

Another noteworthy finding regarding b is its difference at BT and RT for the same material. For pure Zn, b is more negative at BT because of the cyclic softening of the matrix and the thermal activation of dislocations. Zn is known for being temperature sensitive due to its low melting point. Even at BT, Zn becomes softer and less resistant to cyclic loads, leading to increased damage accumulation per cycle [49–51]. Additionally, higher temperatures facilitate the movement of dislocations within the material, accelerating the initiation and growth of fatigue cracks [51,52]. With 1 vol. % TiC nanoparticles reinforcement, while b at BT is still more negative than at RT, the reduction was only 0.003. Whereas for pure Zn, b was reduced by 0.012 at BT. The mechanism behind how nanoparticles reduce temperature sensitivity, especially from a fatigue behavior perspective, can be explained through several aspects. First, ceramic nanoparticles are thermally stable, helping to sustain the nanocomposite’s overall mechanical properties, including fatigue resistance. Secondly, the thermal mismatch between the matrix and reinforcements induces compressive stresses at elevated temperature, delaying fatigue crack initiation and propagation [53]. Thirdly, nanoparticles exert a stronger crack deflection effect in a softer matrix due to the larger mismatch in material toughness and stiffness. Thus, a reversed result of a higher b at BT was observed in the Zn-2TiC samples, suggesting that the beneficial effects of nanoparticles on fatigue resistance outweighed the damage caused by elevated temperatures. In other words, the incorporation of nanoparticles reduced the fatigue temperature sensitivity of pure Zn.

It should be noted that the above-mentioned theories have certain limitations. For example, while incorporating nanoparticles can significantly enhance fatigue performance, there exists an optimal concentration that maximizes improvement without introducing potential drawbacks, such as stress concentration sites and particle agglomerations from overloading nanoparticles. Although the addition of 2 vol. % TiC nanoparticles showed the best fatigue performance in this study, further investigations are necessary to determine the optimal concentration and maximize fatigue and other mechanical performance. Moreover, the conclusions regarding fatigue mechanisms in this work were primarily drawn from fatigue fracture surface observations and analysis based on the Basquin equation. A more direct evaluation of fatigue softening and the stabilizing effects of nanoparticles, particularly under elevated temperatures, could be achieved by monitoring cyclic creep. This could be quantified through the evolution of mean strain, hysteresis loop shape and area, as well as strain amplitude versus cycle number, providing deeper insight into the material’s cyclic deformation behavior.

4.2. Fatigue Fracture Features under Different Conditions

Fatigue fracture surfaces were examined and compared to those shown in Fig. 7, to reveal the effect of different nanoparticle loadings and temperatures on the fatigue performance of Zn systems. Although the samples tested at BT displayed overall similar characteristics to those tested at RT under equivalent applied stresses (50% YS), notable differences were observed in the propagation zones, particularly in the intensity of slip bands and cracks, highlighting variations in local stress distribution.

Fig. 9 presents magnified images of these features in the propagation zone of pure Zn and Zn nanocomposites. For pure Zn, relatively flat and smooth surfaces with large slip bands were discovered in the RT sample (Fig. 9 (a)). In contrast, the BT samples exhibited large cracks parallel to the slip bands (Fig. 9 (d)), indicating a much higher concentration of localized stress and corroborating the detrimental effect of elevated temperature on pure Zn’s fatigue performance. For Zn-1TiC sample, the slip bands observed at both RT and BT were smaller and less dense compared to pure Zn. Although larger slip bands were present in the BT samples (compare Fig. 9 (e) to (b)), no cracks were observed, demonstrating the mitigating effect of nanoparticles on stress localization caused by higher temperature. Encouragingly, the propagation zone of Zn-2TiC appeared similar at both RT (Fig. 9 (c)) and BT (Fig. 9 (f)), suggesting that the negative impact of elevated temperature on fatigue performance were less pronounced with higher nanoparticle reinforcement. In summary, increased nanoparticle loading not only enhanced fatigue resistance by redistributing localized stress but also reduced the temperature sensitivity of fatigue behavior, offering better thermal stability and long-term performance.

Fig. 9.

Magnified SEM images of the propagation zone of 50% YS samples: at RT (a) pure Zn, (b) Zn-1TiC, and (c) Zn-2TiC; at BT (d) pure Zn, (e) Zn-1TiC, and (f) Zn-2TiC. For pure Zn, cracks parallel to slip bands were only observed in BT samples. After being reinforced with 2 vol. % TiC, the negative effect of elevated temperature is less severe.

Fatigue surface roughness was found to have a strong correlation with local stress levels [54]. To evaluate fatigue crack propagation intensity and quantify fatigue resistance, the macroscale fracture surface roughness of all samples in Fig. 9 was measured and compared, as summarized in Table 3). As expected, samples tested at RT exhibited lower propagation zone surface roughness compared to those tested at BT. This trend aligns with the increased stress localization observed at elevated temperatures. The decreasing differences in surface roughness between RT and BT samples with increasing nanoparticle loading further confirm the enhanced thermal stability imparted by nanoparticles. When comparing samples tested at the same temperature, despite under equivalent applied stresses, surface roughness consistently decreased with higher nanoparticle incorporation. This reduction in roughness highlights the nanoparticles’ ability to mitigate crack propagation by redistributing localized stresses more uniformly, thereby enhancing fatigue resistance.

Table. 3.

surface roughness of propagation zones when samples tested under 50% YS at both RT and BT

Sample	Applied stress (MPa)	RT propagation zone surface roughness (μm)	BT propagation zone surface roughness (μm)
Pure Zn	23.2	15.53	19.14
Zn-1TiC	37.05	8.80	12.04
Zn-2TiC	46.4	6.30	9.13

4.3. Zn alloy nanocomposites for biodegradable cardiovascular stent applications

FDA guidelines on the tensile properties of biodegradable stent materials are: UTS >300 MPa and elongation to failure > 15-18% [55]. To develop materials with adequate strength and better fatigue resistance, alloy elements including Al, Cu, and Mg were incorporated in Zn-TiC masters. Pure alloy counterparts without nanoparticle addition were prepared with the same manufacturing route for comparison. These systems were selected due to their establishment in the literature as prominent alloys for biodegradable cardiovascular stents.

ICP-OES results of four different systems are presented in Table 4, confirming the compositions of pure Zn-2Al-4Cu and Zn-0.1Mg, as well as nanocomposites Zn-2Al-4Cu-0.64 vol. % TiC (hereafter Zn-2Al-4Cu-TiC) and Zn-0.1Mg-1.06 vol. %TiC (hereafter Zn-0.1Mg-TiC). Table 5 lists the tensile and fatigue properties of these systems. As reported in previous papers, nanoparticles had significant phase control and modification capability in both alloy systems, increasing not only their strength but also their microstructural homogeneity and long-term stability.

Table 4.

ICP-OES analyses of Zn alloy nanocomposites and their pure alloy counterparts

Sample	Al (wt. %)	Cu (wt. %)	Mg (wt. %)	TiC (vol. %)	Zn (wt. %)
Zn-Al-Cu [44]	2.0446 ± 0.0171	3.9549 ± 0.0233	-	-	Bal.
Zn-Al-Cu-TiC [44]	2.0494 ± 0.0116	3.9512 ± 0.0178	-	0.6370 ± 0.0048	Bal.
Zn-Mg	-		0.1028 ± 0.0001	-	Bal.
Zn-Mg-TiC	0.0283 ± 0.0010		0.0986 ± 0.0010	1.0562 ± 0.0129	Bal.

Table 5.

Tensile and fatigue properties of Zn alloy nanocomposites and their pure alloy counterparts

Sample	YS (MPa)	UTS (MPa)	Elongation (%)	Fatigue limit (MPa)	Fatigue ratio ${\mathit{\sigma }}_{\mathit{a}}/{\mathit{\sigma }}_{\mathit{UTS}}$
Zn-Al-Cu [44]	149.1	259.7 ± 1.3	> 100	44.7	0.172
Zn-Al-Cu-TiC [44]	165.1	340.4 ± 1.3	49.2 ± 4.3	66.0	0.194
Zn-Mg	168.1	319.8 ± 10.2	29.6 ± 3.6	48.6	0.152
Zn-Mg-TiC	155.7	323.5 ± 11.6	32.6 ± 1.4	56.8	0.176

In this study, their fatigue behavior was tested before and after adding TiC nanoparticles. As shown in Fig. 10, the performance of both alloyed systems benefited from nanoparticle additions. The fatigue limit of Zn-2Al-4Cu increased by 47.7% from 44.7 MPa to 66 MPa after adding only 0.6 vol. % TiC. The fatigue limit of Zn-0.1Mg also increased by 16.9 % from 48.6 MPa to 56.8 MPa with the addition of 1vol. % TiC. Encouragingly, the fatigue ratios of both systems increased after adding nanoparticles, suggesting that the nanoparticle strengthening effect was also not the only reason for the increased fatigue limits in alloyed systems. Adding nanoparticles would be a broad solution that is applicable to more Zn systems to address their poor fatigue performance.

Fig. 10.

Fatigue performance of Zn alloy nanocomposites and pure alloy controls: S-N curves of (a) Zn-2Al-4Cu and Zn-2Al-4Cu-TiC and (b) Zn-0.1Mg and Zn-0.1Mg-TiC. Arrows indicate samples that reached 10 million cycles without fracturing. For data points where multiple samples tested at the same stress amplitude reached 10 million cycles without fracture, n is used to indicate the number of overlapping samples. The addition of TiC nanoparticles significantly improved the fatigue limit of both alloy systems.

The findings confirm that nanoparticles significantly enhance the fatigue performance of materials, particularly in Zn systems, which are inherently characterized by low strength and fatigue resistance. Additionally, Zn alloy nanocomposites reinforced with TiC nanoparticles have been previously investigated for their long-term stability, corrosion behavior, and biocompatibility [44]. That study demonstrated that incorporating TiC nanoparticles into the Zn alloy matrix resulted in minimal alterations to the overall corrosion rate. Notably, cell culture tests indicated excellent in vitro biocompatibility of the Zn nanocomposite. Furthermore, the paper discussed potential nanoparticle digestion pathways and preliminary long-term in vivo data, showing favorable biological responses and suggesting possible clearance mechanisms for non-degradable ceramic nanoparticles. These findings help address concerns regarding the safety of TiC nanoparticles in medical implant applications.

Overall, Zn-TiC nanocomposite systems exhibit a promising combination of increased strength, excellent ductility, moderate corrosion rate, superior long-term stability, and favorable biocompatibility. With appropriate alloy designs, these properties position Zn alloy nanocomposites as strong candidates for the next generation of biodegradable cardiovascular stents, as well as broader applications such as orthopedic implants. Corrosion-fatigue testing is widely used to evaluate fatigue behavior in simulated physiological environments. The result would provide more relevant in vitro insights into the performance of these materials. Such testing would serve as a powerful tool in the early-stage development of this material class, offering a deeper understanding of the interplay between corrosion and fatigue behavior in Zn systems, and how nanoparticle reinforcement can mitigate these effects. Further in vivo studies will also be essential to comprehensively assess and validate the long-term performance and safety of these materials under complex physiological conditions.

5. Conclusions and Future Work

This study presented the first systematic investigation into the fatigue performance of various Zn nanocomposites, revealing the fatigue enhancement mechanisms of TiC nanoparticles. Well-dispersed nanoparticles in Zn matrix resulted in significant grain refinement and impeded dislocation movement, leading to enhanced material strength. This strengthening effect significantly contributed to the enhanced fatigue limits observed in nanocomposites.

Further Basquin fitting and fatigue fracture surface analysis provided deeper insights into the mechanisms of nanoparticle-induced fatigue resistance. The calculated fatigue strength coefficient ${\sigma }_f^{\prime }$ confirmed the contribution of increased strength to the improvement of the material’s fatigue limits. The fatigue strength exponent b values and microstructural observations of fracture surfaces further revealed additional mechanisms responsible for the fatigue limit increase by nanoparticles. Firstly, in the crack propagation zone, nanoparticles were found to alter the trajectory of fatigue cracks, deflecting or even stopping microcrack propagation, thereby improving the material’s fatigue resistance. Secondly, local roughness caused by the debonding of nanoparticles during the final fracture was also observed. The energy dissipated by nanoparticle debonding further delayed the fatigue process. Also, nanoparticle-induced grain boundaries acted as dislocation sinks and unified the local stress distribution, resulting in more homogenous cyclic deformation and delayed fatigue. Furthermore, the impact of nanoparticle inclusion at body temperature was examined. It was discovered that nanoparticles had a more pronounced effect in mitigating temperature-induced softening and material fatigue at elevated temperatures, providing a better long-term thermal stability. Analyses of fatigue fracture surface roughness also confirmed these conclusions.

These findings were extended to more biodegradable stent materials, such as Zn-Al-Cu and Zn-Mg alloys. It should be noted that nanoparticles once again positively affected both their tensile and fatigue performances, suggesting that nanoparticle reinforcement could serve as a broad solution to mitigate the poor fatigue performance of Zn alloys. Combined with the demonstrated moderate corrosion behavior, as well as the cyto- and hemocompatibility of the Zn-Al-Cu-TiC system, these findings indicate that Zn alloys reinforced with TiC nanoparticles are promising candidates for biomedical applications.

For future work, continued exploration of the optimal nanoparticle loading and the development of various Zn alloy nanocomposites will be essential to identify the most suitable material. Once promising compositions are selected, it will be critical to bridge fundamental material studies with clinically relevant validation. For example, corrosion-fatigue testing in simulated physiological environments has been widely adopted by many research groups and would serve as a powerful tool to further optimize these materials prior to in vivo assessment [33,34,56,57]. Ultimately, in vivo implantation studies will be necessary to evaluate the long-term performance and safety of these materials and their stent prototypes under complex physiological conditions.

Highlights.

• Novel Zn–TiC nanocomposites were developed as candidates for biodegradable stents.

• Higher TiC content improved strength, fatigue limits, and thermal stability.

• Fatigue improved as TiC refined grains and impeded crack growth.

• Basquin analysis showed TiC reduced temperature sensitivity in fatigue behavior.

• Zn–TiC systems combine strength, fatigue resistance, and excellent biocompatibility.