Thermomechanical Fatigue Loading Research Articles

Isothermal and thermomechanical fatigue crack growth behavior of 316LN stainless steel under load‐ and strain‐controlled modes

AbstractCrack growth tests of 316LN stainless steel under load‐controlled and strain‐controlled thermo‐mechanical fatigue (TMF) loadings at the temperature range of 350°C ~ 550°C were conducted. Moreover, the isothermal fatigue (IF) crack growth tests at the maximum temperature of 550°C were performed for comparison. The differences in IF and TMF crack propagation behavior under different loading conditions were thoroughly discussed, and the underlying mechanisms were revealed by a comprehensive characterization of the deformation state of the crack tip region using digital image correlation (DIC) and electron backscatter diffraction (EBSD) methods. Furthermore, it was found that the EBSD characterization was simply based on a single plane made it difficult to evaluate the complex crack growth behavior. In addition, the limitations of the classical parameter of stress intensity factor under TMF loading were presented.

Effect of thermomechanical loading on fracture resistance and failure mode of new pressable zirconia-reinforced lithium disilicate onlay restoration

<strong data-sider-select-id="4f342ea4-c1fb-46b4-a790-13386de6d875">Background. Insufficient information exists regarding the fracture resistance and failure pattern of newly developed zirconia-reinforced lithium disilicate (ZL, Vita Ambria) onlays. This in vitro study compared the fracture resistance of two types of onlays: monolithic lithium disilicate (LD) and monolithic ZL. <strong data-sider-select-id="70034821-3417-424d-a4c5-f611d8e66f16">Methods. Forty-eight ceramic onlay restorations were fabricated on epoxy dies using a maxillary first premolar model. The samples were divided into two main groups: LD and ZL. Half of each group was subjected to thermomechanical fatigue loading (TML) using a chewing simulator. All the samples were cemented with self-adhesive resin cement. Subsequently, they were loaded until failure in a universal testing machine, and the fracture patterns and resistance were recorded. <strong data-sider-select-id="c8a6c89b-9bfb-476a-96c7-da7fc7628016">Results. Before TML, ZL demonstrated the highest statistically significant mean fracture resistance (499.76±34.14N) compared to LD (470.40±27.38N). After TML, ZL showed the highest non-statistically significant mean fracture resistance (429.27±131.42N), while LD’s mean fracture resistance decreased (377.31±62.18N). <strong data-sider-select-id="4ebc1db1-4d03-4eaa-8d84-ed0c19c6dcdb">Conclusion. Monolithic zirconia-reinforced onlays demonstrated higher fracture resistance and a more favorable failure mode compared to LD. However, the impact of thermomechanical aging resulted in reduced fracture resistance for both materials, with a notable preference observed for ZL

Deformation behavior and crack mechanism of a first generation single-crystal Ni-based superalloys under thermomechanical fatigue loading

A research investigation was conducted to analyze thermomechanical fatigue (TMF) in a first-generation nickel-based single-crystal superalloy. The analysis was conducted within a temperature range of 450–950 °C while employing a strain ratio of −1 during two phases, in-phase (IP) and out-of-phase (OP). TMF life data were collected at varying mechanical strain amplitudes. The IP and OP TMF lifetimes were compared under identical strain conditions. A detailed analysis of the fractures and microstructures of the tested specimens was performed to investigate the crack mechanism. In the IP specimens, the formation of cracks closely followed the direction perpendicular to the principal stress within the material, which was primarily associated with the presence of micropores. Internal cracks tended to develop at locations with strain concentrations and subsequently propagate outward from the surface. The main mechanism behind these cracks was the interaction between creep and fatigue damage. Conversely, in the OP TMF specimens subjected to an oxide scale formed on the fracture surface, leading to fatigue cracking at multiple locations. The cracks gradually extended within the material before transitioning into the (111) crystallographic planes, where localized deformation bands occurred, affecting an area depleted of the γ' phase. This crack mechanism predominantly involved interactions between oxidation and fatigue. Among the various fatigue life prediction models, the Ostergren model exhibited strong agreement with the predicted data points for the fatigue life.

Effect of Thermomechanical Fatigue Loading on the Internal and Marginal Adaptation of Endocrowns Utilizing Different CAD/CAM Restorative Materials

To assess the impact of fatigue loading on the internal and marginal fit of CAD/CAM-fabricated endocrowns for restoring endodontically treated molars using different machinable blocks. A total of 72 mandibular first molars were prepared using a standardized method and were divided into four groups (n = 18), each restored with a different CAD/CAM material: group V = polymer-infiltrated ceramic (Vita Enamic); group K = partially stabilized tetragonal zirconia (Katana); group E = lithium disilicate ceramic (IPS e.max CAD; and group B = polyetheretherketone (BioHPP). Endocrowns were subjected to thermomechanical fatigue loading. Internal and marginal adaptation of the endocrowns were examined at 66 points using the sectioning technique and a stereomicroscope. Data were analyzed using one-way ANOVA and Tukey post hoc test (α = .05). Statistical tests showed that adhesive cementation resulted in significantly increased marginal and internal gap values in all regions in all four tested groups (P < .001). After thermomechanical fatigue loading, all regions in groups B and K showed a statistically significant difference, while no significant differences were found in the regions in group V (P > .05). In group E, the marginal (F = 71.00) and pulpal (F = 28.065) regions showed statistically significant differences (P < .001). Polymer-infiltrated ceramics showed the lowest gap even after thermomechanical fatigue loading and may therefore provide enhanced clinical survival of the restored tooth, favoring the use of this material for fabricating endocrown restorations.

Experimental investigation and constitutive material modelling of low cycle fatigue of EUROFER97 for fusion applications

EUROFER97 is one of the candidate materials for structural components of fusion nuclear reactors (i.e. ITER and DEMO). Due to the plasma instabilities and cyclic operating conditions, these structural components are expected to face asymmetric thermomechanical fatigue loading. In this work, existing models based on the Chaboche viscoplasticity model have been modified in order to represent the fatigue behaviour of EUROFER97 under different strain ratios and ranges. The modified model has been parameterized based on experimental data obtained from fatigue tests of EUROFER97, in which the strain range varied from 0.6 % to 1.5 % and the strain ratio was either -1 or 0 at room temperature or 350 °C. The incorporation of the strain memory effect into the isotropic hardening due to the observed non-Masing behaviour of EUROFER97 enabled the use of unified parameters for every temperature that could cover all the different testing conditions. In order to validate the current model, the simulated results are compared to strain controlled low cycle fatigue test data performed at room temperature and 350 °C, which were not used for the parameterization. A good match between the simulation and experimental results proves the predictive capability of the proposed modified model.

Deformation and damage mechanisms during clockwise diamond and counter clockwise diamond thermomechanical fatigue in Timetal 834 alloy

In the present investigation, the thermomechanical fatigue (TMF) behavior of Timetal 834 alloy was studied under clockwise diamond (CWD) and counter clockwise diamond (CCD) loading conditions. These tests were conducted within 723 K ↔ 873 K temperature range at the strain amplitudes of Δεm/2 = ±0.6% and ±1.0%. The cyclic softening behavior observed under CWD-TMF condition at both strain amplitudes, is attributed to shearing of coherent Ti3Al precipitates. Contrarily, the alloy exhibits both cyclic hardening and softening responses under the CCD-TMF conditions. This additional hardening response is attributed to the occurrence of dislocation–dislocation interactions coupled with dynamic strain aging. The present study reveals that both phase angle and strain amplitude affect the TMF life. The alloy exhibited relatively lower fatigue life under CCD-TMF condition than under CWD-TMF condition, at both strain amplitudes. Transmission electron microscopy analyses of the post-deformed specimens reveal that the dislocation arrangements are similar in both loading conditions at Δεm/2 = ±0.6%, while being distinctively different at Δεm/2 = ±1.0%. At higher strain amplitude, the fatigue life is significantly reduced under CCD-TMF condition. Fractographic analyses indicate the involvement of oxidation damage during both the TMF loading conditions. However, its effect is more pronounced under CCD-TMF than under CWD-TMF condition.

Can Martensitic Phase Transformation Measured by Magnetic Methods be an Indicator of Fatigue Damage in Austenitic Steel at Elevated Temperature and Thermo-Mechanical Loading?

The phenomenon of phase transformation from paramagnetic γ austenite to ferromagnetic α' martensite in metastable austenitic stainless steels under fatigue loading is well investigated, especially at ambient temperature. Generally, it can be said that at ambient temperature, the amount of martensite is proportional to the accumulated plastic strain and, therefore, can be an early indicator of fatigue damage. The problem arises at elevated temperatures when the thermal energy can severely or even completely inhibit the phase transformation. In this case, the amount of α'-martensite can no longer be a good indicator of fatigue damage. The question is, where is the limit the deformation-induced martensite can still be used as a fatigue damage indicator? To answer this question, several fatigue tests have been performed on AISI 347 (X6CrNiNb18-10), a metastable austenitic stainless steel often used in the piping of European nuclear powerplants. Tests included isothermal fatigue tests as well as thermo-mechanical fatigue (TMF) loading in the range of 25 to 320 °C. Deformation-induced martensite was measured in situ by means of a uniaxial magnetic balance, from which the rate of phase transformation was obtained. Fatigue specimens were then cut to investigate the distribution of martensite in the gauge length by means of magnetic force imaging. The results showed that isothermal fatigue loading results in a steady decrease in phase transformation rate with temperature. Initial martensite nucleation was observed to be different at elevated temperatures and occurred only at the surface, where sufficient shear deformation was present. TMF tests showed higher than isothermal rates of phase transformation. Peak temperatures above 300 °C were nevertheless enough to severely reduce the formation of α' martensite, even when cooled back to ambient temperature in the other half of the TMF cycle.

A damage-coupled viscoplastic constitutive model considering tension-compression asymmetry for modeling low cycle fatigue mechanical behaviors of Inconel 617 at elevated temperature

Design and analysis based on the advanced constitutive models capable of simulating the complex mechanical behaviors of materials would improve the operational reliability and safety of high-temperature engineering components that experience thermo-mechanical fatigue loading in aerospace and power industries. Inspired by the experimental observations of a set of low cycle fatigue tests of nickel-based superalloy Inconel 617 at 700 °C, a damage-coupled unified viscoplastic constitutive model considering tension-compression asymmetry is proposed. For a better description of the hysteresis loop shape, the cyclic hardening behavior is modeled by the kinematic hardening rule. The damage is measured by the degradation of elastic modulus, and a low cycle fatigue damage model is employed to describe the cyclic softening behavior and predict the fatigue life. Due to the similarity in the evolution of damage and compressive hardening ratio, a compressive hardening factor, which is a function of damage, is incorporated into the kinematic hardening model to simulate the tension-compression asymmetry, i.e. the mean stress evolution. The strain-range dependence of cyclic hardening and tension-compression asymmetry is also considered. The simulation results show that the proposed model can effectively model the low cycle fatigue mechanical behaviors of the material at elevated temperatures.

Survival and debonding resistance of posterior cantilever resin-bonded fixed dental prostheses for moderately and severely worn dentition during thermomechanical loading.

The purpose of this study was to investigate the survival rate, the debonding resistance, and the failure modes of different occlusal veneer designs when used as a retainer for posterior cantilever, resin-bonded fixed dental prostheses (RBFDPs) at two tooth wear levels. Four test groups were assigned: two groups with occlusal-proximal preparation (PT1 and PT2 for grade 2 and 3 wear), and two groups for occlusal-proximal and lingual preparation (PLT1 and PLT2 for grade 2 and 3 wear) Monolithic zirconia ceramic (3Y-TZP) RBFDPs were luted with an adhesive bonding system (Panavia V5). The specimens underwent a chewing simulation for 1.200.000 cycles with a load of 5kg and thermocycling for 7500 cycles between 5°C and 55°C. The surviving restorations were debonded under quasi-static conditions. The results were analyzed with ANOVA. The specimens exhibited a 100 % survival rate after thermomechanical fatigue loading. The debonding resistance was statistically significant higher for group PLT1 than for group PT1 (P=0.004), and higher for group PT2 than group PT1 (P≤0.001). However, the debonding resistance showed no statistically significant difference between groups PT2 and PLT2 (P=0.343), and groups PLT1 and PLT2 (P=0.222). Groups PT1 and PT2 showed favorable failure modes in 62.5 % and 0.00 % of the specimens, respectively. While groups PLT1 and PLT2 presented 25 % favorable failure modes. Occlusal veneers showed promising results as a retainer for cantilever RBFDPs. The lingual extension might increase debonding resistance. Nevertheless, conservative designs with lingual and proximal bevels are to be recommended, irrespective of the level of tooth wear.

Creep–Fatigue Interaction of Inconel 718 Manufactured by Electron Beam Melting

Electron beam melting of Ni‐base superalloy Inconel 718 allows producing a columnar‐grained microstructure with a pronounced texture, which offers exceptional resistance against high‐temperature loading with severe creep–fatigue interaction arising in components of aircraft jet engines. This study considers the deformation, damage, and lifetime behavior of electron‐beam‐melted Inconel 718 under in‐phase thermomechanical fatigue loading with varying amounts of creep–fatigue interaction. Strain‐controlled thermomechanical fatigue tests with equal‐ramp cycles, slow–fast cycles, and dwell time cycles are conducted in the temperature range from 300 to 650 °C. Results show that both dwell time and slow–fast cycles promote intergranular cracking, gradual tensile stress relaxation, as well as precipitate dissolution and coarsening giving rise to cyclic softening. The interplay of these mechanisms leads to increased lifetimes in both dwell time and slow–fast tests compared to equal ramp tests at higher strain amplitudes. Conversely, at lower mechanical strain amplitudes, the opposite is observed. A comparison with results of conventional Inconel 718 indicates that the electron‐beam‐melted material exhibits superior resistance against strain‐controlled loading at elevated temperatures such as thermomechanical fatigue.

Improved Thermo-Mechanical Fatigue Resistance of Al-Si-Cu 319 Alloys by Microalloying with Mo.

Thermo-mechanical fatigue (TMF) is one of the most detrimental failures of critical engine components and greatly limits their service life. In this study, the out-of-phase TMF (OP-TMF) behavior in Al-Si-Cu 319 cast alloys microalloyed with Mo was systematically investigated under various strain amplitudes ranging from 0.1-0.6% and temperature cycling at 60-300 °C and compared with the base 319 alloy free of Mo. Cyclic stress softening occurred in both experimental alloys when applying the TMF loading, resulting from the coarsening of θ'-Al2Cu precipitates. However, the softening rate of the Mo-containing alloy was lower than that of the base 319 alloy because of its lower θ'-Al2Cu precipitate coarsening rate per cycle. The Mo-containing alloy exhibited a longer TMF lifetime than the base alloy at the same strain amplitude. Microalloying 319 alloy with Mo enhanced the TMF resistance mainly by slowing the coarsening of θ'-Al2Cu precipitates and providing supplementary strengthening from thermally stable Mo-containing α-dispersoids distributed in the Al matrix. The energy-based model was successfully applied for predicting the TMF lifetime with a low life predictor factor, which agreed well with the experimentally measured fatigue cycles.

Cyclic deformation mechanism and fracture behaviour of 316L stainless steel under thermomechanical fatigue loading

Thermomechanical fatigue (TMF) behaviour of 316L is investigated based on the internal stress and microstructural characterization at various cyclic stages. Results reveal that the cyclic stress response shows initial hardening followed by cyclic softening at a high strain amplitude, while presenting continuous cyclic hardening as the strain amplitude decreases. Among these, Out-of-phase TMF always shows cyclic softening irrespective of strain amplitude. It is important to note that the hardening of back stress and friction stress contribute to the initial and subsequent cyclic hardening. In contrast, the subsequent cyclic softening is mainly dominated by the reduced friction stress. It is observed that the proliferation of dislocations at grain boundaries and the enhanced dislocation interactions within the grains are primarily responsible for the hardening of back stress and friction stress, respectively. Specifically, the cyclic softening of friction stress is attributed to the annihilated dislocations as a result of cross-slip. It is also found from the hysteresis curves that the intensity of dynamic strain ageing is related to strain amplitude, loading history and phase angle, which correlates to the occurrence of cross-slip, the diffusion rate of the solute atom, the density of point defect and the mobility of vacancies. Finally, the effect of loading conditions on fracture behaviour is demonstrated from the point view of crack sources, secondary cracks, creep cavities and oxidation damage.

Molecular dynamics simulation of thermomechanical fatigue properties of Ni-based single crystal superalloys

In this paper, the thermomechanical fatigue (TMF) properties of Ni-based single crystal superalloys are studied by molecular dynamics simulations. Two different cyclic deformation mechanisms of superalloys are found under TMF loadings. The sample has a higher cyclic stress range, plastic strain energy density and a shorter fatigue life under Out-of-phase TMF loading than those under In-phase TMF loading. Moreover, the low-temperature tension half-cycle is more favorable for dislocations and stacking faults to cut into the γʹ precipitate phase, causing an earlier failure of superalloys under Out-of-phase TMF loading, which attracts more attention in actual operation.

Life prediction method based on damage mechanism for titanium alloy TC4 under multiaxial thermo-mechanical fatigue loading

In this paper, a life prediction method based on damage mechanism for titanium alloy TC4 under multiaxial thermo-mechanical fatigue loading is proposed. Temperature and time dependent influence factors are proposed and introduced into the strain amplitude-life curve method to consider the temperature and time dependence of damage, respectively. The increase in fatigue and oxidation damages caused by tensile mean stress and non-proportional additional hardening is expressed by the mean stress in Macaulay brackets and by using a multiaxial damage parameter, respectively. A fast cracking influence factor is proposed to consider the influence of fast cracking of materials caused by the combined action of high temperature, tensile stress and shear stress on damage behavior. The material constants are identified by test data under uniaxial and proportional loadings, and are further used for prediction under non-proportional loading. The failure life results of uniaxial isothermal fatigue tests with and without dwell time, uniaxial and multiaxial thermo-mechanical fatigue tests are used to verify the proposed method. The value of the scatter band of life prediction result is less than ± 2, as well as the maximum and mean relative errors between predicted and experimental strain amplitudes are 18.49% and 6.54%, respectively.

Thermomechanical fatigue of additively manufactured 316L stainless steel

An important issue in energy conversion is the performance of materials under complex cyclic loading in a variable temperature field. The present study addresses a new field of research – thermomechanical fatigue of additively manufactured metallic materials, which is crucial for understanding the behaviour of this promising material class under real operating conditions. The material of interest – 316L austenitic stainless steel, commonly used for heat exchangers – was manufactured to bars using laser powder bed fusion. Cylindrical specimens with characteristic hierarchical, non-equilibrium cellular microstructure were machined out of the bars. Two orientations corresponding to the inclination of the building direction to the specimen axis were considered: 0° and 90°. The specimens were subjected to thermomechanical fatigue loading under in-phase (maximum tension coincides with maximum temperature) and out-of-phase (maximum compression coincides with maximum temperature) conditions. The cellular dislocation microstructure showed good stability despite gradual coarsening under the combined effect of thermal loading up to 750 °C and severe plastic deformation. Systematic electron microscopy observations further revealed that basic damage mechanisms – either creep or stress-assisted oxide cracking, the prevalence of which depends on thermomechanical loading conditions – correspond to the behaviour of conventional metallic materials. Under in-phase loading, intergranular creep damage is dominant, hence a key factor affecting the lifetime is the number of grain boundaries in the loading direction. Under out-of-phase loading, fatigue damage is dominant, and the lifetime is determined by transgranular propagation of a principal crack. Comparing the two orientations, the inherent microstructural texture was found to be a crucial factor, also determining the number of grain boundaries and cell walls in the loading direction. Hence, tailoring the microstructure for the service relevant loading conditions via additive manufacturing techniques enables to enhance the component performance in the important field of energy conversion.

Creep-Fatigue Interaction on Small Crack Propagation in a Single Crystal Superalloy under Thermomechanical Fatigue Condition

Recently, the high-efficiency gas turbine combined cycle power generation has become essential in operation as a backup power supply of renewable energy sources. In turbine blades and vane of the gas turbine operated as a backup power supply, there is growing concern about the damage due to thermo-mechanical fatigue (TMF) loading because of expecting increased starting/stopping cycle and frequent load fluctuation. This study investigated the small crack propagation behavior in single crystal Ni-based superalloy under the in-phase TMF loading cycle. The creep-fatigue interaction on the small crack propagation behavior was discussed based on the relationship between the crack growth rate and the creep J-integral, considering the small-scale creep at the crack tip. The experimental results indicated that the crack growth rates under the in-phase TMF loading cycle were significantly higher than those under the out-of-phase TMF condition at a given fatigue J-integral range. In addition, introducing the tension holding at the maximum temperature in the in-phase TMF loading cycle accelerated the crack growth rates. The small-scale creep stress field at the crack tip might dominate the small crack propagation behavior under the in-phase TMF loading with the high-temperature holding as the creep-fatigue interaction.

A Temperature-Dependent Viscoplasticity Model for the Hot Work Steel X38CrMoV5-3, Including Thermal and Cyclic Softening under Thermomechanical Fatigue Loading.

In this paper, a temperature-dependent viscoplasticity model is presented that describes thermal and cyclic softening of the hot work steel X38CrMoV5-3 under thermomechanical fatigue loading. The model describes the softening state of the material by evolution equations, the material properties of which can be determined on the basis of a defined experimental program. A kinetic model is employed to capture the effect of coarsening carbides and a new isotropic cyclic softening model is developed that takes history effects during thermomechanical loadings into account. The temperature-dependent material properties of the viscoplasticity model are determined on the basis of experimental data measured in isothermal and thermomechanical fatigue tests for the material X38CrMoV5-3 in the temperature range between 20 and 650 ∘C. The comparison of the model and an existing model for isotropic softening shows an improved description of the softening behavior under thermomechanical fatigue loading. A good overall description of the experimental data is possible with the presented viscoplasticity model, so that it is suited for the assessment of operating loads of hot forging tools.

Viscoplastic constitutive model considering strain rate sensitivity under multiaxial thermomechanical fatigue loading

AbstractStrain rate sensitivity will change the cyclic mechanical response of materials under multiaxial thermomechanical fatigue loading; thus, it will lead to errors in fatigue life prediction if strain rate sensitivity is not considered in constitutive simulation. In order to more accurately express the time/rate dependence of viscoplastic behavior, a strain rate sensitivity factor is proposed to modify the viscoplastic function of Chaboche model first. On this basis, a viscoplastic constitutive model considering strain rate sensitivity under multiaxial thermomechanical fatigue loading is proposed, which can describe the varying influence of strain rate sensitivity on cyclic mechanical behavior. Meanwhile, the exponential isotropic hardening rule with the linear term is adopted to characterize the initial rapid softening and subsequent stable softening of the material under fatigue loading at elevated temperature. Moreover, the material‐dependent nonproportional hardening coefficient and the loading path‐dependent rotation factor are used in the kinematic hardening rule to consider the effect of nonproportional additional hardening on the cyclic mechanical behavior of materials. Finally, the proposed method is verified by the stress–strain data of titanium alloy TC4 and Ni‐based superalloy GH4169 under uniaxial and multiaxial thermomechanical fatigue loadings, and a good agreement is obtained.

Influence of T6 heat-treating and over-ageing on out-of-phase thermo-mechanical fatigue behaviors of Al-Si-Cu alloy

In this study, the Influence of the T6 heat-treating and over-ageing process on the material behavior of cylinder head Al-Si-Cu alloy under out-of-phase thermo-mechanical fatigue at maximum temperatures of 250 °C to 350 °C was investigated. The specimen is subjected to a conventional T6 process that includes a 5 hours solution at 510 °C, an air quench, and 3 hours of ageing at 200 °C, some specimens (with and without the T6 process) were over-aged at 250 °C for 100 hours. The fracture surfaces of specimens under thermo-mechanical fatigue (TMF) conditions are examined using scanning electron microscopy (SEM). The Al-Si-Cu alloy exhibited a ductile characteristic, as shown by the SEM images. The quick cyclic softening happened early in the TMF cyclic stress response behavior, and afterward, the cyclic stress remained steady at decreasing strain amplitudes. Cracks in TMF damage behavior were mostly initiated by broken and debonded Si and intermetallic phases, and they commonly nucleated near the primary Si-matrix interface. The T6 treatment improved the fatigue lifetime considerably under TMF loadings for non-over-aged alloys. However, the T6 treatment had no significant effect on the over-aged alloy. • The material hardened and softened at <250°C, but softened at >250°C. • The A380-T6 alloy has more cyclic softening than the A380-T0 alloy (without heat-treating). • The reduction in crack initiations and the absence of fatigue striations indicated a slower propagation rate. • Temperature affected the softening rate due to dislocation annihilation and rearrangement. • T6 enhanced fatigue lifetime in non-over-aged alloys, but not in over-aged alloys.

Optimization of Thermo-Mechanical Fatigue Life for Eutectic Al–Si Alloy by the Ultrasonic Melt Treatment

The eutectic cast Al–Si alloys with excellent high-temperature and casting performance are widely used in engine pistons. During frequent starts and stops, the thermo-mechanical fatigue (TMF) is the most important failure cause. Ultrasonic melt treatment (UT) was chosen to compare and investigate the influence of micro-structures on fatigue life and damage mechanisms of as-cast (AC) eutectic Al–Si alloys under TMF loading. After UT, the grain size, primary Si, and intermetallic particles are reduced significantly in the alloy; fatigue life increases obviously. As a result of pilling-up of dislocations, the competitive effects of the critical strain/stress for fatigue crack nucleation can be found. There are two different crack initiation mechanisms under TMF: one is primary Si fracture for AC alloys with limited critical strain/stress for fatigue crack nucleation at fractured Si particles, and the other is primary Si debonding for UT alloys with increasing critical fracture strain/stress. After the crack initiation, the fractured or debonded primary phases provide the advantages for the further development of main cracks for both alloys. The UT alloy (805 ± 253 cycles) has about twice the TMF life of the AC alloy (403 ± 98 cycles). The refinement of micro-structures is instrumental in improving the fatigue resistance and life of TMF for the UT alloy.