Thermomechanical Fatigue Resistance Research Articles

Enhanced thermomechanical fatigue resistance in W10Re alloys: Microstructural and surface engineering approaches

Thermomechanical fatigue of refractory metals is commonly observed in high-temperature environments like first walls and divertors for proposed fusion reactors or X-ray anodes for medical imaging. Typically, alloying with rhenium or optimized microstructures are employed to counteract or delay the detrimental effects of fatigue in tungsten. Additionally, a novel concept is the utilization of surface structures to compensate cyclic thermal stresses. This work aimed to investigate the combination of these approaches, by comparing two W10Re alloys with vastly different microstructures, as well as engineered surface conditions of varying scales. Samples were subjected to thermocyclic fatigue under pulsed electron beam exposure, mimicking the surface temperatures typically encountered in a rotating X-ray anode. Analysis of several in-situ data streams, post-mortem investigations by metallography, and finite element methods revealed the interplay between microstructure and surface modifications. The columnar microstructure exhibited higher resistance to severe surface damage compared to the globular one, linked to the deflection of cracks along grain boundaries and subsequent melting. Coarse structuring was found to partly relieve the surface stresses during thermal cycling, preventing most of the damage accumulation. A full damage relief and performance equivalence between columnar and globular microstructure was achieved by engineering fine-structured surfaces, which led to a ten-fold increase in fatigue resistance over the non-structured condition.

3D Investigation of Damage During Strain‐Controlled Thermomechanical Fatigue of Cast Al–Si Alloys

The strain‐controlled thermomechanical fatigue behavior is investigated for three cast near‐eutectic Al–Si alloys with different Ni, Cu, and Mg contents. Synchrotron tomography and neutron diffraction experiments are used to correlate 3D microstructural features with damage initiation and evolution. The results show that the alloy with lower Cu, Ni, and Mg concentrations has up to 45% higher thermomechanical fatigue resistance for cooling/heating rates of 5 and 15 K s−1. In addition, this alloy also exhibits damage formation at later stages during thermomechanical fatigue and slower damage accumulation compared to other alloys. This difference in behavior is a consequence of its higher ductility, which is a result of the lower volume fraction and global interconnectivity of the 3D hybrid networks formed by Si and intermetallics and the absence of large primary Si clusters which act as preferred crack initiation sites during the early stages of 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.

Active Crack Obstruction Mechanisms in Crofer® 22H at 650 °C.

Increased cyclic loading of components and materials in future thermal energy conversion systems necessitates novel materials of increased fatigue resistance. The widely used 9–12% Cr steels were developed for high creep strength and thus base load application at temperatures below 620 °C. At higher temperature, these materials present unstable grain structure, prone to polygonization under thermomechanical fatigue loading and limited resistance to steam oxidation. This seminal study compares thermomechanical fatigue resistance and long crack propagation of the advanced ferritic-martensitic steel grade 92 and Crofer® 22H, a fully ferritic, high chromium (22 wt. %) stainless steel, strengthened by Laves phase precipitation. Crofer® 22H features increased resistance to fatigue and steam oxidation resistance up to 650 °C. Both thermomechanical fatigue (crack initiation) and residual (crack propagation) lifetime of Crofer® 22H exceeded that of grade 92. The main mechanisms for improved performance of Crofer® 22H were increased stability of grain structure and “dynamic precipitation strengthening” (DPS). DPS, i.e., thermomechanically triggered precipitation of Laves phase particles and crack deflection at Laves phase-covered sub-grain boundaries, formed in front of crack tips, actively obstructed crack propagation in Crofer® 22H. In addition, it is hypothesized that local strengthening may occur near the crack tip because of grain refinement, which in turn may be impacted by testing frequency.

Challenges of Developing Ductile and Lighter Refractory High Entropy Alloys

In the recent past refractory high-entropy alloys (RHEAs) have attracted a lot of attention due to their interesting mechanical properties. These are simple solid solution alloys either with face-centered cubic (FCC) or body-centered cubic (BCC) crystal structures. BCC alloys exhibit relatively higher strengths at high temperatures and therefore, can be considered as structural materials for ultrahigh temperature applications (≥ 1300 ºC). However, the main challenge in developing BCC structured RHEAs for structural applications is balancing their high temperature (HT) strength with their room temperature (RT) tensile ductility and density. Therefore, searching for new and lighter RHEAs with optimized strength and tensile ductility is still a challenge. Improvement of oxidation resistance RHEAs is also an important issue as refractory metals are prone to oxidation in air at high temperatures. This paper describes various theoretical and experimental research works to develop stronger, ductile and lighter RHEAs alloys with novel compositions. The paper also describes various attempts in the development of RHEAs with high creep strength, RT ductility, fracture toughness, lower density, thermo-mechanical fatigue resistance, and host of other physical properties. It will also describe the advance manufacturing techniques in fabricating the components out of these compositions.

Thermomechanical fatigue resistance of low temperature solder for multiwire interconnects in photovoltaic modules

Novel interconnect technologies leveraging low melting temperature solders, such as multiwire interconnects, are being deployed in photovoltaic (PV) modules for improved reliability through interconnect redundancy and lower thermal loads during interconnection and lamination. However, the equivalency of standardized accelerated testing to field conditions has not yet been established for these emerging technologies. In this study, the thermomechanical fatigue resistance of low temperature solder alloys is investigated and compared to that of conventional SnPb to assess the acceleration behavior of these alloys. While InSn is shown to have sufficient thermomechanical fatigue resistance on the order of that of SnPb, these results indicate Sn–Bi alloys may have poor thermomechanical fatigue resistance at field conditions. The results also show that Sn–Bi alloys have thermal cycling acceleration factors of less than one. This indicates that the standardized accelerated thermal cycling test, such as that in IEC 61215, will produce misleading results for Sn–Bi alloys and that unique testing is required for this PV module architecture. Though accelerated thermal cycling may be a meaningful qualification test for SnPb solder joints, these results suggest that mechanical loading may be a more appropriate test for Sn–Bi multiwire interconnects. This is due to the distinct processing and geometry of multiwire interconnects which may allow for mechanical, rather than strictly metallurgical interconnections.

Temperature-Invariant Superelastic and Fatigue Resistant Carbon Nanofiber Aerogels.

Superelastic and fatigue-resistant materials that can work over a wide temperature range are highly desired for diverse applications. A morphology-retained and scalable carbonization method is reported to thermally convert a structural biological material (i.e., bacterial cellulose) into graphitic carbon nanofiber aerogel by engineering the pyrolysis chemistry. The prepared carbon aerogel perfectly inherits the hierarchical structures of bacterial cellulose from macroscopic to microscopic scales, resulting in remarkable thermomechanical properties. In particular, it maintains superelasticity without plastic deformation even after 2 × 106 compressive cycles and exhibits exceptional temperature-invariant superelasticity and fatigue resistance over a wide temperature range at least from -100 to 500 °C. This aerogel shows unique advantages over polymeric foams, metallic foams, and ceramic foams in terms of thermomechanical stability and fatigue resistance, with the realization of scalable synthesis and the economic advantage of biological materials.

Assessment of Thermo-Mechanical Fatigue Performance of an Exhaust Manifold through Simulation

Exhaust manifold of an IC engine typically experiences cyclic thermal and mechanical loading and are prone to TMF failure. Thermo-mechanical fatigue resistance needs to be ensured for exhaust manifold to meet the requirement of durability in order to meet the demanding needs of recent trends in IC engine design. Considering reduced product development cycle time, more accurate design procedure for exhaust manifold through simulation route is preferred. The present work is an attempt in this direction, where methodology to carry out TMF analysis of exhaust manifold through simulation is formulated and successfully implemented. The complete simulation process involved four important stages, namely simplified 1-D simulation of engine, thermal analysis of exhaust manifold, and structural analysis of exhaust manifold and TMF life evaluation of exhaust manifold. The developed methodology has been successfully implemented considering a typical exhaust manifold. Thermal inputs obtained through simplified engine simulation using 1-D gas dynamics and engine simulation software required for thermal analysis of exhaust manifold are found to be satisfactory and has eliminated the need for complex 3-D CFD simulations. TMF life valuated at critical locations indicate that except one location all other locations have life exceeding the LCF limit.

Thermo-mechanical fatigue resistance characterization and materials ranking from heat-flux-controlled tests. Application to cast-irons for automotive exhaust part

In a context where the mass, the cost and the mechanical strength of materials must be jointly optimized, it is necessary to have experimental data quickly available and sufficiently robust to make efficient conception choices. For thermomechanical fatigue, standard tests usually allow comparing material for the same temperature and strain ranges although differences in thermal properties such as conductivity or thermal expansion could make significant deviations when the same thermal flux is applied particularly for structure with forced heat flux operating regimes. A new protocol is then proposed in order to compare the specific resistances of metallic materials against thermomechanical fatigue. It can easily rank materials according to their lifetime under thermomechanical loadings where strain range and temperature amplitude are determined by the heat flux applied on an industrial part. The method is based on a complete numerical analysis to determine experimental loading conditions as a temporal evolution of temperature and mechanical strain representative of thermomechanical loading observed in TMF critical areas for the part. TMF tests on hollow specimens are carried out to rank the materials: temperature and strain amplitude are different for each alloys whereas heat flux is identical. A materials ranking list based on TMF resistance is then determined according to their lifetimes under “heat-flux-controlled” tests. The method is tested for exhaust parts and demonstrates the superiority of some cast irons over others, whereas the intrinsic isotherm mechanical properties suggested an alternative classification. The obtained ranking is confirmed by experimental tests on industrial structures.

A new ferritic stainless steel with improved thermo-mechanical fatigue resistance for exhaust parts

Over the past few years, car manufacturers have been considering ever higher service temperatures for the engine in order to comply with the constraints of depollution standards. The requirements in terms of exhaust gas temperature could easily reach and overtake the limits of common stainless steel grades used for such applications in the coming years.A new ferritic stainless steel – named K44X – with increased high temperature resistance has therefore been developed to withstand service temperature up to 1000 °C. K44X belongs to EN 1.4521 and AISI 444 classifications and is composed of approximately 19% Cr, 2%Mo and 0.6% Nb. This specific composition leads to better mechanical properties, higher creep and fatigue resistance than EN 1.4509, while keeping comparable weldability and formability. Its coefficient of thermal expansion is lower in comparison to austenitic stainless steel grades and its resistance to cyclic oxidation is improved significantly.High-temperature properties (mechanical properties, creep, cyclic oxidation resistance, and high cycle fatigue) of K44X are presented in this paper and compared with common ferritic and austenitic stainless steels used in the hot end of exhaust lines. A thermal fatigue test – designed to reproduce exhaust manifold service conditions – has also been carried out with the highest temperatures of the cycle in the range of 850–1000 °C. The results of these thermal fatigue tests were compared with the above-mentioned stainless steels. A thermal fatigue damage criterion was then identified based on these experimental results and using a cyclic behaviour law obtained from isothermal low cycle fatigue tests.

Stress relaxation of compacted graphite iron alloyed with molybdenum

In a previous study, the thermomechanical fatigue resistance of four compacted graphite irons (CGIs) and one grey cast iron was investigated. The molybdenum content of the four CGIs varied between 0 and 1·01 wt-%. It was observed that during thermal cycling, the maximum value of the compressive stress continuously decreased while the value of the maximum tensile stress continuously increased. The continuous decrease in compressive stresses showed that stress relaxation occurs at elevated temperatures during thermal cycling. The goal of the present investigation was to investigate the phenomenon of stress relaxation at elevated temperatures. The tests were performed at 350 and 600°C respectively. The results of the stress relaxation tests performed at 600°C showed the same trend observed at thermomechanical fatigue testing. The tests showed that additions of molybdenum improved the fatigue resistance of CGI by lowering the stress relaxation rate.

A new ferritic stainless steel with improved thermo-mechanical fatigue resistance for exhaust parts

A new ferritic stainless steel with improved thermo-mechanical fatigue resistance for exhaust parts

Thermocracks®, a Specific Testing Machine for Evaluation of the Thermal Fatigue Resistance of Materials

Cracking by thermal fatigue is one of the major problems encountered during the operation of numerous parts such as exhaust manifolds or brake discs. One way to improve their lifespan is to develop new materials solutions. Estimating thermo-mechanical fatigue resistance of new materials can be performed with a specific test machine developed by CTIF, associating a multi-position wheel, an induction heating and a cooling by air blowing. Test consists in measuring the number of thermal cycles corresponding to the first crack appearance and to the complete failure of a specific versatile sample.Two examples of studies performed with this machine are shown hereafter.First tests were performed to compare the thermo-mechanical fatigue resistance of different grades of ferritic spheroidal graphite cast iron. These materials are an interesting alternative to alloyed steels or nickel-alloyed cast irons, for the constitution of exhaust manifolds and turbine housings working in the upper-middle temperatures (up to 950°C). The compared materials are SG irons alloyed with silicon (with or without molybdenum), and aluminium.Truck brakes dissipate several megajoules of energy every few seconds, which leads to high thermal stresses in the rubbing discs. Therefore, truck brake discs are mainly damaged by thermal fatigue cracking. One improvement way is to evolve disc material with the aim of increasing their thermal fatigue resistance while keeping the braking performance. For the thermal fatigue tests being as close as possible to real brake conditions, a specific geometry of sample and inductor was developed, to reproduce the thermal gradients observed during tests on real brake discs. The test parameters were then chosen after a coupled numerical-experimental study on full-scale braking. Behaviour of different materials under cycling loading (microstructural changes and cracking) is compared with regard to the results obtained during braking tests.

Influence of molybdenum alloying on thermomechanical fatigue life of compacted graphite irons

In this study four compacted graphite irons (CGIs) and one grey cast iron (FGI) were produced and tested in the laboratory. The molybdenum content of the four CGI grades was varied between 0 and 1··01 wt-%. The purpose of the investigations was to examine the effect of the different molybdenum contents of the CGI on the thermomechanical fatigue (TMF) behaviour. The TMF tests were performed by cycling a constrained specimen between 110 and 600°C. For every material three tests were performed on specimens machined from a ∅20 mm cylinder. Other tests were performed on specimens machined from ∅55 mm and ∅85 mm cylinders respectively. The tests showed that additions of molybdenum improved the fatigue resistance of CGI. It was observed that additions of molybdenum refined the pearlite and that the specimens with a finer metallic matrix had a higher TMF resistance.

A reliability analysis method in thermomechanical fatigue design

The design of structures against thermomechanical fatigue (TMF) is a relatively new concern and research has generally concentrated on deterministic methods to ensure the resistance of structures undergoing thermal–mechanical loadings. Many studies have thus been conducted to better represent the nonlinear behaviour of materials or to develop thermomechanical fatigue criteria. However, fatigue is a phenomenon which is random in nature: manufacturing processes, geometric tolerances and usage conditions can affect the lifetime of a structure. Typically, the use of a car by a customer is unique (type of roads, weather conditions, drivers behaviour, etc.) and thus thermomechanical loads for instance on a cylinder head become probabilistic. Similarly, the intrinsic strength of a structure is variable (casting and machining process, specific microstructures, etc.). It is therefore necessary to be able to guarantee the TMF resistance of a particular structure itself undergoing a particular load, whatever the structure is. The work presented here consists of the development of a complete protocol analysis of the risk of failure of a structure subjected to thermomechanical fatigue when either exact loading conditions or strength for a given structure are uncertain. The proposed method relies on the stress–strength interference analysis and also on numerical techniques based on finite element calculation and engine load analysis which enable us to compute a local damage from a global loading. The method is successfully applied to a cylinder head.

On the Mechanical Behavior of a New Single-Crystal Superalloy for Industrial Gas Turbine Applications

The mechanical behavior of a new single-crystal nickel-based superalloy for industrial gas turbine (IGT) applications is studied under creep and out-of-phase (OP) thermomechanical fatigue (TMF) conditions. Neutron diffraction methods and thermodynamic modeling are used to quantify the variation of the gamma prime (γ′) strengthening phase around the γ′ solvus temperature; these aid the design of primary aging heat treatments to develop either uniform or bimodal microstructures of the γ′ phase. Under creep conditions in the temperature range 1023 K to 1123 K (750 °C to 850 °C), with stresses between 235 to 520 MPa, the creep performance is best with a finer and uniform γ′ microstructure. On the other hand, the OP TMF performance improves when the γ′ precipitate size is larger. Thus, the micromechanical degradation mechanisms occurring during creep and TMF are distinct. During TMF, localized shear banding occurs with the γ′ phase penetrated by dislocations; however, during creep, the dislocation activity is restricted to the matrix phase. The factors controlling TMF resistance are rationalized.

On Localized Deformation and Recrystallization as Damage Mechanisms during Thermomechanical Fatigue of Single Crystal Nickel-Based Superalloys

Thermomechanical fatigue (TMF) in superalloys is growing in importance due to the introduction of advanced cooling systems but also due to the changes in demand and competition within the power generation market; this is requiring many power plants to operate under cyclic conditions. In this paper the TMF behaviour of three different single crystal nickel-based superalloys are compared. It is demonstrated that the deformation and damage mechanisms occurring during TMF are rather different from those traditionally reported for creep or isothermal fatigue. In all cases examined, the deformation is localized within a rather small number of deformation bands. While these bands were found to consist mainly of micro-twins in some alloys, in others they might be better described as slip or shear bands. Furthermore, in some circumstances these bands are prone to recrystallization. In CMSX-4, the intersection points of twins of different orientation act as initiation sites for this process. In the SCA425 alloy – of smaller gamma’ content, lower creep resistance and less great oxidation resistance – twinning is observed infrequently; however the deformation is still very localized and in the distorted gamma-gamma’ microstructure, along the shear bands, recrystallization is observed. Furthermore the recrystallization is enhanced by oxidation due to the development of a gamma’-depleted zone. In CMSX-4, TCP phases precipitated during long term ageing cause a more dispersed deformation behaviour which prevents recrystallization. Our findings confirm the importance of an inhomogeneous microstructure for good TMF resistance.

The study of 430Ti and 430LNb ferritic welding wires for application in the cold part of automotive exhaust systems

Ferritic stainless steels have been extensively used in automotive exhaust systems. Ferritic stainless steel wires have been recently introduced in the fabrication of these components to reduce costs (hot and cold parts) and improve the quality of hot part components, mainly thermo-mechanical fatigue resistance, and their use has been growing since. This study compares mechanical and metallurgical characteristics of fusion zones obtained with 430Ti and 430LNb welding wires. The influence of Ar 2%O2 and Ar 2%N2 3%CO2 shielding gases was also studied. Metallographic analysis was performed by optical and electronic microscopy, revealing the main characteristics of the fusion zone obtained with these welding wires and shielding gases. Welded joint ductility was measured using the Erichsen test. The results indicated that ferritic stainless steel wires could be used to produce welds with adequate ductility. However, this depends critically on the choice of the welding wire and the shielding gas to maximize mechanical and metallurgical properties and to avoid any future problems.

A Reliability Study of Nanoparticles Reinforced Composite Lead-Free Solder

This work looks at the development and investigation of a reinforced composite solder with low melting point. The composite solder was prepared by adding Sn-3.0Ag-0.5Cu nanoparticles into Sn-58Bi solder paste. The Sn-3.0Ag-0.5Cu nanoparticles were manufactured using a self-developed Consumable-electrode Direct Current Arc (CDCA) technique. The test FR-4 Printed Circuit Board (PCB) with Cu pad and Electroless Nickel Immersion Gold (ENIG) surface finish were fabricated, and fifty SR1206 chip resistors were mounted on pads of test PCB with the reinforced composite solder paste by using conventional surface mount technology. The differential scanning calorimetry (DSC) was used to analyze the constituent of the composite solder joint after reflow. A scanning electron microscope (SEM), transmission electron microscope (TEM) and optical microscope (OM) were employed in order to observe the morphology of nanoparticles; the microstructure of reinforced composite solder joint; the crack initiation and propagation in solder joint; and the fracture mode after shear test. The thermal cycling (TC) was carried out with a temperature range of 40C and 125C. The contact resistance of the solder joint was measured during thermal cycling, and the shear test of solder joints was performed before and after 500 thermal cycles. After the shear test, all fracture surfaces were inspected to identify the fracture mode of the composite solder joint. The results of the experiments detailed in this work indicate that the shear strength of the composite solder increased 2 times in comparison to Sn-58Bi. Meanwhile, the thermomechanical fatigue (TMF) resistance of the composite solder with 1 mass% nanoparticles was 16 times stronger than Sn-58Bi and 4 times stronger than Sn-3.0Ag-0.5Cu. However, the tendency of forming micro-cracks between nanoparticles and solder matrix and the fracture within solder was increased for solder joints with more than 3 mass% nanoparticles after thermal cycling. [doi:10.2320/matertrans.MJ201002]

Modeling of mechanical properties of alloy CMSX-4

High efficiency in gas turbines requires increased gas temperatures. Gas temperatures above 1100 °C can only be handled using air-cooled structures to keep the metal temperatures below 1000 °C. Single-crystalline superalloys meet the requirements for creep strength and thermo-mechanical fatigue resistance. Single-crystalline superalloys are used for gas turbine blades. Modeling of deformation is a key issue in order to develop life prediction tools. In this study modeling of creep deformation and creep rupture as well as fatigue contribute to a description of the mechanical behavior of CMSX-4 at three different temperatures. Creep deformation was described by modified Garofalo equation and a Larson–Miller approach. Cyclic elastic–plastic deformation as well as fatigue life were described by a new engineering approach. Here the elastic–plastic deformation work was considered in order to determine a temperature-dependent master curve for fatigue behavior.