Improving the efficiency of advanced ultra-supercritical power plants requires hotside materials that maintain mechanical propertiesMechanical properties in high pressure and high temperature environments. Nickel superalloysNickel superalloy are a viable yet expensive option, but weldingWelding challenges can lead to premature joint failures. The goal of this work was to experimentally evaluate nickel superalloysNickel superalloy computationally designed to lower alloy cost and for improved weldabilityWeldability while maintaining mechanical propertiesMechanical properties. Two optimized nickel alloys were designed to these targets and compared to benchmark nickel superalloysNickel superalloy (Nimonic 263, Waspaloy, and/or 740H). Both optimized alloys (5Co-4Ti, 7Co-3Ti) had lower cost than benchmark superalloysSuperalloy. The 7Co-3Ti alloy had similar elastic modulus and room temperature hardnessHardness to the benchmark alloys, while the 5Co-4Ti alloy exceeded room temperature hardnessHardness and elastic modulus values. Solidification crackingSolidification cracking and strain-age cracking were assessed, and 7Co-3Ti had increased strain age and solidification crackingSolidification cracking resistance compared to commercial alloys.
A novel testing approach for cyclic crack-growthcyclic crack-growth characterization is presented, enabling faster data generation and evaluation of crack propagation behavior at both room and elevated temperatures. This is particularly relevant for superalloysSuperalloy used in critical high-temperature applications such as turbine engines, where fatigue cracks can lead to catastrophic failure. Due to their complex microstructuresMicrostructures, superalloysSuperalloy are especially sensitive to crack initiation and propagation, requiring accurate and efficient fatigue life prediction. Conventional fatigue crack-growth tests using compact tension or corner crack specimens under constant load amplitude are often time-consuming. However, particularly during alloy development and optimization, when rapid screening of material properties is necessary, faster methods would be useful. The new method presented here employs single-edge notched bend specimens following ASTM E1820, with cyclic loading, unloading, and reloading at a constant stress rate. The maximum stress increases incrementally with each cycle until fracture. At each maximum loading reversal point, the differential elastic modulus ( E_D ) is determined. Progressive crack growth reduces the load-bearing cross section and further decreases E_D . The stress dependence of the stiffness in the undamaged state E_D0 must be considered. The E_D/E_D0 ratio allows estimation of crack length and cross-sectional area reduction over time. For validation of this new approach, Alloy 718Alloy 718 was tested, stress-intensity factors were calculated according to ASTM E1820, and cyclic crack-growthcyclic crack-growth rates ( da/dN ) were plotted against the stress-intensity range ( Δ K_I ), producing classic fatigue crack-growth curves. The very good agreement with literature data demonstrates the strong potential of this method for accelerated material screening.
The influence of forgingForging conditions on grain growthGrain growth behavior during heat treatmentHeat treatment of Ni-based alloys for turbine disksTurbine disk was investigated. It was found that grain growthGrain growth behavior during super-solvus heat treatmentHeat treatment after forgingForging is affected by the following parameters, which are determined by the forgingForging conditions: -Grain growthGrain growth driving force (strain energy) and its balance with neighboring grains, -initial grain size distribution. ForgingForging temperature determined the average grain size after forgingForging, and the strain distribution determined the grain size distribution after solution treatmentSolution treatment. Even when the average grain size before heat treatmentHeat treatment was different, the final average grain size after solution treatmentSolution treatment was nearly constant as long as the strain distribution after forgingForging was uniform. This is considered to be because the driving forces for grain growthGrain growth became balanced among neighboring grains. To achieve the required properties for turbine disksTurbine disk, it is necessary to control the grain size after heat treatmentHeat treatment to an appropriate value. This study revealed that not only the heat treatmentHeat treatment conditions, but also the microstructureMicrostructure (grain size distribution) after forgingForging must be properly controlled. In addition to conventional process design methods based on preventing cracking and ensuring formability, a process selection method that considers grain growthGrain growth behavior after heat treatmentHeat treatment was proposed.
The manufacturing of gamma primeGamma prime (γ′) strengthening nickel-based superalloysSuperalloy using additive manufacturingAdditive manufacturing (AM) processes has received significant attention from both the industrial and research communities due to its outstanding high-temperatureHigh-temperature properties. Nevertheless, high crack susceptibilityCrack susceptibility remains a considerable challenge for superalloysSuperalloy with high γ′ volume fractions manufactured by the laser powder bed fusionLaser Powder Bed Fusion (PBF-LB) (PBF-LB) process. Post-heat treatmentHeat treatment (PHT) plays a key role in shaping γ′ morphology and minimizing crystallographic anisotropy to achieve the desired mechanical propertiesMechanical properties. However, studies focusing on microstructural and γ′ morphology changes in the sub-solvus PHT for PBF-LB-manufactured Alloy 247Alloy 247 are limited. This study investigates the effects of exposure to sub-solvus temperatures, ranging from 600 to 900 °C, with varying holding times of 30 to 180 min, on microstructural changes, γ′ morphology, crystallographic textureTexture, and hardnessHardness. The microstructural changes were analyzed using optical microscopy, scanning electron microscopy (SEM), and electron backscatter diffraction (EBSDElectron Backscatter Diffraction (EBSD)). The results show that the as-manufactured 247 alloy exhibits a fine cellular dendritic structure with a strong <001> columnar textureTexture, which is parallel to the build direction, achieving a hardnessHardness of 463 HV1.0. Exposure to 800 °C resulted in cuboidal γ′ precipitates, resulting in the highest hardnessHardness of 545 HV1.0. The EBSDElectron Backscatter Diffraction (EBSD) results confirmed a gradual decrease in low-angle grain boundaries. These results contribute to a deeper understanding of the kinetics ofΓ′ precipitation γ′ precipitationPrecipitation and identify the temperature ranges in which nucleation and growth of γ′ occur at an accelerated rate.
Understanding the microstructural evolutionMicrostructural evolution of Ni-base superalloysSuperalloy during cooling from solution heat treatmentHeat treatment is essential for optimizing turbine disc performance. In this study, a multi-technique approach was employed to investigate theγ′ precipitation γ′ precipitationPrecipitation behavior of Udimet 720LIUdimet 720LI under controlled continuous coolingContinuous cooling conditions. A series of cooling experiments were conducted using a quenching dilatometer to apply precisely defined cooling rates between 0.7 and 100 °C/s. Selected experiments were complemented by synchrotron-based small-angle X-ray scattering (SAXSSmall-Angle X-ray Scattering (SAXS)) measurements to track the precipitation kineticsPrecipitation kinetics during cooling. Post-cooling microstructural characterization by scanning electron microscopy (SEM) showed that the grain size and primary γ′ precipitates remained unaffected by the cooling rate, whereas the secondary γ′ precipitates exhibited a strong cooling rate sensitivityCooling rate sensitivity. Their average diameter followed a power-law relationship with cooling rate, with an exponent consistent with diffusion-controlled growth. The size distributions were broad at slow cooling and narrow at high cooling rates, in agreement with the nucleation burst theory described in literature. The SAXSSmall-Angle X-ray Scattering (SAXS) analysis showed very good agreement with the SEM results and additionally revealed the presence of fine tertiary γ′ precipitates, with diameters below the SEM resolution limit. These findings highlight the importance of combining SEM with in-situ SAXSSmall-Angle X-ray Scattering (SAXS) to capture the full precipitate population across multiple size scales. By linking in-situ phase transformation kinetics with post-cooling microstructural analysis, this study establishes a framework for quantifying the relationship between cooling rate and γ′ evolution and supports the development of optimized heat treatmentHeat treatment strategies for turbine disc alloys.
