The Influence of Grain Boundary Elements on Properties and Microstructures of P/M Nickel Base Superalloys (original) (raw)
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Metallurgical and Materials Transactions A, 1998
The principal limitation of today's Ni-and Fe-based superalloys continues to be their susceptibility to intergranular degradation arising from creep, hot corrosion, and fatigue. Many precipitationstrengthened superalloys are also difficult to weld, owing to the formation of heat-affected zone (HAZ) cracks during postweld heat treatments (PWHTs). The present work highlights significant improvements in high-temperature intergranular degradation susceptibility and weldability arising from increasing the relative proportion of crystallographically ''special'' low-⌺ CSL grain boundaries in the microstructure. Susceptibility to intergranular degradation phenomena is reduced by between 30 and 90 pct and is accompanied by decreases in the extent and length of PWHT cracking of up to 50-fold, with virtually no compromise in mechanical (tensile) properties upon which the functionality of these specialty materials depends. Collectively, the data presented suggest that ''engineering'' the crystallographic structure of grain boundaries offers the possibility to extend superalloy lifetimes and reliability, while minimizing the need for specialized welding techniques which can negatively impact manufacturing costs and throughput.
Materials
Inconel 738LC (IN738LC) is a nickel-based superalloy specially used in the hot section components of turbine engines. One of its main drawbacks relies on the cracking susceptibility when it is manufactured by laser powder bed fusion (LPBF). This paper analyzes the influence of minor alloying element concentration on cracking tendency of IN738LC superalloy manufactured by LPBF. For that objective, samples were manufactured using two powders, which presented different minor alloying elements concentration (Si, Zr and B). It was shown that the samples crack tendency was very different depending on the powder used for their manufacturing. In fact, the measured crack density value was 2.73 mm/mm2 for the samples manufactured with the powder with higher minor alloying elements concentration, while 0.25 mm/mm2 for the others. Additionally, a special emphasis has been put on elemental composition characterization in cracked grain boundaries in order to quantify possible Si or Zr enrichment....
Crack growth behaviour of a nickel-based powder metallurgy superalloy under elevated temperature
International Journal of Fatigue, 2011
Crack growth behaviour of a nickel-based powder metallurgy superalloy has been studied under elevated temperatures. Test results show that there is a turning point for RT, 550°C and 650°C crack growth curves. Above this point, the three curves merge together and influence of elevated temperature disappears. Material characteristic microstructure length (grain size) is considered to be responsible for this phenomenon. However, this turning point is suppressed at 750°C possibly by grain boundary weakening. Dwell tests show that effect of 90 s holding time on the crack growth rate is limited at 650°C but much significant at 750°C due to creep damage.
Fatigue crack growth in nickel-based superalloys at elevated temperatures
Materials & Design, 2011
In the present work, fatigue crack growth in two nickel-base mono-materials and one bi-material has been investigated at 450 and 550 °C. The electric potential drop technique was found to better estimate the crack length during cycling as compared to the compliance method. This finding is supported by microscopic observations of the fracture surface and also by the numerical simulation using finite element code Castem2000. The crack was found to grow faster in the coarse grained material than in the fine grained one. The fracture surface observation showed that the performance of the bi-material is linked to the mono-material content at the interface. In addition, the content of each mono-material at the interface was found to be very stochastic. This heterogeneity, due to the assembly process, strongly affects the behaviour of the biomaterial. Finite element computation showed a good agreement between numerical and experimental results in term of stress intensity factor.► Crack growth measurements are better estimated by the potential drop technique. ► Crack propagation is faster at 550 than 450 °C for all the materials. ► The crack propagates faster in the U720LC than in the U720PM mono-material. ► The content of U720PM and U720LC mono-materials is very stochastic at the interface. ► The heterogeneity at the interface affects the behaviour of the joint material.
Crack growth in a new nickel-based superalloy at elevated temperature - Part III - Characterisation
J Mater Sci, 2005
Crack growth mechanisms at elevated temperature have been examined in a new nickel-based superalloy. Scanning electron microscopy has been used to identify the fracture modes in specimens tested under triangular, fast-slow, slow-fast, dwell and sustained loading conditions at 650 and 725 • C. A model has been developed considering a predominant oxidation-driven crack growth mechanism, as revealed from the SEM analysis. An oxidation sensitive time has been adopted to characterise the rate-dependent crack growth, together with an Arrehenius relationship to take account of the influence of temperature. The model predictions have been compared with the experimental results. C 2005 Springer Science + Business Media, Inc.
In the current study, the effect of strain distribution in a simple forging geometry on the propensity for recrystallization, and its impact on mechanical properties has been investigated in a newly developed experimental nickel-based superalloy. The new alloy was produced via a Powder Metallurgy (PM) route and was subsequently Hot Isostatic Processed (HIP), isothermally forged, and heat treated to produce a coarse grain microstructure with average grain size of 23-32 μm. The alloy was examined by means of Electron Back-Scatter Diffraction (EBSD) to characterise the microstructural features such as grain orientation and morphology, grain boundary characteristics and the identification of potential Prior Particle Boundaries (PPBs) throughout each stage of the processing route. Results at the central region of the cross-section plane parallel to the loading direction showed significant microstructural differences across the forging depth. This microstructural variation was found to be highly dependent on the value of local strain imparted during forging such that areas of low effective strain showed partial recrystallisation and a necklace grain structure was observed following heat treatment. Meanwhile, a fully recrystallised microstructure with no PPBs was observed in the areas of high strain values, in the central region of the forging.
STRUCTURE AND ITS INFLUENCE ON Ni – BASE SUPERALLOY MECHANICAL PROPERTIES
2013
The structure of polycrystalline Ni – base superalloys, depending on a heat – treatment, consist of solid solution of elements in Ni (γ phase, also called matrix), primary carbides MC type, intermetallic precipitate Ni3(Al, Ti) (γ phase), and secondary carbides M23C6 type. Shape and size of these structural components have a significant influence on final mechanical properties of alloy. For instance the precipitate γ size greater than 0.8 mm significantly decreasing the creep rupture life of superalloys and also carbides size greater than 50 mm is not desirable because of fatigue cracks initiation.
Effect of HIP Parameters on the Micro-Structural Evolution of a Single Crystal Ni-Based Superalloy
Advanced Materials Research, 2011
For reducing the porosity of single crystal (SX) nickel-based superalloys, Hot Isostatic Pressing (HIP) is used. High pressures of about 100-170 MPa lead to local deformation, which close the pores. However, since HIP also requires high temperatures (1000-1200°C) it has a pronounced effect on the microstructure and the local distribution of elements. This contribution analyses the effect of different HIP treatments on both the microstructure and the segregation of the SX superalloy LEK94 in the as-precipitation-hardened state. In addition, the effects of rapid or slow cooling are analyzed. To distinguish the effect of pressure from those of temperature, the HIPed samples are compared with specimens annealed at atmospheric pressure.
Acta Materialia 166 (2019) 158-167, 2019
The microstructural and compositional evolution of intergranular carbides and borides prior to and after creep deformation at 850 C in a polycrystalline nickel-based superalloy was studied. Primary MC car-bides, enveloped within intergranular g 0 layers, decomposed resulting in the formation of layers of the undesirable h phase. These layers have a composition corresponding to Ni 3 Ta as measured by atom probe tomography and their structure is consistent with the D0 24 hexagonal structure as revealed by transmission electron microscopy. Electron backscattered diffraction reveals that they assume various mis-orientations with regard to the adjacent grains. As a consequence, these layers act as brittle recrystallized zones and crack initiation sites. The composition of the MC carbides after creep was altered substantially, with the Ta content decreasing and the Hf and Zr contents increasing, suggesting a beneficial effect of Hf and Zr additions on the stability of MC carbides. By contrast, M 5 B 3 borides were found to be micro-structurally stable after creep and without substantial compositional changes. Borides at 850 C were found to coarsen, resulting in some cases into g 0-depleted zones, where, however, no cracks were observed. The major consequences of secondary phases on the microstructural stability of superalloys during the design of new polycrystalline superalloys are discussed.