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Yttria stabilized tetragonal zirconia polycrystal (Y-TZP) owes its high toughness to transformation toughening, a mechanism that requires the development of a process zone. It is important to measure if and to what extent the size of components can be reduced. In this study, tests were carried out using focused ion beam or picosecond milled pre-notched, 3 mol percent Y2O3 (3Y-) TZP micro-cantilever (10–250 μm) beams. The tests show clearly that the maximum fracture resistance is size dependent and the plateau toughness was not reached in any of the small-scale samples. A correlation between transformability of the tetragonal phase and the measured fracture resistance became visible only for the largest micro cantilever but did not reach the values measured in macroscopic samples. Based on these results, it is not advantageous to use very tough zirconia materials in components with dimensions smaller than ∼0.25 mm, as the high toughness is not fully realized.

Bond coats are essential in gas turbine technology for oxidation protection. Freestanding MCrAlY (M = Ni, Co) bond coats were investigated with respect to their creep strength at elevated temperatures. Three types of MCrAlY, a Ni-based bond coat Amdry 386, a Co-based bond coat Amdry 9954 and Amdry 9954 + 2 wt% Al2O3 (ODS = oxide dispersion strengthened) produced by low pressure plasma spraying, were analyzed. The two phase microstructure of the bond coats consists of a fcc γ-Ni solid solution and a B2 β-NiAl phase. Constant load experiments were performed in a thermomechanical analyzer at temperatures between 900 and 950 °C. Microtensile test specimens with a diameter of 450 µm were produced by a high-precision grinding and polishing process. Creep rupture was mainly due to void nucleation along the β–γ interfaces and grain boundaries. The time to failure is larger in Ni-based Amdry 386 compared to that in Co-based Amdry 9954 due to a higher fraction of the high-strength β-NiAl phase at test temperatures. The addition of ODS-particles in the Co-based bond coat Amdry 9954 resulted in a better creep resistance but lower ductility in comparison to ODS-particle-free Amdry 9954.

The creep resistance of single-crystalline Ni-base superalloys at elevated temperatures depends among others on solid solution strengthening of the γ-matrix. To study the influence of various solid solution strengtheners on the mechanical properties, a series of Ni-base superalloys with the same content of different alloying elements (Ir, Mo, Re, Rh, Ru, W) or element combinations (MoW, ReMo, ReW) was investigated. Nanoindentation measurements were performed to correlate the partitioning behavior of the solid solution strengtheners with the hardness of the individual phases. The lowest γ′/γ-hardness ratio was observed for the Re-containing alloy with the strongest partitioning of Re to the γ-matrix. As a result of the creep experiments in the high-temperature/low-stress regime (1373 K (1100 °C)/140 MPa), it can be concluded that solid solution hardening in the γ-phase plays an essential role. The stronger the partitioning to the γ-phase and the lower the interdiffusion coefficient of the alloying element, the better the creep resistance. Therefore, the best creep behavior is found for alloys containing high contents of slow-diffusing elements that partition preferably to the γ-phase, particularly Re followed by W and Mo.

This paper presents the high temperature yield and fatigue strength as well as thermophysical properties of two polycrystalline wrought γ/γ′ Co-base superalloys developed for application in a temperature regime above 700 °C. The alloys CoWAlloy1 (Co42Ni32Cr12Al6W3Ti2.5Ta1.5 + Si,C,B,Zr,Hf) and CoWAlloy2 (Co41Ni32Cr12Al9W5 + Ti,Ta,Si,C,B,Zr,Hf) exhibited solidus temperatures of 1070 °C and 1030 °C, respectively, and a γ′ fraction of about 50%. CoWAlloy2 displayed a rather high Young’s modulus of 250 GPa. The two Co-base superalloys showed a good high temperature strength exceeding Ni-base disc alloys U720Li and Waspaloy at temperatures above 800 °C. When comparing the yield strength from tensile and compression tests, no asymmetry could be found. The low-cycle fatigue life of CoWAlloy2 at a total strain amplitude of 0.5% is similar to that of U720Li and Waspaloy (about 370 cycles, R = −1). During long-term aging for 1024 h at 750 °C, no additional phases were formed and the room temperature hardness barely changed.

PDF of https://doi.org/10.1002/adem.202001464 Lamellar eutectic NiAl–Cr(Mo) alloys show an increased fracture toughness due to different toughening mechanisms. These mechanisms result from the fibrous or lamellar microstructure of the two constituting phases α‐Cr(Mo) and β‐NiAl. However, the fracture toughness of the individual phases and the evolution from early crack growth to the toughening mechanisms have not yet been systematically studied. Herein, bending tests on focused ion beam (FIB)‐notched microcantilever beams are used to characterize the small‐scale fracture properties. The micromechanical investigations reveal that the fracture toughness of the α‐Cr(Mo) phase (7.5−9.1 MPa𝑚⎯⎯⎯√7.5−9.1 MPam) is much higher than the fracture toughness of β‐NiAl (2.2−2.9 MPa𝑚⎯⎯⎯√2.2−2.9 MPam). Larger cantilevers in the crack arresting orientation show an enhanced fracture toughness with up to14.4 MPa𝑚⎯⎯⎯√14.4 MPam, which is still lower than the one of macroscopic experiments. This is attributed to the small interaction volume of the crack, which hinders the full exploitation of potential extrinsic toughening mechanisms.

The mechanical properties of low-pressure plasma sprayed (LPPS) MCrAlY (M = Ni, Co) bond coats, Amdry 386, Amdry 9954 and oxide dispersion strengthened (ODS) Amdry 9954 (named Amdry 9954 + ODS) were investigated after annealing in three atmospheres: Ar&ndash, O2, Ar&ndash, H2O, and Ar&ndash, H2&ndash, H2O. Freestanding bond coats were investigated to avoid any influence from the substrate. Miniaturized cylindrical tensile specimens were produced by a special grinding process and then tested in a thermomechanical analyzer (TMA) within a temperature range of 900&ndash, 950 &deg, C. Grain size and phase fraction of all bond coats were investigated by EBSD before testing and no difference in microstructure was revealed due to annealing in various atmospheres. The influence of annealing in different atmospheres on the creep strength was not very pronounced for the Co-based bond coats Amdry 9954 and Amdry 9954 + ODS in the tested conditions. The ODS bond coats revealed significantly higher creep strength but a lower strain to failure than the ODS-free Amdry 9954. The Ni-based bond coat Amdry 386 showed higher creep strength than Amdry 9954 due to the higher fraction of the &beta, NiAl phase. Additionally, its creep properties at 900 &deg, C were much more affected by annealing in different atmospheres. The bond coat Amdry 386 annealed in an Ar&ndash, H2O atmosphere showed a significantly lower creep rate than the bond coat annealed in Ar&ndash, O2 and Ar&ndash, H2O atmospheres.