In this study, compressive dwell (C-D) and no-dwell (N-D) lowcycle fatigue (LCF) behaviours of several single crystal Ni-base superalloys, including CMSX-4, LSC-11 and LSC-15, were studied under strain-controlled zero-compression (R  = -) loading at 1100°C. LSC-11 and LSC-15 are new alloys developed by IHI Corporation, Japan with 0.8 wt% Re and without Re addition, respectively, as reduced-cost alternatives to the second generation single crystal Ni-base superalloys. The fatigue experiments were conducted with or without a two-minute dwell (hold) in compression and total strain ranges of 0.7%, 0.6% and 0.5% on uncoated specimens in the [001] orientation. Examination of the cyclic stress-strain behavior revealed that the initially compressive mean stress relaxed to approximately zero stress in N-D tests, while compressive hold resulted in the development of a tensile mean stress during C-D fatigue. Cyclic stress softening was observed under all test conditions. Microstructural analysis of tested specimens showed that N-D fatigue promoted isotropic coarsening of the ' precipitates, while C-D loading resulted in the formation of discontinuous ' rafting parallel to the loading direction. Fatigue cracks initiated from the specimen surface from regions of localized oxide attack. All alloys were compressive dwell sensitive. C-D fatigue lives were 415× shorter than N-D when the same alloys were considered. CMSX-4 exhibited 1.5-3× N-D fatigue life advantage over alloys LSC-11 and LSC-15. Under C-D fatigue the life advantage of CMSX-4 was 2050% greater than alloys LSC-11 and LSC-15. The differences in these behaviours could be attributed to Re content and oxidation.

No abstract

Mechanical metamaterials have attracted great interest due their ability to attain material properties outside the bounds of those found in natural materials. Many promising mechanical metamaterials have been designed, fabricated, and tested, however, these metamaterials have not been subjected to the rigorous requirements needed to certify their use in demanding industrial applications that require multifunctional behavior. This paper details an auxetic multifunctional metamaterial that has been optimized to outperform conventional designs for cooling systems commonly used in the space, the transportation, the energy and the nuclear industry. Experimental tests performed to certify this material for use in gas turbines have shown that in comparison to conventional designs, the metamaterial increases structural life by orders of magnitude while also providing more efficient cooling and maintaining similar acoustic damping characteristics. This metamaterial offers an agile and economical solution for the realization of next generation components.

In the braze repair of gas turbine components, substituting the parent metal with dissimilar metals can improve the mechanical properties of the refurbished parts. In this study, the microstructure and high temperature tensile properties of brazed joints made of X-40 (the parent metal) and IN738 (the substitute metal), were evaluated in the narrow gap as brazed and heat treated conditions. The resulting joint contained primary γ-Ni phase, discrete refractory element rich carbides/carboborides and eutectic phases. The carbides and eutectic phases exhibited Vickers hardness values several times higher than those of IN738 and γ-Ni phase particles. The high temperature yield strength of the as brazed joints was also greater than that of the X-40 base metal, but the ductility was significantly lower. The fracture surfaces revealed that cracking occurred in both intergranular (among the γ-Ni grains) and transgranular modes. Crack initiation was found to be associated with discrete carbides/carboborides and eutectic phases. It was observed that post-braze heat treatment at 950°C for 120 h improved the high temperature tensile ductility due to a reduction of carbides and eutectic phases in the brazed joint. Increasing the heat treatment time reduced the size and the amount of carbides and eutectic phases and contributed to an increase in transgranular fracture. However, extending the heat treatment time up to 840 h reduced the ductility due to oxidation damage in the braze joint.

A framework of integrated creep-fatigue (ICF) modeling is proposed based on the deformation decomposition rule that the total inelastic strain (in a polycrystalline material) consists of intragranular deformation (ID) and grain boundary sliding (GBS). With consideration of the respective deformation mechanisms, the resulting constitutive laws are given in 3D tensor forms such that fatigue damage (ID) and creep damage (GBS) are represented in different strain spaces, respectively. Then, the creep-fatigue life consumption can be evaluated using a physics-based formula that captures the intricate interaction between a propagating fatigue crack and distributed creep damage, leading to final fracture.