E. Alabort

In-situ high-temperature tensile testing of a polycrystalline nickel-based superalloy

An in-situ testing method for high-temperature time-resolved surface observations inside the scanning electron microscope is described and used to improve the mechanistic understanding of a new polycrystalline nickel-based superalloy. The in-situ observations are able to elucidate surface mechanisms of damage in different deformation regimes. Tensile tests from ambient conditions to 750 °C determine the effect of temperature on the prevalent modes of deformation. At low and intermediate temperatures, the formation of discrete slip bands and the cracking of the harder precipitates (carbides) are observed directly. Under this regime, both inter- and trans-granular failure are observed. At the highest temperatures tested, the mechanism of damage is different; the grain boundaries, their orientation and their precipitates play a crucial role. Void growth is directly observed as a consequence of small hard-particle decohesion from the matrix, within those grain boundaries of relative orientation which promotes the shear deformation. As deformation increases, voids grow and coalesce, forming grain boundary cracks thus leading to catastrophic intergranular failure: this manifests itself as low-ductility particularly at the high-temperature conditions.

In Situ Diagnostics of Damage Accumulation in Ni-Based Superalloys Using High-Temperature Computed Tomography

The design, operation, and performance of a laboratory-scale X-ray computed tomography arrangement that is capable of elevated-temperature deformation studies of superalloys to 800xa0°C and possibly beyond are reported. The system is optimized for acquisition of three-dimensional (3D) backprojection images recorded sequentially during tensile deformation at strain rates between 10−4 and 10−2xa0s−1, captured inxa0situ. It is used to characterize the evolution of damage—for example, void formation and microcracking—in Nimonicxa080A and Inconelxa0718 superalloys, which are studied as exemplar polycrystalline alloys with lesser and greater ductility, respectively. the results indicate that such damage can be resolved to within 30 to 50xa0μm. Collection of temporally and spatially resolved data for the damage evolution during deformation is proven. Hence, the processes leading to creep fracture initiation and final rupture can be quantified in a novel way.

On the Rapid Assessment of Mechanical Behavior of a Prototype Nickel-Based Superalloy using Small-Scale Testing

An electro-thermal mechanical testing (ETMT) system is used to assess the mechanical behavior of a prototype single-crystal superalloy suitable for industrial gas turbine applications. Miniaturized testpieces of a few mm2 cross section are used, allowing relatively small volumes to be tested. Novel methods involving temperature ramping and stress relaxation are employed, with the quantitative data measured and then compared to conventional methods. Advantages and limitations of the ETMT system are identified; particularly for the rapid assessment of prototype alloys prior to scale-up to pilot-scale quantities, it is concluded that some significant benefits emerge.