Superalloys 2024: General Session 4: Disk Alloy Mechanical Behavior II
Program Organizers: Jonathan Cormier, ENSMA - Institut Pprime - UPR CNRS 3346
Monday 8:10 PM
September 9, 2024
Room: Exhibit Hall
Location: Seven Springs Mountain Resort
Session Chair: Howard Stone, University of Cambridge; Michael Fahrmann, Haynes International
8:10 PM
High Temperature Crack Growth Characteristics Under Dwell-fatigue in a PM Superalloy for Disc Applications: Hangyue Li1; 1University of Birmingham
Crack growth characteristics under dwell-fatigue loading are investigated in this paper for a new PM nickel superalloy developed by Rolls-Royce plc. In order to achieve a good balance of mechanical properties, it is critical that an appropriate heat treatment be defined for the alloy. Here, the influences of cooling rate after solution heat treatment on γ' size and distribution, grain boundary serration and dwell-fatigue crack growth resistance are first investigated. A medium cooling rate has been found to produce good dwell-fatigue crack growth resistance and has been carried forward to a comprehensive matrix of dwell-fatigue growth testing in air with varied temperatures (700 and 760 C), initial K values, and testing procedures (constant amplitude loading and load shedding). Tests have also been interrupted to allow detailed examination of crack tips using scanning electron microscopy. Notably the dwell time is 120 s (positioned at maximum load), and the applied stress ratio is fixed at 0.1. Significant variations in behaviour are observed at both temperatures. It is concluded that the variation in crack growth rates results from the selection and interactions between two different time-dependent mechanisms: environmentally assisted oxide forming and cracking, and environmental independent creep deformation and creep crack growth. When the environmentally related mechanism operates alone, the fastest crack growth rates are obtained. This study demonstrates that the environmentally assisted, sustained fast crack growth can be inhibited with a combination of an appropriate microstructure and a defined range of mechanical driving force.
8:35 PM
Strain Rate Effect on Strain Localization in Alloy 718 Ni-based Superalloy at Intermediate Temperature: Malo Jullien1; Rephayah Black2; Marc Legros3; Jean-Charles Stinville2; Damien Texier1; 1Institut Clement Ader; 2University of Ilinois Urbana-Champaign; 3CEMES
Tensile tests on Alloy 718 Ni-based superalloy at 650 °C at different strain rates revealed a strain-rate dependency on the fracture mode. A change from intergranular to transgranular fracture was observed in air as the strain rate increased, mainly when Portevin-Le-Chatelier (PLC) mesoscopic deformation bands were present. To better understand the link between strain rate and fracture mode, a description of the strain localization in the early deformation stage is needed. In this study, high-resolution digital image correlation (HR-DIC) was carried out at the onset of strain localization at low strain rate (LSR, ˙ε = 10−4 s−1) and at high strain rate (HSR, ˙ε = 10−2 s−1), this latter condition aimed at investigating the microplasticity development within PLC bands. The in-plane and out-of-plane displacement components of each single plastic event were measured to accurately assess and distinguish morphological sliding at grain boundaries (i.e., grain boundary sliding) and dislocation slip. The deformation within the PLC bands was examined at macro, meso, and microscales. Statistical analyses highlighted the distribution and partitioning of these strain localization events related to different microstructural features, including grains, and grain and twin boundaries. Grain boundary sliding was found to be more prominent at LSR. Interestingly, events near and parallel to twin boundaries are particularly intense regardless of the strain rate. At HSR, grain boundary sliding is less pronounced, and a high density of intragranular slip bands developed within the PLC bands based on observations before and after the occurrence of the PLC band.
9:00 PM
Using “Microstructure Informatics” to Understand Abnormal Grain Growth Factors in Powder Metallurgy Ni-based Superalloys: Luis Arciniaga1; Pascal Thome1; Kevin Severs2; Sammy Tin1; 1University of Arizona; 2ATI Forged Products
Advanced electron back scatter diffraction (EBSD) and electron dispersive spectroscopy (EDS) techniques were used to systematically quantify meso-scale microstructural descriptors in an advanced powder processed polycrystalline Ni-base superalloy containing elevated levels of refractory alloying additions. The microstructural changes of the alloy as a function of effective strain were tracked and related to the subsequent heat-treated microstructures. This emerging field of “microstructure informatics” extends beyond the conventionally used metrics of grain and precipitate sizes and distributions. Due to the multidimensional nature of the data, manual microstructure characterization becomes virtually impossible, especially when a multitude of different material states must be considered. This motivated the development of an automated microstructure characterization procedure, which extracts useful geometric, crystallographic, and chemical microstructure features through a batch process. These features provide a level of microstructure detail that has not traditionally been demonstrated at a statistically significant scale capable of effectively capturing the level of intrinsic heterogeneity that is present in polycrystalline Ni-base superalloys. In this study, microstructural descriptors from the deformed material were evaluated and used to understand the grain growth response during super-solvus heat treatment. Compared to traditional qualitative and semi-quantitative approaches for characterizing microstructures, the innovative methodology used in this investigation provide insightful, quantitative microstructure metrics that lead to the generation of new knowledge and scientific understanding.