The Microstructure and Creep Behavior of Cold Rolled Udimet 188 Sheet

Abstract

Udimet 188 was subjected to thermomechanical processing (TMP) in an attempt to understand the effects of cold-rolling deformation on the microstructure and tensile-creep behavior. Commercially available sheet was cold rolled to varying amounts of deformation (between 5-35% reduction in sheet thickness) followed by a solution treatment at 1,464 K (1,191 °C) for 1 h and subsequent air cooling. This sequence was repeated four times to induce a high-volume fraction of low-energy grain boundaries. The resultant microstructure was characterized using electron backscattered diffraction. The effect of the TMP treatment on the high-temperature [1,033-1,088 K (760-815 °C)] creep behavior was evaluated. The measured creep stress exponents (6.0-6.8) suggested that dislocation creep was dominant at 1,033 K (760 °C) for stresses ranging between 100-220 MPa. For stresses ranging between 25-100 MPa at 1,033 K (760 °C), the stress exponents (2.3-2.8) suggested grain boundary sliding was dominant. A significant amount of grain boundary cracking was observed both on the surface and subsurface of deformed samples. To assess the mechanisms of crack nucleation, in situ scanning electron microscopy was performed during the elevated-temperature tensile-creep deformation. Cracking occurred preferentially along general high-angle grain boundaries (GHAB) and less than 25% of the cracks were found on low-angle grain boundaries (LAB) and coincident site lattice boundaries (CSLB). Creep rupture experiments were performed at T = 1,088 K (815 °C) and σ = 165 MPa and the greatest average time-to-rupture was exhibited by the TMP sheet with the greatest fraction of LAB+CSLB. However, a clear correlation was not exhibited between the grain boundary character distribution and the minimum creep rates. The findings of this work suggest that although grain boundary engineering may be possible for this alloy, simply relating the fraction of grain boundary types to the creep resistance is not sufficient.

Identifying the microstructural features associated with void nucleation during elevated‐temperature deformation of copper

“…In this study, each void that nucleated at a twin/HAGB triple junction subsequently grew along the HAGB. Prior investigations, including the in situ studies of Boehlert et al, assessed the relative fraction of HAGBs and twins with voids but did not examine where voids originated 37,48 . This study thus demonstrates the importance of detailed in situ observations to determine where voids nucleate during mechanical loading at elevated temperatures.…”

“…Because cavities form throughout the thickness of uniformly loaded specimens, surface‐based microscopy techniques can characterize the microstructural features associated with incipient voids in situ 47 . Boehlert et al were the first to take advantage of this by using in situ EBSD to characterize the formation of the cavities formed during high‐temperature deformation of nickel‐based alloys 37,48 . Grain‐boundary voids were observed primarily along HAGBs while twins were relatively void‐free.…”

“…47 Boehlert et al were the first to take advantage of this by using in situ EBSD to characterize the formation of the cavities formed during high-temperature deformation of nickelbased alloys. 37,48 Grain-boundary voids were observed primarily along HAGBs while twins were relatively voidfree. Because these and other 49 studies focused on void growth rather than the features within grains associated with voids, for example, subgrain boundaries, they provide little insight into the mechanisms behind this observation.…”

Isolation, characterization and analysis of bacteriophages from the haloalkaline lake Elmenteita, Kenya

“…Previous studies on the Soda lakes include isolation of phages from Lake Magadi by Jamison et al [35] and Muruga et al [36]. Moulton et al [37] also isolated and studied a phage infecting an alkaliphilic Vibrio metschnikovii from Lake Magadi. Peduzzi et al [38] carried out an electron microscopic study of cyanophages that affect African flamingo population in Lake Nakuru.…”