Bicrystal growth and characterization of copper twist grain boundaries

Abstract Copper bicrystals with twist character were grown using the vertical Bridgman technique. Cu bicrystals were grown such that the grain boundary in each sample had a nominal twist misorientation consisting of either a low angle (108), a special angle (S5=36.878), or a high angle (458). The grain boundary plane in all cases was (1 0 0). The grain boundaries were grown using single-crystal seeds that were oriented to within AE 0.58 using the Laue back-reflection X-ray diffraction method. The misorientation of each twist boundary was characterized using electron backscattering diffraction patterns in a scanning electron microscope. All grain boundary misorientations were determined to be within the limits defined by the Brandon criterion. # 2001 Published by Elsevier Science B.V

Growth of oriented C11b MoSi2 bicrystals using a modified Czochralski technique

Oriented bicrystals of pure C11b MoSi2 have been grown in a tri-arc furnace using the Czochralski technique. Two single crystal seeds were used to initiate the growth. Each seed had the orientation intended for one of the grains of the bicrystals, which resulted in a 60° twist boundary on the (110) plane. Seeds were attached to a water-cooled seed rod, which was pulled at 120 mm/h with the seed rod rotating at 45 rpm. The water- cooled copper hearth was counter-rotated at 160 rpm. Asymmetric growth ridges associated with each seed crystal were observed during growth and confirmed the existence of a bicrystal. It was also found that careful alignment of the seeds was needed to keep the grain boundary from growing out of the boule. The resulting boundary was characterized by imaging and crystallographic techniques in a scanning electron microscope. The boundary was found to be fairly sharp and the misorientation between the grains remained within 2° from the disorientation between the seeds

Investigation of Grain Boundary Segregation and Embrittlement Mechanisms of the Cu-Bi System by Analytical Electron Microscopy

Grain boundary (GB) segregation and embrittlement of copper (Cu) by small amounts of bismuth (Bi) has been investigated on 6°, 13°, and 33° Cu twist bicrystals. The results from micro-mechanical double edge notched testing showed no embrittlement effects in the 6° GB. The 33° GB has been shown to be significantly embrittled by the introduction of Bi. Single edge notch testing of the 13° GB also showed a reduction in fracture toughness. These mechanical results have been interpreted through the use of analytical electron microscopy (AEM) studying the GB geometry, the atomic structure, the electronic structure, and the chemical compositions of the GBs. The 6° and 33° GBs were found to be close to pure twist boundaries but with more accurate twist angles of 4.3° and 38.0°, respectively. The electronic structure of the GBs was not found to be a good indication of the presence of Bi, which was confirmed on the 13° and 33° GBs. The Bi GB coverage was confirmed via quantitative XEDS on the 33° GB to correspond to 0.12 ± 0.03 monolayers of Bi and through 3-dimensional scanning transmission electron microscope (STEM) through focus imaging to be 0.02 – 0.09 monolayers of Bi. The presence of edge dislocations along the 33° GB was confirmed with Bi segregating to edge dislocation cores. The Bi atoms on the dislocation cores embrittle the GB by increasing the energy required to move a dislocation in response to an applied stress resulting in reduced plasticity at the crack tip which promotes GB cleavage

The Role of Grain Boundaries in the Tensile Deformation Behavior of CoCrFeMnNi High Entropy Alloys

High Entropy alloys (HEAs) are metal alloys consisting of multiple base metals in equimolar or near equimolar concentrations. HEAs exhibit unique combinations of properties that render them an attractive choice in many engineering applications. Among HEAs, a single phase face centered cubic (FCC) CoCrFeMnNi alloy, known as the Cantor alloy, shows simultaneous strength and ductility specifically at cryogenic temperatures. This has been attributed to the activation of deformation twinning as an additional mode of plastic deformation. Experimentally it has been observed that grain boundaries (GBs) facilitate the nucleation of deformation twins in HEAs. However, the role of GB geometry in the deformation behavior of HEAs remains unexplored. In this thesis work, we leverage atomistic simulations to systematically investigate the role of GB geometry in the deformation behavior of the Cantor alloy at 77 K. To this end, a series of Cantor alloy bicrystals with \u3c110\u3e and \u3c111\u3e symmetric twist GBs are constructed and used in tensile deformation simulations. Simulation results reveal that plastic deformation proceeds by the nucleation of partial dislocations from GBs, which then grow with further loading by bowing into the bulk crystals leaving behind stacking faults. Variations in the nucleation stress exist as function of GB character, defined in this work by the twist angle. Our results provide future avenues to explore GBs as a microstructure design tool to develop HEAs with tailored mechanical properties

Small-Scale Fracture Toughness Studies of Grain Boundary Embrittlement in Cu-Bi Alloys

Grain boundary embrittlement in the Cu-Bi alloy system was investigated using small-scale fracture toughness tests that were based on commonly used bulk-scale tests. Tests were conducted on pure and Bi-doped \u3c001\u3e twist Cu bicrystals with misorientation angles of 6, 13, and 33 ̊ in order to determine the effect of misorientation angle on the degree of embrittlement. The results of these tests showed that the 6 ̊ grain boundary was nearly immune to embrittlement, showing little to no differences in fracture toughness values and failure mechanisms between the pure and doped specimens. However, the 33 ̊ boundary exhibited a significant amount of embrittlement, with a nearly 40% decrease in fracture toughness in the doped specimens compared to the pure and a distinct shift in the failure mechanism from transgranular shear to intergranular fracture. The 13 ̊ boundary exhibited an intermediate amount of embrittlement with a measurable drop in toughness, but not a clear shift in the failure mechanism. These results are consistent with previously published results from tests on bulk-scale bicrystals.Furthermore, a single-crystal plasticity model was incorporated into a commercial finite element software package (ABAQUS) in order to investigate the development of the plastic zone in front of the notches created in the test specimen. It was found that the size of this zone was likely constrained by the specimen dimensions, which had a significant impact on the measured fracture toughness values