In-Situ Neutron Diffraction Study of the Bauschinger Effect in B2 Structured CoZr

Abstract

A combination of in-situ neutron diffraction and elastoplastic self-consistent (EPSC) modeling have been used to elucidate the role played by intergranular stresses in the Bauschinger effect in B2 structured CoZr at room temperature and 423 K (150°C). It is shown that, when insufficient slip modes are present to accommodate arbitrary strains, the large intergranular stresses built up due to inhomogeneous plastic deformation are responsible for the observed Bauschinger effect. Upon the onset of secondary deformation mechanism(s), the stresses are more uniformly distributed among the grains and the influence of intergranular stresses on the Bauschinger effect diminishes. On the other hand, it is speculated that the contribution of intragranular (dislocation-based) stresses is responsible for the persistent Bauschinger effect past the transition point. Similar results are obtained at both room temperature and 423 K (150°C), and while the yield strength decreases with temperature, the high-temperature stress-strain curve progressively becomes harder than the room temperature one. In light of this, the previously characterized yield strength anomaly in CoZr has been re-examined.

High-Resolution Single-Grain Diffraction of Polycrystalline Materials

“…The procedure is illustrated by an example: Single crystals of CoTi and CoZr intermetallics of the B2 type, CsCl structure, can only deform plastically if they are oriented for a particular slip-mode such that <001>{110} slip systems become activated [28,29,30]. When deformed in other orientations they respond with early brittle fracture [31,28].…”

“…This would not be sufficient for the generalized criterion of the number of slip systems required to accommodate arbitrary strains in polycrystalline specimens [32,33]. Despite the limited number of slip systems in the soft slip-mode, the polycrystalline specimens of these alloys exhibit significant ductility [29,30]. Lattice strain measurements by in-situ neutron diffraction experiments supported by polycrystal plasticity modeling have shown that during deformation of polycrystalline CoTi and CoZr the slip systems of the soft mode are accompanied by the activation of <110>{110} and/or <111>{111} slip systems [29].…”

Three dimensional line profile analysis

“…As a first approach, in case of tensile tested CoTi and CoZr B2 structure ordered alloys presented in chapter 4, possible dislocation types were grouped into smaller groups based on the physical criteria of plastic deformation of the B2 alloy system and earlier TEM observations [Wollmershauser et al 2010;Mulay et al 2011;Mulay & Agnew 2012]. Care was taken to deliberately mix different Burgers vector types in each group.…”

Thermomechanical Response of Polycrystalline alpha-Uranium

“…One set of experiments was conducted with in-situ lattice strain monitoring, and the other set was conducted without the neutron di↵raction monitoring, referred to here as ex-situ. The ex-situ experiments were conducted to failure and were used to illustrate the changes in ductility between the charging levels, while the in-situ experiments will reveal any changes in the onset of the di↵erent deformation The change in lattice spacing during deformation for di↵erent hkl di↵raction peaks provides insight into the plastic deformation mechanisms active, and when used in conjunction with micromechanical modeling, can be used to quantify the relative strengths, as seen in work by the current authors on uranium [30,31], beryllium [66], zirconium [67], stainless steels, intermetallics [68], and magnesium [48]. Here, the measurements are used to show any changes in deformation mechanisms between the di↵erent levels of hydrogen charging.…”