Deformation response of ferrite and martensite in a commercially produced dual-phase sheet steel with a nominal composition of 0.15% C–1.45% Mn–0.30% Si (wt.%) was characterized by nanoindentation ...and uniaxial compression of focused ion beam-milled cylindrical micropillars (1–2μm diameter). These experiments were conducted on as-received and pre-strained specimens. The average nanoindentation hardness of ferrite was found to increase from ∼2GPa in the as-received condition to ∼3.5GPa in the specimen that had been pre-strained to 7% plastic tensile strain. Hardness of ferrite in the as-received condition was inhomogeneous: ferrite adjacent to ferrite/martensite interface was ∼20% harder than that in the interior, a feature also captured by micropillar compression experiments. Hardness variation in ferrite was reversed in samples pre-strained to 7% strain. Martensite in the as-received condition and after 5% pre-strain exhibited large scatter in nanoindentation hardness; however, micropillar compression results on the as-received and previously deformed steel specimens demonstrated that the martensite phase in this steel was amenable to plastic deformation and rapid work hardening in the early stages of deformation. The observed microscopic deformation characteristics of the constituent phases are used to explain the macroscopic tensile deformation response of the dual-phase steel.
The intermetallic SmCo5 phase is well known for its unique magnetic properties, but so far, research on the mechanical properties and the underlying lattice defects is comparably scarce. This study ...aims to investigate the deformation mechanisms of the SmCo5 phase through nanoindentation and micropillar compression on two single crystals with different orientations. We find two active slip systems in compression, the pyramidal slip system {2 1¯1¯1¯} 〈2¯ 1 1 6¯〉 and the basal slip system (0 0 0 1) 2 1¯1¯ 0. Additionally, basal and pyramidal stacking faults and partial dislocations were observed by transmission electron microscopy. Using ab initio and atomistic modelling we investigated the underlying defect structures and energy barriers. These allowed us to rationalize the observed slip systems and suggest that the pyramidal stacking faults may form by a synchro-shear slip mechanism. Our findings also provide a basis to understand the deformation behaviour of structurally related phases in the Sm-Co system, such as Sm2Co7, which comprises SmCo5 and Laves SmCo2 building blocks.
Display omitted
Helium irradiation blistering in tungsten is of intense interest for plasma-facing materials under fusion-relevant conditions. Previous studies have revealed that the surface blistering is closely ...related to the evolution of subsurface microstructures (e.g., bubbles and cracks), but its dependence on the surface orientation and deformation remains unclear. The present work reports the helium irradiation blistering on the surface planes of tungsten {100}, {110} and {111}. A systematic study of helium irradiation blistering was conducted at room temperature by utilizing a Helium Ion Microscope. The cross-sectional microstructural analyses at different ion fluences confirmed that the blistering deformation starts with the nanocrack nucleation, initiated at the peak in the bubble depth distribution via interbubble fracture, and is followed by the gas buildup in a primary cavity. In addition, the comparison of blistering behaviors between 1-µm-diameter and 2-µm-diameter circular areas revealed that only the blisters formed on tungsten {100} can grow to a larger size, whereas those formed on tungsten {110} or {111} tend to become wrinkled or burst when the irradiated area increases. Combined with the micropillar compressions in a Scanning Electron Microscope, it was also found that the surface deformation is orientation-dependent and associated with the number, symmetry and distribution of effective slip systems in body-centered-cubic transition metals. Our findings provide new evidence and insights into the mechanisms of helium irradiation blistering and surface deformation in tungsten.
Display omitted
3D interfaces are a new type of interface containing nanoscale crystallographic, structural, and chemical heterogeneities in all spatial dimensions. Recently, 3D interfaces have been shown to enhance ...strength and deformability simultaneously by frustrating shear instability under layer-normal micropillar compression in Cu/Nb nanolaminates. However, quantification of deformed microstructure and effects of loading orientation were not explored in that work. Here, we address these shortcomings by performing post mortem TEM characterization of micropillars compressed at normal and 45° inclination to layers. We find high strength and deformability in both loading geometries and show that 3D interfaces enhance mechanical behavior under multiple loading orientations. In layer-normal compression, post mortem characterization allows for quantification of key quantities correlating well to the severity of shear localization across nanolaminates with different layer thickness and interface type. In 45° compression, TEM results demonstrated no strong plastic instability. This motivated analytical computation of Schmid factors and simulation of slip system activity via crystal plasticity finite element modeling (CPFE). The CPFE model demonstrates that most slip activity occurs non-parallel to layers, indicating that dislocation-3D interface interactions must mediate the observed mechanical behavior of micropillars. This work lays the foundation for further study of 3D interface-driven deformation physics in nanostructured alloys.
Display omitted
The room temperature deformation behavior of single crystals of WC has been investigated as a function of crystal orientation and specimen size by micropillar compression tests. Plastic flow is ...successfully observed at room temperature by the operation of slip on {011¯0}〈21¯1¯3〉 when the specimen size is reduced to micrometer orders. This slip system is the only operative slip system in WC at room temperature, and thus plastic flow is not observed for crystal orientations close to the c-axis orientation. The bulk critical resolve shear stress (CRSS) is estimated to be 1.2 ± 0.3 GPa from the extrapolation of the size-dependent CRSS. The 1/3〈21¯1¯3〉 dislocation carrying slip on {011¯0} dissociates into two identical collinear partial dislocations separated by a stacking fault with the energy of 211∼264 mJ/m2. The preference of slip along 〈21¯1¯3〉 among possible directions (such as 〈21¯1¯0〉 and 0001) on the {011¯0} prism plane is discussed in terms of dislocation self-energy based on anisotropic elasticity, stacking fault energy, dislocation dissociation and Peierls stress for dislocation motion. The lowest Peierls stress arising from the shorter Burgers vector (1/6〈21¯1¯3〉) of dislocations as a result of collinear dissociation of dislocations with b = 1/3〈21¯1¯3〉 on the {011¯0} slip plane is considered to be the main reason for the preference of the {011¯0}〈21¯1¯3〉 slip system.
Display omitted
The interactions between δ-hydrides and plastic slip in a commercial zirconium alloy, Zircaloy-4, under load were studied using in situ secondary electron microscope (SEM) micropillar compression ...tests of single crystal samples and ex situ digital image correlation (DIC) macroscale tensile tests of polycrystalline samples. The hydrides decorate near basal planes in orientation, and for micropillars orientated for 〈a〉 basal slip localised shear at the hydride–matrix interface is favoured over slip in α-Zr matrix due to a lower shear stress required. In contrast, for pillars oriented for 〈a〉 prismatic slip the shear stress needed to trigger plastic slip within the hydride is slightly higher than the critical resolved shear stress (CRSS) for the 〈a〉 prismatic slip system. In this case, slip in the hydride is likely achieved through 〈110〉-type shear which is parallel to the activated 〈a〉-type shear in the parent matrix. At a longer lengthscale, these results are used to inform polycrystalline samples analysed using high spatial resolution DIC. Here localised interface shear remains to be a significant deformation path which can both cause and be caused by matrix slip on planes closely-oriented to the phase boundaries. Matrix slip on planes nearly perpendicular to the adjacent hydride–matrix interfaces can either result in plastic slip within the hydrides or get arrested at the interfaces, generating local stress concentration. Through these mechanisms, the presence of δ-hydrides leads to enhanced strain localisation in Zircaloy-4 early in the plastic regime.
Display omitted
Although laser powder bed fusion (LPBF) can produce complex structures with promising properties, there still exists a concern on the diversity in materials, especially composites, for LPBF. This ...work innovatively used NiTi and Nb powders to prepare NiTi–Nb eutectic-type alloy by LPBF, which provides a new method to prepare NiTi–Nb shape memory alloy and could be an alternative for tailoring microstructure. The semi-molten Nb particle phase was retained during LPBF, so that β-Nb and the eutectic structure display a gradient distribution in the Nb diffusion concentration. The heat treatment process (annealed at 850 °C for 0.5 h) makes NiTi–Nb samples have high yield strength (~1640 MPa), high compressive strength (~2380 MPa), and high compressive strain (~39%), which are superior to the corresponding ones of its as-built and as-cast counterparts. Large Nb phase particles with a size of 10–30 μm are retained and accelerated the formation of eutectic phase and β-Nb precipitate phase. Overall, the presence of β-Nb phase has a beneficial effect on the alloy, and a large number of intertwined dislocations and stacking faults around the β-Nb phase promote the formation of martensite under stress loading. The eutectic phase makes an important contribution to the high strength and plasticity of the NiTi–Nb alloy.
Display omitted
Fe–Cr is a model alloy for ferritic steels used in thermal power generation systems and envisaged as primary structural material for future fusion reactors. However, upon heating and, further, under ...irradiation, Fe–Cr can suffer from phase decomposition leading to the formation of Fe-rich and Cr-rich regions that induce simultaneous hardening and embrittlement. In this study, we examine the origins of the degradation in mechanical properties by performing room-temperature in situ micropillar compression tests on single-crystalline Fe–40wt.%Cr alloys in solid solution, and in the spinodally decomposed state obtained after annealing at 500 °C for 1008 and 2016 h, respectively. The compressed micropillars are subjected to correlative nanoscale structural characterization using transmission electron backscattered diffraction and transmission electron microscopy. Dislocation slip occurs unequivocally on the {110}〈11¯1〉 slip system for all conditions. While the 2016 h annealed state exhibits a more evolved nanoscale phase modulation than the 1008 h annealed condition, both microstructures display approximately double the yield strength of the solid-solution state without any concurrent loss of ductility. Our findings reveal a fundamental change in plasticity mechanism across the three different microstructures. Deformation in the solid-solution state is associated with kink-mediated local plasticity occurring on multiple glide systems, which activate sequentially based upon the largest instantaneous Schmid factor. This gradually transforms into a less localized Lüders-band like plasticity associated with single-slip activation in the 1008 h annealed state, while the deformation in the 2016 h annealed state is marked by uniform strain hardening related to a homogeneous polycrystalline-like dislocation motion occurring simultaneously on multiple slip systems. Correlations between the spatial and compositional fluctuations in Cr and the associated plasticity dynamics are established. It is shown that the spatial fluctuations in Cr strongly influence dislocation strengthening and the relative mobility of the edge and screw components across all microstructural states. It is further concluded that the phase-separation effect, despite promoting strengthening, does not act as the primary cause of embrittlement, but rather plays a contrary role of enhancing ductility.
Display omitted
PDF
