Dislocation Boundary Interactions Research Articles

Tuning the microstructure and mechanical properties of magnetron sputtered Cu-Cr thin films: The optimal Cr addition

Grain boundary (GB) engineering via alloying opens a pathway to design the interfaces in alloys for tuning their mechanical properties, thus it is quite critical to determine the optimum addition of alloying element. Here, we prepared immiscible Cu-Cr alloyed thin films by non-equilibrium magnetron sputtering deposition to study Cr alloying effects on the microstructure and mechanical properties of Cu thin films. It is found that Cr doping can notably refine the grains and promote the formation of fully nanotwinned columnar grains at low Cr volume concentrations (≤∼3.7 at.%) associated with the average GB Cr concentrations ≤ ∼15 at.%, beyond which the formation of nanotwins is significantly suppressed. The role of Cr atoms/clusters played in the twinning process is rationalized in terms of dislocation rebound-promotion mechanism. Importantly, a maximum hardness is discovered at the optimum Cr addition of ∼2.0 at.%. The Cr concentration-dependent hardness of Cu-Cr alloyed films was quantified by a combined strengthening model. The achievement of the maximum hardness was related to the greatest GB solute segregation-induced strength contribution. It is further uncovered that the Cu-Cr system exhibits strain rate sensitivity (SRS, m) monotonically reduced with increasing the Cr concentration, ranging from a positive m of 0.0314 (for pure Cu) to a negative m of −0.0245, which is attributed to a competition between the dislocation-boundary interactions (increasing m) and the dislocation-solute atoms interactions (decreasing m). These findings provide valuable insights into tuning the composition of alloyed thin films to achieve optimized mechanical performance.

Subgrains, micro-twins and dislocations characterization in monolike Si using TEM and in-situ TEM

A critical aspect in the development of mono-like Silicon for photovoltaic (PV) application lies in the control of its defect microstructure. While micro-twins do not seem to represent a major concern regarding electrical activity, some dislocations and low angle grain boundaries need to be avoided in the growing process because of their recombination properties. Here, we have studied how these defects grow and interact together in order to understand what are the mechanisms responsible for their presence in the cast ingots. We show that, in our specific convex liquid-solid interface growth mode, micro-twins probably originate from this interface while dislocations and subgrains build up both by epitaxy at the solid liquid interface and through dislocation-boundary interactions in the solidified region. Post-growth TEM characterization allowed identifying constitutive dislocations of some subgrain boundaries for the case of [001] seeds. In-situ TEM straining of Si at high temperature also allowed measuring dislocation mobility at several temperatures. Those are compared to the dislocation velocities in pure Czochralski Si found in literature.

Local Stress States and Microstructural Damage Response Associated with Deformation Twins in Hexagonal Close Packed Metals

The current work implements a correlative microscopy method utilizing electron back scatter diffraction, focused ion beam and digital image correlation to accurately determine spatially resolved stress profiles in the vicinity of grain/twin boundaries and tensile deformation twin tips in commercially pure titanium. Measured local stress gradients were in good agreement with local misorientation values. The role of dislocation-boundary interactions on the buildup of local stress gradients is elucidated. Stress gradients across the twin-parent interface were compressive in nature with a maximum stress magnitude at the twin boundary. Stress profiles near certain grain boundaries initially display a local stress minimum, followed by a typically observed “one over square root of distance” variation, as was first postulated by Eshelby, Frank and Nabarro. The observed trends allude to local stress relaxation mechanisms very close to the grain boundaries. Stress states in front of twin tips showed tensile stress gradients, whereas the stress state inside the twin underwent a sign reversal. The findings highlight the important role of deformation twins and their corresponding interaction with grain boundaries on damage nucleation in metals.

Structure of A-C Type Intervariant Interface in Nonmodulated Martensite in a Ni-Mn-Ga Alloy.

The structure of A-C type intervariant interface in nonmodulated martensite in the Ni54Mn25Ga21 alloy was studied using high resolution transmission electron microscopy. The A-C interface is between the martensitic variants A and C, each of which has a nanoscale substructure of twin-related lamellae. According to their different thicknesses, the nanoscale lamellae in each variant can be classified into major and minor lamellae. It is the boundaries between these lamellae in different variants that constitute the A-C interface, which is thus composed of major-major, minor-minor, and major-minor lamellar boundaries. The volume fraction of the minor lamellae, λ, plays an important role in the structure of A-C interfaces. For major-major and minor-minor lamellar boundaries, they are symmetrical or asymmetrical tilt boundaries; for major-minor boundary, as λ increases, it changes from a symmetrical tilt boundary to two asymmetrical microfacets. Moreover, both lattice and misfit dislocations were observed in the A-C interfaces. On the basis of experimental observations and dislocation theory, we explain how different morphologies of the A-C interface are formed and describe the formation process of the A-C interfaces from λ ≈ 0 to λ ≈ 0.5 in terms of dislocation-boundary interaction, and we infer that low density of interfacial dislocations would lead to high mobility of the A-C interface.

The effects of reducing specimen thickness on mechanical behavior of cryo-rolled ultrafine-grained copper

Tensile tests on ultrafine-grained (UFG) Cu prepared by equal channel angular pressing (ECAP) and cryo-rolling at liquid nitrogen temperature were performed at various strain rates in order to investigate size effect on their tensile plastic deformation behaviors. It was found that the tensile strength decreases with reducing specimen thickness, which is called as the first-order size effect due to the increased surface softening. A surface layer model is developed to evaluate the relationship between the degree of surface softening and the number of grains across thickness direction. Compared to coarse-grained Cu subjected to rolling at room temperature, both tensile strength and ductility of the UFG Cu after cryo-rolling are enhanced, which is ascribed to the grain refinement and the increased fraction of high angle grain boundaries. The elevated strain rate sensitivity and decreased activation volume of UFG Cu suggest that its plastic deformation mechanism is dominated by dislocation–boundary interactions.

Twinning/detwinning-mediated grain growth and mechanical properties of free-standing nanotwinned Ni foils: Grain size and strain rate effects

Both twinning and its reversible process, i.e., detwinning, play important roles in crystalline plasticity, especially for nanostructured metals and alloys. Contrary to the popular belief that the nanotwinned (NT) microstructure is quite stable, in this work, we reported the twinning/detwinning-mediated grain growth (GG) and dislocation nucleation-controlled strength softening in the high stacking fault energy (SFE) NT-Ni foils with different grain sizes but nearly identical twin lamellar thickness during tension at room temperature. We further proposed two possible dislocation–boundary interaction mechanisms that cause GG via twinning/detwinning processes, and elucidated strain rate effect on the double-inverse grain size dependence of twinning/detwinning in nanostructured Ni samples. Our findings provide in-depth insights into the manipulation of internal features of metals and alloys with high SFE to achieve optimum robust performance for practical applications and fundamental understanding of the twinning/detwinning behavior of NT-materials to predict their stability and evolution of microstructures.

Differences in deformation behavior of bicrystalline Cu micropillars containing a twin boundary or a large-angle grain boundary

Micrometer-sized compression pillars containing a grain boundary are investigated to better understand under which conditions grain boundaries have a strengthening effect. The compression experiments were performed on focused ion beam fabricated micrometer-sized bicrystalline Cu pillars including either a large-angle grain boundary (LAGB) or a coherent twin boundary (CTB) parallel to the compression axis and additionally on single-crystalline reference samples. Pillars containing a LAGB show increased strength, stronger hardening and smaller load drops compared to single crystals and exhibit a bent boundary and pillar shape. Samples with a CTB show no major difference in stress–strain data compared to the corresponding single-crystalline samples. This is due to the special orientation and symmetry of the twin boundary and is reflected in a characteristic pillar shape after deformation. The experimental findings can be related to the dislocation–boundary interactions at the different grain boundaries and are compared with three-dimensional discrete dislocation dynamics simulations.

The Hall–Petch law investigated by means of in situ straining experiments in lamellar TiAl and deformed Al

Dislocation-boundary interactions are studied in TiAl and Al by means of in situ straining experiments in transmission electron microscopes (TEM). The results in TiAl allow us to measure the strength of domain boundaries against the motion of ordinary dislocations and twins. The results in Al are used to analyze the interaction between dislocations and cell walls. Parameters involved in the Hall-Petch law are then deduced from these observations.

The role of harmonic functions in dislocation–boundary interactions by the method of images

We present a theoretical basis for the link between the harmonic functions and the method of images, which was effectively used in Head's analysis [A.K. Head, Proc. Phys. Soc. London Ser. B 66 (1953) 793–801] of dislocation–boundary interaction. We also report the solution of the displacement field of an edge dislocation near a plane surface, and illustrate the profiles of its boundary displacements.

Boundary element method with internal stresses for the investigation of dislocation-boundary interactions

In the theory of dislocations, most formulas and solutions are for an infinite body. When the body containing dislocations is finite and the boundary effect is significant, the mathematical treatment becomes much more involved and, for most cases, excludes the possibility of closed form solutions. A general numerical method, boundary element method with internal stresses (BEMIS), is developed for investigating dislocation-boundary interactions. In this method, the internal stresses caused by the dislocations are obtained from the infinite body solution, while the boundary element method is used to satisfy the boundary conditions. With BEMIS, the shape of the dislocations, the geometry of the finite body, and the loading conditions may be general, while the boundary may be a free surface or a bi-material interface. The accuracy and versatility of the method are examined through comparison with known closed form solutions.

Deformation processes at crack tips in NiAl single- and bicrystals

Pre-cracked single- and bicrystals were examined in order to understand the quasi brittle behaviour of NiAl single crystals and the strength of grain boundaries. Single crystals were tested in-situ within a modified scanning force microscope (SFM). Sequences of elastic deformation, dislocation emission, followed by discontinuous crack growth on a submicron scale were observed within the SFM. In addition the fracture toughness of bicrystals with stoichiometric and off-stoichiometric composition pre-cracked along and perpendicular to the boundary was measured. The results proved that NiAl is inherently ductile. However, depending on purity, discontinuous crack growth occurred with local crack opening displacements (COD) of less than 100 nm and slip band lengths of less than 10 μm, showing clearly that local dislocation pinning can initiate brittle crack growth after blunting. In all cases the toughness of grain boundaries was more than a factor of two lower than the toughness of the adjacent component crystals. This proves that grain boundary brittleness is an inherent property of pure NiAl. However, boundaries with a surplus in Ni were tougher. For cracks precracked perpendicular to the boundaries dislocation-boundary interaction can either increase or decrease the toughness depending on the orientation of the adjacent grains.

Dislocation boundary interactions in a Σ13 grain boundary in silicon. II. Grain boundary structures and dislocations produced during deformation

Abstract A Σ13, 22·6°/[001]½, grain boundary in Si with plastically deformed at 850°C under a range of loading orientations and strains. The resulting deformation was characterized in the transmission electron microscope, including the determination of the operating slip systems, and the Burgers vectors of the various grain boundary dislocations produced. For small strains, the observed grain boundary dislocations are consistent with a dissociation reaction in the boundary of the incoming crystal dislocations to produce two ¼⟨111⟩ partial grain boundary dislocations. This provides a good compromise between the overall reduction in elastic strain energy and the increase in step height area in the boundary. As deformation proceeds, the different sets of grain boundary dislocations interact and further dissociate, so that a new network of grain boundary dislocations is produced. Finally, partial dislocations having a ⅛ component along the [001]½ disorientation axis are also observed.