Synergy of grain boundary sliding and shear-coupled migration process in nanocrystalline materials

An attempt has been made to illustrate a new physical mode of plastic deformation in nanocrystalline materials that synergizes two processes, i.e. grain boundary (GB) sliding and stress-driven shear-coupled GB migration. The latter process incorporates a rotational and a translational plastic flow, in which a normal migration is coupled with a shear. The energy change resulting from the above-mentioned mode was calculated, and showed that the new deformation mode was much more energetically favorable than both the pure GB sliding mode and the cooperative process of GB sliding and migration without a coupling shear. In addition, the new deformation mode considerably enhances the ductility of nanocrystalline materials compared with the other two above-mentioned processes.     

Anatomy of nanomaterial deformation: Grain boundary sliding,plasticity and cavitation in nanocrystalline Ni

The deformation of nanocrystalline metals is a complex process that involves a cascade of plastic events, including dislocation motion, grain boundary activity and cavitation. These mechanisms act simultaneously and synergistically during fracture, masking their individual roles and often resulting in a wide range of failure modes in the same material. Using large-scale molecular dynamics simulations, we dissect the size-dependent deformation of nanocrystalline Ni nanowires for a range of diameters spanning a few nanometers to the bulk. By analyzing the localization of von Mises shear strain and stress triaxiality, we identify the key nanostructural features, the role of each elementary process and the dominant deformation mechanism as a function of sample diameter. Our atomic level analysis not only provides a fundamental understanding of the deformation of nanocrystalline Ni, but also demonstrates that large-scale simulations can be an essential complement for modern in situ electron microscopy/atom-probe tomography.     

Tensile deformation of an ultrafine-grained aluminium alloy: Micro shear banding and grain boundary sliding

This work investigates the relationship between the strain rate and the ductility and the underlying deformation mechanisms in an ultrafine-grained Al6082 alloy. At room temperature the uniform elongation of the material exhibits a marked increase with decreasing strain rate. This effect is related to the activation of micro shear banding, which is controlled by grain boundary sliding. The contribution of these mechanisms to uniform elongation is estimated. It is proposed that the grain boundary sliding suppresses the transformation of micro shear bands into macro shear bands. The activity of other deformation mechanisms during plastic deformation of the ultrafine-grained material is also discussed.     

Inter-granular liquid phase aiding grain boundary sliding in superplastic deformation of fine-grained ZK60 Mg alloy

For 7475 Al alloy,there were micrographs showing filaments or whiskers formation during the separation stage of superplastic elongation.This indicates the presence of liquid phase which accommodates grain boundary sliding to reach superplasticity.On the other hand,there is no such phenomenon reported regarding Mg alloy in literatures.Scanning electron microscopic(SEM)fractography exceptionally exhibits a mark of grain boundary sliding and its accommodating mechanism of inter-granular liquid phase.Under the testing conditions of 350℃ and 1×10- 4s -1,the initially fine-grained structure(3.7μm)yields 642%superplastic elongation and exhibits fluffy appearance on the fractured surface.For other specimens showing less superplasticity,their fractured surfaces exhibit partial fluffy appearance.     

Tailoring and patterning the grain size of nanocrystalline alloys

Nanocrystalline alloys that exhibit grain boundary segregation can access thermodynamically stable or metastable states with the average grain size dictated by the alloying addition. Here we consider nanocrystalline Ni–W alloys and demonstrate that the W content controls the grain size over a very broad range: ∼2–140 nm as compared with ∼2–20 nm in previous work on strongly segregating systems. This trend is attributed to a relatively weak tendency for W segregation to the grain boundaries. Based upon this observation, we introduce a new synthesis technique allowing for precise composition control during the electrodeposition of Ni–W alloys, which, in turn, leads to precise control of the nanocrystalline grain size. This technique offers new possibilities for understanding the structure–property relationships of nanocrystalline solids, such as the breakdown of Hall–Petch strength scaling, and also opens the door to a new class of customizable materials incorporating patterned nanostructures.     

Mechanisms of plastic deformation in microcrystalline and nanocrystalline TiNi-based alloys

Mechanisms of plastic deformation of a high-temperature B2 phase that act upon tension, compression, and high-pressure torsion in TiNi-based single crystals have been studied depending on the crystal orientation. For the crystals with orientations located near the [$ \bar 1 $ \bar 1 11] and [$ \bar 1 $ \bar 1 12] poles in the standard stereographic triangle, multiple dislocation slip prevails upon both compression and tension. In “hard” crystals with the deformation axis close to the [001] direction, in which the Schmid factors for dislocation slip are close to zero, the main deformation mechanisms are the mechanical twinning in the B2 phase and the stress-assisted B2 → B19′ martensitic transformation. All the above listed mechanisms take part in the formation of the {111}〈hkl〉 texture. The mechanism of the change in the orientation of “hard” polycrystalline grains upon the formation of a nanocrystalline and amorphous-crystalline state has been demonstrated on the example of the evolution of the structure of [001] crystals upon severe plastic deformation in a Bridgman cell.     

On possible mechanisms of nanostructure evolution upon severe plastic deformation of metals and alloys

Nondislocation mechanisms of deformation-induced fragmentation of nanostructures in metals upon plastic deformation are discussed. Conditions under which the refinement of nanograins can effectively occur via deformation twinning and/or deformation-induced phase transformations of a martensitic type are considered. It is shown that for each metal system and each deformation method, there exists a limiting nanostructure with a minimum possible average size of nanocrystallites.     

Molecular dynamics simulations of grain boundary sliding: The effect of stress and boundary misorientation

Molecular dynamics simulations were used to study the effect of applied force and grain boundary misorientation on grain boundary sliding in aluminum at 750 K. Two grains were oriented with their 〈1 1 0〉 axes parallel to their boundary plane and one grain was rotated around its 〈1 1 0〉 axis to various misorientation angles. For any given misorientation, increasing the applied force leads to three sliding behaviors: no sliding, constant velocity sliding and a parabolic sliding over time. The last behavior is associated with disordering of atoms along the grain boundary. For the second sliding behavior, the constant sliding velocity varied linearly with the applied stress. A linear fit of this relationship did not intersect the stress axis at the origin, implying that a threshold stress for sliding exists. This threshold stress was found to decrease with increasing grain boundary energy. The ramifications of this finding for modeling grain boundary sliding in polycrystals are discussed.     

Structural refinement and deformation mechanisms in nanostructured metals

Deformation mechanisms in metals deformed to ultrahigh strains are analyzed based on a general pattern of grain subdivision down to structural scales 10 nm. The materials analyzed are medium- to high-stacking fault energy face-centered cubic and body-centered cubic metals with different loading conditions. The analysis points to dislocation glide as the dominant deformation mechanism at different length scales supplemented by a limited amount of twinning at the finest scales. With decreasing deformation temperature and increasing strain rate, the contribution of twinning increases.     

Cavitation and grain boundary sliding during creep of Mg-Y-Nd-Zn-Mn alloy

Creep of squeeze-cast Mg-3Y-2Nd-1Zn-1Mn alloy was investigated at the constant load in the stress range of 30-80 MPa. Tensile creep tests were performed at 300℃up to the final fracture.Several tests at 50 MPa were interrupted after reaching the steady state creep;and another set of creep tests was interrupted after the onset of ternary creep.Fraction of cavitated dendritic boundaries was evaluated using optical microscopy.Measurement of grain boundary sliding by observation of the offset of marker lines ...     

Critique of mechanisms of formation of deformation,annealing and growth twins: Face-centered cubic metals and alloys

The formation of deformation, annealing and growth twins in face-centered cubic materials is discussed. Slip precedes deformation twinning, and twins form from the interaction between primary and secondary slip dislocations having co-planar, but different, Burgers vectors. The influence of several metallurgical variables on twinning can only be rationalized in terms of the model. Annealing twins form due to growth accidents on differently inclined {1 1 1} facets present on a migrating grain boundary. Growth twins also form by growth accidents on the {1 1 1} planes.     

Formation of single and multiple deformation twins in nanocrystalline fcc metals

Deformation twins are often observed to meet each other to form multi-fold twins in nanostructured face-centered cubic (fcc) metals. Here we propose two types of mechanism for the nucleation and growth of four different single and multiple twins. These mechanisms provide continuous generation of twinning partials for the growth of the twins after nucleation. A relatively high stress or high strain rate is needed to activate these mechanisms, making them more prevalent in nanocrystalline materials than in their coarse-grained counterparts. Experimental observations that support the proposed mechanisms are presented.     

Phenomenology of shear-coupled grain boundary motion in symmetric tilt and general grain boundaries

Shear-coupled grain boundary motion is examined for a large number of grain boundaries including 73 〈1 0 0〉, 〈1 1 0〉 and 〈1 1 1〉 symmetric tilt boundaries. In the present work, the grain boundary motion is induced by a synthetic driving force as opposed to prior studies of shear coupling induced by applied shear. For those boundaries that are observed to undergo shear-coupled motion, the results based on the two driving forces agree well, both for experiments and simulations. This agreement also confirms the generality of the shear coupling mechanism over numerous boundaries and boundary types. The examination of boundary structure provides insight into the different trends that are observed. Shear coupling according to modes not predicted by the Frank–Bilby equation are also demonstrated. The temperature dependence of shear coupling is examined, and is consistent with prior work for symmetric tilt boundaries. While prior studies have emphasized symmetric tilt boundaries, some general grain boundaries exhibit shear coupling as well. In these boundaries, it is found that the shear coupling is either temperature independent, decreases in magnitude with increasing temperature or, in some cases, changes direction with temperature.     

A comparison of grain boundary evolution during grain growth in fcc metals

Grain growth of Cu and Ni thin films, subjected to in situ annealing within a transmission electron microscope, has been quantified using a precession-enhanced electron diffraction technique. The orientation of each grain and its misorientation with respect to its neighboring grains were calculated. The Cu underwent grain growth that maintained a monomodal grain size distribution, with its low-angle grain boundaries being consumed, and the Ni exhibited grain size distributions in stages, from monomodal to bimodal to monomodal. The onset of Ni’s abnormal grain growth was accompanied by a sharp increase in the Σ3 and Σ9 boundary fractions, which is attributed to simulation predictions of their increased mobility. These Σ3 and Σ9 fractions then dropped to their room temperature values during the third stage of grain growth. In addition to the Σ3 and Σ9 boundaries, the Σ5 and Σ7 boundaries also underwent an increase in total boundary fraction with increasing temperature in both metals.