Pre-existing Twin Boundaries Research Articles

Understanding the effects of recrystallization and strain induced boundary migration on ∑3 twin boundary formation in Ni-base superalloys during iterative sub-solvus annealing

The formation of ∑3 twin boundaries in an experimental low stacking fault Ni-based superalloy containing 24 wt% Co was investigated. Normalized microstructures were hot deformed to strain limits of 0.10 and 0.50 at a strain rate of 0.1/s and a temperature of 1020 °C. The hot-deformed microstructures were characterized using electron backscatter diffraction, and were subsequently subjected to a series of iterative sub-solvus anneal heat treatments at 1100 °C for one to two minutes. By characterizing the changes in the microstructure and grain boundary character distribution as a function of annealing time, this investigation sought to quantify and detail how twin boundary generation and growth occurs during annealing. This behavior was investigated in the same alloy, but with two distinct as-deformed starting microstructures associated with the different strain limits. The starting microstructures possessed varying levels of intragranular misorientation networks, which were found to have greatly affected the resultant formation of ∑3 annealing twin boundaries. Strain induced boundary migration (SIBM) was observed to have been restricted in the iteratively annealed sample deformed to a strain of 0.10. Due to the sub-solvus heat treat temperature, grain boundary precipitates were present in the microstructure which limited the mobility of the grain boundaries and suppressed grain growth. As a result, the expansion in the length fraction of ∑3 twin boundaries was limited as a function of annealing time. The sample deformed to 0.50 strain, however, was shown to have recrystallized both upon deformation and subsequent annealing. Although recrystallization was found to largely remove much of the pre-existing twin boundaries, the mobility of the newly formed boundaries during growth of the recrystallized grains resulted in a dramatic increase in both the density and length fraction of twin boundaries.

Evolution of twins and sub-boundaries at the early stage of dynamic recrystallization in a Ni-30%Fe austenitic model alloy

Evolution of twins and sub-boundaries at the early stage of dynamic recrystallization (DRX) is investigated in a Ni-30%Fe austenitic alloy. It is found that the primary nucleation of DRX is accomplished by formation of annealing twins along the bulged pre-existing grain boundaries and at the triple junctions at the early stage of deformation. With the strain increasing, the DRX nucleation due to the formation and evolution of sub-boundaries also plays a significant role. The results also indicate that the ∑9 twin boundaries are generated by mutual interactions between the encountered ∑3 twin boundaries at the initial stage. The pre-existing twin boundaries can gradually convert to general boundaries or twin steps owing to the interaction of twin boundaries with dislocations. Continuous DRX (CDRX) mechanism associated with the sub-boundaries evolution is found to be an assistant mechanism at the early stage of DRX. Besides, formation of sub-boundaries inside the growing DRX grains can stimulate a new round DRX nucleation by impeding migration of grain boundaries and making them serrated.

Anomalous Tensile Detwinning in Twinned Nanowires.

In spite of numerous studies on mechanical behaviors of nanowires (NWs) focusing on the surface effect, there is still a general lack of understanding on how the internal microstructure of NWs influences their deformation mechanisms. Here, using quantitative insitu transmission electron microscopy based nanomechanical testing and molecular dynamics simulations, we report a transition of the deformation mechanism from localized dislocation slip to delocalized plasticity via an anomalous tensile detwinning mechanism in bitwinned metallic NWs with a single twin boundary (TB) running parallel to the NW length. The anomalous tensile detwinning starts with the detwinning of a segment of the preexisting TB under no resolved shear stress, followed by the propagation of a pair of newly formed TB and grain boundary leading to a large plastic deformation. An energy-based criterion is proposed to describe this transition of the deformation mechanism, which depends on the volume ratio between the two twin variants and the cross-sectional aspect ratio.

Direct observation of precipitation along twin boundaries and dissolution in a magnesium alloy annealing at high temperature

Precipitation along twin boundaries and dissolution in a cold-rolled Mg-Y-Nd alloy was directly observed for the first time during annealing at 490°C. Precipitation occurred concurrently with recrystallization and the combined effect of precipitation and solute segregated to twin boundaries modified the recrystallization behaviour. Precipitates later dissolved into the matrix at the point where full recrystallization was nearly complete. The precipitates and higher solute concentration along original twin boundaries hindered grain growth of newly formed recrystallized grains. Even where twin boundaries had been consumed by recrystallization, the size of recrystallized grains were still controlled by the pre-existing twin boundaries.

Single and double twin nucleation, growth, and interaction in an extruded magnesium alloy

The objective of this study was to identify twinning characteristics and mechanisms in an extruded AZ31 magnesium alloy under favorable conditions of profuse 101¯2 extension twinning using in-situ optical microscopy, electron backscatter diffraction, and X-ray diffraction analysis. The propagation of a single twin variant led to a relatively fast saturation of twin nucleation after which the increase in strain was predominantly accommodated by the growth of existing twin lamellas. For distinct twin variants, the intersecting twins led to the confinement of the spaces constrained by the fine twin lamellas. Embryonic twin structures acknowledged theoretically or through atomistic simulations were experimentally observed, including the vanishing of primary112¯1 embryonic twin via the nucleation and growth of either single or multiple 101¯2 secondary extension twin variants during deformation. Newly identified twin-twin interaction scenarios included the ladder-like and branching-like twinning structures which occurred depending on the applied strain, incoming twin paths, and their impingement on the pre-existing twin boundaries. The formation of the branching-like structures involved three or more non-co-zone 101¯2 extension twin variants where the activation and growth of one variant among them were found to be favored by the external stresses via Schmid factor analysis.

Size-dependent plastic deformation and failure mechanisms of nanotwinned Ni3Al: Insights from an atomistic cracking model

The polycrystalline Ni3Al is brittle since the notorious intergranular fracture mode hinders its applications. Here we perform molecular dynamics to highlight the unique role of nanotwin boundary in the plastic deformation and failure mechanisms of Ni3Al via an atomistic cracking model. Surprisingly, the strength, ductility and fracture toughness of the nanotwinned Ni3Al are revealed to increase simultaneously with reducing twin size, possibly evading a traditional tradeoff between ductility/toughness and strength. A possible quasi-brittle fracture mode in single crystalline Ni3Al is recognized as nucleating twinning partials from crack tip. However, the pre-existing twin boundaries can suppress the emission and propagation of successive twinning dislocations. Instead, dislocation avalanches happen and serve as a crack blunting mechanism which leads to the ductile fracture pattern of the nanotwinned Ni3Al. A size-dependent transition of fracture mode from dislocation nucleation to shear localization is observed as twin becomes very small. A physical model combined with energetics analysis is provided to rationalize the transition. Our atomistic insights are in qualitative agreement with recent observations of improved strength and ductility of Ni3Al with disordered nanotwinned structure after severe plastic deformation.

A new mechanism for twin growth in Mg alloys

Twinning is an important deformation mode in lightweight Mg alloys, but the mechanisms of nucleation and growth of twins and their interactions with solutes remain largely unresolved. Here, a new model for thermally activated, stress-driven growth of twin boundaries is presented and the role of random and segregated solutes in controlling this growth is studied analytically and using direct molecular dynamics simulations. Twin growth occurs by the thermally activated nucleation and expansion of twin dislocation loops on a pre-existing twin boundary. Fluctuations in the local concentration of random solutes lowers the energy barrier for this process and thus facilitates twin growth. Segregation of solutes to the twin boundary strongly increases the energy barrier and suppresses twin growth. In random solid solutions at experimental strain rates, the thermally activated nucleation process is sufficiently fast at low applied stresses so that growth of the twin is controlled by solute strengthening of the nucleated twin loop as it expands across the twin boundary. Annealing of a deformed sample leads to solute segregation and very strong pinning so that further twinning can only be accomplished by nucleation of new twins, consistent with experimental observations. The new mechanism also (i) operates for detwinning and rationalizes complex twin shapes observed in experiments, and (ii) suggests a process for dynamic strain aging commonly observed in Mg alloys at elevated temperatures.

Grain refining and improving mechanical properties of a warm rolled AZ31 alloy plate

The mechanical properties of an AZ31 magnesium alloy were improved after warm rolling. The study involved a pre-rolling condition in which the material consisted of twins, playing a key role in recrystallization grain refinement upon subsequent processing. The pre-existing twin boundaries increased the number of nucleation sites for recrystallization during warm rolling, and resulted in complete recrystallization and weakening of the basal texture during subsequent annealing. This led to improved mechanical properties of the warm rolled plates.

Detwining in Mg alloy with a high density of twin boundaries

To investigate the role of preexisting twin boundaries in magnesium alloys during the deformation process, a large number of {10-12} tensile twins were introduced by a radial compression at room temperature before hot compressive tests with both low and high strain rates. Unlike the stable twins in Cu-based alloys with low stacking fault energies, {10-12} twins in Mg alloy are extremely unstable or easy to detwin through {10-12}-{10-12} re-twinning. As a result, non-lenticular residual twins and twin traces with misorientation of 5°–7° were present, as confirmed by electron backscatter diffraction. The extreme instability of the twins during compression indicates that both twin and detwinning require extremely low resolved shear stresses under our experimental conditions.

Doubling the critical current density of high temperature superconducting coated conductors through proton irradiation

The in-field critical current of commercial YBa2Cu3O7 coated conductors can be substantially enhanced by post-fabrication irradiation with 4 MeV protons. Irradiation to a fluence of 8 × 1016 p/cm2 induces a near doubling of the critical current in fields of 6 T || c at a temperature of 27 K, a field and temperature range of interest for applications, such as rotating machinery. A mixed pinning landscape of preexisting precipitates and twin boundaries and small, finely dispersed irradiation induced defects may account for the improved vortex pinning in high magnetic fields. Our data indicate that there is significant head-room for further enhancements.

Microstructural self-organization triggered by twin boundaries during dry sliding wear

A novel microstructural self-organization reaction is observed near the surface of a Cu–Ni–Sn bronze subjected to dry sliding wear. Scanning electron microscopy using electron-backscattered diffraction reveals the formation of near periodic patterns comprising domains with alternating orientation separated by narrow boundaries. Remarkably, orientation patterning takes place only near twin boundaries intercepting the sliding surface, and only on a specific side of the boundaries. The domain period, which ranges from a few to tens of micrometers, increases with the applied load. This orientation patterning is suppressed by promoting steady sliding using solid lubricants such as Ag and self-generated oxide-bearing tribolayers. Transmission electron microscopy reveals planar dislocation glide in the orientation domains, with multiple active slip systems. A rationalization of these results is offered based on an instability triggered by the interaction of elastic stick–slip waves with pre-existing twin boundaries.

Investigation on the mechanical behavior of fivefold twinned silver nanowires

Using molecular-dynamic (MD) simulation, we studied the mechanical responses of fivefold twinned silver nanowire (NW) for tensile and torsional deformations. Although the relaxed configuration of fivefold twinned NW possesses higher potential energy than 〈110〉 single crystalline NW, the internal stable of the unique structure provides a larger energetic barrier to defect formation. Under tensile loading condition, we find that the defect emission and propagation in fivefold twinned NW is prevented by the pre-existing twin boundaries. While the plastic deformation under torsion yield through the nucleation of coaxial dislocations, showing a quite uniformly distribution as observed in the end of the wire. Generally, different external loading leading to different deformation modes, as reported in this paper, might be useful in avoiding for certain deformation mechanisms of metallic NWs.

Direct observation of dislocations originating from perfect twin boundaries

This paper addresses the plasticity deformation mechanisms caused by nanoindentation of pre-existing coherent twin boundaries (CTBs) in copper using atomistic simulation. Initial plasticity caused by the homogeneous nucleation of threefold symmetry twinning partial loops is observed below the critical twin lamella length of 2.50 nm, accompanied by a significant reduction in critical pressure, which indicates that dislocations can originate from perfect CTBs. However, the CTBs will block the dislocation motion following mechanism of twin boundary migration, resulting in further plastic deformation.

주사 힘 현미경에 의한 (001) Pb(Mg1/3Nb2/3)O3-x%PbTiO3단결정의 도메인 구조 및 분극 스위칭 관찰

Domain structure and polarization switching in (001)-oriented Pb(Mg 1/3 Nb 2/3 )O₃-x%PbTiO₃ (PMN -x %PT) crystals for x=20 and 35at% have been investigated in-situ by scanning force microscope (SFM) in a piezoresponse mode under a step DC electrical voltage. In the initial annealed condition, polar nano domains (PND) and domain striations oriented along {110} were observed in x=20 and x=35, respectively. For x=20, domain switching occurs by heterogeneous nucleation, where nucleation is not confined in the vicinity of domain boundaries, but rather can occur throughout the crystal volume. However, for x=35, domain switching tends to be preferentially initiated near pre-existing twin boundaries. With increasing the applying voltage, the nuclei density increased and assembled into {110} striations, indicating a stress-accommodated domain growth process. At higher voltage, nucleation occurs heterogeneously throughout the crystal volume.

Deformation behavior of silver submicrometer-pillars prepared by nanoimprinting

Scaling in material properties is of great importance in microsystems and microelectronics where the device dimensions continuously shrink. Recently, compression tests of micron-sized pillars produced using a focused-ion beam (FIB) have become standard in the investigation of scaling effects. The influence of the fabrication process on the mechanical properties of the samples has, however, not been conclusively resolved. In this study, 130 silver pillars were fabricated using a novel embossing technique that does not pose the issues associated with FIB milling, i.e. surface amorphization and gallium contamination. Displacement-controlled compression tests on pillars with diameters of 130–3000 nm reveal for submicrometer samples an inverse proportionality of flow strength to diameter, which is associated with a wide strength distribution and a deformation taking place in large discrete strain bursts. The largest pillars show instead near bulk-like behavior. Unlike studies on other fcc materials, mechanical twinning is also an important deformation mechanism in Ag due to its very low stacking fault energy. In addition, occasional preexisting twin boundaries constituted preferential planes for localized deformation if favorably sloped. Despite the absence of ion-induced artifacts, scaling laws and absolute values of strength are comparable to those obtained via FIB milling.

Dynamic Recrystallization of Austenite in Ni-30 Pct Fe Model Alloy: Microstructure and Texture Evolution

The microstructure and crystallographic texture development in an austenitic Ni-30 pct Fe model alloy was investigated within the dynamic recrystallization (DRX) regime using hot torsion testing. The prominent DRX nucleation mechanism was strain-induced grain boundary migration accompanied by the formation of large-angle sub-boundaries and annealing twins. The increase in DRX volume fraction occurred through the formation of multiple twinning chains. With increasing strain, the pre-existing Σ3 twin boundaries became gradually converted to general boundaries capable of acting as potent DRX nucleation sites. The texture characteristics of deformed grains resulted from the preferred consumption of high Taylor factor components by new recrystallized grains. Similarly, the texture of DRX grains was dominated by low Taylor factor components as a result of their lower consumption rate during the DRX process. The substructure of deformed grains was characterized by “organized,” banded subgrain arrangements, while that of the DRX grains displayed “random,” more equiaxed subgrain/cell configurations.

On a ferroelastic mechanism governing the toughness of metastable tetragonal-prime ( t ′) yttria-stabilized zirconia

This article investigates the toughness of yttria-stabilized zirconia (YSZ) with the tetragonal-prime ( t ′) structure. Such materials are used as durable thermal barriers in gas turbines. Their durability has been attributed to high toughness, relative to materials in the cubic phase field. Based on prior literature, a ferroelastic toughening mechanism is hypothesized and this assertion is examined by characterizing the material in the wake of an indentation-induced crack. Assessment by transmission electron microscopy, Raman spectroscopy and optical interferometry has affirmed the existence of a process zone, approximately 3 μm in width, containing a high density of nano-scale domains, with equal proportions of all three crystallographic variants. Outside the zone, individual grains contain a single variant (no domains) implying that the toughening mechanism is controlled by domain nucleation (rather than the motion of pre-existing twin boundaries). The viability of the ferroelastic toughening mechanism is assessed using a process zone model that relates the observed toughening to the stress/strain hysteresis accompanying domain formation. Based on the measured process zone size, the known tetragonality of t ′-7 wt% YSZ and the enhancement in toughness relative to cubic YSZ, consistency between the model and the observed toughening is demonstrated for a reasonable choice of the coercive stress.

Uniaxial Compression Behavior of Bulk Nano-twinned Gold from Molecular Dynamics Simulation

AbstractParallel molecular dynamics simulations were used to study the influence of pre-existing growth twin boundaries on the slip activity of bulk gold under uniaxial compression. The simulations were performed on a 3D, fully periodic simulation box at 300 K with a constant strain rate of 4×107 s−1. Different twin boundary interspacings from 2 nm to 16 nm were investigated. The strength of bulk nano-twinned gold was found to increase as the twin interspacing was decreased. However, strengthening effects related to the twin size were less significant in bulk gold than in gold nanopillars. The atomic analysis of deformation modes at the twin boundary/slip intersection suggested that the mechanisms of interfacial plasticity in nano-twinned gold were different between bulk and nanopillar geometries.

Study of deformation micromechanism in cold-deformed TiNi based alloys

The microstructural evolution and deformation micromechanism of cold-deformed TiNi based alloys, both in the martensite and parent phase conditions, have been reviewed in the present paper. The emphasis is placed on the modification of the microstructure through atomic migration such as the coalescence and rearrangement of martensite variants and the adjustment and development of internal twinning. The atomic configuration at the pre-existing twin boundaries after deformation have been revealed and the corresponding deformation micro-mechanisms were discussed.

Polarization switching in (001)-oriented Pb(Mg1∕3Nb2∕3)O3−x%PbTiO3 crystals: Direct observation of heterogeneous nucleation by piezoreponse force microscopy

Polarization switching in (001)-oriented Pb(Mg1∕3Nb2∕3)O3−x%PbTiO3, PMN-x%PT, crystals for x=20 and 35 has been investigated in situ by scanning force microscopy in a piezoresponse mode, under a step-increased dc electrical voltage (Vdc). The results demonstrate that switching initiates by the nucleation of nanosized polar regions with a reversed polarization. For x=20, nucleation occurs heterogeneously throughout the volume of the crystal. However, with increasing PT content, nucleation tends to preferentially initiate near preexisting twin boundaries; with increasing Vdc, the domains becoming increasingly “diffuse” due to heterogeneous nucleation and growth throughout the crystal.