Geometrically Necessary Boundaries Research Articles

Effect of accumulative hot rolling on the microstructure and mechanical properties of dual-phase titanium alloy consolidated via SPS sintering

The improvement of microstructure and mechanical properties of Ti alloys is of great significance for their industrial application. In this study, a developed multi-step hot deformation process was employed to regulate the grain boundaries (GBs), hetero-phase boundaries (PBs) and dislocation structure for dual-phase titanium alloy. The effects of accumulative thermal deformation ratios from 33% to 75% (2–8 cycles) on the microstructure and mechanical properties have been investigated. It is found that the sintered sample shows a typical Widmanstätten structure without obvious pores and micro-cracks. However, with the increase of the deformation ratio, for the hexagonal close packed (HCP) Ti, the crystal orientations of {011‾0} and {1‾21‾0} were gradually transformed into {0001}, with the change of high-angle grain boundaries (HAGBs) to low-angle grain boundaries (LAGBs). The density of stored geometrically necessary dislocations (GNDs) density raised dramatically from 1.95 × 1014 m−2 to 6.5 × 1014 m−2, and various types of dislocations have been characterized. For the β phase, in the thermal deformation process, high density α-precipitates with an average thickness below 20 nm had formed with Burgers relationship of {110}BCC//{0001}HCP and <1‾11 >BCC//<21‾1‾0 >HCP. Finally, the yield strength of the dual-phase titanium alloy increased dramatically from 881 MPa to 1178 MPa, and the ductility can still maintain at above 8.5%. The enhanced strength was mainly contributed by the increase of geometrically necessary boundaries (GNBs) and nano-lamellae boundaries (NLBs). Therefore, this strategy of combining hetero-phase boundaries (PBs) and dislocation engineering could open up new avenues to designing strong and ductile titanium matrix materials.

The synergy between texture evolution and grain refinement in a BCC steel

ABSTRACT This work investigates the synergy between texture evolution and grain refinement in ultra-low carbon body-centred cubic (BCC) ferritic steel. In this regard, experiments and polycrystal plasticity for ambient compressive deformation in low (up to ϵ≈0.22) and moderate (ϵ≈0.22−0.69) strain regimes were carried out. The tests were interrupted at intermediate strains for microstructure and texture characterisation. It was observed that the change in texture to γ -fibre ( { 111 } ∥ Compression direction ( CD ) ) and the formation of θ -fibre ( { 100 } ∥ CD ) up to ϵ ≈ 0.22 contributes towards lattice bending, resulting in grain size reduction from ∼ 104 ( 23 ) μ m to ∼ 23 ( 6 ) μ m . Simultaneously, crystallite size decreases, and statistically stored dislocations (SSD) and geometrically necessary boundaries (GNB) increase rapidly. Thus, resulting in an abrupt decrease in strain hardening rate (SHR). From ϵ ≈ 0.22 − 0.69 , the γ and θ -fibre broadens and θ -fibre intensity increases. The low GNB density increase indicates insignificant deformation-induced lattice curvature formation within grains, resulting in SSD intensification. Thus, leading to elongated grains with sluggish grain size decrease to ∼ 17 ( 4 ) μ m and a lesser decrease in SHR. The α -fibre could not evolve due to the dispersion of 110 axes along the axisymmetric radial direction due to axial deformation. The polycrystal model could appropriately simulate strain-hardening behaviour and texture evolution.

High strength, ductility and sheet formability by normalizing and quenching of low carbon microalloyed dual-phase steel

This work investigates the possibility of developing high strength, ductility, and normal-anisotropy in cold rolled (CR, ∼84%) 0.05 wt% carbon microalloyed dual-phase (DP) steel sheets by normalizing (N, 920°C, 30 min) followed by intercritical annealing and quenching (IA&Q, 80 0°C, 3 min). CR + N produces ferrite with ∼6.9μm grain size and ∼0.23 bainite fraction with ∼5.3μm packet size. A high engg. (true) strength of ∼605(∼798)MPa; uniform engg. (true) strain eU≈0.32 (εU≈0.28); engg. fracture strain, ef≈0.48; with high normal average anisotropy (r‾) of ∼1.84 and low planar anisotropy, Δr of ∼0.30 could be obtained. Further, IA&Q produced ferrite with ∼6.3μm average grain size, ∼0.25 martensite fraction (Ceq≈0.16) with ∼3.2μm island size. A high engg. (true) strength of 690(864)MPa; engg. (true) uniform strain of ∼0.25 (∼0.22); engg. fracture strain ∼0.39; and high r‾ of ∼1.51, low Δr of ∼0.31, suitable for sheet metal formability, were obtained. The high strength could be attributed to (a) fine ferrite grain size, (b) secondary phase fraction (∼0.23B/∼0.25M), packet size and island morphology, and (c) geometrically necessary boundaries (GNB). High ductility could be due to (a) fine ferrite grains with secondary phase morphology, (b) GNB, coincident site lattice boundaries (CSLB), and random high angle grain boundary (RHAGB) networking. Better anisotropic properties could be associated with strong γ-fiber, and suppression of α-fiber and θ-fiber texture. Loci in the stress and strain space indicate that the hydraulic bulge and plane-strain biaxial strength-ductility improves to 950MPa-0.41 and 1030MPa-0.67 after CR + N, and to 968MPa-0.30 and 1070MPa-0.58 after CR + N + IA&Q.

Sequence and mechanism of early microstructure evolution in alloy 690TT subjected to fretting corrosion in high temperature pressurized water

The fretting corrosion-induced early microstructure evolution of Alloy 690TT was investigated. The results indicated that multiple material transfers of 405 SS caused nano-cavities formed, which provided nucleation sites for internal oxidation of the material transfer layer (MTL), and fretting corrosion-induced nano-grains boundaries offered massively high pathways for oxygen diffusion and accelerated oxidation process. An oxide layer developed on MTL by sufficient oxidation and persistent mechanical effect. Under fretting loading, high-density geometrically necessary boundaries (GNBs) appeared inside matrix and developed into deformation layer. With GNBs orientation increased, tribologically transformed structure (TTS) formed. The formation mechanism of TTS was continuous dynamic recrystallization.

Dislocation behavior in Cu single crystal joints under the ultrasonically excited high-strain-rate deformation

• The interfacial evolution for ultrasonically welded joints was used to clarify the coupling effects of ultrasonic excitation and high-strain-rate deformation. • Transmission Kikuchi diffraction and transmission electron microscopy were used to analyze grain orientation and deformed microstructures simultaneously. • Ultrasonic excitation dramatically weakened the constraint of high-strain-rate and characteristic orientations on dislocation motion. • The first-principle calculation and applied strain-rate analysis were conducted to discuss the preferred deformation mechanism to accommodate the imposed plastic strain. The coupling effects of ultrasonic excitation and high-strain-rate deformation are the core factors for weld formation during ultrasonic welding. However, interfacial deformation behavior still shrouds in uncertainty because of the contradictory features between mutual dislocation retardation caused by severely frictional deformation and ultrasonic-accelerated dislocation motion. [101] and [111]-oriented Cu single crystals which tended to form geometrically necessary boundaries (GNBs) were selected as the welding substrates to trace the uniquely acoustoplastic effects in the interfacial region under the ultrasonically excited high-strain-rate deformation. It was indicated that for a low energy input, micro-welds localized at the specific interface region, and equiaxed dislocation cells substituting for GNBs dominated in the initial single crystal rotation region. As the welding energy increased, continuous shear deformation drove the dynamic recrystallization region covered by equiaxed grains to spread progressively. Limited discrete dislocations inside the recrystallized grains and nascent dislocation cells at the grain boundaries were observed in [101] and [111] joints simultaneously, suggesting that the ultrasonic excitation promoted motion of intragranular dislocation and pile-up along the sub-grain boundaries. The interfacial morphology before and after expansion of recrystallization region all exhibited the weakening of orientation constraint on dislocation motion, which was also confirmed by the similar micro-hardness in joint interface. The first-principle calculation and applied strain-rate analysis further revealed that ultrasonic excitation enhanced dislocation slipping, and enabled dislocation motion to accommodate severe plastic deformation at a high-strain-rate.

Kink mechanism in Cu/Nb nanolaminates explored by in situ pillar compression

Nano metallic laminates (NMLs) exhibit different failure modes depending on the loading conditions due to their mechanical anisotropies. Kinking is a typical failure mode in many NMLs compressed along a layer-parallel direction. However, a detailed description of the microstructure evolution during kink band (KB) formation and an in-depth understanding of the formation mechanisms are lacking. In this work, the KB process is investigated in Cu/Nb NMLs by in situ micro pillar compression in the scanning electron microscope (SEM) along a layer-parallel direction. Post-mortem S/TEM and transmission Kikuchi diffraction (TKD) analyses show that kink banding leads to significant microstructure changes characterized by an accumulation of geometrically necessary dislocations (GNDs) and of tilt geometrically necessary boundaries (GNBs) near KB boundaries (KBBs). The distinct microstructure evolution implies that KB formation is facilitated by the inhomogeneous microstructures resulting in constrained deformation modes. Specifically, dislocations active on slip planes nearly parallel to the interfaces make a major contribution to kink evolution after the onset of kinking. Once layer-parallel slip systems are activated, preexisting lattice dislocations and dislocations nucleating from interfaces will accumulate as GNDs near KBBs via the stochastic storage of lattice dislocations that have certain Burgers vectors. GNDs can further transform into GNBs via cross-slip and climb driven processes near the KBB. Furthermore, GNBs near KBBs can grow by incorporating more GNDs or by coalescence to accommodate the KB evolution. We further hypothesize that microstructural perturbations and their ensuing stresses can initiate KB formation in Cu/Nb NMLs.

Effects of dislocation boundary spacings and stored energy on boundary migration during recrystallization: A phase-field analysis

The local migration of recrystallization boundaries into spatially varying deformation fields which are typical for metals deformed to low and high strains is investigated using quantitative phase-field simulations. It is found that the average migration velocities as well as the local velocities of the recrystallization boundaries critically depend on the spacing between planar geometrically necessary boundaries (GNBs) which are present in the deformation microstructure. Also the morphology of the deformation field reflecting low and high strain deformation cases strongly affect the migrating boundary during recrystallization. Additional simulations were performed as a function of the two critical parameters- stored energy and GNB spacing - independently. Two regimes are found suggesting that these two parameters are closely linked. Previous experimental results are interpreted based on these novel simulation results.

Influence of Severe Plastic Deformation on Static Recrystallization Microstructure of Pure Iron

In recent years, ultrafine-grained steel has been gaining increasing attention as a high-performance material. Accordingly, it is necessary to develop an efficient production method for ultrafine-grained steel. Severe plastic deformation is a critical factor that causes grain subdivision into ultrafine grains less than 1 µm in diameter. In this study, the effects of plastic deformation on the microstructure and static recrystallization of pure iron were studied by comparing orthogonal cutting and rolling. Orthogonal cutting yielded ultrafine grains with a diameter of 0.2 µm. It was found that a high strain rate in the thin shear plane generated during the cutting process caused a uniform subdivision of grains, and this uniform plastic deformation resulted in the uniform recrystallization of grains. In addition, a theoretical model was developed, and it was revealed that the number of recrystallized grains depended on the fraction of a large-misorientation area constructed with geometrically necessary boundaries (GNBs). It was suggested that the cutting process was more advantageous than rolling in producing ultrafine recrystallized grains because cutting could apply severe plastic strain uniformly on a work material, effectively generating GNBs.

Strain-dependence of deformation microstructures in ultra-low-C IF steel deformed to high strains by torsion at elevated temperatures

Single-phase bcc-ferrite in an interstitial free (IF) steel was deformed to different strains in a wide range from low to high strains ( ε = 1–7) by torsion under different Zener-Hollomon ( Z ) conditions. The specimens were rapidly quenched after the torsion to preserve microstructures formed under different deformation conditions. The results showed that during high-Z (low-temperature) deformation, grains were subdivided by geometrically necessary boundaries (GNBs) via the grain subdivision mechanism. Deformation to high strains ( ε > 5) led to the ultrafine lamellar structures (with grain sizes < 1 μm) mainly composed of GNBs having high misorientation angles. Decreasing Z with increasing temperature and/or decreasing strain rate accelerated thermally activated processes, such as dynamic recovery and boundary migration. Unlike the ultrafine lamella formed under the high- Z condition, a variety of microstructures having equiaxed morphologies with fine to coarse grain sizes (>1 μm) were realized with decreasing Z . The significance of the grain subdivision and the thermally activated phenomena on the formation of various microstructures under different deformation conditions is discussed.

FTMP-based simulations and evaluations of Geometrically-Necessary Boundaries (GNBs) of dislocation

The present study targets four representative GNBs (Geometrically Necessary Boundaries) in terms of their unified and quantitative evaluations based on the field theory of multiscale plasticity (FTMP). Discrete dislocation dynamics simulations on GNBs 2, 3, 4 and 7, whose details about the consisting dislocations have been experimentally identified and theoretically evaluated by Hong, Winther, et al., are conducted. Applying the FTMP-based duality diagram representation scheme to the formation processes reveals a systematic interrelationship that further governs the stability of the GNBs, i.e., all the GNBs exhibit a common tendency to converge ultimately to a single master curve. This leads us to propose a postulate about the stability/instability criterion that involves their dynamic interactions with external disturbances.

Evolution of nano-laminated structure formed by the thermally-assisted plastic deformation in dry sliding wear

Abstract The evolution of nano-laminated structure generated in dry sliding wear of a high strength martensite steel was investigated by using Scanning Electronic Microscopy (SEM) and Transmission Electron Microscopy (TEM). Gradient microstructure along the depth was observed with the top layer consisting of nano-laminated structure. During the sliding wear-induced plastic deformation, grains were refined and lamellar structures were formed as a result of generation of dislocation cell blocks bounded by geometrically necessary boundaries (GNBs) and incidental dislocation boundaries (IDBs). With increasing plastic shear strain and strain gradient, there is a continuous reduction in the spacing and length in dislocation cell blocks, leading to nano-laminated structures. The ultrafine nano-laminated structure with laminate spacing of 20 nm was observed at the sliding time of 120 min. However, the grain refinement is not a merely mechanical deformation process. The formation of Fe3O4 and FeO gave direct evidence that the highest temperature due to frictional heat could reach up to 600 °C. The result suggested that the formation of nano-laminated structure was governed by thermally-assisted plastic deformation.

Evaluation of Microstructure due to Addition of Carbon in Ni–Cr–Mo Steel Mechanically Through Surface Mechanochemical Case Carburizing Treatment (SMCT)

Mechanochemical reaction of Fe–C is achieved in Ni–Cr–Mo steel during surface mechanochemical case carburizing treatment (SMCT) at room temperature. Gradient nano-carbide layers are formed in the surface and near surface up to few microns. The Ni–Cr–Mo steel was treated for 15, 30, 45 and 60 min, respectively, to find out the alteration in microstructure. The surface and cross-sectional microstructures were investigated by Olympus optical microscope (Japan), 6390-A scanning electron microscope and JEOL F-2100 transmission electron microscope (TEM). It is observed that mechanical alloying of carbon in iron lattice is achieved and surface carbon concentration increased from 0.17 to 0.8 wt% with 1 hour of SMCT operation time. The mechanochemical reaction between Fe and C produces Fe3C (cementite) in the surface and undersurface up to 500–800 microns along with nano-particulate on the surface and near surface in 1-hour operation time. It is observed that the carbide cell blocks (CB) and geometrically necessary boundaries (GNB) are produced due to severe plastic deformation created by SMCT operation.

Formation of Σ3{110} incoherent twin boundaries through geometrically necessary boundaries in an Al-8Zn alloy subjected to one pass of equal channel angular pressing

For coarse grained (CG) alloys with high stacking fault energies (SFEs), like aluminum, deformation twins can rarely form. Here, we report that Σ3{110} incoherent twin boundaries (ITBs) could be generated in a CG Al-8Zn alloy by one pass of ECAP. A systematic investigation shows that the Σ3{110} ITBs are formed by gradual evolution from geometrically necessary boundaries (GNBs) delineating deformation bands (DBs) by lattice rotation via <111>-twist CSL boundaries. This is a new deformation mechanism in Al alloys, which has never been reported.

The microstructural origin of work hardening stages

The strain evolution of the flow stress and work hardening rate in stages III and IV is explored by utilizing a fully described deformation microstructure. Extensive measurements by transmission electron microscopy reveal a hierarchical subdivision of grains by low angle incidental dislocation boundaries (IDBs) and medium to high angle geometrically necessary boundaries (GNBs). This universal evolution is demonstrated for nickel, copper, and aluminum deformed by cold rolling from strains of 0.05–5.5. Microstructural morphology evolves with increasing strain through a transition resulting in a lamellar cell-block structure aligned with the deformation. This transition is caused by the emergence of new slip systems and a stable texture. Four parameters describe the microstructure, the misorientation angle across each boundary type and their respective spacing. Universal scaling characterizes the normalized distributions of three separate parameters. A new scaling law connects the strain evolution of two strength parameters: the dislocation density of IDBs and the spacing between GNBs. Strengthening mechanisms and strength contributions for those two parameters are expressed respectively as a linear addition of the classical Taylor and Hall-Petch formulations. Model predictions agree closely with experimental values of flow stress and work hardening rate in stages III and IV. Strong connections between the evolutionary stages of the deformation microstructure and work hardening rates create a new (modern) basis for the classic problem of work hardening in metals and alloys. These connections lead the way for the future development of ultra high strength ductile metals produced via plastic deformation.

Evolution mechanism of dislocation boundary and characteristic micro-structure of commercial pure titanium during the projectile impact

Dislocation boundary response and characteristic micro-structure were investigated in commercial pure titanium during the penetration impact and dynamic compression. The initial dislocation boundary induced by cold rolling can strengthen the adiabatic shearing sensitivity of commercial pure titanium during the penetration impact, which is consistent with the mechanical response and micro-structure during the dynamic compression. The compression with a loading direction vertical to the initial geometrically necessary boundaries (GNBs) results in the decrease in the spacing between GNBs, while the compression with a loading direction parallel to the initial GNBs attributes to the generation of new dislocation boundary crossed with the initial one. Then, a model of distribution and morphology of dislocation boundary around crater is proposed.

Formation of nano-laminated structures in a dry sliding wear-induced layer under different wear mechanisms of 20CrNi2Mo steel

The microstructures of 20CrNi2Mo steel underneath the contact surface were examined after dry sliding. Scanning Electronic Microscopy (SEM), Transmission Electron Microscopy (TEM), Electron Backscattered Diffraction (EBSD) and an ultra-micro-hardness tester were used to characterize the worn surface and dry sliding wear-induced layer. Martensite laths were ultra-refined due to cumulative strains and a large strain gradient that occurred during cyclic loading in wear near the surface. The microstructure evolution in dominant abrasive wear differs from that in adhesive wear. In dominant abrasive wear, only bent martensite laths with high-density deformation dislocations were observed. In contrast, in dominant adhesive wear, gradient structures were formed along the depth from the wear surface. Cross-sectional TEM foils were prepared in a focused ion beam (FIB) to observe the gradient structures in a dry sliding wear-induced layer at depths of approximately 1–5μm and 5–20μm. The gradient structures contained nano-laminated structures with an average thickness of 30–50nm and bent martensite laths. We found that the original martensite laths coordinated with the strain energy and provided origin boundaries for the formation of gradient structures. Geometrically necessary boundaries (GNBs) and isolated dislocation boundaries (IDBs) play important roles in forming the nano-laminated structures.

Statistical analysis of dislocations and dislocation boundaries from EBSD data

Electron BackScatter Diffraction (EBSD) is often used for semi-quantitative analysis of dislocations in metals. In general, disorientation is used to assess Geometrically Necessary Dislocations (GNDs) densities. In the present paper, we demonstrate that the use of disorientation can lead to inaccurate results. For example, using the disorientation leads to different GND density in recrystallized grains which cannot be physically justified. The use of disorientation gradients allows accounting for measurement noise and leads to more accurate results.Misorientation gradient is then used to analyze dislocations boundaries following the same principle applied on TEM data before. In previous papers, dislocations boundaries were defined as Geometrically Necessary Boundaries (GNBs) and Incidental Dislocation Boundaries (IDBs). It has been demonstrated in the past, through transmission electron microscopy data, that the probability density distribution of the disorientation of IDBs and GNBs can be described with a linear combination of two Rayleigh functions. Such function can also describe the probability density of disorientation gradient obtained through EBSD data as reported in this paper. This opens the route for determining IDBs and GNBs probability density distribution functions separately from EBSD data, with an increased statistical relevance as compared to TEM data. The method is applied on deformed Tantalum where grains exhibit dislocation boundaries, as observed using electron channeling contrast imaging.

Effects of Initial Micro-Structures on Deformation Behaviors of Commercial Pure Titanium

In this paper, effects of initial micro-structures on deformation behaviors of commercial pure titanium were elaborated by investigating the evolution of dislocation boundary and its adiabatic shear sensitivity. At the low to medium stain rates, the main plastic deformation mechanism of as-annealed commercial pure titanium is dislocation slipping. Meanwhile, geometrically necessary boundaries (GNBs) with different directions are generated and crossed with each other. However, new dislocation boundaries are formed in as-cold rolled plates, which are parallel to the initial ones induced by cold rolling. When the strain rate is up to 1000 s-1, the initial dislocation boundary playes an adverse role in the adiabatic shear sensitivity of commercial pure titanium. The adiabatic shear band is the high-speed deformation characteristic micro-structure in commercial pure titanium. In addition, dynamic recrystallized grains are generated in the center of an adiabatic shear band, which is consistent with the sub-grain rotation mechanism.

Mechanism of grain refinement of aluminium alloy in shear spinning under different deviation ratios

To investigate the grain refinement and its mechanism in shear spinning, microstructures of shear spun parts made by aluminium alloy under different deformation conditions, induced by different shear spinning deviation ratios, are studied. The results show that, after shear spinning, the microstructure is distributed symmetrically about a zone in sheet thickness defined as the neutral zone which is located between the inner surface and the middle plane of spun sheet thickness. Various deviation ratios in shear spinning can lead to grain refinement in different regions along thickness direction of the spun part. The microstructure characteristics indicate that the mechanism of grain refinement is due to the formation of deformation bands (DBs). It is observed that in DBs, parallel geometrically necessary boundaries (GNBs) formed by a zero deviation ratio and crossed GNBs formed by positive and negative deviation ratios are due to the different stress states induced by various deviation ratios in shear spinning. Due to the influence of grain refinement, micro hardness increases with the decreasing of the deviation ratio. The average value is increased by 16.04% under a negative deviation ratio compared to the initial micro hardness of the sheet.

Study of Dislocation Boundary Structure in Al–Li Alloy During Bending

The dislocation boundary structures of 2060-T8 alloy during bending were investigated by backscattered electron imaging, electron backscattered diffraction, and misorientation axes maps. Experimental result shows that typical dislocation boundary structures, which depend on grains’ orientation, are formed in grains during bending. The microstructure of type A is mainly observed in grains near brass, copper, and Goss orientations; microstructure of type B is mainly found in grains near S orientation; microstructure of type C is mainly seen in grains near Cube orientation. The angle between geometrically necessary boundaries (GNBs) and force axis is in the range of −45° to −30° and 30° to 45°. Most of the GNBs are approximately parallel to the trace of {111} slip planes which are identified by Schmid factor analysis.