Kink Bands Research Articles

Non-local large-strain FFT-based formulation and its application to interface-dominated plasticity of nano-metallic laminates

This paper presents a novel formulation and its robust numerical implementation of strain-gradient (SG) crystal plasticity within a large-strain (LS) elasto-viscoplastic (EVP) fast Fourier transform (FFT)-based micromechanical model. The resulting non-local SG-LS-EVPFFT formulation is used to model and understand the process of kink band formation during layer-parallel compression of nano-metallic laminates (NMLs). NMLs are layered composites with nanoscale thicknesses, thus requiring consideration of the interaction between dislocations and interfaces within the micromechanical model. The length-scale parameter of the SG model is calibrated by simulating a double pile-up and comparing predictions to analytical solution. This required new expressions for the defect energy, resulting in more accurate double pile-up predictions. The calibrated SG-LS-EVPFFT model is then used to simulate layer-parallel compression of copper–niobium NML. Formation of kink bands is predicted, and the model is used to rationalize the microscopic mechanisms enabling the formation process. It is found that accumulation of dislocations at interfaces leads to activation of layer-parallel slip, which in turn leads to kink band formation.

Evaluation of the Failure Mechanism in Polyamide Nanofibre Veil Toughened Hybrid Carbon/Glass Fibre Composites.

The interface of hybrid carbon/E-glass fibres composite is interlayered with Xantu.layr® polyamide 6,6 nanofibre veil to localise cracking to promote a gradual failure. The pseudo-ductile response of these novel stacking sequences examined under quasi-static three-point bending show a change to the failure mechanism. The change in failure mechanism due to the interfacial toughening is examined via SEM micrographs. The incorporation of veil toughening led to a change in the dominant failure mechanism, resulting in fibre yielding by localised kinking and reduced instances of buckling failure. In alternated carbon and glass fibre samples with glass fibre undertaking compression, a pseudo-ductile response with veil interlayering was observed. The localisation of the fibre failure, due to the inclusion of the veil, resulted in kink band formations which were found to be predictable in previous micro buckling models. The localisation of failure by the veil interlayer resulted in a pseudo-ductile response increasing the strain before failure by 24% compared with control samples.

Effects of microstructure heterogeneity on tensile properties and failure mechanism of GTAW joint of as-cast Ti2531

In this study, as-cast dual-phase titanium alloy Ti2531 was welded by Gas Tungsten Arc Welding, and the effects of microstructure heterogeneity on tensile properties and failure mechanism was analyzed. Tensile test results showed that the specimens fractured in the base metal region, and the tensile strength R m of the welded plate was 855 MPa ~ 859 MPa, which was almost the same as that of the base metal. It was found that, compared with the base metal region, the fine and basketweave phase in the weld zone and heat-affected zone resulted in higher hardness and yield strength. Therefore, plastic strain concentration occurred in the base metal region and a large number of kink bands were found, which accounts for the relatively good plasticity with the fracture strain of 0.073–0.09. In addition, by means of intergranular orientation analysis, it was demonstrated that [0001] α and [101] β were the rotation axes of kink bands in the α and β phases, respectively. Furthermore, by observing the high-resolution electron microscopy, it was found that these kink bands were formed by the arrangement of edge dislocations. As a consequence, when the lattice distortion was severe, the unstable expansion of kink bands occurred and cracks were formed in widmanstätten colonies, inducing the failure of the specimen in the base metal region and great tensile strength equivalent to that of base material. • The weld joint of Ti2531 had good tensile strength close to that of base material. • Plastic deformation was concentrated in base metal region rather than other regions. • Kink bands were formed to coordinate severe plastic deformation in base metal region. • The instable expansion of kink bands resulted in the fracture of the welded specimen.

Taylor impact of high-entropy alloy Al0.1CoCrFeNi: Dynamic severe plastic deformation and bulk gradient structure

Taylor impact experiments are conducted on a typical face-centered cubic single-phase high-entropy alloy (HEA) Al0.1CoCrFeNi, to investigate its dynamic severe plastic deformation and gradient structure. Parameters of the Johnson–Cook constitutive model are obtained, and finite element modeling based on this model is consistent with the experimental observations. A bulk gradient structure with deformation gradient is generated directly via the Taylor impact. The deformation mechanisms include the {111}〈112〉 deformation twins and kink bands with the 〈110〉 or 〈112〉 rotation axes, and vary considerably along the impact direction. Subsequent annealing at 800∘C and the impact-induced gradient in the degree of re-crystallization lead to a new bulk gradient structure with gradients in grain size and proportion of the non-recrystallized region, providing a novel approach to developing bulk gradient structured materials.

Deformation kinking in duplex titanium alloy with widmannstatten laths under high strain rates

Deformation kinking acts as an important complementary deforming mechanism which is usually discovered in low-symmetrical metallic materials with hexagonal close-packed (HCP) crystallographic structure. Interestingly, for titanium alloys, rather than taking place in α-Ti with HCP structure, deformation kinking prefers to occur in β-Ti with the body-centered cubic (BCC) structure of higher symmetry. in this paper, the duplex titanium alloy with the widmannstatten microstructure was prepared, and uniaxially compressed at a strain rate of 3000 s−1. Kink bands are independently observed for the first time in duplex titanium alloy. The 30°<101¯0> KBs contain an unfavorable orientation for plastic deformation, while the KBs of about 35° misorientation angle possess a favorable orientation, which consist of two subordinate deformation bands, i.e. 20°<22¯01> and 15°<11¯01> respectively. The detailed analysis of KBs is of great significance for both the fundamental understanding of duplex titanium deformation and their potential engineering applications.

High-temperature deformation behavior and microstructural evolution of NbZrTiTa refractory high entropy alloy

The hot deformation behaviors and microstructural evolution of an equiatomic NbZrTiTa refractory high entropy alloy (RHEA) were studied by using isothermal compression tests in a range of temperatures (900 °C∼1200 °C) and strain rates (10−3 s−1∼1 s−1). A category of Arrhenius power law relationship was constructed and the activation energy was calculated to be 336–403 kJ mol−1 throughout the strain. The dislocation piling-up and kink bands found in local microstructural areas at low temperatures and high strain rates contributed to intense strain hardening and subsequent strain softening. As the temperature increased and the strain rate decreased, a sharp drop in flow stress was observed after the peak stress. This may be explained by dislocations pinned by short-range ordering (SRO) structures being unlocked. The higher dislocation density and newly emerging fine grains near deformed grain boundaries suggested that the dynamic recrystallization (DRX) was responsible for the continuous flow softening. Discontinuous DRX (DDRX) was the main DRX mechanism in this work, however, the isolated equiaxed grains in initial grains and large cumulative misorientation along the interior of the deformed grains were also found at the high temperatures and low strain rates, which demonstrated that the continuous DRX (CDRX) occurred.

The microstructure evolution and tensile properties of Ti–43Al–4Nb–1Mo-0.2B alloy during hot rolling

Ti–43Al–4Nb–1Mo-0.2B (at.%) (TNM) alloy sheets were fabricated, and the corresponding microstructure developed from lamellar to duplex structure with the rolling reduction increased from 50% to 72.5%. Interestingly, a novel Z-type lamellar structure developed at 67.5% deformation strain, and the formation process was discussed by calculating the Schmid factor. Moreover, α2+γ→βo phase transformation occurred at the kink band boundaries. Subsequently, as the reduction increased to 72.5%, the coarse βo phase decomposed and transformed to α2+γ phases due to the distortion energy stored after multi-pass rolling. Finally, the room-temperature tensile properties of the TNM sheets were tested. With the comparison of the as-forged alloy, the strength and ductility increased significantly by hot rolling, and the ultimate tensile strength reached 880 MPa at 60% total reduction. The relationship between the rolling process, microstructure evolution, and corresponding properties was established.

Kink-band formation in the directionally-solidified Mg/LPSO two-phase alloys

ABSTRACT The variation in the mechanical properties with the volume fraction of the long-period stacking ordered (LPSO) phase in directionally solidified (DS) Mg/LPSO two-phase alloys was examined. Unexpectedly, the yield stress of the DS alloys increases non-monotonically with an increase in the volume fraction of the LPSO phase. The LPSO phase is considered an effective strengthening phase in Mg alloys, when the stress is applied parallel to the growth direction. Nevertheless, the highest strength was obtained in alloys with 61–86 vol.% of the LPSO phase, which was considerably higher than that in the LPSO single-phase alloy. It was clarified that this complicated variation in the yield stress was generated from the change in the formation stress of kink bands, which varied with the thickness of the LPSO-phase grains. Furthermore, the coexistence of Mg in the LPSO phase alloy induced the homogeneous formation of kink bands in the alloys, leading to the enhancement of the ‘kink-band strengthening’. The results demonstrated that microstructural control is significantly important in Mg/LPSO two-phase alloys, in which both phases exhibit strong plastic anisotropy, to realize the maximum mechanical properties.

Compression properties of unidirectional over-braided multilayer composites

A unidirectional (UD) braided structure was developed to improve the in-plane mechanical performance of the two-dimensional (2D) braided composites. The basic compression properties of UD braided composites need to be studied to broaden their application. In UD over-braided multilayer composites (UDMLCs), the braid angle gradually increases from the first layer to the last one. This work found that a slight range variation (2°–5°) in the mean braid angle (calculated by averaging the braid angles of all layers) of UDMLCs had a significant effect on the compression modulus and little effect on the compression strength. The reinforcement (braided) yarn off-axis orientation introduced shear under compression load. The gradual increase of the braid angle from the first to the last layer created a load gradient in the thickness direction. These helped to reveal the formation mechanism of the kink band which was the key reason leading to catastrophic compression failure. Compared with 2D over-braided multilayer composites (2DMLCs), the compression modulus and strength of UDMLCs increased by 9.08% and 15.84%, respectively. However, the interweaving of reinforcement yarns and the nesting between layers in 2DMLCs made their integrity better than that of UDMLCs. Meanwhile, the interwoven structure of 2DMLCs also made their structural stability better than that of UDMLCs.

Impact damage assessment of sandwich structures with CFRP face sheets reinforced by CNTs in thermal environment

The damage behaviour of honeycomb sandwich structures with carbon nanotube (CNT) reinforced composite face sheets under high velocity impact at elevated temperatures is investigated in the present study. In order to endure the out-of-plane loading events such as bird hit, runway debris and hail stone impact, CNTs are introduced into the matrix system with the intention of improving the damage resistance. CNT reinforced carbon-epoxy composites with various weight percentages such as 0.2, 0.4, 0.6 and 0.8 were manufactured as quasi-isotropic laminates. Sandwich structures were prepared and tested under high velocity impact at room temperature and elevated temperature (393 K) using a single stage gas gun. Macro structural observations were performed on the perforated specimens and the damage area, depth, central deflection and bulging height were measured. At room temperature, the impact damage pattern was characterized by matrix cracking, fiber breakage, fiber debris, fiber push away, fiber peel up and push delamination. Plastic deformations of the matrix and kink band formation are the significant damage modes in the thermal environment. Adding a higher proportion of nanoscale reinforcement is not always judicious to get improved impact resistance; rather, the temperature dependency must be considered when designing the CFRP composite for structural applications.

Remarkably high fracture toughness of HfNbTaTiZr refractory high-entropy alloy

• The fracture behavior of the Hfnbtatizr refractory HEA was investigated. • The HEA exhibits a remarkably high fracture toughness of K JIC = 211 MPa•m 1/2 . • Fully-ductile fracture mode with the formation of kink bands was observed. • The excellent damage-tolerance plus high strengths in a wide temperature range endow this HEA a promising structural material. High-entropy alloys (HEAs) with face-centered cubic (FCC) phase such as CoCrFeMnNi or CoCrNi generally exhibit ultra-high fracture toughness but relatively low strength. In contrast, body-centered cubic (BCC) HEAs often display higher strengths, but the few reports on fracture toughness measurement demonstrate low toughness values. Here we show that the BCC HfNbTaTiZr refractory HEA, which possesses a combination of high strength, good tensile ductility and excellent high-temperature properties, also exhibits a remarkably high fracture toughness. By using the "single specimen" compliance method for J -integral measurement according to the ASTM E1820–17 standard, its fracture toughness K JIC was experimentally determined to be 210 MPa m 1/2 , which renders this HEA among the toughest metallic materials. The excellent damage tolerance makes the HEA promising for applications as high-temperature structural materials such as in aerospace field.

Effect of LPSO orientation on the compression behavior and workability of the Mg-5Gd-3Y–1Zn-0.5Zr alloy

To investigate the effect of LPSO orientation on the compression behavior and workability of Mg alloys, a Mg-5.3Gd-3.3Y-1.1Zn-0.5Zr (wt%) alloy was extruded to obtain the bimodal microstructure with a certain amount of directionally arranged LPSO phases. The subsequent compression tests indicate that the values of activation energy of deformation (Q) are approaching (180–194 kJ/mol) along different orientations (ED, 45°, and TD), which are much lower compared with the as-homogenized Mg-Gd-Y-Zn-Zr alloys due to the sufficient initial grain boundaries (I-GBs). Enhanced kinking and DRX at kink bands at the higher Zener-Hollomon (Z) parameters result in the slight decrease of Q and the movement of optimal processing domains for the ED orientation. The I-GBs overshadow the effect of LPSO orientation via kinking deformation on the DRX ratio and workability. The increase of Z parameter tends to reduce the DRXed grain size and the homogeneity of both the microstructure and hardness. Different α-Mg and LPSO orientations of the extruded alloy induce the anisotropy of peak flow stress (σp) during compression and the extraordinary elliptical shape of the compressed TD specimens. This work provides useful information for the industrial production of wrought LPSO-containing Mg alloys.

Experimental investigation on high-velocity impact damage and compression after impact behavior of 2D and 3D textile composites

This paper investigates the impact resistance and damage tolerance of 2D triaxially braided composites and 3D angle-interlock woven composites panels using high-velocity impact tests and compression after impact tests. Both materials present equivalent impact resistance with similar ballistic limit and energy absorption ratio. Nevertheless, the damage patterns of the two materials are quite different, among which, 2DTBC is characterized by global response and yarn debonding, while 3DAWC is characterized by localized damage without yarn debonding. However, in compression after impact tests, 3DAWC exhibits higher damage tolerance because the binder yarns resist buckling failure and kink bands formation during compression, resulting in higher residual compressive strength. In addition, how the damage morphology varies spatially due to the different deformation and damage modes caused by high-velocity impact is learned from X-ray CT scanning. Different from the relationship between impact energy and residual compressive strength pointed out by most research, this study also reveals a unique trend of residual compressive strength as a function of impact energy, considering both penetration and rebound conditions.

Conjugate, Riedel-shear vein arrays: A new type of en echelon vein system

This paper presents the geometric analysis of sigmoidal en echelon vein system that formed by non-coaxial deformation within conjugate Riedel-shear zones. Both sinistral S-type and dextral Z-type vein arrays, which have a subparallel enveloping surface, are observed. Individual asymmetric quartz veins consist of (1) a central, thin slickenfibre shear vein that is separating (2) pennant-shaped extensional vein tips characterised by strong dilation. Our observations closely resemble the results from published analogue experiments, which suggest that dilation occurs along R and R′ Riedel shear fractures during progressive rotation and slip within brittle-ductile shear zones. The shear vein formed by incremental pull-apart in the extensional step of overlapping R′ shear fractures, as marked by the presence of solid inclusion trails and bands. The pennant veins opened in the obtuse angle between intersecting R’ and R shears. The close association with a kinked pre-existing tectonic foliation, which acted as a passive marker of non-coaxial deformation, allows a kinematic restoration of the initial conjugate shear zone geometry that formed under approximate NE-SW compression. This model could warrant the reassessment of the classic interpretation of sigmoidal en echelon vein systems that are associated with kink bands.

Simulation of crystal plasticity in irradiated metals: A case study on Zircaloy-4

A classical crystal plasticity formulation based on dislocation slip was extended to include the mechanisms of dislocation channelling, with associated strain softening which is observed in many alloys post irradiation. The performance of the model was evaluated against experimental data on Zircaloy-4, which included engineering stress-strain response and high-resolution digital image correlation strain mapping. Variants of the model were developed to evaluate the influence on the strain hardening law used, comparing hardening based on a linear relationship for effective plastic strain with that based on the evolution of geometrically necessary dislocations. In addition, governing equations for simulating the interaction between gliding dislocations with various types of irradiation defect were investigated; this included the comparison of isotropic and anisotropic interactions based on the resultant reaction segments for each interaction. It was shown that the engineering stress-strain response measured by experiment could be captured by many of the model variants, but the simulation of characteristic strain heterogeneity was more sensitive to the model used. Direct modelling of the HRDIC experiments indicated that the model successfully predicted the activation of slip systems in many cases and exhibited localised strain distribution as observed in the experiment. In all models localised kink band formation was predicted, which is not observed experimentally, which highlights limitations in modelling of softening materials with a local crystal plasticity approach and a required area of development going forward.

Microstructural factors governing the significant strengthening of Al/Al2Cu mille-feuille structured alloys accompanied by kink-band formation

Development of high-strength Al alloys that can be used at temperatures above 150 °C is strongly desired. To achieve this, Al/Al2Cu mille-feuille structured alloys, in which soft (Al) layer and hard (Al2Cu) layer are alternatively stacked in lamellar form, is recently focused, via inducing kink-band formation as a novel strategy for controlling the mechanical properties. However, the localized formation of large kink bands reduces the yield stress and degrades the ductility of the alloy. There was little information to derive a "general law" for quantitatively predicting an increase in yield stress in mille-feuille structured alloys from a microstructure perspective. In this study, we successfully distinguished the effects of lamellar thickness and colony size on the strength. The results demonstrated that the yield stress governed by kink-band formation is less related to the lamellar thickness in the Al/Al2Cu alloys but it highly depends on the colony size. The decrease in lamellar colony size obtained by increasing the growth rate during the directional solidification of the Al/Al2Cu eutectic alloy can induce the homogeneous formation of tiny kink bands in the alloy, leading to a drastic increment of the yield stress accompanied by ductility. In addition, the formation of tiny kink bands was demonstrated to contribute to tensile deformation. That is, the control of the morphology of kink band changes its role from a fracture mode to a deformation mode. Furthermore, such microstructure control can induce “kink-band strengthening” in the alloys. These experimental results demonstrate that controlling the colony size while maintaining the alignment of lamellae is the key factor for inducing superior mechanical properties of the mille-feuille structured alloy via tiny kink-band formation.

Deformation kinking and highly localized nanocrystallization in metastable β-Ti alloys using cold forging

• Microstructural refinement involving deformation kinking was investigated. • Formation of kink band involves dislocation gliding, pre-kinking and its ripening. • Nanograins ∼15 nm in diameter are produced but highly localized in kink bands. • Localized nanocrystallization results from the intrinsic soft nature of kink bands. • The softening of kink bands originates from the inner degraded dislocation density. Deformation kinking as an uncommon plastic deformation mechanism has been reported in several materials while the relevant microstructure evolution and grain refinement behavior at a large strain remain unclear so far. In this study, the issue was systematically investigated by utilizing cold forging to impose severe plastic deformation (SPD) on Ti-11 V metastable β -Ti alloys. It is found that the formation of kink bands experiences dislocation gliding, pre-kinking and the ripening of pre-kinks in sequences. The kink bands are subsequently thickened through the coalescence of multiple kink bands in a manner of high accommodation. Ordinary dislocation slip is developed as a dominant deformation mechanism when deformation kinking is exhausted. The resulting grain refinement involves transverse breakdown and longitudinal splitting of dislocation walls and cells, which fragment kink bands into small β -blocks. Further refinement of the β -blocks is still governed by dislocation activities, and finally nanograins with a diameter of ∼15 nm are produced at a large strain of 1.2. Alternatively, it is revealed that nanocrystallization is highly localized inside kink bands while the outer microstructure maintains original coarse structures. Such localized refinement characterization is ascribed to the intrinsic soft nature of kink bands, shown as low hardness in nanoindentation testing. The intrinsic softening of kink bands is uncovered to originate from the inner degraded dislocation density evidenced by both experimental measurement and theoretical calculation. These findings enrich fundamental understanding of deformation kinking, and shed some light on exploring the deformation accommodation mechanisms for metal materials at large strains.

Microstructure and mechanical properties of directionally ordered porous Ti3SiC2 prepared using freeze casting

Directionally ordered porous Ti3SiC2 embodied in lamellar porous and scaffold structures were prepared using freeze casting. The maximum compression stress was higher than that with a disordered structure, and increased from 22 MPa to 89 MPa as the solid content increased from 15 to 25 vol%. The elastic deformation exhibited an opposite trend. The Young’s modulus of the directionally ordered porous Ti3SiC2 was nearly linear and obeyed the parallel model (E=122–147δ). The deviation from the theoretical value was due to the difference between the real scaffold structure and that of the theoretical model. Cyclic loading of the porous Ti3SiC2 at the elastic stage indicated that the energy dissipation (Wd) per loop increased from 0.0009 to 0.0139 MJ/m3 with a porosity increase. The fracture failure behaviours were mainly splitting and fragmentation. The absorbed-in-fracture energy increase mechanisms during the irreversible deformation of the plastic stage included kink bands, crack extension, and delamination.

Formation of Millefeuille Structure in Titanium-based Alloys and its Development to the Introduction of Kink Bands

Formation of Millefeuille Structure in Titanium-based Alloys and its Development to the Introduction of Kink Bands

Crystal Plasticity Analysis for Kink Band Formation and Kink Strengthening

Crystal Plasticity Analysis for Kink Band Formation and Kink Strengthening