Shear Band Deformation Research Articles

Deformation-induced phase transition and nanotwins in SS 304 steel during cryogenic laser shock peening without coating

The influence of Laser shock peening without coating at a cryogenic temperature (CLPwC) on SS 304 alloy was investigated. Three different laser energies, such as 150 mJ, 300 mJ, and 450 mJ were chosen to investigate the effects of CLPwC in SS 304 steel. The impact of CLPwC on the residual stress, surface roughness, hardness, phase transformation and microstructural changes was studied. The XRD studies show the peak broadening and strain-induced martensite (γ→α′) of 36.3% after CLPwC at 9 GW cm−2. The work-hardening effect was observed up to 300–400 μm depth from the peened surface. The microhardness could effectively increase to 309.43 HV at 9 GW cm−2 when it compared to unpeened hardness (192.1 HV). Relaxation of compressive residual stress at cryogenic temperature was distinguished, and the maximum compressive stress of −138 MPa was obtained at 50 μm from the surface. Microstructural studies were carried out by different techniques such as EBSD and TEM. An increase in low angle grain boundary fraction (43%) and grain size reduction was observed using EBSD after CLPwC. The TEM investigations revealed deformation twins (width of 6–70 nm), shear bands, stacking faults, and martensitic phase. Strain induced martensitic phase is witnessed in XRD, EBSD, TEM studies.

Evolution of local densities during shear banding in Zr-based metallic glass micropillars

Shear bands (SBs) play an important role in plastic deformation of metallic glasses (MGs), but the evolving mechanism of SBs is still elusive. We study the structural formation and evolution of SBs inside MGs using synchrotron X-ray nano-computed tomography with the combination of high-resolution transmission electron microscopy. It is found that discrete anisotropic low-density regions, as manifestation of the activations of accumulated shear transformation zones, are created in the elastic deformation regime, which form SBs of density fluctuations under large plastic deformation. With further increasing plastic strain, the thickness of SBs increases from ∼ 105 nm to ∼ 180 nm, and their local densities decrease. Before the formation of cracks, the low-density regions in SBs gradually evolve into nanovoids with the matrix around densified. Compared to the unaffected matrix, densified regions with more crystal-like order (CLO) structure appear around thick SBs of severe bond breaking, and the densification is more apparent near crack surface with CLO structure further increasing. The results are significant for understanding the structural evolution during shear banding and plastic deformation mechanism for glassy materials.

Shock compression and spallation of a medium-entropy alloy Fe40Mn20Cr20Ni20

Shock compression and spallation of a low-cost cobalt-free medium-entropy alloy (MEA), Fe40Mn20Cr20Ni20 (at%), with a face-centered-cubic structure are investigated via plate impact experiments, to reveal its dynamic mechanical properties and corresponding microscopic deformation/damage mechanisms. The Hugoniot equation of state, yield strength, spall strength and pullback rate are obtained from free-surface velocity histories. Post-deformation samples are characterized with transmission electron microscopy, scanning electron microscopy and electron backscatter diffraction. The Fe40Mn20Cr20Ni20 MEA exhibits a good balance in spall strength and ductility (low pullback rate) at a relatively low cost, compared to several other types of medium/high entropy alloys and steels. At sufficiently high impact velocity (e.g., 500 ms−1 here), nanoscale deformation twinning becomes a key deformation mechanism for Fe40Mn20Cr20Ni20 MEA in addition to dislocation slip. During spallation, voids (i.e., damage) nucleate preferentially at grain boundary triple junctions, and grow isotropically with increasing loading. Owing to fine grains and strong plastic deformation capability of the Fe40Mn20Cr20Ni20 MEA, void coalescence is accomplished by intragranular shear deformation bands and cracks, which contributes to its high ductility.

Shear banding-induced 〈c+a〉 slip enables unprecedented strength-ductility combination of laminated metallic composites

Shear bands in metallic materials have been reported to be catastrophic because they normally lead to non-uniform plastic deformation. Ductility of laminated metallic composites deteriorates with increasing processing strain, particularly for those having hexagonal-close-packed (hcp) constituents due to inadequate slip systems and consequently prominent shear banding. Here, we propose a design strategy that counterintuitively tolerates the bands with localized strains, i.e. the shear banded laminar (SBL) structure, which promotes 〈c+a〉 dislocation activation in hcp metals and renders unprecedented strength-ductility combination in hcp-metal-based composites fabricated by accumulative roll bonding (ARB). The SBL structure is characterized with one soft hcp metal constrained by adjacent hard metal in which dislocations have been accumulated near the bimetal interfaces. High-energy X-ray diffraction astonishingly reveals that more than 90% of dislocations are non-basal in Ti layers of the SBL Ti/Nb composite processed by eight ARB cycles. Moreover, 〈c+a〉 dislocations occupy a high fraction of ∼30%, promoting further 〈c+a〉 cross slip. The unique stress field tailored by both shear banding and heterophase interface-mediated deformation accommodation triggers important 〈c+a〉 slip. This SBL design is of significance for developing hcp-based laminates and other heterostructured materials with high performances.

Microstructure evolution and mechanical properties of severely deformed TA15 alloy by multi‐directional forging and annealing

AbstractMulti‐directional forging technology, as a representative severe plastic deformation technology, is urgent to be developed because it has strong microstructure refinement and performance improvement effects. In this paper, multi‐directional forging experiments of TA15 titanium alloy with different passes were carried out at 700 °C using unrestricted multi‐directional forging die structure. Then the samples of TA15 titanium alloy after multi‐directional forging deformation were subjected to a high‐temperature vacuum annealing treatment. The test results reveal TA15 alloy was effectively refined through multi‐directional forging without any cracking with the increasing of deformation passes. The mechanism of grain refinement during multi‐directional forging included dynamic recrystallization, grain crushing and adiabatic shear deformation bands refinement. The amount and grain size of recrystallization grains increased when the annealing time increased from 1 hour to 4 hours. Supplemented by annealing, the adiabatic shear deformation bands microstructure with the mixed microstructure of coarse αp and fine αs were obtained and the hardness after multi‐directional forging and annealing was investigated. The yield strength and ultimate tensile strength increased by 26.1 % and 25.5 % respectively when the deformation pass increased to 3 passes compared with the initial specimen. The present work revealed that multi‐directional forging deformation combined with appropriate annealing could represent an efficient route to improve the microstructure and mechanical property of TA15 alloy.

Effects of deep cold rolling on the evolution of microstructure, microtexture, and mechanical properties of 2507 duplex stainless steel

A commercial ferritic and austenitic duplex stainless steel was severely cold rolled at room temperature from 50% to 87.5% thickness reduction. The microstructure and microtexture evolutions of the cold-rolled steel were investigated by scanning electron microscopy, X-ray diffraction, transmission electron microscopy and electron backscattered diffraction. The results revealed that both constituent phases were severely deformed and morphologically elongated along the rolling direction. The plastic strain was preferentially partitioned into austenite phase during cold rolling due to the involvement of multiple deformation modes. As the deformation strain increased, the twin-matrix lamellae gradually rotated to coincide with the rolling plane, after which the localized shear deformation contributed to the formation of “eye” shaped domains composed of nanotwins/fragmented fine structures. After 87.5% cold rolling, austenite phase was constituted by Brass {110}<112>, Goss {110}<001>, and S {123}<634> textures, and the formation of both deformation twins and shear bands leaded to a continuous texture transition from the Copper {112}<111> to the near-Goss and Brass orientations. A strong α-fiber (<110>//RD) accompanied with weak γ-fiber (<111>//ND) texture was dominated in ferrite. Cold rolling significantly improved the strength of duplex stainless steel at the sacrifice of ductility. With the increasing of deformation strain, the fracture manner of the material changed from a mixture of ductile and brittle fracture to typical brittle fracture.

A constitutive model for metallic glasses based on two-temperature nonequilibrium thermodynamics

We develop a new finite deformation constitutive model for metallic glass within the framework of irreversible nonequilibrium thermodynamics. To consider the intrinsically out-of-equilibrium characteristics of metallic glass, its total internal energy is divided into two weakly coupled configurational and kinetic subsystems, and configurational temperature coupling with configurational degrees of freedom is introduced as a thermodynamic state variable to characterize the evolution of the disordered structure. Furthermore, the classical shear transformation zone theory is extended by reasonably considering the reverse shear transformation as a form of the relaxation of the strain field, which is stored in the elastic matrix and produced by the constraint of the matrix on shear transformation. With the help of the finite element implementation for the new model, the effectiveness of the proposed model is validated by comparing the modeling stress–strain responses of the macroscopic deformations under different temperatures and strain rates with the experimental results, and the utility of the model for predicting the shear banding behavior of metallic glasses is examined as well. The results therefore show that the constructed constitutive model can not only effectively predict the deformations of metallic glass under different ambient temperatures and applied strain rates, but also reasonably explain the mechanisms of deformation mode evolution and shear band formation.

Unveiling the mechanical response and accommodation mechanism of pre-rolled AZ31 magnesium alloy under high-speed impact loading

Split Hopkinson pressure bar (SHPB) tests were conducted on pre-rolled AZ31 magnesium alloy at 150–350 ℃ with strain rates of 2150s -1 , 3430s -1 and 4160s -1 . The mechanical response, microstructural evolution and accommodation mechanism of the pre-rolled AZ31 magnesium alloy under high-speed impact loading were investigated. The twin and shear band are prevailing at low temperature, and the coexistence of twins and recrystallized grains is the dominant microstructure at medium temperature, while at high temperature, dynamic recrystallization (DRX) is almost complete. The increment of temperature reduces the critical condition difference between twinning and DRX, and the recrystallized temperature decreases with increasing strain rate. The mechanical response is related to the competition among the shear band strengthen, the twin strengthen and the fine grain strengthen and determined by the prevailing grain structure. The fine grain strengthen could compensate soften caused by the temperature increase and the reduction of twin and shear band. During high-speed deformation, different twin variants, introduced by pre-rolling, induce different deformation mechanism to accommodate plastic deformation and are in favor for non-basal slip. At low temperature, the high-speed deformation is achieved by twinning, dislocation slip and the following deformation shear band at different deformation stages. At high temperature, the high-speed deformation is realized by twinning and dislocation slip of early deformation stage, transition shear band of medium deformation stage and DRX of final deformation stage.

Interface dominated deformation transition from inhomogeneous to apparent homogeneous mode in amorphous/amorphous nanolaminates

• A length-scale-dependent mechanical properties were observed in the A/ANLs. • The hardness of A/ANLs was closely related with their deformation mode. • The deformation mode of A/ANLs was determined by the interface. The amorphous/amorphous nanolaminates (A/ANLs) have aroused great attentions owing to their tunable structure and enhanced mechanical properties. However, the plastic deformation mechanism of A/ANLs have yet been clarified. Here, we systematically examined the mechanical properties and deformation behavior of series of NiNb/ZrCuNiAl A/ANLs via nanoindentaion test. It was found that both the amount and morphology of amorphous/amorphous interface (A/AIs) played crucial roles in the plastic deformation of A/ANLs. Less and straighter A/AIs facilitated multiple shear banding deformation, of which the hardness increased with decreasing layer thickness, as the A/AIs hindered the propagation of shear bands (SBs). Whilst, more and wavier A/AIs promoted homogeneous deformation, of which the hardness stayed at a much lower value and was relatively irrelevant with the layer thickness, for the promoted activation of shear transformation zones by A/AIs. Our results provide guidance for modifying the mechanical properties of amorphous alloys with interface engineering design.

Microstructure and texture of severely warm-rolled and annealed coarse-grained CoCrNi medium entropy alloy (MEA): A perspective on the initial grain size effect

Microstructure and texture formations were investigated in a coarse-grained starting material (average grain size ~100 µm) of equiatomic CoCrNi MEA after severe warm-rolling and annealing. The results were compared with a fine-grained starting material (FGSM with average grain size ~7 µm) to highlight the effect of starting grain size. Progressive microstructural evolution accompanied by deformation bands, shear bands, and nano-twins formation resulted in an ultrafine microstructure in the heavily warm-rolled CGSM. The hardness of CGSM after 90% warm-rolling was similar to FGSM, indicating a rather comparable ultrafine microstructure. The deformation texture of the CGSM was rather similar to the FGSM featured by a weak brass component. Remarkably, the annealed grain sizes of the CGSM and FGSM series of specimens were rather similar. These observations amply corroborated with comparable deformation microstructure in the two materials. Both CGSM and FGSM showed weak recrystallization texture featured by the retention of deformation texture components. The results indicated rather limited effect of the starting grain size on microstructure and texture formation in warm-rolled and annealed CoCrNi MEA. Further, a remarkable strength-ductility balance could be obtained in annealed CGSM due to ultrafine microstructure and solid solution strengthening.

Infrared thermographic assessment of heat release phenomena in steel parts subjected to quasi-static deformation

The paper describes the mechanism of heat release during deformation and main features of passive infrared thermographic diagnostics of steel parts subjected to elastic, elastic–plastic and plastic deformation under quasi-static loading. A basic equation for evaluating temperature signals caused by quasi-static deformation is derived. It is stated that the evaluation of steel structures by passive thermal methods should be performed when the heat is generated by mechanical stresses appearing in structures subjected to mechanical loading. The analysis of the loading kinetics and temperature anomalies has shown that thermal nondestructive testing technique is a useful tool for determining stress state in steel structures during the initiation and evolution of fracture. It is shown that the quasi-static deformation of steel structures is accompanied by the formation of elastic clusters separated by shear bands, and the plastic deformation in shear bands causes heat release resulting in a temperature increase of 4–12.5 °C.

An Insight into Amorphous Shear Band in Magnetorheological Solid by Atomic Force Microscope.

Micro mechanism consideration is critical for gaining a thorough understanding of amorphous shear band behavior in magnetorheological (MR) solids, particularly those with viscoelastic matrices. Heretofore, the characteristics of shear bands in terms of formation, physical evolution, and response to stress distribution at the localized region have gone largely unnoticed and unexplored. Notwithstanding these limitations, atomic force microscopy (AFM) has been used to explore the nature of shear band deformation in MR materials during stress relaxation. Stress relaxation at a constant low strain of 0.01% and an oscillatory shear of defined test duration played a major role in the creation of the shear band. In this analysis, the localized area of the study defined shear bands as varying in size and dominantly deformed in the matrix with no evidence of inhibition by embedded carbonyl iron particles (CIPs). The association between the shear band and the adjacent zone was further studied using in-phase imaging of AFM tapping mode and demonstrated the presence of localized affected zone around the shear band. Taken together, the results provide important insights into the proposed shear band deformation zone (SBDZ). This study sheds a contemporary light on the contentious issue of amorphous shear band deformation behavior and makes several contributions to the current literature.

Microstructure and Texture Evolution During Cold Rolling of 316L Stainless Steel

The evolution of the deformation microstructure and the local crystallographic orientations are investigated for 10, 30, 50, and 80 pct cold rolled 316L austenitic stainless steel strips and the results are related to the overall macro-texture evolution. The microstructures are characterized by scanning electron microscopy (SEM), including electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). The macro-textures are characterized by X-ray diffraction (XRD) measurements. It is found that the deformation leads to both slip and twinning followed by martensite transformation and shear banding. The deformation twinning occurs on planes with the highest twinning Schmid factors and shows a strong orientation dependence, in the sense deformation twinning occurs preferentially in grains with near Copper orientation rather than Brass orientation. It is furthermore found that the formation of both deformation twins and shear bands have a significant effect on the texture evolution. The correlations between microstructure, local crystallographic orientations and macro-textures are discussed.

Penetration Damaging Behavior of ATI425 Titanium Alloy

The penetration damaging behavior of ATI425 titanium alloy was studied by 7.62 mm diameter armor piercing projectiles. The damage characteristics and the mechanism were analyzed by observing and analyzing the craters of ATI425 titanium alloy target. It can be found that local temperature-rise of the target plate occurred, even sputtering phenomenon in the opening stage. The shear bands extended upward along the cater wall could be seen in the stable stage. The large non-homogeneous deformation in adiabatic shear bands caused microcracks and micropores. A large number of macro-cracks were observed on the side wall and at the bottom of the crater.

Study on the effects of transverse ultrasonic vibration on deformation mechanism and mechanical properties of riveted lap joints

In this paper, a new and special transverse ultrasonic vibration-assisted riveting (TUVR) system is developed to improve the plasticity and qualification of titanium alloy rivets. Comparing with conventional riveting (CR), the formed driven heads by TUVR are not only suffered from the riveting force and hole boundary constrain, but also the acoustic softening and dragging friction force. Consequently, to disclose the deformation mechanism and mechanical properties in driven head, a series of studies were carried out. First, the dimensions of driven head for different amplitudes and roughness were investigated. On this basis, the typical position microstructures in driven head of TUVR and CR were contrastively analyzed. Moreover, the interference sizes of laminates riveted lap joints under CR and TUVR were compared. Finally, the micro-hardness and pull-out strength in joints of two riveting processes were tested, and illustrating the mode and microstructure of failure. Results show that ultrasonic vibration can act as a new and effective auxiliary riveting method, which contributes to achieving plasticity improvement and higher mechanical properties of joints. Although the TUVR has significant effects on the deviation of the driven head, which can be restrained well with a reasonable amplitude and roughness. The grains deformation in typical positions of TUVR is much severer than CR, except in the lower shear band and difficult deformation area. Besides, the uniformity of interference sizes of riveted sample is well improved by TUVR. The experimental results show that the micro-hardness and yield load of TUVR riveted lap joints increase more than 10% and 4%, respectively.

Evaluation of the interface stability of Al/AlN multilayered composites under thermal stress

Abstract Al/AlN multilayered composites with different individual layer thicknesses (25, 250 nm) on Si substrate were fabricated by magnetron sputtering and subjected on thermal cycle testing from room temperature up to 500 °C and then tested under nanoindentation. There is a significant layer thickness effect on the interface damage behavior of the Al/AlN multilayer: when the individual layer thickness is 250 nm, it was found that the hillocks damage on the surface of the multilayer, while the damage behavior was dominated by the interface delamination and fracture between ceramic layers and metallic layers with smaller individual layer thickness (25 nm). The deformation behavior transferred from inhomogeneous shear band deformation (severe thinning and thickening of layer thickness caused by dislocation pile-ups in Al layer) at submicron scale to homogeneous shear bands deformation (heterogeneous interface instability) caused by the grain boundary sliding and the grain rotation in Al layers.

Anisotropic strengthening of nanotwin bundles in heterogeneous nanostructured Cu: Effect of deformation compatibility

Anisotropic tensile behaviors of a heterogeneous nanostructured Cu composed of isotropic nanograin matrix and anisotropic nanotwin bundles were investigated under loading directions parallel, normal, and 45° inclined to the twin boundaries (TBs). Distinct from the anisotropic strengthening effect in the homogeneous nanotwinned (NT) counterpart, the heterostructure exhibits the highest strength under parallel tension, and moderate strength under normal tension. High-resolution digital image correlation analysis indicated an orientation-dependent deformation compatibility between the nanotwin bundles and nanograin matrix, i.e., compatible deformation in the parallel orientation; but significant incompatibility in orientations both normal and 45° inclined to TBs. The results reveal that the strengthening effect of nanotwins in heterogeneous nanostructure is not only dependent on the strength of themselves, but also influenced by the deformation compatibility with surrounding components. The orientation-dependent deformation compatibility was attributed to the interactions between isotropic shear bands in nanograin matrix and anisotropic deformation in nanotwin bundles.

Strain rate-dependent hardness and deformation behavior in the nanocrystalline/amorphous Ti2AlNb film

Abstract Building nanocrystalline/amorphous biphase nanostructure has recently emerged as a new strengthening-toughening route that can combine the strengthening benefits of nanocrystallinity and amorphization. Therefore, it is required exploring on the strain rate-dependent deformation behavior with above different nanostructure in certain system. Herein, three typical nanostructures: polycrystalline, uniform nanocrystalline/amorphous core-shell nanostructure and amorphous matrix with nanoclusters, have been achieved in the sputtered Ti2AlNb films by individually regulating the sputtering bias voltage (Ubias). Note that a high Ubias can promote formation of Nb–rich amorphous tissues via Al preferential resputtering and Nb segregation. Moreover, microstructural evolution and strain rate-dependent hardness and deformation behavior were further explored. Firstly, at a low Ubias (−40 V), Ti2AlNb film exhibited polycrystalline character; it yielded a relatively low hardness (~10.0 GPa) but a high strain sensitivity coefficient of 0.1505. Subsequently, the novel nanocrystalline/amorphous core-shell nanostructure consisting of Ti2AlNb nanocrystalline cores uniformly encapsulated by thin amorphous shells was achieved at a higher Ubias (−120 V); this nanostructure provided a remarkable hardness enhancement (~15.2 GPa) without sacrificing its ductility and intermediate strain sensitivity coefficient of 0.1382. Finally, a transition to amorphous matrix with nanoclusters occurred with further increasing Ubias to −200 V, thus yielding slight decrease in hardness (~12.5 GPa) and a minimum strain sensitivity coefficient of 0.0915 when shear-band deformation was activated.

Micro-Compression Characterization and Thermal Stability of Electrolessly Plated Nickel Phosphorus Alloy

The micro-compression characteristic of an electrolessly plated Ni-P alloy was evaluated in this study. Specimens used in the micro-compression test were micro-pillars having a square cross-section fabricated by focused ion beam system. One side of the square cross-section was varied from 5 to 20 μm and the aspect ratio of the micro-pillars was kept at 2:1 to evaluate the sample size effect on the mechanical property. The as-plated Ni-P alloy was amorphous. The sample size effect commonly observed in mechanical properties of metallic micro-specimens was not obvious for micro-pillars fabricated from the as-plated Ni-P alloy, and all of the micro-pillars showed shear band deformation with the 0.2% yield strength at roughly 2.2 GPa. Heat treatment was conducted to study the influence on the sample size effect. After heat treatment at 300 °C for 2 h, an increase in the strength was observed in micro-pillars with all sizes. In addition, for the heat-treated specimens, the fracture strength increased from 3.1 to 4.7 GPa when cross-section of the micro-pillar decreased from 20 × 20 to 10 × 10 μm2, which revealed the sample size effect.

On the dependence of crack surface morphology and energy dissipation on microstructure in ductile plate tearing

The two distinct tearing mechanisms observed in ductile metal plates are the void-by-void advance of the crack tip, and the simultaneous interaction of multiple voids on the plane ahead of the crack tip. Void-by-void crack advance, which leads to a cup-cup crack surface morphology, is the dominant mechanism if the plate contains a low number of small void nucleation sites (i.e., second phase particles). Conversely, a large number and/or size of nucleation sites trigger the simultaneous interaction of multiple voids resulting in a slanted crack. The present work aims to provide further insight into the parameters controlling the mechanisms and energy dissipation of plate tearing by focusing on the shape and, thereby, the orientation of the nucleation sites. The study uses a two-dimensional plane strain finite element domain to model the cross section of a plate, subject to mode I tearing, with discretely modeled, randomly distributed, finite-sized elliptic void nucleation sites. The developed finite element setup can capture the dependence of the crack surface morphology on the microstructure of the plate. The simulation results confirm that cup-cup crack propagation develops by intense plastic straining throughout the thinning region of the plate. Conversely, slanted and cup-cone cracks propagate in thin localized shear deformation bands. The energy dissipation is, therefore, greater for cup-cup cracks. The study demonstrates that the damage-related microstructure has a significant role in determining the overall hardening capacity of a plate, which in turn dictates the tearing mode and energy.