Shear Band Deformation Research Articles

Altering strength and plastic deformation behavior via alloying and laminated structure in nanocrystalline metals

Nanoindentation and electron microscope techniques have been performed on sputtering deposited monolayered nanocrystalline CuNb and multilayered CuNb/Cu thin films. Microstructural features, hardness and surface morphologies of residual indentation have been evaluated to identify the effects of alloying and laminated structure on strength and plastic deformation behavior of nanocrystalline metals. By altering the content of Nb in CuNb alloy and adding crystalline Cu layers into CuNb alloy, the volume fraction of amorphous phase in CuNb alloy and interface structures changed dramatically, resulting in various trends that are related to hardness, indentation induced pileup and shear banding deformation. Based on the experimental results, the dominant deformation mechanisms of the CuNb and CuNb/Cu thin films with various Nb contents were proposed and extended to be discussed.

Hot Workability and Processing Maps of 7150 Aluminum Alloys with Zr and V Additions

The hot workability and processing maps of 7150 aluminum alloys with different Zr additions (0-0.15 wt.%) and V additions (0.01-0.15 wt.%) were investigated using uniaxial compression tests conducted at various temperatures (300-450 °C) and strain rates (0.001-10 s−1). The results reveal that the processing map of the 7150 base alloy exhibits a single domain (Domain I) associated with dynamic recovery and partially dynamic recrystallization. With the increasing Zr and V additions, Domain I shrinks toward higher temperatures and higher strain rates and exhibits decreases in efficiency of power dissipation due to a restrained level of dynamic recovery caused by the pinning effect of Al3Zr and Al21V2 dispersoids. When the added Zr and V contents reach 0.15%, another domain (Domain II) is formed, corresponding to cavity formation in the microstructure. Flow instability during hot deformation of 7150 alloys is attributed to the formation of adiabatic shear bands and deformation bands. The instability region extends toward lower strain rates when alloyed with Zr and V. The optimum hot-working parameters for those alloys are determined to be a deformation temperature of 450 °C and a strain rate of 0.01 s−1.

The hardness and related deformation mechanisms in nanoscale crystalline–amorphous multilayers

Built upon recent findings that nanocrystalline–amorphous multilayers could possess both high strength and ductility due to nano-layer-limited structures, depth-sensing nanoindentation was employed to evaluate the hardness and shear band deformation behavior of nanoscale crystalline–Cu/amorphous–CuZr (C/A) multilayers among a wide range of amorphous layer thickness with constant crystalline Cu layer thickness. By varying individual layer thickness of amorphous layers, both weakening and strengthening effects on the hardness of C/A multilayers were observed, comparing with that derived from the rule of mixture, for which discontinuity of dislocation motion played a crucial role. A critical amorphous layer thickness of 20nm was identified, above which interface–dislocation–absorption dominated plastic deformation, causing the weakening effect, below which, continuous-dislocation-impediment took over and was responsible for the strengthening effect. Mechanisms underlying the observed shear band morphologies under indentation were also extendedly discussed.

Deformation induced alloying in crystalline – metallic glass nano-composites

We study the mechanisms of deformation driven chemical mixing in a metallic nanocomposite model system. More specific, we investigate shear banding at the atomic scale in an amorphous CuZr/ crystalline Cu nanolaminate, deformed by microindentation. Three CuZr/Cu multilayer systems (100nm Cu/100nm CuZr, 50nm Cu/100nm CuZr, and 10nm Cu/100nm CuZr) are fabricated to study the effect of layer thickness on shear band formation and deformation induced alloying. The chemical and structural evolution at different strain levels are traced by atom probe tomography and transmission electron microscopy combined with nano-beam diffraction mapping. The initially pure crystalline Cu and amorphous CuZr layers chemically mix by cross-phase shear banding after reaching a critical layer thickness. The Cu inside the shear bands develops a high dislocation density and can locally undergo transition to an amorphous state when sheared and mixed. We conclude that the severe deformation in the shear bands in the amorphous layer squeeze Zr atoms into the Cu dislocation cores in the Cu layers (thickness <5nm), resulting in local chemical mixing.

Extremely-low-cycle fatigue behaviors of Cu and Cu–Al alloys: Damage mechanisms and life prediction

The extremely-low-cycle fatigue (ELCF) behaviors of pure Cu and Cu–Al alloys are comprehensively studied following the cyclic push–pull loading tests with extremely high strain amplitudes (up to ±9.5%). Compared with the common low-cycle fatigue (LCF) region, several unique features in the ELCF regime can be noticed, including the deviations of fatigue life from the Coffin–Manson law, the non-negligible proportion occupied by the cyclic hardening stage of the whole fatigue life, special microstructures formed by cyclic loading containing deformation twins, shear bands and ultra-fine grains and the transformation of fatigue cracking modes. All these characteristics indicate the existence of special interior fatigue damage mechanisms of ELCF. To help discover the new damage mechanisms under ELCF, a model of fatigue life prediction with a hysteresis energy-based criterion is proposed. Based on the analysis of the experimental and modeling results, two intrinsic factors determining the ELCF properties were concluded: the capacity of ELCF damage, and the defusing and dispersion ability of the external mechanical work. The former can be evaluated by a parameter of the model called the intrinsic fatigue toughness W0, which is related to the microstructure evolution condition, the cyclic hardening ability, the deformation homogeneity and possibly the static toughness. The latter can be represented by the damage transition exponent β, which can be enhanced by improving the planarity, reversibility and uniformity of plastic deformation, reflecting the decline in the degree of surface damage and the dispersion of fatigue cracks. For Cu–Al alloys with increasing Al content, cooperation between an increasing damage capacity and a decreasing damage accumulation rate leads to a comprehensive improvement in the ELCF properties.

Numerical analysis of strain rate sensitivity in ball indentation on cohesive powder Beds

In the shear deformation of powder beds beyond the quasi-static regime the shear stress is dependent on the strain rate. Extensive work has been reported on the rapid chute flow of large granules but the intermediate regime has not been widely addressed particularly in the case of cohesive powders. However in industrial powder processes the powder flow is often in the intermediate regime. In the present work an attempt is made to investigate the sensitivity of the stresses in an assembly of cohesive spherical particles to the strain rate in ball indentation using the Distinct Element Method. This technique has recently been proposed as a quick and easy way to assess the flowability of cohesive powders. It is shown that the hardness, deviatoric and hydrostatic stresses within a bed, subjected to ball indentation on its free surface, are dependent on the indentation strain rate. These stresses are almost constant up to a dimensionless strain rate of unity, consistent with trends from traditional methods of shear cell testing, though fluctuations begin to increase from a dimensionless strain rate of 0.5. For dimensionless strain rates greater than unity, these stresses increase, with the increase in hardness being the most substantial. These trends correlate well with those established in the literature for the Couette device. However the quantitative value of the strain rate boundary of the regimes differs, due to differences in the geometry of shear deformation bands. Nevertheless, this shows the capability of the indentation technique in capturing the dynamics of cohesive powder flow.

Evolution of shear bands into cracks in metallic glasses

In the present work, the cracks formation and propagation behavior of a ductile Zr-based bulk metallic glass with single dominant shear band deformation was investigated. From the ideal compressive load–displacement curve of the sample with single shear band deformation, the instant at which the shear band turns into cracks and the total area of cracked-regions prior to failure were analyzed. Further investigations revealed that the micro-cracks might have originated from the local planar faults or defects within the shear plane along the shear direction. The existence of such planar faults (steps) was consistent with the observations of the surfaces of unfractured and fractured samples. Based on these results, a new interpretation of how shear bands evolve into the present micro-cracks was proposed. These novel results provide a new perspective to understand the deformation behavior of metallic glasses.

Effect of large plastic deformation on microstructure and mechanical properties of a TWIP steel

The effect of cold rolling on the microstructure evolution and mechanical properties of a cold rolled Fe-0.3C-17Mn-1.5AI TWIP steel was studied. The plate samples were cold rolled with reductions of 20, 40, 60 and 80%. The structural changes were associated with the development of deformation twinning and shear bands. The average spacing between twin boundaries in the transverse section of the rolled plates decreased from ~190 to 36 nm with an increase in the rolling reduction from 20 to 40%. Upon further rolling to 80% reduction the twin spacing remained at about 30 nm. The cold rolling resulted in significant increase in strength as revealed by tensile tests at an ambient temperature. The offset yield stress approached 1440 MPa, and the ultimate tensile strength increased to 1630 MPa after rolling reduction of 80%. Such significant strengthening was attributed to the development of specific structure consisting of deformation nanotwins with high dislocation density.

Size Effect on the Deformation Characteristic and Shear Band of Granular Material Based on Hypoplastity

The deformation characteristic of granular material is important. It is important factor to analysis the bearing capacity and the thickness of granular material footings. There are many factors influence the deformation of granular material, such as material characteristic, content of water, degree of compaction, test method, etc. For analyzing size effect on the deformation characteristic of granular material, hypoplastic constitutive model considering void ratio is introduced. The influence of an initial void ratio, pressure and a mean grain diameter on the deformation is investigated by polar quantities: rotations, curvatures, couple stresses and a characteristic length. Thus, the influence of density and pressure on the initiation of shear bands can be investigated with a single set of constitutive constants.

Effect of Cold Rolling on Microstructure and Mechanical Properties of an Al-Mg-Sc-Zr Alloy

The microstructural evolution and mechanical properties of an Al-5.4Mg-0.4Mn-0.2Sc-0.09Zr alloy subjected to cold rolling with a total strain up to ~1.6 was studied using high resolution EBSD analysis and TEM. It was shown that cold rolling induces elongation of initial grains and the formation of deformation bands along rolling direction in addition to dramatic increase in density of lattice dislocations. Two types of deformation bands evolve. Deformation bands initially bounded by low-angle boundaries (LABs) with misorientation higher than 2o and spacing ranging from 0.8 to 4 μm gradually transform to lamellas delimited by high-angle boundaries (HABs). Thin deformation bands delimited by LABs with misorientation of 2o or less evolve within these coarse bands. First type of deformation bands is subdivided due to mutual intersection with second order deformation bands or shear bands to elongated crystallite evolving to micron scale grains with strain. The thin deformation bands may be also subdivided to nanoscale crystallites. It was shown that the formation of well-defined deformation bands yield very high anisotropy in strength and ductility, while a strong increase in lattice dislocation density with strain diminishes this anisotropy.

Textures of rectangular extrusions and their effects on the mechanical properties of thermo-mechanically treated, lamellar microstructure, Ti–47Al–2Cr–2Nb–0.15B

Textures of α2 and γ phases in Ti–47Al–2Cr–2Nb–0.15B (at.%) rectangular billet, extruded below its α transus temperature, were evaluated and their relation to the mechanical properties of TMTL microstructure was investigated. Strong α2 phase texture {112¯0}<101¯0> was found after extrusion and it evolved into {101¯0}<112¯0> recrystallization texture via oriented growth during α solution treatment. The α/α2 phase texture changes α grain growth behavior at Tα, such that it is slow at first but becomes rapid when the solution time exceeds a critical value, creating a new method of obtaining fine-grained TMTL microstructure. Mechanical properties of the TMTL microstructure were shown to be closely related to the Kearns texture factor of the α2 phase along the loading direction. The center and edge of as-extruded billet have distinct γ phase textures, dominated by β fiber (center) and featuring multiple texture components (edge), respectively. In the highly strained edge region, the presence of C-twin, Y, and Z texture components suggests the occurrence of mechanical twinning and shear band deformation, while B/G auxiliary component was found to act as a transition band from deformation to cubic recrystallization texture, in support of the oriented nucleation hypothesis for the formation of cubic texture.

Shear Band Creation in Amorphous Ribbon under Bending

Metallic glasses subjected to bending loads often exhibit the significant strength through the creation of multiple shear bands. Amorphous metallic glasses in the form of thin ribbon (about 40 μm in thickness) can be bent to large curvatures without the failure. The thickness of metallic glass is also an important factor that influences the exceptional properties of these materials. The shear bend formation under bending of Fe40Ni40B20 (at. %) amorphous ribbon was studied. There is highly localized plastic deformation in shear bands as a function of the curvature radius. The development of primary and secondary shear bands is related to the inhomogeneous plastic deformation that occurs at the room temperature. The total deformation increases with the decreasing the bending radius. Shear bands in the bent ribbons were observed using scanning electron microscopy.

A systematic study of grain refinement during impact of 4340 steel

Inhomogeneous plastic deformation at high strain rates and large strains results in the concentration of deformation and strain localization in adiabatic shear bands (ASBs). In the current study, AISI 4340 steel specimens were impacted at increasing strain rates/strains (impact momentums) to capture the formation of adiabatic shear bands. Advanced specimen preparation techniques using Focused Ion Beam (FIB) and extensive electron microscopy were used to identify the sequence of evolution of the shear bands. It was observed that the structure of the shear bands that evolve after strain localization starts out with elongation of the grains due to grain reorientation in the shear direction with the initiation of random and transverse dislocation boundaries along the elongated grains. For higher strain rates/strains during impact, the elongated grains break along the initiated dislocation boundaries resulting in the creation of smaller elongated broken grains and nanograins. Boundary refinement of the broken grains occurring through grain rotation and adiabatic heating results in the evolution of refined grains, subgrains and nanograins. The presence of elongated grains, broken grains, refined grains, subgrains and nanograins within the shear band structures demonstrates that the local deformation is inhomogeneous and that these mechanisms occur concurrently. It is concluded that the evolution of the shear band structure can be considered as a simultaneous layering of microstructures initially driven by dislocations which produce the final structures observed in the shear bands.

Numerical investigation of the influence of shear band localization on the resonant behavior in the VHCF regime

The monitored resonant behavior of fatigue specimens of metastable austenitic stainless steel (AISI304) is correlated with its damage accumulation in the very high cycle fatigue (VHCF) regime. The resonant behavior is studied experimentally and shows a distinct transient characteristic. Microscopic examinations indicate that during VHCF a localized plastic deformation in shear bands arises on the specimen surface. Hence, this work focuses on the effect of damage accumulation in shear bands on the resonant behavior of AISI304 in the VHCF regime. A microstructural simulation model is proposed that takes into account specific mechanisms in shear bands proven by experimental results. The simulation model is solved numerically using the two-dimensional boundary element method and the resonant behavior is characterized by evaluating the force-displacement hysteresis loop. Simulation of shear bands agrees well with microscopic examinations and plastic deformation in shear bands influences the transient characteristic of the resonant behavior.

Simulation of Deformation-induced Martensite Formation and its Influence on the Resonant Behavior in the Very High Cycle Fatigue (VHCF) Regime

The exploration of fatigue mechanisms in the VHCF regime is gaining importance since many components have to withstand a very high number of loading cycles due to high frequency or long product life. In this regime, particular attention is paid to the period of fatigue crack initiation and thus the localization of plastic deformation. The resonant behavior of a metastable austenitic stainless steel (AISI304) is studied experimentally in the VHCF regime and shows a distinct transient characteristic. The major contribution of this work is to obtain a physically-based understanding of this characteristic by modeling the underlying microstructural mechanisms and their influence on the resonant behavior. Microscopic examinations indicate that AISI304 undergoes deformation-induced martensite formation starting mostly at intersecting shear bands during fatigue. Therefore, a microstructural shear band model [Hilgendorff et al. (2013)] is extended regarding the mechanism of deformation-induced martensite formation. The model accounts for the microstructural mechanisms occurring in shear bands as documented by experimental results, and nucleation of martensite is assumed to occur at intersecting shear bands following the Olsen-Cohen nucleation model (1972) in combination with the Bogers-Burgers mechanism (1964). The simulation model is numerically solved using the two-dimensional (2-D) boundary element method. By using this method, a 2-D microstructure can be modeled considering grain orientations as well as individual anisotropic elastic properties in each grain. The resonant behavior is characterized by evaluating the force-displacement hysteresis loop. Results show that plastic deformation in shear bands and deformation-induced martensite formation have a major impact on the resonant behavior in the very high cycle fatigue (VHCF) regime.

Structure vs Chemistry: Friction and Wear of Pt-Based Metallic Surfaces

In comparison of a Pt57.5Cu14.7Ni5.3P22.5 metallic glass with a Pt(111) single crystal we find that wearless friction is determined by chemistry through bond formation alloying, while wear is determined by structure through plasticity mechanisms. In the wearless regime, friction is affected by the chemical composition of the counter body and involves the formation of a liquid-like neck and interfacial alloying. The wear behavior of Pt-based metallic surfaces is determined by their structural properties and corresponding mechanisms for plastic deformation. In the case of Pt(111) wear occurs by dislocation-mediated homogeneous plastic deformation. In contrast the wear of Pt57.5Cu14.7Ni5.3P22.5 metallic glass occurs through localized plastic deformation in shear bands that merge together in a single shear zone above a critical load and corresponds to the shear softening of metallic glasses. These results open a new route in the control of friction and wear of metals and are relevant for the development of self-lubricated and wear-resistant mechanical devices.

Hot working behavior of a super high strength Cu–Ni–Si alloy

Hot working behavior of a novel super high strength Cu–6.0Ni–1.0Si–0.5Al–0.15Mg–0.1Cr alloy has been studied systematically. Experiment of hot compression deformation with temperature ranging from 750°C to 900°C and strain rate from 0.001s−1 to 1s−1 indicated that both strain rate and deforming temperature affect the flow stress. Flow stress increased to a peak value, and then decreased with increase of deformation time, showing continuous dynamic recrystallization behavior. In the deformation temperature zone of 750–800°C, nucleation of the recrystallization grains occurred in the interior grains at special locations such as deformation bands and shear bands. At the high temperature zone (850–900°C), nucleation of the recrystallization grains occurred both at the grain boundaries and inside the grains of the matrix. Three typical textures appeared after hot compressive deformation and the order of the texture intensity from largest to smallest was: Copper texture of {112}〈111〉, S texture of {123}〈634〉, Gauss texture of {011}〈100〉. On the basis of established processing map, appropriate hot working temperature range was 850–875°C. With the optimised hot working parameters and appropriate thermal mechanical treatment, the designed alloy has achieved excellent properties: hardness 341HV, tensile strength 1090MPa, yield strength 940MPa, elongation 3.5%, electrical conductivity 26.5% IACS, and the stress relaxation rate 8.52% as tested at 150°C for 100h.

Interaction of deformation shear bands with nanoparticles in amorphous-nanocrystalline alloys

The interaction of shear bands with crystalline nanoparticles in amorphous-nanocrystalline Fe-Ni-B alloys is analyzed theoretically and experimentally. The following five interaction mechanisms are revealed: absorption, bypassing, cutting, retardation, and accommodation. The nanocrystal size is shown to be the key factor of this interaction. The experimental results obtained are satisfactorily described by the proposed theoretical models.

A review of microstructural evolution in the adiabatic shear bands induced by high speed machining

Investigations made by the authors and collaborators into the microstructural and fracture aspects of adiabatic shear bands (ASBs) of the hardened steels and Ti alloys induced by high speed machining (HSM) are briefly reviewed. The principal findings are the following: (a) the microstructure inside the ASBs varies from the band center to the normal chip material, the gradient microstructures are found; (b) the HSM can produce two types of ASBs with increasing in cutting speed, the deformed shear bands formed at lower cutting speed and the transformed shear bands formed at higher cutting speed; (c) the very small equiaxed recrystallized grains are observed in the center of the ASBs, the dynamic recrystallization and phase transformation may occur simultaneously during the formation of the transformed ASBs; (d) The dynamic rotational recrystallization is the origin of the equiaxed grains in the center of the ASBs. A microstructural evolution model in ASBs produced during HSM for the harden steel is proposed; (e) the microstructural pattern of fracture surface is characterised by the elongated dimples. A microcosmic adiabatic shear fracture model during HSM of the hardened steel is built up.

High tensile ductility of Ti-based amorphous matrix composites modified from conventional Ti–6Al–4V titanium alloy

Three Ti-based amorphous matrix composites containing ductile dendrites were fabricated by adding alloying elements of Ti, Zr, V, Ni, Al and Be into a conventional Ti–6Al–4V alloy, and the deformation mechanisms related to the improvement of tensile ductility were investigated by focusing on how the effective size of ductile dendrites affected the initiation and propagation of deformation bands or shear bands. The composites contained ∼73–76vol.% dendrites ∼63–103μm in size, and had excellent tensile properties with a yield strength of over 1.3GPa and an elongation of over 7%. In the composite containing very large dendrites, deformation bands were formed at dendrites in the same direction. In the composite containing small dendrites, however, many deformation bands were actively formed inside dendrites in the several directions, and cross each other to form widely deformed areas. This wide and homogeneous deformation in both dendrites and amorphous matrix enhances the tensile ductility, resulting in high strength and elongation occurring simultaneously. In order to theoretically explain the enhanced tensile ductility, a finite-element method (FEM) analysis based on the real microstructures considering dendrite crystal orientations was performed. The FEM simulation results of deformation bands or shear bands were in good agreement with the experimental findings. The reasons for such a good match between the simulation and experimental results are discussed in detail.