Shear Band Branching Research Articles

Improved ductility of annealed Fe-based metallic glass with good soft magnetic property by cryogenic thermal cycling

The brittleness after annealing treatment severely restricts the subsequent processing and application of Fe-based magnetic metallic glass. Through proper annealing treatment combined with cryogenic thermal cycling, we obtained excellent comprehensive properties of Fe-based metallic glass ribbons with not only good soft magnetic properties but also high bending deformation ability. After thermal cycling, the sample rejuvenated with higher energy state and larger size of soft regions, which facilitated the initiation and branching of shear bands, leading to the improvement of bending plasticity. Although the large soft regions may act as pining sites for domain walls, the domain structure and coercivity of the sample remained nearly identical because of the reduced number of crystal-like structures. Our study indicates that the cryogenic thermal cycling is an effective method to regulate mechanical and magnetic properties, which makes it powerful means to obtain excellent performance of Fe-based magnetic metallic glasses.

Tuning Glass Formation and Mechanical Properties of ZrCoAl(Nb) Bulk Metallic Glass with Nb Microalloying Process

In the present study, the effect of Nb addition on mechanical behavior and glass formation in (Zr56Co28Al16)100−xNb(x=0,2,4) bulk metallic glasses was studied. The differential scanning calorimetry (DSC) was used to investigate thermal properties of samples. Further, mechanical behavior of prepared samples was analyzed using quasi-static compressive tests. The results showed that 2 at.% minor addition of Nb enhances the glass forming ability (GFA), but larger amount of Nb (4%) deteriorates GFA. The X-ray diffraction patterns also confirmed that the structures of whole of studied compositions are amorphous. In addition, the compressive test results revealed that 2% addition of Nb significantly improves compressive ultimate strength (up to 2.5 GPa) and increases the plastic strain from 1.1% in the composition having no Nb content to ~ 6%. The observation of shear band branching and also the increase in the density of vein-like patterns in the fracture surface (studied by scanning electron microscopy) also confirm the improvement of plasticity in (Zr56Co28Al16)98Nb2 sample. Our finding highlights the effect of minor alloying on the formation and mechanical properties of bulk metallic glasses.

An atomic-level perspective of shear band formation and interaction in monolithic metallic glasses

Understanding the relationship between nanoscale structural heterogeneities or elastic fluctuations and strain localization in monolithic metallic glasses remains a long-standing underlying issue. Here, an atomic-level investigation of the correlation between elastic and structural heterogeneities and the mechanisms of shear banding in CuZr metallic glass is conducted using molecular dynamics simulations. The shear band formation and propagation processes and the intersection mechanism of multiple shear bands are evaluated by means of local entropy-based structural identification and von Mises stress calculation. The shear band follows the path of lower order and high entropy while shear deflection and branching occur when approaching regions of low entropy. The local von Mises stress calculation allows predictions on the shear band direction and the propensity for activation and propagation prior to yielding and sheds light on shear band branching and multiplication processes.

Shear banding and fracture behaviors of a bulk metallic glass studied via in-situ bending experiments with notched and un-notched specimens

The shear banding and fracture behaviors of Ti32.8Zr30.2Ni5.3Cu9Be22.7 bulk metallic glass (BMG) were studied via in-situ three-point bending experiments using notched and un-notched specimens. A notch strengthening effect was found based on comparing the nominal flexural strength of the notched and un-notched specimens. From in-situ observations of the notched specimens, it was found that the first shear band initiated near the notch tip was followed by a spate of shear band initiations in a wider range of locations, leading to macroscopic yielding. The shear band zone continued to grow until the specimen fractured, with plastic strain being accommodated by repeated branching of shear bands and increase of shear offset in the earlier formed parts of shear bands. Significant differences in the 3-dimensional orientation and shear offset direction of shear bands were observed between the notched and un-notched specimens. These differences were analyzed by relating the elastic stress fields in the general direction of shear band growth, obtained by finite element modeling, to the ellipse criterion.

Atomic-scale origin of shear band multiplication in heterogeneous metallic glasses

Using molecular dynamics simulations, we provide an atomistic description of the shear band multiplication mechanisms in a heterogeneous metallic glass consisting of two distinct amorphous regions with different amounts of free volume and degrees of short-range order. The structural differences and elastic fluctuations encountered by the developing shear band while crossing the interface between the two regions alter the autocatalytic activation of the shear transformation zones (STZs) and, subsequently, lead to shear band branching. The two-unit STZ-vortex mechanism sheds light on the correlation between the strain distribution during tensile deformation and the directional characteristics of the STZ activation process.

Probing structural changes during ductile fracture in metallic glasses via in situ straining inside a MeV transmission electron microscope

Structural changes in the vicinity of a crack during ductile fracture in various metallic glasses (MGs) with the propensity of forming shear bands were investigated by in situ straining inside a MeV transmission electron microscope (TEM). During tensile loading, a plastic deformation zone with a width of ∼100 nm and shear bands with a width of ∼15 ± 5 nm developed from crack tips of ductile fracture in MGs. In particular, crack propagation by nanovoid formation and coalescence was observed in Ti40Zr29Cu9Ni8Be14 MG with the highest compressive plasticity (∼6.7 ± 0.5%) among the tested MGs, whereas this process was hardly observable in other MGs. This difference may be attributed to atomic scale heterogeneities in Ti40Zr29Cu9Ni8Be14 MG induced by icosahedral short range orders (ISRO), which cause pronounced shear band branching and thus delay fracture process. Meanwhile, no crystallization of MGs even with ISRO or with lower Tx (crystallization onset temperature) was observed during in situ straining inside the MeV TEM and uniaxial tensile test with ribbon samples; a possible effect of normal stress and nanovoid formation during in situ straining is discussed to compare with crystallization during uniaxial compression test of the same MGs. Our study provides an insight into intrinsic toughening of MGs and thus can give useful guidelines on how to delay ductile fracture processes of MGs by tuning nanovoid formation, taking advantage of structural features through customized design of MGs.

Ductile bulk metallic glass by controlling structural heterogeneities

A prerequisite to utilize the full potential of structural heterogeneities for improving the room-temperature plastic deformation of bulk metallic glasses (BMGs) is to understand their interaction with the mechanism of shear band formation and propagation. This task requires the ability to artificially create heterogeneous microstructures with controlled morphology and orientation. Here, we analyze the effect of the designed heterogeneities generated by imprinting on the tensile mechanical behavior of the Zr52.5Ti5Cu18Ni14.5Al10 BMG by using experimental and computational methods. The imprinted material is elastically heterogeneous and displays anisotropic mechanical properties: strength and ductility increase with increasing the loading angle between imprints and tensile direction. This behavior occurs through shear band branching and their progressive rotation. Molecular dynamics and finite element simulations indicate that shear band branching and rotation originates at the interface between the heterogeneities, where the characteristic atomistic mechanism responsible for shear banding in a homogeneous glass is perturbed.

Effect of minor Nb addition on mechanical properties of in-situ Cu-based bulk metallic glass composite

In-situ formed (Cu0.6Zr0.3Ti0.1)95Nb5 bulk metallic glass (BMG) composite with Nb-rich dendrite randomly dispersed in hard glassy matrix was prepared by casting into a water-cooled copper mold. The dendrite has much smaller hardness and elastic modulus than glassy matrix, and the stress concentration at interface provides a channel for the initiating and branching of shear bands upon loading, thus leading to a high compressive fracture strain of 6.08% and fracture strength about 2200 MPa. Comparing with other Cu-based BMG composite, the fracture strength of present (Cu0.6Zr0.3Ti0.1)95Nb5 composite is not significantly reduced, indicating that the addition of Nb in the current work is an effective and effortless way to fabricate new practical BMG composites with enhanced strength and good plasticity.

The effect of liquid phase separation on the Vickers microindentation shear bands evolution in a Fe-based bulk metallic glass

The Vickers microindentation experiments and associated plastic deformation in as-cast and annealed (Fe0.9Ni0.1)77Mo5P9C7.5B1.5 bulk metallic glass was conducted. In addition to the bulk indentation behavior, the shear band morphology underneath the Vickers microindenter was examined by employing the bonded interface technique. Microstructural characterization revealed that a liquid phase separation occurred during melting process. Atomic force microscopy of the glassy matrix of the as-cast specimen reveals the composition inhomogeneity induced by the liquid phase separation. This effect generates shear band branching or deflection during the shear band propagation. For the bulk indentation, the trends in the hardness vs. indentation load were found related to the pressure sensitive index and the phase separation process simultaneously. The results show that the as-cast as well as the annealed specimens are deformed through semi-circular and radial shear bands. In addition, in the partially crystalized specimen, the change in the properties and microstructure of the BMG induced by the partial crystallization treatment and phase separation process resulted in tertiary shear bands formation.

Deformation patterns in Zr-based and Ti-based metallic glasses under scratch processes

The plastic deformation features of Zr- and Ti-based bulk metallic glasses (BMGs) are investigated by scratch method under different applied load and scratch velocity. The results show that the two BMG systems exhibit quite different shear band features during scratch processes. Shear band branching are found in Ti-based BMG, and the branching are more prominent at high applied load and rate during the scratch process. In contrast, the Zr-based BMG shows primary shear band dominating pattern, which becomes more straight and regular at high load and rate. Furthermore, the effect of structural relaxation on the shear band feature during scratch tests is studied. It is found that relaxation gives rise to a distinct transition from primary shear band dominating pattern to a branching and irregular pattern in the Zr-based BMG. The mechanism of change of the shear band pattern is discussed in the view of free volume theory.

Microstructure and Compressive Properties of <I>In-Situ</I> Martensite CuZr Phase Reinforced ZrCuNiAl Metallic Glass Matrix Composite

In-situ micro-level martensite CuZr phase reinforced Zr50:5Cu27:45Ni13:05Al9 metallic glass matrix composite was synthesized by copper mold casting. Microstructure and compressive properties of the composite are investigated. The martensite CuZr phase possesses numerous coherent twin boundaries, and holds strong interfacial cohesion with the matrix. They both effectively induce the seeding and branching of shear bands during compression. The compressive properties of the composite are therefore improved. The fracture strength and plastic strain of composite with a diameter of 3 mm is up to 2190 MPa, 5.7% respectively. [doi:10.2320/matertrans.M2010031]

High-strength and ductile glassy-crystal Ni–Cu–Zr–Ti composite exhibiting stress-induced martensitic transformation

We present a Ni-based crystal-glassy composite material having superior strength paired with a considerable ductility of 15%. The formation of a metastable crystalline phase in a glassy matrix during solidification has been proven capable of promoting a strain-induced martensitic transformation leading to enhanced plasticity under compression at room temperature. Underlying mechanisms of plastic deformation are discussed in terms of the interplay between dislocation slip in the crystalline phase and shear deformation in the glassy matrix. We suppose that the strain-induced martensitic inclusions serve as strong barriers for shear band propagation, promoting shear band branching and multiple shear band formation, thus extending the ductility and preventing a premature brittle fracture. The acoustic emission technique has been employed to clarify the kinetics of transformation and stages of plastic deformation.

Mesoscopic theory of shear banding and crack propagation in metallic glasses

We propose a phenomenological theory to model shear banding, shear-band propagation and branching on mesoscopic scales in metallic glasses by using Ginzburg-Landau formulism. The disordering caused by mechanical or thermal agitation is represented by atomic volume dilatation and used as an order parameter. This model captures several important features in the deformation process, namely, shear localization or banding, shear-band propagation and branching, and crack propagation and its velocity. We also assessed the relation between the crack propagation velocity and local heating and the connection between the serrated flow and shear band branching.

<i>In Situ</i> Fracture Behavior of AZ51 and AZT513 Alloy

The fracture behavior of Mg-5Al-1Zn and Mg-5Al-1Zn-3Sn alloy was investigated by direct observation of microfracture process using an in-situ loading stage installed inside a scanning electron microscope chamber. Crack was initiated at the interface of Mg/second-phase particles or second-phase particles. Fracture of the alloys was predominantly dimple or/and quasi-cleavage failure. The improvement of what could be explained by mechanisms of blocking of crack or shear band propagation, formation of multiple shear bands, crack blunting and shear band branching.

CHARACTERIZATION OF SHEAR BANDS IN TWO BULK METALLIC GLASSES WITH DIFFERENT INHERENT PLASTICITY

Shear banding characterization of Zr 64.13 Cu 15.75 Ni 10.12 Al 10 and Zr 65 Cu 15 Ni 10 Al 10 bulk metallic glasses (BMGs) with significant difference in inherent plasticity and quite similar chemical composition was studied by depth sensitive macroindentaion tests with conical indenter. Well-developed shear band pattern can be found for both BMGs after indentation. Distinct difference in the shear band spacing, scale of plastic deformation region and the shear band branching in the two BMGs account for the different plasticity.

Microfracture Observation of Zr-Based Bulk Metallic Glasses

Microfracture mechanisms of Zr-based bulk metallic glass (BMG) alloy containing ductile crystalline particles were investigated by directly observing microfracture processes using an in situ loading stage. Strength of the BMG alloy containing crystalline particles was lower than that of the monolithic BMG alloy, while ductility was higher. According to the direct microfracture observation, crystalline particles initiated shear bands, acted as blocking sites of shear band or crack propagation, and provided the stable crack growth which could be confirmed by the R-curve analysis, although they negatively affected apparent fracture toughness. This increase in fracture resistance with increasing crack length improved overall fracture properties of the alloy containing crystalline particles, and could be explained by mechanisms of blocking of crack or shear band propagation, formation of multiple shear bands, crack blunting, and shear band branching.

In situ fracture observation and fracture toughness analysis of Zr-based bulk amorphous alloys

The fracture property improvement of Zr-based bulk amorphous alloy containing ductile crystalline particles was explained by directly observing microfracture processes using an in situ loading stage installed inside a scanning electron microscope (SEM) chamber. Strength and apparent fracture toughness measured from the in situ fracture test of the amorphous alloy containing crystalline particles were lower than those of the monolithic amorphous alloy, whereas its ductility was higher. According to the microfracture observation, shear bands were initiated from ductile crystalline particles, and the propagation of the shear bands or cracks was blocked by crystalline particles, thereby resulting in stable crack growth which could be confirmed by the fracture resistance curve (R-curve) behavior. This increase in fracture resistance with increasing crack length improved fracture properties of the alloy containing crystalline particles, and could be explained by mechanisms of blocking of crack or shear band propagation, formation of multiple shear bands, crack blunting, and shear band branching.

A Catastrophic Yield Theory of Shear Band Formation in α-Brass

This paper emphasises the role that dislocation pile-ups have in promoting instability that leads to shear banding‚ and the Dillamore formalism is used to demonstrate their importance. The model is capable of being applied to both laminar structures and particle hardened alloys. By incorporating the idea of a critical length in which shear band branching can dissipate shear imposed from neighbouring grains‚ it is possible to estimate the rolling strain at which shear banding changes from being grain scale to sample scale.