Localized plastic deformation in amorphous films on a ductile substrate

Bulk metallic glasses (BMGs) exhibit superior mechanical properties as compared with other conventional materials and have been proposed for numerous engineering and technological applications. Zr/Hf-based BMGs or tungsten reinforced BMG composites are considered as a potential replacement for depleted uranium armor-piercing projectiles because of their ability to form localized shear bands during impact, which has been known to be the dominant plastic deformation mechanism in BMGs. However, in conventional tensile, compressive and bending tests, limited ductility has been observed because of fracture initiation immediately following the shear band formation. To fully investigate shear band characteristics, indentation tests that can confine the deformation in a limited region have been pursued. In this thesis, a detailed investigation of thermal stability and mechanical deformation behavior of Zr/Hf-based BMGs is conducted. First, systematic studies had been implemented to understand the influence of relative compositions of Zr and Hf on thermal stability and mechanical property evolution. Second, shear band evolution under indentations were investigated experimentally and theoretically. Three kinds of indentation studies were conducted on BMGs in the current study. (a) Nano-indentation to determine the mechanical properties as a function of Hf/Zr content. (b) Static Vickers indentation on bonded split specimens to investigate the shear band evolution characteristics beneath the indention. (c) Dynamic Vickers indentation on bonded split specimens to investigate the influence of strain rate. It was found in the present work that gradually replacing Zr by Hf remarkably increases the density and improves the mechanical properties. However, a slight decrease in glass forming ability with increasing Hf content has also been identified through thermodynamic analysis although all the materials in the current study were still found to be amorphous. Many indentation studies have revealed only a few shear bands surrounding the indent on the top surface of the specimen. This small number of shear bands cannot account for the large plastic deformation beneath the indentations. Therefore, a bonded interface technique has been used to observe the slip-steps due to shear band evolution. Vickers indentations were performed along the interface of the bonded split specimen at increasing loads. At small indentation loads, the plastic deformation was primarily accommodated by semi-circular primary shear bands surrounding the indentation. At higher loads, secondary and tertiary shear bands were formed inside this plastic zone. A modified expanding cavity model was then used to predict the plastic zone size characterized by the shear bands and to identify the stress

Competing mechanisms in fracture of amorphous films resting on ductile substrates

The microstructure evolution and formation of shear bands in Zr40Ti60 alloy during different cold rolling processes were studied in this article. Results indicate that the width of the shear band formed in the sample increases rapidly with the increasing of the thickness reduction of the sample, and the temperature rise caused by plastic deformation played an important role in the evolution process of the shear band. The microstructure within the shear bands much finer than that in the surrounding matrix has been confirmed, and nanosized subgrains were observed at the centre of the shear band with a width of 15 μm. Hardness tests showed that the microhardness within the shear band with a width of 15 μm (537 HV) was markedly higher than that in the surrounding matrix (410 HV).

In the light of increasing research into amorphous composites and their applications, as-cast specimens of multicomponent Ti48Zr18V12Cu5Be17 amorphous composites were prepared via water-cooled copper mold suction casting. Subsequently, the as-cast specimens were subjected to semi-solid isothermal treatment to obtain semi-solid specimens. Taking the semi-solid specimens as the research object, room temperature compressive deformation behavior was investigated by analyzing the shear band characteristics on the side surfaces of the compressed specimens. The evolution of shear bands at various stages of plastic deformation was investigated via scanning electron microscopy (SEM). Additionally, significant work hardening was observed after yielding. Surface deformation morphologies indicate that the work-hardening behavior is associated with plastic deformation, interactions between shear bands, and interactions between shear bands and β-Ti crystals. Experiments have demonstrated that at a specific deformation extent, shear bands preferentially initiate at the crystal–amorphous matrix interface. In the final stage of plastic deformation, shear bands propagate through work-hardened β-Ti crystals into the amorphous matrix, with their propagation retarded by the β-Ti crystals. When shear bands in the amorphous matrix are obstructed by β-Ti crystals and can no longer propagate, some evolve into cracks. These cracks then propagate exponentially, leading to eventual fracturing of the specimens and termination of plastic deformation. The research findings provide a theoretical basis for analyzing the deformation capacities of various amorphous composites.

Most geotechnical structures failed by formation and development of shear bands in soils. Thus, shear deformation and shear bands development evaluation are necessary to understand shear failure mechanism. During shearing, deformation behaviour analysis for soil particles within entire soil specimen are evaluated to understand the soil behaviour and shear strength characteristics. In this paper, a series of plane strain compression tests using Nevada sand and Ottawa sand were conducted to identify the shear strain and shear failure mechanism. With the results of plane strain compression tests, image analyses using Particle Image Velocimetry (PIV) were carried out in order to measure the change in position of soil particles and shear bands development. Deformation vectors and contours were constructed to see the entire deformation mechanism in the soil specimen. During shearing, shear band was identified after peak stress and most visually distinctive at residual state. However, shear band started to develop invisibly immediately after starting loading and this invisible development was able to be observed by horizontal and vertical movement analyses of PIV. Soil particles moved actively in horizontal and vertical direction to generate shear band in the beginning of shearing. After development of shear band, soil particles moved along the shear band.

Metals and their alloys, except for a few intermetallics, are inherently ductile, i.e., plastic deformation precedes fracture in these materials. Therefore, resistance to fracture is directly related to the development of the plastic zone at the crack tip. Recent studies indicate that the fracture toughness of single crystals depends on the crystallographic orientation of the notch as well as the loading direction. In general, the dependence of crack propagation resistance on crystallographic orientation arises from the anisotropy of (i) elastic constants, (ii) plastic deformation (or slip), and (iii) the weakest fracture planes (e.g., cleavage planes). Because of the triaxial stress state at the notch tips, many slip systems that otherwise would not be activated during uniaxial testing become operational. The plastic zone formation in single crystals has been tackled theoretically by Rice and his co-workers [Rice, J. R., 1987, Mech. Mater. 6, pp. 317–335; Rice, J. R., and Saeedvafa, M., 1987, J. Mech. Phys. Solids 36, pp. 189–214; Saeedvafa, M., and Rice, J. R., 1988; ibid., 37, pp. 673–691; Rice, J. R., Hawk, D. E., Asaro, R. J., 1990, Int. J. Fract. 42, pp. 301–321; Saeedvafa, M., and Rice, J. R., 1992, Modell. Simul. Mater. Sci. Eng. 1, pp. 53–71] and only limited experimental work has been conducted in this area. The study of the stresses and strains in the vicinity of a fcc single-crystal notch tip is of relatively recent origin. We present experimental and numerical investigation of three-dimensional (3D) stress fields and evolution of slip sector boundaries near notches in fcc single-crystal PWA1480 tension test specimens and demonstrate that a 3D linear elastic finite element model, which includes the effect of material anisotropy, is shown to predict active slip planes and sectors accurately. The slip sector boundaries are shown to have complex curved shapes with several slip systems active simultaneously near the notch. Results are presented for surface and mid-plane of the specimens. The results demonstrate that accounting for 3D elastic anisotropy is very important for accurate prediction of slip activation near fcc single-crystal notches loaded in tension. Results from the study will help establish guidelines for fatigue damage near single-crystal notches.

Finite element analysis (FEA) of tensile loading of nano-scale double edge notched (DEN) metallic glass (MG) specimens are conducted using a non-local plasticity model. The effects of notch acuity on the plastic deformation response of the samples are studied. Transition is observed from localized plastic deformation by single shear band (SSB) to necking of the ligament and further to double shear bands (DSBs) with increase in notch acuity. These results corroborate with molecular dynamics (MD) simulations and are rationalized from spread of plastic zone in the ligament.

Effects of texture on shear band formation in plane strain tension/compression and bending

Dynamic elastic-plastic behaviour of a frame including coupled bending and torsion

The full time response of a space frame under impact loading perpendicular to the frame plane is discussed. Theoretical solutions and experimental results are presented and compared. A space frame clamped at its two ends is loaded by a 0.22 lead bullet that hits a mass in the middle of the transversal beam of the frame. The loading time is about 40 to 60 ?sec and the impact impulses in experiment from 0.5 to 1 Ns. The time response of this frame can be divided into four phases where different physical effects are dominant: (a) the ‘loading’ phase where elastic wave motion dominates the time response. Because of the high impact impulses, plastic deformation occurs in the vicinity of the mass and must be included in a theoretical model. The influence of reflections at the corners on the time response is shown; (b) the ‘evolution’ phase. Within this phase, a plastic collapse mechanism develops. Most of this phase is dominated by elastic deformation but local plastic deformations beside the mass are also present. Because many reflections at the corners and the clampings occur within this phase, a modal analysis method is used to predict time histories; (c) the ‘plastic’ phase with plastic zones at the clampings. This phase sets in after the bending wave reaches the clampings. It is characterized by plastic deformation near the clampings and elastic deformation of the other parts of the frame. We used a modal analysis including plastic ‘modes’ to get accurate results; (d) the ‘elastic vibration’ phase.

Microstructural evolution of adiabatic shear bands in pure copper during impact at high strain rates

Based on the facts that the thickness of a shear band in bulk metallic glasses (BMGs) is a few tens of nanometers, the shear displacement across the band is few micrometers, and the time for their formation is in submicrosecond duration, the local strain rates within the shear band can be as high as 109∕s. To capture such dynamic effects, a thermo-micromechanical model based on momentum diffusion mechanism, free-volume theory, and heat diffusion analysis is proposed. The model has been shown to capture the characteristic rate effects, i.e., significant local temperature rise and a dramatic drop in viscosity during shear band evolution in BMGs. The model also takes into account the effects of normal stress component on the deformation behavior of BMGs. While the predicted maximum temperature rise under quasistatic deformation in the absence of normal component of stress is low (300 K), significant temperature rise well above 1000 K accompanied by a sudden drop in viscosity has been predicted under dynamic loads at high normal pressures. It is also predicted that temperature rise and viscosity drop are negligible during the early phase of shear band formation but increase significantly towards the final phase of shear band evolution and cause subsequent fracture, as has been theorized by many researchers in the literature.

Experimental observation and numerical modeling of formation of local plastic zones in hardened surface layers due to contact overloading

A finite element analysis of evolution of stress–strain and martensite transformation in front of a notch in shape memory alloy NiTi

Failure assessment of a hard brittle coating on a ductile substrate subjected to cyclic contact loading