Shear bands mediate cavitation in brittle metallic glasses

Recent experimental studies have revealed nanoscale cavities and periodic corrugations on the fracture surfaces of brittle metallic glasses. How such cavitation in these materials leads to brittle failure remains unclear. Here we show, using atomistic and continuum finite element simulations, that a shear band can mediate cavity nucleation and coalescence owing to plastic flow confinement caused by material softening. This leads to brittle fracture as cavities nucleate and coalesce within a shear band, causing the crack to extend along it. (c) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

On the mechanism and the length scales involved in the ductile fracture of a bulk metallic glass

Some bulk metallic glasses (BMGs) exhibit high crack initiation toughness due to shear band mediated plastic flow at the crack tip and yet do not display additional resistance to crack growth due to the lack of a microstructure. Thus, at crack initiation, the fracture behavior of BMGs transits from that of ductile alloys to that of brittle ceramics. In this paper, we attempt to understand the physics behind the characteristic length from the notch root at which this transition occurs, through testing of four-point bend specimens made of a nominally ductile Zr-based BMG in three different structural states. In the as-cast state, both symmetric (mode I) and asymmetric (mixed mode) bend specimens are tested. The process of shear band mediated plastic flow followed by crack initiation at the notch root was monitored through in situ imaging. Results show that stable crack growth occurs inside a dominant shear band through a distance of, similar to 60 mu m, irrespective of the structural state and mode mixity, before attaining criticality. Detailed finite element simulations show that this length corresponds to the distance from the notch root over which a positive hydrostatic stress gradient prevails. The mean ridge heights on fractured surfaces are found to correlate with the toughness of the BMG. The Argon and Salama model, which is based on the meniscus instability phenomenon at the notch root, is modified to explain the experimentally observed physics of fracture in ductile BMGs. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Densification mechanisms during hot pressing of ZrB2-20 vol.% SiC composite

The dominant densification mechanisms for hot pressing of ZrB2-20 vol.% SiC composite at different hot-pressing temperatures and pressures was identified. The dominant densification mechanisms were found to change over a very short temperature range. For hot pressing at 1700 degrees C, the dominant densification mechanism was found to be mechanically driven particle fragmentation and rearrangement only, whereas at 1850 degrees C a plastic flow mechanism started to become dominant after initial particle fragmentation and rearrangement. At 2000 degrees C, the dominant mechanism changed from plastic flow to grain boundary diffusion. (c) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Nanoindentation behavior of nanotwinned Cu: Influence of indenter angle on hardness, strain rate sensitivity and activation volume

The influence of strain on the mechanical properties and deformation kinetic parameters of nanotwinned (at) copper is investigated by a series of nanoindentation experiments, which were performed by employing sharp indenters with five varying centerline-to-face angles (psi). Comparison experiments were also conducted on (1 1 0) single crystalline Cu. Experimental results indicate that, unlike coarsegrained materials, nt-Cu is prone to plastic flow softening with large material pile-up around the indentation impression at high levels of strains. Localized detwinning becomes more significant with decreasing psi, concomitant with reduced strain-rate sensitivity (m) and enhanced activation volume (V*). The m of nt-Cu is found to depend sensitively on psi with a variation of more than a factor of 3, whereas V* exhibits a much less sensitive trend. This paper discusses the validation of the experimental techniques and the implications of various deformation kinetic parameters on the underlying deformation mechanisms of nt-Ca. 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Deformation field heterogeneity in punch indentation

Plastic heterogeneity in indentation is fundamental for understanding mechanics of hardness testing and impression-based deformation processing methods. The heterogeneous deformation underlying plane-strain indentation was investigated in plastic loading of copper by a flat punch. Deformation parameters were measured, in situ, by tracking the motion of asperities in high-speed optical imaging. These measurements were coupled with multi-scale analyses of strength, microstructure and crystallographic texture in the vicinity of the indentation. Self-consistency is demonstrated in description of the deformation field using the in situ mechanics-based measurements and post-mortem materials characterization. Salient features of the punch indentation process elucidated include, among others, the presence of a dead-metal zone underneath the indenter, regions of intense strain rate (e. g. slip lines) and extent of the plastic flow field. Perhaps more intriguing are the transitions between shear-type and compression-type deformation modes over the indentation region that were quantified by the high-resolution crystallographic texture measurements. The evolution of the field concomitant to the progress of indentation is discussed and primary differences between the mechanics of indentation for a rigid perfectly plastic material and a strain-hardening material are described.

Hot deformation behaviour and microstructure control in AlCrCuNiFeCo high entropy alloy

An AlCrCuNiFeCo high entropy alloy (HEA), which has simple face centered cubic (FCC) and body centered cubic (BCC) solid solution phases as the microstructural constituents, was processed and its high temperature deformation behaviour was examined as a function of temperature (700-1030 degrees C) and strain rate (10(-3)-10(-1) s(-1)), so as to identify the optimum thermo-mechanical processing (TMP) conditions for hot working of this alloy. For this purpose, power dissipation efficiency and deformation instability maps utilizing that the dynamic materials model pioneered by Prasad and co-workers have been generated and examined. Various deformation mechanisms, which operate in different temperature-strain rate regimes, were identified with the aid of the maps and complementary microstructural analysis of the deformed specimens. Results indicate two distinct deformation domains within the range of experimental conditions examined, with the combination of 1000 degrees C/10(-3) s(-1) and 1030 degrees C/10(-2) s(-1) being the optimum for hot working. Flow instabilities associated with adiabatic shear banding, or localized plastic flow, and or cracking were found for 700-730 degrees C/10(-3)-10(-1) s(-1) and 750-860 degrees C/10(-1.4)-10(-1) s(-1) combinations. A constitutive equation that describes the flow stress of AlCrCuNiFeCo alloy as a function of strain rate and deformation temperature was also determined. (C) 2014 Elsevier Ltd. All rights reserved.

Effect of Zirconium on the Densification of Reactively Hot-Pressed Zirconium Carbide

Densification mechanisms involved during reactive hot pressing (RHP) of zirconium carbide (ZrC) have been studied. RHP has been carried out using zirconium (Zr) and graphite (C) powders in the molar ratios 1:0.5, 1:0.67, 1:0.8, and 1:1 at 40MPa, 800 degrees C-1200 degrees C for different durations. The volume fractions of phases formed, including porosity, are determined from the measured density and from Rietveld analysis. Increased densification with an increasing nonstoichiometry in carbon has been observed. Microstructural and X-ray diffraction observations coupled with the predictions of a model based on the constitutive laws governing plastic flow of zirconium suggest that the better densification of nonstoichiometric compositions arise from the higher amount of starting Zr and also the longer duration of its availability for plastic flow during RHP. Volume shrinkage due to reaction between Zr and C and the gradual elimination of the soft metal phase limit the final density achievable. Based on these observations, a two-step RHP carried out at 800 degrees C and 1200 degrees C leads to a better densification than a single RHP at 1200 degrees C.

Stress states in individual Si particles of a cast Al-Si alloy: Micro-Raman analysis and microstructure based modeling

The stress states in Si particles of cast Al-Si based alloys depend on its morphology and the heat treatment given to the alloy. The Si particles fracture less on modification and fracture more in the heat treated condition. An attempt has been made in this work to study the effect of heat treatment and Si modification on the stress states of the particles. Such understanding will be valuable for predicting the ductility of the alloy. The stress states of Si particles are estimated by Raman technique and compared with the microstructure-based FEM simulations. Combination of Electron Back-Scattered Diffraction (EBSD) and frequency shift, polarized micro-Raman technique is applied to determine the stress states in Si particles with (111) orientations. Stress states are measured in the as-received state and under uniaxial compression. The residual stress, the stress in the elastic-plastic regime and the stress which causes fracture of the particles is estimated by Raman technique. FEM study demonstrates that the stress distribution is uniform in modified Si, whereas the unmodified Si shows higher and more complex stress states. The onset of plastic flow is observed at sharp corners of the particles and is followed by localization of strain between particles. Clustering of particles generates more inhomogeneous plastic strain in the matrix. Particle stress estimated by Raman technique is in agreement with FEM calculations. (C) 2014 Elsevier B.V. All rights reserved.

High-Temperature Deformation Processing Map Approach for Obtaining the Desired Microstructure in a Multi-component (Ni-Ti-Cu-Fe) Alloy

An equiatomic NiTiCuFe multi-component alloy with simple body-centered cubic (bcc) and face-centered cubic solid-solution phases in the microstructure was processed by vacuum induction melting furnace under dynamic Ar atmosphere. High-temperature uniaxial compression experiments were conducted on it in the temperature range of 1073 K to 1303 K (800 degrees C to 1030 degrees C) and strain rate range of 10(-3) to 10(-1) s(-1). The data generated were analyzed with the aid of the dynamic materials model through which power dissipation efficiency and instability maps were generated so as to identify the governing deformation mechanisms that are operative in different temperature-strain rate regimes with the aid of complementary microstructural analysis of the deformed specimens. Results indicate that the stable domain for the high temperature deformation of the multi-component alloy occurs in the temperature range of 1173 K to 1303 K (900 degrees C to 1030 degrees C) and (epsilon) over dot range of 10(-3) to 10(-1.2) s(-1), and the deformation is unstable at T = 1073 K to 1153 K (800 degrees C to 880 degrees C) and (epsilon) over dot = 10(-3) to 10(-1.4) s(-1) as well as T = 1223 K to 1293 K (950 degrees C to 1020 degrees C) and (epsilon) over dot = 10(-1.4) to 10(-1) s(-1), with adiabatic shear banding, localized plastic flow, or cracking being the unstable mechanisms. A constitutive equation that describes the flow stress of NiTiCuFe multi-component alloy as a function of strain rate and deformation temperature was also determined. (C) The Minerals, Metals & Materials Society and ASM International 2015

A constitutive model for transversely isotropic foams, and its application to the indentation of balsa wood

An elastic-plastic constitutive model for transversely isotropic compressible solids (foams) has been developed. A quadratic yield surface with four parameters and one hardening function is proposed. Associated plastic flow is assumed and the yield surface evolves in a self-similar manner calibrated by the uniaxial compressive (or tensile) response of the cellular solid in the axial direction. All material constants in the model (elastic and plastic) can be determined from a combination of a total of four uniaxial and shear tests. The model is used to predict the indentation response of balsa wood to a conical indenter. For the three cone angles considered in this study, very good agreement is found between the experimental measurements and the finite element (FE) predictions of the transversely isotropic cellular solid model. On the other hand, an isotropic foam model is shown to be inadequate to capture the indentation response. © 2005 Elsevier Ltd. All rights reserved.

Deformation behavior and microstructure effect in 2124Al/SiCp composite

Dynamic compression tests were performed by means of a Split Hopkinson Pressure Bar (SHPB). Test materials were 2124Al alloys reinforced with 17% volume fraction of 3, 13 and 37 μm SiC particles, respectively. Under strain rate ε = 2100 l/s, SiC particles have a strong effect on σ_0.2 of the composites and the σ_0.2 increases with different SiC particle size in the following order: 2124Al-alloy → 124Al/SiC_p (37 μm) → 2124Al/SiC_p (13 μm) → 2124Al/SiC_p (3 μm), and the strain hardening of the composites depends mainly on the strain hardening of matrix, 2124A1 alloy. The results of dimensional analysis present that the flow stress of these composites not only depends on the property of reinforcement and matrix but also relates to the microstructure scale, matrix grain size, reinforcement size, the distance between reinforcements and dislocations in matrix. The normalized flow stress here is a function of inverse power of the edge-edge particle spacing, dislocation density and matrix grain size. Close-up observation shows that, in the composite containing SiC particles (3 μm), localized deformation formed readily comparing with other materials under the same loading condition. Microscopic observations indicate that different plastic flow patterns occur within the matrix due to the presence of hard particles with different sizes.

Deformation Behavior Of Fe-Based Bulk Metallic Glass During Nanoindentation

Fe-based bulk metallic glasses (BMGs) normally exhibit super high strength but significant brittleness at ambient temperature. Therefore, it is difficult to investigate the plastic deformation behavior and mechanism in these alloys through conventional tensile and compressive tests due to lack of distinct macroscopic plastic strain. In this work, the deformation behavior of Fe52Cr15Mo9Er3C15B6 BMG was investigated through instrumented nanoindentation and uniaxial compressive tests. The results show that serrated flow, the typical plastic deformation feature of BMGs, could not be found in as-cast and partially crystallized samples during nanoindentation. In addition, the deformation behavior and mechanical properties of the alloy are insensitive to the applied loading rate. The mechanism for the appearance of the peculiar deformation behavior in the Fe-based BMG is discussed in terms of the temporal and spatial characteristics of shear banding during nanoindentation.

Effect of quasicrystalline phase on the deformation behavior of Zr62Al9.5Ni9.5Cu14Nb5 bulk metallic glass

Quasicrystalline phase with different volume fraction were formed by isothermally annealing the as-castZr(62)Al(9.5)Ni(9.5)Cu(14)Nb(5) bulk metallic glass at 723 K for different times. The effects of quasicrystals on the deformation behavior of the materials were studied by nanoindentation and compression test. It revealed that the alloys with homogeneous amorphous structure exhibit pronounced flow serrations during the nanoindentation loading, while no obvious flow serration is observed for the sample with quasicrystals more than 10 vol.%. However, further compression tests confirm that the no-serrated flows are formed due to different reasons. For annealed samples containing quasicrystals less than 35 vol.%, continuous plastic deformation occurs due to propagation of multiple shear bands. While the disappearance of serrated flow cannot be explained by the generation of multiple shear bands for samples containing quasicrystals more than 35 vol.%, which will fracture with a totally different fracture mode, namely, dimple fracture mode under loading instead of shear fracture mode. (c) 2005 Published by Elsevier B.V.

Numerical analyses for the material bifurcation in plane sheet under tension

The various patterns (shear banding, surface wrinkling and necking) of material bifurcation in plane sheet under tension are investigated in this paper by means of a numerical method. It is found that numerical analysis can provide better ground for searching for the lowest critical loads. The inhomogeneity caused by void damage and the nonuniformity in the stress distribution across sheet thickness are proved to have detrimental effects on the material bifurcation. Nevertheless, material stability can be promoted by any means of depressing void damage or alleviating stress, even locally across the thickness. Besides, the peculiar behaviour of material bifurcation under slight biaxiality state is demonstrated. Copyright (C) 1996 Elsevier Science Ltd

Formation of shear bands in plane sheet

The formation of shear bands in plane sheet is studied, both analytically and experimentally, to enhance the fundamental understanding of this phenomenon and to develop a capability for predicting material failure. The evolution of voids is measured and its interaction with the process of shear banding is examined. The evolving dilatancy in plasticity is shown to have a vital role in analysing the shear-band type of bifurcation, and tremendously reduces the theoretical value of critical stresses. The analyses, referring to both localized and diffuse modes of bifurcation, fairly explain the corresponding observations obtained through testing a dual-phase steer sheet and provide a justification of the constitutive model used.