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Abstract: We reported the formation of nanocrystalline structure in the surface layer of a pure tantalum plate under sliding friction treatment. Nanocrystalline grains of 7nm with amorphous boundaries were formed in the topmost surface layer of Ta plate. Deformation twins were also observed in the nanocrystalline grains with size below 20nm. Deformation twinning and localized amorphization are the possible deformation mechanisms in the top surface layer of tantalum induced by sliding friction. [Copyright &y& Elsevier]

Abstract: This paper aims to reveal the material removal mechanisms and the mechanics behind the vibration-assisted cutting (VAC) of unidirectional fibre reinforced polymer (FRP) composites. Through a comprehensive analysis by integrating the core factors of the VAC, including fibre orientation and deformation, fibre–matrix interface, tool–fibre contact and tool–workpiece contact, a reliable mechanics model was successfully developed for predicting the cutting forces of the process. Relevant experiments conducted showed that the model has captured the mechanics and the major deformation mechanisms in cutting FRP composites, and that the application of ultrasonic vibration in either the cutting or normal direction can significantly decrease cutting forces, minimise fibre deformation, facilitate favourable fibre fracture at the cutting interface, and largely improve the quality of a machined surface. When the vibrations are applied to both the cutting and normal directions, the elliptic vibration trajectory of the tool tip can bring about an optimal cutting process. There exists a critical depth of cut, beyond which the fibre–matrix debonding depth is no longer influenced by the vibration applied on the tool tip. [Copyright &y& Elsevier]

Abstract: This paper investigates the effect of compressive strain rate on the mechanical behaviour of single crystalline silicon nanowires using molecular dynamics simulation. It was found that of the whole range of the strain rates studied, the initial deformation of a nanowire is elastic. At lower strain rates the nanowire exhibits greater elasticity, and simple constitutive equations can be developed to describe the nanoscale structure and its deformation mechanism. With the increase in strain rate, the buckling stress increases and becomes steady at medium strain rates. On applying a very high strain rate, which is equivalent to a mechanical shock, the maximum buckling stress has a sudden rise and the silicon nanowire undergoes ballistic annihilation at both ends. [Copyright &y& Elsevier]

Abstract: Inelastic deformation of metallic glass is through shear banding, characterized by significantly localized deformation and emerged expeditiously under certain stress state. This study establishes a new constitutive model addressing the physical origin of the shear banding. In the modeling, the atomic structural change and the free volume generation are embodied by the plastic shear strain and the associated dilatation. The rugged free energy landscape is adopted to naturally reflect the rate-independent flow stress and flow serrations. Based on this, the conditions for the onset of shear banding instability are established, which enables the explicit calculation of the shear band inclination angle and its extension speed. The study concludes that shear band angle is significantly influenced by the diltancy factor and pressure sensitivity, that a shear band does not increase its thickness once emanated from a deformation unit, that the spreading speed of a shear band is intersonic, and that more shear bands, which lead to higher ductility, can be induced by high strain rates or by the introduction of a second material phase. The analysis also demonstrates that the ductility of metallic glass depends on the sample geometry and/or the stress state. [Copyright &y& Elsevier]

In this paper, we present the results of using selective laser melting (SLM) to produce biomedical beta Ti–24Nb–4Zr–8Sn components, including the manufacture of a sample acetabular cup. The density of the material increases with increasing incident laser energy (i.e. decreasing laser scan speed) and reaches a near full density value of >99% without any post-processing. The mechanical properties of the as-processed material are also compared to those of conventionally processed material. [Copyright &y& Elsevier]

Abstract: Pronounced grain refinement to ultrafine or nanoscale size in initially coarse-grained Co–25Fe and Co–35Fe alloys has been achieved upon conventional compression at room temperature. Both Co–Fe alloys exhibit a large plasticity of more than 140% without fracture at room temperature, which is related to a multiscale-grained microstructure resulting from deformation-induced grain refinement. The grains refine with increasing deformation during compression. The microstructure features indicate that the possible mechanism for the strain-induced grain refinement under conventional compression is a consequence of the propagation and coalescence of dislocation cell structures with strain accumulation. [ABSTRACT FROM AUTHOR]

Abstract: This paper proposes a new constitutive model to describe the dynamic plasticity of FCC metals using the thermal activation mechanism of dislocation motion. In the model development, the constitutive parameters were directly linked with the characteristics of microstructures of materials. As an example of its application, the model was used to describe the behavior of OFHC copper. To determine the globally optimized parameters of the constitutive model for OFHC copper, an improved multi-variable optimization method of constrained nonlinear programming was used based on the flow stress of the material measured experimentally. A comparison with some models and experimental data in the literature shows that the new model is simple to apply and is much better in terms of its prediction accuracy. It was shown that compared with the MTS model the new constitutive equation is explicit and can be easily embedded into a computational code of material dynamics; while compared with the Zerilli–Armstrong and Johnson–Cook models the new one reflects more precisely experimental observations. It was concluded that the new model is applicable to a wide range of problems with temperature variation from 77K to 1096K, strain rates ranging from 10−3 s−1 to 104 s−1, and strain as high as 1. [Copyright &y& Elsevier]

Abstract: Using the cutting of long fibre reinforced polymer and ceramic particle reinforced aluminium as the examples, this paper tends to understand some common features in the mechanics modelling of machining composites. It demonstrates that an accurate characterisation of matrix deformation, matrix–reinforcement interaction, and reinforcement deformation are key factors for the establishment of a model to reflect the principal material removal mechanisms. A precise understanding of these factors can be achieved through a logic process of mechanism exploration, model derivation and verification. [Copyright &y& Elsevier]

Abstract: This review paper discusses some basics in using continuum mechanics and molecular dynamics to characterize the deformation of single-walled carbon nanotubes (SWCNTs). It identifies that the van der Waals force between SWCNTs in a bundle distributes symmetrically and influences the bundle formation, and that to avoid misleading results from a molecular dynamics simulation, the interaction potential, thermostat scheme and simulation parameters must be carefully selected. The paper then points out that when the necessary condition proposed by Vodenitcharova and Zhang and a compatibility condition for elastic constants are satisfied, the intersect of the bending and in-plane stiffness curves in the modulus-thickness plane can determine a unique effective wall thickness of an SWCNT and hence its Young''s modulus. [Copyright &y& Elsevier]

Abstract: This paper clarifies a common misunderstanding of the phase transformation in monocrystalline silicon under nanoindentation, namely that a pop-out represents the onset of a phase transition. Through a detailed investigation into the indentation-induced deformation of monocrystalline silicon using a Berkovich indenter, it was found that a pop-out does not correspond to the onset of the transformation. The critical contact pressure for initiating phase transformation during unloading is independent of the maximum indentation load or of the unloading rate. The size of a pop-out depends on the time it takes place (earlier and later), and its location alters the proportion of the transferred phases (amorphous and crystalline phases) after complete unloading. A lower unloading rate or a higher maximum indentation load promotes the occurrence of a pop-out. [Copyright &y& Elsevier]

Abstract: This paper investigates the deformation mechanisms of MMCs subjected to micro-indentation by a spherical indenter using a three-dimensional finite element modeling. It was found that deformation behavior, hardness and work hardening of MMCs were highly dependant on the location of indentation relative to particles, volume percentage of the particle, and the size ratio of indenter to particle. The hardness of an MMC varied in a complex manner depending on the restriction on the matrix flow by reinforced particles and work hardening of the matrix material. Hardness increased with the increase of volume percentage of reinforced particles and decrease of the size ratio of indenter to particle. Matrix flow due to indentation was highly non-uniform which generated an inhomogeneous strain filed in an MMC. These pose a question that the conventional definition of micro-hardness is not very appropriate for characterizing MMCs. [Copyright &y& Elsevier]

Bulk (Ti0.65Fe0.35)100−x Sn x (x =0, 2.5, 5at.%) ultrafine composites with high strength and plasticity have been prepared. All composites have a hypereutectic microstructure with primary FeTi or/and Ti3Sn phases and a (β-Ti+FeTi) eutectic matrix. The ultrafine composites exhibit a high fracture strength of ∼2350–2650MPa and a large plasticity of ∼7.4–12.5% under compression. The structure of the phases, their refinement and volume fraction are the crucial factors determining the mechanical properties. [Copyright &y& Elsevier]

Abstract: The plastic deformation behavior of Ce68Al10Cu20Nb2 and Ce70Al10Cu20 bulk metallic glasses (BMGs) at room temperature was studied by depth-sensing nanoindentation and microindentation. It is shown that the two BMGs exhibit a continuous plastic deformation without distinct serration at the all of the studied loading rates during nanoindentation. An obvious creep displacement was observed during the holding-load segment at the maximum load for the two alloys, and the magnitude of creep during holding-load increases with loading rate. The subsurface plastic deformation zone of the two BMGs after indentation at various loading rates was investigated through bonded interface technique using depth-sensing microindentation. A highly developed shear banding pattern can be observed in the plastic deformation region, though the global load–depth curves illuminate a “homogeneous flow”. The plastic deformation behavior of the Ce-based BMGs during indentation measurements is discussed in terms of localized viscous flow. [Copyright &y& Elsevier]

Abstract: This paper studies the pure bending and bending-induced local buckling of a nanocomposite beam reinforced by a single-walled carbon nanotube (SWNT). The Airy stress-function method was employed to analyse the deformation of the matrix, and the cross-sectional change of the SWNT in bending was taken into account. A particular consideration was given to the effect of the SWNT’s radial flexibility on the strain/stress states and buckling. It was found that in thicker matrix layers the SWNT buckles locally at smaller bending angles and greater flattening ratios. This causes higher strains/stresses in the surrounding matrix and in turn degrades the strength of the nanocomposite structure. [Copyright &y& Elsevier]

Abstract: This study analyses the microstructure of monocrystalline silicon after indentation with a Berkovich and spherical indenter. Transmission electron microscopy on cross section view samples was used to explore the detailed distributions of various phases in the subsurfaces of indented silicon. It was found that an increase of the P max would promote the growth of the crystalline R8/BC8 phase at the bottom of the deformation zone. Microcracks were always generated in the range of the P max studied. It was also found that the deformation zones formed by the Berkovich and spherical indenters have very different phase distribution characteristics. A molecular dynamics simulation and finite element analysis supported the experimental observations and suggested that the distribution of the crystalline phases in the transformation zone after indentation was highly stress-dependent. [Copyright &y& Elsevier]

The mechanisms of macroscopic ductile deformation of brittle ceramics during grinding operations are still unclear although the microcracking-free processes of grinding have been used widely in industry. This paper analyses the dislocation structure in the plastic zones of scratched alumina. It is shown that there exist three independent slip systems (two basal and one prism) and two twin systems even far below the critical onset temperatures of these systems that is usually expected. This new finding explains the formation mechanism of the macroscopic deformation induced by scratching and indicates that it is the stress field around the cutting edge which plays an important role in the activation of the slip systems. The present study offers a new insight into the mechanism understanding of material removal in ductile-regime grinding of ceramic materials. [ABSTRACT FROM AUTHOR]

Nanoscratching-induced deformation in monocrystaline silicon is found to depend on the loading conditions. Molecular dynamics simulations reveal that amorphous phase transformation and nanotwins are the two major mechanisms. At a relatively low scratching depth, amorphous transformation occurs on the surface; however, when the scratching depth is greater than 1nm, nanotwinning also emerges in the subsurface along 〈110〉 direction and its formation is associated with the body-centred-tetragonal-5 Si phase transformation. The twinning deformation stops at a Shockley partial dislocation. [ABSTRACT FROM AUTHOR]

Abstract: A grain size-dependent model is theoretically suggested to describe the combined effects of special rotational deformation and dislocations near a mode I crack tip on the fracture toughness of nanocrystalline metals and ceramics. In the model, the special rotation deformation is driven by the external stress concentration near the crack tip, and serves as a toughening mechanism by releasing part of local stresses. The lattice dislocations consist of triple junction dislocation produced by intergrain sliding and edge dislocations emitted from the crack tip. The emitted dislocations are stopped at grain boundaries. The stress fields of these dislocations suppress future dislocation emission, and the suppression depends on grain size. The results indicate that the combination of special rotational deformation and dislocations near the crack tip can lead to an increase of critical crack intensity factor (effective fracture toughness) by several times in nanocrystalline materials at finest grain size. It is also found that the fracture toughness of nanocrystalline materials is highly sensitive to grain size and there is an ideal grain size corresponding to the best toughening effects, which is qualitatively consistent with the conclusion in previous work. [Copyright &y& Elsevier]

Abstract: The deformation behavior and the effect of the loading rate on the plastic deformation features in (numbers indicate at.%) Ce60Al15Cu10Ni15, Ce65Al10Cu10Ni10Nb5, Ce68Al10Cu20Nb2, and Ce70Al10Cu20 bulk metallic glasses (BMGs) were investigated through nanoindentation. The load–displacement (P–h) curves of Ce65Al10Cu10Ni10Nb5, Ce68Al10Cu20Nb2, and Ce70Al10Cu20 BMGs exhibited a continuous plastic deformation at all studied loading rate. Whereas, the P–h curves of Ce60Al15Cu10Ni15 BMG showed a quite unique feature, i.e. homogeneous plastic deformation at low loading rates, and a distinct serrated flow at high strain rates. Moreover, a creep deformation during the load holding segment was observed for the four Ce-based BMGs at room temperature. The mechanism for the appearance of the “anomalous” plastic deformation behavior in the Ce-based BMGs was discussed. [Copyright &y& Elsevier]

Abstract The hot deformation behavior and microstructural evolution of a core-shell (soft coarse-grained Ti cores and hard Ti-N solid solution shells) structured titanium alloy were investigated under 800–950 °C at a strain rate range of 0.001–1 s−1. The core-shell structured titanium alloys exhibit a higher flow stress (195 MPa deformed at 800 °C/0.1 s−1) compared with that of pure Ti (48 MPa deformed at 800 °C/0.1 s−1), suggesting that core-shell structure promotes a strengthening effect of titanium alloys. The nitrogen solution strengthened hard shells squash along compression direction playing a roll of skeleton and the soft cores accommodate the deformation, leading to inhomogeneous deformation. Dynamic recrystallization takes place mainly adjacent to the shells due to stress and strain concentration. The improvement compression stress of core-shell structured titanium alloys is attributed to the nitrogen-induced solution strengthening and core-shell structure itself, together with the inhabited plastic deformation and dynamic recrystallization by core-shell structure. Highlights • The core-shell structure enhances compression strength of titanium alloys. • The hard shells squash and soft cores accommodate deformation. • Dynamic recrystallization takes place mainly adjacent to the shells. [ABSTRACT FROM AUTHOR]

Abstract Laves phase alloys are promising materials for several structural applications, but the extreme brittleness is the predominant shortcoming of a Laves matrix. One potential solution to overcome this shortcoming is to alloy Laves matrix with some soft matrix. A group of Ti-35Zr-5Fe-xMn (x = 0, 2, 4, 6, 8 wt%) alloys was cast with an aim to improve deformation in Laves alloy compositions. The phase and microstructure analyses reveal dual phase matrices, including a β phase and a C14 type Laves phase in the investigated alloys. The mechanical properties such as yield strength, hardness and plastic strain for the investigated alloys are found to be significantly sensitive to volume fraction of the Laves phase. Ti-35Zr-5Fe shows impressive ultimate compressive strength (~1.7 GPa), yield strength (1138 MPa) and large plastic strain (23.2 %). The fracture mechanisms are dependent on the microstructure of the alloys. Additionally, the work-hardening ability of the investigated alloys have also been evaluated based on the analyses of slip band patterns formed around the micro-hardness indentations. Notably, the extreme brittleness is not encountered in all the Ti-35Zr-5Fe-xMn alloys and all exhibit very good compressive elongation including the maximum (32.5 %) in Ti-35Zr-5Fe. Graphical abstract Unlabelled Image Highlights • The C14 type Laves phase is formed in all the Ti-35Zr-5Fe-xMn alloys. • Yield strength, hardness and plastic strain are affected by the content of Mn. • Plastic strain correlates inversely with the size of dimples formed on fractured surfaces. • Ti-35Zr-5Fe displays impressive elongation (32.5%), ultimate strength (~1.7 GPa) and yield strength (~1.1 GPa). [ABSTRACT FROM AUTHOR]

We report that the strength and plasticity of Ti 62 Nb 12.2 Fe 13.6 Co 6.4 Al 5.8 alloy can be modulated by selectively controlled micron-sized phases, i.e. β -Ti phase and twined (CoFe)Ti 2 phase. It is found that the alloy with micron-sized β -Ti phase embedded in nanostructured matrix possesses lower yield strength but higher ultimate strength relative to the alloy with micron-sized (CoFe)Ti 2 phases. The plastic strain and ultimate strength of the alloy with micron-sized β -Ti phases is as large as 22.1% and 2826 MPa, respectively. The work-hardening exponent n of the alloy with micron-sized β -Ti phase, 0.12, is far greater than 0.008 for the alloy with (CoFe)Ti 2 phase. Such significant and different mechanical properties are attributed to the roles and response of micron sized phases during deformation process. [ABSTRACT FROM AUTHOR]

Abstract: Commercial-purity aluminum was processed by equal-channel angular pressing (ECAP) at room temperature (RT-ECAP) and cryogenic temperature (CT-ECAP) with liquid nitrogen cooling between two successive passes. It was found that the RT-ECAPed samples showed equiaxed microstructure after 4 and 8 ECAP passes, while the CT-ECAPed samples displayed slightly elongated microstructure and slightly smaller grain size. Moreover, the CT-ECAPed samples had higher hardness values than the RT-ECAPed samples subjected to the same amount of deformation. Positron annihilation lifetime spectroscopy (PALS) was used to investigate the evolution of vacancy-type defects during the ECAP deformation process. The results showed that three types of defects existed in the ECAPed samples: vacancies associated with dislocations, bulk monovacancies and bulk divacancies. The CT-ECAPed samples had a higher fraction of monovacancies and divacancies. These two types of defects are the major vacancy-type defects that can work as dislocation pinning centers and induce hardening, resulting in higher hardness values in the CT-ECAPed samples. A quantitative relationship between material hardness and the defect concentration and defect diffusion coefficient has been established. [Copyright &y& Elsevier]

Deformation behaviors of monocrystalline Si induced by nanoscratching were systematically investigated by use of cross-sectional transmission electron microscopy and Raman spectroscopy. The study demonstrated that the lateral load in nanoscratching played a key role in the amorphization of Si, which led to a different phase transformation behavior when compared with the well-documented phase transformation route in nanoindentation. [ABSTRACT FROM AUTHOR]

The layer-wise fracture is a common fracture in porous metal structures, where the whole layer with stress concentration fractures firstly, and then the adjacent layers fracture sequentially. The brittle layer-wise fracture has always been a major obstacle that restricts the development of metallic porous structures, which may induce catastrophic failure under occasional overloading. This work reports that the non-layer-wise fracture was achieved in a beta-type Ti–25Nb–3Zr–3Mo–2Sn (TLM) titanium alloy with 50% porosity cubic lattice structure manufactured by selective laser melting (SLM), mainly due to the stress-induced progressive martensitic transformation (β→α″) supplemented by deformation twinning, resulting in uniform deformation without cracks in the struts even under a large compression strain of ~50%. The in situ synchrotron high-energy X-ray diffraction (HE-XRD) was utilized to study the dynamic deformation behavior of a strut, revealing that the martensitic transformation initiates at the early stage of the compression with a strain of 1.47%, followed by progressive martensitic transformation upon further straining, which probably contributes to the "double-yielding" platform. TEM microstructural observations of the {112}<111> β twins in the strut implies that deformation twinning was activated as a supplementary deformation mechanism for the accommodation of large strain. The findings in this work provide an alternative strategy of introducing non-layer-wise fracture in porous metallic materials through the coupled effect of phase transformation and twinning. [ABSTRACT FROM AUTHOR]