Enhanced gradient crystal-plasticity study of size effects in B.C.C. metal

Owing to continuous miniaturization, many modern high-technology applications such as medical and optical devices, thermal barrier coatings, electronics, micro- and nano-electro mechanical systems (MEMS and NEMS), gems industry and semiconductors increasingly use components with sizes down to a few micrometers and even smaller. Understanding their deformation mechanisms and assessing their mechanical performance help to achieve new insights or design new material systems with superior properties through controlled microstructure at the appropriate scales. However, a fundamental understanding of mechanical response in surface-dominated structures, different than their bulk behaviours, is still elusive. In this thesis, the size effect in a single-crystal Ti alloy (Ti15V3Cr3Al3Sn) is investigated. To achieve this, nanoindentation and micropillar (with a square cross-section) compression tests were carried out in collaboration with Swiss Federal Laboratories for Materials Testing and Research (EMPA), Switzerland. Three-dimensional finite element models of compression and indentation with an implicit time-integration scheme incorporating a strain-gradient crystal-plasticity (SGCP) theory were developed to accurately represent deformation of the studied body-centered cubic metallic material. An appropriate hardening model was implemented to account for strain-hardening of the active slip systems, determined experimentally. The optimized set of parameters characterizing the deformation behaviour of Ti alloy was obtained based on a direct comparison of simulations and the experiments. An enhanced model based on the SGCP theory (EMSGCP), accounting for an initial microstructure of samples in terms of different types of dislocations (statistically stored and geometrically necessary dislocations), was suggested and used in the numerical analysis. This meso-scale continuum theory bridges the gap between the discrete-dislocation dynamics theory, where simulations are performed at strain rates several orders of magnitude higher than those in experiments, and the classical continuum-plasticity theory, which cannot explain the dependence of mechanical response on a specimen s size since there is no length scale in its constitutive description. A case study was performed using a cylindrical pillar to examine, on the one hand, accuracy of the proposed EMSGCP theory and, on the other hand, its universality for different pillar geometries. An extensive numerical study of the size effect in micron-size pillars was also implemented. On the other hand, an anisotropic character of surface topographies around indents along different crystallographic orientations of single crystals obtained in numerical simulations was compared to experimental findings. The size effect in nano-indentation was studied numerically. The differences in the observed hardness values for various indenter types were investigated using the developed EMSGCP theory

Dynamic behavior of advanced Ti alloy under impact loading: experimental and numerical analysis

Industrial applications of Ti-based alloys, especially in aerospace, marine and offshore industries, have grown significantly over the years primarily due to their high strength, light weight as well as good fatigue and corrosion-resistance properties. A combination of experimental and numerical studies is necessary to predict a material behavior of such alloys under high strain-rate conditions characterized also by a high level of strains accompanied by high temperatures. A Split Hopkinson Pressure Bar (SHPB) technique is a commonly used experimental method to characterize a dynamic stress-strain response of materials at high strain rates. In a SHPB test, the striker bar is shot against the free end of the incident stress bar, which on impact generates a stress pulse propagating in the incident bar towards the specimen sandwiched between the incident and transmitted bars. An experimental study and a numerical analysis based on a three-dimensional finite element model of the SHPB experiment are performed in this study to assess various features of the underlying mechanics of deformation processes of the alloy tested at high-strain and -strain-rate regimes

Indentation studies in b.c.c. crystals with enhanced model of strain-gradient crystal plasticity

An enhanced model of strain-gradient crystal plasticity is used to study the deformation behaviour of b.c.c. single crystals of a β-Ti alloy under indentation. In this model, both incipient strain gradients, linked to a component’s surface-to-volume ratio, and strain gradients evolving in the course of deformation were characterized. The results of numerical simulations are in a good agreement with the obtained experimental data demonstrating an anisotropic nature of surface topographies around the indents performed in different crystallographic orientations. The influence of evolved strain gradients on the surface profile of indents is demonstrated

Strain-gradient crystal-plasticity modelling of micro-cutting of b.c.c. single crystal

In recent years thanks to enhancements in design of advanced machines, laser metrology and computer control, ultra-precision machining has become increasingly important. In micromachining of metals the depth of cut is usually less than the average grain size of a polycrystalline aggregate; hence, a cutting process can occur entirely within a single crystal. The respective effect of crystallographic anisotropy requires development of machining models that incorporate crystal plasticity for an accurate prediction of micro-scale material removal under such conditions. To achieve this, a 3D finite-element model of orthogonal micro-cutting of a single crystal of b.c.c. brass was implemented in a commercial software ABAQUS/Explicit using a user-defined subroutine VUMAT. Strain-gradient crystal-plasticity theories were used to demonstrate the influence of evolved strain gradients on the cutting process for different cutting directions

Influence of strain gradients on lattice rotation in nano-indentation experiments: a numerical study

In this paper the texture evolution in nano-indentation experiments was investigated numerically. To achieve this, a three-dimensional implicit finite-element model incorporating a strain-gradient crystal-plasticity theory was developed to represent accurately the deformation of a body-centred cubic metallic material. A hardening model was implemented to account for strain hardening of the involved slip systems. The surface topography around indents in different crystallographic orientations was compared to corresponding lattice rotations. The influence of strain gradients on the prediction of lattice rotations in nano-indentation was critically assessed

Numerical modelling of micro-machining of f.c.c. single crystal: influence of strain gradients

A micro-machining process becomes increasingly important with the continuous miniaturization of components used in various fields from military to civilian applications. To characterise underlying micromechanics, a 3D finite-element model of orthogonal micro-machining of f.c.c. single crystal copper was developed. The model was implemented in a commercial software ABAQUS/Explicit employing a user-defined subroutine VUMAT. Strain-gradient crystal-plasticity and conventional crystal-plasticity theories were used to demonstrate the influence of pre-existing and evolved strain gradients on the cutting process for different combinations of crystal orientations and cutting directions

Computational study of ultrasonically-assisted turning in Ti alloys

The industrial applications of titanium alloys especially in aerospace, marine and offshore industries has grown significantly over the years primarily due to their high strength, light weight as well as good fatigue and corrosion-resistance properties. The machinability of these difficult-to-cut metallic materials using conventional turnning (CT) techniques has seen a limited improvement over the years. Ultrasonically-assisted turnning (UAT) is an advanced machining process, which has shown to have specific advantages especially in the machining of high-strength alloys. In this study a three-dimensional finite element model of ultrasonically-assisted oblique cutting of a Ti-based super alloy (Ti15V3Cr3Al3Sn) is developed. The nonlinear temperature-sensitive material behaviour is incorporated in our numerical simulations based on results obtained with split-Hopkinson pressure bar tests. Various contact conditions are considered at the tool tip-workpiece interface to get an indepth understanding of the mechanism influencing the cutting parameters. The simulation results obtained are compared for both CT and UAT conditions to elucidate main deformation mechanisms responsible for the observed changes in the material’s responses to cutting techniques

Thermally enhanced ultrasonically assisted machining of Ti alloy

Recently, a non-conventional machining technique known as ultrasonically assisted turning (UAT) was introduced to machine modern alloys, in which low-energy, high-frequency vibration is superimposed on the movement of a cutting tool during a conventional cutting process. This novel machining technique results in a multi-fold decrease in the level of cutting forces with a concomitant improvement in surface finish of machined modern alloys. Also, since the late 20th century, machining of wear resistant materials that soften when heated has been carried out with hot machining techniques. In this paper, a new hybrid machining technique called hot ultrasonically assisted turning (HUAT) is introduced for the processing of a Ti-based alloy. In this technique, UAT is combined with a traditional hot machining technique to gain combined advantages of both schemes for machining of intractable alloys. HUAT of the Ti alloy was analysed experimentally and numerically to demonstrate the benefits in terms of reduction in the cutting forces and improvement in surface roughness over a wide range of industrially relevant speed-feed combinations for titanium alloys. © 2014 CIRP

Enhanced gradient crystal-plasticity study of size effects in a β-titanium alloy

A calibrated model of enhanced strain-gradient crystal plasticity is proposed, which is shown to characterize adequately deformation behaviour of b.c.c. single crystals of a β-Ti alloy (Ti-15-3-3-3). In this model, in addition to strain gradients evolving in the course of deformation, incipient strain gradients, related to a component's surface-to-volume ratio, is accounted for. Predictive capabilities of the model in characterizing a size effect in an initial yield and a work-hardening rate in small-scale components is demonstrated. The characteristic length-scale, i.e. the component's dimensions below which the size effect is observed, was found to depend on densities of polar and statistical dislocations and interaction between them

Finite-element simulations of split Hopkinson test of Ti-based alloy

Ti-based alloys are extensively used in aerospace and other advanced engineering fields due to their high strength and toughness, light weight, excellent corrosion resistance and ability to withstand extreme temperatures. Since these alloys are hard to machine, there is an obvious demand to develop simulation tools in order to analyse the material's behaviour during machining and thus optimise the entire machining process. The deformation processes in machining of Ti-alloys are typically characterized by high strains and temperatures. Split Hopkinson Pressure Bar (SHPB) technique is a commonly used experimental method to characterize the material behaviour at high strain rates; the stress-strain relation of the material is derived from the obtained experimental data. A computational study on a three-dimensional finite element model of the SHPB experiment is performed to assess various features of the underlying mechanics of deformation processes at highstrain and -strain-rate regimes. In the numerical analysis, an inhomogeneous deformation behaviour is observed in the workpiece at the initial stages of compression contrary to a standard assumption of stress and strain homogeneity in the specimen