J.V. Fernandes

Ultra-microhardness testing procedure with Vickers indenter

Abstract Depth-sensing indentation equipment is widely used for evaluation of the hardness and Youngs modulus of materials. The depth resolution of this technique allows the use of ultra-low loads. However, aspects related to the determination of the contact area under indentation should be cautiously considered when using this equipment. These are related to the geometrical imperfections of the tip, the diamond pyramidal punch and the formation of pileup or the presence of sink-in, which alter the shape and size of the indent. These and other aspects, such as the thermal drift of the equipment and the scattering at the zero indentation depth position related to surface finishing, are discussed in this work. A study concerning the hardness and the Youngs modulus results determined by Vickers indentation on different materials was performed. Samples of fused silica, BK7 glass, aluminium, copper and mild steel (for which the values of Youngs modulus were previously known) were tested using indentation loads in the range 10–1000 mN. Moreover, two methods are proposed for performing the indentation geometrical calibration of the contact area; these are compared with a former method proposed by Oliver and Pharr (OP). The present methods are based on: (i) analysis of the punch profile using atomic force microscopy (AFM); and (ii) a linear penetration-depth function correction (LM), based on knowledge of the values of the Youngs modulus of several materials. By applying these methods to the indentation load/indentation depth results, it was possible to draw some conclusions about the benefit of the AFM and LM methods now under proposal.

Effects of ion bombardment on properties of d.c. sputtered superhard (Ti, Si, Al)N nanocomposite coatings

d ´˜ Abstract A d.c. reactive magnetron sputtering technique was used to deposit (Ti, Si, Al)N films. The ion current density in the substrate was varied by the superimposition of an axially symmetric external magnetic field between the substrate and target. It was found that the variation of the magnetic field strength induced changes in the ion current density in the substrate with a consequent change in film properties. XRD patterns of sputtered films revealed changes of the lattice parameter (from 0.418 nm to approx. 0.429 nm) with the increase of the ion yatom arrival rate ratio. As already reported for samples prepared by r.f. sputtering, both can be assigned to a cubic B1 NaCl structure, typical for TiN. The lowest lattice parameter corresponds to a metastable phase where Si and Al atoms occupy Ti positions, while the highest lattice parameter corresponds to a system where at least a partial segregation of TiN and SiN phases already occurred, leading to the formation of a nanocomposite film of the type nc-TiAlNya- x Si N. The mixture of the metastable phase with nanocomposite coating phases in some samples indicates that, in general, the 34 segregation of TiN and SiN phases is not complete. Hardness values as high as 45 GPa were measured. Small Si additions to x (Ti, Al)N coatings induce a reduction in the pin-on-disk sliding wear rate. � 2002 Elsevier Science B.V. All rights reserved.

Effect of grain size on substructural evolution and plastic behaviour of copper

Abstract Previous study has established that close correlations exist in copper single crystals between dislocation cells and active slip systems. In grains of a polycrystalline sample, comparison between cells and slip systems may indicate the level of intragranular stress field. In order to reach a better understanding of cell development in polycrystalline metals, tensile tests were performed on copper sheets with different grain sizes (between 20 and 250 μm). Transmission electron microscopy observations have shown that for a 250 μm grain size the dislocation substructure in similar to that which has been observed in single crystals; in some grain orientations only one family of parallel dislocation walls is observed. For grain sizes equal to or lower than 65 μm the plastic behaviour is strongly influenced by the interaction between grains; closed cells are present whatever the grain orientation. Such observations may be explained through a composite model taking into account the statistical and geometrical dislocation densities. However, a strain limit value appears in the applicability of this model; the limit is dependent on the grain size. For large strain amounts the accomodation between dislocation cells seems to control the stress-strain curves.

Plastic behaviour of copper sheets during sequential tension tests

Abstract Sequences of two uniaxial tension tests along different axes were performed on a polycrystalline copper sheet. In order to reach a better understanding of the physical mechanisms occurring during the second deformation, the effect of the strain path change on subsequent yield and flow behaviour has been investigated using optical and transmission electron microscopy. The value of the reloading yield stress is a function of the angle φ between the two tensile axes. For φ >15° the requirement of glide of dislocations with a new Burgers vector implies that a very low density of potentially mobile dislocations is available at the beginning of the reloading deformation. This effect is moderated at φ values of about 90° by the inverse activity on some slip systems during reloading. The transient observed in the work-hardening behaviour after the path change is concomitant with the disappearance of some dislocation walls developed during the prestrain. A more homogeneous dislocation substructure appears for φ > 15° as a result of the interactions between the mobile dislocations on the new active slip systems and the previous dislocations walls; the dissolution of the previous dislocation arrangement is also promoted by the inversion of the slip direction in some systems, depending on the value of the angle φ. This results in an increase in dynamic recovery rate during the early stage of reloading, particularly when φ approaches 90°.

Numerical study of the plastic behaviour in tension of welds in high strength steels

The influence of the mismatch between material properties and constraint on the plastic deformation behaviour of the heat affected zone of welds in high strength steels is investigated in this study, using finite element simulations. An elastoplastic implicit three-dimensional finite element code (EPIM3D) was used in the analysis. The paper presents the mechanical model of the code and the methodology used for the numerical simulation of the tensile test of welded joints. Numerical results of the tensile test of welded samples with different hypothetical widths for the Heat Affected Zone and various material mismatch levels are shown. The analysis concerns the overall strength and ductility of the joint and in relation to the plastic behaviour of the heat affected zone. The influence of the yield stress, tensile strength and constraint on the stress and plastic strain distribution in the soft heat affected zone is also discussed.

A modified swift law for prestrained materials

Abstract A modified Swift law to describe the evolution of the mechanical behaviour in reloading of prestrained materials is proposed in this work. This equation is deduced from the original Swift law by including a parameter that accounts for the effect of strain path change. This parameter depends on the value of the yield stress and the subsequent work-hardening behaviour in reloading. The new equation predicts well the general mechanical behaviour in the second path for copper and steel. In particular, it predicts accurately the strain value for which necking occurs during reloading and fits experimental stress-strain curves well. The flow equation formulated remains sufficiently simple to be applied in finite element modelling of prestrained materials. However, since the parameter, which is needed for the modified Swift law, must be previously known, the strain path change itself cannot be part of the simulation.

Estimation of young's modulus and of hardness by ultra-low load hardness tests with a vickers indenter

The evaluation of the elastic-plastic properties of a material by using an ultra-low load hardness test requires a geometrical calibration that must take into account the imperfect form of the diamond indenter. In the present work, the Vickers indenter offset of the microindentation equipment was estimated using differently heat-treated steel samples. To this end, the dimensions of the indentations have been evaluated by two different methods: optical measurement of the diagonals and direct measurement of the penetration depth during the test. The elastic-plastic properties are then calculated from the analysis of the penetration depth/indentation load curves. The Youngs modulus values determined for the different high-speed steel samples were very similar and close to the literature value for steel if the appropriate corrections are performed. The hardness values decrease when the determination procedure includes the geometrical correction of the indenter offset, and still further when using the total correction obtained by means of optical measurements of the indenter diagonal. Variation of the hardness values with the applied load is much less when the corrections are carried out.

Evolution of the microstructure, residual stresses, and mechanical properties of W-Si-N coatings after thermal annealing

W-Si-N films were deposited by reactive sputtering in a Ar + N-2 atmosphere from a W target encrusted with different number of Si pieces and followed by a thermal annealing at increasing temperatures up to 900 degrees C. Three iron-based substrates with different thermal expansion coefficients, in the range of 1.5 x 10(-6) to 18 x 10(-6) K-1 were used. The chemical composition, structure, residual stress, hardness (H), and Youngs modulus (E) were evaluated after all the annealing steps. The as-deposited film with low N and Si contents was crystalline whereas the one with higher contents was amorphous. After thermal annealing at 900 degrees C the amorphous film crystallized as body-centered cubic alpha-W. The crystalline as-deposited film presented the same phase even after annealing. There were no significant changes in the properties of both films up to 800 degrees C annealing. However, at 900 degrees C, a strong decrease and increase in the hardness were observed for the crystalline and amorphous films, respectively. It was possible to find a good correlation between the residual stress and the hardness of the films. In several cases, particularly for the amorphous coating, H/E higher than 0.1 was reached, which envisages good tribological behavior. The two methods (curvature and x-ray diffraction) used for calculation of the residual stress of the coatings showed fairly good agreement in the results.

The effect of strain path change on the mechanical behaviour of copper sheets

Abstract Oxygen-free high purity copper sheet (99.95% Cu) has been prestrained in tension, rolling, shear and equibiaxial stretching. Effects of the prestrain value, nature of prestrain and path change on the total homogeneous deformation were evaluated by subsequently performing uniaxial tensile tests with the axis parallel to a reference direction on the sheet. After a critical prestrain value, the path change measured by a parameter α influences the residual homogeneous deformation during the subsequent path. A phenomenological model, based on the estimation of the reloading limit stress with respect to the reference stress corresponding to the prestrain amount, was proposed to predict the occurrence of the early plastic instability during the subsequent path. According to the model, the dependence of the residual uniform strain on the amplitude of the strain path change is only due to the relation existing between the reloading stress and the parameter α.

Mechanical behaviour and the evolution of the dislocation structure of copper polycrystal deformed under fatigue-tension and tension-fatigue sequential strain paths

Two sequences of tension-fatigue and fatigue-tension tests were performed on copper polycrystal sheet, with a mean grain size of 32 mum. For the angle between the two successive loading directions, two typical values (0 and 45degrees) have been chosen. The effect of strain path change on subsequent initial work hardening rate and saturation stress in tension-fatigue, as well as the effect of strain path change on subsequent yield and flow behaviour in fatigue-tension have been investigated. The strain rate for the tension tests was 5 x 10(-3) s(-1), while the fatigue tests were performed under constant plastic strain amplitude control with different values of amplitudes (epsilon(pl) = 6 x 10(-4), 1.5 x 10(-3), 3 x 10(-3)). Slip morphology and dislocation microstructure were observed by optical and transmission electron microscopy (TEM) after mechanical tests. Under these conditions, in the case of fatigue-tension, it was found that fatigues prestraining influences the subsequent yield and flow behaviour in tension. However, the subsequent mechanical behaviour of samples seems only to be affected by the magnitude of strain path change (namely, the angle between the two successive loading directions), and not by the value of the plastic strain amplitude of the preceding fatigue tests. In the case of tension-fatigue, the strain amount of preloading in tension obviously affects the initial cyclic hardening rate, while it has almost no effect on the saturation stress of subsequent fatigue tests, irrespective of the value of the angle between the two successive loading directions. The occurrence of microbands in the saturation fatigue dislocation structures of samples prestrained in tension implies that fatigue is a more effective loading mode than tension, in causing intense glide on the activated slip systems. The correlation between mechanical properties and microstructural observations is discussed