Manuel F. Vieira

In situ TEM study of grain growth in nanocrystalline copper thin films.

Nanocrystalline metals demonstrate a range of fascinating properties, including high levels of mechanical strength. However, as these materials are exposed to high temperatures, it is critical to determine the grain size evolution, as this process can drastically change the mechanical properties. In this work, nanocrystalline sputtered Cu thin films with 43 +/- 2 nm grain size were produced by dc-magnetron sputtering. Specimens were subsequently annealed in situ in a transmission electron microscope at 100, 300 and 500 degrees C. Not only was grain growth more evident at 500 degrees C but also the fraction of twins found. An analysis of grain growth kinetics revealed a time exponent of 3 and activation energy of 35 kJ mol(-1). This value is explained by the high energy stored in the form of dislocation, grain boundaries and twin boundaries existing in nanocrystalline copper, as well as the high probability for atoms to move across grains in nanocrystalline materials.

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°.

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.

Multilayered interface in Ti/Macor® machinable glass-ceramic joints

Macor ® machinable glass-ceramic and commercially pure (cp.) titanium were joined by active metal brazing, using a 64Ag‐34.5Cu‐1.5Ti (wt.%) filler alloy. The influence of the brazing temperature and holding stage on the microstructure and hardness profile of the interface, as well as on the shear strength of the joint, were assessed. Brazing was performed in a high vacuum furnace at 850, 890 and 930°C for 10 and 30 min. The reaction between the braze alloy and both materials led to the formation of a multilayered interface. The interfacial microstructure was analysed in a scanning electronic microscope (SEM) and the composition of each reaction layer was investigated by energy dispersive X-ray scans (EDS). The interfacial hardness profile was determined by a series of microhardness tests on each reaction layer. The mechanical strength of the joint was assessed from shear tests conducted at room temperature. Brazing at 850°C with a 10 min holding stage produced stronger joints, with an average shear strength of more than 85% of the glass-ceramic bulk strength.

Joining of Superalloys to Intermetallics Using Nanolayers

Joining nickel based superalloys to gamma-TiAl intermetallic alloys will contribute to a more efficient application of these advanced materials, particularly in extreme environments. In this study, Inconel alloy and gamma-TiAl are joined using as filler alternated nanolayer thin films deposited onto each base material. The nanolayers consisted in Ni/Al exothermic reactive multilayer thin films with periods of 5 and 14 nm deposited by d.c. magnetron sputtering in order to improve the adhesion to the substrates and to avoid the reaction between Ni and Al. Diffusion bonding experiments with multilayer coated alloys were performed under vacuum at 800°C by applying 50 MPa during 1h. Bonding was achieved in large areas of the centre of the joints where regions without cracks or pores were produced, especially when using multilayer thin films with a 14 nm modulation period.

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.

Joining Ti-47Al-2Cr-2Nb with a Ti/(Cu,Ni)/Ti clad-laminated braze alloy

The joining of Ti-47Al-2Cr-2Nb using Ti-15Cu-15Ni (wt%) as braze alloy was investigated. Experiments were conducted at 980 and 1000°C for 10 min. The microstructure and the chemical composition of the interfaces were studied by scanning electron microscopy (SEM) and by energy dispersive X-ray spectroscopy (EDS), respectively. For both processing conditions the reaction between the γ-TiAl alloy and the braze alloy produced layered interfaces, which are essentially composed of α2-Ti3Al and of Ti-Ni-Cu-Al and Ti-Ni-Cu intermetallic compounds. Microhardness tests showed that all reaction layers are harder than either the γ or the (α2 + γ) lamellar grains of the intermetallic alloy.

Further development of the hybrid model for polycrystal deformation

Abstract The Kocks–Mecking hybrid model for polycrystal deformation has been developed further, in order to obtain an easy definition of polycrystalline behaviour, from knowledge of the single crystal stress–strain curves. The hybrid model is a one-parameter theory that considers the flow stress governed by total dislocation density. This is the addition of the statistically stored dislocations, independent of the grain size, and the geometrical dislocations, necessary to accommodate the deformation, and thus dependent on the grain size (as proposed by Ashby). The condition for the onset of necking is used to determine the parameters of the equation of the model. The analytical description of the resulting flow law was used to analyse the tensile behaviour of copper sheets, with five different grain sizes, which were deformed at three different strain rates.

The effect of brazing temperature on the titanium/glass-ceramic bonding

Abstract The aim of this work is to study the effect of time and brazing temperature on the interfacial microstructure and mechanical properties of the joint obtained by active metal brazing between c.p. titanium and a fluorosilicate machinable ceramic–glass using a 64Ag–34.5Cu–1.5Ti (wt%) brazing alloy. The reaction between the brazing alloy and the two materials leads to the formation of several interfacial reaction layers with different compositions, morphologies and extensions. These layers are constituted by various reaction products that ensure chemical bonding between the two materials, their stability and capability to accommodate the discontinuity of properties across the interface determining the success of the joining. The interfacial microstructure was analysed by SEM and the composition of each reaction layer was investigated by EDS. Microhardness tests were performed across the interfacial zone and the global interfacial mechanical behaviour was evaluated by means of shear tests.

Yield stress after double strain-path change

Two different types of complex loading paths, with three strain paths (rolling-rolling-tension), were carried on a polycrystalline copper sheet. The first and second paths in rolling were performed perpendicular to each other. Depending on the case, the last strain path in tension was considered to be parallel to the first or second rolling path. The effect of double strain path change on the subsequent reloading yield stress is studied in this work. The results show that the behavior after prestraining depends mainly on the orientation relationship between the previous (rolling-rolling) and the subsequent (tension) paths and less on the order in which they have been performed. The mechanical behavior in reloading is discussed on the basis of a statistical study of the slip systems that remain active after path change.