Cast aluminium single crystals cross the threshold from bulk to size-dependent stochastic plasticity

Metals are known to exhibit mechanical behaviour at the nanoscale different to bulk samples. This transition typically initiates at the micrometre scale, yet existing techniques to produce micrometre-sized samples often introduce artefacts that can influence deformation mechanisms. Here, we demonstrate the casting of micrometre-scale aluminium single-crystal wires by infiltration of a salt mould. Samples have millimetre lengths, smooth surfaces, a range of crystallographic orientations, and a diameter D as small as 6 mu m. The wires deform in bursts, at a stress that increases with decreasing D. Bursts greater than 200nm account for roughly 50% of wire deformation and have exponentially distributed intensities. Dislocation dynamics simulations show that single-arm sources that produce large displacement bursts halted by stochastic cross-slip and lock formation explain microcast wire behaviour. This microcasting technique may be extended to several other metals or alloys and offers the possibility of exploring mechanical behaviour spanning the micrometre scale.

Tensile properties of irradiated Cu single crystals and their temperature dependence

Zhongwen Yao

Single crystal copper was irradiated with 590 MeV protons to a dose of 0.01 dpa at room temperature. Irradiated and unirradiated tensile samples were deformed and relaxation tests were performed at temperatures between 77 and 293 K. The tests show a strong temperature dependence of the flow stress of irradiated samples as compared to the unirradiated case. Deformed microstructures in the unirradiated and irradiated samples were investigated by transmission electron microscopy. The plastic deformation mechanism of the irradiated samples is dislocation slip, while in the unirradiated sample at large strains and at 77 K, twinning is observed. In the irradiated case, strain localization in the form of defect-free channel takes place, over the temperature range from 77 to 293 K. Deformation processes are analyzed through the determination of the activation volumes and energies of the deformation mechanisms, as deduced from relaxation tests. The activation energy in unirradiated Cu has an approximate value of 1.6 eV. In the irradiated samples it is suggested that more than one deformation process is operative, in the temperature range from 77 to 293 K. (C) 2004 Elsevier B.V. All rights reserved.

2004

On the relationship between the microstructure and the mechanical properties of an ODS ferritic/martensitic steel

Amuthan Ramar

In the quest of materials for the first wall of the future fusion reactor, it has been shown that oxide dispersion strengthened (ODS) ferritic / martensitic (F/M) steels appear to be promising candidates. The inherent good mechanical properties supported by a good thermal conductivity, swelling resistance and low radiation damage accumulation of the base material, such as EUROFER97, are further enhanced by the presence of a fine dispersion of nanometric oxide particles. They would allow in principle for a higher operating temperature of the fusion reactor, above 550°C, which improves its thermal efficiency. In effect, their strength remains higher than the base material at high temperatures. There are however limitations to their application in the reactor, which relate to the intimate link between the microstructure and the mechanical properties. These limitations, such as an increased ductile to brittle transition temperature above room temperature relative to the base material, come directly from the role played by the oxide particles but also from their indirect impact on the overall microstructure, stemming from the procedure of elaboration of these materials. In addition, there is a lack of knowledge on their response to irradiation, which, even though it appears to be promising with a reduced irradiation induced hardening, is not very well understood. In this work we have developed and evaluated ferritic/martensitic steel, EUROFER97, strengthened by a dispersion of yttria particles. (1) The powder metallurgical process used to obtain it was optimized, (2) the basic mechanisms of hardening were identified and (3) the impact of irradiation on its microstructure and mechanical properties were investigated. In order to understand the behaviour of this class of material other oxide dispersion strengthened (ODS) ferritic/martensitic steels based on EUROFER97 and model alloys were also investigated. For the elaboration of the ODS steel the powders of atomized EUROFER97 and yttria are ball milled in a planetary ball mill and the material is compacted by hot isostatic pressing (HIP). The mechanical properties are evaluated by uniaxial tensile test, Charpy test and micro-indentation while the microstructure is investigated by optical microscopy, scanning electron microscopy and transmission electron microscopy. The focused ion beam (FIB) technique is also used. In the study of the ball milling it appears that the optimal time is 20 hours, as the X-ray peak broadening, crystallite size and hardness of the particles remain unchanged at longer times. Yttria nanoparticles clusters are no more detectable by X-ray after 2 hrs of milling, which is attributed here to their incorporation in the matrix, as opposed to dissolution in the matrix as is often suggested in the open literature. HR-TEM performed on the yttria particles before and after incorporation in the steel by ball milling shows that the yttria maintain its bcc equilibrium structure throughout the process. This could suggest that they are not dissolved during ball milling. We have clarified this point by a cross sectional examination of the EUROFER97 ball milled particles with 0.3wt% of yttria in an FIB. By X-ray EDS analysis it shows nanometric regions reach in yttrium and oxygen, which are the yttria particles embedded in the EUROFER97 particles, proving that they are not dissolved by ball milling. The mechanism at the origin of the alloy compaction during HIP was elaborated. The alloy compaction was done through HIPing at 1150°C for 2.5 hrs at a pressure of 190 MPa. This condition is shown to provide the highest bulk density, to a value close to 99.9%. In the optimization process, it is found that temperature plays a larger role than pressure for alloy compaction. The compaction appears to be mainly due to diffusion. Also, it appears that the ball milled particle size plays a major role in the alloy compaction. It is shown that smaller particles size induces better compaction, provided that the densification proceeds through diffusion flow. This further suggests to terminate the ball milling process by 20 hrs as the particles size reaches its minimum. The impact of the ball milling process on the mechanical properties was evaluated. EUROFER97 with and without ball milling, with and without oxide addition presents an average density around 99.8%. In the as received state, casted EUROFER97, atomized EUROFER97, ball milled EUROFER97 and ODS Yttria present microstructure morphology typical of tempered martensite, whereas ODS Ti presents a nano sized grain microstructure with an average grain size of 200 nm. Ball milling and HIPping does have a slight influence up to 400°C, due to the higher dislocation density induced by ball milling relative to casted EUROFER97. It becomes negligible above 400°C, since both casted and ball milled EUROFER97 present similar hardness and microstructure. In ODS Yttria, yttria presents an average particle size of 25 nm. In ODS Ti, Y-Ti-O presents an average particle size of 7 nm. In the following the identification of the hardening mechanisms in ODS F/M steels. A new model was developed to quantify the collective hardening due to different obstacles, assuming that they provide independent hardening mechanisms. The model helped identifying the dominating source of hardening in ODS F/M steels. The base of the model is the dispersion barrier strengthening model, which is given by Δσsparticle or defects = Mαμb(Nd)1/2 for the hardening due small defects or oxide particles and is given by Δσd-d = Mαμb(ρ)1/2 for the hardening due to dislocations. In this approximation, the total hardening is then given by Δσtotal = {(Δσd-d)2 + (Δσparticle or defects)2}1/2 . The model is used to explain the hardening due to oxides, dislocations and irradiation induced defects. It appears that dispersed oxides is dominating hardness up to 800°C, while following a heat treatment at 1000°C it is the dislocations that then dominate the hardness. Deformation mechanisms were investigated using differential strain rate experiments. The activation parameters, i.e. the activation volume and activation energy, were calculated. It appears that there is a decrease in the activation volume from room temperature to 400°C, which is less than 3%. At about 500°C there is a sudden drop in the activation volume that reaches a minimum at 600°C. With further increase in temperature, there is a significant raise in the activation volume. This abrupt decrease between 400°C and 600°C hints at a change in dislocation-rate controlling mechanism, marking two regimes, a low temperature one and a high temperature one. In situ TEM observations show that at low temperature the deformation is mainly due to the formation of the Orowan loops around the particle. At high temperature the activation energy is about 3.4 eV for ODS, whereas it is 1.3 eV for EUROFER97. The calculated activation energy corresponds well to the one of dislocation climb mechanism. This indicates that at high temperature the yttria particles could be overcome by climb, while at low temperature they are overcome by the Orowan mechanism. The peculiar response of the yield stress to irradiation for this class of material was rationalized using the total hardening model we developed. At a dose of 2 dpa, irradiation induced defects are higher in number density than the dispersed yttria particles, which although they are weaker than the yttria particles they can explain the significant hardening observed in the material. Conversely, the shallow impact of irradiation on ODS F/M steels at low doses, relative to base F/M steels, is explained by the fact that the amount of dispersoid is found to be higher than the density of irradiation induced defects.

EPFL2008

Deformation mechanism in nanocrystalline FCC metals studied by atomistic simulations

Christian Brandl

Polycrystalline materials with crystallite diameters below hundred nanometer exhibit extraordinary strength which goes along with a decrease in ductility. In order to tailor tough materials, which combine strength and ductility, the underlying deformation mechanisms have to be understood. Molecular dynamics suggested partial dislocation mediated plasticity in nanocrystalline face-centred cubic (FCC) metals in terms of 1) dislocation nucleation at the grain boundaries, 2) dislocation propagation through the nano-grain, and 3) eventual dislocation absorption in the neighbouring grain boundary network. The partial dislocation nucleation sites were seen to exhibit stress concentrations such as in triple junctions. Moreover, it has been shown that the partial dislocation nucleation is energetically related to the so-called general stacking fault energies (GSFEs). Transmission electron microscopy, in-situ X-ray diffraction, and deformation experiments can be interpreted in terms of the aforementioned dislocation processes. On basis of this knowledge this thesis utilizes atomistic simulation methods to extend the understanding of the fundamental deformation mechanisms which mediate the plasticity in bulk nanocrystalline FCC metals. In the first part of this thesis, ab-initio simulations are exploited to investigate the effect of the superimposed strains in Ni, Al, and Cu on the GSFEs, which describe the energy penalty for rigidly shearing of the crystal. The energy curves are discussed in terms of a) the second and third order elastic constants b) the energy difference between hexagonal closed packed and face centred cubic structures, c) the GSFE / unstable-twinning-fault-energy interdependence and c) the implication on dislocation nucleation. The second part explores the strain rate, temperature, applied stress, and grain size dependence of nanocrystalline Al utilizing classical molecular dynamics simulations. The deformation behaviour is examined on the basis of the underlying deformation mechanism. The higher the strain rate the more the onset of dislocation mediated plasticity is delayed. This can manifest itself into a stress overshoot that diminishes as the strain rate is lowered. The associated critical resolved shear stresses are seen to cause the strain rate sensitivity in the flow stress. In conjunction with the rate dependence of the critical resolved shear stresses, the temperature dependence of the flow stresses supports the picture of thermally activated dislocation processes which control the deformation of nanocrystalline Al in molecular dynamics. Moreover, the strain rate simulations evidence that a subset of dislocation processes operate at the athermal limit which indicates, once more, the difficulty to extrapolate from the high strain rate regime of molecular dynamics towards common experimental conditions. Furthermore, using an iterative strain/relaxation method, athermal critical stresses are calculated for an isolated nano-grain with a nucleated dislocation therein. Dislocation cross-slip is seen to enable a dislocation to by-pass stress concentration, which can hinder the dislocation propagation through the grain, and allows the dislocation to be deposited in preferred grain regions as e.g. triple junctions. This rationalized the observed lack of strain hardening in the molecular dynamics study. Additionally dislocation cross-slip is more pronounced at lower strain rates which indicate the growing importance of the grain boundary structure for dislocation motion as the stresses are lowered. Slip transfer through a high angle grain boundary is observed and is understood in terms of the grain boundary structure, which exhibits regions of preferred dislocation transmission sites. Such structural features are discussed by means of the implication on mesoscopic models of dislocation transmission. Moreover, the grain size study shows discrete stress-drops, which are associated with dislocation mediated processes, in the so-called "inverse" Hall-Petch regime. The deformation behaviour of nanocrystalline Al in molecular dynamics is rationalized in terms of thermally activated processes, and therein the implications of the grain size dependencies are qualitatively discussed.