Deformation mechanism in nanocrystalline FCC metals studied by atomistic simulations

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.

Nanocrystalline metals: transient testing during in situ X-ray diffraction and molecular dynamics

Zhen Sun

Nanocrystalline (NC) metals have attracted widespread interest in materials science due to their high strength compared to coarse-grained counterparts. It is well know that during uniaxial deformation, the stress-strain behaviour exhibits an extraordinary work-hardening followed by an early observation of constant flow stress. Information on possible deformation mechanisms have been gathered by extensive research combining in situ deformation experiments, electron microscopy observations and computational modelling. Generally, these mechanisms are categorized into two types: dislocation slip and grain boundary (GB) accommodation processes, as for instance, GB sliding based mechanisms and GB migration eventually coupled to the shear stress. However, the interplay between these mechanisms resulting in the constant deformation resistance is not fully understood. Transient testing has proven to be a suitable tool to gather information on the rate limiting deformation mechanisms that are activated during the deformation path. In particular in stress reduction experiments, after an intermediate/large stress drop thermally activated dislocation slip is suppressed so that other underlying mechanisms are brought into foreground during subsequent transient creep. Those mechanisms may play a minor role in the determination of the flow stress but might still be essential to the development of a constant deformation resistance. Within this thesis, transient testing is combined with in situ X-ray diffraction at the Swiss Light Source. Therefore, the transient responses are captured in terms of evolution of macrostrain as well as diffraction peak broadening. Since dislocation slip and GB accommodation have an opposite footprint on the peak broadening, the presence of these two types of mechanisms can be distinguished. Three electrodeposited NC materials with different grain sizes are investigated: two NC Ni batches and one NC Ni50Fe50 batch. The results reveal that the constant flow stress reached during uniaxial deformation of electrodeposited NC metals reflects a quasi-stationary balance between dislocation-based mechanisms and GB-mediated accommodation. The latter plays an important role in producing plastic strain and recovering the defects and internal stress. Depending on the magnitude of the stress drop, a non-monotonic behaviour of the diffraction peak width is observed, suggesting an alternation of mechanisms. Also, by comparing transient responses among different NC materials, the relative contributions of dislocation slip and GB accommodation mechanisms are discussed in terms of grain size and alloying. Finally, different magnitudes of stress reduction are carried out by molecular dynamics (MD) simulations with the aim to verify in situ experimental results and explore the mechanisms responsible for GB accommodation. MD simulations confirm that dislocation slip is reduced after a moderate stress drop, however can continue to operate after adaption of the GB structures by a variety of GB accommodation mechanisms explaining the non-monotonic behaviour of the peak broadening during transient creep.

EPFL2016

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

The relationship between the irradiation induced damage and the mechanical properties of single crystal Ni

Zhongwen Yao

Irradiation is known to lead to a degradation of the mechanical properties of materials. This is particularly crucial in the case of materials that will be used in the future thermonuclear fusion reactor, where extremely high irradiation doses are expected. In the quest of a better understanding between the irradiation induced defects and the mechanical properties Ni single crystal specimens have been irradiated with 590 MeV protons to doses ranging from 10-3 dpa to 0.3 dpa, at room temperature, 250°C and 350°C. The irradiation induced microstructure has been characterized by transmission electron microscopy, and the mechanical properties have been assessed by mechanical testing. Molecular dynamics (MD) simulations have been conducted in Ni and Cu in order to understand the defect formation and accumulation following a displacement cascade. The following conclusions are established. Following irradiation at room temperature, 43% to 55% of the irradiation induced defects in Ni consist in stacking fault tetrahedra (SFTs), 31%∼41% in loops and about 10 % in unidentified black dots. In the case of Ni irradiated at 250 °C, 44%∼53% of the irradiation induced defects consists in SFTs, 35%∼51% in loops, and less than 5% in black dots. In the case of Ni irradiated at 350 °C, 50% of the irradiation induced defects consists in voids. The remaining 50% include ∼ 14% SFTs and 36% loops. Moreover, it appears that these ratios are independent from the irradiation dose, contrary to what was found in the literature. These results allow clarifying a long standing issue, namely that in fact in Ni there is no transition in the ratio between SFTs and loops with increasing dose. In addition, it appears that in fact the irradiation induced defect density is similar to the one found in other irradiated fcc metals for RT irradiation. The data of the irradiation at 250°C are in agreement with previous published results on a neutron irradiation at 230 °C. It appears that the size of SFTs is independent of the irradiation dose, similar to what is found in irradiated Cu. It depends, however, on the irradiation temperature. MD simulations have been performed in order to understand the influence of the interatomic potential's parameters on the formation of defects following displacement cascades. Starting with a known defect configuration, namely the SFT, 4 different potentials are tested. Results of the formation of an SFT from a triangular platelet of vacancies show that i) the platelet of 6 vacancies did not collapse to SFT regardless of the annealing conditions, but formed a void above 800K, for all selected potentials; ii) the platelet of 15 vacancies collapsed into an SFT when simulated with Farkas-I and Farkas-II potentials, which have low stacking fault energies; iii) the platelet of 66 vacancies easily collapsed to an SFT at 500K for all applied potentials, even with a high stacking fault energy (SFE) of 300 mJ·m-2. The common neighbor analysis and Wigner-Seitz defects analysis are used for studying the resulting structures after a displacement cascade, and results show that the largest number of stacking faults is surprisingly obtained with the potential giving the highest stacking fault energy (300 mJ·m-2). An SFT-like structure appears close to the core of cascades and also an isolated interstitial loop are found with Cleri-Rosato potential. It appears that the defects size, configuration and density relates more to the displacement threshold energy than to the stacking fault energy. It appears that a significant irradiation hardening occurs starting at the lowest dose of 10-3 dpa, at room temperature and 250°C. With increasing dose, the yield shear stress increases. The dose dependence of this increase varies from an irradiation temperature of room temperature to 250°C. In fact the irradiation hardening shows a significant temperature dependence. The thermal activation energy was calculated by measuring the activation volume, which is obtained from relaxation tests. In unirradiated Ni, an activation energy of about 0.3 eV can be extrapolated at RT. In case of irradiated Ni, the unsuccessful fitting with a linear relationship between the thermal energy and temperature suggests that multiple deformation mechanisms are operating simultaneously or in turn over the considered temperature range of –196 °C to 423 °C. The dispersion barrier hardening model is used to interpret the irradiation induced hardening in irradiated Ni. According to the equation, Δτ = αμb(Nd)1/2, with α, the obstacle strength. It appears that α is 0.12 for the room temperature irradiation induced hardening, similar to what was previously found for irradiated Cu, and is 0.22 for the hardening following irradiation at 250°C, which is much larger than previous values. This difference in α is suggested to be related to the presence of voids at high temperatures. It is suggested that voids are stronger obstacles than SFTs or dislocation loops. In situ TEM observations were made in irradiated Cu, in order to assess the strength of irradiation induced defects as obstacles to gliding dislocations. It appeared that the strength of obstacles is approximately 100 MPa. Considering that 90% of the damage consists in SFTs, it is suggested that the obstacles presenting a resistance of 100 MPa are SFTs. In particular, the pulling out of a string from the interaction between what was assumed to be an SFT and a moving dislocation could be recorded for the first time. This reaction implies that the gliding dislocation is strongly pinned by SFTs. TEM observation of the microstructure of the irradiated Ni following plastic deformation shows that defect-free channel formation is the deformation mechanism at the beginning of yielding. The channeling explains the observed softening. As deformation proceeds, a dislocation cell structure is initiated. It takes different forms depending on whether the sample is irradiated or not. In the irradiated case, cell formation initiates from mobile dislocation in the defect free channels. This implies that the eventual cell configuration will retain part of the dimensionality of the channels, for the least their width of 100 nm for the cell size, because of the geometrical constraint. In the unirradiated case the dislocation cell size is larger. The higher flow stress observed in the irradiated case is thus related to the dislocation cell size.

EPFL2005