Évolution microstructurale et propriétés mécaniques d'un alliage Co-Ni-Cr

This study concerns the evolution of the thermomechanical properties of new high-performance materials. In particular, we focus on the relationship between the microstructure and the mechanical properties of a superalloy based on Co-Ni-Cr. Superalloys are metallic alloys with superior characteristics to conventional alloys, especially in extreme conditions, and find application in all that areas requiring materials with high elastic modulus, yield strength and resistance. The investigated alloy is used with satisfaction since several years, but the origin of its outstanding mechanical properties is still poorly understood, as well as the role of small alloying elements additions on mechanical properties. To understand the influence, on the mechanical properties, of the elements added to the base composition, the study is carried out simultaneously on two variants of the material, differing for the presence of a small amount of beryllium in one them. The variant containing beryllium exhibits superior mechanical properties with respect to the base variant. The microstructural evolution and mechanical properties was monitored by using several complementary techniques. Series of heat treatments were performed on samples from the two variants, and both TEP and hardness were systematically measured. This has allowed us to establish the different stages of microstructural changes with the temperature. The TEP is a parameter very sensitive to the change in matrix's composition. It indicates that exposure of this material to a temperature between 500°C and 600°C produces a precipitation stage (A), involving titanium. Such a precipitation is responsible for a remarkable hardening, as well as an increase in the elastic limit. Mechanical spectroscopy allows us to obtain information about the mobility of structural defects which dissipate energy when undergone to a periodic stress. By developing a phenomenological model based on the ADIF curves, we can affirm that the hardening comes from the pinning of dislocations by A precipitates. A second precipitation stage (B) occurs only on the variant containing beryllium, undergone a complex sequence of heat treatments up to 700°C. The analysis of samples by TEM and APT provided essential support to the interpretation of results, and to characterization of the precipitation B. Precipitates B are nanostructured intermetallic compounds NiBe, analogous to GP zones. Their growth is favored by the segregation of titanium at the precipitate-matrix interface to relieve the coherency strains. The hardening provided by B precipitates, together with the structural hardening due to the cold working and the precipitation hardening due to the heating at 550°C, allows to obtain the maximum hardness values. The peaks on the internal friction spectra, also provide informations about the influence of beryllium on the mobility of grain boundaries. This parameter determines the recrystallization and the relaxation mechanism at high temperature, which can be observed by mechanical spectroscopy. Finally, the work here presented yields an important contribution to the understanding of the performance's origin of a Co-Ni-Cr superalloy. It also points the way for other studies aiming the optimization of production processes, in terms of temperature and time of annealing, to obtain the desired properties. This thesis allowed to discover a new hardening phase (B) previously unknown, which could lead to even higher hardness values.

Grain Boundary Relaxation in 18-carat Gold Alloys

Ann-Kathrin Audren

Grain boundaries (GBs) play an important role for the mechanical properties of metals. In addition to dislocation motion inside the grains, some metal alloys deform along the GBs at high temperatures. GBs can also be the cause of a brittle behaviour of some metallic alloys, where crack propagation along the GBs is observed. This thesis aims to understand the role of GBs on the thermo-mechanical behaviour of metals and especially to define the microscopic mechanism, which leads to a deformation at the GBs. In this thesis, polycrystals, single crystals and bi-crystals of a yellow gold alloy are studied in detail, primarily by mechanical spectroscopy. The analysis and interpretation of the experimental data identifies different anelastic relaxations (internal energy dissipation processes), that produce peaks in the mechanical loss spectrum. The mechanical loss spectrum of polycrystals shows a relaxation peak P2 at about 780K, which is absent in single crystals made from the same alloy. Stepwise deformation of a single crystal causes an increase of the high temperature mechanical loss background and the appearance of a high temperature peak (P3). Above a critical deformation, P3 disappears and the peak P2, which is normally observed in polycrystals, appears. The increase of the exponential background is interpreted as due to the introduction of new dislocations whereas the peak P3 is attributed to a dislocation relaxation mechanism in the sub-grain boundaries. The peak P2 located at intermediate temperatures depends on the grain size: with grain growth, the peak position shifts to higher temperatures. It is shown that the relaxation time is proportional to the grain size d. Such a grains size dependence is in agreement with the Zener model based on geometrical considerations of an assembly of elastic grains, which can slide against each other. The investigations on bi-crystals containing a single GB with a specific misorientation between crystal lattices and a specific boundary plane orientation show that the relaxation peak P2 is closely related to a mechanism taking place at the boundary. The peak height is proportional to the GB density in a bi-crystal, whereas a single crystalline part cut from a bi-crystal does not show a relaxation peak. Molecular dynamics simulations are performed in order to illustrate the potential microscopic mechanisms responsible for the stress relaxation peak P2 in polycrystals. A Sigma5 grain boundary is submitted to a shear deformation parallel to the boundary plane. The grain boundary shows a migration perpendicular to the boundary plane coupled to shear for temperatures below 700K. Above 1000K, only grain boundary sliding is observed. Two models are developed that provide expressions for the relaxation strength and the relaxation time that are compared to experimental measurements performed on polycrystals. The observed grain size dependence favours the sliding model over the migration model. Measurements as a function of stress allow a refinement of the sliding model. The stress amplitude dependence of the peak P2 indicates a depinning mechanism, which is interpreted as due to GB dislocations between zones where sliding of different amount occurred. Obstacles to the sliding movement like steps or extended coincidence sites in the GB plane act as pinning points. The pinning points can be overcome at high temperatures close to the melting point, which results in the onset of local microscopic creep.

EPFL2014

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

Simulation de la formation des microstructures dans les superalliages à base de nickel de type AM1 durant la solidification et les traitements thermiques

Luc Rougier

Nickel base superalloys are used for the manufacturing of monocrystalline turbine blades, due to their outstanding mechanical properties at high temperature. The mechanical properties strongly depend on the presence of γ’ precipitates in the γ matrix. Improper size or spatial distribution of the precipitates is one of the possible defects that can appear during the processing of turbine blades. In order to anticipate such defects and optimize the processes, numerical simulation can be used to predict the formation and evolution of precipitates as a function of the alloy chemistry and heat treatment conditions. Numerical models can also be used to analyze the influence of microsegregation, which develops during solidification and does not entirely disappear during the solution heat treatment. A modeling approach has been developed to simulate the formation of microstructure in AM1-type nickel based superalloys during the main processing steps. Two distinct numerical models were developed for the simulation of microsegregation, on one hand, and precipitation on the other hand. An existing microsegregation model [1] was modified to take into account important specificities of Ni-base superalloys, such as non-diagonal matrices of diffusion coefficients. The model was applied to several alloys to predict the evolution of the composition profiles, and the formation/dissolution of interdendritic phases during solidification and the solution heat treatment. A new precipitation model was developed in order to predict the formation and evolution of γ' precipitates during heat treatment [2]. The model is based on the description of the precipitate size distribution, which is tracked based on nucleation and growth laws, taking into account coarsening through the Gibbs-Thomson effect. The time evolution of average quantities, such as the average radius, the number density of precipitates and volume fraction is deduced from the particle size distribution. The model is coupled to the phase diagram software Thermo-Calc, which makes it possible to calculate equilibrium concentrations and nucleation driving force in multicomponent industrial alloys. The coupling to Thermo-Calc is also exploited to compute the effective diffusion coefficients from a mobility database. The microsegregation and precipitation models were applied to ternary Ni-Al-Cr alloys and to the industrial CMSX-4 and AM1 superalloys. The results of the simulations were compared with experimental data collected from the literature and atom probe tomography measurements carried out during the project. The simulation results showed to be globally in good agreement with the experiments. The accuracy of the thermodynamic database turned out to be one of the most important factors for quantitative prediction, in particular for precipitation. Chained simulations were performed on Ni-Al-Cr and AM1 alloys in order to assess the sensitivity of precipitation kinetics with respect to microsegregation. The calculations showed that the radius, volume fraction and number density of precipitates are substantially influenced by the residual segregation in industrial heat treatments. The effect of residual segregation on precipitation kinetics decreases however during coarsening at long ageing times. In summary, the simulation approach developed in this project allows for the formation of microstructures in industrial Ni-based superalloys to be analyzed in detail. The application of chained microsegregation and precipitation calculations to AM1 superalloys permitted to analyze the influence of the solidification conditions on the microstructure after heat treatment.

EPFL2013