| 1. |
- Hosseinzadeh Delandar, Arash, 1982-, et al.
(författare)
-
Investigation of loading orientation effect on dynamic deformation of single crystal copper at high strain rates: Discrete dislocation dynamics study
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Uniaxial tensile loading of copper single crystal along [001] and [111] orientations is modeled at two high strain rates of 105 and 106 s-1. Discrete dislocation dynamics method is used to study the anisotropic characteristic of plastic deformation in the model crystal. Furthermore, strain rate sensitivity of the flow stress in copper crystal is examined. Investigation of mechanical response of single crystal to the external loading demonstrates a substantial effect of loading orientation on the plastic flow. We find that at both imposed strain rates flow stress increases significantly when tensile load is applied along [111] crystallographic axis. Similarly, plastic anisotropy is observed in dislocation density evolution such that more dislocations are generated as straining direction of single crystal is changed from [001] to [111] axis. Moreover, plastic flow behavior exhibits a profound strain rate sensitivity at both loading orientations which agrees well with experimental observations regarding strain rate dependency of flow stress in copper single crystal as strain rate exceeds 103 s-1. At both applied strain rates dislocations evolve into a heterogeneous microstructure and highest heterogeneity is observed as model crystal is loaded along [111] direction at strain rate of 106 s-1. Formation of slip bands and consequently localization of plastic deformation are detected for all considered cases. However, at the higher strain rate of 106 s-1, slip band formation is more pronounced for both loading orientations.
|
|
| 2. |
- Hosseinzadeh Delandar, Arash, 1982-
(författare)
-
Numerical Modeling of Plasticity in FCC Crystalline Materials Using Discrete Dislocation Dynamics
- 2015
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Plasticity in crystalline solids is controlled by the microscopic line defects known as “dislocations”. Decisive role of dislocations in crystal plasticity in addition to fundamentals of plastic deformation are presented in the current thesis work. Moreover, major features of numerical modeling method “Discrete Dislocation Dynamics (DDD)” technique are described to elucidate a powerful computational method used in simulation of crystal plasticity.First part of the work is focused on the investigation of strain rate effect on the dynamic deformation of crystalline solids. Single crystal copper is chosen as a model crystal and discrete dislocation dynamics method is used to perform numerical uniaxial tensile test on the single crystal at various high strain rates. Twenty four straight dislocations of mixed character are randomly distributed inside a model crystal with an edge length of 1 µm subjected to periodic boundary conditions. Loading of the model crystal with the considered initial dislocation microstructure at constant strain rates ranging from 103 to 105s1 leads to a significant strain rate sensitivity of the plastic flow. In addition to the flow stress, microstructure evolution of the sample crystal demonstrates a considerable strain rate dependency. Furthermore, strain rate affects the strain induce microstructure heterogeneity such that more heterogeneous microstructure emerges as strain rate increases.Anisotropic characteristic of plasticity in single crystals is investigated in the second part of the study. Copper single crystal is selected to perform numerical tensile tests on the model crystal along two different loading directions of [001] and [111] at two high strain rates. Effect of loading orientation on the macroscopic behavior along with microstructure evolution of the model crystal is examined using DDD method. Investigation of dynamic response of single crystal to the mechanical loading demonstrates a substantial effect of loading orientation on the flow stress. Furthermore, plastic anisotropy is observed in dislocation density evolution such that more dislocations are generated as straining direction of single crystal is changed from [001] to [111] axis. Likewise, strain induced microstructure heterogeneity displays the effect of loading direction such that more heterogeneous microstructure evolve as single crystal is loaded along [111] direction. Formation of slip bands and consequently localization of plastic deformation are detected as model crystal is loaded along both directions.
|
|
| 3. |
- Korzhavyi, Pavel A., et al.
(författare)
-
First-principles evaluation of the effect of alloying elements on the lattice parameter of a 23Cr25NiWCuCo austenitic stainless steel to model solid solution hardening contribution to the creep strength
- 2015
-
Ingår i: Materials Science & Engineering. - : Elsevier BV. - 0921-5093 .- 1873-4936. ; 626, s. 213-219
-
Tidskriftsartikel (refereegranskat)abstract
- By the use of first-principles calculations based on density functional theory, lattice misfit parameters for alloying elements in the austenitic stainless steel 23Cr25NiWCuCo have been derived. These lattice misfit parameters have been applied to determine the solid solution hardening of the elements W, Nb, and Cu in the steel. The model for solid solution hardening is based on work by Hirth and Lothe, where solutes are creating Cottrell clouds around the dislocations and slow down their motion. The model is also verified by comparison to creep tests for Ni-20%Cr and Ni-20%Cr-6W, where W is almost completely in solid solution and no other strengthening mechanism than solid solution hardening should be active. The contribution from the interstitial elements C and N to the solid solution hardening is found to be negligibly small for the studied steel.
|
|
| 4. |
- Källgren, Therese, et al.
(författare)
-
Microstructure and temperature development in copper welded by the FSW-process
- 2003
-
Ingår i: 4th International Symposium on FSW, Park City, USA, April, 2003.
-
Konferensbidrag (refereegranskat)abstract
- The use of Copper canisters with cast iron inserts is one of the proposed methods for long time deposition of nuclear fuel waste. The joining of the lid and possibly the base of the canister can be performed with Friction Stir Welding (FSW). A thorough understanding of the microstructure development in these welds is of major importance, since defects must be avoided. The microstructure and hardness profiles were investigated for two FSW conditions, at the beginning of the weld when there are essentially cold conditions, and when the steady state is reached. In cold welds the features of the nugget are very similar to that observed when FSW-joining aluminium. However, when the welding conditions reach the steady state, the nugget gets wider and the hardness is lower than in the cold weld. Electron Back Scatter Diffraction (EBSD) visualisation has been used to determine the grain size distribution of, twins and misorientation within grains. The orientation maps show a fine uniform equiaxed grain structure. There is no major misorientation within the grains in the nugget or at the root, but 25 mm from the weld centre the misorientation is large. This confirms that this area lies within the Thermal Mechanical Affected Zone (TMAZ) and is partially recrystallised. The root has many more annealing twins than the nugget. This indicates that the nugget has experienced deformation after recrystallisation. A model has been used to correlate heat flow to the grain size and the hardness distributions.
|
|
| 5. |
- Randelius, Mats, et al.
(författare)
-
Fatigue Strength of Conventionally Cast Tool Steels and its Dependence of Carbide Microstructure
- 2012
-
Ingår i: Steel Research International. - : Wiley-Blackwell. - 1611-3683 .- 1869-344X. ; 83:1, s. 83-90
-
Tidskriftsartikel (refereegranskat)abstract
- The axial fatigue strength at two million cycles was experimentally determined for two conventionally cast tool steels and successfully compared with results from a fatigue limit model. Specimens were tested both in the rolling and transverse direction and showed large differences in fatigue properties due to the segregated carbide microstructure. Rolling direction specimens experienced higher fatigue strength than the transverse direction specimens. This is due to smaller carbides present in the load affected cross section of the rolling direction fatigue test bars compared to the cross section of the transverse direction fatigue test bars. Fractographic analysis of failed specimens showed that large carbides had caused fatigue failure, which was also predicted by the model. Measured size distributions of carbides and inclusions were used as input data in the model. The probability that at least one particle will be present in the material volume having a size larger than the threshold value for crack propagation was calculated.
|
|
| 6. |
- Sandström, Rolf, et al.
(författare)
-
Modelling of creep in friction stir welded copper
- 2013
-
Ingår i: Materials research innovations (Print). - 1432-8917 .- 1433-075X. ; 17:5, s. 350-354
-
Tidskriftsartikel (refereegranskat)abstract
- Copper canisters for storage of nuclear waste will be exposed to creep. The canisters will be closed with friction stir welding (FSW). To describe the creep behaviour of the welds, uniaxial creep tests have been performed. A previously developed fundamental creep model for parent metal is applied to the different weld zones. The differences in microstructure and yield strength between the weld zones are taken into account. Creep strain versus time curves for the weld zones have successfully been predicted without the use of any adjustable parameters. It should be noted that the temperature range of interest of 50-100 degrees C is deep down in the power law break down regime with Norton exponents between 25 and 100. The constitutive equations are used in FEM computations of creep in the canister weldments.
|
|
| 7. |
- Jin, Lai-Zhe, et al.
(författare)
-
Steady non-Newtonian flows in copper and iron aluminide at elevated temperatures
- 2007
-
Ingår i: Journal of Materials Processing Technology. - : Elsevier B.V.. - 0924-0136 .- 1873-4774. ; 189:1-3, s. 428-434
-
Tidskriftsartikel (refereegranskat)abstract
- On the basis of dislocation climb and mobility, a steady-state non-Newtonian flow law is derived for two metallic and one intermetallic material,namely electrolytic tough pitch copper, phosphorus alloyed pure copper, and Fe24AlMo iron aluminide. The purpose is to develop a flow lawapplicable to the finite element simulation for hydrodynamic flow of incompressible metals. The model can accurately represent the experimentalflow stress values and the viscosity of the materials. The mathematical form of the model is similar to that of the free-volume approach, which isused for liquids and amorphous metals. The study indicates that in the temperature and strain-rate regimes that are appropriate to the hot-workingprocesses, the Cohen–Grest model, which is essentially related to the total thermal expansion of fluid, can phenomenologically be extended to thecrystalline solid-state materials for the depiction of viscosity data.
|
|
| 8. |
- Sandström, Rolf, et al.
(författare)
-
Grain boundary sliding in copper and its relation to cavity formation during creep
- 2016
-
Ingår i: Materials Science & Engineering. - : Elsevier Ltd. - 0921-5093 .- 1873-4936. ; 651, s. 259-268
-
Tidskriftsartikel (refereegranskat)abstract
- The nucleation of creep cavities, which control the creep ductility is assumed to take place by grain boundary sliding. To determine the grain boundary sliding rate at longer testing times than previously available in the literature, two creep tests have been performed at constant loading rate at 125. °C for oxygen free copper with phosphorus (Cu-OFP). The tests were interrupted after certain strains and the amount of grain boundary sliding (GBS) was measured on flat polished surfaces. The observed amount of GBS per unit strain was 20 to 65 μm. This is of the same order of magnitude as for published tensile tests (Pettersson, 150 and 200. °C) and short time creep tests (Ayensu and Langdon, 400-600. °C). The amount of GBS was modelled based on previously performed FEM investigations. For conditions corresponding to the experiments a value of 52. μm was obtained.A model by Lim for cavity nucleation at junctions between cell and grain boundaries has been adapted to oxygen free pure copper Cu-OF and Cu-OFP. The results show that the gain in free energy at cavity nucleation is much larger for Cu-OF than for Cu-OFP implying that Cu-OF is much more prone to cavity formation. The modelled difference in free energy gain is sufficient to quantitatively explain the much higher creep ductility in Cu-OFP than in Cu-OF.
|
|
| 9. |
- Andersson-Östling, Henrik C.M. 1966-
(författare)
-
Mechanical Properties of Welds at Creep Activation Temperatures
- 2010
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Welds in materials intended for service at temperatures above the creep activation temperature often develop damage before the base metal. The weld is a discontinuity in the material and stresses and strains often accumulate in the weld. Knowledge of the properties of the weld is essential to the safe operation of the component containing the weld. The work in this thesis has been aimed at the study of welds in service at high temperatures: The work is divided into two main chapters. The first chapter deals with welds in stainlesssteels and dissimilar metal welds and includes three papers, and the second chapter dealswith welds in copper intended for nuclear waste disposal, also including three papers. Common to both parts is that the temperature is high enough for most of the damage in the welds to result from creep. In the first part the role of the weld microstructure on the creep crack propagation properties has been studied. Experiments using compact tension specimens have been performed on service exposed, low alloyed heat resistant steels. The results show good correlation with the crack tip parameter, C*, during steady state creep crack growth. The test methodology has also been reviewed and sensitive test parameters have been identified. The results from the creep crack propagation tests on service exposed material has been modeled using uniaxial creep data on both new and ex-service material. The development of the weld microstructure in a dissimilar metal weld between two heat resistant steels has also been investigated. A weld was made between one ferritic and one martensitic steel and the development of the microstructure during welding and post-weldheat treatments has been studied. The results show that the carbon depleted zone that develops near the weld metal in the lower alloyed steel depends on the formation and dissolution of the M23C6-carbide. Variations of the weld parameters and the post-weld heat treatment affect the size and shape of this zone. The process has been successfully modeled by computer simulation. The second part focuses on oxygen free copper intended for nuclear waste disposal containers. The containers are made with an inner core of cast nodular iron and an outer core of copper for corrosion protection. The copper shell has to be welded and two weld methods has been tested, electron beam welding and friction stir welding. Creep specimens taken from both weld types have been tested as have base metal specimens. The technical specifications of the waste canisters demand that the creep ductility of both the copper shell and the welds has to be as high as possible. The creep test results show that base material doped with at least 30 ppm phosphorus has high creep ductility, and friction stir welds made from this material has almost as high creep strength and creep ductility. Copper without phosphorus does not exhibit the same ductility. The creep properties evaluated from testing has been modeled and extrapolated for the intended purpose
|
|
| 10. |
- Chen, Kaixuan, et al.
(författare)
-
Slow strain rate tensile tests on notched specimens of as-cast pure Cu and Cu–Fe–Co alloys
- 2020
-
Ingår i: Journal of Alloys and Compounds. - : Elsevier. - 0925-8388 .- 1873-4669. ; 822
-
Tidskriftsartikel (refereegranskat)abstract
- Microstructure evolution in the as-cast pure Cu, Cu-(1.0, 2.0, 3.0)Fe-0.5Co (wt. %) alloys were characterized in the former work. The aim of the present study is to investigate the slow strain rate tensile (SSRT) performance and fracture behavior of the Cu–Fe–Co alloys reinforced with fined grains (FG) and iron-rich nanoparticles (NP), referred as NPFG structure. The plastic deformation and fracture characteristics were examined by multiaxial SSRT tests at 75 and 125 °C on notched specimens. The addition of Fe and Co enhanced the ultimate tensile strength and yield strength almost by double to triple times the properties compare to pure Cu, along with an acceptable reduction in ductility, both at 75 and 125 °C. The SSRT properties of the copper samples varied as a function of temperature and alloying content. The analysis of fracture surface indicates the effect of iron-rich nanoparticles and grain boundaries on the deformation and fracture processes. The Kocks-Mecking model was applied to describe the SSRT experimental results with fitting parameters. The model predicted the dynamic recovery ability of the copper samples with different Fe, Co content and temperature. The evolution mechanism of SSRT properties upon alloying content and temperature was discussed in terms of the microstructure characterization, fractographic observation, deformation modeling, strengthening models as well as the analysis of strain-hardening curves. The results indicate through further microstructural engineering the NPFG Cu–Fe–Co alloy is promising in utilization as the canister for the storage of the nuclear waste.
|
|
