| 1. |
- Sandström, Rolf
(författare)
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Basic Modelling of General Strength and Creep Properties of Alloys
- 2024
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Ingår i: Crystals. - : MDPI AG. - 2073-4352. ; 14:1
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Forskningsöversikt (refereegranskat)abstract
- There are excellent methods for modelling physical and elastic properties, for example, those based on ab initio atomistic procedures. For mechanical properties that are controlled by the motion of the dislocations, such methods have not been available in the past. One has been forced to resort to fitting the experimental data with empirical methods by involving a number of adjustable parameters. However, in recent years, methods based on physical principles have been developed for a number of mechanical properties. These methods can predict properties accurately without the use of fitting parameters. A review of such methods will be given, for example, for the modelling of creep deformation in metallic materials. It will be demonstrated that some properties can be described over a wide range of temperatures and strain rates. The advantage of these new methods is that they can be used for prediction, identification of mechanisms and extrapolation of results for new conditions.
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| 2. |
- Sandström, Rolf
(författare)
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Cavitation
- 2024
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Ingår i: Basic Modeling and Theory of Creep of Metallic Materials. - : Springer Nature. ; , s. 185-203
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- Cavitation is of great technical importance. Nucleated cavities grow and link to form cracks that can cause rupture. During creep, cavities are initiated in the grain boundaries. The nucleation takes place at particles or at subboundary—grain boundary junctions. The main mechanism is believed to be grain boundary sliding (GBS), Chap. 9. According to the double ledge model, cavities are formed when the particles or subboundaries meet other subboundaries. With this assumption quantitative models for cavity nucleation can be derived. They show that the nucleated number of cavities is proportional to the creep strain in good accordance with observations. Cavities can grow by diffusion or by straining. It is important to take into account that cavities cannot grow faster than the surrounding creeping matrix, which is referred to as constrained growth. Otherwise the growth rate can be significantly overestimated. Models both for diffusion and strain controlled growth have been available for a long time. A recently developed model for strain controlled growth is presented based on GBS. It has the advantage that is associated with a well-defined initiation size of cavities and that constrained growth is automatically taken into account, features that some previous strain controlled models miss.
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| 3. |
- Sandström, Rolf
(författare)
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Cells and Subgrains : The Role of Cold Work
- 2024
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Ingår i: Basic Modeling and Theory of Creep of Metallic Materials. - : Springer Nature. ; , s. 145-167
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- In almost all metals and alloys, dislocations are concentrated to narrow regions after plastic deformation that divide the material into cells or subgrains. The cell walls consist of tangles whereas the subgrains are surrounded by thin regular networks of dislocations. The cells are transferred to subgrains with increasing temperature. Although these substructures have been analyzed for many years, basic models of their development have only appeared recently. Models for substructures are presented for plastic deformation at constant stress and at constant strain rate. During straining the dislocations can move in opposite directions creating a polarized structure, where the possibility for recovery of dislocations is reduced. This can be expressed in term of a back stress. Its presence explains why creep curves at near ambient temperatures could have an appearance that is similar to that at elevated temperatures. It is also the basis for the effect of cold work on creep. The models can quantitatively describe why the creep rate can be reduced by up to six orders of magnitude for Cu after cold work.
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| 4. |
- Sandström, Rolf
(författare)
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Creep Ductility
- 2024
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Ingår i: Basic Modeling and Theory of Creep of Metallic Materials. - : Springer Nature. ; , s. 257-273
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- For a number of creep resistant steels, the creep ductiliy decreases with increasing temperature and time. As a function of stress, the ductiity is often describe with an S-shaped curve with an upper and a lower shelf level. As a function of time, the S-shape is inverted. If the ductility is high, the rupture is referred to as ductile, and for low ductility levels as brittle. Ductile rupture is believed to be due to a plastic instability such as necking. Brittle rupture on the other hand is controlled by the nucleation, growth and linkage of creep cavities. With the help of the basic models for creep deformation and cavitation, the rupture stress and ductility can be predicted. Several models exist for the influence of multiaxiality on the creep ductility. Although the models are based on different principles, they predict approximately the same behavior, which is verified by comparison to rupture data for notched bars.
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| 5. |
- Sandström, Rolf
(författare)
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Creep with Low Stress Exponents
- 2024
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Ingår i: Basic Modeling and Theory of Creep of Metallic Materials. - : Springer Nature. ; , s. 83-114
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- Primary creep models predict that at low stresses a stress exponent of 1 can be obtained for dislocation creep. Also experimentally this has been observed for an austenitic stainless steel. The time dependence of the primary creep verifies that it is dislocation creep. An other example is for Al at very high temperatures (Harper-Dorn creep), where at sufficiently low stresses, the stress exponent approaches 1. For both materials higher stresses give larger stress exponents as expected for dislocation creep. Obviously, diffusion and dislocation creep can be competing processes. The validity of creep models at low stresses and high temperatures as well as at high stresses and low temperatures demonstrates their wide range of usage. Since this in reality represents an extensive extrapolation, it can be consider as a direct verification of the basic creep models. In cases for Cu and stainless steels, the predicted creep rate by diffusion creep (Coble) exceeds the observed creep rate as well as the predicted one by dislocation creep by an order of magnitude. The likely explanation is that constrained boundary creep is taken place, i.e. the grain boundary creep rate cannot be essentially faster than that of the bulk.
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| 6. |
- Sandström, Rolf
(författare)
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Extrapolation
- 2024
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Ingår i: Basic Modeling and Theory of Creep of Metallic Materials. - : Springer Nature. ; , s. 275-310
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- The extrapolation of creep data to longer times is technically very important. The traditional way of extrapolating creep rupture data is to use time temperaturer parameters (TTPs). In this way data from several test temperatures are combined to a single master curve that can be used to assess rupture strengths at long times. Recently, there is much focus on machine learning techniques (neural networks, NNs). Both types of procedures can generate accurate results, but a detailed analysis is required. A good way to assess the quality of the results is to use the post assessment tests (PATs) developed by ECCC. Without such tests arbitrary results can be obtained. They are important for both TTPs and NNs. It has been shown that by putting requirements on the derivatives of the creep rupture curves, the PATs can more or less automatically be satisfied. In addition, the error in the extrapolated values should be estimated. Using the basic creep models presented in this book, prediction of rupture strength and ductility can be made in a safer way. It is demonstrated for Cu that accurate extrapolation of many order of magnitude in the creep rate can be made, which is never possible with empirical models.
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| 7. |
- Sandström, Rolf
(författare)
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Grain Boundary Sliding
- 2024
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Ingår i: Basic Modeling and Theory of Creep of Metallic Materials. - : Springer Nature. ; , s. 169-184
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- During plastic deformation at elevated temperatures, grains move relative to each other which is referred to as grain boundary sliding (GBS). The amount of GBS is proportional to the creep strain with a proportionality constant that is known from finite element analyses, and found to agree with experiments for Cu. The most import effect of GBS is that it gives rise to the initiation of creep cavities, Chap. 10. GBS is also the main mechanism for superplasticity. A basic model for superplasticity is presented.
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| 8. |
- Sandström, Rolf
(författare)
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Precipitation Hardening
- 2024
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Ingår i: Basic Modeling and Theory of Creep of Metallic Materials. - : Springer Nature. ; , s. 131-144
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- Models for precipitation hardening (PH) at room temperature have been available for a long time. In spite of the importance of PH, it took a long time to establish models for elevated temperatures. In fact, empirically the room temperature models have also been used at higher temperatures. This gives the wrong temperature dependence and overestimates PH. It was for a long time thought that it was an energy barrier for climb across particles that was the controlling mechanism, but it was gradually appearing that this effect was so small that it could be neglected. Instead it is time it takes for dislocations to climb across particles that is the critical factor. Small particles are readily passed and do not contribute to the strengthening. Particles larger than a critical size have to be passed by the Orowan mechanism, because there is not time enough for dislocations to climb across these particles. This mechanism was finally verified for Cu–Co alloys.
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| 9. |
- Sandström, Rolf
(författare)
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Preface
- 2024
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Ingår i: Springer Series in Materials Science. - : Springer Science and Business Media Deutschland GmbH.
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Bokkapitel (övrigt vetenskapligt/konstnärligt)
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| 10. |
- Sandström, Rolf
(författare)
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Primary Creep
- 2024
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Ingår i: Basic Modeling and Theory of Creep of Metallic Materials. - : Springer Nature. ; , s. 59-81
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- For many materials, primary creep can be described with the phi (ϕ) model and tertiary creep with the Omega (Ω) model (discussed in Chap. 12 ). According to the phi model, the creep rate is linear in strain and time in a double logarithmic diagram. When using empirical descriptions of the creep curves, these models are recommended. Several basic models for primary creep are derived. They are based on the creep rate in the secondary stage. This means that primary creep can be derived without any new data. The primary creep models are in agreement with the phi model and can describe experimental data. For the martensitic 9–12% Cr steels at least two dislocation densities are needed to represent primary creep because the initial dislocation density is high contrary to the situation for annealed fcc materials.
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