| 1. |
- Borgenstam, Annika, et al.
(författare)
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Critical temperature for growth of martensite
- 1995
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Ingår i: Acta Metallurgica Et Materialia. - Oxford : Pergamon. - 0956-7151. ; 43:3, s. 945-954
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Tidskriftsartikel (refereegranskat)abstract
- Ms may be defined as the temperature below which the formation of martensite starts upon cooling. It may also be useful to define Mg, the temperature below which martensite can grow if it is already nucleated. In order to analyze the mechanism of martensite formation, it is essential to know the difference Mg - Ms. We have tried to evaluate Mg - Ms for an Fe-C alloy with a decarburized surface zone in order to induce nucleation. The samples were studied by means of electron microprobe, serial sectioning and optical microscopy. The results indicate that Mg is surprisingly close to Ms. The possibility that Mg is controlled by growth rather than nucleation is discussed.
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| 2. |
- Hillert, Mats, et al.
(författare)
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Escape of carbon from ferrite plates in austenite
- 1993
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Ingår i: ACTA METALL MATER. - 0956-7151. ; 41:7, s. 1951-1957
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Tidskriftsartikel (refereegranskat)abstract
- A newly developed computer program for the simulation of diffusional transformations has been applied to study the escape of carbon from a plate of ferrite assuming that the plate initially formed by a partitionless reaction from an Fe-C austenite. Thereafter the ferrite austenite interface was assumed to be immobile and local equilibrium was assumed for carbon but not for iron. The process first follows a parabolic rate law and is there controlled by the rate of diffusion in ferrite. Later stages are not parabolic and are controlled by the diffusivity in austenite. Its concentration dependence was taken into account. It was found that the rate could be estimated analytically using the maximum value rather than the average value.
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| 3. |
- Ion, John, et al.
(författare)
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Diagrams for laser materials processing
- 1992
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Ingår i: Acta Metallurgica et Materialia. - 0956-7151. ; 40:7, s. 1539-1551
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Tidskriftsartikel (refereegranskat)abstract
- An analytical heat flow model is used to identify dimensionless parameter groups which determine the temperature field produced in a material by a scanning laser beam. The groups are used to plot experimental data for metallic alloys on a processing diagram for a range of continuous CO2 laser treatments. Practical operating regions for each type of treatment are thus identified, which coincide with those predicted using the heat flow model. The model is extended in order to construct more detailed diagrams for transformation hardening, surface melting and keyhole welding, which quantify the depth of treatment. By using realistic estimates of certain poorly-known process variables, good agreement is observed between measured and predicted data. Methods for optimising processing parameters with respect to various criteria are presented for transformation hardening. The diagrams, which are constructed on a personal computer, are a useful tool for summarising current data, optimising practical processing parameters, and assessing the potential of novel laser treatments on new materials
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| 4. |
- Larker, Richard, et al.
(författare)
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Diffusion bonding reactions between a SiC/SiC composite and two superalloys during joining by hot isostatic pressing
- 1992
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Ingår i: Acta Metallurgica et Materialia. - 0956-7151. ; 40:11, s. 3129-3139
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Tidskriftsartikel (refereegranskat)abstract
- Diffusion reactions during solid state joining of a ceramic SiC/SiC composite to two superalloys, Hastelloy X and Incoloy 909, by hot isostatic pressing (HIP) have been investigated. The HIP pressure was 200 MPa in all joining cycles, and the temperature/dwell time were either 800°C/15 min, 900°C/1 h or 1000°C/1 h. The reaction zones formed consisted of a thin layer of carbides surrounded by several layers containing silicides and free carbon. The thicknesses of the reaction layers increased with increasing temperature, but were more affected by the composition of the alloy. With more carbide formers in the alloy, the thickness of the reaction layer decreased. The SiC composite was found to be considerably more prone to reactions with these superalloys during HIP as compared to Si3N4 under similar conditions
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| 5. |
- Li, W.-B, et al.
(författare)
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Influence of particle shape on zener drag
- 1990
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Ingår i: Acta Metallurgica et Materialia. - 0956-7151. ; 38:6, s. 1045-1052
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Tidskriftsartikel (refereegranskat)abstract
- Practically all papers concerned with grain refinement based on pinning by particle dispersions have considered spherical particles. This paper avoids this restriction and develops Zener pinning equations for ellipsoidal particles of any shape, orientation and degree of coherency. Theoretical predictions are verified using soap bubble model experiments. It is found that within well defined conditions of particle orientation, disk or needle-shaped particles may offer greater pinning resistance, or Zener drag to migrating boundaries than spherical ones. However, the pinning resistance of non-spherical particles is sensitive to particle orientation. Their use in alloys is best utilised in systems based on mono-orientation dispersions
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| 6. |
- Li, W.B., et al.
(författare)
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Mechanisms of indentation creep
- 1991
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Ingår i: Acta Metallurgica et Materialia. - 0956-7151. ; 39:12, s. 3099-3110
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Tidskriftsartikel (refereegranskat)abstract
- A physically-based theoretical model is developed for describing the phenomenon of indentation creep over the whole temperature range, from 300 K to melting. In agreement with experimental data collected, the model predicts that most materials, including ceramics, exhibit indentation creep at temperatures down to 300 K. It is established that the principal mechanism causing indentation creep is dislocation glide plasticity. The dominance of this mechanism over the whole temperature range is due to the very high stresses involved in indentation creep. If, however, the grain size is small (typically less than 0.3-0.4 μm) indentation creep may be dominated by grain boundary (Coble) diffusive creep instead. The implications of these results in terms of the design, forming and application of the so-called "hard materials" is discussed.
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| 7. |
- Li, W.B., et al.
(författare)
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Model for nano-indentation creep
- 1993
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Ingår i: Acta Metallurgica et Materialia. - 0956-7151. ; 41:10, s. 3065-3069
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Tidskriftsartikel (refereegranskat)abstract
- A new model is developed to describe that stage of nano-intentation creep prior to `pop-in', i.e. when the indentation load induces purely elastic stresses within the sample. Expressions for the stress fields due to a conical indenter are derived and the resulting chemical potential and diffusion fluxes established. The considerations lead to a creep rate equation which is found to fit well with existing experimental data on tungsten. The model is hence used to predict nano-indentation creep rates at various temperatures. It is found, for example, that at a temperature as low as 100 °C, the indentation creep rate of tungsten can be considerable
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| 8. |
- Ringer, S.P., et al.
(författare)
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On the rotation of precipitate particles
- 1992
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Ingår i: Acta Metallurgica et Materialia. - 0956-7151. ; 40:2, s. 275-283
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Tidskriftsartikel (refereegranskat)abstract
- An energetically unfavourable situation can develop when coherent particles are bypassed by migrating grain boundaries. This is due to the precipitates being exposed to incoherent interfaces in the new matrix. In this work, the rotation of precipitate particles to low energy, coherent orientations in the new matrix is shown to be one of several possible responses to this situation. A physical and kinetic model for the rotation is put forward and the results of calculations of rotation rate presented. It is shown that particle rotation is controlled by interfacial diffusion and depends upon alloy composition, time, temperature and particle size and shape. The possibility of particle rotation occurring during particle/boundary contact is also discussed. This is shown to depend upon the nature of the boundary type. Generally, boundaries moving under large driving pressures with high velocities (e.g. phase transformation interfaces), are less likely to allow rotation during particle/boundary contact than less mobile boundaries such as those in grain growth and recrystallization. Experimental results, from SAD and TEM of a Ti stabilized austenitic stainless steel containing a dispersion of coherent TiC precipitates is also presented and supports the particle rotation model
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| 9. |
- Xie, Z.L., et al.
(författare)
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Isothermal martensitic transformation under hydrostatic pressure in an Fe-Ni-C alloy at low temperatures
- 1993
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Ingår i: Acta Metallurgica Et Materialia. - Oxford : Elsevier Science Ltd. - 0956-7151. ; 41:8, s. 2283-2290
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Tidskriftsartikel (refereegranskat)abstract
- The isothermal martensitic tranformation under applied hydrostatic pressure has been investigated with an Fe-21.5Ni-0.95C alloy single crystal by electrical resistivity, magnetic susceptibility, X-ray diffraction and optical microscopy. The martensitic transformation starting temperature, M(s), is lowered by applied hydrostatic pressure. The isothermal martensitic transformation occurs first as a burst under a critical pressure and it is continued by further transformation with decreasing hydrostatic pressure The critical pressure under which the isothermal martensitic transformation starts changes curvilinearly with decreasing temperature. The morphology of the isothermal martensite formed under hydrostatic pressure is similar to that of athermal martensite. The temperature dependence of the critical pressure under which martensitic transformation starts has been calculated based on the homogeneous nucleation model and the heterogeneous nucleation model.
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