| 1. |
- Akhtar, Farid, et al.
(författare)
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On the processing, microstructure, mechanical and wear properties of cermet/stainless steel layer composites
- 2007
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 55:4, s. 1467-1477
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Tidskriftsartikel (refereegranskat)abstract
- This study deals with layer composites of carbide reinforcements and stainless steel prepared successfully by powder technology. The layer material consisted of two layers. The top layer consisted of reinforcements (TiC and NbC) and 465 stainless steel as the binder material for the carbides. The bottom layer was entirely of binder material (465 stainless steel). The microstructure of the composite was characterized by scanning electron microscopy. The microstructural study revealed that the top layer (TiC–NbC/465 stainless steel) showed the typical core–rim microstructure of conventional steel bonded cermets and the bottom layer showed the structure of sintered steel. An intermediate layer was found with a gradient microstructure, having a higher carbide content towards the cermet layer and lower carbide content towards the stainless steel layer. The bending strength of the layered material measured in the direction perpendicular to the layer alignment was remarkably high. The variation of strength as a function of the thickness of the bottom layer revealed that the character of the material changed from the cermet, to a layer composite and then towards metallic materials. The wear resistance of the top layer was studied against high speed steel. The wear mechanisms were discussed by means of microscopical observations on the worn surfaces. The wear was severe at higher wear loads and lower TiC content. Microploughing of the stainless steel matrix was found to be the dominant wear mechanism. Heavy microploughing and rapid removal of material from the wear surface was observed at high wear load. The fracture morphologies of the top, bottom and intermediate layers are reported
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| 2. |
- Asp Grönhagen, Klara, et al.
(författare)
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Phase-field simulation of sintering and related phenomena : A vacancy diffusion approach
- 2006
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 54, s. 1241-1248
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Tidskriftsartikel (refereegranskat)abstract
- A phase-field model of sintering and related phenomena in a two-phase system and in a multi-phase system is presented. We consider diffusion of vacancies as the atomic mechanism for redistribution of material and we will use the familiar model of thermal vacancies in a crystal as our energy formulation. The solid material will thus be characterized by a low vacancy content and the surroundings by a very high vacancy content and a very low content of atoms. The surface of the solid body will be characterized by a continuous variation in vacancy content. The temporal development of particles during solid state sintering with effects such as wetting is shown in various simulations.
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| 3. |
- Boesenberg, Ulrike, et al.
(författare)
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Hydrogen sorption properties of MgH2-LiBH4 composites
- 2007
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Ingår i: Acta Materialia. - : Elsevier BV. - 1873-2453 .- 1359-6454. ; 55:11, s. 3951-3958
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Tidskriftsartikel (refereegranskat)abstract
- A detailed analysis of the reaction mechanism of the reactive hydride composite (RHC) MgH2 + 2LiBH(4) <-> MgB2 + 2LiH + 4H(2) was performed using high-pressure differential scanning calorimetry (HP-DSC) measurements and in situ synchrotron powder X-ray diffraction (XRD) measurements along with kinetic investigations using a Sievert-type apparatus. For the desorption the following two-step reaction has been observed: MgH2 + 2LiBH(4) <-> Mg + 2LiBH(4) + H-2 <-> MgB2 + 2LiH + 4H(2). However, this reaction is kinetically restricted and proceeds only at elevated temperatures. In contrast to the desorption reaction, LiBH4 and MgH2 are found to form simultaneously under fairly moderate conditions of 50 bar hydrogen pressure in the temperature range of 250-300 degrees C. As found in pure light metal hydrides, significant improvement of sorption kinetics is possible if suitable additives are used. (c) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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| 4. |
- Borgenstam, Annika, et al.
(författare)
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Metallographic evidence of carbon diffusion in the growth of bainite
- 2009
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 57:11, s. 3242-3252
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Tidskriftsartikel (refereegranskat)abstract
- There are two paradigms regarding the formation of bainite. One is based on the first stage being rapid, diffusionless growth or acicular ferrite and the subsequent formation of carbide occurring by precipitation from the supersaturated ferrite. All assumption that the first stage occurs as a series of subsequent rapid steps resulting in sub-units plays an important role as an explanation of the not so rapid growth observed macroscopically. The other paradigm is based on the first stage being the formation of acicular ferrite under carbon diffusion and on the subsequent growth of carbide and ferrite side by side. Metallographic observations are presented that support the second paradigm. It is difficult to see how they can be accounted for by the first paradigm, in particular the observation of the shapes of sub-units.
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| 5. |
- Chen, Q., et al.
(författare)
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Analytical treatment of diffusion during precipitate growth in multicomponent systems
- 2008
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 56:8, s. 1890-1896
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Tidskriftsartikel (refereegranskat)abstract
- We propose an approximate growth rate equation that takes into account both cross-diffusion and high supersaturations for modeling precipitation in multicomponent systems. We then apply it to an Fe-alloy in which interstitial C atoms diffuse much faster than substitutional solutes, and predict a spontaneous transition from slow growth under ortho-equilibrium to fast growth under the non-partitioning local equilibrium condition. The transition is caused by the decrease in the Gibbs-Thomson effect as the growing particle becomes larger. The results agree with DICTRA simulations where full diffusion fields are calculated.
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| 6. |
- Eklund, Per, 1977-, et al.
(författare)
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Ta4AlC3 : Phase determination, polymorphism and deformation
- 2007
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 55:14, s. 4723-4729
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Tidskriftsartikel (refereegranskat)abstract
- Ta4AlC3, a new member of the Mn+1AXn-phase family, has been synthesized and characterized (n = 1-3, M = early transition metal, A = A-group element, and X = C and/or N). Phase determination by Rietveld refinement of synchrotron X-ray diffraction data shows that Ta4AlC3 belongs to the P63/mmc space group with a and c lattice parameters of 3.10884 ± 0.00004 Å and 24.0776 ± 0.0004 Å, respectively. This is shown to be the α-polymorph of Ta4AlC3, with the same structure as Ti4AlN3. Lattice imaging by high-resolution transmission electron microscopy demonstrates the characteristic MAX-phase stacking of α-Ta4AlC3. Three modes of mechanical deformation of α-Ta4AlC3 are observed: lattice bending, kinking and delamination. © 2007.
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| 7. |
- Emmerlich, Jens, et al.
(författare)
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Thermal stability of Ti3SiC2 thin films
- 2007
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 55:4, s. 1479-1488
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Tidskriftsartikel (refereegranskat)abstract
- The thermal stability of Ti3SiC2(0 0 0 1) thin films is studied by in situ X-ray diffraction analysis during vacuum furnace annealing in combination with X-ray photoelectron spectroscopy, transmission electron microscopy and scanning transmission electron microscopy with energy dispersive X-ray analysis. The films are found to be stable during annealing at temperatures up to 1000 °C for 25 h. Annealing at 1100–1200 °C results in the rapid decomposition of Ti3SiC2 by Si out-diffusion along the basal planes via domain boundaries to the free surface with subsequent evaporation. As a consequence, the material shrinks by the relaxation of the Ti3C2 slabs and, it is proposed, by an in-diffusion of O into the empty Si-mirror planes. The phase transformation process is followed by the detwinning of the as-relaxed Ti3C2 slabs into (1 1 1)-oriented TiC0.67 layers, which begin recrystallizing at 1300 °C. Ab initio calculations are provided supporting the presented decomposition mechanisms.
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| 8. |
- Erneman, J., et al.
(författare)
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The evolution of primary and secondary niobium carbonitrides in AISI 347 stainless steel during manufacturing and long-term ageing
- 2006
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 54:1, s. 67-76
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Tidskriftsartikel (refereegranskat)abstract
- Nb(C,N) precipitates were studied in a niobium-stabilised stainless steel (AISI 347) statically aged at 700 degrees C. Scanning electron microscopy and energy filtered transmission electron microscopy were used to determine the volume fraction and precipitate size of primary and secondary Nb(C,N) after ageing times between 0 and 70,000 h. The experimental data were correlated with simulations of Nb(C,N) formation based on the assumption that the process is controlled by diffusion. These simulations provide a rationale for the existence of two sets of mobium carbonitrides in commercial tubes of AISI 347. Growth of primary Nb(C,N) occurred essentially during manufacturing, with no significant growth at 700 degrees C. Rapid dissolution and re-precipitation of secondary Nb(C,N) occurred during manufacturing. Coarsening at 700 degrees C of secondary particles was modelled using the Lifshitz-Slyozov-Wagner theory, which overestimated the coarsening rate. These problems were overcome with a model developed by the authors. This model takes both growth and coarsening into account.
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| 9. |
- Grönhagen, Klara, et al.
(författare)
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Grain-boundary segregation and dynamic solute drag theory : A phase-field approach
- 2007
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 55, s. 955-960
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Tidskriftsartikel (refereegranskat)abstract
- We propose a model based on a phase-field approach to study grain-boundary segregation and solute drag. We will show that it is possible to model the dynamics of grain-boundary segregation to a stationary boundary as well as solute drag on a moving boundary with the same phase-field model. We shall achieve this by introducing a concentration dependency in the height of the double-well potential in the Gibbs-energy expression. As the model then will be able to treat the build-up of a concentration spike in the boundary as well as its disappearance we shall term this treatment dynamic solute-drag theory.
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| 10. |
- Hallström, Samuel, et al.
(författare)
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Thermodynamic reassessment of the Ni-Ru system and assessment of the Al-Ni-Ru system at 1273-1523 K using ab initio calculations
- 2008
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 56:15, s. 4062-4069
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Tidskriftsartikel (refereegranskat)abstract
- The Ni-Ru and Al-Ni-Ru systems are assessed with a combined CALPHAD and ab initio approach. Particular attention is paid to the possible existence of a miscibility gap in the B2 phase. Both face-centered cubic and body-centered cubic ordering are analyzed within the compound energy formalism. Ab initio calculations for the B2 phase show a similar trend as calorimetric measurements but the magnitude is much smaller. It is found that the calorimetric measurements cannot be reconciled with any reasonable phase diagram, whereas the ab initio results can. From the parameters obtained, isothermal sections in reasonable agreement with experimental phase diagrams are calculated. We have concluded that there is no miscibility gap in the B2 phase at 1273 K and higher temperatures.
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