| 1. |
- Åkerfeldt, Pia, et al.
(författare)
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Electron backscatter diffraction characterization of fatigue crack growth in laser metal wire deposited Ti-6Al-4V
- 2018
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Ingår i: Materials Characterization. - : Elsevier BV. - 1044-5803 .- 1873-4189. ; 135, s. 245-256
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Tidskriftsartikel (refereegranskat)abstract
- By additive manufacturing (AM) there is a feasibility of producing near net shape components in basically one step from 3D CAD model to final product. The interest for AM is high and during the past decade a lot of research has been carried out in order to understand the influence from process parameters on the microstructure and furthermore on the mechanical properties. In the present study laser metal wire deposition of Ti-6Al-4V has been studied in detail with regard to its fatigue crack propagation characteristics. Two specimen orientations, parallel and perpendicular to the deposition direction, have been evaluated at room temperature and at 250 °C. No difference in the fatigue crack growth rate could be confirmed for the two specimen orientations. However, in the fractographic study it was observed that the tortuosity varied between certain regions on the fracture surface. The local crack path characteristic could be related to the alpha colony size and/or the crystallographic orientation. Moreover, large areas exhibiting similar crystallographic orientation were observed along the prior beta grain boundaries, which were attributed to the wide alpha colonies frequently observed along the prior beta grain boundaries. © 2017 Elsevier Inc.
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| 2. |
- Åkerfeldt, Pia, et al.
(författare)
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The effect of crystallographic orientation on solid metal induced embrittlement of Ti-6Al-1Mo-1V in contact with copper
- 2013
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Ingår i: IOP Conference Series: Materials Science and Engineering. - : IOP Publishing Ltd. - 1757-8981 .- 1757-899X. ; 48:1, s. 012011-
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Konferensbidrag (refereegranskat)abstract
- Solid metal induced embrittlement (SMIE) occurs when a metal experiences tensilestress and is in contact with another metal with lower melting temperature. SMIE is believed tobe a combined action of surface self-diffusion of the embrittling species to the crack tip andadsorption of the embrittling species at the crack tip, which weakens the crack tip region. In thepresent study, both SMIE of the near alpha alloy Ti-8Al-1Mo-1V in contact with copper and itsinfluence by crystallographic orientation have been studied. U-bend specimens coated withcopper were heat treated at 480°C for 8 hours. One of the cracks was examined in detail usingelectron backscatter diffraction technique. A preferable crack path was found along high anglegrain boundaries with grains oriented close to [0001] in the crack direction; this indicates thatthere is a connection between the SMIE crack characteristics and the crystallographicorientation.
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| 3. |
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| 4. |
- Gaddam, Raghuveer, et al.
(författare)
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Influence of high-pressure gaseous hydrogen on the low-cycle fatigue and fatigue crack growth properties of a cast titanium alloy
- 2014
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Ingår i: Materials Science & Engineering A: Structural Materials: Properties, Microstructure and Processing. - : Elsevier BV. - 0921-5093 .- 1873-4936. ; 612, s. 354-362
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Tidskriftsartikel (refereegranskat)abstract
- In the present study, the effect of gaseous hydrogen on the fatigue properties of a commonly used aerospace titanium alloy (Ti–6Al–4V) was studied. The low-cycle fatigue and fatigue crack growth properties were investigated at room temperature in ambient air and 15 MPa gaseous hydrogen. Results showed that the low-cycle fatigue life was significantly reduced in hydrogen, and the detrimental effect was larger at higher strain amplitudes. The fatigue crack growth rate in hydrogen remained unaffected below a critical stress intensity ΔKn E 17 MPa√m, while beyond this value, the fatigue crack growth rate fluctuated and increased with increasing ΔK. Fractography analysis clearly showed that gaseous hydrogen mainly affected the fatigue crack growth rate. On the fracture surfaces, striations were noted over the entire crack growth region in air, whereas in hydrogen striations were noted at stress intensities lower than ΔKn. Above ΔKn, secondary cracks and brittle flat surfaces with features similar to crack arrest marks were mostly observed in hydrogen. Microstructural analysis along the crack growth direction showed that the crack followed a transgranular path in air, i.e. through α colonies. In hydrogen, the crack also grew along the prior β grain boundaries and at α/β interface within the α colonies. Thereby, the detrimental effect of hydrogen in cast titanium alloy was attributed to a change in the fracture process during crack propagation.
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| 5. |
- Gaddam, Raghuveer, et al.
(författare)
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Influence of hydrogen environment on fatigue crack growth in forged Ti-6Al-4V : fractographic analysis
- 2013
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Ingår i: 7th EEIGM International Conference on Advanced Materials Research. - : IOP Publishing Ltd. ; 48, s. 012010-
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Konferensbidrag (refereegranskat)abstract
- In this study, the influence of hydrogen environment (15 MPa) on the fatigue crack growth in forged Ti-6A1-4V at room temperature is investigated. It is observed that at 21 < ΔK > 25 MPa√m, there exists a change of fatigue crack growth rate (FCGR) in hydrogen environment, and it is accelerated at ΔK > 25MPa√m. FCGR in hydrogen environment is dependent on the stress intensity levels (ΔK). Detailed fractographic analysis of the fracture surfaces were performed at different ΔK using high-resolution scanning electron microscope (HR-SEM). Fatigue striations were observed in air and hydrogen at ΔK < 21MPa√m. At ΔK > 21MPa√m, secondary cracks were observed in hydrogen environment. The differences in appearances of fracture surfaces in air and hydrogen are discussed.
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| 6. |
- Kero, Ida, et al.
(författare)
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Preparation and firing of a TiC/Si powder mixture
- 2009
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Ingår i: 5th International EEIGM/AMASE/FORGEMAT Conference on Advanced Materials Research. - Bristol : IOP Publishing Ltd. ; 5, s. 012016-
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Konferensbidrag (refereegranskat)abstract
- This paper describes how the preparation and heat treatment of TiC/Si powders influences the phase reactions during firing. The powders are prepared by milling and some effects of powder preparation are discussed. A solid state displacement reaction according to: 3TiC + 2Si → Ti3SiC2 + SiC is a priori expected to take place during heat treatment. The firing procedure is investigated with respect to the effect of heat treatment time and temperature on the phases produced, especially Ti3SiC2. Samples were heat treated in a graphite lined furnace. Heat treated samples are analysed by x-ray diffraction, scanning electron microscope and energy dispersive spectroscopy. Ti3SiC2, TiC and SiC are dominant in the final products. The highest amount of Ti3SiC2 is achieved for short holding times (2-4 hours) at high temperatures (1350-1400°C). Ti3SiC2 appears to decompose at elevated temperatures or extended times, through a Ti3SiC2 → TiC + Si(g) type reaction. The activation energy of Ti3SiC2 phase formation is determined to be 289 kJ/mol, using the Mehl-Avrami-Johnson model.
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| 7. |
- Edin, Emil, et al.
(författare)
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Rapid method for comparative studies on stress relief heat treatment of additively manufactured 316L
- 2022
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Ingår i: Materials Science & Engineering. - : Elsevier. - 0921-5093 .- 1873-4936. ; 847
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Tidskriftsartikel (refereegranskat)abstract
- The additive manufacturing method laser powder bed fusion (L-PBF) is known to introduce large residual stresses in the built component. Optimization of process parameters and subsequent heat treatment is crucial to relieve these residual stresses. However, many of the available tools used to analyze these residual stresses are either prohibitively expensive, or too time consuming for initial prototyping stages.A qualitative method for rapid evaluation of the effectiveness of stress relief heat treatment of L-PBF manufactured 316L has been tested. Residual stress induced distortion has been measured with contact and non-contact methods to study the effect of different stress relief heat treatment temperatures (600 – 950 °C, fixed holding time: 1 h). Over the examined temperature interval, at which deformation was measured, distinct differences were observable at each temperature with both methods. Based on the distortion, shape stability was considered reached after subjecting the test geometry to a heat treatment temperature of 900 °C for 1 hour. Complementary mechanical testing and microstructural characterization were carried out to provide a more general understanding of the implications of each heat treatment temperature. Microstructural characterization revealed that complete dissolution of the cellular sub-grain features occurred at the same temperature as where the minimum magnitude of distortion was obtained.
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| 8. |
- Alvi, Sajid
(författare)
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Synthesis and Characterization of High Entropy Alloy and Coating
- 2019
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- High entropy alloys (HEAs) are a new class of alloys that contains five or more principal elements in equiatomic or near-equiatomic proportional ratio. The configuration entropy in the HEAs tends to stabilize the solid solution formation, such as body-centered-cubic (BCC), face-centered-cubic (FCC) and/or hexagonal-closed-pack (HCP) solid solution. The high number of principal elements present in HEAs results in severe lattice distortion, which in return gives superior mechanical properties compared to the conventional alloys. HEAs are considered as a paradigm shift for the next generation high temperature alloys in extreme environments, such as aerospace, cutting tools, and bearings applications.The project is based on the development of refractory high entropy alloy and film. The first part of the project involves designing high entropy alloy of CuMoTaWV using spark plasma sintering (SPS) at 1400 oC. The sintered alloy showed the formation of a composite of BCC solid solution (HEA) and V rich zones with a microhardness of 600 HV and 900 HV, respectively. High temperature ball-on-disc tribological studies were carried out from room temperature (RT) to 600 oC against Si3N4 counter ball. Sliding wear characterization of the high entropy alloy composite showed increasing coefficient of friction (COF) of 0.45-0.67 from RT to 400 oC and then it decreased to 0.54 at 600 oC. The wear rates were found to be low at RT (4 × 10−3 mm3/Nm) and 400 oC (5 × 10−3 mm3/Nm) and slightly high at 200 oC (2.3 × 10−2 mm3/Nm) and 600 oC (4.5 × 10−2 mm3/Nm). The tribology tests showed adaptive behavior with lower wear rate and COF at 400 oC and 600 oC, respectively. The adaptive wear behavior at 400 oC was due to the formation of CuO that protected against wear, and at 600 oC, the V-rich zones converted to elongated magneli phases of V2O5 and helped in reducing the friction coefficient.The second part of the project consists of sintering of novel CuMoTaWV target material using SPS and depositing CuMoTaWV refractory high entropy films (RHEF) using DC-magnetron sputtering on silicon and 304 stainless steel substrate. The deposited films showed the formation of nanocrystalline BCC solid solution. The X-ray diffraction (XRD) studies showed a strong (110) preferred orientation with a lattice constant and grain size of 3.18 Å and 18 nm, respectively. The lattice parameter were found to be in good agreement with the one from the DFT optimized SQS (3.16 Å). The nanoindentation hardness measurement at 3 mN load revealed an average hardness of 19 ± 2.3 GPa and an average Young’s modulus of 259.3 ± 19.2 GPa. The Rutherford backscattered (RBS) measurement showed a gradient composition in the cross-section of the film with W, Ta and Mo rich at the surface, while V and Cu were found to be rich at the substrate-film interface. AFM measurements showed an average surface roughness (Sa) of 3 nm. Nano-pillars of 440 nm diameter from CuMoTaWV RHEFs were prepared by ion-milling in a focused-ion-beam (FIB) instrument, followed by its compression. The compressional yield strength and Young’s modulus was calculated to be 10.7 ± 0.8 GPa and 196 ± 10 GPa, respectively. Room temperature ball-on-disc tribological test on the CuMoTaWV RHEF, after annealing at 300 oC, against E52100 alloy steel (Grade 25, 700-880 HV) showed a steady state COF of 0.25 and a low average wear rate of 6.4 x 10-6 mm3/Nm.
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| 9. |
- Escalera, Edwin, et al.
(författare)
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Thermal treatment and phase formation in kaolinite and illite based clays from tropical regions of Bolivia
- 2012
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Ingår i: 6th EEIGM International Conference Advanced Materials Research. - Bristol : IOP Publishing Ltd.
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Konferensbidrag (refereegranskat)abstract
- The aim of this study was to compare the thermal behaviour of clays containing illite and kaolinite in various proportions. The clays contained small amounts of K and Fe, which act as fluxing agents. In order to investigate the phase formations during heating, the samples were examined in a differential scanning calorimeter at temperatures up to 1300°C. The thermal expansion of the samples was determined by dilatometer measurements from room temperature up to 1150°C. Phases were identified using x-ray diffraction and scanning electron microscopy. In all samples, most of the kaolinite was transformed into metakaolinite during heating up to 650°C, while illite remained unchanged up to 950°C. There was no influence of K and Fe on dehydroxylation. Metakaolinite formed at temperatures above 950°C leading to a Si-Al spinel. Furthermore, mullite was formed in the temperature interval 1050-1150°C. In this temperature range, the mechanism of mullite formation depended on the amount of K and Fe in the samples, changing the temperature of formation of mullite. It was observed by x-ray diffraction that most of the illite was transformed into a Si-Al spinel phase at 1050°C, and during further heating transformed into mullite. An increased amount of illite in the clays slightly decreased the melting temperature. The dilatometer measurements showed expansion and shrinkage for the dehydroxylation and spinel-phase formation, respectively.
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| 10. |
- Fargas, G., et al.
(författare)
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Influence of cyclic thermal treatments on the oxidation behavior of Ti-6Al-2Sn-4Zr-2Mo alloy
- 2018
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Ingår i: Materials Characterization. - : Elsevier BV. - 1044-5803 .- 1873-4189. ; 145, s. 218-224
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Tidskriftsartikel (refereegranskat)abstract
- Ti-6Al-2Sn-4Zr-2Mo is one of the most common titanium alloys for aerospace industry. This alloy experiences oxidation phenomenon at elevated temperatures. In the present study, cyclic thermal treatments were performed in air at 500, 593 and 700 °C, up to 500 cycles, in order to determine the oxidation kinetics and to analyze the oxide scale and alpha-case formation. Moreover, results were compared to those achieved under isothermal conditions to elucidate differences between both thermal conditions. In this sense, metallographic techniques and X-ray diffraction, together with a detailed advanced characterization of the microstructure by Field Emission Scanning Electron Microscopy and Focus Ions Beam, were used to analyze surface oxidation evolution. Results pointed out that cyclic treatments induced a strong increase of the weight gain compared to isothermal treatments. The analysis of the oxide scale revealed the formation of not only rutile, as isothermal treatments, but also anatase. Thickness of the oxide scale was higher for cyclic conditions, while alpha case did not exceed values reached by isothermal treatments and even became lower at 500 °C.
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