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Incubation time for iron-nitride layer formation upon gaseous nitriding of iron-based alloys
Jung, Minsu,Meka, Sai Ramudu,Mittemeijer, Eric Jan Abingdon; Taylor & Francis Ltd 2016 PHILOSOPHICAL MAGAZINE Vol.96 No.14
<P>A model was developed to predict quantitatively the influence of alloying element (Me) dissolved in the ferrite (alpha) matrix on the incubation time for iron-nitride layer formation upon gaseous nitriding of iron-based alloys. The model incorporates the coupled, concurrent processes of inward diffusion of nitrogen and the depth dependency of the time dependency of the precipitation of alloying-element nitride particles in the a matrix. Experimental results were obtained by gaseous nitriding of an Fe-2.23 at.% V alloy. The incubation time for iron-nitride formation on Fe-Me alloy is generally much larger than that for iron-nitride formation on pure iron due to a pronouncedly lesser rate of increase of dissolved N content at the surface of Fe-Me alloy. The extent of segregation of N at the MeN/alpha-Fe interfaces has distinct influence on the incubation time.</P>
Crystallisation kinetics of Zr41.2Ti13.8Cu12.5Ni10Be22.5 BMG alloy during heating
Lee, Sang Bok,Kim, Nack J. Abingdon; Taylor & Francis Ltd 2005 PHILOSOPHICAL MAGAZINE Vol.85 No.2-4
<P> The crystallisation behaviour of Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 bulk metallic glass (BMG) alloy during heating from the amorphous state has been investigated. Upon heating, the nucleation process was not consistent with classical nucleation theory. This problem can be overcome by substituting the nucleation density with the number density of nuclei, N, for growth of the crystalline phase, which is estimated from the length scale of the microstructure. The growth of crystalline phase is the governing process for crystallisation of the Zr41.2Ti13.8Cu12.5Ni10Be22.5 alloy upon heating. The growth process is more important than the nucleation process for analysing the crystallisation mechanisms of the amorphous state. Non-isothermal heating DSC data have been analysed with a modified crystallisation kinetics model based on the non-isothermal transformation theory. Using this model, the onset crystallisation temperatures at various heating rates have been calculated. The calculated critical heating rate to avoid crystallisation is 190 K s-1, which matches well with the experimental value. The onset crystallisation times for isothermal annealing conditions have also been calculated and compared with the isothermal DSC data. Time temperature transformation (TTT) and continuous heating transformation (CHT) diagrams have been constructed quantitatively. The crystallisation kinetics of BMG alloys can be more reliably analysed with our modified crystallisation kinetics model than with several nucleation models.</P>
Hwang, Jiyong,Lee, Hoseong,Yi, Seonghoon Abingdon; Taylor & Francis Ltd 2016 PHILOSOPHICAL MAGAZINE Vol.96 No.24
<P>Fe-based amorphous ribbons with excellent soft magnetic properties and mechanical properties were prepared in the Fe-Si-P ternary system. Enhanced soft magnetic properties could be achieved through annealing treatment of the ribbons for 1h at 325 degrees C, which is far below the glass transition temperatures (462-474 degrees C). Icosahedral medium-range ordering with a size range of around 2nm occurred throughout the amorphous matrix during the low-temperature annealing treatment. The annealed ribbons exhibited improved magnetic saturation of over 185emu/g while maintaining good mechanical flexibility. During icosahedral ordering, the distance between the Fe atoms and the coordination number within the amorphous ribbon can be optimised for achieving high magnetic saturation. However, nanocrystallisation of the SiP and Fe2P transition phases embedded within the amorphous matrix occurred after the annealing treatment for 1h at 385 degrees C, which caused deterioration of the soft magnetic properties and mechanical flexibility of the ribbons. Therefore, the combination of high magnetic saturation and mechanical flexibility of the amorphous ribbons could be optimised through low-temperature annealing treatment without any nanocrystallisation.</P>
Heterogeneous nucleation of icosahedral phase from FCC phase in cast Al 87 Mn 4 Si 2 Be 7 alloy
Fleury, E.,Chang, H.-J.,Kim, D.-H. Abingdon; Taylor & Francis Ltd 2006 PHILOSOPHICAL MAGAZINE Vol.86 No.3-5
<P>The formation of the icosahedral (i) quasicrystalline phase has been studied in the cast Al 87 Mn 4 Si 2 Be 7 alloy. Under low cooling rates, i-phase particles of about 5 to 15?µm formed in the a Al matrix. A thorough examination of the microstructure revealed the existence of cubic crystalline phases embedded in the i-particles. Microstructural and thermal analyses indicated that the i-particles nucleated heterogeneously from the cubic AlMn(Si)Be 4 phases by a peritectic reaction, however with a slight misorientation.</P>
Hydrogen-induced softening in nanocrystalline Ni investigated by nanoindentation
Zhao, Yakai,Seok, Moo-Young,Lee, Dong-Hyun,Lee, Jung-A,Suh, Jin-Yoo,Jang, Jae-il Abingdon; Taylor & Francis Ltd 2016 PHILOSOPHICAL MAGAZINE Vol.96 No.32
<P>The influence of hydrogen on the plastic deformation of nanocrystalline nickel was analysed by recourse to nanoindentation on the uncharged and hydrogen-charged samples. It was revealed that, in nanocrystalline Ni, hydrogen significantly decreases hardness but does not alter the strain rate sensitivity. Through thermal desorption spectroscopy measurement, charged hydrogen was expected to reside in face-centred cubic lattice, grain boundaries (GBs) and vacancies rather than dislocations. The hydrogen-induced softening behaviour is discussed in terms of the possible roles of hydrogen in GB-assisted dislocation flow mechanism.</P>