Browsing by Author "Marceau, RKW"

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    The competition between metastable and equilibrium S (Al2CuMg) phase during the decomposition of Al-Cu-Mg alloys
    (Elsevier, 2015-10-01) Styles, MJ; Marceau, RKW; Bastow, TJ; Brand, HEA; Gibson, MA; Hutchinson, CR
    The decomposition sequence of the supersaturated solid solution leading to the formation of the equilibrium S (Al2CuMg) phase in Alsingle bondCusingle bondMg alloys has long been the subject of ambiguity and debate. Recent high-resolution synchrotron powder diffraction experiments have shown that the decomposition sequence does involve a metastable variant of the S phase (denoted S1), which has lattice parameters that are distinctly different to those of the equilibrium S phase (denoted S2). In this paper, the difference between these two phases is resolved using high-resolution synchrotron and neutron powder diffraction and atom probe tomography, and the transformation from S1 to S2 is characterised in detail by in situ synchrotron powder diffraction. The results of these experiments confirm that there are no significant differences between the crystal structures of S1 and S2, however, the powder diffraction and atom probe measurements both indicate that the S1 phase forms with a slight deficiency in Cu. The in situ isothermal aging experiments show that S1 forms rapidly, reaching its maximum concentration in only a few minutes at high temperatures, while complete conversion to the S2 phase can take thousands of hours at low temperature. The kinetics of S phase precipitation have been quantitatively analysed for the first time and it is shown that S1 phase forms with an average activation energy of 75 kJ/mol, which is much lower than the activation energy for Cu and Mg diffusion in an Al matrix (136 kJ/mol and 131 kJ/mol, respectively). The mechanism of the replacement of S1 with the equilibrium S2 phase is discussed. © Crown Copyright 2015 Published by Elsevier Ltd. on behalf of Acta Materialia Inc.
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    The effect of molybdenum on clustering and precipitation behaviour of strip-cast steels containing niobium
    (Elsevier B. V., 2019-12) Jiang, L; Marceau, RKW; Guan, B; Dorin, T; Wood, K; Hodgson, PD; Stanford, N
    Two high-strength low-alloy (HSLA) steels containing Nb-carbonitrides were produced, one contained Mo and the other was Mo-free. The alloys were produced by simulated direct strip casting, and were fully bainitic in the as-cast condition. Isothermal ageing treatments were carried out to precipitate harden the alloy, and the strength was measured using a shear punch test. The dislocation density was measured with X-ray diffraction (XRD), and was found to be larger in the alloy containing Mo in all ageing conditions. Atom probe tomography (APT) showed the presence of solute clusters in the as-cast condition, and the addition of Mo increased both size and volume fraction of these clusters. The solute clusters provided significant strengthening increments of up to 112 MPa, and cluster strengthening was larger in the Mo-containing alloy. Precipitation of Nb-carbonitrides was observed after longer ageing times, which were refined by the addition of Mo. This was attributed to the higher dislocation density that increased the number of nucleation sites. Precipitate chemistry was similar for both alloys, and contrary to some literature reports, minimal Mo was observed to segregate to the precipitates. A thermodynamic rationale is presented which describes the reasons that Mo segregates to the Nb-carbide in some alloys but not in others, despite the alloy chemistries being relatively similar. © 2019 Acta Materialia Inc.
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    Effect of molybdenum on the precipitation in ferritic niobium-containing steels produced by strip casting
    (Australian Institute of Nuclear Science and Engineering (AINSE), 2020-11-11) Jiang, L; Wood, K; Dorin, T; Marceau, RKW; Stanford, N
    Molybdenum (Mo) is often alloyed into the steels containing niobium (Nb), in order to enhance the formation of harder microstructures, such as bainite and acicular ferrite, and denser and finer precipitates. However, the effect of Mo on the nano-precipitates formed in the ferrite of Nb steels is still subject to debate, mostly due to its experimentally challenging nature. In addition, direct strip casting is a revolutionary casting technique that integrates casting and subsequent rolling together with rapid solidification and cooling rates, which not only simplifies the process, but also confers superior energy-saving as compared to conventional alloy thermomechanical processing. In this work, therefore, we have studied the effect of Mo on the precipitation in the ferrite of a Nb-containing micro-alloyed steel produced by strip casting using various advanced characterisation techniques. Isothermal ageing treatments were carried out at 650 ℃ up to 10,000 s to form precipitates, and the strength was measured using shear punch test. Transmission electron microscopy (TEM) observation showed that precipitates were formed along dislocations in both steels with and without Mo. Atom probe tomography (APT) analyses revealed that the addition of Mo increased both size and volume fraction of solute clusters after short ageing times, which provided a much higher cluster strengthening. Precipitation of Nb-rich carbonitrides were found after longer ageing treatments. However, no significant Mo was observed to segregate to the precipitate. Small-angle neutron scattering (SANS) results indicated that the addition of Mo reduced the average precipitate size. X-ray diffraction (XRD) results suggested. © The Authors
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    Investigate the effect of molybdenum on precipitation in coiled strip cast niobium steels using correlative microscopy
    (Australian Microscopy and Microanalysis Society, 2016-02-04) Jiang, L; Dorin, T; Marceau, RKW; Wood, K; Hodgson, P; Stanford, N
    Steels containing both Nb and Mo have been reported to exhibit superior strength as compared to those containing only Nb. This has been explained by the formation of harder microstructures, such as bainite and acicular ferrite, and denser and finer precipitates promoted by the addition of Mo. For further improvement of this strengthening method, a fundamental understanding of the effect of Mo on precipitation in steels is necessary. Transmission electron microscopy (TEM) is commonly used for precipitation studies and provides direct imaging of the precipitates, however it only allows local observation and it is thus non statistical by nature and makes it difficult to estimate precipitates volume fraction. On the other hand, small angle neutron scattering (SANS) can be used as a statistical technique to quantitatively study precipitation. Consequently, TEM and SANS provide complementary information and are thus powerful when used in combination. SANS measurement was used to determine the size distribution and volume fraction of particles. Meanwhile, TEM observation was performed to obtain the distribution, morphology, chemistry and size of particles on the samples investigated by SANS. In this research, the effect of Mo on precipitate formation is studied in steels processed with the recently developed direct strip casting process and subsequent coiling treatment. Direct strip casting is a revolutionary casting technique that integrates casting and subsequent rolling together with rapid solidification and cooling rates, which not only simplifies the process, but also confers superior energy-saving as compared to conventional alloy thermomechanical processing. The strip cast samples were isothermally coiled at 700 ̊C for durations up to 10000 seconds. TEM results show that interphase precipitates were formed in both alloys and the size of particles were in the range of 10 nm. Additionally, SANS was used to quantify the precipitate size and volume fraction evolution during coiling.