Louis J. Santodonato: "A Bragg-Williams model to describe site-ordering behavior in a
high-entropy alloy"
Site-ordering behavior in binary intermetallic systems is well known, with models dating back the work of Bragg
and Williams in 1934 (1). The models are constructed in terms of order parameters, representing the
probability that atoms occupy the “correct” lattice sites. The present talk describes the application of a BraggWillliams model to interpret neutron diffraction data obtained on a high-entropy alloy (HEA), and to understand
the underlying ordering behavior. HEAs have multiple principal components, and they ideally form random solid
solutions with simple crystal structures, such as body-centered cubic (2, 3). However, site ordering (i.e., nonrandom mixing) has been detected in many HEAs, and there is a great interest in understanding and minimizing
this behavior, in order to realize the practical applications of this promising class of materials.
1. W. L. Bragg, E. J. Williams, Proceedings of the Royal Society of London. Series A, Containing Papers of a
Mathematical and Physical Character 145, 699 (1934).
2. J. W. Yeh et al., Advanced Engineering Materials 6, 299 (May, 2004).
3. Y. Zhang, Y. J. Zhou, J. P. Lin, G. L. Chen, P. K. Liaw, Advanced Engineering Materials 10, 534 (Jun, 2008).
Zhiqian Sun and Gian Song: "The Development of NiAl and/or Ni2TiAl-Strengthened Ferritic Steels for
Fossil-Energy Power Plants"
In order to limit greenhouse gases and promote the energy transfer efficiency, the operation steam temperature
of supercritical plants is expect to reach 760 ℃with the steam pressure of 35 MPa [1, 2]. Ferritic steels are
preferred for steam-piping and heater materials due to their lower thermal expansion coefficients and higher
thermal conductivity, compared with austenitic steels [2]. Similar to Ni-based superalloys, coherent co-planar
precipitates of an ordered superlattice phase in a disordered solid-solution matrix have been adopted as a
strengthener for high-temperature creep resistance in ferritic alloys, such as NiAl and Ni2TiAl precipitates[3-5].
In the present study, effects of two different precipitates on mechanical properties, such as deformation
behavior at room and elevated temperatures and creep characteristics, has been studied. Investigation on the
NiAl-strengthened ferritic alloy shows that the ferritic alloy is susceptible to brittle fracture at room and
intermediate temperatures. In-situ tension test was performed under neutron diffraction at 370 ℃in order to
reveal underlying fracture mechanisms of NiAl-strengthened ferritic steels at room and intermediate
temperatures. Based on the lattice-strain evolution during the in-situ experiment, it is proposed that NiAl
precipitates are likely to fracture at the beginning, and then these cracks grow and coalescence, resulting in
transgranular fracture without plastic elongation. Two-phase microstructure consisting of Ni2TiAl or
Ni2TiAl/NiAl precipitates in a α-Fe matrix has been developed by the addition of Ti element [2, 4, and 6 weight
percent (wt.%)] into the NiAl-strengthened ferritic alloy. Transmission electron microscopy and high energy Xray diffraction measurements have been conducted for the microstructural identification, lattice-parameter, and
misfit measurement between the precipitate and the matrix. Compression step-loading creep and tension creep
rupture tests were carried out in order to assess the creep resistance at 973K. Creep results revealed that the
creep resistance of the two-phase ferritic alloys is considerably improved, compared to the NiAl-strengthened
ferritic alloys.
[1] Viswanathan R, Coleman K, Rao U. Materials for ultra-supercritical coal-fired power plant boilers. International
Journal of Pressure Vessels and Piping 2006;83:778-83.
[2] Viswanathan R, Henry JF, Tanzosh J, Stanko G, Shingledecker J, Vitalis B, et al. U.S. program on materials
technology for ultra-supercritical coal power plants. Journal of Materials Engineering and Performance
2005;14:281-92.
[3] Huang S, Brown D, Clausen B, Teng Z, Gao Y, Liaw P. In Situ Neutron-Diffraction Studies on the Creep Behavior
of a Ferritic Superalloy. Metallurgical and Materials Transactions A 2011;43:1497-508.
[4] Teng ZK, Ghosh, G. Miller, M. K. Huang, S., Clausen, B., Brown, D. W., Liaw, P. K. Neutron-diffraction study and
modeling of the lattice parameters of a NiAl-precipitate-strengthened Fe-based alloy. Acta Mater 2012;60:5362-9.
[5] Sun Z, Liebscher CH, Huang S, Teng Z, Song G, Wang G, et al. New design aspects of creep-resistant NiAlstrengthened ferritic alloys. Scr Mater 2013;68:384-8.