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A mean-field model based on the elasticity theory of point defects has been developed to investigate the role of uniform stress fields on the long-range ordering of carbon atoms in bct-iron. From an analysis of the thermodynamic equilibria, composition – temperature – stress state diagrams are derived. We demonstrate that ferrite, martensite and supercritical iron are various instances of the same bct-iron phase region. A coherent mapping of the phase transitions is drawn, identifying (i) continuous transitions such as ferrite ordering, martensite enhanced ordering and ferrite – martensite transformation, and (ii) discontinuous transitions such as temperature-induced martensite and stress-induced martensite. Our analysis is supported by rigid-lattice Monte Carlo simulations. Recently published experimental results on highly-drawn perlitic wires are re-interpreted in terms of supercritical iron, rather than strain-induced martensite. Novel low-temperature thermomechanical treatments of supersaturated ferrite are suggested, for improved nanostructure design of martensitic steels. [ABSTRACT FROM AUTHOR]

The existence of an L1 0 –L1 2 hybrid ordered phase for face-centered cubic (fcc) crystal symmetry was first theorized by William Shockley in 1938, but has not been verifiably observed to date. Ordered magnetic thin films of Fe 38.5 Pd 61.5 , the expected L1 0 –L1 2 eutectoid composition, grown epitaxially on MgO(0 0 1) substrates by pulsed laser deposition, are shown to exhibit the first empirical confirmation of L1′ type ordering. A formal treatment of L1′ ordering requiring the definition of two independent ordering parameters is discussed in conjunction with the crystallographic and magnetic characterization of the Fe 38.5 Pd 61.5 thin films. Two theories regarding origination of the unique metastable ordering behavior induced by the non-equilibrium strain conditions are proposed. [ABSTRACT FROM AUTHOR]

Mechanical property degradation due to the disorder–order phase transformation is of potential concern for alloys based on the Ni–Cr binary system, particularly in nuclear power applications, where component lifetimes can exceed 80 years. In the present research, a disorder–order phase transformation has been studied in the Ni–33 at.% Cr model alloy by a combined experimental and computational approach. The multiscale modeling framework utilizes grand canonical and kinetic Monte Carlo simulation techniques based upon density functional theory calculations to treat both the thermodynamic and kinetic aspects of the phase transformation. The simulation results are used to generate a simple model for the ordering kinetics based upon the Kolmogorov–Johnson–Mehl–Avrami equation. Experimental measurements of the change in lattice parameter as a function of aging time and temperature are obtained in order to assess the model accuracy. The resulting model shows reasonable agreement with experimental data at 470 and 418 °C; however, additional experimental data at longer aging times are needed to confirm the accuracy of the model at lower temperatures. The model predicts that the initiation of the ordering transformation will occur in Ni–33Cr at temperatures and timescales relevant to nuclear power systems, though longer times are required for the transformation to proceed to completion. [ABSTRACT FROM AUTHOR]

The crystal structure of the δ 1k phase in the Fe–Zn system was investigated by scanning transmission electron microscopy. The δ 1k phase has a superlattice structure based on the δ 1p phase having a tripled periodicity along the a -axis direction of the δ 1p phase, accompanied by one-dimensional stacking disorder of structural blocks (called order–disorder (OD) packets) along the c -axis direction. The crystal structure can be described crystallographically in terms of the OD theory so as to belong to the category IV OD structures composed of two types of non-polar OD layers ( L 2 n and M 2 n +1 ). The tripled periodicity along the a -axis direction is due to chemical ordering of the constituent Fe and Zn atoms in the OD layer M 2 n +1 . Because of the tripled periodicity of the OD layer M 2 n +1 , three different equivalent stacking positions are generated in stacking an OD layer M 2 n +1 on top of the OD layer L 2 n . Depending on the stacking order, the crystal structure of the δ 1k phase can be ordered with various periodicities along the stacking direction or completely disordered. Based on the OD theory, two maximum degree of order (MDO) polytypes belonging to the space groups of P 6 3 / mcm (MDO1) and R 3 c (MDO2) are deduced for the δ 1k phase. The most stable MDO polytype in the OD family of the δ 1k phase is determined experimentally to be the MDO2 polytype. [ABSTRACT FROM AUTHOR]

Abstract: Real-time observations of Al–Al2O3 dynamic liquid–solid interfaces on the atomic scale indicate the presence of structural ordering in the liquid at the solid–liquid interface. The main problem with direct high resolution transmission electron microscopy (HRTEM) interpretation is that the imaging conditions and aberrations in the imaging system have a significant influence on the contrast in the image, and may lead to inaccurate conclusions about the structure examined. New quantitative results based on using a single image iterative wave function reconstruction are presented. This technique requires only a single experimental image, and allows extraction of reliable and aberration-free structural information from experimental HRTEM micrographs. This numerical phase retrieval method was successful in analysis of the experimental data and allowed, for the first time, direct extraction of quantitative information regarding the degree of ordering (parallel and perpendicular to the interface) at liquid–solid interfaces. The degree of ordering at the Al2O3–Al interface at 750°C was quantified and both layering and in-plane ordering were found. The layering in the liquid extends to about four to five layers (about 1nm from the edge of the crystal). The in-plane ordering, which was observed only in the first three layers of the liquid, decays faster than the layering. In addition, the interlayer spacings measured in the liquid indicate that the liquid atoms at the interface are influenced by the structure of the crystal, while further away the ordering of the liquid atoms gradually disappears, until they adopt the characteristics of the bulk liquid. [Copyright &y& Elsevier]

Abstract: Ternary Au–Cu–Pt alloys, of unequal gold and copper atomic contents and containing 2.5wt.% Pt, are ordered isothermally at 250 or 400°C after annealing at 650°C and quenching in water. The alloys are characterized by transmission electron microscopy, X-ray diffraction, hardness and tensile testing. With 75 and 76.5wt.% Au, the microstructural evolution depends on the ordering temperature. At 400°C, a classical nanotwinned “polytwin” structure with, locally, two L10 crystal orientation variants is formed; this evolves into a two-phase, L10 +A1, checkerboard-like microstructure after 106 s. At 250°C, ordering develops a stable (up to 105 s) structure locally containing all three L10 crystal variants arranged in a network of {110} twins roughly 30nm wide. At both ordering temperatures the 78wt.% Au alloy develops a similar three-variant nanotwinned structure that also remains stable up to 105 s. The 75 and 76.5wt.% Au alloys display a peak in hardness after roughly 120 s at both 400°C and 250°C; with 78wt.% Au, peak hardness is not reached at 105 s. With all three alloys, superior hardness and tensile strength, coupled with lower ductility, are obtained with the three-variant nanotwinned structure formed at 250°C compared with the more classical polytwin structure that develops at 400°C. [Copyright &y& Elsevier]

Abstract: The equilibrium phase diagram of solid Pt–Rh is calculated by means of semi-grand canonical lattice Monte Carlo simulations using thermodynamic integration. Configurational energies are described by a refined BOS model (bond-order simulation mixing), which is fitted to reference data obtained from first-principles calculations. The calculated equilibrium phase diagram shows the ordered face-centered cubic structure 40 and structure as stable phases at low temperatures (<250K) and has no miscibility gap at higher temperatures, which is in contrast to existing assumptions. The calculated short-range order parameters of the solid solution are in full agreement with experimental values and thus confirm that the BOS model provides a sound description of configurational energies. [Copyright &y& Elsevier]

Abstract: The Al–Fe system is important for many alloys and the new interest in iron aluminides makes it necessary to improve the modeling of the different ordered forms on the body-centered cubic lattice in this system. This has now been done using a four-sublattice model based on the compound energy formalism, which can describe disordered A2 and the B2, and B32 ordering. The chemical and ferromagnetic ordering transitions can be both first and second-order and they have a strong interaction. Almost all available experimental and theoretical data for all phases in the system have been fitted within estimated uncertainties. [Copyright &y& Elsevier]

Abstract: The effect of interfacial ordering kinetics on growth morphology is examined for undercooling solidification of intermetallics. The focus is on NiAl, which has been found to solidify into completely irregular seaweed-like morphologies at growth velocities of ∼2ms−1. The transition from regular dendritic to seaweed growth in this material is simulated using a hybrid deterministic–probabilistic model of microstructure evolution, taking account of the interfacial kinetic effects. Modelling shows that seaweed formation in intermetallic forming systems can be attributed in part to transient entrapment of the chemical and orientational disorder at the solid/liquid interface. Both types of disorder trapping give rise to strongly preferential depression of the interfacial temperature, while orientational disorder trapping also reduces the definitiveness of crystal orientation at the interface. These effects counterbalance the role of crystallographic anisotropy in promoting growth into preferred directions, and hence lead to relatively easy breakdown of regular dendritic patterns in highly ordered materials. [Copyright &y& Elsevier]

Abstract: The grain growth and ordering behaviors in nanocrystalline Fe75Si25 powders produced by mechanical alloying was studied. In the temperature range 573–703 K, the disordered α-Fe(Si) showed only limited growth with a small activation energy of 32.3 kJmol−1. However, in the range 773–853 K ordering to DO3 structure occurred accompanied by a sharp increase in average grain size to a maximum value that depended on the temperature. A grain growth model available in the literature for nanocrystalline materials yielded a good fit to the experimental data. Activation energy of 278.9 kJmol−1 determined using this model for grain growth is similar to that for lattice diffusion of Si in DO3 Fe3Si. The reason for the concurrence of ordering and grain growth is attributed to the Si desegregation at grain boundaries upon ordering transformation. The levels of segregation are estimated from lattice parameter measurements. [Copyright &y& Elsevier]

Abstract: This paper reports the evolution of order in stoichiometric Ni–25at.%V alloys with different thermal histories. In these alloys, the face-centred cubic structure of the disordered matrix transforms to a tetragonal (D022) structure during ordering. In such types of transformation, the process of order evolution involves a number of nucleation and growth processes, which are often competitive in nature. In the present work, evolution of the order has been studied in two sets of samples, one in which samples were subjected to order annealing treatment in the “melt-spun state” and the other in which samples were annealed in the “solid solution state”. Different sequences of the order evolution have been observed in the two sets of samples. In the first set of samples, grain boundary aided heterogeneous nucleation and growth of ordered domains dominated the order evolution process, whereas in the second set of samples the intragranular nucleation and growth of ordered domains dominated the ordering process. As a manifestation of different evolution sequences, different morphological changes occurred in the two sets of samples. The difference in the evolution sequences has been rationalized on the basis of driving forces influencing the ordering transformation in the two cases. [Copyright &y& Elsevier]

The ordering behaviour of a Ni-Mo alloy in the presence of a ternary additive, viz. Cr, has been studied. The sequence of ordering transformations in binary Ni–Mo alloys has been shown earlier to be controlled by a competition between several f.c.c.-based superlattices, viz. Ni2Mo (Pt2Mo type), Ni3Mo (DO22), Ni4Mo (D1a) and the so-called short range ordered (SRO) structure characterized by the presence of diffraction intensity maxima at the 11/20f.c.c. positions in reciprocal space. The effect of ternary addition of chromium in the selection of the superlattice structure has been examined in this paper in an alloy of composition Ni–24 at.% Mo–6 at.% Cr. The presence of Cr has been experimentally found to favour the formation of Ni2(Mo,Cr) (Pt2Mo-type) phase in preference to the Ni3Mo (DO22) and Ni4Mo (D1a) superlattices. This leads to a sequence of transformation different from that obtained in binary alloys. The effect of Cr on the ground state phase stability is determined, in this alloy, using the first-principles local-density based full-potential augmented plane wave (FP-LAPW) method. From the differences in the electronic structures, densities of states and total energies of binary Ni2Cr, Ni2Mo (Pt2Mo type), Ni3Mo (DO22) and Ni4Mo (D1a) phases and the corresponding ternary superlattice structures, an attempt has been made to predict the hierarchy of relative phase stabilities. Theoretical predictions based on these electronic structure calculations have been found to be consistent with the experimental microstructural observations of the evolutionary stages of ordering in the ternary Ni–24 at.% Mo–6 at.% Cr alloy. [ABSTRACT FROM AUTHOR]