B2 Order Research Articles

Spray deposition of an iron aluminide

High velocity oxyfuel (HVOF) spray forming of an iron aluminide [Fe–12.5 Al–2.93 Ni–0.02 B (wt%), containing 300 p.p.m. oxygen], followed by heat treatment for 24 h at 500°C, 18 h at 600°C and 20 min at 800°C, and multipass hot rolling at 800°C has been studied. Three different thicknesses (0.43, 0.93 and 1.33 mm) of sprayed deposit were produced by spraying for different times (approximately 10, 20 and 30 min). The spray-deposited layers exhibited some oxide and some porosity. This porosity was reduced by heat treatment. The as deposited layer had a high degree of B2 order, and a B2 antiphase domain size of 4.5 nm. On hot rolling this material to a reduction of 38%, it was found to be more susceptible to edge cracking than similar material processed by an ingot–extrusion–hot rolling route. In heat treatment, the aluminide-sprayed layer formed a non-protective Fe2O3 oxide, rather than the usual Al2O3 that forms on the binary alloy. This is attributable to the Ni content of the iron aluminide powder employed. © 1998 Kluwer Academic Publishers

Pressure and temperature dependence of nickel diffusion in Fe 3Al: effect of ordering

The pressure and temperature dependence of 63Ni heterodiffusion was investigated in the temperature range 888–1282 K, which includes the critical temperature of the A2–B2 order–disorder transition. All the experiments were carried out using the radiotracer method combined with a serial sectioning technique. The high-pressure measurements have shown that the diffusion process is controlled by mono-vacancies both in the ordered and disordered structures. The Arrhenius plot presents a downward curvature as the result of B2-type atomic ordering and the lattice diffusion coefficients can be fitted by a continuous curve as a function of temperature and long-range order parameter. Because of the site preference of Ni for α-sites, nickel diffusivity is slower and the activation energy of diffusion is higher than the corresponding values for iron self-diffusion.

Ordering temperatures in Cu–Al–Ni shape memory alloys

The ordering temperatures Tc1 (disordered β to B2 order) and Tc2 (B2 to L21 order) have been obtained in Cu–Al–Ni shape memory alloys with different concentrations by electrical resistivity. The dependence of the ordering temperatures on the concentration has been established. Also, a modification of the theoretical calculations has been proposed to predict the ordering temperatures in Cu–Al–Ni ternary alloys. A good agreement between the theoretical ordering temperatures and the experimental results has been found.

A Monte-Carlo study of B2 ordering and precipitation via vacancy mechanism in b.c.c. lattices

Atomistic Monte-Carlo simulations of precipitation and B2 ordering in a binary alloy on a b.c.c. lattice are performed using a vacancy mechanism. They reproduce the following experimental observations: for a composition close to the A2 solubility limit, precipitation and ordering proceed simultaneously and lead to subsequent coarsening of B2 precipitates. At higher concentrations, a rapid short range ordering is observed before A2 phase appears via thickening of the antiphase boundaries. Closer to the B2 solubility limit, one also observes nucleation of A2 phase inside B2 domains. Moreover, it is observed that the very early stages of ordering are strongly dependent on atomic mobility parameters. Depending on the relative exchange frequency of vacancies with A and B atoms, and depending on the composition, two extreme regimes of ordering are identified, leading either to localized or delocalized ordering.

Experimental Study by Transmission Electron Microscopy of Antiphase Boundaries in Fe-Al27% Close to the DO<sub>3</sub>→B2 Order Phase Transition

In this paper, we present the first quantitative determination of the divergence law of the correlation length ξ, when approaching the second order phase transition DO 3 →B2, by studying the behaviour of the APBs by transmission electron microscopy.

The relationship of microstructure and magnetic properties in cold-rolled 6.5% Si-Fe alloy

The effect of microstructures on the magnetic properties in the 6.5% Si-Fe alloy was studied. The alloy was subjected to two patterns of heat treatments, and the microstructures were observed by means of TEM. The alloy after an appropriate treatment has shown excellent soft magnetic properties, with the maximum permeability of 7.25/spl times/10/sup 4/ and the core loss W/sub 10/50/ of 0.39 W/kg. The corresponding microstructure was characterized by the extremely fine domains of DO/sub 3/ order phase existing in B2 order phase. However, the coarsening of the DO/sub 3/ domains was found to reduce the maximum permeability and increase the core loss of the alloy. It is suggested that the coarsening of the DO/sub 3/ domains be accompanied with the formation of magnetic contrast around the boundary of the DO/sub 3/ and B2 phases. Therefore, it is assumed that the phase boundary tends to oppose the displacement of magnetic domain walls due to the existence of the magnetic contrast.

Mössbauer effect in atomic ordered and disordered Fe0.75−xMnxAl0.25 alloys

AbstractA Mössbauer effect study of the effect of chemical order on hyperfine magnetic field distributions in the quasi‐binary alloy system Fe0.75−xMnxAl0.25 (based on substitution of Fe by Mn in Fe3Al) shows that a preferential substitution of Mn at one of the sites leads to a B2 order in the ternary system. The kinetics of the ordering process in Fe0.45Mn0.3Al0.25 shows distinctly different relaxation behaviours at the two possible inequivalent sites in the ordered structure.

Cycling Effects on Transformation Behaviour in Shape Memory CuZnAl Alloys

Shape memory alloys transform martensitically to close packed structures and have internally faulted structures in martensitic condition, and they are very sensitive to cycling and ageing effects. In noble metal copper basal alloys, martensitic transformation occurs from a cubic structure with B2 (or DO 3 ) order to a long period layered structure 9R (or 18R). In a single crystal of parent phase, 24 martensite variants grouped around six {110} bcc matrix planes can be formed at temperatures below M s . These alloys deform by variant transformations, in case they are stressed in the martensitic condition. Cycling effects on transformation behaviour in two shape memory CuZnAl alloys received in the closed-coil form called shape memory actuator which have originally open-coil form in austenitic condition have been investigated. Shape memory alloys retrain in the deformed shape in martensitic condition and recover the original form in austenitic condition and the shape memory actuators cycle between the open and closed-coil configurations on heating above A f and cooling below M f temperatures. These effects are attributed to the reversible transition between the ansenite and martensite phases. These alloys are metastable in martensitic condition, and cycling effects also contribute to the martensite stabilization.

Phase diagrams of bcc alloys at low temperatures with ballistic atom movements.

Kinetic Monte Carlo simulations were used to determine the steady states of equiatomic bcc alloys with first- and second-nearest-neighbor (1NN and 2NN) effective pair interactions. Atom movements occurred with a vacancy mechanism, and were of two types: thermal and ballistic. The phase diagram in the space spanned by the 1NN and 2NN interaction strengths was determined for thermal atom movements alone, and with some ballistic atom movements. With increasing temperature, the region of B32 phase receded against the adjacent regions of B2 order and unmixed states. With some ballistic atom movements, however, the region of B32 order encroached on the adjacent regions of B2 order and unmixed state. The shifts in phase boundaries are attributed to differences in how the internal energies of the different phases are affected by ballistic atom movements. Local fluctuations in the internal energy density in the presence of ballistic atom movements caused two-phase regions in the phase diagram.

A Monte-Carlo Study of B2 Ordering and Precipitation on a Bcc Lattice

ABSTRACTAtomistic Monte-Carlo simulations of precipitation and B2 ordering in a binary alloy on a BCC lattice via vacancy mechanism are presented. Evolution of simulated patterns is in good agreement with the experimentally observed morphologies. Moreover, it is observed that the very early stages of ordering are strongly dependent on atomic mobility parameters. Depending on the relative exchange frequency of vacancies with A and B atoms, two extreme regimes oiordering are identified, leading either to localized or delocalized ordering.

B2 and B32 ordering transformations of equiatomic bcc alloys with ballistic and thermal atom movements

We used Monte Carlo simulations to study the kinetics and steady states of B2 and B32 ordering in an equiatomic binary bcc alloy having both thermal and ballistic atom movements. Atom movements occurred by a vacancy mechanism, where first-neighbor atoms exchanged sites with the vacancy. In the thermodynamic case, where this jump probability was set by a Boltzmann factor alone, we found the formation of large amounts of transient B2 (B32) during disorder → B32 (B2) order transformations. The transient order developed in distinct regions, but vanished completely when equilibrium was attained. Ballistic jumps were included as random interchanges of the vacancy with one of its neighboring atoms. Even with small fractions of ballistic jumps, there were large changes in the transient states and steady states of order in the alloy. A kinetic explanation is proposed, in which the presence of ballistic jumps contributed a greater amount of internal energy to the B2 phase than to the B32 phase when there was a strong asymmetry in the second-nearest neighbor pair potentials (VAA2 ↗ VBB2). A two-phase coexistence in the steady state was explained by fluctuations in the local density of ballistic jumps.

Critical Behavior of Antiphase Boundaries in Fe-A127% Close to the D03→B2 Order Phase Transition

ABSTRACTIn this paper, we present the first quantitative determination of the divergence law of the correlation length ξ, when approaching the second order phase transition DO3→B2, by studying the behaviour of the APBs by transmission electron microscopy.

First-Principles Calculations of Heusler Phase Precursors in the Atomic Short-Range Order of Disordered BCC Ternary Alloys

AbstractWe apply our recently developed first-principles theory of atomic short-range order in disordered multicomponent alloys to CuAuZn2 and AgAuZn2, two alloys which display partially-ordered B2 phases as well as fully-ordered Heusler structures. The calculated Warren-Cowley pair correlation functions for both alloys peak at the special points 〈½ ½ ½〉 and 〈100〉, with the peak at 〈100〉 dominant. This is indicative of a tendency to B2 order at high temperatures, and a subsequent tendency to Heusler-type order at lower temperatures. An analysis in terms of effective interactions shows that the Au Zn interaction drives the 〈100〉 ordering tendency, while the Ag Au and Cu Au interactions contribute more to the 〈 ½ ½ ½〉ordering tendency - this effect is larger in CuAuZn2 and should lead to a slightly higher transition temperature for the B2 to Heusler transition in CuAuZn2 as compared to AgAuZn2.

Phases and phase stabilities of Fe3X alloys (X=Al, As, Ge, In, Sb, Si, Sn, Zn) prepared by mechanical alloying

Mechanical alloying with a Spex 8000 mixer/mill was used to prepare several alloys of the Fe3X composition, where the solutes X were from groups IIB, IIIB, IVB, and VB of the periodic table. Using x-ray diffractometry and Mössbauer spectrometry, we determined the steady-state phases after milling for long times. The tendencies of the alloys to form the bcc phase after milling are predicted well with the modified usage of a Darken–Gurry plot of electronegativity versus metallic radius. Thermal stabilities of some of these phases were studied. In the cases of Fe3Ge and Fe3Sn, there was the formation of transient D03 and B2 order during annealing, although this ordered structure was replaced by equilibrium phases upon further annealing.

Phase transformations in the (Ti, Nb) 3 Al section of the TiAlNb system—II. Experimental tem study of microstructures

In Part I of this paper possible transformation paths that involve no long range diffusion and their corresponding microstructural details were predicted for TiAlNb alloys cooled from the high temperature b.c.c./B2 phase field into close-packed orthorhombic or hexagonal phase fields. In the present paper experimental TEM results show that two of the predicted transformation paths are indeed followed for different alloy compositions. For Ti25Al12.5Nb (at.%), the path includes the formation of intermediate hexagonal phases, A3 and D0 19, and subsequent formation of a metastable domain structure of the low-temperature O phase. For alloys close to Ti25Al25Nb (at. %), path involves an intermediate B19 structure and subsequent formation of a translational domain structure of the O phase. The path selection depends on whether B2 order forms in the high temperature cubic phase prior to transformation to the close-packed structure. The paper also analyzes the formation of a two-phase modulated microstructure during long term annealing 700°C. The thermodynamics underlying the path selection and the two-phase formation are also discussed.

Microstructural influence of Mn additions on thermoelastic and pseudoelastic properties of CuAlNi alloys

The thermoelastic and mechanical properties of CuAlNiBMn shape memory alloys have been studied as a function of manganese concentration and of heat treatment. Below a limiting value of manganese content, the loss of thermoelastic and pseudoelastic properties has been observed, in particular in the quenched specimens. The partial transformation and its degradation during thermal cycling observed in the low manganese content alloy has been attributed to the lower degree of B2 order achieved during the quench, leading to slower kinetics of DO 3 ordering. The accomodation of strains between martensite variants and between the martensite/austenite phases appear to need dislocation accumulation at their interfaces. The presence of dislocations observed during the reverse transformation seem responsible for the degradation of the transformation and the loss of pseudoelastic properties of this alloy.

Kinetics of B2, DO3, and B32 ordering: Results from pair approximation calculations and Monte Carlo simulations

A kinetic theory of ordering based on the path probability method was implemented in the pair (Bethe) approximation and used to study the kinetics of short- and long-range ordering in alloys with equilibrium states of B2, DO3, or B32 order. The theory was developed in a superposition approximation for a vacancy mechanism on a bcc lattice with first- (1nn) and second-nearest neighbor (2nn) pair interactions. Chained 1nn conditional probabilities were used to account for the entropy of 2nn pair configurations. Monte Carlo simulations of ordering were also performed and their results compared to predictions of the pair approximation. Comparisons are also made with predictions from an earlier kinetic theory implemented in the point (Bragg-Williams) approximation. For all three calculations (point, pair, and Monte Carlo), critical temperatures for B2 and DO3 ordering are reported for different 1nn and 2nn interaction strengths. The influence of annealing temperature on the kinetic paths through the space of B2, DO3, and B32 order parameters was found to be strong when the thermodynamic preferences for the ordered states were of similar strengths. Transient states of intermediate order were also studied. A transient formation of B32 order in an AB3 alloy was found when 2nn interactions were strong, even when B32 order was neither a Richards-Allen-Cahn ground state nor a stable equilibrium state at that temperature. The formation of this transient B32 order can be argued consistently from a thermodynamic perspective. However, a second example of transient B2 order in an AB alloy with equilibrium B32 order cannot be explained by the same thermodynamic argument, and we believe that its origin is primarily kinetic.

Influence of Mn Additions on the Thermoelastic and Pseudoelastic Behaviour of Cu-Ai-Ni Alloys

AbstractThe transformation properties of Cu-Al-Ni alloys modified by the additions of boron and manganese have been interpreted by studying the reversibility and stability of the martensitic transformation as a function of heat treatment and of manganese concentration between 2 and 4 wt%. The alloy containing 2% Mn exhibits a lack of thermoelasticity due to a decrease of the degree of B2 order and supression of DO3 order during quenching and the increase of both types of order during annealing. From the determination of the activation energies responsible for the martensitic and reverse transformations, we have deduced that, in both cases, the kinetics of the transformation are controlled by an atomic jump at the interface between the two phases.

The nature of dislocations and effect of order in rapidly solidified Fe–(5.5–7.5)wt.%Si alloys

The results of an investigation on the nature of dislocations in rapidly solidified Fe–(5.5–7.5)wt.%Si has been presented as a function of the ordered state achievable during rapid solidification processing. The comparative study involves three rapid solidification processing routes for each alloy: melt spinning, planar flow casting and twin rolling. It is shown that for Fe–5.5wt.%Si alloy, ordering can be completely suppressed, while, in Fe–6.5wt.%Si alloy, the B2 ordering cannot be suppressed. The DO3 state is present in twin-rolled Fe–wt.%Si alloy while, in rapidly solidified Fe–7.5wt.%Si alloy, it is the predominant ordered state. The disordered Fe–5.5wt.%Si alloy contains dislocation dipoles and vacancy loops, while rapidly solidified samples of Fe–6.5wt.%Si composition exhibit B2 order with a0/2<111>type superdislocations where ain0 is the DO3 lattice parameter. The paper reports the first observation of a four dislocation set in Fe–Si alloys with the superdislocation burgers vector a0<111>. They are predominantly of mixed type, with some in edge orientation. Analysis of the experimental superdislocation separations in terms of existing models of interaction energies is presented. It is shown that separations reflect the non-equilibrium order parameters prevalent in the alloy. Furthermore it is pointed out that the dislocation separations observed in the DO3-ordered state cannot be explained by the existing models.

Transformation of BCC and B2 High Temperature Phases to HCP and Orthorhombic Structures in the Ti-Al-Nb System. Part II: Experimental TEM Study of Microstructures.

Possible transformation paths that involve no long range diffusion and their corresponding microstructural details were predicted by Bendersky, Roytburd, and Boettinger [J. Res. Natl. Inst. Stand. Technol. 98, 561 (1993)] for Ti-Al-Nb alloys cooled from the high temperature BCC/B2 phase field into close-packed orthorhombic or hexagonal phase fields. These predictions were based on structural and symmetry relations between the known phases. In the present paper experimental TEM results show that two of the predicted transformation paths are indeed followed for different alloy compositions. For Ti-25Al-12.5Nb (at%), the path includes the formation of intermediate hexagonal phases, A3 and DO19, and subsequent formation of a metastable domain structure of the low-temperature O phase. For alloys close to Ti-25Al-25Nb (at%), the path involves an intermediate B19 structure and subsequent formation of a translational domain structure of the O phase. The path selection depends on whether B2 order forms in the high temperature cubic phase prior to transformation to the close-packed structure. The paper also analyzes the formation of a two-phase modulated microstructure during long term annealing at 700 °C. The structure forms by congruent ordering of the DO19 phase to the O phase, and then reprecipitation of the DO19 phase, possibly by a spinodal mechanism. The thermodynamics underlying the path selection and the two-phase formation are also discussed.