Order-discharge transition in Ni3Fe alloy

The order-disorder phase transition in Ni₃Fe alloy (26.4 at.% Fe) is investigated experimentally, determining the transition temperature, the size of the antiphase domains, the temperature dependence of the degree of long-range order, and the lattice parameter. The differing and common features of the order -disorder transition in the alloys Ni₃Fe and Cu₃Au are discussed, and the temperature dependence of the degree of long-range order in these alloys is compared with theoretical predictions, including results obtained by the Green's function method in the present work.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 32–39, May, 1977.     

Special features of the long-range order – short-range order phase transition in FCC-based alloys

Results of x-ray analysis of the long-range order in FCC-based alloys with L1₂, L1₂(M), L1₂(MM), and D1_a superstructures are generalized. Regularities of the behavior of the long-range order parameter and average size of the thermal and periodic antiphase boundaries (APB) are traced, main mechanisms of temperature order-disorder phase transition are elucidated, and influence of the thermal and periodic APB on the special features of the order-disorder phase transition is revealed. The character of change of the periodic antiphase boundary density determines the special features of the order-disorder phase transition in alloys with longperiod superstructures.     

Neutronographic investigation of the characteristics of phase transition in the alloy Ni3Mn,alloyed with small additions of aluminum and copper

The temperature dependence is studied of the extent of the long-range order and size of the domain in the alloy Ni₃Mn, alloyed with small additions (a few atoms percent) of aluminum and copper. It is shown that A1 stabilizes the superlattice of Ni₃Mn but Cu destroys it. In alloys of Ni₃(Mn, Al) a monotonic effect is observed of the aluminum concentration in the alloy on the nature of the phase transition.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 16–25, November, 1973.     

Order-disorder transitions in alloys near Au3Cu

We applied diffractometry to the order-disorder transition in Cu-72.5 at.% Au alloy with a Ll₂ superlattice; the long-range order parameter and the lattice parameter have been measured in relation to annealing temperature. Disordering occurs through a two-phase Ll₂ + Al region; the upper bound to the phase-transition temperature is 208±3°C. A study has been made of the dimensions of the antiphase domains as defined by superlattice lines.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 70–74, March, 1980.     

Ordering kinetics in nickel alloys with tungsten and molybdenum at t=800°C

The kinetics of the transition of alloys Ni-W and Ni-W-Mo along the slit Ni+20 at. % (W+Mo) from the disordered into the ordered state after deformation and isothermal annealing at t=800°C for 1, 2, 5, 10, 20, 50 and 100 hours is studied in this paper. It is established that ordering occurs in the alloys according to the type Ni₄Mo. The degree of long-range order was estimated according to the degree of tetragonality (c/a) of the elementary cells of Ni₄W and Ni₄(W, Mo) that depended on the annealing time and alloy composition. The presence of a ternary superstructure of Ni₄(W, Mo) of the type Dl_a in ternary Ni-W-Mo alloys with 20 at.% (W+Mo) is confirmed by x-ray diffraction.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 78–82, June, 1990.     

Critical wetting and surface-induced continuous phase transition on Cu3Au(001)

In the bulk Cu₃Au is known to undergo adiscontinuous order-disorder phase transition atT ₀=663 K. Spin polarized low-energy-electron diffraction (SPLEED) reveals acontinuous surface-induced order-disorder phase transition-equivalent to a critical wetting transition—on the surface of Cu₃Au. The results indicate that nearT ₀ the observed singularity is related to the surface order parameter which can be described by a functional dependence of the formt ¹ with ¹=0.77±0.06.     

Kinetics of ordering in Ni1.50Sn (‘Ni3Sn2’) as revealed by the variation of the lattice parameters upon annealing

X-ray powder diffraction was employed to reveal the structural changes occurring upon annealing of quenched, hexagonal Ni_1.50Sn below its equilibrium ordering temperature, leading finally to long-range ordered, orthorhombic Ni_1.50Sn. The changes in the diffraction patterns indicate that the features of the low-temperature phase develop gradually in two main stages. Ex situ and in situ time-resolved X-ray powder diffraction analysis of the first stage was employed to monitor at various temperatures the time dependence of the lattice axial ratio c/a associated with the formation of long-range order in small domains. Data evaluation using an ‘equivalence-time’ method gave a value of 165–170?kJ/mol for the activation energy of the ordering, indicative of lattice-site changes of Ni atoms.     

Peculiarities of structural–phase states in ternary alloys Ni<Subscript>3</Subscript>(Mn,Ti)

The results of investigation of structural and physical properties of ternary alloys Ni₃(Mn, Ti) are presented. A correlation between their physical and structural properties is revealed. The presence of a minimum is established in the curve for the long-range atomic order parameter in the superlattice L1₂ in the concentration dependence plot. It is assumed that an introduction of titanium would give rise to degradation of antiferromagnetic manganese–manganese interaction.     

Specific features of the strain-induced order-disorder phase transition and its mechanisms

In the present paper, the methods of x-ray diffractometry are used to investigate the influence of plastic deformation on the long-range atomic order in alloys with L1₂, L1₂(M), and L1₂(MM) superlattices. The influence of ordering energy, initial values of the long-range order parameter, and average sizes of antiphase domains on the specific features of the dependence of long-range order parameter on the strain degree is considered. It is demonstrated that the wetted strain-induced order-disorder phase transition occurs with rapid accumulation of antiphase boundaries that play an important role in the destruction of long-range atomic order.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 21–32, September, 2004.     

Mechanism of Strain-Induced Breaking the Long-Range Atomic Order in Alloys with the L12 Superlattice Caused by Accumulation of Dislocation Walls

The mechanism of strain-induced breaking the long-range atomic order in alloys with the L1₂ superlattice caused by rearrangement of dislocations in dislocation walls is examined. A mathematical model of strain-induced breaking the long-range atomic order caused by accumulation of dislocation walls is constructed. The dependence of the long-range order parameter on the strain degree is calculated for alloys with high- and low-energy antiphase boundaries.     

Internal energy and parameters of the order-disorder phase transition in titanium monoxide TiO y

Quantum-mechanical ab initio calculations are used to simulate the free energy functions for titanium monoxide TiO _y . The effect of the long-range order of the Ti₅O₅ type superstructure on the internal energy of the compound is studied by the supercell method. The dependences of the configuration entropy and free energy on the long-range order parameter are determined. It is found that the order-disorder phase transition in titanium monoxide must occur in accordance with the mechanism of the first-order phase transition with a critical value of the long-range order parameter of 0.971. The calculated parameters of the phase transition are compared with the experimental data and the results obtained using the model of point charges and by calculating the Madelung energy. It is concluded that the short-range order and the phonon entropy must be taken into account in calculating the equilibrium phase diagrams for strongly nonstoichiometric compounds.     

Special features of the order-disorder phase transformation and the role of associated antiphase boundaries

Experimental investigations of the states of long-range order in alloys with the L1₂, L1₂(M), L1₂(MM), and D1_a superlattices are discussed. The results obtained enable basic mechanisms involved in the thermal order-disorder transformation to be found and demonstrate an important role of antiphase boundaries in this transition.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 48–58, October 2004.     

Properties of ordering alloys ofβ-brass type in a model incorporating correlation and the order dependence of lattice parameters

The statistical theory is applied to ordering in a B2 superlattice (CuZn alloy) with allowance for correlation and the distance dependence of the pair-interaction energy. The theoretical temperature dependence for the long-range-order parameter agrees quite well with that observed, as does the lattice parameter, the configuration specific heat, and the specific-heat discontinuity at the order-disorder transition for CuZn.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 9, pp. 7–11, September, 1971.The authors are indebted to L. E. Popov for useful discussion.     

An anomalous martensitic transformation in Mn85Ni9C6

It is found that the antiferromagnetic alloy Mn₈₅Ni₉C₆ can undergo its cubic-to-tetragonal crystal-structure transformation in two different ways - one corresponding to the familiar process whereby as a function of temperature the a and c dimensions of the crystallographic unit cell deviate continuously from equality, and another in which the a dimension changes continuously through the transition temperature but where the c dimension jumps discontinuously. This second effect has previously been recorded in the literature for the closely related alloy Mn₈₀Ni₂₀, but passed unremarked. Both kinds of transition are of second order. However, the two types of transformation have completely different critical behaviour, the second kind moreover tending to show two temperature ranges with different critical indices. Analysis of the data indicates that the magnetism of the alloy is fundamentally involved.     

The Si(111) 7 × 7 TO “1 × 1” transition

We have made a detailed study of the Si(111) 7 × 7 to “1 × 1” transition at 1140 K using low-energy electron diffraction. Excess diffuse scattering appears during the transition whose temperature dependence and magnitude are consistent with the loss of the 7th order superlattice intensities showing the transition is to a disordered surface phase rather than a true unreconstructed ×1. This strongly favors the class of models for the 7 × 7 selvedge involving an ordered arrangement of subunits like vacancies or adatoms rather than models with periodic distortion. The superlattice intensity shows the transition is continuous and reversible at least for temperatures below 0.995 T_c. The critical exponent for the long-range order is β = 0.11 ± 0.015 similar to those for 2D Ising or Potts models. However, the system does not show the critical scattering properties expected for simple models. The relaxation time seems to diverge near T_c but only for negative temperature increments.     

Order-Disorder Transition in A(B'B')O3 Perovskite Type Ceramics with Second Nearest Neighbour Interactions

The approach based on an order-disorder transition model combined with the coulomb electrostatic interaction method allows investigating of the order-disorder phase transition in A(B′B′)O₃ complex perovskite type ceramics. An analytical analysis is given on how the second nearest neighbour interactions are connected to the quantities characterising the phase transition. While, the criterion proposed by Zhang et al. (Zhang, X.W., Wang, Q. and Gu, B.L. (1991). Study of the order-disorder transition in A(B′B′)O₃ perovskite type ceramics. J Am Ceram Soc., 74(11), 2846-2850.) in order to predict the phase transition and the behaviour of the curve which describes the quantity ?′ F ′₀ T corresponding to order parameter in function of the ionic radius variation and ionic radius ratio are discussed.     

Nucleation of the D1a type ordered phase in the near-surface of A4B alloy: II. Nucleation at surface

Nucleation in the D1_a type A₄B ordering alloy was investigated by means of an atomistic calculation based on a phenomenological nucleation theory and the Bragg-Williams approximation. The nucleation rate at the Ni₄Mo surface is presented as a function of temperature, surface orientation, and type of the plane to which the surface is parallel (fundamental plane or superlattice plane). The radius, degree of order and activation free energy of the critical nucleus are also given as a function of temperature. The value of the nucleation rate at the surface is compared with that in the bulk. The results are as follows: (i) at temperatures near T_c. the nucleation rate is higher at the surfaces than in the bulk and the {200}_FCC surfaces are the highest in nucleation rate; (ii) however, at temperatures near the instability temperature t₀, the nucleation rate is lower at the surface than in the bulk; (iii) the present calculated results give reasonable explanations to our experimental results on the Ni₄ Mo alloy: (a) preferential surface ordering at high temperatures near T_c; (b) surface disordering at intermediate temperatures near the nose temperature.     

Order-disorder transformation mechanism in aluminum-doped Ni3Mn alloy

The kinetics of order-disorder transformation in Ni₃(Mn, Al) alloys prequenched from 700°C has been studied by neutron-diffraction analysis. Within the framework of the I. M. Lifshits model, it is shown that in Ni₃(Mn, Al) alloys containing 2.0 and 4.3 at.% aluminum, ordering proceeds by the homogeneous mechanism at 400° and by nucleation and growth at 450°. The causes are discussed of the sharp speedup of the order-disorder transformation in its first stage in aluminum-doped Ni₃Mn alloy as compared with the binary alloy.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 24–28, April, 1977.     

First-principles calculation of elastic and thermodynamic properties of Ni2MnGa Heusler alloy

The equilibrium lattice parameter, heat capacity, thermal expansion coefficient and bulk modulus of Ni 2 MnGa Heusler alloy are successfully obtained using the first-principles plane-wave pseudopotential (PW-PP) method as well as the quasi-harmonic Debye model. We analyse the relationship between bulk modulus B and temperature T up to 800 K and obtain the relationship between bulk modulus B and pressure at different temperatures. It is found that the bulk modulus B increases monotonically with increasing pressure and decreases with increasing temperature. The pressure dependence of heat capacity C v and thermal expansion α at various temperatures are also analysed. Finally, the Debye temperature of Ni 2 MnGa is determined from the non-equilibrium Gibbs function. Our calculated results are in excellent agreement with the experimental data.     

Temperature dependence of the short-range order parameter and the concentration dependence of the order-disorder temperature for NiPt and NiFe systems in the improved statistical pseudopotential approximation

The calculations for the temperature dependence of the first shell short- range order (SRO) parameter for Ni₃Fe using the cubic approximation of Tahir Kheli, and the concentration dependence of order-disorder temperature T_c for NiFe and NiPt systems using the linear approximation, have been carried out in the framework of pseudopotential theory. It is shown that the cubic approximation yields a good agreement between the theoretical prediction of the α₁ and the experimental data. Results for the concentration dependence of the T_c show that improvements in the statistical pseudopotential approach are essential to achieve a good agreement with experiment.