Volume 101, Issue 11

In this work, multi-phase field and molecular dynamics simulations have been used to investigate nanoscale grain growth mechanisms. Based on experimental observations, the combination of grain boundary expansion and vacancy diffusion has been considered in the multi-phase field model. The atomistic mechanism of boundary movement and the free volume redistribution during the growth process have been investigated using molecular dynamics simulations. According to the multi-phase field results, linear grain growth in nanostructured materials at low temperature can be explained by vacancy diffusion in the stress field around the grain boundaries. Molecular dynamics simulations confirm the observation of linear grain growth for nanometresized grains. The activation energy of grain boundary motion in this regime has been determined to be of the order of onetenth of the self-diffusion activation energy, which is consistent with experimental data. Based on the simulation results, the transition from linear to normal grain growth is discussed in detail and a criterion for this transition is proposed.

The Co–Gd system was re-assessed using the CALPHAD technique. The solution phases (liquid, body-centered cubic, face-centered cubic and hexagonal close-packed) were described by the substitutional solution model. The temperature dependence of the interaction parameters of the liquid phase was separately expressed by the linear function and Kaptay equation. The intermetallic compounds Co 17 Gd 2 and Co 5 Gd, which have the same CaCu 5 -type structure, were treated as one phase and described by a three-sublattice model (Co 2 , Gd)(Co 2 , Gd) 2 Co 15 , with Co 2 and Gd mixing on the first and second sublattices and the third sublattice occupied by Co. The other compounds (Co 7 Gd 2 , Co 3 Gd, Co 2 Gd, Co 3 Gd 4 and CoGd 3 ) were treated as stoichiometric compounds. Two sets of self-consistent thermodynamic parameters of the Co–Gd system were obtained.

The Y 2 O 3 /Ti-5Si alloy composites were prepared by nonconsumable vacuum arc melting through the synthesis reaction from Ti, Si, Y and SiO 2 . The thermodynamics of the insitu synthesis reaction were analyzed. Microstructural analysis showed that the Ti 5 Si 3 and Y 2 O 3 reinforcements were in-situ synthesized in the Ti matrix. Continuous and quasicontinuous network-shape Ti 5 Si 3 + Ti eutectic cells are abundant, and Y 2 O 3 grows from near-equiaxed shape to dendritic shape with increasing Y content in the composite. The introduction of in-situ Y 2 O 3 particles results in a strong interfacial bonding between the reinforcement and matrix, which can significantly improve the wear resistance and frictional stability of Ti–Si alloy, due to the load-bearing function of the Y 2 O 3 particles and the Ti 5 Si 3 phase. Homogeneous distribution of the in-situ reinforcements helps to resist the propagation of subsurface microcracks and then improve the wear properties of in-situ composites.

The thermodynamic parameters of the Nd 2 O 3 –Y 2 O 3 system were re-assessed for better reproduction of experimental data. The thermodynamic parameters were combined from binary descriptions to calculate phase diagrams for the ternary system ZrO 2 –Nd 2 O 3 –Y 2 O 3 . The calculated phase diagrams were used to select compositions for the experimental studies at 1 250, 1400 and 1 600°C. The samples were synthesised by co-precipitation and heat treated at 1 250–1 600 °C, investigated by X-ray diffraction and scanning electron microscopy combined with energy dispersive Xray spectroscopy. It was found that solubility of the Y 2 O 3 in the pyrochlore phase exceeds 10 mol.%. The experimental data obtained for phase equilibra were used to derive thermodynamic parameters for fluorite, Y 2 O 3 cubic phase C, monoclinic B and Nd 2 O 3 hexagonal A phases by CALPHAD method. The isothermal sections and liquidus surface were calculated for the ZrO 2 –Nd 2 O 3 –Y 2 O 3 system.

Molecular dynamics simulations, along with the modified analytic embedded atom method, have been employed to study the bcc → fcc phase transition of nanocrystalline iron. The Gibbs free energies of bulk fcc and bcc iron phases are calculated as a function of temperature, and used to determine the bulk phase-transition temperature. Furthermore, the transformation temperature in the nanocrystalline iron, with a mean grain size of 3 nm, is determined to be 975 ± 25 K using the bond-order parameter method. The radial-distribution function and common neighbor analysis are used to understand the phase structure of the nanocrystalline iron and the evolution of local atomic structure. The snapshots of a two atomic layer thick slice provide a visible scenario of structural evolution during phase transition.

A thermodynamic assessment of the Cr–Al–Nb system was performed using the CALPHAD technique on the basis of new experimental data on three isothermal sections at 1 150, 1300 and 1 450 °C as well as the available earlier literature data. The ternary phase diagram is dominated by an extended phase field of the Laves phase Nb(Cr, Al) 2 . Both the cubic C15 and the hexagonal C14 structure type of Laves phases occur as stable phase in the ternary system. A two-sublattice model was used for their thermodynamic description. The ternary solubilities of the (Nb) and (Cr) solid solutions and the binary phases NbAl 3 , Nb 2 Al and Nb 3 Al were taken into account in the ternary assessment. Several isothermal sections were calculated using the evaluated parameters. Good agreement between the calculations and the experimental data is achieved.

The phase equilibria of the Cu–Mn–Zn system at 550 °C were investigated using 4 diffusion couples and 25 equilibrated alloys. The preliminary phase relations resulting from diffusion couples were used to select the compositions of the equilibrated alloys. The samples were examined by means of X-ray diffraction, optical microscopy, scanning electron microscopy with energy dispersive X-ray spectrometry and electron probe microanalysis. The experimental results show no existence of any ternary compound at 550 °C. The phase separation of the β(CuZn)-phase (bcc_A2 structure based on approximate Cu 49 Zn 51 , in wt.%) into β and β 2 was observed in the diffusion couple of Cu 73 Mn 27 (in wt.%)/Zn and in the ternary alloy of Cu 42.4 Mn 8.8 Zn 48.5 (in wt.%). There is a debate on such a phase separation in the literature. An order/disorder (A2/B2) transition was inferred to exist according to the present experimental result. All the experimental phase diagram data available from the literature were critically reviewed. A set of self-consistent thermodynamic parameters for the Gibbs energies of individual phases in the Cu–Mn–Zn system was then obtained by using the CALPHAD approach taking into account the reliable data from the literature and from the present work. Comprehensive comparisons between the calculated and measured phase diagrams showed that most of the experimental information is satisfactorily accounted for by the present thermodynamic modeling.

The lattice constants and elastic constants for Al–Mg–Si alloys have been calculated by using first-principles total energy calculations within the generalized gradient approximation. The calculated results are in good agreement with available experimental and theoretical results. The polycrystalline shear modulus, Young's modulus and Poisson's ratio are also estimated from the calculated single crystalline elastic constants. The Young's modulus and shear modulus increase following the precipitation sequence in Al–Mg–Si. The Debye sound velocity, Debye temperature, Grüneisen constant, heat capacity and linear coefficients of thermal expansion are predicted for the considered Al–Mg–Si alloys based on the Debye–Grüneisen model. The calculated values of Mg 2 Si agree well with the previous experimental and theoretical results.

Recent developments in modeling the interaction of thermodynamics and kinetics during microsegregation formation in multicomponent alloys are reported. A model accounting for solid state back diffusion, dendrite arm coarsening, dendrite tip undercooling, eutectic undercooling and thermodynamic correction of the interface concentrations is presented. By coupling the solution of the diffusion equation in the liquid and solid phases with the thermochemistry library ChemApp, the phase equilibria at the solid–liquid interface are calculated at each time step. Using an activity based method for identifying multiphase equilibria and metastable extensions of the stability range of phases, a thermodynamically consistent implementation of eutectic undercooling in multicomponent systems is achieved. The model is equipped with a userfriendly interface that allows setting the model parameters interactively and that presents the results (solute distribution, phase fractions, secondary arm spacing) in a pertinent way.

Dense SiC with addition of CeO 2 was obtained by liquid phase sintering using different additives (Y 2 O 3 , Al 2 O 3 ) and (Y 2 O 3 , AlN). The total amounts of additives (Al 2 O 3 + Y 2 O 3 ) and (AlN + Y 2 O 3 ) were fixed at 10 mass.% and 10 vol.%, respectively. Two different molar ratios of Al 2 O 3 :Y 2 O 3 additives, 1:1 and 5:3, were selected for investigation. Ratios of AlN:Y 2 O 3 were selected as 3:2 and 4:1. Influences of both different ratios of Al 2 O 3 :Y 2 O 3 and AlN:Y 2 O 3 and amounts of CeO 2 on sintering behaviour were investigated. The phase reaction products were identified by X-ray diffraction and microstructures were investigated using scanning electron microscopy with energy dispersive X-ray spectroscopy techniques. In the samples using Al 2 O 3 + Y 2 O 3 as sintering additives, the CeO 2 was dissolved in Y 4 Al 2 O 9 phase during sintering. The oxidation state of Ce 4+ changed to Ce 3+ and Ce 3+ occupies Y 3+ positions, causing increases in lattice parameters for Y 4 Al 2 O 9 with CeO 2 content. In samples with AlN + Y 2 O 3 as sintering additives, Y 10 Al 2 Si 3 O 18 and Y 2 Si 3 N 4 O 3 were formed in the CeO 2 -free and CeO 2 containing compositions. During sintering, the CeO 2 dissolves in oxynitrides via a similar mechanism as in the Y 4 Al 2 O 9 phase.

In order to study the phase diagram of the ternary Fe–Ti–V system, the isothermal sections at 1 000 °C and 1 200 °C have been determined. The nature and composition of the phases involved in the different equilibria were investigated by means of X-ray powder diffraction and electron probe microanalysis. More than 15 at.% Fe could be substituted in the body centered cubic Ti–V solid solution at both temperatures. The C14, Fe 2 Ti-derived phase extends to the composition Ti 33 V 33 Fe 33 . A linear regression between the lattice parameter and the composition of the studied body centered cubic phase is proposed. The data provided by the isothermal sections, together with the differential thermal analysis measurements, highlighted the presence of a quasi-peritectic reaction U, and a tentative liquidus projection is presented, based on a metallographic analysis.

The thermodynamic optimization of the Ce–La and Ce–La–Mg systems has been carried out using the CALPHAD technique. The solution phases (liquid, bcc, fcc, hcp and dhcp) were described with a substitutional solution model. The isostructural compounds MgCe in the Ce–Mg system and MgLa in the La–Mg system with B2 structure form a continuous range of solid solution in the Ce–La–Mg system. The order–disorder transition between the solutions with A2 structure and compounds with B2 structure in the system has been taken into account and thermodynamically modeled. The other isostructural compounds Mg 12 Ce and Mg 12 La, Mg 17 Ce 2 and Mg 17 La 2 , Mg 41 Ce 5 and Mg 41 La 5 , Mg 3 Ce and Mg 3 La, and Mg 2 Ce and Mg 2 La were described as such formula Mg 12 (Ce, La), Mg 17 (Ce, La) 2 , Mg 41 (Ce, La) 5 , Mg 3 (Ce, La, Mg), Mg 2 (Ce, La), respectively. A set of self-consistent thermodynamic description of the Ce–La–Mg system was obtained.

The results of the molar volume calculation of two galliumbased ternary systems, Ga–Bi–Sn and Ga–Bi–In, are presented in this paper. The calculation has been done according to the general solution model, using known experimental data for constitutive binary systems at a temperature of 873 K, for the considered ternary alloys in the sections with molar ratio of Bi:X (X = Sn, In) = 1: 1; 1: 3; 3: 1. The calculated values for the molar volume of liquid alloys have been compared with available experimental data and good agreement was found.

Brazing of copper to copper using phosphor–copper brazing filler metal has been carried out using vacuum brazing technology. The wettability, melting performance, microstructure and phase constitution in the joint were studied by means of differential thermal analysis, metallography, energy dispersive X-ray detector and X-ray diffraction. Experimental results obtained showed that the structure in the joint consisted of hypoeutectic alloy. The primary phase is copper solid solution (α(Cu)) and silver solid solution (α(Ag)). The eutectic phase is copper solid solution (α(Cu)), silver solid solution (α(Ag)) and Cu 3 P. The optimum process parameters are: brazing temperature T = 820–850°C, vacuum level p = 10 −3 Pa, holding time t = 5–10 min.

A study of the internal strain (stress) evolution during uni-axial cyclic deformation along the prior extrusion axis with fully reversed total constant strain amplitudes ε A (0.5% < ε A < 5%) was investigated by using in-situ high energy synchrotron X-ray diffraction. The deformation is dominated by {10 1 2} 〈10 1 1〉 twinning and detwinning mechanisms within the textured hot extruded magnesium alloy AZ31. The results show a strong load partitioning between the internal stresses of (10 1 0) and (11 2 0) parent grains and of the (0002) twinned daughter grains that are relaxed. Following the evolution of the twinning/detwinning behavior and the hkil dependent microstresses as a function of cycling numbers, two different regimes were observed whereby it was found that the transition between these regimes is marked by an applied strain amplitude of eA = 0.625%.