Ordering in Ni-Mo alloys—First-principles calculations versus experimental observations

An extensive amount of experimental work has been reported in the literature on the ordering behaviour of Ni-Mo alloys containing 8–33 at% of Mo, which exhibit both short-range and long-range ordering phenomena and a competition among several fcc-based long-range ordered structures. We have used local-density-based tight binding linear muffin-tin orbital (TB-LMTO) method in conjunction with ‘augmented space recursion + orbital peeling’ (ASR + OP) for the determination of ground state energies of these superstructures in terms of effective pair interactions up to the fourth nearest neighbour pairs. The ordering behaviour of the four competing fcc-based superstructures has been studied using the mean-field-based ‘static concentration wave’ (SCW) model in terms of the free energy-order parameter plots (Landau plots) and the free energy-composition plots. The instability domains with respect to concentration fluctuations, both short wavelength (ordering) and long wavelength (clustering) have been identified from these calculations. This information has been used to predict the sequence of transformation events in the Ni-Mo alloys undergoing ordering and/or clustering and the results are compared with those obtained experimentally.     

Characteristic features of silumin alloys crystallization

The results differential thermal analysis (DTA) of crystallization of multicomponent silumin alloys with 8÷22 wt %Si and Mg, Cu, Ni and Fe minor admixtures are presented in the paper. It has been proved that to any one of the solid phases crystallizing from liquid (, β, Mg₂Si, Al₆Cu₃Ni, Al₂Cu, Al_gFe₂Si) a particular thermal effect can be deduced and identified. A new hypothesis concerning hypereutectic silumin alloys crystallization has been proposed in the work.     

Elaboration of platinum-modified NiAl coatings by combustion synthesis: Simultaneous repairing and coating of Ni-based superalloys

The “SHS build-up process” under high gas pressure was previously reported in literature by the authors in order to repair damaged turbine blades. In this process, a brazing foil is inserted between a substrate of Ni-based superalloy and a reactant compact made of the 50 at % Ni-50 at % Al mixture. The heat released during the combustion synthesis of the Ni + Al compact gives rise to interdiffusion of the elements and consequently to joining at the interface. This study reports the effect of Pt addition to the Ni + Al compact on the SHS build-up process. Indeed, the turbine blades need to be coated after repairing to protect them against corrosion. The aim of this investigation was to show the feasibility of this process for simultaneous repairing and coating. Good results were obtained: the joint quality was found to be good and even with a content of 18 at % Pt, the Pt-aluminide coatings retained the β-NiAl structure without PtAl₂ precipitates. Moreover, these coatings are free of porosity and after 100 h of air treatment, a homogenous oxide layer free of porosity was formed.      

Microtomographic characterization of columnar Al–Cu dendrites for fluid flow and flow stress determination

During the twin roll casting of Al alloys, the interdendritic liquid may flow as the two solidification fronts are compressed together between the rolls. This can lead to defects such as centerline segregation. To understand the flow properties of the interdendritic liquid, samples of Al–12 wt.% Cu were solidified directionally in a Bridgman furnace and quenched to capture the growing columnar dendritic structures. The quenched samples were scanned using a laboratory X-ray microtomography (XMT) unit to obtain the 3D structure with a voxel resolution of 7.2 μm. Image analysis was used to separate the Al dendrite from the interdendritic Al–Al₂Cu eutectic. Flow between the dendrites was simulated by solving the Stokes equation to calculate the permeability tensor as a function of the fraction solid. The results were compared to prior experimental measurements and calculations using synchrotron tomography observations of equiaxed structures. Elasto–plastic finite element (FE) simulations were performed on the dendritic structures to determine flow stress behavior as a function of fraction solid. It was found that the standard approximations for the reduction in flow stress in the semi-solid have a variation in excess of 100% from that calculated using the true structure. Therefore, it is critical to simulate the actual dendrite for effective flow stress determination.     

Synthesis and compressive behavior of Cr2GeC up to 48 GPa

M_n+1AX_n compounds have gathered huge momentum because of its exciting properties. In this paper we report the synthesis of ternary layered ceramic Cr₂GeC, a 211 M_n+1AX_n compound by hot-pressing. Scanning electron microscopy and X-ray diffraction have been employed to characterize the new synthesized phase. High-pressure compressibility of Cr₂GeC were measured using diamond anvil cell and synchrotron radiation at room temperature up to 48 GPa. No phase transformation was observed in the experimental pressure range. The bulk modulus of Cr₂GeC calculated using the Birch–Murnaghan equation of state is 169 ± 3 GPa, with K′ = 3.05 ± 0.15.     

Influence of stress condition on adiabatic shear localization of tungsten heavy alloys

Dynamic deformation and failure behavior of a tungsten heavy alloy (93W) under complex stress condition are studied using a split Hopkinson pressure bar (SHPB) apparatus. Cylindrical, step-cylindrical and truncated-conic specimens are used to generate different stress condition in an attempt to induce strain localization in the alloy. The microstructure of the specimens after tests is examined by optical microscopy and scanning electronic microscopy (SEM). It is found that in all the specimens, except the cylindrical ones, intense strain localization in the form of shear bands is initiated at stress concentration sites. In order to analyze the stress condition of different specimen geometry, finite element simulations are also presented. The Johnson-Cook model is employed to simulate the thermo-viscoplastic response of the material. It is found that dynamic deformation and failure modes are strongly dependent on the geometry of the specimens. The stress condition controlled by specimen geometry has significant influence on the tendency for shear band formation. The adiabatic shear band has general trends to initiate and propagate along the direction of maximum shear stress. It is suggested that further studies on the control of the stress condition to promote shear band formation be conducted in order to improve the penetration performance of the tungsten heavy alloy.     

Microstructural evolution during upward and downward transient directional solidification of hypomonotectic and monotectic Al–Bi alloys

Upward directional unsteady-state solidification experiments were performed with both a hypomonotectic Al–2.0 wt% Bi alloy and a monotectic Al–3.2 wt% Bi alloy. Besides, the monotectic composition (3.2 wt% Bi) was directionally solidified under downward transient heat flow conditions, which enables the effects of melt convection on the final microstructure to be evaluated since the collective downward movement of Bi-rich particles is favored in such case. This is due to the density differences between the two coexisting liquid phases. The thermal parameters such as cooling rate, growth rate and thermal gradient were experimentally determined by data collected from cooling curves recorded along the casting length. The monotectic features observed in the Al–3.2 wt% Bi alloy castings, i.e. the interphase spacing and Bi-rich particles diameter were correlated with the growth rate and thermal gradient. The cell spacing was experimentally determined for the Al–2.0 wt% Bi alloy as a function of both the cooling rate and tip growth rate. These experimental data were compared with the main predictive cellular growth models from the literature. A comparison between upward and downward unsteady-state solidification results for the interphase spacing and Bi-rich particles diameter has also been conducted.     

高分子合金反应性共混体系研究近况

综述了反应性增容技术研制高分子合金的最新进展 ,分别讨论了各种反应性增容技术 ,着重阐述了带有活性官能团的反应性聚合物的增容作用 ,包括含酸酐官能团的反应性共混、含羧酸官能团的反应性共混、含羧酸衍生物官能团的反应性共混及含有环氧基团或羧基的反应性共混等