B-rich Layer Research Articles

On the role of Zr and B addition on solidification cracking of IN738LC produced by laser powder bed fusion

The demand for manufacturing increasingly complex geometries for high temperature applications drives the increasing interest into additive manufacturing of nickel-based superalloys. Of particular interest are superalloys with high contents of the strengthening phase γ' such as IN738LC. Previous research suggests that especially B and Zr have a detrimental influence on crack formation during the laser powder bed fusion (LPBF) process. The present study investigates solidification cracks in an IN738LC derivative with increased B (0.03 wt.%) and Zr (0.07 wt.%) in more detail using high resolution techniques such as transmission electron microscopy (TEM) and atom probe tomography (APT). Analysis of the bulk material shows a high number of MC carbides containing Ti and Cr. The concentration profiles indicate non-equilibrium carbide compositions by suggesting that Cr is pushed out of these particles. The carbides are surrounded by a thin B-rich layer at the metal/carbide interface. Analysis of the fracture surface shows both Zr and small amounts of B in the formed oxide layer. The presence of these elements together with thermodynamic calculations and previously reported findings of the same material variant support the hypothesis that low-melting phases are likely reasons for cracking of IN738LC.

The responsive behaviors of bilayer membrane under uniaxial mechanical probe.

In experiments, atomic force microscopy technology was used to measure the modulus of the membrane. However, these studies mainly focus on the linear responsive behavior. In the present work, a theoretical study is performed to show the nonlinear responsive behavior, which includes the stretching induced structural transitions. It demonstrates that the structural transition of the bilayer membrane takes place during the stretching process of the mechanical probe. A vertical cylindrical micelle can be obtained by stretching the membrane under deep compression conditions, and the cylindrical micelle can grow continuously along the axial direction. Moreover, under shallow compression conditions, the probe pulls a spherical micelle from the membrane, and then, the membrane returns to flatness. A comprehensive study is performed to show the mechanism of the responsive behaviors of the structural transition during the compression and stretching processes. When the probe acts on the B-rich layer, it is more likely to pull out a regular micelle. However, when the probe acts on the bottom A-rich layer, complex vesicles are more likely to be pulled out from the bilayer membrane. This study provides a comprehensive diagram of the mechanical responsive behavior of the membrane, which would be a guide for an experiment of biomembranes and the design of new self-assembled structures.

On the Negative Surface Tension of Solutions and on Spontaneous Emulsification.

The condition of negative surface tension of a binary regular solution is discussed in this paper using the recently reconfirmed Butler equation (Langmuir 2015, 31, 5796-5804). It is shown that the surface tension becomes negative only for solutions with strong repulsion between the components. This repulsion for negative surface tension should be so strong that this phenomenon appears only within a miscibility gap, that is, in a two-phase region of macroscopic liquid solutions. Thus, for a macroscopic solution, the negative surface tension is possible only in a nonequilibrium state. However, for a nano-solution, negative surface tension is also possible in equilibrium state. It is also shown that nano- and microemulsions can be thermodynamically stable against both coalescence and phase separation. Further, the thermodynamic theory of emulsion stability is developed for a three-component (A-B-C) system with A-rich droplets dispersed in a C-rich matrix, separated by the segregated B-rich layer (the solubility of B is limited in both A and C while the mutual solubility of A and C is neglected). It is shown that when a critical droplet size is achieved by forced emulsification, it is replaced by spontaneous emulsification and the droplet size is reduced further to its equilibrium value. The existence of maximum temperature of emulsion stability is shown. Using low-energy emulsification below this maximum temperature, spontaneous emulsification can appear, which is enhanced with further decrease of temperature. This finding can be applied to interpret the experimental observations on spontaneous emulsification or for the design of stable micro- and nanoemulsions.

Particles with selective wetting affect spinodal decomposition microstructures.

We have used mesoscale simulations to study the effect of immobile particles on microstructure formation during spinodal decomposition in ternary mixtures such as polymer blends. Specifically, we have explored a regime of interparticle spacings (which are a few times the characteristic spinodal length scale) in which we might expect interesting new effects arising from interactions among wetting, spinodal decomposition and coarsening. In this paper, we report three new effects for systems in which the particle phase has a strong preference for being wetted by one of the components (say, A). In the presence of particles, microstructures are not bicontinuous in a symmetric mixture. An asymmetric mixture, on the other hand, first forms a non-bicontinuous microstructure which then evolves into a bicontinuous one at intermediate times. Moreover, while wetting of the particle phase by the preferred component (A) creates alternating A-rich and B-rich layers around the particles, curvature-driven coarsening leads to shrinking and disappearance of the first A-rich layer, leaving a layer of the non-preferred component in contact with the particle. At late simulation times, domains of the matrix components coarsen following the Lifshitz-Slyozov-Wagner law, R1(t) ∼ t1/3.

Phase separation in antisymmetric films: A molecular dynamics study

We have used molecular dynamics (MD) simulations to study phase-separation kinetics in a binary fluid mixture (AB) confined in an antisymmetric thin film. One surface of the film (located at z = 0) attracts the A-atoms, and the other surface (located at z = D) attracts the B-atoms. We study the kinetic processes which lead to the formation of equilibrium morphologies subsequent to a deep quench below the miscibility gap. In the initial stages, one observes the formation of a layered structure, consisting of an A-rich layer followed by a B-rich layer at z = 0; and an analogous structure at z = D. This multi-layered morphology is time-dependent and propagates into the bulk, though it may break up into a laterally inhomogeneous structure at a later stage. We characterize the evolution morphologies via laterally averaged order parameter profiles; the growth laws for wetting-layer kinetics and layer-wise length scales; and the scaling properties of layer-wise correlation functions.

A method to calculate equilibrium surface phase transition lines in monotectic systems

When a surface active component (B) is being gradually added to a component with a higher surface tension (A) at any constant temperature (being lower than the so-called surface critical point, T cr ∗ ) in any A–B system with a miscibility gap, the first order surface phase transition (SPT) will occur at a certain bulk concentration of component B, within a one-phase, A-rich liquid region of the phase diagram. This SPT is connected with an abrupt change of the surface concentration of component B from a low value to a high value, i.e. with the formation of a thin film of a B-rich layer on the surface of the A-rich bulk liquid alloy. In this paper a general method is developed, based on the Butler equation, to calculate the equilibrium SPT line for any monotectic system, using optimized CALPHAD thermodynamic properties of the liquid alloy and the surface tension and molar volume values of the pure components, as initial data. This new SPT line should be included in all equilibrium phase diagrams with miscibility gaps, for which the T cr ∗ point appears in the A-rich one-phase liquid region of the diagram. The behavior of liquid alloys is expected to be qualitatively different on the two sides of the SPT line. The temperature coefficient of the surface tension is shown to change its sign from negative to positive in the vicinity of the SPT line, which has a serious impact on the Marangoni convection of the alloy. The method is validated on the Ga–Pb system, for which a reasonable agreement is found between the calculated SPT line and the experimentally determined points, as published in 1998 by Chatain, Wynblatt and co-workers. The method of calculation is shown to be applicable for multi-component liquid alloys and solid solutions, as well.

Micro-phase separation of diblock–copolymer melts confined in a slit from simulation calculations—effect of coarse-graining scale

Two methods with different levels of coarse-graining are used to simulate microphase separation in diblock copolymer melts. The first uses Monte Carlo simulation (MC) based on particles and the second uses the cell-dynamical-system method (CDS) based on fluid elements. Results are presented for symmetric and asymmetric A– B diblock-copolymer melts ( f A =0.5, 0.4, 0.3 and 0.2) confined in a slit. Several microdomain morphologies are obtained. A lamella structure with alternate A- and B-rich layers appears for the symmetric case. Mesh-like, bi-continuous cylinder-like and dispersed sphere-like layers appear for asymmetric block copolymers with different compositions. Both methods of calculation give similar morphologies, indicating their mutual consistency. However, the calculation efficiency of CDS is remarkably higher, and it is therefore used to provide the pattern evolutions in time, the order-parameter distribution in real space, and the dynamic scattering function in Fourier space.

Spinodal decomposition in fine grained materials

We have used a phase field model to study spinodal decomposition in polycrystalline materials in which the grain size is of the same order of magnitude as the characteristic decomposition wavelength (Xsu). In the spirit of phase field models, each grain (i) in our model has an order parameter (η i) associated with it;η i has a value of unity inside the ith grain, decreases smoothly through the grain boundary region to zero outside the grain. For a symmetric alloy of composition,c = 0–5, our results show that microstructural evolution depends largely on the difference in the grain boundary energies, ygb, of A-rich (a) and B-rich (β) phases. If Y αgb is lower, we find that the decomposition process is initiated with an a layer being formed at the grain boundary. If the grain size is sufficiently small (about the same as λsd), the interior of the grain is filled with the β phase. If the grain size is large (say, about 10λSD or greater), the early stage microstructure exhibits an A-rich grain boundary layer followed by a B-rich layer; the grain interior exhibits a spinodally decomposed microstructure, evolving slowly. Further, grain growth is suppressed completely during the decomposition process.

Microdomain Morphology of Symmetrical Diblock Copolymer Thin Films Confined in a Slit

The microphase separation and morphology of symmetric diblock copolymer thin films confined in a slit with neutral or attractive surfaces were studied by the cell dynamic system method (CDS), and Monte Carlo simulation. The size effect, especially in CDS, was carefully investigated indicating that excessively small sizes in the X- and Y-directions will give incorrect results although periodic boundary conditions are imposed. When the walls are neutral, parallel ordered lamella structure only exists over a short range, while irregular microdomain morphology occurs over the whole region. When directional quenching is applied, or the walls are attractive to one of the blocks, a periodical lamellar structure of altermating A-rich and B-rich layers occurs over the whole region of the film. Changing the slit width and the strength of interaction will influence the period and arrangement of lamellae. Agreement between the results from CDS and those from simulation is satisfactory indicating the reliability of the CDS method. Comparisons with corresponding experimental results are also dis- cussed.

Properties of cubic boron nitride films with buffer layer control for stress relaxation using ion-beam-assisted deposition

Significant ion irradiation during film growth is required for the formation of cubic boron nitride (cBN) films. Meanwhile, a huge level of intrinsic stress possibly induced by the ion bombardment has been frequently reported to result in cracking and/or lack of adhesion of deposited cBN films. The present work has been performed to investigate the interfacial and/or the buffer layer structures with better matching to the cBN film by relaxation of the film stress using ion-beam-assisted deposition (IBAD). Boron nitride films have been synthesized on Si(100) wafer and tungsten carbide (WC) substrates by depositing boron vapor under simultaneous bombardment with nitrogen ions and nitrogen–argon mixture ions in the energy range of 0.5–10 keV. Cubic BN films with enhanced tribological properties have been explored by inserting a BN layer with various B/N compositions as a controlled buffer at the interface. Significant relaxation of the film stress has been observed for the buffer layer with a boron-rich (B-rich) composition (B/N∼10) with hardness maintained at a relatively high value (∼30 GPa). A structural analysis by Fourier transform infra-red spectroscopy (FTIR) and cross-sectional transmission electron microscopy (TEM) confirms that polycrystalline films with a high cBN fraction were synthesized on the B-rich layer. Formation of the cBN films with a B-rich buffer layer enabled the tribological characterizations to be performed, and the tribological properties have been observed to be significantly enhanced with the insertion of the B-rich buffer and the enhancement of atomic intermixing at the initial stage of cBN layer growth. Using the nanoindentation method, the hardness of the cBN films with a high cubic fraction was found to be as high as the values for the bulk cBN.

Interfacial structure control of cubic boron nitride films prepared by ion-beam assisted deposition

Boron nitride films were prepared by ion beam assisted deposition (IBAD). The films were synthesized by depositing boron vapor under simultaneous bombardment with nitrogen ions and nitrogen-argon mixture ions. Cubic boron nitride (c-BN) films with enhanced tribological properties had been explored by inserting a B-rich layer as a controlled buffer at the interface. Tribological characterizations of the buffer layer and the double-layered BN films consisting of the c-BN layer underneath with the B-rich buffer layer have been performed. Successful growth of c-BN layer has been observed on the B-rich layer and the hardness of the films increased almost linearly with increasing fraction of the sp 3 bonded cubic phase in the c-BN layer. The control of the interfacial structure exhibited a significant effect on the improvement of the tribological properties of the films due to the effective relaxation of internal stress of the c-BN films.

Monte Carlo simulation of polymers at interfaces

Polymers at interfaces pose challenging problems to statistical physics because their configurations often differ greatly from the bulk. Computer simulation of coarse-grained models then gives valuable insight and allows stringent tests of various theoretical predictions. Three examples are briefly treated: chain configurations of B-chains in the surface-enriched B-rich layer of an (AB) binary polymer mixture; “frustrated” lamellar ordering in ultra-thin block-copolymer films; and the collapse of polymer brushes in bad solvents.

Interdiffusion in binary polymer mixtures

The interface between two partially miscible polymer species (A+B) is characterized theoretically under different thermodynamic conditions using the standard model for spinodal decomposition and interdiffusion. Two quantities are introduced to characterize the mass transport process across the interface between the A-rich and B-rich layers. One is the interfacial width W defined as related to the reciprocal of the maximal gradient in composition and is most sensitive to the local structure cline to the A/B interface. The other is mass transport M due to diffusion and is mint insensitive to detailed profiling of the composition

Characterization and growth mechanisms of boron nitride films synthesized by ion-beam-assisted deposition

We have studied boron nitride films deposited at room temperature by ion-beam-assisted deposition in an ultrahigh vacuum apparatus, with ion accelerating voltages ranging between 0.25 and 2 kV. By using complementarily in situ Auger electron spectrometry and ex situ nuclear analyses to determine the respective surface and bulk N concentrations in the deposited films, we were able to identify the different phases of the mechanism leading to the nitridation of evaporated boron by nitrogen ions. For low nitrogen/boron flux ratios, the incorporation of nitrogen seems to be only governed by ion implantation, and, with respect to the depth of the deposit, the surface is found largely depleted in nitrogen, while the N-incorporation yield remains close to one whatever the ion energy. Such a behavior is well verified as long as a critical bulk nitrogen concentration close to 5.5×1022 cm−3 has not been achieved. For concentrations greater than this, superstoichiometric material is obtained up to a saturation which corresponds to a bulk N incorporation ranging from 6 to 7×1022 cm−3. Further increase of the N/B flux ratio induces a strong diffusion process from N-rich bulk to N-depleted surface, which results in the nitridation of surface boron atoms and a loss of nitrogen by sputtering or desorption. The density measurements seem to indicate that the synthesized phase is close to h-BN. However, the density of B-rich layers ([N]/[B]≊0.2–0.3) is found to be very close to that calculated for a mixture of pure boron and c-BN. The transparency and microhardness of the synthesized BN have satisfying values for its application as a wear-resistant optical coating, but it is not c-BN.