Effect of interfacial thickness and stiffness on the stress distributions in fibre reinforced cementitious composites

The different microstructure of the fibre–cement interface might result in different failure mechanisms. It is expected that improvement of strength and toughness in fibre-reinforced cementitious composites will depend on their interfacial thickness and stiffness. A three-phase model, subject to a transversely uniform tensile stress, was utilized to investigate the effect of interfacial thickness and stiffness on the stress distributions near the fibre–cement interface and the corresponding failure mechanism. The results suggest that optimum interfacial microstructure of fibre-reinforced cementitious composites can be tailored to obtain a higher strength and toughness. Optimum interfacial thickness and stiffness was evaluated for various reinforcements, including steel, carbon, glass and polypropylene fibres. This revised version was published online in November 2006 with corrections to the Cover Date.     

Investigations on transient directional solidification under microgravity on sounding rocket missions

Transient growth conditions are common to a variety of technical solidification processes and lead to modified materials properties. In directional solidification the microstructure at the solid-liquid interface of an alloy is a result of the interaction of diffusive and convective heat and mass transport in the bulk and of interface and thermophysical properties. We have carried out experiments under diffusive conditions without convection in microgravity during the sounding rocket missions TEXUS-36 and 40. The used transparent alloy succinonitrile-acetone freezes like metals and the solidification process was observed in-situ. Within a gradient furnace the solid-liquid interface is forced to move accelerated and to transform from planar into cellular and dendritic structures. The dynamics of the planar interface and of the spacing and the amplitude of diffusive grown cells and dendrites were observed directly with cameras and analyzed. A comparison of the TEXUS-40 results to predictions taken from a macroscopic thermal model, a coupled heat-mass transfer model and a phase-field model was carried out. A good agreement is found for the planar interface dynamics for the coupled heat-mass transfer model and the phase-field model, when using additional information from the thermal modelling. In the cellular and dendritic growth regime typical microstructure features can be reproduced by the phase-field model. The experimental results thus serve as important bench-marks for the validation of numerical models describing time-dependent solidification processes.     

In-situ observation of solid-liquid interface transition during directional solidification of Al-Zn alloy via X-ray imaging

The morphological instability of solid/liquid(S/L) interface during solidification will result in different patterns of microstructure. In this study, two dimension(2 D) and three dimension(3 D) in-situ observation of solid/liquid interfacial morphology transition in Al-Zn alloy during directional solidification were performed via X-ray imaging. Under a condition of increasing temperature gradient(G), the interface transition from dendritic pattern to cellular pattern, and then to planar growth with perturbation was captured. The effect of solidification parameter(the ratio of temperature gradient and growth velocity(v), G/v) on morphological instabilities was investigated and the experimental results were compared to classical "constitutional supercooling" theory. The results indicate that 2 D and 3 D evolution process of S/L interface morphology under the same thermal condition are different. It seems that the S/L interface in 2 D observation is easier to achieve planar growth than that in 3 D, implying higher S/L interface stability in 2 D thin plate samples. This can be explained as the restricted liquid flow under 2 D solidification which is beneficial to S/L interface stability. The in-situ observation in present study can provide coherent dataset for microstructural formation investigation and related model validation during solidification.     

The solidification process of Al–Mg–Si alloys

The microstructure and solidification process of three Al–Mg–Si alloys with different magnesium contents have been studied using optical microscopy and the electron probe X-ray microanalysis. The results showed that Al–Mg–Si alloys possessed fairly complicated solidification path: L→α-Al+L1→α-Al+Al15Si2(FeMn)3+L2→α-Al+Al15Si2(FeMn)3+ (α-Al+Mg2Si)+L3→α-Al+Al15Si2(FeMn)3+(α-Al+Mg2Si)+(α-Al+Mg2Si+Al15Si2(FeMn)3), and wide solidification temperature of 75 °C. The magnesium content in the alloys greatly influenced the as-cast microstructure. The higher the magnesium content, the more Mg2Si structure was present. Iron and manganese segregated to the finally solidified zone, which resulted in the formation of ternary eutectic structure. Although their content in the alloys was very low, their effect on solidification behaviour cannot be ignored. This revised version was published online in November 2006 with corrections to the Cover Date.     

FIB and TEM studies of interface structure in diamond–SiC composites

The microstructure of diamond–SiC interfaces was studied by transmission electron microscopy (TEM). Specimens were prepared by focused ion beam (FIB) etching from a diamond–SiC composite bulk material. The diamond–SiC interfaces were easily located by high contrast in FIB images of the bulk surface, and site-specific specimen preparation was possible. The possible origin of this high contrast in FIB images compared to SEM images is discussed. TEM images and electron diffraction patterns showed that the diamond and SiC crystals away from the interface region are relatively defect-free, but numerous defects are present at the diamond–SiC interface over a dimension of 600 nm, much larger than the physical interface.     

Surface energy controlled α–γ–α transformation texture and microstructure character study in ULC steels alloyed with Mn and Al

In this article, an ultralow-carbon steel grade alloyed with Mn and Al has been investigated during α–γ–α transformation annealing in vacuum. Typical texture and microstructure has evolved as a monolayer of grains on the outer surface of transformation-annealed sheets. This monolayer consists of <100>//ND and <110>//ND fibre, which is very different from the bulk texture components. The selective driving force is believed to reside in the anisotropy of surface energy at the metal–vapour interface. The grain morphology is very different from the bulk grains. Moreover, 30–40% of the grain boundary interfaces observed in the RD–TD surface sections are tilt incoherent <110> 70.5° boundaries, which are known to exhibit reduced interface energy. Hence, the conclusion can be drawn that the orientation selection of surface grains is strongly controlled by minimization of the interface energy; both metal/vapour and metal/metal interfaces play a roll in this.     

Properties and microstructure of Sn–0.7Cu–0.05Ni solder bearing rare earth element Pr

Effects of trace amount of rare earth element Pr on properties and microstructure of Sn–0.7Cu–0.05Ni solder were investigated in this paper. The solderability of Sn–Cu–Ni–xPr alloy and shear strengh of Sn–Cu–Ni–xPr soldered micro-joints were determined by means of the wetting balance method and shear test, respectively. Moreover, microstructure of solder alloys bearing Pr, as well as intermetallic compound (IMC) layer formed at solder/Cu interface after soldering were observed. It was concluded that the major benefits of rare earth element Pr on Sn–Cu–Ni lead-free solder are: improving solderability, refining microstructure, and depressing IMC (IMC) growth, which exhibited improved mechanical properties. It also revealed that (Cu,Ni)₆Sn₅ is the majority IMC phase at the interface of Sn–Cu–Ni–xPr/Cu solder joints. Ni added into the solder effectively suppressed the growth of Cu₃Sn and consequently also the total IMC layer thickness. Above all, the thickness and morphology of the interfacial (Cu,Ni)₆Sn₅ IMC were optimized due to alloying Pr. It can be inferred that Pr and Ni would play an important role in improving the reliability of Sn–Cu–Ni lead-free solder joints.     

The Dependence of the Melting Temperature of Cobalt–Carbon Eutectic on the Morphology of its Microstructure

Fixed points provide a reliable way to realize and verify temperature scales. High-temperature fixed points are being developed based upon alloys, since single-phase materials are impractical to use above the copper freezing point. In particular, eutectic alloys have been shown to be sufficiently reproducible to warrant consideration as a way to significantly improve high-temperature metrology. However, eutectic alloys have certain characteristics requiring that they are used differently from the current ITS-90 fixed points. As their freezing temperature depends on the freezing rate, the melting temperature is preferred, though it has been shown that for some alloys, notably iron–carbon and cobalt–carbon, the apparent melting temperature can depend on the rate of the preceding freeze. This behavior will need to be explained and quantified if such fixed points are to be acceptable. The effect of varying the freezing rate on subsequent melting has been investigated for cobalt–carbon eutectic fixed points. The apparent melting temperature varies by up to 50 mK. Measurements were made of two different fixed-point blackbodies with very similar results. Optical microscopy of samples produced at different freeze rates shows a change in scale of the microstructure. Electron back-scatter diffraction (EBSD) shows evidence of high levels of residual strain in rapidly frozen samples. The effect of annealing on the melting behavior and microstructure has also been investigated. It is suggested that disordered phase boundaries and residual strain lead to changes in the melting behavior as nonequilibrium conditions may lead to a significant level of pre-melting. Whether this actually changes the liquidus temperature, or whether the melting temperature variation is due to the way the melting point is defined, is also discussed. The variation requires consideration if the highest accuracy is to be achieved, and will be a contributory factor to any uncertainty budget.     

Numerical model for nucleation of peritectic alloy during unidirectional solidification

Based on transient nucleation theory, a numerical model has been constructed to describe the nucleation process of a new phase in front of the liquid–solid interface of a prior steady-growth phase in peritectic alloy with the combination of the concentration field calculated by a self-consistent numerical model for cellular/dendritic growth. The results show that the nucleation incubation time of a new phase varies with the solidification rate during unidirectional solidification. During unidirectional solidification of the Zn–4.0 wt.% Cu alloy, the incubation time changes very slightly when the solidification rate increases from 50 to 500 μm/s, but it increases significantly when the solidification rate exceeds 500 μm/s. The calculated results show a reasonable agreement with the experimental ones. This model reveals that nucleation of a new phase is time-dependent and reasonably explains the effect of the solidification velocity on the behaviors of nucleation and growth of ɛ dendrites in the matrix of the η phase in unidirectional solidification of Zn rich Zn–Cu alloys.     

Microstructure and texture assessment of Al–Mn–Fe–Si (3003) aluminum alloy produced by continuous and semicontinuous casting processes

Aluminum sheets are currently produced by the direct-chill process (DC). The need for low-cost aluminum sheets is a challenge for the development of new materials produced by the twin roll caster (TRC) process. It is expected that sheets produced from these different casting procedures will differ in their microstructure. These differences in microstructure and in the crystallographic texture have great impact on sheet mechanical properties and formability. The present study investigated microstructure and evaluated texture of two strips of Al–Mn-Fe–Si (3003) aluminum alloy produced by TRC and by hot-rolling processes. It was possible to notice that the microstructure, morphology, and grain size of the TRC sample were more homogenous than those found in hot-rolled samples. Both strips, obtained by the two processes, showed strong texture gradient across the thickness.     

Ultrasonic Non-destructive Evaluation of Titanium Diffusion Bonds

Diffusion bonds offer several advantages over alternative welding methods, including the ability to produce near-net shapes and achieve almost parent metal strength. However, voids remnant from the joining process can be tens of microns in their lateral dimension, making them difficult to detect with conventional pulse-echo immersion inspection at any significant metal depth. In titanium the inspection is particularly challenging; the anisotropic microstructure is highly scattering and the diffusion bond itself forms an interface between regions of preferred crystallographic orientation (macrozones), which can act as a weak spatially coherent reflector. A simple interfacial spring model predicts that, for partial bonds (sub-wavelength voids distributed on the bond line) and at certain frequencies, the phase of the signal can be used to separate the component of the signal due to the change in texture at the interface and the component due to the flaw. Here it is shown that the phase of the signal from an interface is also affected by the anisotropic microtexture of Ti–6Al–4V. Good separation between well-bonded and partially bonded samples was achieved using a symmetric inspection, where the magnitude and phase of the reflection coefficient were calculated for normal incidence from opposite sides of the diffusion bond.     

Static and dynamic properties of the water/amorphous silica interface: a model for the undissociated surface

Amorphous silica–water interfaces are found ubiquitously in nanoscale devices, including devices fabricated from silica as well as from silicon that acquire a surface oxide layer. The surface silanol groups serve as hydrogen-bonding sites for a variety of chemical species, and their reactivity enables convenient chemical modification, making silica surfaces strategic in bio-sensing applications. We have extended the popular BKS and SPC/E models for bulk silica and water to describe the hydrated, hydroxylated amorphous silica surface. The parameters of our model were determined using ab initio quantum chemical studies on small fragments. Our model will be useful in empirical potential studies, and as a starting point for ab initio molecular dynamics calculations. At this stage, we present a model for the undissociated surface. Our calculated value for the heat of immersion, 0.6Jm⁻², falls within the range of reported experimental values of 0.2–0.8Jm⁻². We also study the perturbation of water properties near the silica–water interface. The disordered surface is characterized by regions that are hydrophilic and hydrophobic, depending on the statistical variations in silanol group density. We report non-equilibrium molecular dynamics simulations of Poiseuille flow of water near an amorphous silica surface.     

Electron microscopy study of chemically deposited Ni-P films

The structure of electroless thin films of Ni-P has been studied. The microstructure and the selected area diffraction pattern of the samples reveal that certain samples transform to crystalline Ni with P in solid solution by nucleation and growth, whereas others transform to crystalline state by growth alone. The former set of thin films having a P-content of 19–21 at.% is characterized as amorphous. Films with a P-content of 13–15 at.% fall in the latter category and are characterized as microcrystalline. Those with a P-content of 16–18 at.% contain both amorphous and microcrystalline regions.     

Microstructure and bonding properties of diffusion–bonded composite comprising an Fe–Al alloy and carbon steel

This study discusses microstructure evolution, diffusion behavior and bonding strength of a couple comprising of an iron aluminium alloy (Fe–Al) and high carbon-steel (FeCMn) during diffusion bonding. A columnar microstructure evolves from the joint interface toward FeCMn and disappears in couples bonded for a long period. Aluminium diffusion from Fe–Al to FeCMn and columnar microstructure evolution are retarded as compared to a couple consisting of an Fe–Al and ferrite steel. The carbide in the FeCMn impedes the aluminium diffusion and retards the columnar grain evolution. When the carbide is dissolved in the ferrite during the aluminium diffsuion from Fe–Al, coarse grains evolve due to the coalescence of the columnar grains and a high-bonding strength is obtained. The hardness variation is minimum in the FeCMn of a couple bonded for a short period, which is explained by the microstructural changes in the columnar grain evolution and carbide dissociation.     

Numerical analysis of constitutional supercooling during directional solidification of alloys

Some limitations of Tiller’s morphological stability criterion are discussed in the present study. This criterion assumes a purely diffusive regime in the melt as well as a planar solid–liquid interface and a constant solidification rate. But experimental works in agreement with previous numerical modeling have shown a significant decrease of the growth rate and a variable interface curvature during the concentrated semiconductor alloys solidification. The mathematical expression of the morphological stability criterion was derived by using Tiller’s equation, predicting the solute distribution in the liquid. The numerical computations performed in this study show a significant disagreement between the numerical results and Tiller’s formula. Numerical modeling conducted in conditions when the supercooling should occur, show that the Tiller’s stability criterion cannot predict the moment of interface destabilization. The interface destabilization is numerically observed when some fluctuations appear in the liquid solutal profiles and cause the appearance of a supercooled zone inside the liquid at small distance from the interface. The present numerical results are not in contradiction with the basic elements of the classical constitutional supercooling theory, providing only that the stability criterion cannot predict the moment of the interface destabilization.     

Volume change during the solidification of SG iron: comparison between experimental results and simulation

In order to understand the shrinkage behaviour of spheroidal graphite (SG) iron during solidification, a volume change kinetic model was set up to simulate the volume change during the eutectic solidification, which was presented in an earlier paper. Furthermore in the present work experiments were carried out for comparison with theoretical prediction. The microstructure of the mushy zone during the solidification of SG cast iron was obtained by the quenching method and analysed by normal metallography and image analysis. The results show that the mushy zone exists in front of the interface between the liquid and the solid. The study by quantitative stereology shows that the graphite fraction in the mushy zone has the same trend as that of the theoretical prediction and the silicon content in cast iron strongly influences graphitization during the solidification. A heat-transfer model to stimulate the heat transfer of the experimental apparatus was developed. A modified Rappaz’s model was used to simulate the eutectic growth under fully equilibrium conditions. The theoretical prediction has been compared with the experimental results, and found to be in good agreement with each other. This revised version was published online in November 2006 with corrections to the Cover Date.     

Control of solidification structure of wear-resistant austenite-bainite polyphase steel with nodular eutectic

A new austenite-bainite polyphase steel with nodular carbides can be obtained by controlling the solidification structure of the steel melt, which only contains manganese and silicon, with modification of Si-Ca-B compound and air-hardening. The result indicates that the nodular carbide is in the eutectic form of austenite and (Fe, Mn)3C, which is formed between the austenitic dendrites during solidification due to element segregation. The modifying elements (calcium, silicon, etc.) have the following functions: (1) their chemical compounds (CaS, SiO2) are formed preferentially during solidification to act as heterogeneous nuclei for nodular eutectic crystallization, (2) the eutectic can be turned into the nodular shape after modification because of the decrease in the amount of the adsorbed impurity elements (oxygen and sulphur) and silicon enriched on the eutectic growth interface. The quantity of nodular eutectic makes up 10%–20%, with a size of 15–25 μm. The hardness and the toughness of this steel are 40–50 HRC and 20–40 J, respectively, and hence its wear-resistance can be more greatly increased than that of the austenite-manganese steel and the austenite-bainite steel. This revised version was published online in November 2006 with corrections to the Cover Date.     

The interphase and interface microstructure and chemistry of isothermal/isobaric chemical vapour infiltration SiC/BN/SiC composites: TEM and electron energy loss studies

The boron nitride interphase and its interfaces in two-dimensional-SiC/BN/SiC composites have been analysed by transmission electron microscopy and electron energy loss spectroscopy. BN was deposited by isothermal/isobaric chemical vapour infiltration from BCl₃–NH₃–H₂ mixture at moderate temperature. BN and the fibre/BN interface exhibit different features depending on the nature of the Nicalon^TM fibre surface, raw or treated prior to the BN deposition. When untreated fibres are used, a carbon-rich layer and silica clusters are formed during the manufacturing of the composite. In that case, the interphase is poorly organized and presents a porous microstructure and a large carbon content. With the treated Nicalon^TM fibre, no formation of a new interlayer is observed at the fibre–BN interface and the interphase exhibits a better organized turbostratic microstructure with no voids. Additionally, in both types of composites, a carbon-rich layer is formed at the BN–matrix interface during the SiC infiltration step at about 1000 °C. This revised version was published online in November 2006 with corrections to the Cover Date.     

The Convective Instabilities in a Liquid–Vapor System with a Non-equilibrium Evaporation Interface

Rayleigh–Marangoni–Bénard instability in a system consisting of a horizontal liquid layer and its own vapor has been investigated. The two layers are separated by a deformable evaporation interface. A linear stability analysis is carried out to study the convective instability during evaporation. In previous works, the interface is assumed to be under equilibrium state. In contrast with previous works, we give up the equilibrium assumption and use Hertz–Knudsen’s relation to describe the phase change under non-equilibrium state. The influence of Marangoni effect, gravitational effect, degree of non-equilibrium and the dynamics of the vapor on the instability are discussed.     

The relationship between fracture toughness and microstructure of fully lamellar TiAl alloy

Fully lamellar (FL) Ti–46.5Al–2Cr–1.5Nb–1V (at%) alloy is used to study the relationship between microstructure and fracture toughness. A heat treatment process is adopted to control the microstructural parameters of the studied alloy. Fracture toughness experiments and scanning electron microscope (SEM) in-situ straining experiments are carried out to determine the influence of lamellar spacing and grain size on the fracture toughness of FL TiAl alloys. It is found that ligament length depends on the lamellar spacing, and fracture toughness varies non-monotonously with the increase of grain size. The results are ascribed to the competition between the microcrack nucleation and microcrack propagation. Finally a semi-empirical relationship between the fracture toughness and microstructure parameters was established.