Role of zirconium in the phase formation at the interdendritic zone in nickel-based superalloys

A commercial Alloy 80A and a modified alloy with lower titanium and zirconium contents, e.g. 1.4% and <0.01%, respectively, were subjected to unidirectional solidification followed by a rapid quench in water to preserve the structure at elevated temperatures. The slow solidification results in a coarse dendritic structure with a high enrichment of sulphur (up to 2%), aluminium (up to 4%), titanium (up to 13%) and zirconium in the interdendritic zones. In the commercial Alloy 80A, the zirconium and sulphur present in the interdendritic zone result in a eutectic containing ZrS and NiCrTi-matrix. Excess zirconium forms an NiZr-intermetallic. No TiS is formed in this case. In the absence of zirconium, the titanium enriched in the interdendritic residual melt forms a eutectic of TiS. In both cases the effective solidus temperature corresponding to the composition of the residual melt is 1170°C, compared to 1330 °C, which is the solidus temperature of the bulk material.     

Equilibrium segregation coefficient and solid solubility of B in Czochralski Ge crystal growth

This paper reports reevaluated results of the segregation coefficient of boron (B) and a solid solubility of B at melting point of Ge. The crystal growth was carried out from a B-saturated Ge melt, and a solid solubility of B in Ge crystal was estimated to be 3 ± 0.5 × 10¹⁸ cm^− 3. The effective segregation coefficients of 4 and 2.5 were obtained at pulling rates of 10 and 30 mm/h, respectively. An equilibrium segregation coefficient was estimated to be 6.2 and it was considerably smaller than those reported previously. Therefore, boron atoms in Ge melt easily interact with residual O atoms existing in the Ge melt, resulting in various values reported of the segregation coefficient. A phase diagram of Ge and B at Ge-rich region estimated from obtained experimental results is proposed.     

A model for distribution of aluminum in silicon refined by vacuum directional solidification

We propose a simple numerical model for distribution of Aluminum (Al) in silicon ingot during vacuum directional solidification, including segregation from silicon crystal to silicon melt as well as evaporation from silicon melt to vacuum atmosphere. According to the model, the effective segregation K_eff and the total evaporation coefficient k_T(Al)of Al under the experimental conditions are 0.0137 and 2.6755 × 10⁻⁶ m·s⁻¹, respectively. In comparison with experimental results, the segregation of Al is dominated at the beginning of solidification, whereas at the last stage of solidification the removal of Al is mainly depended on the evaporation. It is also found that the critical influences on aluminum removal during vacuum directional solidification are temperature and solidification rate.     

Effects of Re on surface eutectic formation for Ni-base single crystal superalloys during directional solidification

Effects of Re on the formation of surface eutectics have been investigated by using Ni-base single crystal superalloys with different Re additions. It was found that Re promotes the segregation of Al and Ta to the eutectic melt, leading to an increase of the surface and internal eutectics. In addition, the addition of Re also increased the freezing range, the local solidification time, and the permeability of the dendritic network within the mushy zone. These factors ultimately promoted the outflow of the interdendritic residual liquid with the action of solidification shrinkage, and led to the formation of more surface eutectics. In contrast, the addition of Re had no obvious influence on the surface eutectic microstructures.     

Numerical Simulation of Morphology and Microsegregation Evolution during Solidification of Al-Si Alloy

A stochastic model coupled with transient calculations for the distributions of temperature, solute and velocity during the solidification of binary alloy is presented. The model can directly describe the evolution of both morphology and segregation during dendritic crystal growth. The model takes into account the curvature and growth anisotropy of dendritic crystals. Finite difference method is used to explicitly track the sharp solid liquid (S/L) interface on a fixed Cartesian grid. Two-dimensional mesoscopic calculations are performed to simulate the evolution of columnar and equiaxed dendritic morphologies of an AI-7 wt pct Si alloy. The effects of heat transfer coefficient on the evolution of both the dendrite morphology and segregation patterns during the solidification of binary alloys are analyzed. This model is applied to the solidification of small casting. Columnar-to-equiaxed transition is analyzed in detail. The effects of heat transfer coefficient on final casting structures are also studi     

A scanning electron microscopy study of laser coating microstructures

Investigations were made, using scanning electron microscopy, to examine microstructural features of laser coatings. The scanning electron microscope makes it possible to observe clearly the detail of the structure in the laser-irradiated zone. It is shown that the microstructures in the laser-coated zone have various features, including a dendritic eutectic, dendritic solid solution, and residual graphite, and that the microstructures in the transitional zone are more complex, including cellular dendritic-cellular, dendritic eutectic, planar, and residual graphite. The laser coating process changes the nature of solidification of the coating melt, compared with common solidification.     

Two types of boron segregation at austenite grain boundaries and their mutual relation

The distribution of boron in 10 p.p.m. B-doped Fe-30 wt% Ni austenitic alloy was investigated by means of particle tracking autoradiography (PTA). It was shown that when quenched from a certain temperature between 750° C and 1220° C with medium cooling rates, both equilibrium and non-equilibrium grain boundary segregation of boron appeared in the alloy. The degree of segregation represented the sum of the two kinds of segregation of boron. A transition temperature, of approximately 950° C, was found below which the equilibrium grain-boundary segregation was dominant and above which the non-equilibrium grainboundary segregation was dominant. For the degree of segregation achieved by these two types of segregation of boron there exists a temperature from 1000° C to 1050° C at which a minimum amount of segregation is obtained. It was also shown that non-equilibrium segregation of boron can considerably promote the precipitation of boride at grain boundaries.     

Micro-segregation and Precipitation of Alloy 690 during Isothermal Solidification:the Role of Nitrogen Content

The segregation and precipitation behavior of Alloy 690 containing 0.001-0.11 wt% nitrogen during isothermal solidification at 1370 and 1355 ℃ have been investigated using optical microscopy (OM),electron probe microanalysis (EPMA) and transmission electron microscopy (TEM).The results indicate that the volume fraction of TiN-type nitride formed during isothermal solidification increases with the nitrogen content of Alloy 690.Segregation of Ti and Cr exists in samples solidified at 1370 and 1355℃.The Ti content in the residual liquid markedly decreases and the concentration of Cr increases when the nitrogen content of Alloy 690 increases.Furthermore,N and S also show segregation to some extent in the residual liquids at 1355℃.Accompanying by the segregation of Cr,Ti,C,N and S,sulfides and chromium nitrides form.In a low nitrogen content Alloy 690,sulfur segregates and precipitates in the form of Ti 4 C 2 S 2 and (Cr,Ti)S,but in the form of (Cr,Ti)S or CrS in a high nitrogen content Alloy 690.(Cr,Ti)N-type nitrides with an fcc crystal structure have been identified in a sample with 0.11 wt% nitrogen.     

Experimental study of radial segregation caused by weak convection

Under conditions of weak convection in the liquid during metal solidification and crystal growth, large chemical inhomogeneities can occur due to insufficient mixing of the melt. In this project a series of experiments with increasing melt flow, driven by thermo-capillary forces, have been performed in a Get Away Special flight. The level of convection was evaluated quantitatively from analysis of axial segregation profiles. As expected a significantly stronger radial segregation has occurred in the space grown material compared to ground based reference experiments. It was also found that there was a shift of the position of maximum concentration when the flow was increased. Unexpected axial solute maxima were also revealed in the space samples. A comparison was made to numerical simulations, and good correlations were found to first order effects.     

Effect of ultrasonic vibration on the grain refinement and SiC particle distribution in Zn-based composite filler metal

Refining the matrix microstructure of the composite is an effective method to avoid severe reinforcement particle pushing by the advancing solid–liquid interface during solidification. The effect of the ultrasonic vibration, which was injected into the melt at various stages of solidification, on the grain refinement and particle distribution in a Zn-based solidified composite filler metal was investigated. Perfect grain refinement was obtained with the application of continuous ultrasonic vibration. However, severe particle pushing by the sound radiation pressure was observed, resulting in serious particle segregation. Uniform distribution of SiC particles as well as grain refinement was obtained when proper intermittent ultrasonic treatment was applied.     

Numerical simulation of hydrogen evolution upon interaction between zirconium melt and water

The process of hydrogen generation upon interaction between a zirconium melt and water is studied. An oxidation model for zirconium liquid droplets in a water–steam medium was developed. The model was incorporated into the VAPEX computational code developed for calculation of interactions between core material melts and water during a severe accident at nuclear power plants equipped with pressurized water reactors. The modified VAPEX code was used to analyze the ZREX experiments on molten zirconium–water interactions. It was shown that the VAPEX code satisfactorily predicts the amount of the hydrogen generated during such an interaction.     

强制均匀凝固组织精确控制技术

针对合金熔体,尤其是大体积合金熔体凝固组织存在的晶粒粗大、成分偏析、组织不均匀和目前传统电磁搅拌存在的趋肤效应问题,提出了实现大体积合金熔体凝固组织细化、均匀化的新思路和环缝式电磁搅拌制备新方法,并对其技术原理做了深入研究和分析。研究结果表明:与常规电磁搅拌技术相比,环缝式电磁搅拌技术可使合金熔体获得较大的剪切速率,有利于获得更加均匀的温度场和成分场,得到的合金熔体温度梯度小,温度场分布均匀,初生相组织细小、形貌圆整、分布均匀;环缝式电磁搅拌技术连铸过程中的温度梯度减小,温度场更为均匀,可流变铸造出表面质量好,初生相组织细小、分布均匀的半固态坯料。     

Uptake of iron and its effect on grain refinement of pure magnesium by zirconium

Abstract

The uptake of iron by molten magnesium from uncoated new mild steel crucibles at temperatures 680°C, 730°C, and 780°C has been investigated. It was shown that the uptake of iron was sluggish at 680°C and the use of 0.05% zirconium addition could effectively suppress the increase in iron content within the first 2 h of holding at temperature. Rapid and severe uptake of iron was observed at 780°C. As a consequence, it was found that the grain refinement of pure magnesium achieved by 1% zirconium addition nearly vanished after 60 min hold at 780°C due to the depletion of soluble zirconium. The uptake of iron at 730°C was conspicuous but it was still controllable by use of 0.05% zirconium addition within the first 60 min of holding at temperature. The work conducted using an aluminium titanite crucible and a boron nitride coated mild steel crucible at 730°C further confirmed the highly detrimental influence of the uptake of iron on the grain refinement of pure magnesium by zirconium. The characteristic zirconium rich coring structures developed from circular to rosette like when the melt was held at 730°C in an uncoated mild steel crucible, while no such evolution was observed when held in an aluminium titanite crucible at the same temperature. Recommendations to minimise the consumption of zirconium by the uptake of iron were made based on the results obtained from this investigation. The mechanism of grain refinement of magnesium by a low concentration of zirconium is discussed.     

Microstructure of Ni-10Co-8Cr-4W-13Zr alloy and its bonding behaviour for single-crystal nickel-base superalloy

In the later stages of solidification of zirconium-containing superalloys, the concentration of zirconium in the interdendritic melts is above 10 wt%. The dendrites formed in the early stage of solidification may be considered to be joined by the interdendritic zirconium-rich melt. Based on the composition of the zirconium-rich melt, an Ni-10Co-8Cr-4W-13Zr (wt %) alloy was selected as an interlayer alloy for brazing and transient liquid-phase (TLP) bonding of single crystal superalloys. All the elements in the interlayer alloy are beneficial to the single-crystal superalloys. A bonding microstructure which is very similar to that of the base alloy was obtained by means of the TLP process using this interlayer alloy. In the present work, the microstructural characteristics of the interlayer alloy and the phase relationships in the bond during brazing and TLP processes were investigated.     

Crystal growth from melt in space

A review of experiments on crystal growth from melt in nearly zero gravity environments in space is given. The review includes experiments from skylab and ASTP missions. The results discussed are morphological observations, melt interfaee observations, dopant segregation, alloy solidification and containerless experiments.     

Phase formation under non-equilibrium processing conditions: rapid solidification processing and mechanical alloying

Rapid solidification processing (RSP) of metallic alloys, involving solidification of liquid metals at very high rates, results in the formation of a variety of metastable phases such as supersaturated solid solutions, crystalline intermetallic compounds, quasicrystalline phases, and metallic glasses. Additionally, significant refinement of the grain sizes and segregation patterns also occurs. Mechanical alloying (MA), another powerful non-equilibrium processing technique, utilizes repeated cold welding, fracturing, and rewelding of powder particles in a high-energy ball mill. MA also results in the formation of metastable phases and microstructural refinement similar to what happens during RSP. Consequently, comparisons are frequently made between the phases produced by RSP and MA and the general understanding is that they both result in similar metastable effects. A detailed analysis of the metastable phases produced by RSP and MA is made in the present work, and it is shown that even though the effects may appear similar, the mechanisms of formation and the composition ranges in which particular phases form are quite different. These two methods also have some unique features and produce different phases. The differences have been ascribed to the fact that RSP involves solidification from the melt while MA is a completely solid-state process that is not restricted by the phase diagram.     

First-principles Study on the Ductility Effect of Zirconium and Its Distinct Behavior from Boron to Restrain Hydrogen-induced Embrittlement in Ni-Ni3Al Alloys

By studying a cluster model containing Ni region （phase）, NiaAI region （phase） and Ni/Ni3Al region （interface） with a first-principles method, the occupation behavior and the ductility effect of zirconium in a Ni-Ni3Al system were investigated. It is found that zirconium has a stronger segregation tendency to Ni region than to Ni3Al region. The bond order analyses based on Rice-Wang model and the maximum theoretical shear stress model, however, show that zirconium has different degrees of ductility effect in these three regions, which originates from its different ability to increase the Griffith work of interracial cleavage 2γint and to decrease the maximum theoretical shear stress τmax. In addition, it is revealed in this paper that the distinct behavior of zirconium from boron to restrain hydrogen-induced embrittlement can be attributed to their different influences on the crystalline and electronic structures in Ni-Ni3Al alloys.     

Zirconium diboride nanofiber generation via microwave arc heating

Ultrahigh temperature zirconium diboride nanofibers were produced by microwave arc heating using micron-sized raw powder. While microwave heating the ZrB(2) powder, the development of local arcing led to rapid heating and solidification of the samples, along with the creation of nanofibers. The morphology of these high aspect ratio nanofibers was characterized using scanning electron microscopy and transmission electron microscopy. Energy dispersive x-ray spectroscopy, electron energy loss spectroscopy and selected area electron diffraction showed the composition to contain zirconium, boron, nitrogen, aluminum and oxygen as well as the crystallographic orientation. ZrB(2) nanofiber applications include aerospace and other harsh environments.     

Segregation and eutectic formation in solidification of Fe-1C-1 5-Cr steel

Abstract

The microstructure and solute segregation have been investigated in a continuously cast bloom and laboratory cast ingot of Fe–1C–1.5Cr (wt-%) steel. Eutectic carbide formation was observed only in the centreline region in the continuously cast bloom. In both specimens, the maximum chromium level detected was 3% in the columnar and 5% in the equiaxed region, while the minimum remained at 1.2% in both regions. The corresponding segregation ratios (C _max/C _min) were 2.5 and 5, in agreement with many previous studies. By numerical modelling of microsegregation it has been shown that the equilibrium partition coefficient of chromium k _Cr, which changes with carbon content, has a significant effect on chromium distribution during solidification. The carbon distribution may be taken to be in equilibrium during solidification, while that of chromium develops a concentration gradient in the solid. Numerical predictions of segregation behaviour, assuming local equilibrium at the liquid/solid interface, backdiffusion in the solid and complete mixing in the residual liquid, are consistent with experimental results in the columnar and equiaxed regions. The conclusion that eutectic carbide observed in the centreline region must have resulted from macrosegregation is supported by an estimate of the composition of the enriched liquid.     

Wettability of nickel alloys by Ni-Si-B fillers: influence of substrate oxide reduction and isothermal solidification

The process of wetting of nickel and nickel-chromium substrates by nickel-silicon-boron filler metals is examined. In this system, wetting behaviour is dominated by interactions between the filler metal and the oxide layer on the substrate (which is not completely removed during conventional vacuum brazing). Boron emanating from the filler metal reduces this residual oxide. The extent of wettability is controlled by the onset of isothermal solidification which is a consequence of the removal of boron from the filler metal due to the reaction with the substrate oxide layer.