Effect Of Microsegregation Research Articles

Solidification and Room Temperature Microstructure of a Fully Pearlitic Compacted Graphite Cast Iron

Compacted graphite cast irons are rapidly developing for they have better mechanical properties than lamellar graphite cast irons and present less porosity than spheroidal graphite cast irons. For many applications, an as-cast fully pearlitic matrix would be desired which can hardly be achieved when graphite is compacted. Addition of manganese, copper and tin are thus made as these elements are known to be pearlite promoters. However, their amount should be limited so as to avoid detrimental effects amongst which are heterogeneities in the matrix properties which impede easy machining. In the present work, a compacted graphite cast iron containing 0.3 wt% Mn, 0.8 wt% Cu and 0.1 wt% Sn was cast in sand mould and in standard thermal analysis cup. The cup sample showed a nearly fully pearlitic matrix and was selected for further study. The characterization consisted of measuring and correlating the distributions of pearlite interlamellar spacings and microhardness values. An attempt was made to look for the effect of solidification microsegregation on microhardness which did not reveal any trend.

Study of austempering kinetics of high silicon bainitic cast steels

ABSTRACTThis study investigates the effect of chemical composition and microsegregation on the bainitic transformation of high Si cast steels at two different austempering temperatures. The advance of the bainite precipitation as a function of time and temperature was monitored through measurements of the amount of retained austenite by X-ray diffraction, the evolution of hardness and microstructural characterization. The results showed that the bainitic transformation starts preferentially at zones with lower contents of alloying elements but later covers the entire matrix. The austempering heat treatments resulted in an apparently homogeneous bainitic microstructure throughout the entire matrix after a short period of time. The results encourage the continuation of the development of high Si steels suitable to produce carbide-free austempered cast parts.

Characterization of microstructure and mechanical properties of nickel based superalloy 617 by pulsed current gas tungsten arc welding technique

Nickel based superalloy (alloy 617) exhibits high strength and oxidation resistance at elevated temperatures. The major limitation of this alloy is microsegregation of alloying elements due to the formation of Molybdenum (Mo) rich secondary phases in the interdendritic region of the weldments. The present study investigates the possibilities to mitigate the microsegregation effect. In order to achieve this, weld joints were fabricated with pulsed current gas tungsten arc welding (PCGTAW) techniques with three different filler wires. The filler wires employed with present study are ERNiCrMo-4, ERNiCrMo-10 and ERNiCrMo-14. The macro examination was carried out to reveal the defects in the weld joints. Optical and Scanning electron microscope (SEM) analysis were performed to evaluate the micro structural changes in the fusion zone and Heat Affected Zone (HAZ). Microstructure shows the fine equiaxed dendrite structure in the fusion zone. Energy Dispersive x-ray Spectroscopy (EDS) analysis was also carried out in the weldments to quantify the level of microsegregation of alloying elements. Tensile test reveals the strength and ductility of the weld joints. Root and face bend test was carried out to check the ability of the weld joints to withstand bending forces. The result shows the defect free weld was achieved in all the welded joints. EDS analysis revealed the presence of Mo rich phases in the ERNiCrMo-4 filler wire. There is no evidence for the formation of secondary phases in the filler wires. The tensile strength of ERNiCrMo-14 filler shows the marginally higher strength compare to other filler wires. Root and Face bend test did not show the presence of cracks and other defects in the weldments.

Effect of Micro-Segregation on Impact Toughness of 2.25Cr-1Mo Steel after Post Weld Heat Treatment

2.25Cr-1Mo steel with high strength at high temperatures and superior hydrogen resistance is widely used as power generation boiler material in high-temperature and high-pressure environments. Following the test evaluation of the ASME Boiler and Pressure Vessel Code, specimens from the base metal of a boiler pipe were found to have impact toughness values of 386 and 28J, which are drastically different values. The analysis of the fracture surface of the 28J test specimen revealed MnS inclusions and it was found that cracks were initiated at the inclusions. Observation of the cross-section of the crack propagation front revealed that cracks propagated along the ferrite regions and precipitate voids. Inclusions were also found in the 386J impact specimen. However, the volume fraction of the inclusions was significantly less than that of the 28J specimen. It was also found that the ferrite and carbide content of the 386J specimen was less than the 28J specimen. The reason that the inclusions, ferrite, and carbide content differed in the two adjacent impact test specimens was analyzed. The effects of micro-segregation such as MnS inclusions on ferrite and carbide were compared and analyzed.

Microstructure control and creep behavior of Ti-47Al-6Nb-0.1C alloy by directional solidification

Newly designed Ti-47Al-6Nb-0.1C at.% alloy was directionally solidified by cold crucible under 45 kW at the growth rate of 5–16.67 μm/s. The macro/microstructure evolution, room-temperature tensile property and creep behavior are investigated. The results show that this alloy was well directionally solidified by cold crucible at the growth rate of 5–11.67 μm/s, and they display primary β solidification at this growth-rate range. It is also found that the lamellar spacing is mainly controlled by growth rate, and the lamellar spacing decreases with the increasing of growth rate according to relationship d=1751V−0.58. Moreover, the extent of microsegregation increases with the increasing of growth rate, and thus the effect of microsegregation on creep behavior is discussed. After controlling directional solidification process at growth rate of 5–8.33 μm/s, this alloy exhibits good creep properties with creep life more than 800 h at 800 °C/200 MPa, and the acceptable room-temperature tensile property with yield strength 436–452 MPa and elongation 1.62–1.68%.

Differential Scanning Calorimetry (DSC) and Thermodynamic Prediction of Liquid Fraction vs Temperature for Two High-Performance Alloys for Semi-Solid Processing (Al-Si-Cu-Mg (319s) and Al-Cu-Ag (201))

There is a need to extend the application of semi-solid processing (SSP) to higher performance alloys such as 319s (Al-Si-Cu-Mg) and 201 (Al-Cu-Ag). The melting of these two alloys was investigated using differential scanning calorimetry (DSC) and thermodynamic prediction. The alloys had been processed by magneto-hydrodynamic (MHD) stirring before receipt to produce a microstructure suitable for SSP. The DSC results for the as-received MHD material were compared with those for material which has been taken through a complete DSC cycle and then reheated for a second DSC run. The effects of microsegregation were then analyzed. A higher liquid fraction for a particular temperature is found in the second DSC run than the first. Microstructural observations suggest this is because the intermetallics which form during the first cooling cycle tend to co-located. Quaternary and ternary reactions then occur during the second DSC heat and the co-location leads to enhanced peaks. The calculated liquid fraction is lower with 10 K/min DSC heating rate comparing with 3 K/min at a given temperature. The DSC scan rate must therefore be carefully considered if it is to be used to identify temperature parameters or the suitability of alloys for SSP. In addition, the starting material for DSC must represent the starting material for the SSP. With thermodynamic prediction, the equilibrium condition will provide better guidance for the thixoforming of MHD stirred starting material than the Scheil condition. The Scheil mode approximates more closely with a strongly microsegregated state.

Synthesis and mesomorphic properties of liquid crystals containing a perfluorinated segment via different linkers

This study presents the synthesis and mesomorphic properties of a series of biphenyl benzoate liquid crystals carrying a perfluorinated segment, via three uncommon flexible linkers. In particular, a (E)-2-propenyloxy linker was used which was issued from an elegant regioselective dehydrohalogenation reaction. The remarkable microsegregation effect of the perfluorinated segment, led to the formation of highly stable untilted smectic mesophases. For all mesophases, the molecular packing corresponds to a monolayer arrangement of the mesogenic parts and a partial intercalation of the perfluorinated chains, measured by a intercalation ratio parameter τF. Geometrical calculations based on Xrays data led to τF comprised between 1.48 and 1.68 depending of the mesophase and temperature.

Application of Thermo-Calc TCFE7 to High-Alloyed Mottled Cast Iron

The thermodynamic modeling of alloy systems consisting of stable and metastable phases e.g. high-alloyed mottled cast iron can be problematic. Thermodynamic databases are rather well-developed for low, medium and high alloyed steels (e.g. HSS) but the application of those databases is not yet very common for high-alloyed (mottled) cast irons. The Thermo-Calc software together with the TCFE7 database is used to calculate isopleth and property diagrams, using the CALPHAD method. Additionally Scheil-Gulliver calculations are performed to simulate the effects of microsegregation during solidification. The results from the thermodynamic calculations are compared with measurements on own samples and with literature values. Those measurements include quantitative light-optical analysis, SEM with BSE detector, EDX measurements for the distribution of the alloying elements as well as XRD and DSC measurements. The investigations show the possibilities which are offered by thermodynamic calculations for high-alloyed mottled cast iron as well as the limitations and the compromises which have to be taken into account when calculating stable and metastable phases existing next to each other.

Phase Field Modeling of Microstructure Banding in Steels

A phase field model (PFM) is applied to simulate the effects of microsegregation, cooling rate, and austenite grain size on banding in a C-Mn steel. The PFM simulations are compared with experimental observations of continuous cooling transformation tests in the investigated steel. Using electron probe microanalysis, the microsegregation characteristics of Mn were determined and introduced into the model. Ferrite nucleation is assumed to occur at austenite grain boundaries, and ferrite growth is simulated as mixed-mode reaction for para-equilibrium conditions. The driving pressure for the austenite to ferrite transformation depends on Mn concentration and thus varies between the alternating microsegregation layers. In agreement with experimental observations, the simulation results demonstrate that by increasing the cooling rate and/or austenite grain size, banding tends to disappear as the transformation shifts to lower temperatures such that ferrite also forms readily in the layers with higher Mn levels. Further, a parametric study is conducted by changing thickness and Mn content of the bands. In accordance with experimental observations, it is shown that for sufficiently large band thickness, band splitting takes place where ferrite grains form close to the center of the Mn-rich band. Changing the degree of Mn segregation indicates that a segregation level of 0.2 wt pct is necessary in the present case to achieve banded microstructures.

On the Thermodynamics and Kinetics of TCP Phase Precipitation in Re‐ and Ru‐ Containing Ni‐base Superalloys

AbstractThree experimental Ni‐base superalloys of the second, third and fourth generation are studied in this work to clarify the effect of Re and Ru on the precipitation of TCP phases. Thermodynamic calculations using the CALPHAD method are performed based on the local chemical composition obtained with the use of electron probe microanalysis. The effects of microsegregation and γ′‐precipitation are taken into account. The results, complemented with scanning electron microscopy observations, show that Re increases, while Ru decreases the inclination of the material to precipitate TCP phases. The effect of both elements is assigned to thermodynamic rather than kinetic reasons. The CALPHAD‐calculations do predict the effect of Re on the TCP phase formation in the investigated alloys correctly, but not the effect of Ru.

Interplay between columnar and smectic stability in suspensions of polydisperse colloidal platelets

The phase behavior of a model suspension of colloidal polydisperse platelets is studied using density-functional theory. Platelets are modelled as parallel rectangular prisms of square section $l$ and height $h$, with length and height distributions given by different polydispersities $\delta_l$ and $\delta_h$. We obtain the phase behavior of the model, including nematic, smectic and columnar phases and its dependence with the two polydispersities $\delta_l$ and $\delta_h$. When $\delta_l>\delta_h$ we observe that the smectic phase stabilises first with respect to the columnar. If $\delta_h>\delta_l$ we observe the opposite behavior. Other more complicated cases occur, e.g. the smectic stabilises from the nematic first but then exists a first-order transition to the columnar phase. Our model assumes plate-rod symmetry, but the regions of stability of smectic and columnar phases are non-symmetric in the $\delta_l-\delta_h$ plane due to the different dimensionality of ordering in the two phases. Microsegregation effects, i.e. different spatial distribution for different sizes within the periodic cell, take place in both phases.

Microsegregation effects on the thermal conductivity of silicon-germanium alloys

A silicon-germanium (SiGe) alloy is a promising candidate for thermoelectric materials; while it shows a significantly reduced thermal conductivity (κ) as compared to pure Si and Ge, the κ values obtained from previous experiments and computations tend to be widely scattered. We present here a computational analysis of thermal transport in SiGe, particularly the effects of the local segregation (microsegregation) of alloying elements. Our nonequilibrium molecular dynamics simulations confirm the strong dependence of κ on the Si:Ge ratio and the occurrence of the minimum κ around Si0.8Ge0.2, consistent with existing experimental observations. Moreover, our study clearly demonstrates that the κ of Si0.8Ge0.2 increases substantially and monotonically as Ge atoms undergo segregation; that is, the magnitude of alloy scattering is found to be sensitive to homogeneity in the distribution of alloying elements. Nonequilibrium Green's function analysis also shows that such microsegregation enhances phonon transmiss...

The Distribution and Transport of Alpha Activity in Tin

The presence of alpha emitters in packaging materials poses an increased risk to device reliability as device size decreases and flip chip and 3D packaging technology becomes more widespread. Particularly troubling is the observation that alpha emission increases over time in some reported instances. Several experiments were conducted to examine alpha flux behavior in tin materials and determine the cause for the temporal alpha emissivity increase. The experiments determined the distribution of the alpha emitters within the material volume, and a mechanism based on microsegregation is presented to explain the nonuniform distribution. Data describing differential isotope distribution within tin are also presented, providing confirmation of 210Po transport within a solid tin matrix. The effects of micro-segregation and 210Po migration on alpha emissivity over time are discussed. Potential impacts from these mechanisms on alpha emissivity in packaging applications are presented.

Integration of Alkyl-Substituted Bipyridyl Benzenedithiolato Platinum(II) Complexes with Cadmium(II) Ion via Selective Dative Bond Formation

The presence of lone pairs on the Pt and S atoms of [Pt(Bdt)(DTBbpy)] (1) (Bdt = 1,2-benzenedithiolato and DTBbpy = 4,4'-di-tert-butyl-2,2'-bipyridine) and [Pt(Bdt)(C13bpy)] (2) (C13bpy = 4,4'-ditridecyl-2,2'-bipyridine) led to selective dative bond formation with Cd(II). Complexes 1 and 2 show no binding interaction with Zn(II), while they bind selectively with Cd(II) to give a twisted trinuclear complex, [Cd{Pt(Bdt)(DTBbpy)}2(ClO4)(H2O)](ClO4) (3), and a shuttlecock-shaped tetranuclear complex, [Cd{Pt(Bdt)(C13bpy)}3(H2O)](ClO4)2·CH2Cl2 (4), respectively, depending upon the alkyl groups substituted on the 2,2'-bipyridine. The two platinum moieties in 3 are connected to the seven-coordinated Cd atom through Pt → Cd (2.7331(7) and 2.7936(7) Å) and S → Cd (2.690(3), 2.940(3), and 3.067(3) Å) dative bonds, while the three moieties in 4 are connected to the tetrahedral Cd atom only by S → Cd (2.552(4) Å) dative bonds. These structural variations found in 3 and 4 are caused not only by steric hindrance of the t-Bu groups but also by the microsegregation effect derived from the tridecyl chains. The three platinum moieties in 4 align so as to form a parallel orientation of their dipole moments, in contrast to the twisted arrangement found in 3. The dative bonds formed in 3 and 4 are commonly stable in the solid state and in less coordinative solvents such as dichloromethane, while dissociation behavior of platinum moieties with Cd(II) was observed in more coordinative THF. UV-vis and NMR spectroscopy unsealed the characteristic association/dissociation properties depending on the coordination abilities of solvents. Finally, the present study revealed that the formation of dative bonds between the platinum moieties with Cd(II) plays important roles not only in stabilizing the ground states, which leads to blue shifts in both absorption and emission energies, but also in electronic interactions between the moieties, which are revealed by electrochemical studies.

INFLUENCE OF SEGREGATION ON LIQUID DENSITY IN THE MUSHY ZONE OF DZ483 Ni--BASED SUPERALLOY

Ni-based superalloys have been widely applied in advanced aeroengine as gas turbine blades and vanes. The freckles in superalloys formed during directional solidification have deleterious influence on the properties of the alloys. The generation of freckles is associated with the local liquid density gradient in the mushy zone, which is obviously influenced by microsegregation of alloy elements. However, the individual contributions of the various elements to the total density variation are still not well known. Therefore, the effect of microsegregation on the liquid density variation in DZ483 Ni-based superalloy was investigated by isothermal solidification together with liquid quench method. Solidification microstructures were observed by optical microscope and SEM, and the compositions of the solids and the residual liquid were determined by EDS. Based on the compositions of residual liquids, the densities of liquids at different temperatures were calculated. The results show that the onset solidification temperature of DZ483 alloy is a little bit below 1335 ,a ndM C, which is enriched with Ta and Ti, formed at about 1325 . The segregation coefficients of different elements show that W and Co are negative segregation elements, Ta and Ti positive segregation elements, while Al and Cr show little segregation. The density of the residual liquid generally decreases as the decrease of temperature, with the exception that it increases somewhat from 1325 to 1315 . Calculation results show that temperature has insignificant influence on liquid density, and variation of density is mainly due to microsegregation. Segregations of Mo and Ta lead to the increase of density, but segregations of Ti and W present opposite effect. Contribution of each element to the variation of the liquid density

The 2013 Joseph W. Richards Summer Research Fellowship -- Summary Report: Microsegregation Effects on the Thermal Conductivity of Silicon-Germanium Alloys

Due to increased need for renewable energy sources in recent years, a significant number of both experimental and theoretical efforts have been undertaken to find effective ways to enhance the performance of thermoelectric (TE) energy conversion. Since the 1960s, SiGe alloys have received much attention as one of the promising candidates for TE materials, due to their low thermal conductivity (κ), as compared to pure Si and Ge. Earlier studies1,2 demonstrated that the low κ is mainly attributed to phonon scattering as a result of the mass difference between Si and Ge atoms (the so-called alloy scattering). While the strength of alloy scattering is a strong function of the Si/Ge ratio, previous experiments3 also showed evidence that Si and Ge atoms often remain locally segregated in bulk SiGe samples prepared by mechanical alloying. However, no research has been reported regarding the microsegregation effect on κ. As a part of the ECS summer project, we performed a computational analysis to explore how the local atomic arrangement affects thermal transport in bulk SiGe; some results of this work are presented herein. To investigate the microsegregation effect, we prepared several Si0.8Ge0.2 samples by embedding spherical Ge particles of different sizes (ranging from 5 to 293 atoms) in the Si matrix. As illustrated in Fig. 1, embedded Ge particles were randomly positioned but not allowed to overlap each other. A nonequilibrium MD (NEMD) method4 with the Stillinger-Weber (SW) potential model5 was used to calculate the κ of SiGe alloys at 300 K, while the SW parameters were modified using the first-principles-based force-matching method.6 For each of the Si1-xGex systems considered, five independent NEMD simulations were performed with different atomic arrangements and initial velocity distributions. All NEMD simulations were performed using LAMMPS (Large-Scale Atomic and Molecular Massively Parallel Simulator)7 with a time step of 1 fs; a detailed description of the simulation steps can be found in Ref. 8 and 9. Figure 2 shows the variation of κ for the Si0.8Ge0.2 samples as a function of Ge particle diameter (De); here, De is approximated by (6NGeVGe/π) 1/3, where NGe is the number of Ge atoms in the particle and VGe is the volume per atom for Ge (= 0.0238 nm3 from our DFT-GGA calculation). The κ is predicted to monotonically increase with Fig 1. The various Si0.8Ge0.2 configurations show a random distribution of Si and Ge atoms (random) and embedded Ge particles with different diameters (De = 0.91, 1.58, and 2.37 nm) in the Si matrix. Green (black) balls and yellow lattices represent Ge and Si atoms, respectively.

Studies on pitting corrosion and sensitization in laser rapid manufactured specimens of type 316L stainless steel

The pitting corrosion and sensitization in laser rapid manufactured specimens of type 316L stainless steel are studied and compared with its wrought and weld deposit (by plasma transferred arc welding-PTAW) counterparts. With respect to wrought specimens, the laser rapid manufactured specimens exhibited lower pitting corrosion resistance as well as higher degree of sensitization. However, pitting corrosion resistance of laser rapid manufactured specimens is largely similar to that of PTAW-deposited specimens. Relatively lower pitting corrosion resistance of laser rapid manufactured and PTAW-deposited specimens are attributed to their cast microstructure and associated micro-segregation effects as expected. Repeated exposure to sensitization temperature range (773–1073K) during multiple layer deposition coupled with slower rate of cooling during later part of laser rapid manufacturing resulted in sensitization, giving rise to continuous corrosion attack along the grain boundaries. Solution annealing is found to be helpful in healing the associated chromium depletion and decreasing the degree of sensitization with modest improvement in pitting resistance. Primary mode of solidification in laser rapid manufactured specimens of type 316L stainless steel influenced potential sites for initiation of pitting corrosion. In primary ferrite solidified regions, initiation of pitting corrosion occurred preferentially at δ-ferrite/austenite boundaries whereas core of austenite cells/dendrites and grain boundaries were the preferred sites for initiation of pitting corrosion in primary austenite solidified regions. These studies provide insight into the basic corrosion mechanisms of laser rapid manufactured specimens of type 316L stainless steel.

The Distribution and Transport of Alpha Activity in Tin

The presence of alpha emitters in packaging materials pose an increased risk to device reliability as device size decreases and flip chip and 3-D packaging technology becomes more widespread. Particularly troubling is the observation that alpha emission increases over time in some reported instances. Several experiments were conducted to examine alpha flux behavior in tin materials and determine the cause for the temporal alpha emissivity increase. The experiments determined the distribution of the alpha emitters within the material volume, and a mechanism based on microsegregation is presented to explain the non uniform distribution. Data describing differential isotope distribution within tin are also presented, providing confirmation of 210Po transport within a solid tin matrix. The effects of microsegregation and 210Po migration on alpha emissivity over time will be discussed. Potential impacts from these mechanisms on alpha emissivity in packaging applications will also be presented.

Effect of Microsegregation on Porosity Formation in High Cr Steel under Pressurized Atmosphere

The effect of microsegregation on porosity formation in high nitrogen steels containing high chromium under pressurized atmosphere was investigated using pressurized directional solidification furnace. In this study, it was successful to add over 0.70 mass% nitrogen in the ingot with no porosity. Nitrogen distributions in unidirectional solidification ingots were almost homogeneous regardless of the presence of porosities. In the case of low nitrogen content and peritectic solidification, porosities were observed inside the ingot solidified under nitrogen–argon gas mixture in the total pressure below certain critical value. In the case of high nitrogen content and austenitic solidification, porosity was not observed inside the ingot solidified under nitrogen gas condition. Liquidus and solidus temperature, equilibrium partition coefficient, solid fraction and temperature at peritectic transition, the relationship between solid fraction and temperature during solidification were calculated with Thermo-calc. The microsegregation tendency of nitrogen and alloying elements was simulated using Scheil equation and the modified method of Ohnaka's microsegregation model in this study. As a result of solidification simulation, in high chromium steel, it is suggested that supersaturated solution of nitrogen caused by bubble formation during solidification is likely to occur near the peritectic transition. Defined critical pressure of porosity formation in this study would be effective to determine the pressure conditions to manufacture high nitrogen steel.

Kinetics of rafting in a single crystal superalloy: effects of residual microsegregation

The kinetics of directional γ′ coarsening, known as rafting, are examined in the nickel based single crystal superalloy CMSX-4. Electron microscopy and image analysis are used to characterise heat treated specimens, prestrained in creep to a plastic strain beyond the threshold required to initiate rafting. The kinetics of rafting are found to be sensitive to the presence of microsegregation from casting, incompletely removed by the heat treatment process; thus the dendritic regions raft appreciably faster than the interdendritic ones – this despite rhenium enrichment at the dendrite cores consistent with its preferred partitioning during solidification. Estimates of the activation energy required for rafting in the two regions are established and Avrami-type equations developed. Since the important γ′ forming elements aluminium and tantalum are known to partition interdendritically causing an enhanced γ′ fraction, it is suggested that the correspondingly smaller γ channel width so caused is responsible for the greater resistance to rafting displayed by the interdendritic regions.