Directional Solidification Samples Research Articles

Microstructure and Properties of Ni70.2Si29.8 Eutectic Alloy Produced by Directional Solidification

The directional solidification of Ni70.2Si29.8 eutectic alloy was carried out, and the solidification microstructure and compression properties of Ni2Si – Ni31Si12 eutectic composite were systematically studied. Directional solidification samples were obtained by induction heating and down – draw process. During directional solidification, well-aligned lamellar structure with a micron scale was obtained in directional solidified Ni70.2Si29.8 eutectic alloy. Despite it is a kind of faceted – faceted eutectic alloys, highly regular and homogeneous structure was obtained through directional solidification. Compared with other methods, the compress test results showed that fracture strength of Ni70.2Si29.8 eutectic alloy obtained through directional solidification was improved obviously, reaching 2100 MPa.

Tailored microstructure creation of TiAl-based refractory alloys within VGF solidification

The work is aimed at the study of the formation and refinement of primary microstructure appearing in the refractory lightweight structural TiAl-based intermetallics during their solidification from the melt. The alloy with Ti–46Al–8Nb (at.%) nominal composition is selected for applied study in microstructure engineering. For tailored microstructure development, the Directional Solidification (DS) of pre-synthesized alloy was performed in the vertical multizone electro-furnace by Vertical Gradient Freezing (VGF) power-down technique in pure argon environment. Both columnar-dendrite, and equiaxed-granular reproducible as-cast microstructures have been produced in DS ingots, basing onto Columnar-to-Equiaxed Transition (CET) diagram exploration. Particular attention was paid further to equiaxed microstructure improvement by combination of modifying doping of alloy with boron grain refiner and VGF processing. As a result the perfect inoculated microstructure of Ti–44Al–7Nb–2B (at.%) ingots was produced with 120 μm mean grain diameter, low scattering of dimensional characteristics and high tolerance to DS process parameters variation. DS samples were examined by SEM microanalysis along with EBSD and Auger spectrometry of boride particles in the alloy matrix. The nature, state and exact composition of precipitated borides are discussed along with the nucleation mechanism in relation to the literature data.

The Effects of the Withdrawal Rate and Heat Treatment on the Microstructure of Directionally Solidified Nb-14Si-24Ti Alloy

AbstractIn this work, an Nb-14Si-24Ti alloy (at.%) was prepared by liquid-metal-cooled directional solidification (DS) and then heat treated (HT) at 1450°C for 100 h. The effects of the withdrawal rate and heat treatment on the microstructure were investigated. The results show that the DS samples consisted of NbSS dendrites, Nb3Si blocks and eutectic NbSS/Nb3Si. As the withdrawal rate increased, the microstructure became finer, the volume fraction of eutectic NbSS/Nb3Si increased and the morphology of eutectic cell transformed from petaloid to regularly arranged granular morphology. The microstructure of the HT samples consisted of NbSS and Nb3Si. The metastable phase of Nb3Si did not discompose through the eutectoid reaction. The dendritic NbSSconnected through diffraction and the secondary dendrite arm got coarser. The boundary of eutectic NbSS/Nb3Si became blurry and the black Ti-rich phase of the DS samples in this region disappeared. The Nb3Si connected to form the continuous matrix. The granular NbSS of eutectics spheroidized and the lamellar NbSS of eutectic interconnected and became more bending distributed uniformly in the Nb3Si matrix.

The microstructure evolution of directionally solidified Nb-22Ti-14Si-4Cr-2Al-2Hf alloy during heat treatment

To obtain a homogenous and optimizing microstructure, non-equilibrium directionally solidified (DS) samples at high withdrawal rate were heat treated (HT). The heat treatments were carried out at 1500 °C for 3–100 h. The DS and HT samples consisted of NbSS, α-Nb5Si3 and γ-Nb5Si3, and the DS morphology altered significantly after heat treatment. The well developed NbSS dendrites were compromised and the size decreased from more than 100 μm to less than 60 μm when the heat treated time was 6 h. On the other hand, the NbSS in eutectic interconnected and meanwhile connected with the dendrites, and the frequency of small size got lower with the heat treated time increasing. When heat treated for 100 h, the size of NbSS was comparatively uniform and concentrate in the region of 5–20 μm with a value of 63% for Ostwald ripening. The dendric and eutectic morphologies exited in the DS sample disappeared, turning into a net-work structure. The components of primary Nbss dendrites and Nbss in the intercellular regions were homogenized after heat treated for 3 h. However the enrichment of Ti, Cr and Hf in γ-Nb5Si3 compared to α-Nb5Si3 could not be eliminated by homogenizing treatment even for 100 h.

On the state of boride precipitates in grain refined TiAl-based alloys with high Nb content

Abstract The work is aimed at the refinement of primary microstructure appearing in the refractory lightweight structural TiAl-based intermetallics during their solidification from the melt. The green alloy with Ti–46Al–8Nb (at.%) nominal composition is selected for applied study in microstructure engineering. For grain refining, the metallurgical alloying of the material was performed with microcrystalline titanium diboride TiB2 additive, along with subsequent Directional Solidification (DS) in the vertical multizone electro-furnace by power-down technique in pure argon environment. As a result the uniform equiaxed microstructure of Ti–44Al–7Nb–2B (at.%) ingots was produced with low dimensional scattering and characteristic grain diameter of 110–130 μm. DS samples were examined by SEM microanalysis along with EBSD and Auger spectrometry of boride particles in the alloy matrix. It was stated that initial diboride TiB2 additive was completely dissolved and has been afterwards re-precipitated in the course of DS into orthorhombic (Ti,Nb)B monoboride single-crystalline micro-particles of B27 structure, acting as randomly distributed point seeds for inoculated solid grains nucleation. The nature, exact composition and nucleation potency of newly formed borides are discussed along with the nucleation mechanism in relation to the literature data. The addition of trace amount (below 0.1 at.%) of beryllium solute is proposed for the enhancement of β-grain refinement at solidification stage.

On the Role of Copper and Cooling Rates on the Microstructure, Defect Formations and Mechanical Properties of Al-Si-Mg Alloys

This paper aims to assess the role of Cu on Al-Si-Mg alloys, in a range of 0 - 5 wt%, qualitatively on microstructure, defect formation, in terms of porosity, and strength in the as-cast conditions. The ternary system of Al-Si-Mg, using the A356 alloy as a base material, were cast using the gradient solidification technique; applying three different solidification rates to produce directional solidified samples with a variety of microstructure coarsenesses. Microstructural observations reveal that as the Cu levels in the alloys are increased, the amounts of intermetallic compounds as well as the Cu concentration in the α-Al matrix are increased. Furthermore, the level of porosity is unaffected and the tensile strength is improved at the expense of ductility.

Effect of withdrawal rate on the microstructure of directional solidified Ni-43Ti-4Al-2Nb-2Hf single crystal alloys

Single crystals of Ni-43Ti-4Al-2Nb-2Hf (atomic fraction, %) alloy were directionally solidified (DS) using liquid Single crystals of Ni-43Ti-4Al-2Nb-2Hf (atomic fraction, %) alloy were directionally solidified (DS) using liquid metal cooling method by the seeding technique. The preferred crystallographic orientations of single crystals were determined by Electron Backscattered Diffraction (EBSD). The effect of withdrawal rates (5×10-6 m•s-1,1×10-5 m•s-1 and 1.5×10-5 m•s-1) on microstructures was also investigated. The results show that the primary dendritic spacing and the morphology of Ti2Ni phase are markedly influenced by withdrawal rates at the heating temperature of 1600°C even also the solidification pathway of the alloy is changed. As except, the primary dendritic spacing decreases with the increasing withdrawal rate. Moreover, the Ti2Ni phases distribute in interdendritic region in DS samples withdrawn at 5×10-6 m•s-1 and 1×10-5 m•s-1, whereas the Ti2Ni phase solidifies as the primary phase when the withdrawal rate increases to 1.5×10-5 m•s-1.

Effect of casting temperature on microstructure in a directionally solidified Ni-44Ti-5Al-2Nb-1Mo alloy

Ni-44Ti-5Al-2Nb-1Mo (atomic fraction, %) alloy was directional solidified (DS) at different temperature (1350, 1450, 1550, 1650 °C) and a constant withdrawing rate of 100 μm/s, by liquid metal cooling method. The microstructural characteristics indicate that the DS temperature do not influence the phase composition, but rather to change the phase morphology. The phase composition of directional solidified samples is the same as the as-cast sample, which is composed of NiTi (B2), Ti2Ni and Nb-rich phases. After DS, [100] becomes the preferred orientation in NiTi matrix. With the casting temperature increasing, [100] orientation is significantly enhanced, the solid/liquid (S/L) interface morphology changes from cellular dendrite to cellular, and the interface shape changes from convex to plane. Meantime, the volume fraction of the Nb-rich phase decreases with the temperature increasing.

Effect of Y 2O 3 crucible on contamination of directionally solidified intermetallic Ti–46Al–8Nb alloy

The effect of Y 2O 3 crucible on contamination of Ti–46Al–8Nb (at.%) alloy directionally solidified (DS) in a Bridgman-type apparatus was studied. Directional solidification experiments were performed in dense Y 2O 3 crucibles using different growth rates, melt temperatures and various reaction time between the melt and the crucible. The main mechanism responsible for the contamination of the DS samples is diffusion controlled dissolution of the Y 2O 3 crucible in the melt which leads to an increase of oxygen and yttrium content in γ(TiAl) + α 2(Ti 3Al) matrix and precipitation of non-metallic particles in interdendritic region. Transmission electron microscopy (TEM), energy-dispersive spectroscopy (EDS) and X-ray diffraction analysis (XRD) showed that these particles are Y 2O 3 phase. The oxygen content and volume fraction of Y 2O 3 particles increase with increasing melt temperature and reaction time. The activation energy for increase of oxygen content is calculated to be Q O = 412.1 kJ/mol and the kinetics of this process is suggested to be controlled by long-range diffusion with the oxygen content exponent of 3. The activation energy for Y 2O 3 particle formation is calculated to be Q Y = 421.8 kJ/mol and the time exponent is determined to be m = 0.55. Vickers microhardness measurements in lamellar γ + α 2 matrix without Y 2O 3 particles can be used as an indirect evidence of the level of contamination of DS samples with statistically identical α 2–α 2 interlamellar spacing.

On the properties of the eutectic alloy Al3(Nb,Cr)+Cr(Al,Nb)

Abstract The eutectic alloy Al 3 (Nb,Cr) + Cr(Al,Nb) forms an in situ composite and the Al 3 Nb presents high specific strength and low oxidation rate that may be improved by the combination with other phases. The purpose of this work is to investigate physical, mechanical and oxidation properties of the eutectic alloy. Therefore, Rietveld analysis was carried out for furnace cooled and water quenched samples and oxidation tests were performed on directional solidified samples. Compressive tests were performed for the eutectic alloy and also for the Nb–74.8% Cr–24.6% Al alloy in the as-cast condition. The alloy presents 12.9% Cr(Al,Nb) at room temperature, retained from the transformation Cr(Al,Nb) to Al(Nb)Cr 2 . The combination of Al 3 Nb with Cr(Al,Nb) and Al(Nb)Cr 2 considerably improves mechanical behaviour, leading the yield strength to 1525 MPa at 800 °C and 925 MPa at 900 °C. The oxidation tests showed the formation of several oxides at all temperatures studied and that from 900 °C on alfa Al 2 O 3 is formed both in air and O 2 except under O 2 at 1000 °C. It is believed that the Cr(Al,Nb) phase acts as an Al reservoir for the formation of the various Al 2 O 3 scales.

Directional solidification and characterization of the Cd–Sn eutectic alloy

The Cd–67.8 wt.% Sn eutectic alloy was prepared by melting together appropriate amounts of cadmium and tin (both of 99.99% purity) in a graphite crucible in a vacuum melting furnace. Then, this eutectic alloy was poured into graphite crucibles in a hot filling furnace and the samples were solidified. These samples were directionally solidified upward with different growth rate ranges (8.1–165 μm/s) at a constant temperature gradient G (4.35 K/mm) in the Bridgman type directional solidification furnace. The lamellar spacings were measured using directional solidified samples. The variations of λ with respect to V were determined by using linear regression analysis. The dependence of lamellar spacings λ on undercooling, Δ T, was also analysed. The variations of Δ T with V (at constant G) were investigated. According to these results, it has been found that with the increasing values of V, λ decreases, whereas, Δ T increases for a constant G. λ 2 V, Δ Tλ and Δ TV −0.5 values were determined by using λ, Δ T and V values. The results obtained in this work have been compared with the Jackson–Hunt eutectic theory and experimental results studied by Borland and Elliott, Clark and Elliott for the same alloy system, and also the similar alloy systems studied by Çadırlı and Gündüz, Kaya et al.

Effect of titanium additions on the microstructure of DC-cast aluminium alloys

The effect of the Ti content on the solidification structures of aluminum alloys of AA3003 type was investigated. Some improved corrosion properties can be obtained from increasing the Ti contents in aluminium alloys to a level above the normal practice for grain refinement. However, increasing the Ti content above the peritectic point, 0.15%, can influence the grain refinement and cause casting difficulties. The investigation was made by both characterizing the grain structure over the width of DC-cast rolling ingots of different alloys, and by studying the solidification microstructure of Bridgman directional solidified samples, grown to simulate ingot solidification. Structure properties such as grain size, distances between secondary phases and microstructure coarsening were studied. It was found that with normal Ti contents in the range of 0.015%, the grain refinement is effective. However, upon larger Ti additions to levels around 0.15% the grain structure becomes coarser.

Orientations of Y 2BaCuO 5 and YBCO within melt-textured and directional solidified samples studied by EBSD

By means of automated electron backscatter diffraction (EBSD) analysis, we studied the local orientations of embedded Y 2BaCuO 5 (2 1 1) particles within melt-textured YBCO samples, and also the orientations of embedded YBCO particles in directional solidified 211 samples. On both systems, we obtained high-quality Kikuchi patterns, allowing the automated mapping of the crystal orientations and a multi-phase analysis. In melt-textured YBCO with (0 0 1) orientation, we find that the embedded 211 particles do not have any preferred orientation, but the maps also reveal that at certain orientations of the 211 particles the YBCO growth is not altered. In directional solidified 211 samples, where the 211 is mainly oriented in (0 0 1) direction, the embedded YBCO particles show only some specific orientations.

TEM and XAS Characterization of Hard Magnetic Phase in Nd-Fe Alloys

ABSTRACTNd80Fe20 alloys were prepared by copper mold casting and directional solidification (DS) to obtain a coarse microstructure, which can be identified unambigously. The mold-cast samples exhibit a fine globular eutectic structure and show high room temperature coercivity (∼ 5 kOe). Transmission electron microscopic (TEM) investigations reveal the presence of fcc (a = 5.32 Å) Nd-rich and nanocrystalline non-cubic Fe-rich phases. The directionally solidified (DS) samples exhibit large grains (composition close to NdFe2). The NdFe2 grains (formed in the DS samples) show clear magnetic domains with uniaxial anisotropy indicating that the NdFe2-type phase observed in the present samples is not a cubic phase. X-ray absorption spectroscopy (XAS) was used to analyze the local configuration around Fe atoms in the mold-cast samples. The XAS patterns do not match with the bcc and fcc standards, which supports the presence of NdFe2 phase with uniaxial symmetry in the DS samples.

Effect of small amounts of nitrogen on properties of a Ni-based superalloy

The microstructural, tensile, and stress-rupture lives of conventionally and directionally solidified (DS) cast specimens of a nickel-based superalloy have been investigated using materials of normal commercial composition and two melts with additions of 22 and 30 ppm nitrogen. It was found that the ductility, tensile life, and stress-rupture life of the conventional castings dramatically decreased with the increase of nitrogen content, but, in the longitudinal direction of DS samples, these properties were independent of nitrogen within the contents studied. The transverse tensile rupture life of DS castings had no significant change with nitrogen, except for some decrease in elongation of the sample containing 30 ppm N. The stress-rupture life and elongation in the trasverse direction of the DS samples were improved at the 22 ppm nitrogen level, which might be attributed to the dendrite refining. Increasing the nitrogen content resulted in a change in carbide morphology from that of a “Chinese script” to a blocky form and a change in microporosity formation, especially in the conventional castings. The increased nitrogen content resulted in a significant reduction in the volume conventional castings. The increased nitrogen content resulted in a significant reduction in the volume fraction of eutectic microconstituents and the in dendrite in a significant reduction in the volume fraction of eutectic microconstituents and the in dendrite refinement in DS samples, but nitrogen content did not affect the morphology of γ′ precipitates in the heat-treated alloys.