Eutectic spacing and faults of directionally solidified Al–Al3Ni eutectic

The Al–Al₃Ni eutectic was directionally solidified at a thermal gradient of 4.5 K/mm in a vacuum Bridgman-type furnace in order to study eutectic spacing selection criterion. The microstructure was examined in transverse and longitudinal sections and the interrod spacings were measured at different growth velocity. It has been shown that the interrod spacing is not unique and displays a limited range for rodlike Al–Al₃Ni eutectic alloy. The initial growth velocities are not responsible for the eutectic spacing range, while such faults as branching, ending and diameter change have a significant influence on the eutectic spacing adjustment.     

Microstructure and mechanical properties of directionally solidified Al–Si eutectic alloys with and without antimony

Abstract

Hardness H, interjlake spacing λ, and tensile properties are reported for Al–12·7Si and Al–12·7Si–0·2Sb (all wt-%) eutectic alloys directionally solidified at growth velocities of up to 250 μm s^?1 and under temperature gradients in the liquid of up to 12·9 K mm^?1. The hardness is related to interflake spacing by the equation H=H^o+Kλ^?0·2, where H^o is the initial hardness of the alloy. This behaviour contradicts previous results, which suggest that a Hall–Petch relationship is followed. The tensile properties are shown to follow similar behaviour, confirming that hardness shows the same dependence as proof stress on interflake spacing. However, the nature of the relationship depends on the Si morphology and caution should be exercised in using hardness or interflake spacing to indicate proof stress.

MST/1585     

Microstructures and fracture toughness of directionally solidified Mo–ZrC eutectic composites

Microstructures and fracture toughness of arc-melted and directionally solidified Mo–ZrC eutectic composites were investigated in this study. Two kinds of directionally solidified composites were prepared by spot-melting and floating zone-melting. Microstructure of the arc-melted composite (AMC) consists of equiaxed eutectic colonies, in which ZrC particles are dispersed. The spot-melted composite (SMC) exhibits spheroidal colony structure, which is rather inhomogeneous in size and morphology. ZrC fibers in the eutectic colonies are aligned almost parallel to the growth direction. Well aligned, homogeneous columnar structure with thin ZrC fibers evolves in the floating zone-melted composite (FZC). Texture measurement by X-ray diffractometry revealed that the growth direction of Mo solid solution (Mo_SS) in FZC is preferentially 〈100〉, while that of SMC is scattered. Fracture toughness K_Q evaluated by three point bending test using the single edge notched beam method is >13 MPa m^1/2 for AMC, 20 MPa m^1/2 for SMC and 9 MPa m^1/2 for FZC. Intergranular fracture along colony boundaries is often observed in AMC. In contrast, transgranular fracture is dominant in SMC and FZC, although significant gaps caused by intergranular fracture are occasionally observed in SEM micrographs of SMC. Fracture surface in FZC is wholly flat. Pull-out of ZrC occurs owing to Mo/ZrC interfacial debonding in intergranularly fractured regions of AMC and SMC.Coarse elongated colonies in SMC and FZC induce transgranular fracture instead of intergranular fracture. Intergranular fracture and interfacial debonding in AMC and SMC causes frequent crack deflection accompanied by ligament formation and crack branching, which is responsible for the high fracture toughness of the composites. Preferred 〈100〉 growth of Mo_SS phase in FZC leads to brittle {100} cleavage fracture associated with low fracture toughness.     

Microstructural characteristics of directionally solidified Ni–43Ti–7Al alloys

Abstract

Ni–43Ti–7Al (at-%) alloy was directionally solidified at different withdrawal rates (2, 20 and 100 μm s^?1) and a constant temperature of 1550°C by liquid metal cooling method. Results show that as the withdrawal rate decreases from 100 to 2 μm s^?1, the cellular arm spacing increases from 39·5 to 126 μm, the size of Ti₂Ni and the stability of the liquid/solid interface also increase, while the volume fraction of Ti₂Ni decreases from 3·1 to 0·9%. Moreover, microstructural analysis reveals that a NiTi+Ti₂Ni anomalous eutectic structure is formed in intercellular regions of directionally solidified samples withdrawn at 20 and 100 μm s^?1. However, in the sample withdrawn at 2 μm s^?1, Ti₂Ni phases represent strip and liquid droplet morphologies in the intercellular region. Finally, the possible explanation to the change of microstructure is discussed.     

Microstructure evolution of directionally solidified Ni–43Ti–7Al alloy during heat treatment

The microstructure change of directionally solidified Ni–43Ti–7Al alloy after heat treatment has been investigated by transmission electron microscopy and back scattered electron imagings in this paper. After solution and aging treatment, the NiTi + Ti₂Ni anomalous eutectic structure nearly cannot be observed and Ti₂Ni phases become spheroidized. The β′-Ni₂TiAl precipitates are nearly spherical at the early stage of aging at 800 °C (0.1–1 h); however, they become aligned along 〈100〉 directions and change to cubic shape after aging for 20 h. In the course of further aging, the coarsened semi-coherent β′ precipitates occur preferentially in intercellular regions. Then the coarsened β′ precipitates begin to occur in intracellular regions with the increasing aging time and aging temperature. These β′ precipitates change the shape to sphere and plate, accompanied with loss of their coherency by introducing interface dislocations surrounding them. Finally, the formation mechanism of β′ precipitates are discussed compared to other studies in the Ni–Ti–Al system.     

Microstructure and mechanical properties of directionally solidified high-Nb containing Ti–Al alloys

Two high-Nb containing Ti–Al alloys, Ti–16Al–8Nb and Ti–16Al–8Nb–1Sn were fabricated using directional solidification. Their microstructures and mechanical properties at both room and high temperatures were studied. Results showed that the addition of 1% Sn promoted the formation of laths and contributed remarkably to the enhancement in room-temperature strength and high temperature ductility of Ti–Al alloy. The alloys exhibited the feature of quasi-cleavage fracture at room temperature and they experienced significant plastic deformation at high temperatures.     

Corrosion resistance of directionally solidified Al–6Cu–1Si and Al–8Cu–3Si alloys castings

The aim of this article is to compare the electrochemical corrosion resistance of two as-cast Al–6 wt.% Cu–1 wt.% Si and Al–8 wt.% Cu–3 wt.% Si alloys considering both the solutes macrosegregation profiles and the scale of the microstructure dendritic arrays. A water-cooled unidirectional solidification system was used to obtain the as-cast samples. Electrochemical impedance spectroscopy (EIS) and potentiodynamic anodic polarization techniques were used to analyze the corrosion resistance in a 0.5 M NaCl solution at 25 °C. It was found that the Al–8Cu–3Si alloy has better electrochemical corrosion resistance than the Al–6Cu–1Si alloy for any position along the casting length. At the castings regions where the Cu inverse profile prevailed (up to about 10 mm from the castings surface) the corrosion current density decreased up to 2.5 times with the decrease in the secondary dendrite arm spacing.     

Growth direction and Si alloying affecting directionally solidified structures of Al–Cu–Si alloys

The roles of growth direction and Si content on the columnar/equiaxed transition and on dendritic spacings of Al–Cu–Si alloys still remain as an open field to be studied. In the present investigation, Al–6 wt-%Cu–4 wt-%Si and Al–6 wt-%Cu alloys were directionally solidified upwards and horizontally under transient heat flow conditions. The experimental results include tip growth rate and cooling rates, optical microscopy, scanning electron microscopy energy dispersive spectrometry and dendrite arm spacings. It was found that silicon alloying contributes to significant refinement of primary/secondary dendritic spacings for the upward configuration as compared with corresponding results of the horizontal growth. Experimental growth laws are proposed, and the effects of the presence/absence of solutal convection in both growth directions are discussed.     

Microstructure,texture, and crystallography in Ni–GDC and Co–GDC porous cermets obtained from directionally solidified eutectic ceramics

Lamellar NiO–GDC (Gadolinium-doped Ceria) and CoO–GDC directionally solidified eutectic ceramics (DSECs) were produced by the laser floating-zone technique and subjected to reduction in order to obtain porous cermets of Ni–GDC and Co–GDC, which have potential applications as anodes in solid oxide fuel cells (SOFC). The reduction of these DSECs into porous cermets was studied at 650 °C in NiO–GDC and at 500 and 700 °C in CoO–GDC, all of them processed with similar reduction kinetics. In comparison to similar Ni–YSZ and Co–YSZ lamellar cermets previously studied, no sharp reduction front was observed. The interface between the reduced and nonreduced zones is broader, with pores homogenously distributed in wide areas. Afterwards, the microstructure, texture, and crystallography of the samples were studied by electron microscopy as well as by electron and X-ray diffraction when completely reduced. The single crystal NiO and CoO lamellae transformed into porous polycrystalline metallic lamellae. Moreover, microscopy observations revealed a porous nanostructure of Co particles obtained by reduction at low temperatures (500 °C). Many of the Co and Ni particles seemed to have roughly maintained the previous crystallographic orientation with respect to the GDC phase, although the disorder of the crystallographic orientation increased significantly. In addition, a significant amount of the Ni particles reoriented to form an epitaxial interface with the (100)–GDC surface.     

Effect of magnesium content on thermal and structural parameters of Al–Mg alloys directionally solidified

In this study, the influence of magnesium content on thermal and structural parameters during the unsteady-state unidirectional solidification of Al–Mg alloys is analyzed. Using a special device, Al–Mg alloys containing 5, 10, and 15 wt% Mg were submitted to unidirectional solidification. Using a data acquisition system, the temperature variations along the casting during solidification were measured. From these results, the variations of solidification parameters as growth rate of dendrite tips, thermal gradient, cooling rate, and local solidification time were determined. The variation of global heat transfer coefficient at metal/mould interface was estimated through the adjustment of experimental temperature variation close to the interface and numerical predictions. Primary and secondary dendrite arms spacing variations during solidification were measured by optical microscopy. From these results, comparative analysis were developed to determine the influence of magnesium content.     

Dependency of eutectic spacings and microhardness on the temperature gradient for directionally solidified Sn–Ag–Cu lead-free solder

Sn–3.5 wt.%Ag–0.9 wt.%Cu alloy was directionally solidified upward at a constant growth rate (V = 7.20 μm s⁻¹) with different temperature gradients (G = 2.48–6.34 K mm⁻¹) by using a Bridgman type directional solidification furnace. The eutectic microstructures of directionally solidified Sn–3.5 wt.%Ag–0.9 wt.%Cu alloy were observed to be plate and rod structures from quenched samples. The values of eutectic spacings (λ) and microhardness (H_V) were measured from both transverse and longitudinal sections of the samples. The dependence of eutectic spacings (λ) and microhardness (H_V) on the temperature gradient (G) were determined by using linear regression analysis. According to these results, it has been found that, the value of λ decreases with the increasing the value of G and whereas, the value of H_V increases for a constant growth rate. The results obtained in the present work were also compared with the previous similar experimental results obtained for binary and ternary alloys.     

Dependence of electrical and thermal conductivity on temperature in directionally solidified Sn–3.5 wt% Ag eutectic alloy

Sn–3.5 wt% Ag alloy was directionally solidified upward with a constant growth rate (V = 16.5 μm/s) and a temperature gradient (G = 3.3 K/mm) in a Bridgman-type growth apparatus. The variations of electrical resistivity (ρ) with temperature in the range of 293–476 K for the directionally solidified Sn–3.5 wt% Ag eutectic alloy was measured. The measurements indicate that the electrical resistivity of the directionally solidified Sn–Ag eutectic solder increases with increasing temperature. The variations of thermal conductivity of solid phases versus temperature for the same alloy was determined from the Wiedemann-Franz and Smith-Palmer equations by using the measured values of electrical conductivity. From the graphs of electrical resistivity and thermal conductivity versus temperature, the temperature coefficient of electrical resistivity (α _TCR ) and the temperature coefficient of thermal conductivity (α _TCT ) for the same alloy were obtained. According to experimental results, the electrical and thermal conductivity of Sn–Ag eutectic solder linearly decrease with increasing the temperature. The enthalpy of fusion (ΔH) and the change of specific heat (ΔC _P ) during the transformation at the studied alloy were determined from heating curve during the transformation from eutectic solid to eutectic liquid by means of differential scanning calorimeter (DSC).     

The effects of temperature gradient and growth rate on the microstructure of directionally solidified Sn–3.5Ag eutectic solder

The mechanical properties (microhardness, tensile strength) of alloys are controlled by their microstructure, which depends strongly on temperature gradient (G) and growth rate (V). Thus, it is important to understand the relationships among G, V and microstructure (rod eutectic) of Sn–Ag solders. The Sn–3.5 wt% Ag eutectic alloy was directionally solidified upward with a constant growth rate, V (16.5 μm/s) at different temperature gradients, G (1.43–4.28 K/mm) and with a constant temperature gradient, G (3.93 K/mm) at different growth rates, V (8.3–500 μm/s) in a Bridgman–type directional solidification furnace. The rod spacings (λ) have been measured from both longitudinal section (parallel to the growth direction, λ _L ) and transverse section (perpendicular to the growth direction, λ _T ) of the samples. The undercooling values (ΔT) were calculated by using V, λ and system parameters (K ₁ and K ₂). It was found that the values of λ (λ _T , λ _L ) decrease while V and G are increasing. The relationships between rod spacing and solidification parameters (G and V) were obtained by linear regression analysis. The dependences of eutectic spacings λ on undercooling (ΔT) are also analyzed. λ² V, λΔT, ΔTV ^−0.5 and ΔTG ^−0.5 values were determined by using λ, ΔT, V and G values. The results obtained in this work are compared with the Jackson–Hunt eutectic theory and the similar experimental works. The experimental l_\textT ² \textV \lambda_{\text{T}}^{ 2} {\text{V}} value (159.3 μm³/s) is slightly lower than the result 174.6 μm³/s calculated from Jackson–Hunt eutectic theory.     

Microstructure evolution of directionally solidified Nb–Si alloy

Abstract

By taking the method of liquid–metal cooled directional solidification, alloys with a nominal composition of Nb–14Si–24Ti–10Cr–2Al–2Hf (at-%) were prepared under different conditions. Alloys were initially directional solidified with different withdrawal rates (R?=?1·2, 6, 18 mm min^?1) at 1750°C and subsequently heat treated at 1450°C for 10 h. These processes aimed to investigate the microstructure of directionally solidified (DS) and heat treated (HT) alloys by XRD, SEM, and EDS. The microstructure of DS alloy was composed of (Nb,Ti)_SS, (Nb,Ti)₅Si₃, and Laves phase Cr₂Nb, and the former two components formed (Nb,Ti)_SS+(Nb,Ti)₅Si₃ eutectics. In addition, (Nb,Ti)₅Si₃ laths only presented in DS1·2 alloy. With the increasing withdrawal rates, the microstructure of alloy altered from hypereutectic into pseudo-eutectic, accompanied with the eutectic morphology transformation from petaloid into coupled. Also, the dimension of constituent phases reduced. However, after heat treatment, the constituent phases did not change. The petaloid morphology of eutectics in DS specimens disappeared and coupled eutectic transferred into network. The block or needle-like Cr₂Nb gathered along the boundary between (Nb,Ti)₅Si₃ and (Nb,Ti)_SS, and the overall alloy composition became homogenisation.     

Influence of temperature gradient and growth rate on the mechanical properties of directionally solidified Sn–3.5 wt% Ag eutectic solder

The Sn–3.5 wt% Ag eutectic alloy was directionally solidified upward with a constant growth rate (V = 16.5 μm/s) at different temperature gradients (G = 1.43–4.28 K/mm) and with a constant temperature gradient (G = 3.93 K/mm) at different growth rates (V = 8.3–500 μm/s) in a Bridgman-type directional solidification furnace. The rod spacings (longitudinal section, λ _L and transverse section, λ _T ) and mechanical properties (microhardness, HV and ultimate tensile strength, σ _UTS ) of Sn–3.5 wt% Ag eutectic alloy were measured. The dependency of the microhardness, ultimate tensile strength on the temperature gradient, growth rate and rod spacings were determined. According to experimental results, the microhardness and ultimate tensile strength of the solidified samples increase with increasing G and V, but decrease with the increasing the rod spacing.     

Constitution of Ni3Al–Ni3Mo–Ni3W section of Ni–Al–Mo–W system

Abstract

The isothermal section of the Ni–Al–Mo–W system has been studied at 75 at.-%Ni at temperatures of 1523 and 1273 K. Constitutional data have been determined using electron probe microanalysis, X-ray diffraction, and microscopical examination. The alloys studied lay in the range 12·5–15 at.-%Al, 2·5–7·5 at.-%Mo, and 2·5–7·5 at.-% W. The phases present at 1523 K were γ, γ′, and α (based on the Mo–W continuous series of solid solutions); at 1273 K, NiMo(δ′) was also encountered. The γ/γ′ mismatch values lay in the range ?0·03 to ?0·75%. In the as-solidified state, the alloys consisted predominantly of γ-phase containing γ′-precipitates formed in the solid state.

MST/462     

Mechanical behaviour of zirconia–mullite directionally solidified eutectics

Laser floating zone technique (LFZ) is used to grow directionally solidified eutectic (DSE) zirconia–mullite composite fibres (30:70 in wt.%). A notable increase in hardness is observed from 11.3 to 21.2 GPa as the pulling rate increases from 10 to 500 mm/h, due to the ultra-fine eutectics developed at very high growth rates. The indentation fracture toughness reaches a maximum value of 3.5 MPa m^1/2 for the fibre pulled at 100 mm/h, almost three times the value of 1.2 MPa m^1/2 determined for LFZ single-crystal mullite. The eutectic dendrites that develop along the growth direction are immersed in a glassy phase whose brittleness is counteracted by the beneficial ultra-fine morphology, giving a bending strength maximum of 534 MPa. Yet, the soft nature of the glassy matrix prevails at the high temperature testing (1400 °C), causing a decrease to about one-half of the RT value in the fibres with less glassy phase content.