High rate directional solidification and its application in single crystal superalloys

In the present paper there are two parts contributing to the discussion of high rate directional solidification and its application. The first part aims to characterize the high rate directional solidification of various kinds of alloys. It was found that the relevant cooling rate of the high rate directional solidification is defined to be within 1–10³ K/s (solidification rate is 10^-4–10^-1 m/s as G_L= 100 K/cm) and that it is located in the region between the near-equilibrium slow growth rate and the rapid solidification rate beyond the equilibrium condition, whilst at the same time there occurs a series of turning effects of interface stability and morphologies. With the increase in the growth velocity the interface with the plane front evolves to cells and dendrites at the stage of near-equilibrium and with a further increase in growth rate they transformed reversibly from dendrites to cell structure and then to the absolute stability of a planar interface. The change of solute segregation reveals the same from a lowsegregation, then increased and finally reduced again. An explanation based on effective constitutional supercooling about the evolution of interface morphologies with respect to the changes of growth rate is proposed.

The second part is devoted to introducing experimental results for single crystal superalloys using the rate directional solidiication principle. It is shown that the single crystal superalloys CMSX–2 and NASAIR 100 exhibit significant improvement in microstructure segregation and mechanical properties at high temperature both in the as-cast and after-heat-treatment conditions with the high rate directional solidification technique.     

Effect of cooling rate on the morphology of γ′ precipitates in a nickel-base superalloy under directional solidification

The morphological evolution of γ′ precipitates in a nickel-based superalloy K5 was studied by zone melting directional solidification under vacuum conditions. The results show that at the lower cooling rate of 12.42 K s⁻¹, γ′ precipitates remand big cuboids. γ′ particles become smaller at the cooling rate ranges from 12.42 to 38.80 K s⁻¹. For a rather fast cooling rate of 50.16 K s⁻¹, γ′ particles retain a spherical shape. The experiments show that big cuboids will become unstable and split into several small ones at the lower cooling rate of 1.1 K s⁻¹. The mechanism of the evolution of the γ′ morphologies is also analyzed by introducing a new parameter-shape factor which classifies the total energy into several energy levels. Based on this, the effect of the cooling rate on the γ′ morphology is discussed.     

Characteristics of S/L Interface Evolution during High Rate Directional Solidification

The present paper aims to the characterization of high rate direction solidification on AI-Mn and AI-Cu alloys. It is indicated that the relevant cooling rate of high rate directional solidification is defined within 10(0)～10(3) K/s that is located in the region between near-equilibrium slow growth rate and rapid solidification rate beyond equilibrium condition, and at the meantime there occurred a series of turning effect of interface stability and morphologies. With the increase of growth velocity the interface with planar front evolved to cells and dendrites at the stage of near-equilibrium and with further increase of growth rate they transformed reversely from dendrites to cell structure and then to absolute stability of planar interface. An explanation based on effective constitutional supercooling about the evolution of interface morphologies with the change of growth rate was proposed.     

Effect of growth rate on microstructure and tensile properties of Ti–45Al–2Cr–2Nb prepared by electromagnetic cold crucible directional solidification

Ti–45Al–2Cr–2Nb alloy was directionally solidified at different growth rates varying from 0.2 to 1.2 mm/min by applying an electromagnetic cold crucible directional solidification technique. It was determined that well-aligned α₂ (Ti₃Al)/γ(TiAl) lamellar structures, B₂ phase and blocky γ phase were generated in columnar grains. The interlamellar spacing (λ) decreases with the increasing growth rate (V) according to the relationship λ ∝ V^− 0.48, but the volume fraction of B₂ is increased as growth rate is increasing. Results of uniaxial tensile tests show that the B₂ phase and the blocky γ phase have significant influence on tensile failure when they are presented in the matrix of α₂/γ lamellar structures, because they are usually employed to act as cracking sources during the tension process.     

Effects of Co content on tensile properties and deformation behaviors of Ni-based disk superalloys at different temperatures

Tensile properties of three Ni-based disk superalloys with 5 wt.%, 15 wt.% and 23 wt.% Co contents were investigated from room temperature (RT) to 725 °C with a constant strain rate of 3 × 10^− 4 s^− 1. It is found that addition of Co enhances the yield strength and the strain hardening capacity of the alloy in the studied temperature regime. It is due to the following two reasons: i.e. the interactions between a large volume fraction of fine secondary and tertiary γ′ precipitates with the dislocations in slip bands at lower temperatures and the formation of deformation microtwins at higher temperatures. However, the formation of deformation microtwins in the high Co-containing content alloy sharply decreases the ductility at higher temperatures.     

Tribological behavior of Ti3SiC2/(WC–10Co) composites prepared by spark plasma sintering

The dry sliding friction and wear behavior of Ti₃SiC₂/(WC–10Co) composites (TWCs) against GCr15 steel pair at room temperature was investigated through the determination of friction coefficient and wear rate under different conditions and the analysis of the morphologies and compositions of wear debris, worn surfaces of TWC and GCr15 steel. The friction coefficients of TWC with 3 wt.% WC–10Co were in the range of 0.40–0.48, and the wear rate varied from 0.6 × 10⁻⁴ mm³ (N m)⁻¹ to 1 × 10⁻⁴ mm³ (N m)⁻¹. At the load of 10 N and sliding speed of 0.353 m/s, the glazes were formed on the worn surfaces of TWC. The wear mechanisms were complicated, including micro-cutting and abrasive wear of TWC, oxidation wear of GCr15 steel, as well as adhesive wear caused by the glaze flaking.     

GaN epilayer on separately deposited buffer layer by hot wall epitaxy

High quality GaN epilayers were grown on a sapphire substrate using a hot wall epitaxy method. We have investigated the crystal, optical, and electrical properties of GaN epilayers grown as functions of the nitridation condition of the substrate and the growth condition of GaN buffer layer. In order to study an effective method to grow a buffer layer for the growth of high quality GaN epilayer, the buffer layers were formed on the nitridated substrate using two different methods. One is separately deposited buffer layer (SDBL), and the other is co-deposited buffer layer (CDBL). It was observed that the growth condition of the buffer layer had a strong influence on the crystal and optical properties of GaN epilayer. A strong band edge emission peak at 3.474 eV was observed from the photoluminescence spectrum measured at 5 K for GaN epilayer grown at the optimum condition of the buffer layer. The carrier concentration and mobility of undoped GaN epilayer grown with a growth rate of 0.5 μm h⁻¹ were 2 × 10¹⁸ cm⁻³ and >50 cm³ V⁻¹ s⁻¹ at room temperature, respectively.     

High temperature tensile deformation behavior and failure mechanisms of an Al–Si–Cu–Mg cast alloy — The microstructural scale effect

In this study the high temperature tensile deformation behavior of a commercial Al–Si–Cu–Mg cast alloy was investigated. The alloy was cast with two different cooling rates which resulted in average secondary dendrite arm spacing of 10 and 25 μm, which is typical of the microstructure scale obtained from high pressure die casting and gravity die casting. Tensile tests were performed at different strain rates (10^− 4 s^− 1 to 10^− 1 s^− 1) and over a wide temperature range from ambient temperature to 500 °C. The fine microstructure had superior tensile strength and ductility compared to the coarse microstructure at any given temperature. The coarse microstructure showed brittle fracture up to 300 °C; the fracture mode in the fine microstructure was fully ductile above 200 °C. The fraction of damaged particles was increased by raising the temperature and/or by microstructure coarsening. Cracks arising from damaged particles in the coarse microstructure were linked in a transgranular-dominated fashion even at 500 °C. However, in the fine microstructure alloy the inter-dendritic fracture path was more prevalent. When the temperature was raised to 300 °C, the concentration of alloying elements in the dendrites changed. The dissolution rates of Cu- and Mg-bearing phases were higher in the fine microstructure.     

Prediction and characterization of growth temperatures in Al–Zn–Mg alloys

Growth temperatures of α-Al, intermetallic τ and eutectic α + τ phases in Al-12 wt.% Zn 6 wt.% Mg alloy has been determined as a function of growth velocity in the range of 3 × 10^? 5 to 1 × 10^? 3 m/s at a temperature gradient of 2500 K/m, using a directional solidification technique. The experimental results are found to be in good agreement with predictions of growth temperatures of competing constituents for multicomponent systems.     

Effect of CuO nanostructure morphology on the mechanical properties of CuO/woven carbon fiber/vinyl ester composites

The effect of CuO nanostructure morphology on the mechanical properties of CuO/woven carbon fiber (WCF)/vinyl ester composites was investigated. The growth of CuO nanostructures embedded in the surface of woven carbon fibers (WCFs) was carried out by a two-step seed-mediated hydrothermal method; i.e., seeding and growth treatments with controlled chemical precursors. CuO nanostructural morphologies ranging from petal-like to cuboid-like nanorods (NRs) were obtained by controlling the thermal growth temperature in the hydrothermal process over a growth time of 12 h. The Cu²⁺/O⁻ ratio and the rate of reaction greatly influenced the formation of CuO nanostructures as self-assembled shapes on the crystal planes in the order L[0 1 0] > L[1 0 0] > L[0 0 1]. Morphological variations were analyzed by scanning electron microscopy, X-ray diffraction, and Brunauer–Emmett–Teller surface area analysis. The impact behavior, in-plane shear strength, and tensile properties of the CuO/WCF/vinyl ester composites were analyzed for different CuO NR morphologies at various growth temperatures and molar concentrations. The CuO/WCF/vinyl ester composites had improved impact energy absorption and mechanical properties because the higher specific surface area of CuO NRs grown as secondary reinforced nanomaterials on WCFs enhanced load transfer and load-bearing capacity.     

Experimental study on the dynamic tensile behavior of a poly-crystal pure titanium at elevated temperatures

A new technique for measuring dynamic tensile behavior of metallic materials at elevated temperatures was developed. This technique employs a rapid contact heating method to obtain a stable and nearly homogenous high temperature field in the testing gage of the specimen. As an application of this new technique, a commercially pure titanium (CP-Ti) was tested in the strain rate range of 300 s⁻¹–1400 s⁻¹ and in a temperature range of 298 K–973 K. Quasi-static experiments (10⁻³ s⁻¹, 10⁻² s⁻¹) were also performed in the same temperature range for comparison. The testing results indicated that both temperature and strain rate have pronounced influence on the mechanical behavior of CP-Ti.     

Anaxial columnar dendrites in directional solidification of an Al–15 wt.% Cu alloy

A new dendrite morphology, anaxial columnar dendrites, was found in directional solidification using the real-time X-ray imaging technique. The dendrites are composed of a pair of stems, which are divided by a narrow liquid zone located in their center. The formation process of a free dendrite in the melt indicates that it grows along < 110> directions. According to the basic morphology of the dendrites, a tip model was developed to explain the growth preference of the secondary arms. The formation of anaxial columnar dendrites indicates that the dendrite morphology is closely related to the selection of the growth direction of the dendrites.     

Strain rate dependent plastic mutation in a bulk metallic glass under compression

This paper reported a strain rate dependent plasticity in a Zr-based bulk metallic glass (BMG) under axial compression over a strain rate range (1.6 × 10⁻⁵–1.6 × 10⁻¹ s⁻¹). The fracture strain decreased with increasing strain rate up to 1.6 × 10⁻³ s⁻¹. A “brittle-to-malleable” mutation occurred at strain rate of 1.6 × 10⁻² s⁻¹, subsequently, the macro plasticity vanished at 1.6 × 10⁻¹ s⁻¹. It is proposed that the result is strongly related to the combined action of the applied strain rate, the compression speed, and the propagating speed of the shear band. When the three factors coordinated in the optimal condition, multiple mature shear bands were initiated simultaneously to accommodate the applied strain, which propagated through the specimen and distributed homogeneously in space, dominating the overall plastic deformation by consuming the entire specimen effectively.     

Characterization of hot compression behavior of a new HIPed nickel-based P/M superalloy using processing maps

Isothermal forging was a critical step process to fabricate the high-performance nickel-based superalloy. The temperature and strain rate served the most critical role in determining its microstructure and mechanical properties. In this article, we employed the hot compression to simulate the isothermal forging process upon the temperature ranging from 1000 °C to 1100 °C in combination with a strain rate of 0.001–1.0 s^− 1 for a new P/M nickel-based alloy. The activation energy was determined as 903.58 kJ/mol and the processing maps at a strain range of 0.4–0.7 were developed. The instability domains were more inclined to occur at strain rates higher than 0.1 s^− 1 and manifested in the form of adiabatic shear bands. The map further demonstrated that the regions with peak efficiency of 55% were located at 1080 °C/0.0015 s^− 1 and 1095 °C/0.014 s^− 1, respectively. Obvious dynamic recrystallization could be detected at the strain rate 0.01 s^− 1 leading to a significant flow stress drop and the grain growth was remarkably triggered under 1100 °C. The findings can shed light on the forging processing optimization of the new nickel-based superalloy.     

One-step preparation of six-armed Fe3O4 dendrites with carbon coating applicable for anode material of lithium-ion battery

Six-armed Fe₃O₄ dendrites with carbon coating were synthesized by a simple one-step reaction between ferrocene and urea at 550 °C. Electron microscopy examinations indicate the formation of large numbers of Fe₃O₄ dendrites with mutually vertical arms and uniform carbon shells. Electrochemical measurement demonstrates that the dendrites using as anode materials for lithium-ion battery exhibit an initial capacity of 658 mAh g^? 1 and a reversible capacity of 473 mAh g^? 1 after 100 cycles at a rate of C/10, as well as a high cycling efficiency of 97% after the forth cycle. The formation mechanism of the six-armed dendrites was also discussed.     

Microstructure and orientation variation during cell/dendrite transition in directional solidification of a single crystal nickel-base superalloy

The crystallographic orientation and microstructure variation during cell/dendrite transition of a nickel-base single crystal superalloy have been analyzed. The crystal with the off-<0 1 1> orientation grew with a cellular interface at 6 μm s^?1 while with a dendrite interface at 100 μm s^?1. The crystal kept the orientation of bottom seed in the dendritic growth stage, and there was a short transitional zone. The deviation angle between the cellular orientation and the heat flow direction became small in the cellular growth stage. The cellular growth direction was in accordance with the heat flow direction and was independent of the crystallographic orientation. At the same crystallographic orientation, the dendrites had a large primary dendrite arm spacing during the cell/dendrite transition. The cellular crystal with a smaller orientation deviation from the cylindrical axis showed a larger primary arm spacing.     

Modeling quasi-static and high strain rate deformation and failure behavior of a (±45) symmetric E-glass/polyester composite under compressive loading

Quasi-static (1 × 10⁻³–1 × 10⁻² s⁻¹) and high strain rate (∼1000 s⁻¹) compressive mechanical response and fracture/failure of a (±45) symmetric E-glass/polyester composite along three perpendicular directions were determined experimentally and numerically. A numerical model in LS-DYNA 971 using material model MAT_162 was developed to investigate the compression deformation and fracture of the composite at quasi-static and high strain rates. The compressive stress–strain behaviors of the composite along three directions were found strain rate sensitive. The modulus and maximum stress of the composite increased with increasing strain rate, while the strain rate sensitivity in in-plane direction was higher than that in through-thickness direction. The damage progression determined by high speed camera in the specimens well agreed with that of numerical model. The numerical model successfully predicted the damage initiation and progression as well as the failure modes of the composite.     

Effects of temperature and strain rate on microstructure and mechanical properties of high chromium cast iron/low carbon steel bimetal prepared by hot diffusion-compression bonding

The objective of this study is to develop a hot diffusion-compression bonding process for cladding low carbon steel (LCS) to high chromium cast iron (HCCI) in solid-state. The influence of temperature (950–1150 °C) and strain rate (0.001–1 s⁻¹) on microstructure, hardness and bond strength of the HCCI/LCS bimetal were investigated. The interface microstructure reveals that the unbonded region can only be found for 950 °C due to lack of diffusion, while the intergrowth between the constituent metals occurred at and above 1100 °C. When bonding temperature increases to 1150 °C, a carbide-free zone was observed near the interface on the HCCI layer, and the thickness of the zone decreases with an increase of bonding strain rate. These evolutions indicate that the bond quality was improved by raising temperature and reducing strain rate due to the increase of element diffusion. The hot compression process of the bonding treatment not only changes the carbide orientation of the HCCI, but also increases the volume fraction of Cr–carbide. Based on the microstructural examinations and mechanical tests, the optimum bonding temperature and bonding strain rate are determined to be 1150 °C and 0.001 s⁻¹, respectively.     

Unidirectional solidification of a Zn-rich Zn–2.17 wt%Cu hypo-peritectic alloy

Unidirectional solidification of a Zn-rich Zn–2.17 wt%Cu hypo-peritectic alloy has been carried out to investigate the microstructure evolution over the growth velocity range 0.02–4.82 mm/s at a temperature gradient of 15 K/mm by means of the Bridgman technique. Regular and plate-like two-phase cellular structures were observed in samples grown at growth velocities V above 0.48 and 2.64 mm/s, respectively. The dominant microstructure in samples grown below 0.22 mm/s was dendrites of primary ε in a matrix of secondary η. Intercellular spacing Λ decreased with increasing growth velocity V such that ΛV^1/2 is a constant of 316±55 μm^3/2/s^1/2. Secondary dendrite arm spacing λ₂ of primary ε decreased with increasing V such that λ₂V^1/3 is a constant of 14.9±0.9 μm^4/3/s^1/3. The observed transition from regular cells to plate-like cells of η is discussed on the basis of competitive growth and crystallographic effect.     

Improvement of magnetic properties of an Fe–6.5 wt.% Si alloy by directional solidification

The magnetic properties of an Fe–6.5 wt.% Si alloy can be improved through texture and microstructure control during directional solidification process. With the increasing of directional solidification rate, the main texture of the Fe–6.5 wt.% Si alloy along specimen withdrawing direction evolved in the way of < 130> → < 100> → < 142>, and the coercivity initially decreased and then increased. For the directional solidification rate of 1 mm/min, a homogeneous microstructure of the Fe–6.5 wt.% Si alloy specimen with low energy boundaries between columnar grains was obtained. The main texture of the specimen was < 100>, and the coercivity of the alloy was reduced by 44% compared with that of the alloy consisting of equiaxed grains.