Rapid Solidification Process Research Articles

Phase stability behavior of nanoscaled lead-bismuth peritectic alloys embedded in zinc matrix

Pb-Bi alloy inclusions of nanometric size with peritectic composition embedded in zinc matrix are prepared by rapid solidification processing to study their phase stability and solid-state transformation behavior within the Zn matrix. The experimental results indicate the inclusions to be single phase having hcp crystal structure with a mode of size distribution of 25nm. In-situ transmission electron microscopy using heating stage and high temperature X-ray diffraction results suggest that at small length scales of the confined inclusions, the coexistence of a liquid with two solid phases does not take place at the peritectic temperature.

Direct Observation of Inherent Atomic-Scale Defect Disorders responsible for High-Performance Ti1-x Hfx NiSn1-y Sby Half-Heusler Thermoelectric Alloys.

Structural defects often dominate the electronic- and thermal-transport properties of thermoelectric (TE) materials and are thus a central ingredient for improving their performance. However, understanding the relationship between TE performance and the disordered atomic defects that are generally inherent in nanostructured alloys remains a challenge. Herein, the use of scanning transmission electron microscopy to visualize atomic defects directly is described and disordered atomic-scale defects are demonstrated to be responsible for the enhancement of TE performance in nanostructured Ti1-x Hfx NiSn1-y Sby half-Heusler alloys. The disordered defects at all atomic sites induce a local composition fluctuation, effectively scattering phonons and improving the power factor. It is observed that the Ni interstitial and Ti,Hf/Sn antisite defects are collectively formed, leading to significant atomic disorder that causes the additional reduction of lattice thermal conductivity. The Ti1-x Hfx NiSn1-y Sby alloys containing inherent atomic-scale defect disorders are produced in one hour by a newly developed process of temperature-regulated rapid solidification followed by sintering. The collective atomic-scale defect disorder improves the zT to 1.09 ± 0.12 at 800 K for the Ti0.5 Hf0.5 NiSn0.98 Sb0.02 alloy. These results provide a promising avenue for improving the TE performance of state-of-the-art materials.

Tribological Characterization of Hypereutectic Al–25Si Alloy Under Dry and Lubricated Sliding Conditions

Friction and wear properties of hypereutectic Al–25Si alloy were studied under dry and lubricated sliding conditions. Hypereutectic Al–25Si alloys were prepared by rapid solidification process (RSP) under the T6 condition. Experimental studies were conducted using a ball on disk type tribometer. The effect of the sliding distance and normal load on the friction and wear were investigated. The coefficient of friction (COF) remained stable with an increase in the sliding distance (250–1500 m) and decreased with an increase in the normal load (10–50 N), whereas the wear rate decreased with an increase in the sliding distance, and increased with the increase in the normal load up to 40 N and then attained a steady-state value under dry and lubricated sliding conditions. The improvements in COF and wear rate were mainly attributed to the morphology, size, and distribution of hypereutectic Si particles due to its fabrication process. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), optical microscopy, and three-dimensional (3D)-surface profilometer were used for characterization of the wear tracks. The dominant wear mechanisms for a hypereutectic Al–25Si alloy were adhesive wear, abrasive wear, and plastic deformation.

Effects of Forming Processes on the Microstructure and Solderability of Sn-3.5Ag Eutectic Solder Ribbons as well as the Mechanical Properties of Solder Joints

Two kinds of Sn-3.5Ag eutectic solder ribbons of 0.13 mm thickness were prepared by a casting–rolling process and a rapid solidification process. The microstructure, phase constitution, melting characteristics, wetting behavior and soldering strength were compared using optical microscopy, scanning electron microscopy, x-ray diffraction, energy dispersive spectroscopy, differential scanning calorimetry and a MTS ceramic testing system. The results show that the microstructure of rapidly solidified solder is finer and more uniform, and the eutectic structure has a higher solid solubility and more homogeneous distribution of Ag in a Sn matrix. The solidus and liquidus temperature decreased, resulting in a 3.3% reduction of pasty range. In addition, the wettability and shear strength of the solder joints increased by 13.2% and 7.9%, respectively.

Epitaxial growth during the rapid solidification of plasma-sprayed molten TiO2 splat

In this study, epitaxial growth during the rapid solidification of plasma-sprayed molten TiO2 droplet was studied. The crystallographic structure of the TiO2 splats deposited on rutile and α-Al2O3 substrates at 150, 300 and 500 °C was characterized by high resolution transmission electron microscopy and electron back scattering diffraction. The results reveal that homo-epitaxial and hetero-epitaxial TiO2 splats can be formed at the deposition temperature of 500 °C. Crystal orientation is another key factor influencing the epitaxial growth process. It is easier to form an epitaxial TiO2 splat with the 〈001〉 orientation in the crystal growth direction. Based on the experimental results, a competition mechanism between heterogeneous nucleation and epitaxial growth was proposed to understand the phenomena of epitaxy during the rapid solidification process. The effect of undercooling degree, crystal orientation and deposition temperature on the epitaxial growth of TiO2 splat was examined. The simulation results are in close agreement with the experimental observations.

Determining the kinetics of transient liquid phase bonding (TLPB) of inconel 625/BNi-2 couples using differential scanning calorimetry

A cyclic DSC method was developed which can measure the rate of isothermal solidification (IS) of the centerline eutectic in a Ni-based superalloy TLPB couple using a single sample. This was done by measuring the enthalpy of solidification of the remaining eutectic liquid, in a half braze joint when cooled from the brazing temperature. The analysis revealed that a rapid diffusional solidification (DS) process occurs during initial melting such that approximately 50% of the eutectic liquid is already diffusionally solidified by the time the peak brazing temperature is reached. This initial DS process greatly reduces the total time for solidification at the brazing temperature. Experimental DSC determinations of the complete isothermal solidification time agree well with theoretical predications.

Domain structure analysis of (AgSbTe2)15(GeTe)85 thermoelectric compounds fabricated by rapid solidification process (RSP) and spark plasma sintering (SPS)

The domain structure of the thermoelectric compound (AgSbTe2)15(GeTe)85 was analyzed using high resolution transmission electron microscopy. Numerous coherent domain boundaries of two different types were observed, formed by the polar phase transition from the cubic NaCl-type phase to a rhombohedral structure with an angular distortion of 0.5–1.5°. One type has twin boundaries parallel to [001] and [110] directions, the other has inversion domain boundaries parallel to [111] and [112] directions. This high density of internal interfaces could reduce lattice thermal conductivity due to phonon scattering on the interfaces, and less deleterious on electronic transport processes, because they are coherent.

Effect of substrate orientations on microstructure evolution and stability for single crystal superalloys in rapid solidification process

Recast layer (RL), a main by-product in electrical discharge machining (EDM) drilling, has been criticized for high oxidation and poor thermal stability in advanced engine supercharging systems. In this study, it is found that the substrate crystallographic orientation has a significant effect on rapid solidification microstructure, including cellular-dendrite distribution characteristic of stack structure, and the cellular-dendrite microstructure characteristic around holes. By means of EBSD technique, and with the structural feature, it is proved that RL-substrate has imperfect single crystal (SX) structure. The low-angle grain boundaries are distributed at RL-substrate interface and second discharge region interface. The former is only 1–3° while the latter is about 7–10°. Precisely because of the different property characteristics, three types of boundaries almost take on various thermal stability that correspond to different fault mechanisms. Among these, the cellular-dendrite boundary plays an important role in structural damage in primary discharge region. The experiments show that the (100) inner RL has better thermal stability than (110) and (111). The result is in accord with the research purpose approximately that the probabilistic damage of structures could be deduced quantitatively on different substrates.

The enhanced microhardness in a rapidly solidified Al alloy

The objective of the present study is to explore the feasibility of producing 7075 Al alloy ribbons with the increased microhardness via rapid solidification processing, namely melt spinning. Our results show that the melt spun+aged 7075 Al alloy ribbons exhibit approximately 17% (307MPa) higher microhardness than that of the coarse-grained (CG) counterpart solid-solution treated+aged under the same conditions. The higher microhardness of the melt spun+aged 7075 Al alloy stems from its microstructural features, including (i) sub-micrometric/micrometric sized grains, (ii) intragranular subgrains, and (iii) nanoscale precipitates with inter-precipitate distance slightly smaller than that in the CG counterpart. In an effort to understand the microstructure in melt spun+aged 7075 Al alloy, the microstructural evolution during aging of the as-melt spun 7075 Al alloy was analyzed and discussed. By quantifying the contributions of precipitation, grain boundary, dislocation, solid solution and subgrain boundary strengthening to the yield strength based on the microstructural analysis, the higher microhardness of the melt spun+aged 7075 Al alloy ribbons was rationalized.

In-situ ultrafine three-dimensional quasi-continuous network microstructural TiB reinforced titanium matrix composites fabrication using laser engineered net shaping

In this study, we created an innovative ultrafine three-dimensional quasi-continuous network (3DQCN) microstructure by in-situ laser engineered net shaping (LENS) of TiB reinforced titanium matrix composites (TiB-TMCs). As a rapid solidification process, the LENS process enabled high degree of Ti grain refinement. The in-situ processed eutectic TiB aggregated at these Ti grain boundaries, forming the ultrafine 3DQCN microstructure. The microstructural characterizations of the ultrafine 3DQCN microstructure were investigated. Effects of the TiB reinforcement and the ultrafine 3DQCN microstructure on the mechanical performance of the fabricated parts were studied. The results demonstrated that the TiB-TMCs with the ultrafine 3DQCN microstructure exhibited superior mechanical properties.

Structural evolutions and hereditary characteristics of icosahedral nano-clusters formed in Mg70Zn30 alloys during rapid solidification processes

To investigate the structural evolution and hereditary mechanism of icosahedral nano-clusters formed during rapid solidification, a molecular dynamics (MD) simulation study has been performed for a system consisting of 107 atoms of liquid Mg70Zn30 alloy. Adopting Honeycutt-Anderson (HA) bond-type index method and cluster type index method (CTIM-3) to analyse the microstructures in the system it is found that for all the nano-clusters including 2~8 icosahedral clusters in the system, there are 62 kinds of geometrical structures, and those can be classified, by the configurations of the central atoms of basic clusters they contained, into four types: chain-like, triangle-tailed, quadrilateral-tailed and pyramidal-tailed. The evolution of icosahedral nano-clusters can be conducted by perfect heredity and replacement heredity, and the perfect heredity emerges when temperature is slightly less than Tm then increase rapidly and far exceeds the replacement heredity at Tg; while for the replacement heredity, there are three major modes: replaced by triangle (3-atoms), quadrangle (4-atoms) and pentagonal pyramid (6-atoms), rather than by single atom step by step during rapid solidification processes.

Microstructure and hardness studies of Inconel 718 manufactured by selective laser melting before and after solution heat treatment

The microstructure of Additive Manufactured (AM) Inconel 718 in general and Selective Laser Melting (SLM), in particular is different from the material produced by conventional methods due to the rapid solidification process associated with the former. As a result, the widely adapted standard solution heat treatment temperature (<1100°C) for conventional material is found to be not high enough for materials fabricated with SLM method in order to dissolve Laves and other microsegregated phases for releasing the ageing constituents (Nb, Ti, Al) sufficiently into the alloy matrix. In this study, sample of Inconel 718 fabricated with SLM method were solution heat-treated to 1100°C or 1250°C at different hold times to investigate the dissolution of macro- and micro-segregated precipitates. Investigations of microstructure and segregation in as-printed and solution heat-treated states have been studied using Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM) and Optical Microscopy (OM). Measurement of material hardness was performed with Vickers hardness tests. The microstructure of the as-printed parts exhibit non-columnar grains, but contain well-shaped columnar/cellular sub-grains. The intergranular boundaries are decorated with high density of dislocations and segregated particles. Tremendous stress relief and grain coarsening were observed with solution heat treatment. In particular, at 1250°C annealing, the sub-grains, including precipitates and dislocation networks along the sub-grain boundaries, were entirely dissolved. However, the 1100/1250°C solution heat treatment scheme could not dissolve microsegregated precipitates and carbides completely. Details of the analysis on microstructure, dissolution of precipitates and hardness are presented.

Effects of microstructure on the hydrogen storage properties of the melt-spun Mg-5Ni-3La (at.%) alloys

Nanocrystalline and amorphous microstructures prepared by rapid solidification (RS) processing are effective in improving the absorption/desorption kinetics of the Mg-based hydrogen storage alloys. In this paper, the as-cast, nanocrystalline and amorphous Mg-5Ni-3La (at.%) alloys were prepared, and effects of microstructure on the hydrogen storage properties of the alloys were studied. The nanocrystalline and amorphous Mg-5Ni-3La (at.%) alloys show better absorption/desorption kinetics than that of the as-cast alloy due to the smaller grain size and more grain boundaries. The values of ΔH (enthalpy) for the Mg-H2 system of the nanocrystalline and amorphous alloys are −74.0 ± 1.2 kJ mol−1 and -66.1 ± 3.4 kJ mol−1, respectively, which are significantly lower compared to that of the as-cast alloy (−77.7 ± 0.3 kJ mol−1).

Size-dependent solubility and phase transformation behavior of Sn–Cd nanoparticles in an Al matrix

Nanoscaled Sn–Cd alloy particles with a nominally equiatomic composition and size in the range of 5–120 nm, embedded in an Al matrix, have been synthesized by rapid solidification processing. The orientation relationship between tin, cadmium and aluminum is given by $$ \begin{aligned} & (10\bar{1})_{\text{Al}} ||(0\bar{1}0)_{\text{Sn}} ||(1\bar{2}10)_{\text{Cd}} \\ & [111]_{\text{Al}} ||[101]_{\text{Sn}} ||[0001]_{\text{Cd}} \\ \end{aligned} $$ . Particles in the size range 5–25 nm display a single-phase hcp structure, while particles with size range of 25–120 nm size are found to be phase-separated into a tin-rich phase with bct structure and a cadmium-rich phase with hcp structure. In situ heating studies in the transmission electron microscope indicate that particles having a size range of 5–18 nm always display a single-phase microstructure right up to the melting point and subsequent cooling. The in situ heating experiments revealed complex pathways for transformation for particles having size between 18 and 30 nm. The as-solidified microstructures of over 75% of the particles analyzed, exhibit a two-phase structure in the melt-spun samples. While more than 75% of the particles analyzed by in situ heating and cooling experiments reveal formation of a single-phase structure during resolidification, some of the particles on cooling from molten state solidify into a two-phase microstructure of cadmium and tin solid solution. These particles, on cooling in the microscope, transform to single-phase particles of cadmium-rich solid solution around 110 °C suggesting a kinetically dominated microstructure evolution. In contrast, the particles in the size range 30–120 nm exhibit a two-phase microstructure both during heating and cooling. Size-dependent alloying behavior of the particles could be explained based on models that consider capillary pressure and interfacial energies.

A DFT study on the competition and evolution characteristics between icosahedra and FCC clusters in rapid solidification of liquid Ag

Abstract Based on molecular dynamics (MD) simulations for rapid solidification processes of liquid silver, the energetics, chemical stability and electronic structures of icosahedra short-range orders (ISROs), icosahedra medium-range orders (IMROs), fcc short-range orders (FSROs) and fcc medium-range orders (FMROs) at high and low temperature are further investigated by a DFT calculation. Transition states (TS) search shows that the evolution from ISRO to FSRO in super-cooled liquid must overcome an energy barrier and this transformation is an endothermic process. A high chemical hardness should be responsible for the large population of ISROs and IMROs over FSROs and FMROs in super-cooled liquid and the dominant FMROs in a crystal of silver, and their high chemical stabilities can be attributed to larger energy gaps, more low highest occupied molecular orbital (HOMO) energy levels and more perfect full fill of electrons in the HOMO energy level.

Co-extrusion of Dual Aluminum Alloys with Special Surface Properties

Abstract Co-extrusion method is used for production of a dual Aluminum rod. The surface layer material for co-extrusion is prepared by rapid solidification process. The alloys Al-4.5Mg-1.0Ag (wt%) and Al-4.5Mg are selected as the surface and the bulk materials, respectively, and are co-extruded at high temperature range (450-480°C). The microstructures of rapid solidified Al in co-extruded rod consist of small grains (50µm) and some small particles (0.1-1µm). A proper annealing on the extruded rod increases the surface hardness more than 30%. Since the co-extruded material contains small T-phase particles, this increase in hardness indicates precipitation of a metastable Mg-Ag phase or a Mg-Ag-Al phase at 160°C during annealing.

Liquid-Liquid Phase Separation in Quenched Fe&lt;sub&gt;71&lt;/sub&gt;Cu&lt;sub&gt;10&lt;/sub&gt;P&lt;sub&gt;10&lt;/sub&gt;B&lt;sub&gt;9&lt;/sub&gt; Multi-Component Alloy

Liquid-liquid phase separation (LLPS) in the rapidly solidified Fe71Cu10P10B9 alloy under different casting conditions is investigated based on the XRD, DSC, and SEM measurements. It is found that during rapid solidification process, Cu-rich globules precipitated in the matrix which mainly consists of α-Fe and Fe3B0.82P0.18 crystals. With increasing cooling rate, LLPS becomes weaker, leading to less precipitation of Cu-rich globules, while the microstructure of the matrix became finer. Magnetic measurements show that the saturation induction and the coercivity of the present samples increase first and then decrease with increasing cooling rate. The corresponding mechanisms related to magnetic performance are also discussed details.

Influence of rapid solidification on Sn–8Zn–3Bi alloy characteristics and microstructural evolution of solder/Cu joints during elevated temperature aging

The effects of rapid solidification on the microstructure and melting behavior of the Sn–8Zn–3Bi alloy were studied. The evolution of the microstructural characteristics of the solder/Cu joint after an isothermal aging at 150 °C was also analyzed to evaluate the interconnect reliability. Results showed that the Bi in Sn–8Zn–3Bi solder alloy completely dissolved in the Sn matrix with a dendritic structure after rapid solidification. Compared with as-solidified Sn–8Zn–3Bi solder alloy, the melting temperature of the rapid solidified alloy rose to close to that of the Sn–Zn eutectic alloy due to the extreme dissolution of Bi in Sn matrix. Meanwhile, the adverse effect on melting behavior due to Bi addition was decreased significantly. The interfacial intermetallic compound (IMC) layer of the solder/Cu joint was more compact and uniform. Rapid solidification process obviously depressed the formation and growth of the interfacial IMC during the high-temperature aging and improved the high-temperature stability of the Sn–8Zn–3Bi solder/Cu joint.

Surface modification of Q550 HSLA steel with Co–Fe–Cr composite coatings manufactured by fibre laser cladding

The surface of Q550 steel was modified with Co–Fe–Cr/WC composite coatings deposited by fibre laser cladding. Coating microstructure, elemental distribution, phase constitution and microhardness were investigated. Results showed that the composite coating was mainly composed of dendrites and eutectic lamellar of carbides. Phases of composite coatings were comprised of Co3Fe7, Fe–Cr, CFe15.1, Cr7C3, and M6C (M represents for W, Co and Fe). Owing to the dilution of base metal, large amount of Fe was transformed into molten pool. Martensite laths were easily formed at the front of fusion line due to the rapid solidification process in laser cladding. Plenty of M6C carbides were synthesised by in-situ reactions between W, Co, Fe and C. Carbides were prone to form eutectic lamellar with growth directions perpendicular to each other. The microhardness of coatings added with wt.50% WC was significantly increased to 668HV0.1, which was about 2.4 times that of Q550 steel.

Phase formations in low density high entropy alloys

Phase formations in high entropy alloys (HEAs) with at least two light elements in literature are predicted by CALPHAD (CALculation of PHAse Diagrams) thermodynamic calculations and the results are compared with experimental observations. The comparison suggests that the applicability of traditional CALPHAD calculations depends on the manufacturing processes of HEAs. Factors such as solute trapping, energies of defects need to be considered while predicting phases in HEAs prepared by non-equilibrium processes. The effects of light elements (Al, Ti, Si, alkali and alkaline earth metals) on the phase formations in HEAs are discussed. Especially, intermetallics predicted for Si-containing HEAs by traditional CALPHAD calculation can be suppressed in rapid solidification process, due to the solute trapping effect. Mg or other alkali and alkaline earth metals can lead to the formations of various intermetallics in HEAs prepared by conventional casting, but could be dissolved into solid solutions by non-equilibrium processes such as mechanical alloying. It is proposed that non-equilibrium processes may be an effective way to introduce light elements Si, alkali and alkaline earth metals into HEAs.