Interphase Spacing Research Articles

Synchrotron X-ray radiation study of retained austenite stability and the effect on fatigue behavior of bainitic railway steels

The effect of retained austenite stability on the fatigue performance was studied in bainitic rail steel subjected to different heat treatments, including normalizing at 900 °C and normalizing at 900 °C combined with tempering at 320 °C. The normalized steel had yield strength of 851 MPa, tensile strength of 1131 MPa, uniform elongation 7.0 %, and total elongation 17.5 %. Tempering the steel at 320 °C resulted in similar strength and plasticity compared to the normalized condition. However, the fatigue performance of steel was improved significantly after tempering. The ultimate fatigue stress (endurance limit) after 1 × 107 cycles was 671 MPa, which was 38 MPa greater than the normalized steel. Microstructural analysis revealed similarities between normalized and tempered matrices at crystallographic interfaces and grain boundaries. Synchrotron X-ray radiation indicated that the initial retained austenite content was ∼15.5 % in both normalized and tempered conditions. However, at tensile stress of 800 MPa, the amount of retained austenite transformed in the normalized steel was ∼4 %, which was twice that of the tempered steel. The enhancement of retained austenite stability was an important aspect in improving the fatigue performance. The analysis of interphase spacing showed that the strain in the normalized matrix synchronized with the retained austenite, and the stress was concentrated in the retained austenite, which promoted transformation of retained austenite. The matrix of tempered steel was strained first, which prevented stress concentration in the retained austenite and rendered the retained austenite stable.

An additively manufactured near-eutectic Al-Ce-Ni-Mn-Zr alloy with high creep resistance

A new additively manufactured (AM) Al-7.5Ce-4.5Ni-0.4Mn-0.7Zr (wt.%) near-eutectic alloy is reported, which shows unprecedented creep resistance up to 400 °C (a homologous temperature of 0.72). The eutectic solidification microstructure comprises ∼ 27 vol% of coarsening-resistant second phase network with an ultrafine (<100 nm) inter-phase spacing. Both Mn and Zr contribute to creep resistance of the alloy. Small amount of Mn addition promotes selection of coarsening resistant phases without compromising the alloy processability. Zr not only improves hot-tearing resistance, but further enhances the second phase coarsening resistance resulting in improved creep resistance. Neutron diffraction performed during creep deformation reveals that the underlying mechanism for creep resistance in this alloy is impedance to dislocation motion stemming from the ultrafine eutectic solidification microstructure, whereas load transfer strengthening becomes less effective as the creep temperature increases. The second phase forms a continuous network in the as-fabricated condition, which is maintained during long-term creep at 300 °C. However, this network is fragmented into fine dispersoids at higher temperatures. It is proposed that the rate-limiting deformation mechanism at 300–400 °C is (i) dislocation climb for the alloy with fragmented second phase dispersoids and (ii) Orowan looping for the alloy with a continuous second phase network. The present design of an AM-processable multicomponent eutectic alloy with high creep resistance can be applied to other metallic systems exhibiting eutectic reactions, with expected extreme creep resistance.

Assessment of extremely low frequency electric and magnetic field of 400kV transmission lines and substations in Oman

The present study is centered on the investigation of longitudinal and lateral profiles of Extremely low-frequency Electromagnetic field experiments conducted at three distinct locations in Oman. The investigation encompasses findings obtained from 400kV transmission lines and substations. The analysis of electric and magnetic fields emitted by transmission lines was conducted using an EHP-50F E&amp;H field analyzer sensor set and an environmental meter. The measurements were conducted at 400kV transmission lines or to a distance of the insignificant field at the high-voltage substation, various components such as power transformers, CT (current transformers), PT (potential transformers), circuit breakers, reactors, earth switches, transport couplers, lightning arrestors, and other. This research in Oman marks the initial exploration of various factors pertaining to the Oman Electricity Transmission Company (OETC) and its technical standards, which are founded on optimal practices. These factors encompass the interphase spacing, the spacing between conductors within a bundle of twin conductors, the configuration of phases on a tower, the crosssectional area of conductors, and similar considerations. The proposed study aims to support the Sultanate of Oman's objective of establishing a high-quality smart transmission grid by 2030. This system will facilitate the efficient, reliable, and secure transfer and distribution of power, while also offering economic benefits. This research aims to cater to the needs of students and engineers by offering a highly practical approach that utilizes appropriate equipment. This article presents a systematic approach to the implementation of a smart transmission grid, incorporating the aforementioned attributes.

Making high-performance Al2O3-3YSZ ceramics with eutectic/polycrystalline structure by hot-pressing eutectic powders

In this communication, the Al2O3-3YSZ ceramic (AYC) was prepared by hot-pressing the eutectic powders. A continuous transition from raw powders mixture to synthesized eutectic powders to sintered eutectic bulk was traced and investigated. The AYC well preserves the structure and phase composition of the original Al2O3-3YSZ eutectic powders (AYEP), which possess a eutectic/polycrystalline composite structure. The average interphase spacing of eutectic region in AYC reaches 106 nm. The strength, toughness, and hardness of AYC specimen are as high as 768 ± 23 MPa, 8.17 ± 0.30 MPa m1/2, and 19.04 ± 0.45 GPa, which far surpasses other solidified and sintered Al2O3-based eutectic ceramics. The excellent mechanical properties can be attributed to the synergistic toughening mechanisms of intergranular/transgranular fracture, t-m phase transformation and residual stress. These results clearly demonstrate the feasibility of sintering eutectic powders method to fabricate Al2O3-based eutectic ceramics concurrent with fine structure and large dimension.

Equiaxed colony growth in the small undercooled Y2O3-doped Al2O3–ZrO2 eutectic ceramic melt

Al2O3/ZrO2 eutectic bulks (20 mm in diameter) with different Y2O3 contents (0–4.5 mol %) were grown using the conventional casting method. Variations in the microstructural characteristics and colony tip growth behavior caused by Y2O3 addition in the central equiaxed grain zone were investigated. As Y2O3 was added to the Al2O3/ZrO2 binary eutectic, except for a decrease in colony size, the colony tip radius reduced drastically and then increased slightly. Colony growth behavior can be well predicted using the Lipton–Glicksman–Kurz (LGK) model. A critical Y2O3 content (0.5–1.1 mol%) exists below which the eutectic colony growth is accelerated owing to the Y2O3 addition, but slowed down otherwise. However, the average interphase spacing and ZrO2 rod diameter inside the colonies varied with Y2O3 content, converse to the growth rate.

Regulation of Y2O3 addition on structure and properties of Al2O3/ZrO2(Y2O3) directionally solidified eutectic ceramic

Al2O3/ZrO2(Y2O3) directionally solidified eutectic ceramics (DSECs) with different Y2O3 addition were prepared. Three polymorphs of ZrO2: m-ZrO2, t-ZrO2 and c-ZrO2 appeared and disappeared in order with the increase of Y2O3 addition, the effects of Y2O3 addition amount on the mass fraction of ZrO2 polymorphs were quantitatively analyzed. The gradual decrease of area fraction and average size of colony was attributed to the increase of constitutional undercooling region, the variation of interphase spacing in the colonies was explained via selection mechanism of dendrite tip. The maximum hardness and fracture toughness were 18.35 GPa and 8.55 MPa m1/2 obtained at 9 mol% and 3 mol% Y2O3 addition, respectively. The hardness was determined by the mass fraction of m-ZrO2 phase, the area fraction of intercolony and the residual compressive stress, and the toughening mechanisms were composed of microcrack toughening, t-m transformation toughening and residual stress toughening.

Formation of Al2O3–GdAlO3 eutectic ceramics with a fine anisotropic structure in a flash event

AbstractAn Al2O3–GdAlO3 (GAP) composite with a fine eutectic microstructure was obtained through local melting and rapid solidification during a flash event. An AC field of 1 kV/cm at a frequency of 1 kHz was applied to a calcined Al2O3–GAP body at a furnace temperature of 1400°C to induce the flash event. The Al2O3–GAP body exhibited local melting through the current path. Solidification proceeded with the maximum cooling rate of about 140°C/s just after the power supply was turned off. Using this flash treatment, a refined eutectic structure with rodlike GAP phases in the Al2O3 matrix was obtained within 1 min. The interphase spacing of the obtained structure was approximately 170 nm, comparable with the finest Al2O3–GAP eutectic material obtained in previous studies. A flash event and subsequent rapid cooling can contribute to forming fine eutectic ceramics with a relatively low furnace temperature and short processing time.

A correlation of eutectic growth mechanism with resultant microhardness enhancement for undercooled Ni-47.7%Mo alloy

Liquid Ni-47.7 % Mo alloy was undercooled by 328 K (0.21 TE) and solidified under microgravity condition to investigate the correlation of microhardness enhancement with eutectic growth mechanism. The microstructural morphology transformed from (Ni) + NiMo lamellar eutectic to a duplex mixture of lamellar and anomalous eutectics, and finally into anomalous eutectic. With the increase of alloy undercooling, the eutectic interphase spacings decreased significantly, leading to a remarkable structural refinement and a conspicuous rise of Vickers hardness. The dependence of microhardness upon eutectic spacing was revealed for lamellar and anomalous eutectics, respectively.

Lamella-rod pattern transition and confinement effects during eutectic growth

We present an in situ experimental study of the lamellar-to-rod transition during directional solidification of a model eutectic transparent alloy –the succinonitrile-(d)camphor (SCN-DC) system. Coupled-growth patterns, and their morphological transformations upon changes of the pulling velocity V (3.5−70.0nms−1) were observed in real time in a series of flat-wall samples with different thicknesses δ from 15 (thin samples) to 350 µm (thick samples). At relatively large velocity, rod-like patterns formed systematically. In contrast, stable lamellae were observed at sufficiently low velocity, regardless of the sample thickness in the investigated range. In confined-geometry samples (δ=30−140 µm), lamellar patterns were stable over a large range of the interphase spacing λ at low V. The lamella in contact with the sample walls were stabilized against a propagative breakup, and the formation of rods was mediated by secondary instabilities such as a zigzag mode and a spatial-oscillation mode. In the bistable range, a hysteresis upon increasing or decreasing V was observed, as well as complex phenomena involving oscillatory, mixed and hybrid patterns. The (reverse) transition from rods to lamellae involved a rod elongation instability. A discussion on the elementary mechanisms that determine the lamellar-rod transition, and their dependence on λ and V is initiated.

Impact of External Pressure on the Performance and Expansion of State-of-the-Art and Next-Generation Battery Materials Using Operando Dilatometry

The demand for higher energy densities in lithium-ion batteries leads to the use of next-generation anode materials, e.g. lithium metal or silicon anodes, in order to achieve those energy targets. When comparing with today’s state-of-the-art (SoA) electrode materials, one downside of the increased energy density of the next-generation materials is the drastically increased volume changes during formation and cycling. These next-generation materials exhibit volume changes theoretically up to ~300% for silicon, for example, and ~4.9 µm per mAh cm-2 for lithium metal [1]. The volumetric changes can favor aging mechanisms and increase the electrochemical/mechanical interactions inside the cells.In this work, the interactions between (in)homogeneous mechanical pressure on the cell and volume expansion are studied for SoA materials. In addition, the volumetric changes of next-generation materials are examined and compared with SoA materials. The influence of the volume changes, primarily of the anode, on the electrochemical processes occurring in the cell is investigated using a dilation sensitive test set-up [2] based on the system reported by Sauerteig et al. [3], but with the addition of a reference electrode which allows the anode and cathode potentials to be distinguished.During initial investigations of graphite vs. NMC 111 [2], an irreversible thickness change of 8% occurs in the first cycle (see Figure 1 (a)) due to formation of the solid electrolyte interphase (SEI), particle rearrangement, delamination and graphene layer spacing in the graphite anode. In the following cycles, the irreversible thickness change decreases and a reversible thickness change between 8% - 10% is observed. Inhomogeneous pressure distribution as well as high applied pressures accelerate aging of the electrodes significantly. During CCCV charging, the anode potential drops below 0 V, causing lithium plating at an earlier state of charge for the inhomogeneous pressure distribution compared to the homogeneous pressure distribution due to localized exchange currents.In contrary to the results of SoA-material, the cycling of lithium metal reveals high volumetric changes during stripping and plating of around 6 µm per mAh cm-2 (see Figure 1 (b)) [4]. The high expansion can be explained on the one hand by the reversible dilation of the lithium metal ions and on the other hand by irreversible dilation due to the formation of the SEI and ‘dead lithium’. By applying high pressure, the irreversible dilation can be suppressed especially at high current rates. Furthermore, the choice of the electrolyte has an impact on the irreversible dilation of the lithium metal and hence the performance during cycling. With the use of a carbonate based electrolyte the irreversible dilation is enhanced and the cell performance is decreased compared to an ether based electrolyte (see Figure 1 (b)).

Microstructures and mechanical properties of V–V3Si eutectic composites

Abstract V–V3Si eutectic alloys were directionally solidified in a high temperature optical floating zone furnace. Depending on the solidification conditions, several microstructures were observed, such as well-aligned broken lamellar, fibrous, or cellular microstructures. The interphase spacings increased with decreasing solidification rates in agreement with the Jackson-Hunt theory. The mechanical properties of the individual phases were investigated by nanoindentation. It was found that the modulus and nanoindentation hardness of V3Si are 214 and 13.8 GPa, respectively, and those of the V solid-solution are 165 and 3.4 GPa, respectively. The high-temperature strength was examined by tensile testing at elevated temperatures. Preliminary results show that the ductile-to-brittle transition temperature is about 800 °C for this composite, and its strength is significantly higher than conventional V solid-solution alloys.

Mechanical properties and microstructure of directionally solidified Fe-4.25%C eutectic alloy

An analysis of mechanical properties of directionally solidified Fe-4.25%C eutectic alloy and its microstructure and fracture mechanisms is presented. Tests were conducted for macro-samples made with different growth rates. Strength tests were performed with the use of micro-samples taken from different cross-sections of macro-samples. Fe-4.25%C eutectic alloy finds application mainly in construction of structures exposed to abrasion under significant compression loads. As specified by the metastable phase diagram, Fe–Fe3C, the white eutectic or austenitic (γ) iron carbide Fe3C can be formed. The type of obtained morphology depends mostly on the temperature gradient G, the chemical composition, and growth rate, which has a substantial impact on the morphology of the γ-Fe3C eutectic. As earlier tests have proven, interphase spacing λ decreases as grow rate v increases, which improves fine-graining of the alloy. We show that directional solidification with controlled growth rate improves mechanical properties of the Fe-4.25%C eutectic alloy, enabling its use as a structural element in a significantly expanded range of applications.

Boundary Diffusion With a Strong Kinetic Effect on Eutectic Growth

The atomic attachment pathway for boundary diffusion is different from that of volume diffusion, thus leading to a stronger kinetic effect. However, it remains challenging to quantitatively determine the kinetic part from the total undercooling. In this article, the kinetic contribution is incorporated into the boundary diffusion model, and the relationships between the growth rate, interphase spacing, and undercooling are obtained, by which the kinetic effect is examined. Moreover, a polymeric eutectic system composed of polyethylene glycol 6000 and 1, 3, 5 - trioxane was chosen to check the kinetic undercooling during the phase transition. This showed that the eutectic grows via boundary diffusion and that kinetic undercooling takes up a majority of the total driving force. Our results can provide in-depth insight into the kinetic contribution during a boundary-diffusion controlled eutectic transformation.

Effect of cooling rate on microstructure and microhardness of hypereutectic Al–Ni alloy

High solidification cooling rates during unsteady-state conditions of solidification of Al-based alloys can induce different microstructural length scales or metastable phases, leading to improved properties. The present study aims to characterize the microstructural arrangement of the hypereutectic Al–8 wt%Ni alloy, unidirectionally solidified in unsteady-state heat flow conditions, examining the influence of the cooling rate in the development of the Al–Al3Ni eutectic and the primary phase. A columnar-to-equiaxed macrostructural transition is shown to occur at a solidification cooling rate (Ṫ) of about 4.8 °C/s, with different microstructures associated with each morphological zone. The observation of microstructures of hypoeutectic, eutectic and hypereutectic Al–Ni alloys, has permitted an asymmetric coupled zone diagram to be proposed. The microstructural interphase spacings of the Al–8 wt%Ni alloy are experimentally determined and correlated to Ṫ, and the Vickers microhardness (HV) is shown to decrease with the increase in such spacings. The higher experimental HV profile of the examined hypereutectic alloy as compared to that of the eutectic Al–Ni alloy is attributed to the formation of a supersaturated solid solution of Ni in α-Al.

Microstructural transition of Nb-Si based alloy during directional solidification upon abruptly decreasing withdrawal rate

Multi-element Nb-Si based alloys are directionally solidified upon abruptly decreasing withdrawal rate from 100 to 10 μm/s and at the constant withdrawal rate of 10 μm/s respectively. With increasing solidification distance after decreasing withdrawal rate, the volume fraction of primary γ-(Nb, X)5Si3 increase from 6.7% to 13.9% firstly and finally decreases to 10.5%, which is similar with the variation in Si concentration of the alloy. On the other hand, the volume fraction of γ-(Nb, X)5Si3 in eutectic cells decreases from 36.7% to 28.5% gradually after abruptly deceasing withdrawal rate. After abruptly decreasing withdrawal rate, the average size of primary γ-(Nb, X)5Si3 increases from 31.1 to 58.0 μm, and the average size of eutectic cells increases from 42.5 μm to 128.2 μm, and the average interphase spacing increases from 1.9 μm to 4.4 μm. The phase proportion, microstructural sizes, hardness and elastic modulus of constituent phases in Nb-Si based alloys directionally solidified upon abruptly decreasing withdrawal rate are similar with that of the alloy at a constant withdrawal rate of 10 μm/s. The average hardness and elastic modulus for NbSS are about 6 and 150 GPa respectively, for γ-(Nb, X)5Si3 are 19 and 280 GPa respectively. However, the results of XRD and EBSD show that the coupling degree of eutectics obtained by abruptly decreasing withdrawal rate is better than that at the constant withdrawal rate of 10 μm/s, which indicates that the method of abruptly decreasing withdrawal rate can modify the microstructure.

Multi-Step Crystallization of Self-Organized Spiral Eutectics.

A method for the solidification of metallic alloys involving spiral self-organization is presented as a new strategy for producing large-area chiral patterns with emergent structural and optical properties, with attention to the underlying mechanism and dynamics. This study reports the discovery of a new growth mode for metastable, two-phase spiral patterns from a liquid metal. Crystallization proceeds via a non-classical, two-step pathway consisting of the initial formation of a polytetrahedral seed crystal, followed by ordering of two solid phases that nucleate heterogeneously on the seed and grow in a strongly coupled fashion. Crystallographic defects within the seed provide a template for spiral self-organization. These observations demonstrate the ubiquity of defect-mediated growth in multi-phase materials and establish a pathway toward bottom-up synthesis of chiral materials with an inter-phase spacing comparable to the wavelength of infrared light. Given that liquids often possess polytetrahedral short-range order, our results are applicable to many systems undergoing multi-step crystallization.

Effect of Microstructure Features on the Corrosion Behavior of the Sn-2.1 wt%Mg Solder Alloy

The Sn–Mg eutectic alloy is a potential replacement for the traditional Sn-38.1 wt%Pb solder alloy, since lead has been banned because of its risk to human health and the environment. However, studies in the literature related to the Sn-2.1 wt%Mg alloy are restricted to mechanical, electrical and thermal properties. Therefore, this study aims to evaluate the corrosion behavior of this alloy, as a function of the microstructural arrangement obtained from directional solidification performed in a transient heat extraction regime. The thermal solidification parameters (solidification growth rate and cooling rate) were determined along the length of the casting for correlations with microstructural features. The resulting microstructure was characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy and X-ray diffraction. Subsequently, the electrochemical impedance spectroscopy, linear polarization, equivalent circuit and evolution of hydrogen release analyzes were performed to evaluate the corrosion behavior of the samples in a 0.5 M NaCl solution at 25 °C. Interphase spacing and Mg2Sn fraction were found to influence the corrosion behavior, showing higher corrosion resistance for a more refined microstructure.

Analysis on the growth and growth-dependent microhardness of Ni3Sn4 intermetallic compound phase in directionally solidified Sn–Ni alloy

The growth morphologies and growth-dependent microhardness of the Ni3Sn4 intermetallic phases were investigated in directionally solidified Sn-15at.%Ni alloy through acid corrosion. To better characterize the growth of Ni3Sn4 phase, the dependences of the inter-phase spacing λ and thickness l on growth velocity v are also presented. The non-linear fit of the experimental results shows λ = 141.93v−0.19 and l = 43.45v−0.32. Comparison between the models describing the variation of primary dendrite arm spacing λ1 as a function of growth velocity and the experiments shows that these models are not appropriate for describing the growth of solidified Ni3Sn4 phase with faceted morphology. In addition, the variation of the microhardness (Hv) of Ni3Sn4 phase on the inter-phase spacing of it λ was measured as a function of growth velocity. The obvious dependence of microhardness on this solidification processing parameter shows that the microhardness (Hv) of Ni3Sn4 phase is obviously influenced by the growth mechanism of Ni3Sn4 phase in this Sn–Ni alloy. The local morphology of Ni3Sn4 phase shows that this lath-shaped phase is composed of sub-structures with faceted morphology. In addition, not only the faceted to non-faceted transition in growth morphology, but also the variation in morphology of the faceted sub-structure with increasing growth velocity can be confirmed through both directionally solidified microstructures with/without acid corrosion. This indicates that the variation in the sub-structure can lead to the change in growth morphology of Ni3Sn4 phase, thus the dependence of Hv on the growth velocity.

Sn-Mg lead-free solder alloy: Effect of solidification thermal parameters on microstructural features and microhardness

In the last decade, several studies have been developed on lead-free alloys as potential candidates to replace Sn–Pb alloys in soldering processes. Sn–Mg alloys arise as promising alternatives due to characteristics such as low melting point, low coefficient of thermal expansion, suitable mechanical properties, electrical resistivity and low cost of Mg as an alloying element. However, the literature is scarce on studies relating the microstructure features of unsteady state cooling conditions during solidification to the resulting properties of Sn–Mg alloys. It is of the utmost importance to know the influence of the solidification cooling rate on the representative length scale of the alloy microstructure, since it varies from about 0.4 to 8 °C s−1 in the soldering process. In the present study the Sn 2.1 wt%Mg eutectic alloy is solidified under unsteady state conditions over a nickel substrate for a range of solidification cooling rates from 0.5 to 12 °C s−1. The microstructure is shown to be formed by a mixture of β–Sn and fibrous Mg2Sn intermetallics (IMC) and an experimental growth law is proposed relating the interphase spacing between Mg2Sn fibers (λF) and the cooling rate. With the decrease in the cooling rate a pronounced decrease in the Mg2Sn IMC fraction is shown to occur; from about 46% to 23%. Consequently, hardness, that is shown to depend on both λF and Mg2Sn fraction, decreases significantly with the decrease in the solidification cooling rate.

Microstructural evolution of supra‐nanostructure Al2O3/ZrO2 eutectic powders by combustion synthesis‐spray cooling

AbstractEutectic powders with fine microstructure are difficult to synthesize by crushing eutectic bulk because of the damage of crystal structure and the introduction of milling media during preparation process. In this work, a novel combustion synthesis‐spray cooling (CSSC) method is developed to fabricate supra‐nanostructure Al2O3/ZrO2 eutectic powders. CSSC is a kind of self‐heating technique, which simplifies operations, reduces costs and supplies ultra‐high cooling rate. The phase composition and the microstructural evolution are investigated using experiment studies and ANSYS simulation. During the process, the t‐ZrO2 are stabilized at room temperature because of the solubility of Al2O3 in ZrO2. The ultra‐high cooling rate greatly refined eutectic structure. Although the eutectic structure coarsens with increases in particle size, the interphase spacing of all particles reaches supra nanoscale. The work provides a route for preparing supra‐nanostructure Al2O3/ZrO2 eutectic powders and for better understanding the microstructural evolution.