In this paper, the amorphous Ce68Al10Cu20Co2 (atom %) alloy was in situ prepared by nanocalorimetry. The high cooling and heating rates accessible with this technique facilitate the suppression of crystallization on cooling and the identification of homogeneous nucleation. Different from the generally accepted notion that metallic glasses form just by avoiding crystallization, the role of nucleation and growth in the crystallization behavior of amorphous alloys is specified, allowing an access to the ideal metallic glass free of nuclei. Local atomic configurations are fundamentally significant to unravel the glass forming ability (GFA) and phase transitions in metallic glasses. For this reason, isothermal annealing near Tg from 0.001 s to 25,000 s following quenching becomes the strategy to tune local atomic configurations and facilitate an amorphous alloy, a mixed glassy-nanocrystalline state, and a crystalline sample successively. On the basis of the evolution of crystallization enthalpy and overall latent heat on reheating, we quantify the underlying mechanism for the isothermal nucleation and crystallization of amorphous alloys. With Johnson–Mehl–Avrami method, it is demonstrated that the coexistence of homogeneous and heterogeneous nucleation contributes to the isothermal crystallization of glass. Heterogeneous rather than homogeneous nucleation dominates the isothermal crystallization of the undercooled liquid. For the mixed glassy-nanocrystalline structure, an extraordinary kinetic stability of the residual glass is validated, which is ascribed to the denser packed interface between amorphous phase and ordered nanocrystals. Tailoring the amorphous structure by nanocalorimetry permits new insights into unraveling GFA and the mechanism that correlates local atomic configurations and phase transitions in metallic glasses.Keywords: crystallization; glass forming ability; nanocalorimetry; Nucleation;

•Li ion doped ZnO was used as cathode buffer in the inverted P3HT:PCBM solar cells.•Li ion doping increased the mobility of ZnO film thus improved the Jsc and FF.•i-PSC using Li ion doped ZnO as cathode buffer resulted in ∼30% enhancement in PCE.We have proposed an approach to improve the photovoltaic performance of inverted polymer solar cells (i-PSC) using lithium ion doped ZnO (LiZnO) as cathode buffer layer (CBL). The LiZnO CBL was prepared using the diffusion technique, performed by inducing the Li ion of 8-hydroxyquinolatolithium (Liq) to diffuse into ZnO film through annealing the bi-layer ZnO/Liq film. Doping concentration of Li ion was controlled by using various thickness of Liq film and annealing temperature. Based on LiZnO CBL, the poly (3-hexylthiophene) [6,6]:-phenyl C61-butyric acid methyl ester (P3HT:PCBM) i-PSC device possessed a optimal power conversion efficiency (PCE) of 4.07%, which was 30% improved than that of the device with neat ZnO as CBL. The enhancement of the device performance could be attributed to the enhanced electron mobility and better band matching of the LiZnO CBL. Our finding indicates that the LiZnO film fabricated with relatively low temperature treatment has great potential for high-performance i-PSCs.In this paper, the photovoltaic performance of the inverted polymer solar cell was significantly improved by using Li ion doped ZnO as cathode buffer layer.

•Fracture location variation with temperature and stress was revealed.•The carbon migration became more serious with increasing temperature.•Carbon-depleted zone determined fatigue life at elevated temperature.This paper studies carbon migration in the welds between 9% Cr steel and CrMoV steel during high-cycle fatigue (HCF) test at various temperatures. The dissimilar joints were fabricated using a narrow gap submerged arc welding (NG-SAW) technique. The HCF tests were carried at room temperature (RT), 400 °C and 470 °C. It was observed that the specimens tested at RT and 400 °C fractured at the CrMoV side while the specimens tested at 470 °C fractured at the 9% Cr fusion line. The microstructure of the 9% Cr fusion zone showed that the fracture occurred in a carbon-depleted zone. In addition, the micro-hardness test revealed that the hardness in the carbon-depleted zone was lower than at other locations of the welds while the hardness in the carbon-enriched zone was the highest in the whole welded joint. It is concluded that the carbon migration at elevated temperatures is considered as a very important factor in high stress HCF performance.

•This is the first overview on the application of nanocalorimetry in metals.•Phase transitions both in rapid heating and cooling processes are reviewed.•Related researches involving amorphous and crystalline materials are reviewed.Nanocalorimetry is profound in studying phase transitions in metallic materials for its ultrafast scanning rates and ultrahigh sensitivity compared with conventional calorimetric methods. This review illustrates recent findings in phase transitions and nucleation in metals employing nanocalorimetry. The melting behavior of nanoparticles was summarized. Furthermore, the glass transition, crystallization and martensite–austenite transformation were reviewed. Attributing to the ultrahigh sensitivity, small thermal changes in the solid reaction of the thin films were possible to be captured, which helped to understand its transformation mechanism. In rapid cooling processes, the nucleation of thin films and single droplets was conducted under controllable conditions, and the undercooling evolution was quantitatively investigated with different nucleation models. Moreover, the internal structures of samples after nanocalorimetric measurements were characterized using different methods. In summary, nanocalorimetry opens up a new field to reveal phase transitions in metallic samples, making it possible to compare the theoretical estimates and experimental results.

In the present work, the creep behavior and microstructure before and after creep test of the dissimilar modified 9Cr–1Mo steels welded joint were investigated in detail. Firstly, creep tests were carried out at different stress values/levels under the temperature of 538 °C and 500 °C, and then the power law equation of steady-state stage were achieved based on the results. The microstructure of welded joint were characterized in detail, the tempered martensite structure is the typical microstructure of whole welded joint, and the δ-ferrite phase is observed in heated affect zone (HAZ) and the base metal without Co and B elements (BM1). A soft zone called over-tempering zone (OTZ) occurred in both HAZs, which was determined by micro-hardness measurement, due to the lower dislocation density and coarsened M23C6 carbides of this zone. The fracture location is in OTZ adjacent to BM1 side determined by microstructure, micro-hardness and geometric calculation. Lots of voids are observed along the triple grain boundaries associated with large second phase particles after creep rupture. The stress concentration caused by the coarsened carbides at the triple boundaries and softening matrix are considered as the main factor of creep voids occurring in the OTZ of welded joint.

In this paper, the fracture toughness and the related microstructure characteristics of dissimilarly welded joint manufactured by advanced 9Cr and CrMoV steels were systematically investigated. The dissimilarly welded joint was fabricated by narrow gap submerged arc welding (NG-SAW) applying multi-layer and multi-pass technique. Fracture toughness, as one of the most important property to assess the reliability of welded joint, was studied for different regions including CrMoV base metal (CrMoV-BM), heat affected zone (HAZ) of CrMoV side (CrMoV-HAZ), weld metal (WM), heat affected zone of 9Cr side (9Cr-HAZ) and 9Cr base metal (9Cr-BM). It was found that the fracture toughness of CrMoV-BM, CrMoV-HAZ and WM was better than that of 9Cr-HAZ and 9Cr-BM. In order to illustrate these results, the microstructure of the whole welded joint was observed by optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM) detailedly. It was found that the fine high-temperature tempered martensite and bainite in WM, CrMoV-BM and CrMoV-HAZ contribute to the higher fracture toughness, while lower fracture toughness for 9Cr-BM and HAZ was caused by coarse tempered lath-martensite. Furthermore, the fracture morphology showed that ductile fracture occurred in WM and CrMoV side, while brittle fracture appeared in BM and HAZ of 9Cr side.

•NiCrMoV nuclear power rotor with heavy section was successfully welded by NG-SAW.•The fatigue properties of WM and HAZ were approximately equivalent to that of BM.•Granular bainite in WM and tempered martensite in HAZ contributed to properties.•NG-SAW exhibited a promising application in manufacture large size rotor.The fatigue property and microstructure characteristics of the welded joint for nuclear power rotor with heavy section, were systematically reported in this paper. The welded joint microstructure is inhomogeneous for NiCrMoV rotor made by narrow gap submerged arc welding (NG-SAW), which could affect the properties in different zones of welded joint. As one of the important indicator to evaluate the running performance of welded rotor, the fatigue crack propagation behavior of the base metal (BM), weld metal (WM) and heat affected zone (HAZ) was comparatively studied. It was found that the fatigue crack propagation threshold (ΔKth) of BM was higher than that of WM and HAZ as stress ratio (R) was 0.1, but ΔKth was very close to each other as R increased. The microstructure, revealed by an optimized corrosive process, was granular bainite in WM and tempered martensite in HAZ, leading to their approximately equivalent resistance of fatigue crack propagation with BM. The experimental results showed that fatigue properties of welded joint for NiCrMoV rotor with heavy section could meet the design requirement, and also push NG-SAW into manufacturing large size rotor.Graphical abstract

Highlights•Regular Fe3O4 octahedrons was successfully fabricated by dealloying technique.•Dealloying factors were compared and optimized to obtain Fe3O4 octahedrons.•The presently prepared Fe3O4 octahedrons revealed good magnetic properties.•The Fe3O4 octahedrons exhibited a promising application prospect.The dealloying processes of Al–15Fe (at.%) alloy ribbons consisting of two distinct phases of α-Al (Fe) and Al13Fe4 in NaOH solutions were investigated. The effects of NaOH solution concentration, dealloying temperature and time on the results were comparatively discussed. The as-dealloyed samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS). It was found that all of the three experimental conditions could affect the morphology of the as-dealloyed samples, and the influence of solution concentration was the crucial factor. The Al could be leached out from both the α-Al (Fe) and Al13Fe4 phases to obtain regular octahedral Fe3O4 under most of the experimental conditions except for that in the 0.5 mol L−1 NaOH solution at ambient temperature. In particular, the Fe3O4 from dealloying Al–15Fe ribbons in 5 mol L−1 NaOH solution for 48 h at ambient temperature shows a uniform octahedral structure (average edge length: 667 ± 158 nm) and special magnetic properties (saturation magnetization: 83.3 emu g−1, residual magnetization: 10.4 emu g−1 and coercive force: 256.9 Oe), implying its potential applications in magnetic fluid, information storage, etc.Graphical abstract

The existence of residual austenite in weld metal plays an important role in determining the properties and dimensional accuracy of welded rotors. An effective corrosive agent and the metallographic etching process were developed to clearly reveal the characteristics of residual austenite in the weld metal of a 9Cr1MoNbV welded rotor. Moreover, the details of the distribution, shape, length, length-to-width ratio, and the content of residual austenite were systematically characterized using the Image-Pro Plus image analysis software. The results revealed that the area fraction of residual austenite was approximately 6.3% in the observed weld seam; the average area, length, and length-to-width ratio of dispersed residual austenite were quantitatively evaluated to be (5.5 ± 0.1) μm2, (5.0 ± 0.1) μm, and (2.2 ± 0.1), respectively. The newly developed corrosive agent and etching method offer an appropriate approach to characterize residual austenite in the weld metal of welded rotors in detail.

Undercooling of Sn droplets in different atmospheres was studied by fast scanning calorimetry (FSC) at cooling rate of 1,000 K/s. It is found that the undercooling decreased with increasing partial pressure of oxygen. Randomly distributed SnO2 islands were observed to form on the droplet surface, which likely has promoted the heterogeneous surface nucleation. As the partial pressure of oxygen changes, the nucleation rate and growth of SnO2 led to different oxide islands, which resulted in various potential catalytic sites for the nucleation of the molten Sn droplet. The results showed that the nucleation process of the Sn droplets was sensitive to the solidification environment, and therefore the atmosphere should be taken into account in the study of the nucleation behavior of the single Sn droplets.

The chemical dealloying of bi-phase Al-35Ag alloy has been investigated within the parting limit. The dealloying of α-Al(Ag) and Ag2Al commenced simultaneously, and all α-Al(Ag) and part of Ag2Al were dealloyed, leaving residual Ag2Al to be dealloyed afterwards. The dealloying of the residual Ag2Al is associated with vacancy controlled mechanism and diffusion of Al atoms. It is revealed that the diffusions of the Al and Ag atoms during dealloying are significant. The Ag skeletons formed at the initial stage, and became coarsened gradually with a time dependence of d ∝ t2/5, illustrating the vital role of diffusion of Ag atoms.Highlights► Selective leaching of α-Al(Ag) and Ag2Al occurs simultaneously during dealloying. ► Diffusion of Al and vacancy controlled mechanism dominate the etching of Ag2Al. ► The coarsening of ligaments in NPS follows a time dependence of d ∝ t2/5.

•FSC was firstly utilized to simulate the laser soldering process.•Size-dependent undercooling for Sn3.5Ag droplets was in situ measured by FSC.•Solidification structure evolution of laser soldering process was simulated.•This research opens a new research approach in rapid solidification field.Fast scanning calorimetry (FSC) has a large scanning rate up to 105 K/s which makes it possible to simulate laser soldering. In this paper, the cooling rate and size dependence of undercooling for Sn3.5Ag droplets were studied. The undercooling increased slightly with increasing cooling rate. When the droplet size was smaller than 25 μm, undercooling increased dramatically. For investigating the solidification microstructure at high cooling rates, scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) were applied. In comparison with the bulk alloys, refined solidification structure instead of coarse β-Sn dendrites and eutectics exists in the droplets. Fine β-Sn dendrites were clearly observed in droplets of 300 μm and 500 μm. Eutectics containing Ag3Sn phase were also detected by SEM. However, the dendrites and eutectics were difficult to be distinguished in droplets of 25 μm and 50 μm, attributing to the higher cooling rate and larger undercooling.

The grain size of prior austenite has a distinct influence on the microstructure and final mechanical properties of steels. Thus, it is significant to clearly reveal the grain boundaries and therefore to precisely characterize the grain size of prior austenite. For NiCrMoV rotor steels quenched and tempered at high temperature, it is really difficult to display the grain boundaries of prior austenite clearly, which limits a further study on the correlation between the properties and the corresponding microstructure. In this paper, an effective etchant was put forward and further optimized. Experimental results indicated that this agent was effective to show the details of grain boundaries, which help analyze fatigue crack details along the propagation path. The optimized corrosion agent is successful to observe the microstructure characteristics and expected to help analyze the effect of microstructure for a further study on the mechanical properties of NiCrMoV rotor steels used in the field of nuclear power.

The crystallization process of Fe78Zr7B15 (at%) amorphous ribbon was investigated by X-ray diffraction (XRD), differential scanning calorimetry and scanning electron microscopy (SEM). The fully amorphous structure of as-quenched (Aq) ribbons was confirmed by XRD pattern. The saturation magnetization (Ms) and Curie temperature of the Aq ribbon were measured as 124.3 (A·m2)/kg and 305 °C with vibrating sample magnetometer (VSM), respectively. When the ribbons was annealed at 550 °C near the first onset temperature (Tx1 = 564.9 °C), the Ms was increased by 17 %, which was caused by the formation of a dual phase structure. The isothermal crystallization kinetics and crystallization mechanism of primary α-Fe phase in the dual phase structure were studied by Arrhenius and Johnson-Mehl-Avrami-Kolmogorov equations respectively. The results showed that the crystallization of α-Fe phase was a diffusion-controlled surface nucleation growth process, and the nucleation rate decreased with longer crystallization time.

The Sn3.0Ag0.5Cu (wt%) lead-free solder alloy is considered to be one of the most promising alternatives to replace the traditionally used Sn–Pb solders. This alloy composition possesses, however, some weaknesses, mainly as a result of its higher melting temperature compared to the eutectic Sn–Pb solders. Nanoparticles of Sn3.0Ag0.5Cu lead-free solder alloy nanoparticles were prepared by chemical reduction with NaBH4 as a reducing agent at room temperature. The melting temperature of the synthesized Sn3.0Ag0.5Cu alloy nanoparticles was determined by differential scanning calorimetry (DSC). The results showed that the calorimetric onset melting temperature of the Sn3.0Ag0.5Cu alloy nanoparticles could be as low as 200 °C, which was about 17 °C lower than that of the bulk alloy (217 °C). The field emission scanning electron microscopy (SEM) images of the as-prepared nanoparticles indicated that the major particle size of Sn3.0Ag0.5Cu nanoparticles is smaller than 50 nm. The structure and morphology of the nanoparticles were analyzed with high resolution transmission electron microscopy (HRTEM). The Ag3Sn and Sn phase were observed in the HRTEM images, which was in good agreement with the XRD results. These low melting temperature Sn3.0Ag0.5Cu alloy nanoparticles show a potential to manufacture high quality lead-free solders for electronic products.

The non-isothermal crystallization kinetics of amorphous Fe78Zr7B15 alloy is investigated to shed light on the crystallization mechanism. The crystallization of amorphous Fe78Zr7B15 alloy exhibits two distinct steps. The apparent activation energy of the first (Ea1) and second step crystallization (Ea2) are determined by Kissinger and Ozawa equations. The comparative value of Ea1 < Ea2 implies that the first step crystallization is easier to be occurred. The calculation based on Kissinger–Akahira–Sunose (KAS) model suggests that the local activation energy [E(x)] decreases with crystallized volume fraction (x). The Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation is extended to non-isothermal transformation to probe the transformation mechanism. The results show that the crystallization of the first and second step is dominated by low and three dimensional nucleation and growth, respectively. Both nucleation rates of these two steps increase firstly and then reduce.Research highlights► The crystallization kinetics of FeZrB amorphous alloy is investigated in continuous heating process. ► The first step crystallization is easier to be occurred than the second step. ► The crystallization of the first and second steps is dominated by low and three dimensional nucleation and growth, respectively. ► The nucleation rates of the first and second steps increase firstly and then reduce.

The Sn3.0Ag0.5Cu (wt.%) lead-free solder alloy is considered to be one of the most promising candidates to replace the traditionally used Sn–Pb solder. However, this alloy composition has some weaknesses, mainly as a result of its higher melting temperature compared to the eutectic Sn–Pb solder. In this paper, lead-free solder alloy nanoparticles of Sn3.0Ag0.5Cu were synthesized by chemical reduction with NaBH4 as reducing agent. The experimental results indicated that the major particle size of Sn3.0Ag0.5Cu nanoparticles was smaller than 100 nm. The melting and solidification properties of the Sn3.0Ag0.5Cu nanoparticles were studied by differential scanning calorimetry at different scanning rates. It was evidenced by the differential scanning calorimetry curves that the melting temperature of Sn3.0Ag0.5Cu nanoparticles was lower than that of the bulk alloy. In addition, the undercooling of the Sn3.0Ag0.5Cu nanoparticles was in the range of 82.0–88.5 °C at different cooling rates, which was much larger than that of the Sn3.0Ag0.5Cu micro-sized particles, showing stronger cooling rate dependence.

The traditional Sn–Pb eutectic solder alloys are being phased out from the electronics industry due to the toxicity of lead (Pb), leading to the development and implementation of lead-free solders. Sn3.5Ag lead-free solder alloy, considered to be one of the promising alternatives to replace the traditionally used Sn–Pb solder, however, still has some weaknesses, such as its higher melting temperature than that of the Sn–Pb solder alloy. A possible way to decrease the melting temperature of a solder alloy is to decrease the alloy particle size to the nanometer range. Sn3.5Ag nanoparticles with different size distribution were synthesized using chemical reduction method by applying NaBH4 as reduction agent. The melting properties of these synthesized nanoparticles were studied by differential scanning calorimetry (DSC), and size-dependent melting temperature depression of these nanoparticles has been observed. Gibbs–Thomson equation was used to analyze the size-dependent melting temperature property, giving a good prediction of the melting temperature depression for the Sn-based lead-free solder alloy nanoparticles.

The size-dependent solidification undercooling was investigated for single micro-sized particles of pure Sn employing differential scanning calorimeter (DSC). The particles were obtained from a solvent-encapsulation remelting and quenching (SERQ) process. Because of the basically unchanged spherical shape of the measured single particles during a series of continuous heating and cooling processes, it allows studying the independent effect of particle size on undercooling. Applying classical nucleation theory in conjunction with available thermodynamic data yields an increasing undercooling with decreasing particle size. The theoretical description is in good agreement with the experimental data.

The heterogeneous nucleation kinetics of a single undercooled tin droplet was measured at a cooling rate of 1 × 104 K/s by fast scanning non-adiabatic calorimetry. The nucleation rates were obtained by a statistical analysis of nearly 1000 identical events. Applying classical heterogeneous theory in conjunction with the available thermodynamic data, it was found that a single mechanism spherical cap model was capable of describing the experimental data. And the present study shows that fast scanning calorimetry allows studying nucleation in micron sized droplets and may open a way for detailed investigations of the influence of droplet size to discriminate surface and bulk nucleation.