Search

A well-designed experimental method has been presented to in-situ visualize the amorphous-to-crystalline phase transformation of two glass formers at the atomic-length scale in the supercooled liquid region using a high voltage electron microscopy (HVEM). Analysis of the HVEM high-resolution images, supported by the in-situ synchrotron diffraction, further confirms previous observations of distinctively different crystallization pathways in the two Zr Cu Al alloys. Moreover, the HVEM results illustrate that isolated distributed nanocrystals with mutual orientation easily grew up from the Zr56Cu36Al8, an average glass former, which follows a classical crystallization pathway; while density population poorly ordered atomic clusters with large misorientation suspended the growth in the Zr46Cu46Al8, a good glass former, which might follow an unusual crystallization pathway. In addition, in-situ synchrotron diffraction measurements confirm that the Zr56Cu36Al8 alloy finally crystallized into an extended structure, in contrast, the final crystalline product of Zr46Cu46Al8 alloy possesses a damped structure. Our study provides a detailed microscopic understanding of the crystallization behaviors in the supercooled liquids, showing that the density population nucleation site with large misorientation and the confined final crystalline structure contribute to the stability of Zr Cu Al supercooled liquids.

Crystallization of Zr-Cu-Al-Ag bulk metallic glasses containing quenched-in crystals was studied by time-resolved in-situ neutron diffraction during isothermal annealing in the supercooled liquid region. The quenched-in crystals grew slowly at the beginning, indicating good thermal stability of the Zr-Cu-Al-Ag metallic glass alloys. The neutron diffraction results demonstrated that crystallization in Zr-Cu-Al-Ag metallic glasses was triggered by quenched-in crystals, rather than traditional heterogeneous nucleation mechanisms such as atomic-level impurities, container wall or surface oxides.

Plants have evolved diverse strategies to ensure their survival and regeneration in specific environments. Although temperature and soil moisture control seed dormancy, most studies have concentrated on temperature and little is known about the influence of moisture for species in the arid region. Responses of seed dormancy and germination of Agriophyllum squarrosum (Amaranthaceae), a pioneer and dominant species in Mu Us Sandland in northern China, to variations in soil moisture and temperature were examined. Our study showed that (1) freshly harvested seeds were in non-deep physiological (conditional) dormancy; (2) seeds in the soil exhibited dormancy cycling being non-dormant in spring and dormant from summer to autumn; (3) dry conditions at cold or warm temperatures alleviated dormancy; (4) germination was promoted by wetting-drying cycles; and (5) dormancy was induced by warm temperature (15/25°) and particularly low soil moisture less than 14.0%. The seasonal pattern of seed dormancy/germination was regulated by seasonal rainfall and soil temperature. At the same time, a range of conditions enable dormancy break and germination regardless of soil moisture conditions allowing the species to persist in an unpredictable environment.

We have studied the crystallization kinetics of Ca40Mg25Cu35 bulk metallic glass under isothermal annealing in the supercooled liquid region, with and without applied stress, by in-situ high-energy synchrotron x-ray diffraction. We observed that without stress, crystallization started from the very beginning of isothermal annealing. However, when a uniaxial compressive stress was applied during isothermal annealing, no change in atomic order was observed at the beginning. Thereafter, the glassy structure relaxed, but still without noticeable evidence of crystallization. It is suggested that the applied compressive stress restricted the atomic mobility, which suppressed crystallization.

Crystallization kinetics of a series of Zr-Cu-Al bulk metallic glasses with different glass-forming abilities were systematically investigated using differential scanning calorimetry, time-resolved neutron diffraction, and high-resolution transmission electron microscopy. Experimental results revealed a transition of classic-to-avalanche types of nucleation for the Zr-Cu-Al glass alloys upon isothermal annealing. The classic mode is characterized by a continuous nucleation and growth of Zr2Cu-type crystals. The avalanche mode features an abrupt formation of massive Cu10Zr7-type precipitates with a complex, low-symmetry crystalline structure. Calorimetric measurements identified an anomalous exothermic peak prior to crystallization for metallic glasses with the Cu10Zr7-type crystalline phase. The formation of a metastable amorphous/semi-crystalline phase was observed using the electron microscopy prior to crystallization for glasses with an anomalous exothermic event. Our results indicate that the formation of a metastable phase might lower the free energy barrier of the crystallization and trigger an avalanche type of nucleation. Neutron diffraction pair distribution function analysis suggests that the easy connection of short-range clusters at the medium-range length scale would favor the avalanche nucleation.

Seed germination is a key life‐history stage of halophytes. Most studies on seed germination of halophytes have focused on the effects of a single salt, while little information is available on the effects of mixed salt in the natural habitat. Due to the contribution of multiple ions in saline soil, we hypothesized that the effect of mixed salt on seed germination will differ from that of a single salt and the mechanism of how germination is affected will differ as well. The effects of mixed salt and NaCl on germination, water imbibition, and ionic concentrations of seeds of Suaeda salsa (L.) Pall. were compared at various salinity levels. Germination percentage (GP) and rate (GR) decreased with increasing salinity level, regardless of salt type. There was no difference in GP or GR between mixed salt and NaCl when the salinity level was below 20 dS m⁻¹. Above 20 dS m⁻¹, GP and GR in NaCl were lower than those in mixed salt. At the same salinity level, Na⁺ concentration in seeds was higher in NaCl than that in mixed salt, but the reverse was true for Ca²⁺ and Mg²⁺ concentrations. Imbibition rate for seeds in NaCl was lower than that in mixed salt at the same salinity level. Addition of Ca²⁺ and Mg²⁺ alleviated the inhibition of NaCl on seed germination. In conclusion, our results suggest that the effects of soil salts and NaCl on seed germination are different, and using NaCl instead of soil salt might not be realistic to show the effect of saline stress on seed germination of halophytes in the natural habitat.

Thermal behaviors and structures of three Zr-based binary glass formers, Zr 50 Cu 50 , Zr 64 Cu 36 and Zr 64 Ni 36 , were investigated and compared using differential scanning calorimetry (DSC), transmission electron microscopy (TEM), high energy X-ray diffraction (XRD) and small angle X-ray scattering (SAXS). The high energy XRD results show that the bulk glass former Zr 50 Cu 50 has a denser atomic packing efficiency and reduced medium-range order than those of marginal glass formers Zr 64 Cu 36 and Zr 64 Ni 36 . Based on TEM observations for the samples after heat treatment at 10 K above their crystallization onset temperatures, the number density of crystals for Zr 50 Cu 50 was estimated to be 10 23 –10 24 m −3 , which was four-orders higher than that in Zr 64 Cu 36 and Zr 64 Ni 36 metallic glasses. SAXS results indicate that Zr 50 Cu 50 has higher degree of nanoscale inhomogeneities than those in Zr 64 Cu 36 and Zr 64 Ni 36 at as-cast state. The observed multiscale structures are discussed in terms of the phase stability and glass-forming ability of Zr-based binary glass formers.

In the comments on “Structure origin of a transition of classic-to-avalanche nucleation in Zr-Cu-Al bulk metallic glasses, Acta Materialia, 149, 108 (2018)”, Altounian et al. argued that the avalanche type of nucleation observed in our paper is due to the presence of oxygen and other impurities. In this letter, we show that the amount of oxygen in our bulk metallic glass samples is far below the level found in ribbon samples that would trigger an explosive nucleation event. Rather, the avalanche type of nucleation reported in our study of Zr-Cu-Al bulk metallic glass is a compositional effect, where the elemental compositions were systematically varied leading to different crystalline phases.

Radix pseudostellariae is a perennial tonic medicinal plant, with high medicinal value. However, consecutive monoculture of this plant in the same field results in serious decrease in both yield and quality. In this study, a 3-year field experiment was performed to identify the inhibitory effect of growth caused by prolonged monoculture of R. pseudostellariae. DGGE analysis was used to explore the shifts in the structure and diversity of soil Fusarium and Pseudomonas communities along a 3-year gradient of monoculture. The results demonstrated that extended monoculture significantly boosted the diversity of Fusarium spp., but declined Pseudomonas spp. diversity. Quantitative PCR analysis showed a significant increase in Fusarium oxysporum, but a decline in Pseudomonas spp. Furthermore, abundance of antagonistic Pseudomonas spp. possessing antagonistic ability toward F. oxysporum significantly decreased in consecutively monocultured soils. Phenolic acid mixture at the same ratio as detected in soil could boost mycelial and sporular growth of pathogenic F. oxysporum while inhibit the growth of antagonistic Pseudomonas sp. CJ313. Moreover, plant bioassays showed that Pseudomonas sp. CJ313 had a good performance that protected R. pseudostellariae from infection by F. oxysporum. In conclusion, this study demonstrated that extended monoculture of R. pseudostellariae could alter the Fusarium and Pseudomonas communities in the plant rhizosphere, leading to relatively low level of antagonistic microorganisms, but with relatively high level of pathogenic microorganisms.

Simultaneous wide- and small-angle X-ray scattering (WAXS/SAXS) measurements were performed to investigate the low-temperature crystallization kinetics of Zr46Cu46Al8 bulk metallic glass during isothermal annealing. Quantitative analysis of the WAXS intensity data indicated that the crystallization process was dominated by steady-state homogeneous nucleation and three-dimensional diffusion-controlled growth. This observation was corroborated by quantitative analysis of the SAXS data showing a saturation of the growth of nanoscale crystallites. Comparison of simultaneously measured WAXS and SAXS data showed evidence of composition fluctuation prior to crystallization, suggesting that the crystallization was facilitated by nanoscale phase separation which established regions of favorable compositions catalyzing crystallization. The temperature dependence of crystallization mechanisms and the resulting microstructures in metallic glass are discussed.

An anomaly in differential scanning calorimetry has been reported in a number of metallic glass materials in which a broad exothermal peak was observed between the glass and crystallization temperatures. The mystery surrounding this calorimetric anomaly is epitomized by four decades long studies of Pd-Ni-P metallic glasses, arguably the best glass-forming alloys. Here we show, using a suite of in situ experimental techniques, that Pd-Ni-P alloys have a hidden amorphous phase in the supercooled liquid region. The anomalous exothermal peak is the consequence of a polyamorphous phase transition between two supercooled liquids, involving a change in the packing of atomic clusters over medium-range length scales as large as 18 Å. With further temperature increase, the alloy reenters the supercooled liquid phase, which forms the room-temperature glass phase on quenching. The outcome of this study raises a possibility to manipulate the structure and hence the stability of metallic glasses through heat treatment., An anomalous exothermal calorimetric peak far below crystallization temperatures in prototypical Pd-Ni-P glasses has been recognized for four decades. Here authors use neutron and high-energy X-ray diffraction to find evidence for a polyamorphous phase transition where medium-range order undergoes large changes while short-range order changes little.