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The high-manganese steels are important structural materials, owing to their excellent toughness at low temperatures. However, the microstructural causes for their unusual properties have not adequately been understood thus far. Here, we report a reversal relationship between impact toughness and grain size in a high-manganese steel and its unrevealed microscopic mechanisms, which result in an excellent low-temperature toughness of the steel. Our investigations show that with increasing grain size the impact toughness of the steel can be improved drastically, especially at low-temperatures. Advanced electron microscopy characterization reveals that the enhanced impact toughness of the coarse-grained steel is attributed to the twinning induced plasticity and transformation induced plasticity effects, which produce large quantities of deformation twins, ehcp-martensite and α′bcc-martensite. Inversely, in the fine-grained steels, the formation of deformation twins and martensite is significantly inhibited, leading to the decrease of impact toughness. Microstructural characterizations also indicate that ehcp-martensite becomes more stable than α′bcc-martensite with decreasing temperature, resulting in characteristic microstructures in the coarse-grained samples after impact deformation at liquid nitrogen temperature. In the coarse-grained samples under impact deformation at -80 °C, ehcp-martensite transformation, α′bcc-martensite transformation and deformation twinning all occur simultaneously, which greatly improves the toughness of the steel.

The crystallography of phase transformation is an important issue for studying metallic alloys. Here, we demonstrate an abnormal orientation relation between face-centered cubic (fcc) and hexagonal close-packed (hcp) phases due to the intersection of two ehcp-martensite variants in a high manganese steel. The corresponding crystallographic characteristics, including invariant line, habit plane and atomic steps, have been characterized by transmission electron microscopy and the quasi-O-line model. In addition, the models of phase transformation about the intersection are proposed based on transmission mechanisms of dislocations. Our findings enrich the theories of phase transformation and implicate the possibility to fabricate stronger and tougher steel.

Generally, the good combination of pre-deformation and aging can improve the mechanical strength of the Al–Cu–Li–Mg alloys. However, the effects of pre-deformation on competitive precipitation relationship and precipitation strengthening have not been clarified in detail in Al–Cu–Li–Mg alloys with high Mg. In the present study, the effects of pre-deformation level on the microstructure and mechanical properties of an Al–2.95Cu–1.55Li–0.57Mg–0.18Zr alloy have been investigated. It is found that the introduction of dislocation by 5% pre-deformation can facilitate the precipitation of new successive composite precipitates and T1 precipitates along the sub-grain boundaries or dislocations and inhibit the precipitation of dispersive GPB zones which is the main precipitates of the alloys without pre-deformation. The introduction of 5% pre-deformation can enhance the mechanical properties considerably. When the pre-deformation level increases from 5 to 15%, the number density of the successive composite precipitates and T1 precipitates increases, and the aspect ratio of T1 precipitates decreases. The decrease in T1 precipitate aspect ratio and the increment of the successive composite precipitates result in the reduction in precipitation strengthening. Therefore, the increase in pre-deformation level from 5 to 15% does not further improve the mechanical properties of the alloys, although the dislocation strengthening increases continuously.

The 2xxx series Al alloys have been widely used in aerospace industry owing to their high strength, good plasticity and superior formability. To ensure a good control of shape, the quenched alloy sheets require a small pre-deformation before artificial aging. However, this pre-deformation considerably deteriorates the mechanical strength of the Al–3.0Cu–1.8Mg–0.5Si (wt%) alloys due to the formation of unfavorable large-sized precipitates at dislocations. To tackle this issue, we designed a pre-aging process prior to the pre-deformation. The thermal–mechanical treatment, involving pre-aging, pre-deformation and subsequent aging, markedly enhanced the ultimate tensile strength up to 521 MPa compared to that (448 MPa) of the alloy without pre-aging. Microstructure characterization revealed that the fine precipitates (~ 2 nm) with a uniform dispersion were promoted within the Al matrix, which in turn partly suppressed the formation of the unfavorable large-sized precipitate (~ 100 nm). Our findings provide a new clue for designing stronger Al alloys with age-hardenability.

Pre-deformation before aging has been demonstrated to have a positive effect on the mechanical strength of the 7N01 alloy in our previous study, which is rather different from the general negative effects of pre-deformation on high-strength 7XXX aluminum alloys. In order to explain the strengthening mechanism relating to the positive effect, in the present study, the microstructure of the aged 7N01 alloy with different degrees of pre-deformation was investigated in detail using advanced electron microscopy techniques. Our results show that, without pre-deformation, the aged alloy is strengthened mainly by the η′ type of hardening precipitates. In contrast, with pre-deformation, the aged alloy is strengthened by the hierarchical microstructure consisting of the GP-η′ type of precipitates formed inside sub-grains, the ηp type of precipitates formed at small-angle boundaries, and the dislocation introduced by pre-deformation (residual work-hardening effect). By visualizing the distribution of the ηp precipitates through three-dimensional electron tomography, the 3D microstructures of dislocation cells are clearly revealed. Proper combinations of ηp precipitates, GP-η′ precipitates and residual dislocations in the alloy are responsible for the positive effect of pre-deformation on its mechanical properties.

Bulk nanocrystalline alloys usually possess enhanced properties than their coarse-grained counterparts. Here, first-principles calculations and aberration-corrected transmission electron microscope (TEM) were employed to investigate the atomic-scale deformation mechanism of Cu-based alloys. The effect of alloying element concentration and temperature-induced solute distribution on the unstable stacking fault energy (γusf), stable stacking fault energy (γisf) and unstable twin fault energy (γutf) were calculated using a Fermi–Dirac distribution of solutes for 42 binary Cu-X alloys. At medium temperature (>200 K) or low solute concentrations (

Owing to the excellent elastic properties and chemical stability, binary metal or light element borides, carbides and nitrides have been extensively applied as hard and low-compressible materials. Researchers are searching for harder materials all the time. Recently, the successful fabrication of nano-twinned cubic BN (Tian et al. Nature 493:385–388, 2013) and diamond (Huang et al. Nature 510:250–253, 2014) exhibiting superior properties than their twin-free counterparts allows an efficient way to be harder. From this point of view, the borides, carbides and nitrides may be stronger by introducing twins, whose formation tendency can be measured using stacking fault energies (SFEs). The lower the SFEs, the easier the formation of twins. In the present study, by means of first-principles calculations, we first calculated the fundamental elastic constants of forty-two borides, seventeen carbides and thirty-one nitrides, and their moduli, elastic anisotropy factors and bonding characters were accordingly derived. Then, the SFEs of the {111} glide system of twenty-seven compounds with the space group F $$\bar{4}$$ 3m or Fm $$\bar{3}$$ m were calculated. Based on the obtained elastic properties and SFEs, we find that (1) light element compounds usually exhibit superior elastic properties over the metal borides, carbides or nitrides; (2) the 5d transition-metal compounds (ReB2, WB, OsC, RuC, WC, OsN2, TaN and WN) possess comparable bulk modulus (B) with that of cBN (B = 363 GPa); (3) twins may form in ZrB, HfN, PtN, VN and ZrN, since their SFEs are lower or slightly higher than that of diamond (SFE = 277 mJ/m2). Our work can be used as a valuable database to compare these compounds.

Continuous SiC fibers have attracted increasing research interest for their significant application in aerospace and nuclear energy. Control of inhomogeneity and decrement of defects are crucial for improving the mechanical and high-temperature performance of SiC fibers. Thus an understanding of fine and detailed microstructural and microchemical distribution of SiC fibers is urgently needed. In the present work, the cross-sectional phase distributions, morphologies and defects of amorphous, non-stoichiometric and near-stoichiometric SiC fibers were well characterized. From surface to core, the amorphous and non-stoichiometric fibers exhibited uniform microstructures, but a gradient change along the radial direction was observed in the near-stoichiometric fiber. Nanosized SiC and turbostratic carbon (2–4 nm) were randomly distributed inside the amorphous fiber. A large amount of graphite surrounding SiC grains appears in the non-stoichiometric fiber which possess low strength and weak thermal stability. The near-stoichiometric fiber has transgranular fracturing, high strength and favorable thermal stability, and the graphite planes are turbostratic inside the fiber but straight on the surface. The results of both energy-dispersive X-ray spectrometry measurements and first-principles calculations demonstrate that Al atoms can occupy Si sites in the SiC grains. Our findings offer fine understanding for fabricating SiC fibers with superior properties.

The T1-precipitates and δ′/θ′/δ′ composite precipitates are the two primary strengthening precipitates and co-exist in the age-hardened 3rd generation Al Li alloys. The relative proportion of these two types of precipitates formed in the alloys is strongly dependent of the thermo-mechanical-treatment (TMT) processes applied. In the present study, using electron microscopy and properties characterization, the competitive precipitation behaviors of these two types of precipitates in an Al-4.0Cu-0.9Li-0.2Mg (wt.%) alloy, and the property of the alloy were investigated systematically in relation to TMT processes employed. It is shown that a 5% pre-deformation can promote the formation of T1 precipitates but depress the δ′/θ′/δ′ composite precipitates, which improves the strength but decreases the plasticity of the alloy significantly. A pre-natural-aging will promote the fine δ′/θ′/δ′ composite precipitates and T1 precipitates to form uniformly during subsequent artificial aging, improving the strength while maintaining good plasticity. A pre-artificial-aging at 180 °C will depress the formation of T1 precipitates, and greatly reduce the plasticity of the alloy. Our study indicates that when the T1 and the δ′/θ′/δ′ precipitates are equally developed, the alloy may achieve optimized properties.

Using thermo-mechanical processing, a heterogeneous microstructure with ultrafine grains and efficient hardening nanoprecipitates was introduced into a transformation induced plasticity (TRIP) steel with composition of Fe-12Mn-2Ni-Mo-Ti-(Al), such that the fabricated steel has an excellent combination of yield strength, ductility and thermal stability. It is shown that the Laves phase precipitates which hindered the ultrafine grains from coarsening were formed at high temperature, whereas the most efficient hardening nanoprecipitates inside these ultrafine grains were introduced at a lower temperature. The former precipitates are hexagonal (Fe,Mn)2(Mo,Ti) Laves phase containing Si and the later nanoprecipitates belong to the B2 phase. Keywords: TRIP steel, Ultrafine grains, Laves phase, Precipitation, Thermo-mechanical processing

In high-performance Al-Cu-Li alloys with low Li-content, δ′-precipitates exist often by enveloping the Guinier-Preston zones and θ′-precipitates, forming so-called composite phases or precipitates. Using atomic-resolution Scanning transmission electron microscopy and energy calculations, we studied these composite phases for their interface structures and microstructure evolution in relation with the mechanical property of the alloys. It is found that all the composite phases have similar interface structures in terms of Cu-Li bonding: interfacial Li-atoms are required to occupy the second nearest neighbor sites to interfacial Cu-atoms. Two types of relations, “anti-phase” and “in-phase” relationships, exist between the two sideward δ′-precipitates in the δ′/θ′/δ′ composite precipitates, depending on the number of Cu-layers contained in the inward θ′-precipitate. Furthermore, it is shown that an “anti-phase” composite precipitate has to become an “in-phase” one when the Cu-layers in the θ′-precipitate increase by growing thick from even to odd number, and vice versa. Since thickening of the θ′-precipitates in these composite precipitates has to be accomplished through a layer-by-layer mechanism, their growth involves the switching of interface relationships with significant structure modifications (until the sideward δ′-precipitate adapts to the final optimized interface structure). As such, the coarsening of these precipitates can generally be depressed upon thermal heating, leading to fine precipitate microstructure of the alloys. It is also demonstrated by first-principles calculations that formation of the composite phases greatly lowers the total system energy, owing to significant decrement of interfacial energy as the θ′/Al interfaces are replaced by the Al/δ′/θ′/δ′/Al interfaces.

Intrinsic stacking-fault energy is a critical parameter influencing the various mechanical performances of austenitic steels with high Mn concentrations. However, quantitative calculations of the stacking-fault energy (SFE) of the face-centered cubic (fcc) Fe, including the changes in concentrations and geometrical distribution of alloying atoms, cannot be obtained by using previous computation models. On the basis of the interaction energy model, we evaluated the effects of a single alloying atom (i.e., Mn, Al, Si, C and N), as well as its aggregates, including the Mn–X dimer and Mn2–X trimer (X = Al, Si, C and N) on the SFE of the fcc Fe via first-principle calculations. Given low concentrations (

The high mutation rates of infectious bronchitis virus (IBV) pose economic threats to the poultry industry. In order to track the genetic evolutionary of IBV isolates circulating in yellow chickens, we continued to conduct the genetic analyses of the structural genes S1, E, M, and N from 64 IBV isolates in southern China during 2009&ndash, 2017. The results showed that the dominant genotypes based on the four genes had changed when compared with those during 1985&ndash, 2008. Based on the S1 gene phylogenetic tree, LX4-type (GI-19) was the most dominant genotype, which was different from that during 1985&ndash, 2008. The second most dominant genotype was LDT3-A-type, but this genotype disappeared after 2012. New-type 1 (GVI-1) isolates showed increasing tendency and there were four aa (QKEP) located in the hypervariable region (HVR) III and one aa (S) insertion in all the New-type 1 isolates. Both the analyses of amino acid entropy and molecular evolutionary rate revealed that the variations from large to small were S1, E, M, and N. Purifying selection was detected in the S1, E, M, and N gene proteins, which was different from the positive selection during 1985&ndash, 2008. Six isolates were confirmed to be recombinants, possibly generated from a vaccine virus of the 4/91-type or LDT3-A-type and a circulating virus. The estimated times for the most recent common ancestors based on the S1, E, M, and N genes were the years of 1744, 1893, 1940, and 1945, respectively. Bayesian skyline analysis revealed a sharp decrease in genetic diversity of all the four structural genes after 2010 and since late 2015, the viral population rapidly rose. In conclusion, the IBVs circulating in southern China over the past decade have experienced a remarkable change in genetic diversity, dominant genotypes, and selection pressure, indicating the importance of permanent monitoring of circulating strains and the urgency for developing new vaccines to counteract the emerging LX4-type and New-type IBVs.

The localised corrosion behaviour and the microstructure in an Al-2.23Cu-0.96Li (wt%) alloy were systematically investigated by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The major strengthening precipitate in the peak-aged alloy is the Guinier-Preston-Bagaryatsky (GPB) zones which nucleate and grow in the Al directions, while the crystallographic corrosion appears and propagates along the {111}Al planes. At the nanometre scale, the features of crystallographic corrosion are made up of many step-like {100}Al-oriented sidewalls which are closely associated with the GPB zones. The GPB zones are unable to affect the overall propagation direction of the crystallographic corrosion, but is related to the formation of the nanoscale localised corrosion morphology of the crystallographic corrosion.

Guinier–Preston–Bagaryatsky (GPB) zones are the well-known strengthening precipitates of AlCuMg alloys formed upon thermal ageing. Here we report that when formed in AlCuLiMg alloys the GPB zones can change significantly in morphology and structure. It is shown that though they do still consist of Al, Cu and Mg elements fundamentally, the GPB zones in AlCuLiMg alloys have a rather different structure due to a featured Li-segregation at their interfaces with the matrix and possible Li-replacement of partial Mg atoms in the structure. As such the Li-containing GPB zones often develop from one-dimensional to quasi-two-dimensional precipitates.

Three types of symmetric (11 $$\bar{2}$$ 0) tilt low-angle grain boundaries (LAGBs) with array of basal, prismatic, and pyramidal edge full dislocations in pure Mg have been studied by using the improved Peierls–Nabarro model in combination with the generalized stacking fault energy curve. The results show that with decreasing distance between the dislocations in all the three types of tilt LAGBs, the stress and strain fields are gradually suppressed. The reduction extent of the stress and strain fields decreases from the prismatic to basal to pyramidal dislocations. The variation of dislocation line energy (DLE) for all tilt LAGBs is divided into three stages: DLE changes slightly and linearly when the distance is larger than 300 A, ~10%; DLE declines exponentially and quickly when the distance goes from 300 to 100 A, ~70%; and finally, the descent speed lowers when the distance is smaller than 100 A and the dislocation core energy is nearly half of the DLE. The grain boundary energy (GBE) decreases when the tilt angle of LAGB increases from 1° to 2° for all cases. The tilt LAGB consists of pyramidal dislocations always has the largest GBE, while that with array of prismatic dislocations has the smallest one in the whole range. The Peierls stress of dislocation in tilt LAGB is nearly unchanged, the same as that of single dislocation. This work is useful for further study of dissociated dislocation, solute segregation, precipitate nucleation in tilt LAGB and its interaction with single dislocations.

The chromized layer on carbon steels demonstrates high hardness and excellent wear-resistance. However, it usually suffers from its thin thickness and the softened underneath diffusion layer owing to the decarburization of the diffusion layer. In the present work, a double strengthened surface with a chromized layer and a N/C-rich diffusion layer was fabricated on a carbon steel by a nitro-chromizing process. The effect of chromizing time and cooling method on the microstructure and property of the nitro-chromized surface layers was investigated. The phase transformations occurring in the double strengthened surface were also studied in detail. Although it is difficult to improve the chromized layer, which consists of CrN, β-Cr2N and (Cr,Fe)7C3 phases, the microstructure of the diffusion layer with a thickness of more than 500 μm can be optimized to contain ultrafine martensite–bainite associated with nanometer-sized precipitates and retained austenite, leading to surface hardness of ~ 1000 HV or higher. The microstructure in the diffusion layer plays a key role in improving the property of the double strengthened surface.

The Al-Cu-Li-(Mg) alloy is a high-performance lightweight material strengthened by complex coexisting precipitates that form in the alloy upon thermal ageing. Using high-resolution (scanning) transmission electron microscopy in association with first-principles energy calculations, we systematically studied the complex coexisting precipitates in the alloys and correlated their precipitation sequences with thermal ageing processes applied. The principal results are the following: (1) eight types of precipitates can be observed in the alloy; (2) of these precipitates, the T 1-phase is most stable. The S-phase precipitates with segregated Li atoms at their interfacial edges are unexpectedly more stable than the σ-phase; (3) the T 1-phase has a characteristic precursor that plays the key role in its nucleation and growth.

7xxx aluminum alloys usually have high quenching sensitivity, which results in the inhomogeneous microstructure and mechanical properties of large-size components. The effect of pre-deformation on the quench-induced inhomogeneity of the microstructure and properties in a high strength 7050 aluminum alloy was investigated by means of the end-quenching technique, transmission electron microscopy (TEM) and three-dimensional electron tomography (3DET). The size of the η′ precipitates and quench-induced precipitates (QP) and the width of the grain boundary (GB) precipitate-free zones (PFZ) increased with the decrease of the quenching rate, leading to a decrease in hardness along the long axial direction of the end-quenching sample. However, after introducing 20% pre-deformation, the PFZ around the GB and QP in the slow-cooled end of the aged samples was significantly reduced, and dispersive η′ precipitates were formed around the GB and QP instead, which greatly improved the strength of the slow-cooled end. Compared with the sample without pre-deformation, the introduction of 20% pre-deformation increased the hardness of the slow-cooled end by 30% without obviously reducing the hardness of the water-quenched end, which means that the 20% pre-deformation effectively reduced the hardness inhomogeneity of the 7050 aluminum alloy. This work provides a new method for reducing the quench-induced inhomogeneity of aluminum alloys.

The formation mechanism of the plate-like strengthening T 1 -phase in an Al–4.15Cu–1.15Li (wt.%) alloy has been revealed by means of advanced imaging techniques and the first-principle calculations. It is shown that T 1 -precipitates nucleate directly from their own GP T1 zones and grow thicker in two possible paths. One is regular by repeating T 1 -cells, and the other is abnormal via T 1 -variants. Structural model of GP T1 zones has been suggested, and refined by image simulation. A possible structural evolution from GP T1 zones to precipitate T 1 at atomic level is suggested and proved to be energetically reasonable.

Although they are among the most important precipitation-hardened materials for industry applications, the high-strength AlZnMg(Cu) alloys have thus far not yet been understood adequately about their underlying precipitation scenarios in relation with the properties. This is partly due to the fact that the structures of a number of different precipitates involved in the alloys are unknown, and partly due to the complexity that the precipitation behaviors of the alloys may be closely related to the alloy's composition. In Part I of the present study, we have determined all the unknown precipitate structures in the alloys. Here in Part II, using atomic-resolution electron microscopy in association with the first principles energy calculations, we further studied and correlated the phase/structure transformation/evolution among these hardening precipitates in relation with the alloy's composition. It is shown that there are actually two coexisting classes of hardening precipitates in these alloys: the first class includes the η′-precipitates and their early-stage Guinier–Preston (GP-η′) zones; the second class includes the precursors of the equilibrium η-phase (referred to ηp, or η-precursor) and their early-stage Guinier–Preston (GP-ηp) zones. The two coexisting classes of precipitates correspond to two precipitation scenarios.

The mechanical properties and precipitation scenarios of Al–Cu–Mg alloys with different Cu/Mg atomic ratios have been investigated. We show that the designed alloy with an intermediate Cu/Mg ratio has better mechanical properties due to the coexistence of S and θ′ series precipitates. Compared to the commercial alloys with higher or lower Cu/Mg ratios, the evolution of θ′ precipitates in the designed alloy is slowed by the S phase, resulting in a longer duration of higher hardness with three peaks.

An Al–7.60Zn–2.55Mg (wt.%) alloy with a characteristically high Mg-to-Zn atomic ratio has been investigated for its strength and hardening precipitates in comparison with that of 7150 alloy. It is shown that this alloy yields a rather high strength upon thermal ageing. Interesting is that this high-strength alloy is hardened by the coherent polyhedral T-phase [(AlZn)49Mg32] particles and their early-stage precipitates, which is very different from that of other high-strength AA7xxx AlZnMg(Cu) alloys hardened by the disc-like precipitates of the η-type phases.

The morphological evolution of the precipitates in Al–7.4Zn–1.7Mg–2.0Cu (wt%) alloy was studied by high-resolution transmission electron microscopy (HRTEM). Statistics reveal that the hardness of the alloy changes accordingly with the change of the average thickness–diameter ratio of precipitates. The GPII zones are mainly responsible for the first and also the highest hardness peak. They grow in diameter and keep 7-atomic-layer in thickness. Once the thickness changes, the phase transformation from GPII zone to η′ or η-precursor would occur. The resultant metastable η′ and η-precursor precipitates grow in both diameter and thickness, but much faster in the former. After the first hardness peak, the metastable η′ precipitates and η-precursor, coexisting with part of GPII zones, are counted as the main hardening precipitates.

The precipitations in an Al–5.0Cu–0.3Mg (wt%) alloy and an Al–5.0Cu–0.3Mg–0.3Si (wt%) alloy have been systematically investigated by high-angle annular dark-field scanning transmission electron microscopy. The results are compared to clarify the effect of Si addition. The nucleation and growth process of θ′ (Al2Cu) phase in Si-containing alloy during isothermal ageing at 180 °C is revealed in detail. The formation of Q″-type precipitates, on which the θ′ precursors nucleate heterogeneously, contributes to the considerable increase in the ageing kinetics and higher strength at the early ageing stage. The thickening of the θ′ precipitate is largely confined due to the rather small size of fine Q″-type precipitate. As a result, a large proportion of θ′ phase precipitates possess a specific thickness of 2cθ′ and change slightly during the entire observed duration of ageing. The θ′ growth mechanism distinct from the Al–Cu–Mg alloy finally leads to a refined θ′ morphology regarding the thickness and aspect ratio (diameter/thickness). As is counterintuitive, the θ′ precipitate thickness distribution is demonstrated to have little effect on the mechanical property steadiness at the late ageing stage of the Al–Cu–Mg–(Si) alloys.

The corrosion resistance and fracture toughness of Al–Zn–Mg–Cu alloys are greatly affected by the remaining large constituent particles with sharp corners and sharp edges. Here, we show that with a careful high-temperature solution treatment, these constituent particles can be formed into spherical rather than irregular shapes. This results in better corrosion resistance and mechanical properties for the alloys than the conventional solution treatment. The complex microstructures of the formed spherical constituent particles and their formation mechanism were studied using focused ion beam (FIB), scanning transmission electron microscopy (STEM) and selected area electron diffraction (SAED). It was revealed that there are five types of spherical constituent particles formed after the special solution treatment, and each type has its own characteristic microstructural features.

The coarsened grain boundaries (GBs) with vein-like morphology frequently appear in the nitrogen diffusion layer of nitrocarburized carbon steels. The electron probe X-ray microanalysis shows that such vein-like GBs are rich in nitrogen and carbon atoms. Transmission electron microscopy and scanning electron microscopy further reveal that the coarsened GBs consist of γ′-nitrocarbide (Fe4(C,N)) and e-nitrocarbide lamellas that are formed during nitrocarburizing due to high nitrogen concentration at GBs. It is shown that many of {111} micro twins exist in the γ′-phase lamellas and thin e-phase slices prefer to nucleate at their twin boundaries with the orientation relationship of {0001}e//{111}γ′. Upon growing large γ′-lamellas may become faceted and thin e-lamellas may grow thicker and become the main portions in the vein-like GBs. The microstructure evolution occurring in the vein-like GBs can be depicted as: α + [N]/[C] → γ′ + [C,N] → e-nitrocarbides.

Ferritic nitrocarburizing is a widely employed industry process by which a strengthened nitrocarburized surface can be formed on mechanical tools or parts made of steels. Mainly composed of the e-Fe2–3(C, N) and γ′-Fe4(C, N) carbonitride phases as well as the α-Fe phase, the nitrocarburized surface has featured microstructures and properties that are directly related to the phase transformations occurring in the surface layers. Thus far, the following phase transformation sequence for the surface has generally been accepted: α-Fe + N/C → e → γ′. In the present work, these phase transformations were systematically revisited by microstructure and property characterizations in association with a controlled nitrocarburizing process by which the microstructures and properties of the surface are adjustable. Our study demonstrates that, to fully understand the microstructure and the property of a nitrocarburized surface, it is necessary to adopt the phase transformation sequence α-Fe + N/C → γ-N/C + N/C → γ′ + N/C → e, in which the existence of a transitional austenite phase containing N/C atoms (γ-N/C) must be assumed and the γ′ phase actually forms prior to the e phase.

We wished to ascertain the prevalence as well as the genetic and antigenic variation of infectious bronchitis viruses (IBVs) circulating in the Guangxi Province of China in recent years. The S1 gene of 15 IBV field isolates during 2012-2013 underwent analyses in terms of the similarity of amino-acid sequences, creation of phylogenetic trees, recombination, and serologic identification. Similarities in amino-acid sequences among the 15 isolates of the S1 gene were 54.3%-99.6%, and 43.3%-99.3% among 15 isolates and reference strains. Compared with the vaccine strain H120, except for GX-YL130025, the other 14 isolates showed a lower similarity of amino-acid sequences of the S1 gene (65.1-81.4%). Phylogenetic analyses of the S1 gene suggested that 15 IBV isolates were classified into eight genotypes, with the predominant genotype being new-type II. Recombination analyses demonstrated that the S1 gene of the GX-NN130048 isolate originated from recombination events between vaccine strain 4/91 and a LX4-like isolate. Serotyping results suggested that seven serotypes prevailed during 2012-2013 in Guangxi Province, and that only one isolate was consistent with the vaccine strain H120 in serotype (which has been used widely in recent years). The serotype of recombinant isolate GX-NN130048 was different from those of its parent strains. These results suggested that not only the genotype, but also the serotype of IBV field isolates in Guangxi Province had distinct variations, and that increasing numbers of genotypes and serotypes are in circulation. We showed that recombination events can lead to the emergence of new serotypes. Our study provides new evidence for understanding of the molecular mechanisms of IBV variations, and the development of new vaccines against IBVs.

Developments of high-strength aluminum alloys have always faced a difficult problem: owing to their small size, the early-stage strengthening precipitates are difficult to characterize in terms of composition, structure and evolution. Even for the widely used AlCuMg alloys, in which the phenomenon of precipitation hardening in metals was first discovered by Wilm more than a century ago, the essential questions remain: how many different precursors exist for the most effective strengthening precipitates (referred to S-phase), and how do they transform to the S-phase? Here we employ atomic-resolution electron microscopy imaging and first-principles energy calculations to address these problems. Our study demonstrates that the early-stage S-phase precipitates are highly dynamic in both composition and structure. Having their own genetic double Cu–Mg atomic walls to guide their evolution, these dynamic precipitates initiate, mature and grow with thermal aging following three evolution paths, leading to the S-phase precipitates formed, without exception, with even numbers of Cu–Mg atomic layers.

Precipitate volume fraction is an important parameter to estimate the strength of precipitation-hardened metals. In this study, a facile method was applied to measure the precipitate volume fractions in an age-hardened AlCuMg alloy. In this method, the precipitate volume fraction values can be obtained by multiplying the volume precipitate number densities with the averaged precipitate volumes, which can be easily measured in scanning electron microscopy and transmission electron microscopy, respectively. Compared with the conventional method, in which the specimen thickness has to be measured in transmission electron microscopy, the method proposed in this study is more facile to perform.

In this paper, the authors study the boundedness of the operator µΩb, the commutator generated by a function b ∈ Lipβ(Rn)(0 < β < 1) and the Marcinkiewicz integral µΩ on weighted Herz-type Hardy spaces.

The S-phase is one of the key strengthening precipitate phases in high-strength 2xxx (AlCuMg) aluminum alloys. Nonetheless, the crystal structure of the S-phase remains controversial, and therefore its physical properties are almost unknown. Using atomic-resolution imaging in electron microscopy and first-principles energy calculations, it was confirmed at last that the structure model proposed by Perlitz and Westgren in 1943 is the only right model for well-developed S-phase precipitates. The structure model proposed by Perlitz and Westgren in 1943 is the only right model. Detailed calculations analysis of the S-phase structure reveals that the characteristics of Cu–Mg bonding and Cu–Al bonding in the structure are basically metallic, but with an ionic character due to electron charge transfer between atoms, leading to a large elastic modulus for the S-phase precipitates to strengthen AlCuMg alloys.

The microstructures and mechanical properties of the new quaternary Mg–6Zn– x Cu–0.6Zr alloys ( x = 0, 0.5, 1.0 and 2.0 wt.%) have been investigated. The results show that the Cu content has a significant effect on the age-hardening response, tensile performance and fracture behavior of the alloys. The addition of 0.5 wt.% Cu resulted in a remarkable age-hardening response and a striking improvement of the room temperature tensile properties after an isothermal ageing at 180 °C. However, an excessive Cu addition of 2.0 wt.% caused the formation of continuous brittle grain-boundary intermetallic particles, thus degrading the age-hardening response and the mechanical properties of the alloy. The microstructural factors associated with the improvements in the mechanical properties are discussed in detail.

Multiple precipitating nanotwins of {1 1 1}/[1 1 2] type have been observed forming on the eutectic Si particles in Al–Si alloys upon annealing. High-resolution transmission electron microscopy (HRTEM) reveals that these precipitating nanotwins may possess very rich microstructures, including 5-fold nanotwins and various twin junctions. The formed Si nanotwins are very thin objects that are naturally appropriate for HRTEM observations. It is very likely that the formation and the morphology of the nanotwins are related to the aging conditions.

The precipitation process in Al-Zn-Mg-based alloys has been revisited using high-resolution transmission electron microscopy. It was found that there is an intermediate phase between the η′ phase and the η phase. This intermediate phase with a hexagonal structure can be termed the η precursor, since its a lattice parameter is the same as that of the η′ phase and its c lattice parameter is approximately the same as that of the η phase. Transformation from the η′ phase to the η phase occurs smoothly with this η precursor existing between them. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

A duplex chromizing process involving the salt bath chromizing of a plasma-nitrided tool steel was carried out at 773 K for 3–24 h. A single-phase CrN layer with a grain size ranging from 20 to 50 nm and an underlying layer consisting of α-Fe and nanosized CrN grains with a grain size ranging from 40 to 80 nm were formed by a 24 h chromizing at 773 K. The duplex chromizing mechanism was discussed.