We report microstructural characterization of a Gd-modified TiAl alloy with a fully lamellar structure by utilizing Cs-corrected transmission electron microscopy. Two types of Gd-contained precipitates, Gd2O3 and Al2Gd, with sizes ranging from several nanometers to micrometers were detected at grain boundaries and lamellar interfaces. Atomic-scale images reveal a well-defined crystallographic relationship between the nano-scale Al2Gd precipitates and γ-TiAl while Gd2O3, formed from TiAl melt during solidification and precipitated from α phase, is incoherent with TiAl matrix. The microstructural characterization provides direct evidence that the microstructure refinement of the Gd-modified TiAl is mainly from both oxide and intermetallic precipitates.Download high-res image (119KB)Download full-size image

In this study, mechanical properties and deformation mechanisms of Mg-Gd-Y-Zr alloy at temperatures ranging from 77 K to 523 K have been investigated. The effects of temperature on the mechanical properties, deformation mechanism, and fracture mechanism are discussed. The results show that the strengths of alloy decrease gradually while the elongations increase progressively with increasing temperature. The maximum ultimate tensile strength of the alloy as high as 442 MPa is obtained at 77 K. As the temperature increases from 77 K to 523 K, the ultimate tensile strength of the alloy decreases from 442 MPa to 254 MPa and the elongations increase from 6.3% to 28.9% gradually. The study verifies that the deformation at 77 K is predominated by basal slip and \({{\left\{ {10\bar{1}2} \right\}} \mathord{\left/ {\vphantom {{\left\{ {10\bar{1}2} \right\}} {\left\langle {10\bar{1}1} \right\rangle }}} \right. \kern-0pt} {\left\langle {10\bar{1}1} \right\rangle }}\) deformation twinning system. At 223 K, lots of twins emerge primarily at grain boundaries. At 373 K, all dislocations are proved to be 〈a〉 dislocations. At 523 K, although basal slip is still the dominant deformation mechanism, non-basal slip systems also become activate.

•Advanced Cs-corrected STEM is applied to obtain atomic-scale images.•The complex interfaces between orientedly-attached Mn3O4 nanoparticles are fully reconstructed.•“Defect transfer” phenomenon is observed for the first time.In this paper, coherent interfaces between orientedly-attached (OA) Mn3O4 nanoparticles are fully reconstructed based on atomic-scale high angle annular dark field (HAADF) - scanning transmission electron microscopy (STEM) direct imaging technique. According to the characterization, five characteristics of the oriented attachment between Mn3O4 are proposed: (1) [100]Mn3O4 direction sharing; (2) crystallographic plane sharing for every two attached particles; (3) preference for O2 − close-packed crystallographic planes to form the OA interface; (4) “defect transfers” compensating the deviation between the macroscopic interface and the atomic interface; (5) possible chemical state changes of the interfacial atoms.

•Calcium phosphate - graphene composite is prepared by electro-deposition.•The preparation is facile, environmental friendly and controllable.•The structures of the composite are studied by various characterization methods.A facile electro-deposition method for the preparation of calcium phosphate – graphene composite (CPG) is proposed and investigated in this paper. Flowerlike calcium phosphate (CaP) compounds grow on graphene sheets during the aqueous electro-deposition. The electro-deposited composite is then investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). To better unveil the structure of CPG, graphene sheets are loaded on the carbon film on copper grid for electrodeposition and following SEM, energy dispersive spectrum (EDS) and, particularly, transmission electron microscopy (TEM) test. The results show that the growth process of CPG resembles blossoming flowers, every petal of which is composed of several thin sheets, and the growth exhibits time dependent morphology development which indicates the high controllability of this preparation method. Furthermore, this method can be utilized in wider applications to prepare CPG coatings on many other materials, such as Mg alloys, a potential candidate for biodegradable materials.

•The precipitation in Mg-Gd-Y-Zr is comprehensively investigated by advanced atomic-scale Cs-corrected HAADF-STEM.•Various novel precipitate structures are unraveled.•Four types of interfaces between the precipitates are fully characterized.•The structural transformation processes of the precipitates are revealed.This paper reports on a comprehensive investigation, using atomic-scaled Cs corrected high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM), of the precipitation in Mg-9.28Gd-2.93Y-0.35Zr, including structures of the precipitates and several structural transformation processes. The precipitation sequence is identified as: Super saturated solid solution → clusters → nucleation β' (major)/ βH (minor)→ precipitate β' (major)/ βM , βT' (minor)→β1 → β(Equilibrium). Under peak-age condition, the strengthening structure is independent defect-free β' with little interaction among each other. Under over-age condition, the β' precipitates start to interact with each other, during which coarsening and orientation transformation of β' precipitates take place. The coarsening and orientation transformation, in which some RE atoms in the precipitates need to change their positions, are achieved by βM and βT' as transitional structures.

•The general morphology of this alloy is 3-D network composed of β′β′ and LPSO.•Three types of interactions between β′β′ and LPSO are fully revealed.•The LPSO that interfaces with α-Mg matrix may have unique characteristics.This paper reports on an atomic-scale investigation into the interactions between β′β′ precipitates and long-period stacking ordered phase (LPSO) in Mg–10Gd–5Y–2Zn–0.5Zr (wt%) alloy, using Cs-corrected high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM). The general morphology within the alloy is LPSO phase intercalated with β′β′ precipitates and the two structures interconnect into a 3-D network. The β′β′/LPSO interactions are fully characterized and categorized into three types: RE-absent gaps parallel to (112¯0)Mg, redistributions of heavy atoms within LPSO structures and interceptions of β′β′ precipitates by LPSO structures from [101¯0]Mg direction.

AZ91/HA composite was prepared by AZ91 magnesium alloy and porous HA using squeeze casting method. The microstructure and mechanical property of the AZ91/HA composite were studied. The results show that the molten AZ91 alloy completely infiltrated the preform without destroying the porous structure of the HA preform. The compressive strength of AZ91/HA composite increased significantly compared with that of the porous HA. The immersion test indicated that AZ91 alloy shows a lower corrosion resistance and is easier to be corroded in comparison with HA.

•One of the most advanced probe Cs corrected STEM in the world is applied to obtain atomic-scaled direct images.•The precipitation behavior of Mg- rare earth alloys during natural ageing is reported for the first time.•Rare earth enrichment areas, dots, zigzag lines and hexagons are characterized to segregate during the natural ageing.•Zirconium-rich cores are proved to facilitate the precipitation during the natural ageing.The segregation of rare earth (RE) atoms in solution-treated Mg-Gd-Y-Zr alloy after a 6-year natural ageing at room temperature has been comprehensively investigated by atomic-scale high angle annular dark field (HAADF)-scanning transmission electron microscopy (STEM) technique. When the aged specimen is viewed from [0001]Mg, RE enriched regions without a definite RE atom arrangement as well as three types of segregation with exact RE atom arrangements, dots, zigzag lines and hexagons, are characterized. The single dots are formed by columns of RE atoms occupying trials of Mg positions along [0001]Mg. The number of atoms in one trial is usually no more than three. The zigzag lines and hexagons are the predecessors of β′ and βM precipitate structures, respectively, which are common in artificially-aged alloys. Moreover, nano-sized zirconium-rich cores in the alloy are observed to facilitate the natural ageing because it provides both elements sources and nucleation sites for the precipitation, particularly under this low-temperature condition which determines the kinetic feasibility to be the crucial factor.

The paper reports on an atomic-scale investigation into the precipitation hardening process in Mg-2.5Sm(wt%) alloy, using advanced Cs-corrected high angle annular dark field – scanning transmission electron microscopy (HAADF-STEM). Various novel precipitate structures existing in different ageing conditions are identified and discussed in depth. In the under-aged sample, three nucleation mechanisms are observed: βLβL, βHβH and β′β′, which are uniformly-scattered atomic-size rare earth clusters, displaying a significant strengthening effect in a very short ageing time of 30 min. In the peak-aged sample, the main strengthening structures are needle-like precipitates composed of zigzag line repetitive units and hexagonal repetitive units. In the over-aged sample, the needle-like precipitates grow coarse and a subset of the precipitates transform into β1β1 precipitates, both of which are detrimental to the mechanical properties. From an engineering perspective, it is of great interest to design advanced Mg-RE materials with the uniformly-scattered atomic-size rare earth clusters, because they are effective strengthening structures requiring short ageing time. In the second place, the paper reveals three types of transformation processes occurring during the precipitation: the transformation between βLβL precipitates and needle-like precipitates, the transformation within needle-like precipitates along [0001]Mg and the transformation between needle-like precipitates and β1β1 precipitates. The RE atoms remain coherent with the α-Mg matrix before they transform into β1β1 precipitates.

•Atomic resolution HAADF-STEM is applied to characterize precipitate phases.•New atomic arrangements of RE elements are observed.•A defect, which is the interface between two growing β′ parts , is imaged.Mg–Gd–Y–Zr alloy has been widely studied due to its excellent properties. Its ageing precipitation sequence was identified as a four-stage precipitation including S.S.S.S.—β″–β′–β1–β. In this paper, the precipitate phases of Mg–Gd–Y–Zr during isothermal ageing at 225 °C are investigated by atomic resolution HAADF-STEM and, as a result, novel structures are observed. Viewed from [0 0 0 1]Mg axis, the general morphology of rare earth (RE) atoms is “main bodies” linked by “bridges”. In bridge areas, some RE atoms form separated chains of hexagons, which has not been clearly reported previously. Furthermore, a defect in the precipitation process, which is the interface between two encountered β′ parts, is characterized.

This paper is aimed to investigate the microstructure of 7005 aluminum sheets joined by friction-stir welding as well as their mechanical properties. Specimens with ten different sets of welding parameters were studied. Tensile test and fracture analysis determined that the joint of the best quality was obtained at the rotation speed of 1000 rpm matching with the travel speed of 200 mm/min, and the travel speed has more impact on the ultimate tensile strength. Optical microscope observation was applied to this high-quality specimen and gave evidence to explaining the formation of the onion ring structure. Electron back-scattered diffraction (EBSD) technique was employed to characterize the textures and revealed the evolution of microstructures during friction stir processing. The EBSD results showed that the grains maintain their original orientations at relatively low deformation while the orientations rotate under increasing strain. Accumulated rotation will turn the textures into mixed shear components, which finally results in grain refinement and contributes to the high quality of the joint.

A Mg95.5Y3Zn1.5 alloy processed via a two-step processing route of extrusion plus ECAP has been investigated. It was found that the ECAP processed Mg95.5Y3Zn1.5 alloy contained ultrafine grains and exhibited excellent mechanical properties. After ECAP, the average grain size of Mg95.5Y3Zn1.5 alloy was refined to about 300 nm. The highest strengths, with yield strength of 444.6 MPa and ultimate tensile strength of 472.7 MPa, were obtained after 1 pass at 623 K. The SAED patterns indicated that the microstructure after ECAP consisted of both high angle and low angle grain boundaries. The fraction of high-angle boundaries increased with increasing numbers of ECAP passes. The Mg95.5Y3Zn1.5 alloy contained a high volume fraction of X-Mg12ZnY phase due to high yttrium and zinc addition. And, it accelerated the growth and coalescence of cracks during tensile testing, resulting in premature fracture and lower elongation of alloy.

This study revealed that the extrusion temperature has a great influence on microstructure and mechanical properties of the Mg97Y2Zn1 alloy. The average grain sizes increased from 3 μm to 8 μm with increasing extrusion temperatures from 623K to 773 K. Both dynamic recrystallization (DRX) and static recrystallization (SRX), which occur during and after deformation, respectively, were observed. The alloy, which extruded at a relatively high temperature, exhibited lower strength because the strain strengthening was balanced by the softening that originated from DRX. Three types of morphologies, namely, big recrystallized grains, fine recrystallized grains, and non-recrystallized grains, were observed in the extruded microstructures obtained at 623 K. The dislocation density was quite high in the fully recrystallized grain. The extruded microstructures obtained at 773 K were composed of large grains with more uniform size. Their degree of recrystallization was higher and the dislocation density also declined. All dislocation in the grain were distinguished as 〈c+a〉 dislocations. Submicron scale precipitates were distributed along the newly formed recrystallized grain boundaries and had a remarkable pinning effect on the recrystallized grain growth after extrusion at 773 K. The precipitates can be divided into two main types: mixed type and single type.

Hot deformation behavior and processing maps of the 2099 Al-Li alloy are investigated by tensile test at the temperature range from 250 to 450 °C and the strain rate range from 0.001 to 5.0 s−1. The typical true stress-true strain curves show that the flow stress increases with increasing the strain rate and decreasing the deforming temperature. All curves exhibit rapid work hardening at an initial stage of strain followed by remarkable dynamic softening. Based on the flow stress behavior, the processing maps are calculated and analyzed according to the dynamic materials model (DMM). The processing maps exhibit an instability domain in the temperature and strain rate ranges: T = 250-260 °C and \(\dot{\upvarepsilon }\) = 0.1-0.5 s−1. The maps also exhibit an optimum hot working condition in the stability domain that occurs in the temperature of 400 °C for a strain rate of 0.001 s−1 and having a maximum efficiency of 60%. The microstructural examinations exhibit the occurrence of dynamic recovery (DRV) during hot deformation of the 2099 alloy which is the dominant softening mechanism in the alloy. The fracture behavior changes from a brittle fracture to a ductile fracture as strain rate decreases and temperature increases.

The processing maps of the Mg97Y2Zn1 alloy have been developed for optimizing hot workability and controlling the microstructure in this paper. Compression tests were conducted in the temperature range from 250 to 450 °C and the strain rate range from 0.001 to 5.0 s−1. The processing maps were calculated based on the flow stress behavior and analyzed according to the dynamic materials model. The maps exhibited a stability domain that occurs in the temperature of 430-450 °C for a strain rate of 0.002-0.003 s−1 and having a maximum efficiency of 50%. A hot extrusion test was carried out to assess the prediction of the processing maps for Mg97Y2Zn1 alloy. The influence of the extrusion temperature on the microstructure and tensile properties of the extruded alloys was analyzed to identify the optimum processing parameters. The results have shown good agreement between the regimes exhibited by the map and the microstructure of the extruded alloy.

Hot compression deformation behavior and processing maps of the Mg-Gd-Y-Zr alloy were investigated in this paper. Compression tests were conducted at the temperature range from 300 to 450 °C and the strain rate range from 0.001 to 1.0 s−1. It is found that the flow stress behavior is described by the hyperbolic sine constitutive equation in which the average activation energy of 251.96 kJ/mol is calculated. Through the flow stress behavior, the processing maps are calculated and analyzed according to the dynamic materials model. In the processing maps, the variation of the efficiency of the power dissipation is plotted as a function of temperature and strain rate. The instability domains of flow behavior are identified by the maps. The maps exhibit a domain of dynamic recrystallization occurring at the temperature range of 375-450 °C and strain rate range of 0.001-0.03 s−1 which are the optimum parameters for hot working of the alloy.

The application of magnesium and its alloy as degradable biomaterials is mainly confined due to its high degradation rate in physiological environment. This research focused on the effects of micro-arc oxidation (MAO) on biodegradable behavior of Mg-Y-Zn magnesium alloy in a simulated body fluid (SBF). The corrosion rate of alloys was gauged by means of hydrogen evolution volume measurement and mass-loss method. Scanning electron microscope (SEM) was utilized to observe the surface of the magnesium alloy and the cross-section of oxidation coating layer before and after corrosion. The Mg-Y-Zn alloy with thicker oxidation coating exhibited greater corrosion resistance during the immersion test for 240 h.

The present study, using advanced Cs-corrected high-angle annular dark field – scanning transmission electron microscopy (HAADF-STEM), reports on a comprehensive investigation into the precipitate structures in an Al-Zn-Mg-Cu alloy aged at 150 ℃, including GP zones, η’ and η precipitates. In the nucleation stage, Zn atoms enrich on the {111}Al-planes abutting spherical Mg clusters that are approximately 3–6 nm in diameter. In the subsequent growth, the as-nucleated structures extend with an increasing diameter and a constant width along [111]Al and grow into platelet precipitates. η’ is proved to be a group of metastable structures existing in the transition from FCC Al to HCP MgZn2 (η). Some metastable structures are assembled by local-ordered rhombohedral units and orthorhombic units as building blocks. Subsequently, the precipitates evolve into η phases with stacking faults.