Three-dimensional size and orientation of the precipitates in AZ91 magnesium alloys measured by TEM techniques

Knowledge of the microscopic structure, including three-dimensional (3-D) size and orientation of the precipitates, is essential to fully understand the mechanical properties of the magnesium alloys and designing the alloys with better performance. Analytical TEM with high spatial resolution offers the simultaneous measurements of 3-D size, structure, orientation, composition of the precipitates from one typical sample along an established crystallographic axis. Besides popular Burgers orientation relationship (OR), other ORs such as Pitsch--Schrader OR, Crawley OR, Potter OR and a new OR with the form of [0001]_α 1.0° from [311]_γ and (11\bar 20)_α 2.0° from (03\bar 3)_γ between the magnesium matrix and the precipitate γ -Mg₁₇Al₁₂ are identified by TEM imaging and diffraction techniques. As a case study, the thicknesses of the individual precipitates with Burgers OR are further measured to be 100--200~nm through both electron energy-loss spectroscopy and x-ray energy dispersive spectroscopy combining differential x-ray absorption and extrapolation, which are in agreement with the overall 3-D size statistic distribution results obtained through analysing various samples along various directions. Furthermore, the fabricated wedge-shape structure provides a platform on which to study the dependence of the interfacial strain on the variation of the thickness.     

Structure and orientation of an intermetallic phase in a W-Ni-Co alloy

The aim of this investigation is crystal structure determination of an intermetallic phase formed in a W-Ni-Co alloy during a heat-treatment carried out at a temperature of 800°C. This intermetallic phase is expected to play a critical role on the final microstructure (fine tungsten particles in an FCC matrix that is present in between large tungsten grains) and thereby, on the properties of the alloy. 92W-5.3Ni-2.7Co alloy was prepared through powder metallurgy route (liquid phase sintering) followed by heat-treatment at 800°C for 5?h. The intermetallic phase formed at this temperature was characterised using transmission and scanning electron microscopes. The intermetallic phase was found to have orthorhombic crystal structure with Pnam (62) space group as determined using automated diffraction tomography along with precession electron diffraction. Chemical analysis in TEM suggested that the intermetallic phase is based on stoichiometry (Co,Ni)₂W. Orientation imaging of the phase was also carried out in TEM and EBSD to understand its evolution. Equiaxed or lath morphology of the intermetallic phase was found to depend on the crystallographic orientation relationship of the phase with the tungsten grains and the matrix phase.     

HRTEM study of the effect of deformation on the early precipitation behaviour in an AA6060 Al–Mg–Si alloy

The effect of 10% pre-ageing deformation on the early precipitation behaviour in an AA6060 Al–Mg–Si alloy aged 10?min at 190°C was investigated by high-resolution transmission electron microscopy (HRTEM) in ?100?Al projections. The precipitate nucleation was heterogeneous since all precipitates were found to grow on dislocation lines. The pre-ageing deformation suppresses growth of Gunier–Preston zones and β″ phase. The resulting precipitates are still largely coherent with the aluminium matrix. They appear with two main morphologies; one consists of independent, small cross-sections arising from needles with disordered β′ and B′ structures. The other morphology is a much more continuous decoration where precipitates have elongated and conjoined cross-sections and where a particular precipitate phase could not be determined. All precipitates in this work were found to contain a common near-hexagonal sub-cell (SC) with projected bases a?=?b?≈?0.4?nm. This strongly indicates that they are built over the same Si network, which recently has been demonstrated to exist in all precipitates in the Al–Mg–Si(–Cu) system. For the discrete morphology type the network has one hexagonal base vector parallel to or very near a ?510?Al direction. For the continuous type, one base vector falls along a ?100?Al direction. This orientation of the network is different from previous studies of ternary Al–Mg–Si alloys and must be a direct consequence of the deformation.     

Effects of torsional deformation on the microstructures and mechanical properties of a CoCrFeNiMo0.15 high-entropy alloy

Abstract

The effects of torsional deformation on the microstructures and mechanical properties of a CoCrFeNiMo_0.15 high-entropy alloy have been investigated. The torsional deformation generates a gradient microstructure distribution due to the gradient torsional strain. Both dislocation activity and deformation twinning dominated the torsional deformation process. With increasing the torsional equivalent strain, the microstructural evolution can be described as follows: (1) formation of pile-up dislocations parallel to the trace of {1 1 1}-type slip planes; (2) formation of Taylor lattices; (3) formation of highly dense dislocation walls; (3) formation of microbands and deformation twins. The extremely high deformation strain (strained to fracture) results in the activation of wavy slip. The tensile strength is very sensitive to the torsional deformation, and increases significantly with increasing the torsional angle.     

Surface nanocrystallization mechanism of a rare earth magnesium alloy induced by HVOF supersonic microparticles bombarding

A nanostructured surface layer with a thickness up to 60 μm was produced on a rare earth Mg-Gd-Y magnesium alloy using a new process named HVOF-SMB (high velocity oxygen-fuel flame supersonic microparticles bombarding). The microstructural features of the treated surface at various depth of the deformed layer were characterized by optical microscopy (OM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) with an aim to reveal the formation mechanism. Results showed that three steps during grain refinement process were found, i.e., twinning dominates the plastic deformation and divides the coarse grains into finer twin platelets at the initial stage, stacking faults are generated and a number of dislocation slip systems are activated leading to the cross slips with increasing strain and strain rate, eventually high-density dislocation networks, dislocation cells and dislocation arrays are formed, which further subdivides the twin platelets and residual microbands into sub-microstructures. As a result, homogeneous nanostructure with a grain size of about 10-20 nm is formed through dynamic recrystallization in the topmost surface layer. Based on the experimental observations, a grain refinement mechanism induced by plastic deformation with higher strain rate during the HVOF-SMB treatment in the rare earth Mg-Gd-Y alloy was proposed.     

Transmission electron microscopy observation of a deformation twin in TWIP steel by an ex situ tensile test

The formation of deformation twins in twinning-induced plasticity steels was observed in transmission electron microscope by an ex situ tensile test. The twinning process initially includes formation of extended dislocations at primary slip plane, then cross-slip to a conjugate slip plane with dissociation of the leading partial into a stair-rod dislocation and an emitted partial, and finally un-faulting of the original faults and formation of Frank partials. Repetition of the operation of the process on successive conjugate planes results in the formation of deformation twins. The formation mechanism of deformation twins can thus be explained by improving the stair-rod cross-slip model.     

The morphology and nucleation kinetics of copper islands during electrodeposition

We describe a study, using in situ transmission electron microscopy, of the shape and nucleation kinetics of three-dimensional islands formed during the electrochemical deposition of copper. By operating an electrochemical cell within an electron microscope, we obtain real-time images of the formation of copper islands on a gold electrode while simultaneously recording electrochemical data such as voltage and current. We first present cyclic voltammetry, where the images show the deposition and stripping processes while the voltammogram demonstrates qualitatively the regimes in which diffusion and surface reaction are the rate limiting steps. We then examine island growth quantitatively under conditions of constant potential. Images recorded during growth at various potentials allow direct visualization of the differences between island shapes in the diffusion limited and kinetically limited growth regimes. Furthermore, a combined analysis of the current transients and the images allows parameters such as the diffusion coefficient, the rate constant and the critical nucleus size to be determined. We discuss these results in the context of electrochemical nucleation and growth models.     

Mg-Sn-Si合金中Mg2(Si,Sn)复合相的结构与性能研究

研究了铸态Mg-Sn-Si合金中Mg₂(Si,Sn)复合相的结构、 特性以及该相对Mg-Sn-Si合金变质作用的影响. 结果表明: Sn原子能取代Mg₂Si中的部分Si生成Mg₂(Si,Sn)复合相, 该三元相与Mg₂Si, Mg₂Sn相的结构相同, 属于面心立方结构, Mg₂(Si,Sn)相的元素含量并不固定, 在Si富集区形成的Mg₂(Si,Sn)相中, Si元素含量高, 在Si贫乏区形成的Mg₂(Si,Sn)相中, Si元素含量低. Si含量较多的Mg₂(Si,Sn)相性能与Mg₂Si相接近, Sn含量较多的Mg₂(Si,Sn)相性能与Mg₂Sn相接近, 实验中发现Mg₂(Si,Sn)复合相的纳米硬度、 弹性模量与维氏硬度等物理性能介于Mg₂Si与Mg₂Sn之间, Mg₂(Si,Sn)相对汉字状Mg₂Si相的变质处理起到桥梁作用. 关键词： Mg-Sn-Si合金 2Si')" href="#">Mg₂Si 2Sn')" href="#">Mg₂Sn 2(Si,Sn)复合相')" href="#">Mg₂(Si,Sn)复合相     

Formation of low-temperature deformation-induced segregations of nickel in Fe–Ni-based austenitic alloys

ABSTRACT

The authors present the results of an investigation in Fe–Ni-Cr austenitic alloys of the low-temperature deformation-induced segregations of nickel that form in the micro regions being (i) located close to grain- and subgrain boundaries and (ii) characteristic of the concentration and magnetic inhomogeneities indicated by the appearance of a dark diffraction contrast at the electron diffraction patterns taken from these regions typical (at the same time) of an enhanced value of Curie temperature. The observed effects were connected with the micro distortions caused by the local change of lattice parameter because of an increase in nickel concentration, as well as in the result of a magnetostriction dilatation. Using methods of the X-ray energy dispersive spectroscopy (XEDS) and atomic-probe body-section radiography (tomography – APT) has made it possible to determine the borders of those regions of austenite that were characteristic of an enhanced concentration of nickel in the fields of the localisation of a deformation-induced segregation of nickel in the vicinity of grain (subgrain) boundaries of austenitic alloys of the types Fe–13Cr–30Ni and Fe–37Ni–3Ti.     

Orientation-dependent microstructure development during high-rate shear deformation of copper

ABSTRACT

The microstructure transformations in copper subjected to high-rate deformation via dynamic channel angular pressing (DCAP) have been examined after one and four DCAP passes using electron backscattering diffraction. The focus was on the interrelation between microstructure and texture evolution and on the role of deformation twinning in these processes. During the first pass, a mesoscopic banding is shown to be the main mechanism of grain fragmentation, while the texture is qualitatively similar to that of face-centered cubic metals subjected to equal channel angular pressing, including the presence of characteristic texture components C, A₁ and A₂. A correspondence between structural elements and texture components occurs. Specifically, the mesobands have orientations near either an A₁ or A₂ ideal orientation, whereas the matrix has an orientation near C. Inside the A₁-bands, microtwins with orientations near A₂ are observed. Detailed analysis of the A₂-bands suggests that they may be of twinning origin. Similar orientation dependence of microstructure, though on a much finer scale, is observed after four DCAP passes. Based on the microstructure examination, it was suggested that mesoscopic banding together with deformation twinning continue to be principal mechanisms of grain refinement until the fourth pass, resulting in the formation of ultrafine-grained structure.     

Electrodeposition of high corrosion resistance Cu/Ni-P coating on AZ91D magnesium alloy

High corrosion resistance Cu/Ni-P coatings were electrodeposited on AZ91D magnesium alloy via suitable pretreatments, such as one-step acid pickling-activation, once zinc immersion and environment-friendly electroplated copper as the protective under-layer, which made Ni-P deposit on AZ91D Mg alloy in acid plating baths successfully. The pH value and current density for Ni-P electrodeposition were optimized to obtain high corrosion resistance. With increasing the phosphorous content of the Ni-P coatings, the deposits were found to gradually transform to amorphous structure and the corrosion resistance increased synchronously. The anticorrosion ability of AZ91D Mg alloy was greatly improved by the amorphous Ni-P deposits, which was investigated by potentiodynamic polarization curve and electrochemical impedance spectroscopy (EIS). The corrosion current density (I_corr) of the coated Mg alloy substrate is about two orders of magnitude less than that of the uncoated.     

Detection of Au precipitates in a Mg-based alloy using electronically simulated hollow cone illumination

Images from electronically simulated hollow cone illumination (or rotating dark field) conditions, obtained under plane wave (or weak) diffraction conditions, are generally assumed to approximate to the compositionally weighted sum of atomic number squared for sufficiently large momentum transfers. However, even for large momentum transfer encountered with a semi-angle of 5° and 300 keV electrons, appropriate numerical integration over condenser and objective aperture configurations indicates that some thermal scattering component is still present. A Mg–Al alloy with minor additions of Zn and Mn, and to which 0.1 at.% Au has been added, is shown to provide a good system for the detection of high Z (atomic number) precipitates within a low Z matrix and on which semi-quantitative calculations may be based. Correlation of absolute rather than relative intensities from small precipitates (3–10 nm diameter) with calculations based on an Einstein model for (incoherent) thermal diffuse scattering show that the small precipitates consist predominantly of Au, a conclusion subsequently supported by EDX analysis and electron diffraction measurements. It is also demonstrated that this incoherent contrast mechanism is ideal for stereographic imaging in the TEM.     

Transmission electron microscopy of dislocations in cementite deformed at high pressure and high temperature

Polycrystalline aggregates of cementite (Fe₃C) and (Fe,Ni)₃C have been synthesised at 10 GPa and 1250 °C in the multianvil apparatus. Further, deformation of the carbides by stress relaxation has been carried out at temperature of 1250 °C and for 8 h at the same pressure. Dislocations have been characterised by transmission electron microscopy. They are of the [1?0?0] and [0?0?1] type, [1?0?0] being the most frequent. [1?0?0] dislocations are dissociated and glide in the (0?1?0) plane. [0?0?1] dislocations glide in (1?0?0) and (0?1?0). Given the plastic anisotropy of cementite, the morphology of the lamellae in pearlitic steels appears to have a major role in the strengthening role played by this phase, since activation of easy slip systems is geometrically inhibited in most cases.     

TEM observations of rhombohedral and monoclinic domains in LaCoO3-based ceramics

Twin structures in rhombohedrally distorted polycrystalline LaCoO₃, La_0.8Ca_0.2CoO_3-δ and La_0.5Sr_0.5Co_0.5Fe_0.5O_3-δ have been studied by transmission electron microscopy. Normal {100} and {110} twinning was predominant in LaCoO₃ and La_0.8Ca_0.2CoO_3-δ. In addition, ferroelastic domains with a one-dimensional superstructure corresponding to a tripling of the pseudocubic lattice parameter, a _c, were found in some grains. The monoclinic superstructure vanished over time and was sensitive to external mechanical stress. In La_0.5Sr_0.5Co_0.5Fe_0.5O_3-δ no normal twinning was observed, but domains with a one-dimensional superstructure of two times a _c were observed. The superstructures have been interpreted as atomic scale twinning, which reduce the crystal symmetry from rhombohedral to monoclinic.     

The effect of Zn on precipitation in Al–Mg–Si alloys

Effects of addition of Zn (up to 1 wt%) on microstructure, precipitate structure and intergranular corrosion (IGC) in an Al–Mg–Si alloys were investigated. During ageing at 185?°C, the alloys showed modest increases in hardness as function of Zn content, corresponding to increased number densities of needle-shaped precipitates in the Al–Mg–Si alloy system. No precipitates of the Al–Zn–Mg alloy system were found. Using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), the Zn atoms were incorporated in the precipitate structures at different atomic sites with various atomic column occupancies. Zn atoms segregated along grain boundaries, forming continuous film. It correlates to high IGC susceptibility when Zn concentration is ~1wt% and the materials in peak-aged condition.     

Bulk interfaces in a Ni-rich Ni-Au alloy investigated by high-resolution Z-contrast imaging

Interfaces between Au-rich precipitates and the Ni-rich matrix in a decomposed Ni-10 at.% Au alloy were investigated by low-magnification and high-resolution Z-contrast imaging. During aging at 923 K, the originally single crystalline sample decomposed and recrystallized resulting in a microstructure consisting of subgrains separated by small-angle grain boundaries. These small-angle grain boundaries are decorated by Au-rich precipitates. The interfaces between the Au-rich precipitates and the Ni-rich matrix were characterized with respect to the orientation relationship between precipitates and matrix, misfit dislocations and concentration gradients. Two transformation modes were identified that are involved in the decomposition of bulk Ni-rich Ni–Au alloys. While in the first mode the interface is semi-coherent, in the second mode the interface corresponds to an incoherent twin boundary. It is further shown that strain fields around misfit dislocations can result in systematic errors in the determination of the concentration gradients across interfaces between precipitates and matrix.