Plate-shaped Phase Research Articles

Microstructural evolution and high temperature flow behaviors of a homogenized Sr-modified Al-Si-Mg alloy

Hot compression experiments are performed to study the flow characteristics of a homogenized Sr-modified Al-Si-Mg alloy at the temperatures of 300–420 °C and strain rates of 0.01–10 s−1. It is found that the deformed grains and Si-containing dispersoids clusters in the matrix are elongated, and incomplete DRX is discovered on the elongated boundaries at high deformation temperatures. The non-uniform distribution of Si-containing dispersoids, soft dispersoid free zones (DFZs) and fragmentation of plate-shaped phases may cause the flow instability at high strain rates. The Si morphology in the homogenized state can be further modified by the subsequent thermo-mechanical deformation. Moreover, the flow stress first rises to the peak with the increase of strain because of strain hardening, and then slowly declines with the further straining owing to the occurrence of incomplete DRX and coarsening of Si-containing dispersoids. The high activation energy is closely associated with the dislocations pinning from Si-containing dispersoids. Additionally, taking the coupling impacts of temperature, strain and strain rate into consideration, an extended phenomenological model compensating softening effects is developed to depict the strain hardening behavior, as well as dynamic softening characteristic. The developed model is validated by the experimental results, and can be applied to accurately predict the flow stresses of the studied Al-Si-Mg alloy during high temperature deformation.

Growth of lamellar pearlite in the weld zone between dissimilar steels

Transmission electron microscopy is used to study the welds between high-carbon pearlitic and chromium–nickel austenitic steel workpieces performed by flash butt welding. It has been established that lamellar pearlite colonies alloyed with chromium and nickel are formed in the weld zones between dissimilar steels. Thin austenite interlayers have been detected in the center of ferrite plates. The structure formed presents the C–F–A–F–C–F–A–F (and so on) sequence of three plate-shaped phases. The ferrite–cementite structure in alloyed-pearlite colonies is finer than that in unalloyed pearlite.

Characterisation of precipitates in a Mg–7Gd–5Y–1Nd–0.5Zr alloy aged to peak-ageing plateau

The precipitates in a Mg–7Gd–5Y–1Nd–0.5Zr alloy aged to peak-ageing plateau (aged at 210°C from 18h to 180h) were studied using transmission electron microscopy (TEM), high-resolution TEM (HRTEM) and high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM). Precipitation at the peak-ageing plateau involves the formation of the β′ phase and a new type of plate-shaped phase precipitated along the (0001)α habit plane. The ellipsoid morphology of β′ was illustrated according to [0001]α and [101¯0]α zone axes observations. The orientation relationship between the β′ precipitates and matrix satisfies [001]β′//[0001]α and (100)β′//(112¯0)α, respectively. The TEM and HRTEM results indicate that the β′ precipitates have a base-centred orthorhombic structure with lattice parameters a=0.64nm, b=2.22nm and c=0.52nm. Further, atomic-scaled HAADF-STEM observations suggest a Mg7RE-type and not a Mg15RE-type structure for the β′ phase. The positive thermal stability of the alloy at 210°C ageing is attributed to the coherent relationship with the matrix and the formation of an interlaced network of the β′ precipitates. Aside from the β′ phase, a large amount of precipitate particles were observed to form on the (0001)α habit plane in the peak-aged samples. The new type of precipitates have a plate shape, of approximately 5nm wide along the [101¯0]α direction of the matrix and approximately 5nm long along the [2¯110]α direction. A completely coherent relationship with the matrix can also be observed.

Microstructure of a newly developed γ′ strengthened Co-base superalloy

The microstructure of a newly developed Co-base superalloy with enhanced high-temperature strength has been investigated using transmission electron microscopy (TEM) and three-dimensional atom probe (3DAP) techniques. It mainly consists of a typical γ/γ′ (FCC/L1 2) structure and a plate-shaped AB 3-type ( Ni , Co , Cr ) 3 ( Ti , Al ) intermetallic compound with hexagonal structure ( a∼5.1 Å and c∼12.5 Å). γ′ is formed with a bimodal distribution and fine γ′ has a cuboidal morphology. Cr and Co are enriched in the γ phase, while Ti, Al and Ni are enriched in the γ′ phase. W and Mo are more or less uniformly distributed in both γ and γ′. Chemical composition analysis by 3DAP suggests that the plate-shaped phase has a higher Ti and lower Al content compared to that of γ′ phase, and the concentration of Ti, Co and Ni has a periodic variation along c-axis with a period of 12.5 Å in the plate-shaped phase.

Effect of Antimony and Ce-Rich Mischmetal Additions on As-Cast Microstructure and Mechanical Properties of AZ91 Alloy

Various amounts of antimony and mischmetal (Ce-rich) were added into AZ91 cast magnesium alloy and the microstructural characteristics of the cast magnesium alloys were studied by optical microscopy, electronic microscopy and X- ray diffraction techniques. Both antimony and mischmetal had grain refinement effect to AZ91 alloy. When they were added together, a further refinement was observed. SEM-EDX and XRD analysis indicated that a plate-shaped phase, Al11RE3, formed at the boundaries of dendritic α-Mg grain. A rod-like Mg3Sb2 compound distributed both within α -Mg dendrites and along grain boundaries. A new spherical particle, ReSb, formed in the magnesium alloy when 0.8wt%RE and 0.4wt%Sb were join added. The individual additions of antimony or mischmetal to AZ91 alloy did not result in a noticeable improvement to its room temperature tensile properties, but when 0.8wt%RE and 0.4wt%Sb were added together, the as-cast tensile properties of the alloy were increased dramatically.

Electron backscattered diffraction and energy dispersive X-ray spectroscopy study of the phase NiSn4

Electron backscattered diffraction (EBSD) and energy dispersive X-ray spectroscopy (EDS) have been performed on a plate-shaped phase formed through the reaction of Sn and Ni. The phase is formed during extended thermal cycling tests on ceramic capacitors having electroplated tin end terminations. The morphology is identical to that of a phase labeled NiSn3 by J. Haimovich. The phase is shown to have a stoichiometry, NiSn4, and a crystal structure isomorphous to PdSn4, PtSn4, and AuSn4 (Aba2, #41, oC20). The structure can also be described with the higher symmetry structure (Ccca, #68, oC20).

Thermodynamics of the Displacive Mechanism of α<SUB>1</SUB> Transformation in a β′ Copper-Zinc Alloy

Thermodynamics of the displacive mechanism of plate-shaped phase α 1 was analyzed in β'Cu-Zn alloys. It was proposed that the displacive transformation of the α 1 plate took place in the solute-depleted region formed in the parent phase during the incubation period. The thermodynamic analysis indicated that the driving force of α 1 transformation, ΔG, increased with the reduction of x d , the solute concentration of the depleted region. And, ΔG could overcome the transformation barrier with solute depletion to a certain degree. In addition, x d was higher than the equilibrium concentration in the phase diagram. Therefore, the shear formation of α 1 plate in the solute-depleted region was thermodynamically supported.