Microstructure and High Temperature Impression Creep Properties of Mg–3Ca–xZr (x = 0.3, 0.6, 0.9 wt%) Alloys

The current study has investigated the influence of zirconium (Zr) addition to Mg–3Ca–xZr (x = 0.3, 0.6, 0.9 wt%) alloys prepared using argon arc melting on the microstructure and impression properties at 448–498 K under constant stress of 380 MPa. Microstructural analysis of as-cast Mg–3Ca–xZr alloys showed grain refinement with Zr addition. The observed grain refinement was attributed to the growth restriction effect of Zr in hypoperitectic Mg–3Ca–0.3 wt% Zr alloys. Heterogeneous nucleation of α-Mg in properitectic Zr during solidification resulted in grain refinement of hyperperitectic Mg–3Ca–0.6 wt% Zr and Mg–3Ca–0.9 wt% Zr alloys. The hardness of Mg–3Ca–xZr alloys increased as the amount of Zr increased due to grain refinement and solid solution strengthening of α-Mg by Zr. Creep resistance of Mg–3Ca–xZr alloys increased with the addition of Zr due to solid solution strengthening of α-Mg by Zr. The calculated activation energy (Q_a) for Mg–3Ca samples (131.49 kJ/mol) was the highest among all alloy compositions. The Q_a values for 0.3, 0.6 and 0.9 wt% Zr containing Mg–3Ca alloys were 107.22, 118.18 and 115.24 kJ/mol, respectively.     

Monte Carlo Simulation Study of Diffuse Scattering in PZT,Pb(Zr,Ti)O<Subscript>3</Subscript>

Transverse polarized diffuse streaks have been observed in diffraction patterns of Pb(Zr_1?x Ti _x )O₃ (PZT) ceramics for compositions ranging from x = 0.3 (rhombohedral phase) to x = 0.7 (tetragonal phase) including the important morphotropic phase boundary (MPB) region (x = 0.48). The streaks correspond to diffuse planes of scattering in three dimensions, and these are oriented normal to the (cubic) \( \langle 111\rangle_{c} \) directions. A Monte Carlo (MC) model has been developed that convincingly reproduces the observed diffraction patterns. In this model, the displacements of Pb ions running in chains along each of the \( \langle 111\rangle_{c} \) directions are directed along the chain and are strongly correlated from cell to cell. There is no evidence of lateral correlation. Neighboring chains are essentially independent. At this stage, it is not clear what role the local order revealed by the scattering might play in governing the exceptional piezo-electric properties of the material, but its presence requires the currently accepted models for the average structure to be reassessed.     

Synthesis and Non-isothermal Carbothermic Reduction of FeTiO<Subscript>3</Subscript>-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> Solid Solution Systems

To investigate the carbothermic reduction behaviors of xFeTiO₃·(1 ? x)Fe₂O₃ solid solutions, the solid solutions with different x values were synthesized and used in the corresponding reactions. With an increase in x, the temperature pertaining to the onset of carbothermic reduction increased, while the rate of reduction of the solid solutions, α, decreased. The lattice parameters calculated from XRD patterns indicated that the solid solution with a higher x led to a larger lattice distortion. The non-isothermal kinetics were calculated, and an average activation energy E value of 3.0 × 10² kJ/mol was obtained.     

Influence of Grain Coarsening on the Creep Parameters During the Superplastic Deformation of a Severely Friction Stir Processed Al-Zn-Mg-Cu Alloy

During grain boundary sliding in ultrafine-grain materials at intermediate temperatures and high strain rates (~10^?2 s^?1), apparent creep parameters usually deviate from the theoretical values, due to microstructural coarsening. An analysis has been carried out in a severely friction stir processed (FSP) 7075 alloy with three different ultra-fine grain sizes (L), obtaining explicit grain size dependence of the creep parameters n _ap = n _ap(L) and Q _ap = Q _ap(L), confirming the validity of the theoretical values of these parameters in the constitutive equation.     

Kinetics study on non-isothermal crystallization of Cu<Subscript>50</Subscript>Zr<Subscript>50</Subscript> metallic glass

In this paper, the crystallization kinetics of melt-spun Cu₅₀Zr₅₀ amorphous alloy ribbons has been investigated using differential scanning calorimetry. Moreover, the Kissinger, Ozawa and isoconversional approaches have been used to obtain the crystallization kinetic parameters. As shown in the results, the onset crystallization activation energy E _x is less than crystallization peak activation energy E _p. The local activation energy E _α increases at the crystallized volume fraction α < 0.2 and decreases at the rest, which suggests that crystallization process is increasingly hard (α < 0.2) at first, after which it become increasingly easy (α > 0.2). The nucleation activation energy E _nucleation is greater than grain growth activation energy E _growth, indicating that the nucleation is harder than growth. In terms of the local Avrami exponent n(α), it lies between 1.27 and 8, which means that crystallization mechanism in the non-isothermal crystallization is interface-controlled one- two- or three-dimensional growth with different nucleation rates.     

Improvement of Creep Resistance at 950 °C and 400 MPa in Ru-Containing Single-Crystal Superalloys with a High Level of Co Addition

Microstructural features, including γ′ volume fraction and size, γ-γ′ lattice misfit, γ channel width, and dislocation substructure, are known to significantly influence the creep performance in Ni-base single-crystal superalloys. In this study, the microstructural characteristics of Ru-containing single-crystal superalloys with different levels of Co, Mo, and Ru additions were quantitatively investigated after ruptured and interrupted creep tests conducted at 1223 K (950 °C) and 400 MPa. The creep lifetime was slightly increased with the high level of Co addition and significantly increased with the coadditions of Mo and Ru. A minor effect of Co content on the γ channel width and γ′ volume fraction was found in experimental alloys. The alloy with high levels of Mo and Ru additions was determined to possess a more negative γ-γ′ lattice misfit, and a high density of stacking faults (SFs) was formed in the γ channels during creep. The combined effects of the SFs in the γ matrix serving as the barriers to dislocation movement, as well as the dense interfacial dislocation networks preventing dislocation to shear the γ′ phase, were considered as the main mechanism responsible for the improvement of creep resistance. Results from this study are helpful to understand the effect of microstructural features on creep performance and contribute to the knowledge of physical metallurgy in Ru-containing single-crystal superalloys.     

Suppression of Martensitic Transformation in Co<Subscript>2</Subscript>Cr(Ga,Si) Heusler Alloys by Thermal Cycling

We have investigated the influence of thermal cycles on martensitic transformation of a Co₂Cr(Ga,Si) ferromagnetic Heusler alloy. The as-quenched specimen exhibits successive L2₁(L)–D0₂₂–L2₁(H) martensitic transformation in the cooling process, which is known as reentrant martensitic transformation. However, heating to 800 K (527 °C) for reverse D0₂₂–L2₁ transformation with a rate of 10 K/min (10 °C/min) stabilizes the parent phase, meaning that the martensitic transformation is suppressed by the thermal cycles. We found precipitate after thermal cycles, and it will be the reason for the stabilization of parent phase.     

Crystal and Electronic Study of Neodymium-Substituted CuFeO<Subscript>2</Subscript> Oxide

Neodymium-substituted CuFeO₂ samples were investigated according to their crystal and electronic properties via the general formula Nd _x Cu_1?x FeO₂. The crystal structure analysis results revealed polycrystalline formations in the sample and a change in crystalline sizes with the substituted heavy fermion “Nd.” Increasing the Nd amount in the sample was determined to cause a disturbance on the Cu-Fe planes that supports the formation of crystal structures with low crystal symmetries such as monoclinic or triclinic geometries. To obtain the background mechanisms of the crystal properties, the X-ray absorption fine structure spectroscopy technique was used to study the electronic properties of the samples. Prominent changes in the crystal structures due to 4f electrons’ contributions from the substituted Nd atoms as the main “role player” in the phase transitions were determined. The Nd atoms were observed as the key element guiding the entire phenomenon as a result of their large size and narrow 4f levels. Also, magnetic properties of the samples were tested at room temperature and without an applied magnetic field by X-ray magnetic circular dichroism study due to previous studies that reported the parent oxide CuFeO₂ to have magnetic ordering at T _N = 11 K (?262 °C). Except the sample for x = 1.0 (NdFeO₃), no magnetic ordering was observed at room temperature; i.e., all of the samples showed paramagnetic behaviors.     

The Interface of TiB<Subscript>2</Subscript> and Al<Subscript>3</Subscript>Ti in Molten Aluminum

In the grain refinement of aluminum, Al₃Ti and TiB₂ particles are introduced to reduce the casting grain size down to 200 micrometer level, which makes cold working possible. The particles are brought in by the addition of Al-Ti-B-type master alloys. It is generally believed that TiB₂ particles are stable and nucleate α-Al grains in solidification in the presence of titanium in solution from the dissolution of Al₃Ti particles in the master alloys. The titanium in solution either forms Al₃Ti layers on the surface of TiB₂ particles to promote the nucleation of α-Al grains or remains as solute to restrict the growth of α-Al grains in solidification. However, a consensus on a grain refinement mechanism is still to be reached due to the lack of direct observation of the three phases in castings. This paper presents finding of the TiB₂/Al₃Ti interfaces in an Al-Ti-B master alloy. It demonstrates a strong epitaxial growth of Al₃Ti on the surface of TiB₂ particles, a sign of the formation of an Al₃Ti layer on the surface of TiB₂ particles in grain refinement practice. The Al₃Ti layer has a crystal coherency with α-Al and hence offers a substrate for heterogeneous nucleation of α-Al grains. However, the layer must be dynamic to avoid the formation of compounded Al₃Ti and TiB₂ particles leading to the loss of efficiency in grain refinement.     

Fabrication of Porous Ti-rich Ti<Subscript>51</Subscript>Ni<Subscript>49</Subscript> by Evaporating NaCl Space Holder

Net-shaped porous Ti-rich Ti₅₁Ni₄₉ alloy with well-controlled porosity, pore size, and pore shape are fabricated by pressing-and-sintering compacts containing fine Ti and Ni powders and coarse NaCl powders. After sintering at 1323 K (1050 °C) for 30 minutes in a high vacuum, the NaCl space holder is removed by evaporation, and the remaining Ti and Ni powders are sintered with about 2.3 vol pct liquid phase. The sintered Ti₅₁Ni₄₉ compacts have porosities of 26, 64, 70, 78, and 85 pct, and no distortion is observed. DSC tests show that the M _S temperature and ΔH are about 347 K (74 °C) and 28 J/g, respectively, and that they are almost independent of the porosity and close to those of wrought Ti-rich TiNi alloys. These porous Ti₅₁Ni₄₉ compacts exhibit a homogeneous microstructure, and the compressive properties and porosity are close to those of human bones.     

Infrared Brazing of Ti<Subscript>50</Subscript>Ni<Subscript>50</Subscript> Shape Memory Alloy and Inconel 600 Alloy with Two Ag-Cu-Ti Active Braze Alloys

Infrared brazing of Ti₅₀Ni₅₀ SMA and Inconel 600 alloy using Cusil-ABA and Ticusil filler metals has been investigated. The joints were dominated by Ag-Cu eutectic with proeutectic Cu in the Cusil-ABA brazed joint and with proeutectic Ag in the Ticusil one. A continuous curved belt composed of a Ni₃Ti layer and a (Cu _x Ni_1?x )₂Ti layer formed in the brazed Ti₅₀Ni₅₀/Ticusil/Inconel 600 joint. On the Ti₅₀Ni₅₀ SMA side, an intermetallic layer of (Cu _x Ni_1?x )₂Ti formed in all joints, with x values around 0.81 and 0.47. Layers of (Cu _x Ni_1?x )₂Ti, Ni₃Ti, and mixed Ni₃Ti and Ni₂Cr intermetallics were observed next to the Inconel 600 substrate in the brazed Ti₅₀Ni₅₀/Cusil-ABA/Inconel 600 joint. The maximum shear strengths of the joints using the Cusil-ABA filler metal and the Ticusil filler metal were 324 and 300 MPa, respectively. In the Cusil-ABA brazed joint, cracks with cleavage-dominated fracture propagated along the (Cu _x Ni_1?x )₂Ti interfacial layer next to the Ti₅₀Ni₅₀ SMA substrate. In the Ticusil brazed joint, ductile dimple fracture occurred in the Ag-rich matrix near the Inconel 600 alloy substrate. The absence of a detrimental Ti-Fe-(Cu) layer on the Inconel 600 substrate side can effectively improve the shear strength of the joint.     

Co Effect on As-cast and Heat-Treated Microstructures in Ru-Containing Single-Crystal Superalloys

The effect of Co on the as-cast and heat-treated microstructures was investigated in two experimental Ni-based single-crystal superalloys containing low levels of Re and Ru. The experimental results indicated that increasing the Co content from 7.9 to 15.8 wt pct decreased the volume fraction of (γ + γ′) eutectic and the solidification segregation ratio of W. High levels of Co additions were also found to decrease the solvus temperatures of the γ′ phase and (γ + γ′) eutectic as well as the solidus temperature. During the long-term thermal exposure at 1373 K (1100 °C), no TCP phases precipitated in either alloy. However, the coarsening and coalescence of γ′ precipitates in the alloy containing 15.8 wt pct Co was slower than that in the other alloy with 7.9 wt pct Co. In the current study, high levels of Co additions decreased the equilibrium volume fraction of γ′ phase, leading to a change in the partitioning ratios of TCP-forming elements Cr, Mo, Re, and W between the γ and γ′ phases. This change resulted in a lower degree of elemental supersaturation in the γ matrix and improved the phase stability of the γ/γ′ microstructure. These experimental results were then compared with those obtained from multi-component thermodynamic calculations, and good agreement was observed.     

The Role of Thermomechanical Routes on the Distribution of Grain Boundary and Interface Plane Orientations in Transformed Microstructures

In the current study, a series of thermomechanical routes were used to produce different microstructures (i.e., ferrite and martensite) in low-carbon low alloy steels. The five-parameter grain boundary character distribution was measured for all microstructures. The thermomechanical processing route altered the texture of the fully ferritic microstructure and significantly influenced the anisotropy of the grain boundary character distribution. Generally, the population of (111) planes increased with an increase in the γ-fiber texture for the ferritic microstructure, but it did not change the shape of the grain boundary plane distribution at specific misorientations. The most commonly observed boundaries in the fully ferritic structures produced through different routes were {112} symmetric tilt boundaries with the Σ3 = 60 deg/[111] misorientation; this boundary also had a low energy. However, the grain boundary plane distribution was significantly changed by the phase transformation path (i.e., ferrite vs martensite) for a given misorientation. In the martensitic steel, the most populous Σ3 boundary was the {110} symmetric tilt boundary. This results from the crystallographic constraints associated with the shear transformation (i.e., martensite) rather than the low-energy interface that dominates in the diffusional phase transformation (i.e., ferrite).     

Microstructure Evolution during Semi-solid Isothermal Holding of Liquidus Cast Strontium-Modified A356 Alloy

An attempt has been made to understand the microstructural evolution during semi-solid isothermal holding of liquidus cast Sr-modified A356 alloy. It was observed that the liquidus casting is effective in producing non-dendritic microstructure with average sphericity of 0.8 and 0.82 for unmodified and modified alloys, respectively. Isothermal holding of the samples at a temperature corresponding to solid fraction of 0.3 for 15 min enhanced the sphericity of unmodified and modified alloys to 0.84 and 0.85, respectively. Further increase in holding resulted grain coarsening and reduction in sphericity. The grain coarsening rate (K) of samples was analysed with Lifshitz–Slyozov–Wanger equation, and the values of K were found to be 58.3 and 98.3 µm³/s for unmodified and modified alloys, respectively. The grain coarsening in unmodified alloy was dominated by Ostwald ripening, whereas in case of modified alloy, grain coarsening was governed by grain coalescence.     

Microstructure and Properties of Fe<Subscript>3</Subscript>Al-Fe<Subscript>3</Subscript>AlC<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Composite Prepared by Reactive Liquid Processing

A Fe₃Al-Fe₃AlC _x composite was prepared using reactive liquid processing (RLP) through controlled mixture of carbon steel and aluminum in the liquid state. The microstructure and phases of the composite were assessed using X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, optical microscopy, and differential scanning calorimetry. In addition, the density, hardness, microhardness, and elastic modulus were evaluated. The Fe₃Al-Fe₃AlC _x composite consisted of 65 vol pct Fe₃Al and 35 vol pct Fe₃AlC _x (κ). The κ phase contained 10.62 at. pct C, resulting in the stoichiometry Fe₃AlC_0.475. The elastic modulus of the Fe₃Al-Fe₃AlC_0.475 composite followed the rule of mixtures. The RLP technique was shown to be capable of producing Fe₃Al-Fe₃AlC_0.475 with a microstructure and properties similar to those achieved using other processing techniques reported in the literature.     

Phase Stability and Transformations in the Zr<Subscript>2</Subscript>Ni<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Cu<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> Amorphous System

Since Ni and Cu differ by only one valence electron, yet have nearly identical atomic sizes (1.27 vs 1.28 Å for Cu and Ni, respectively), the amorphous Zr₂Ni _x Cu_1?x system is ideal for isolating the effects of electronic structure on short- and medium-range order and the concomitant influence of both the structure and order on devitrification pathways. Thermal analysis, time-resolved high-energy X-ray diffraction (HEXRD), and transmission electron microscopy (TEM) were used to follow metastable and stable crystalline phase formation during devitrification. Using HEXRD, we observed that the first devitrification product in the Zr₂Ni system is the C16 structure, if oxygen is kept sufficiently low, while the Zr₂Cu system forms the C11b structure. For x = 0.25, the initial devitrification involves forming coexisting C11b and C16 phases. When Ni is increased to x ≥ 0.50, the initial devitrification only involves the C16 structure. These results are in complete accord with electronic structure calculations showing that the enthalpy of formation for the C11b phase is favored for x = 0, while enthalpies for C11b and C16 are nearly identical for x = 0.25; the C16 phase has the most negative enthalpy for all compositions in which x > 0.25.     

Liquid Inclusions in Heat-Resistant Steel Containing Rare Earth Elements

Abundant thermodynamic data of pure substances were incorporated in the coupled thermodynamic model of inclusion precipitation and solute micro-segregation during the solidification of heat-resistant steel containing rare earth elements. The liquid inclusions Ce_2x Al_2y Si_1?x?y O _z (0 < x < 1, 0 < y < x and z = 1 ? x ? y) were first introduced to ensure the model more accurately. And the computational method for generation Gibbs free energy of liquid inclusions in molten steel was given. The accuracy of accomplished model was validated through plant trials, lab-scale experiments, and the data published in the literature. The comparisons of results calculated by FactSage with the model were also discussed. Finally, the stable area of liquid inclusions was predicted and the liquid inclusions with larger size were found in the preliminary experiments.     

Microstructural Evolution of the Interdiffusion Zone between U-9 Wt Pct Mo Fuel Alloy and Zr-1 Wt Pct Nb Cladding Alloy Upon Annealing

Diffusion couple formed between U-9 wt pct Mo and Zr-1 wt pct Nb alloys, proposed as fuel and clad materials, respectively, in nuclear research reactors, was annealed to investigate the microstructural evolution of the interdiffusion zone (IZ) as a function of temperature. A layered-type IZ microstructure was observed, the mechanism of development of which was elucidated. Mo₂Zr phase, present as dispersoids, in the U-rich part of the as-bonded IZ evolved into a continuous layer and into a “massive” morphology upon annealing. The discontinuous precipitation reaction in the matrix adjoining the Mo₂Zr phase, instigated by Mo depletion, generated lamellae of α-U phase within the γ-U(Mo,Zr) matrix. Zr-rich α-Zr(U) precipitates were observed in U-rich U-Mo-Zr matrix in the IZ next to the U-9Mo base material due to the clustering tendency of the matrix phase. The IZ next to Zr-1Nb base material comprised a “basket weave” microstructure of α-Zr laths with β-Zr(Nb,U) interlath boundaries, wherein an omega like transformation of the latter to δ-UZr₂ was also noticed. The growth rates of the IZ were orders of magnitude lower when compared with the ones reported between the compositionally similar U-10 wt pct Mo alloy and the presently used Al or Al-Si cladding alloys.     

Self-propagating high-temperature synthesis of ceramic materials based on the M<Subscript><Emphasis Type="Italic">n</Emphasis> + 1</Subscript>AX<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> phases in the Ti-Cr-Al-C system

Compact ceramic materials based on the M_{n + 1}AX _n phases in the Ti-Cr-Al-C system are produced by forced self-propagating high-temperature synthesis (SHS) compaction. The mechanisms of the structure and phase formation in synthetic products, as well as the combustion macrokinetics of the SHS mixture, are studied. Complex investigations of the structure, phase composition, and physical and mechanical properties of new Ti_{2 ? x} Cr _x AlC ceramic materials synthesized at different charging parameters (x = 0, 0.5, 1, 1.5, and 2) are performed. The highest content (96–98%) of the M_{n + 1}AX _n phase in the composition of synthetic products is found to be in samples where just one of the host elements (titanium (x = 0) or chromium (x = 2)) is present. The produced materials have a high heat resistance, and the increase in the chromium concentration is favorable to an appreciable growth in resistance to high-temperature oxidation.     

Morphological Stability of <Emphasis Type="Italic">δ</Emphasis>-Ferrite/<Emphasis Type="Italic">γ</Emphasis> Interphase Boundary in Carbon Steel

The morphological changes of the δ-ferrite/γ interphase boundary have been observed in situ with a high-temperature confocal scanning laser microscope (HTCSLM) during δ/γ transformations (δ → γ and γ → δ) of Fe-0.06 wt pct C-0.6 wt pct Mn alloy, and a kinetic equation of morphological stability of δ-ferrite/γ interphase boundary has been established. Thereafter, the criterion expression for morphological stability of δ-ferrite/γ interphase boundary was established and discussed, and the critical migration speeds of δ-ferrite/γ interphase boundaries are calculated in Fe-C, Fe-Ni, and Fe-Cr alloys. The results indicate that the δ-ferrite/γ interphase boundary is very stable and nearly remains absolute planar all the time during γ → δ transformation in Fe-C alloy. The δ-ferrite/γ interphase boundary remains basically planar during δ → γ transformation when the migration speed is lower than 0.88 μm/s, and the interphase boundary will be unstable and exhibit a finger-like morphology when the migration speed is higher than 0.88 μm/s. The morphological stability of δ-ferrite/γ interphase boundary is primarily controlled by the interface energy and the solute concentration gradient at the front of the boundary. During the constant temperature phase transformation, an opposite temperature gradient on both sides of δ-ferrite/γ interphase boundary weakens the steady effect of the temperature gradient on the boundary. The theoretical analysis of the morphological stability of the δ-ferrite/γ interphase boundary is coincident with the observed experimental results utilizing the HTCSLM. There is a good agreement between the theoretical calculation of the critical moving velocities of δ-ferrite/γ interphase boundaries and the experimental results.