Effect of α phase morphology on fatigue crack growth behavior of Ti−5Al−5Mo−5V−1Cr−1Fe alloy

Taking a Ti−5Al−5Mo−5V−1Cr−1Fe alloy as exemplary case, the fatigue crack growth sensitivity and fracture features with various tailored α phase morphologies were thoroughly investigated using fatigue crack growth rate (FCGR) test, optical microscopy (OM) and scanning electron microscopy (SEM). The tailored microstructures by heat treatments include the fine and coarse secondary α phase, as well as the widmanstatten and basket weave features. The sample with coarse secondary α phase exhibits better comprehensive properties of good crack propagation resistance (with long Paris regime ranging from 15 to 60 MPa·m^1/2), high yield strength (1113 MPa) and ultimate strength (1150 MPa), and good elongation (11.6%). The good crack propagation resistance can be attributed to crack deflection, long secondary crack, and tortuous crack path induced by coarse secondary α phase.     

Precipitate evolution in Mg−7Gd−3Y−1Nd−1Zn−0.5Zr alloy during isothermal ageing at 240 °C

The morphology and crystal structure of the precipitates in Mg−7Gd−3Y−1Nd−1Zn−0.5Zr alloy during isothermal ageing at 240 °C were investigated using transmission electron microscopy (TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). After under-ageing for 2 h, the precipitates in the alloy are ordered solute clusters with rare earth atomic columns exhibiting hexagonal ring structure, zigzag GP zones and β' in its early formation. After peak-ageing for 18 h, the precipitates are mainly β' and new rod-like β'_p accompanied with β'. After over-ageing for 100 h, the precipitates are β', β₁, long-period stacking ordered (LPSO) building block known as γ′ and 14H-LPSO. β' has the three-dimensional shape of convex lens with smaller length-to-width ratio viewed along 〈0001〉_α than that in the EW75 alloy. The excellent thermal stability of this alloy can be attributed to the γ' and 14H-LPSO retarding the growth of β' and β₁, low diffusion rate of rare earth atoms and physical character of β' and β₁.     

Experiment and modeling of TiB2/TiB boride layer of Ti−6Al−2Zr−1Mo−1V alloy

The influence of the boriding conditions on the boride layers was examined by boriding Ti−6Al−2Zr− 1Mo−1V alloy in the temperature range of 920−1120°C. The experimental results show that the boride layers were composed of a continuous thin outer layer of TiB₂ and a thick inner layer of TiB with whiskers or needle-like morphologies that extended into the substrate. Thick and compact boride layers were obtained when the boriding temperatures were 1000−1080 °C, and the treatment time exceeded 8 h. The boride layer depth increased with the boriding temperature and time, and the growth kinetics of the boride layers was characterized by a parabolic curve. The growth kinetics of the boride layers, including both TiB₂ and TiB layers, were predicted by establishing a diffusion model, which presented satisfactory consistency with the experimental data. As a result, the activation energies of boron in the TiB₂ and TiB layers were estimated to be 223.1 and 246.9 kJ/mol, respectively.     

Evolution of precipitates in Mg−7Gd−3Y−1Nd−1Zn−0.5Zr alloy with fine plate-like 14H-LPSO structures aged at 240 °C

The morphology and crystal structure of the precipitates in Mg−7Gd−3Y−1Nd−1Zn−0.5Zr (wt.%) alloy with fine plate-like 14H-LPSO structures aged at 240 °C were investigated using transmission electron microscopy (TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Fine plate-like 14H-LPSO structures precipitate after heat treatment at 500 °C for 2 h, and β-type phases precipitate after the alloy is aged at 240 °C. The long-period atomic stacking sequence of 14H-LPSO structures along the [0001]_α direction is ABABCACACACBABA. After being aged at 240 °C for 2 h, the β-type phases are the ordered solution clusters, zig-zag GP zones, and a small number of β′ phases. The peak hardness is obtained at 240 °C for 18 h with a Brinell hardness of 112, the β-type phases are β’ phases and local RE-rich structures. After being aged at 240 °C for 100 h, the β-type phases are β’, β₁ and β’_F phases. β′ phases nucleate from the zig-zag GP zones directly without β″ phases, and then transform into β₁ phase by β’→β’_F→β₁ transformations. The Zn not only can form LPSO structure, but also is the constituent element of β₁ phases. LPSO structures have a certain hindrance to the coarsening of β’ and β₁ along 〈0001〉_α.     

Shear localization behavior in hat-shaped specimen of near-α Ti−6Al−2Zr−1Mo−1V titanium alloy loaded at high strain rate

The microstructure characteristics in early stage shear localization of near-α Ti?6Al?2Zr?1Mo?1V titanium alloy were investigated by split Hopkinson pressure bar (SHPB) tests using hat-shaped specimens. The microstructural evolution and deformation mechanisms of hat-shaped specimens were revealed by electron backscattered diffraction (EBSD) method. It is found that the nucleation and expansion of adiabatic shear band (ASB) are affected by both geometric and structural factors. The increase of dislocation density, structure fragment and temperature rise in the deformation-affected regions provide basic microstructural conditions. In addition to the dislocation slips, the extension twins detected in shear region also play a critical role in microstructural fragmentation due to twin-boundaries effect. Interestingly, the sandwich structure imposes a crucial influence on ASB, which finally becomes a mature wide ASB in the dynamic deformation. However, due to much larger width, the sandwich structure in the middle of shear region is also possible to serve as favorable nucleation sites for crack initiation.     

High-temperature deformation and defect chemistry of (La1−xSrx)1−yMnO3+δ

The creep behavior of (La_1−xSr_x)_1−yMnO_3+δ has been studied as a function of oxygen partial pressure (P_O2) and Sr concentration. Polycrystalline samples (x=0.1, 0.2, 0.3) were deformed at 1523 K in constant-crosshead-speed compression tests in various atmospheres (10⁻²≤P_O2≤10⁵ Pa). The material deformed by grain-boundary sliding accommodated by lattice diffusion with some possible cavitation and/or interface reaction control. The defect-chemistry model which is standard in the literature could not explain the dependence of the stress on P_O2 and the Sr concentration. A modified defect-chemistry model shows that cation vacancies controlled the creep rate at P_O2≤10⁵ Pa for x=0.3 and at low P_O2 for x=0.1 and 0.2, and that oxygen vacancies were rate-controlling at high P_O2 for x=0.1 and 0.2.     

New Fe−Co−Ni−Cu−Al−Ti Alloy for Single-Crystal Permanent Magnets

A new alloy intended for single-crystal permanent magnets has been suggested. The new alloy has been designed based on the well-known Fe?Co?Ni?Cu?Al?Ti system and contains to 1 wt % Hf. The alloy demonstrates an enhanced potential ability for single-crystal forming in the course of unidirectional solidification of ingot. Single-crystal permanent magnets manufactured from this alloy are characterized by a high level of magnetic properties. When designing the new alloy, computer simulation of the phase composition and calculations of solidification parameters of complex metallic systems have been performed using the Thermo-Calc software and calculation and experimental procedures based on quantitative metallographic analysis of quenched structures. After the corresponding heat treatment, the content of high-magnetic phase in the alloy is 10% higher than that in available analogous alloys.     

Dielectric properties of Mg- and Mn-doped (Ba1−xSrx)(Ti1−yZry)O3

This study showed that the substitution of SrO and ZrO₂ in BaTiO₃ results in a change in the unit cell volume, Curie temperature shift and temperature dependence of dielectric constants. The unit cell volume decreased with an increasing SrO content (≤15 mol%), but increased with an increasing ZrO₂ (≤10 mol%). Substitution of Zr⁴⁺ ions for Ti sites in Ba(Ti_1?yZr_y)O₃ shifted the Curie point (T_c) to lower temperatures by 4.1 °C per mol% of ZrO₂ at y≤0.10. SrO substitution in the samples with 10 mol% ZrO₂ (Ba_1?xSr_x)(Ti_0.9Zr_0.1)O₃ brought about relatively small shifts in T_c (2.2 °C/mol%), and their dielectric constants decreased with an increasing SrO content near T_c. Mg- and Mn-doped (Ba_0.9Sr_0.1)(Ti_0.9Zr_0.1)O₃ broadened the dielectric constant peaks at T_c with uniform microstructures, which satisfies the temperature characteristics of Y5V specifications. The leakage currents of the Mg- and Mn-doped samples were significantly suppressed.     

High-performance half-Heusler thermoelectric materials Hf1−x ZrxNiSn1−ySby prepared by levitation melting and spark plasma sintering

Half-Heusler thermoelectric materials Hf_1?xZr_xNiSn_1?ySb_y (x = 0, 0.25, 0.4, 0.5; y = 0.02, 0.04, 0.06) have been prepared by levitation melting followed by spark plasma sintering or hot pressing. X-ray diffraction analysis and scanning electron microscopy observation show that single-phased half-Heusler compounds without compositional segregations have been obtained by levitation melting in a time-efficient manner. A small amount of Sb doping can improve the electrical power factor but undesirably increases the thermal conductivity due to the increased carrier thermal conductivity. The isoelectronic substitution of Zr for Hf substantially decreased the lattice thermal conductivity. A state-of-the-art ZT value of 1.0 has been attained at 1000 K for the levitation-melted and spark-plasma-sintered Hf_0.6Zr_0.4NiSn_0.98Sb_0.02, which is one of the highest achieved ZT values for half-Heusler thermoelectric alloys.     

TEM analysis of Gd5(SixGel−x)4, where x=1/2

The material Gd₅(Si_xGe_l−x)4, with x=1/2 was investigated using transmission electron microscopy. The material possesses a complex crystallographic structure and undergoes a unique, reversible, transformation at low temperatures and an irreversible high temperature transformation. The high, room and low temperature microstructures of arc-melted and heat treated samples are reported. Direct observation of the transformation with a discussion on the mechanism of the reversible, low temperature, phase transformation is detailed.     

Solubility of boron in Mo5+ySi3−y

Using X-ray analysis, optical microscopy, chemical analysis, and electron probe microanalysis (EPMA) it has been determined that the level of boron solubility in Mo_5+ySi_3−y at 1800°C reaches a maximum value of approximately 2 at% in a narrow region within the homogeneity range of −0.08⩽y⩽0.04. Both the B doped and undoped Mo_5+ySi_3−y (T1) have the tetragonal W₅Si₃ crystal structure, which corresponds to the I4/mcm space group. Within the single phase Mo_5+ySi_3−yB_x (T1) region there is a contraction in the lattice volume with increasing boron concentration while there is an increase in cell volume with an increase in Mo:Si ratio.     

Effects of Mg content on microstructure and mechanical properties of low Zn-containing Al−xMg−3Zn−1Cu cast alloys

Effects of Mg content on the microstructure and mechanical properties of low Zn-containing Al?xMg? 3Zn?1Cu cast alloys (x=3?5, wt.%) were investigated. As Mg content increased in the as-cast alloys, the grains were refined due to enhanced growth restriction, and the formation of η-Mg(AlZnCu)₂ and S-Al₂CuMg phases was inhibited while the formation of T-Mg₃₂(AlZnCu)₄₉ phase was promoted when Mg content exceeded 4 wt.%. The increase of Mg content encumbered the solution kinetics by increasing the size of eutectic phase but accelerated and enhanced the age-hardening through expediting precipitation kinetics and elevating the number density of the precipitates. As Mg content increased, the yield strength and tensile strength of the as-cast, solution-treated and peak-aged alloys were severally improved, while the elongation of the alloys decreased. The tensile strength and elongation of the peak-aged Al?5Mg?3Zn?1Cu alloy exceed 500 MPa and 5%, respectively. Precipitation strengthening implemented by T′ precipitates is the predominant strengthening mechanism in the peak-aged alloys and is enhanced by increasing Mg content.     

Onset of plasticity in InxGa1−xAs multilayers

Coherently strained In_xGa_1?xAs multilayers with zero net strain have been investigated in order to determine the influence of coherency strains on the yield stress. The deformation behaviour has been studied using both ex- and in situ nanoindentation and transmission electron microscopy. Nanoindentation showed that the pressure required for the onset of yield decreased with increasing coherency strain, consistent with previous work, while the hardness remained constant. Transmission electron microscopy revealed that deformation was more prevalent in one layer. Using these observations, a straightforward analysis has been developed in which the yield pressure of the multilayer is related to the onset of flow in the weaker layer determined by both its intrinsic yield pressure and coherency strains. This gives good agreement with the experimental observations and is consistent with the observations of the effects of internal stresses in films elsewhere in the literature.     

Microstructure and properties of Cu−2Cr−1Nb alloy fabricated by spark plasma sintering

Cu?2Cr?1Nb alloy was fabricated by spark plasma sintering (SPS) using close coupled argon-atomized alloy powder as the raw material. The optimal SPS parameters obtained using the L₉(3⁴) orthogonal test were 950 °C, 50 MPa and 15 min, and the relative density of the as-sintered alloy was 99.8%. The rapid densification of SPS effectively inhibited the growth of the Cr₂Nb phase, and the atomized powder microstructure was maintained in the grains of the alloy matrix. Uniformly distributed multi-scale Cr₂Nb phases with grain sizes of 0.10?0.40 μm and 20?100 nm and fine grains of alloy matrix with an average size of 3.79 μm were obtained. After heat treatment at 500 °C for 2 h, the room temperature tensile strength, electrical conductivity, and thermal conductivity of the sintered Cu?2Cr?1Nb alloy were 332 MPa, 86.7% (IACS), and 323.1 W/(m·K), respectively, and the high temperature tensile strength (700 °C) was 76 MPa.     

Plasma Spray Physical Vapor Deposition of La1−x Sr x Co y Fe1−y O3−δ Thin-Film Oxygen Transport Membrane on Porous Metallic Supports

Plasma spray physical vapor deposition (PS-PVD) is a very promising route to manufacture ceramic coatings, combining the efficiency of thermal spray processes and characteristic features of thin PVD coatings. Recently, this technique has been investigated to effectively deposit dense thin films of perovskites particularly with the composition of La_0.58Sr_0.4Co_0.2Fe_0.8O_3?δ (LSCF) for application in gas separation membranes. Furthermore, asymmetric type of membranes with porous metallic supports has also attracted research attention due to the advantage of good mechanical properties suitable for use at high temperatures and high permeation rates. In this work, both approaches are combined to manufacture oxygen transport membranes made of gastight LSCF thin film by PS-PVD on porous NiCoCrAlY metallic supports. The deposition of homogenous dense thin film is challenged by the tendency of LSCF to decompose during thermal spray processes, irregular surface profile of the porous metallic substrate and crack and pore-formation in typical ceramic thermal spray coatings. Microstructure formation and coating build-up during PS-PVD as well as the annealing behavior at different temperatures of LSCF thin films were investigated. Finally, measurements of leak rates and oxygen permeation rates at elevated temperatures show promising results for the optimized membranes.     

Microstructure and mechanical properties of high strength Mg−15Gd−1Zn−0.4Zr alloy additive-manufactured by selective laser melting process

In order to verify the feasibility of producing Mg−rare earth (RE) alloy by selective laser melting (SLM) process, the microstructure and mechanical properties of Mg−15Gd−1Zn−0.4Zr (wt.%) (GZ151K) alloy were investigated. The results show that fine grains (~2 μm), fine secondary phases and weak texture, were observed in the as-fabricated (SLMed) GZ151K Mg alloy. At room temperature, the SLMed GZ151K alloy has a yield strength (YS) of 345 MPa, ultimate tensile strength (UTS) of 368 MPa and elongation of 3.0%. After subsequent aging (200 °C, 64 h, T5 treatment), the YS, UTS and elongation of the SLMed-T5 alloy are 410 MPa, 428 MPa and 3.4%, respectively, which are higher than those of the conventional cast-T6 alloy, especially with the YS increased by 122 MPa. The main strengthening mechanisms of the SLMed GZ151K alloy are fine grains, fine secondary phases and residual stress, while after T5 treatment, the YS of the alloy is further enhanced by precipitates.     

Microstructure and shear strength of γ-TiAl/GH536 joints brazed with Ti−Zr−Cu−Ni−Fe−Co−Mo filler alloy

The influence of brazing temperature and brazing time on the microstructure and shear strength of γ-TiAl/GH536 joints brazed with Ti−Zr−Cu−Ni−Fe−Co−Mo filler was investigated using SEM, EDS, XRD and universal testing machine. Results show that all the brazed joints mainly consist of four reaction layers regardless of the brazing temperature and brazing time. The thickness of the brazed seam and the average shear strength of the joint increase firstly and then decrease with brazing temperature in the range of 1090−1170 °C and brazing time varying from 0 to 20 min. The maximum shear strength of 262 MPa is obtained at 1150 °C for 10 min. The brittle Al₃NiTi₂ and TiNi₃ intermetallics are the main controlling factors for the crack generation and deterioration of joint strength. The fracture surface is characterized as typical cleavage fracture and it mainly consists of massive brittle Al₃NiTi₂ intermetallics.