Microstructure and mechanical properties of Mg–5Li–3Al–2Zn–<Emphasis Type="Italic">x</Emphasis>RE alloys

Mg–5Li–3Al–2Zn–xRE alloys were prepared. The microstructure and mechanical properties of as-cast and wrought specimens were studied. RE elements in Mg–5Li–3Al–2Zn alloy cause the microstructure refinement and the formation of Al₃La, which bring about the improvement of mechanical properties of alloys. The optimal RE content for Mg–5Li–3Al–2Zn alloy is 2 wt%, which makes the microstructure the finest and the mechanical properties the best. The further increase of RE content makes the microstructure be coarsened and the morphology of Al₃La change from particular to rod-like and pearlite-like eutectic shape, leading to the poor mechanical properties of alloys.     

Microstructure,tensile properties,and creep behavior of high-pressure die-cast Mg–4Al–4RE–0.4Mn (RE = La,Ce) alloys

High-pressure die-cast (HPDC) Mg–4Al–4RE–0.4Mn (RE = La, Ce) magnesium alloys were prepared and their microstructures, tensile properties, and creep behavior have been investigated in detail. The results show that two binary Al–Ce phases, Al₁₁Ce₃ and Al₂Ce, are formed mainly along grain boundaries in Mg–4Al–4Ce–0.4Mn alloy, while the phase composition of Mg–4Al–4La–0.4Mn alloy contains only α-Mg and Al₁₁La₃. The Al₁₁La₃ phase comprises large coverage of the grain boundary region and complicated morphologies. Compared with Al₁₁Ce₃ phase, the higher volume fraction and better thermal stability of Al₁₁La₃ have resulted in better-fortified grain boundaries of the Mg–4Al–4La–0.4Mn alloy. Thus higher tensile strength and creep resistance could be obtained in Mg–4Al–4La–0.4Mn alloy in comparison with that of Mg–4Al–4Ce–0.4Mn. Results of the theoretical calculation that the stability of Al₁₁La₃ is the highest among four Al–RE intermetallic compounds supports the experimental results further.     

Strong correlation between the cation ordering and magnetic properties of anodically electrodeposited Mn–Co–O nanocrystals

The rock salt-to-spinel structural transformation that occurs in anodically electrodeposited Mn–Co–O nanocrystals involves a rearrangement of Mn/Co cations from octahedral interstices to tetrahedral interstices. The cation ordering process leads to distinct magnetic properties. Curie temperature (T _C) and blocking temperature (T _B) increase dramatically with annealing temperature (200–400 °C), while the corresponding change in particle size for the oxide nanocrystals is rather small. A strong correlation between the magnetic properties and the cation ordering degree in annealed Mn–Co–O nanocrystals was established. These unique magnetic properties can be attributed to the magnetic moment changes induced by Mn/Co cation ordering from octahedral interstices to tetrahedral interstices in the annealed Mn–Co oxide nanocrystals.     

Quasicrystal–crystal structural transformation in Al–5 wt.% Mn alloy

The atomic structure of Al–5 wt.%Mn (Al–5Mn) alloy, prepared by rapid solidification, and pre-annealed at 623 and 773 K for 5 and 1 h, respectively, were characterized by X-ray powder diffraction (XRD) and extended X-ray absorption fine structure (XAFS) techniques. The sample in as-quenched stage was found crystalline, consisting of metastable α-Al (Al–Mn solid solution) and icosahedral quasicrystalline I-Al₆Mn phases. Five hours annealing at 623 K proved thermal stability of both the phases. Pre-annealing at 773 K/1 h on the other hand leads to α-Al phase decomposition and structural transformation of metastable I-Al₆Mn to stable orthorhombic Al₆Mn phase. The EXAFS results indicate that Mn atoms are located preferably on the outer shell of icosahedrons. During the I-Al₆Mn→o-Al₆Mn transformation the total Al atoms coordinating one Mn were found to be constant (∼10). Based on the results, only distance/symmetry changes in atomic arrangement around Mn atoms were suggested.     

Comparison about as-cast microstructures and mechanical properties of Mg–4Y–1.2Mn–0.9Sc and Mg–4Y–1.2Mn–1Zn (wt%) magnesium alloys

In this article, the as-cast microstructures and mechanical properties of the Mg–4Y–1.2Mn–0.9Sc and Mg–4Y–1.2Mn–1Zn (wt%) magnesium alloys are investigated and compared. The results indicate that the Sc-containing alloy is mainly composed of α-Mg and fine particle-like Mg₂₄Y₅, Mn₁₂Y, and Mn₂Sc phases, while the Zn-containing alloy mainly consists of α-Mg and coarse Mg₁₂YZn phases with a continuous network. Furthermore, the grains of the Zn-containing alloy are relatively finer than those of the Sc-containing alloy. In addition, the Sc-containing alloy exhibits relatively higher tensile properties at room temperature and 300 °C than the Zn-containing alloy. However, the creep properties at 300 °C and 30 MPa for 100 h for the Sc-containing alloy are relatively lower than those for the Zn-containing alloy.     

Wear characteristics of a new type austenitic stainless steel

In order to solve the problem of the poor wear resistance in conventional austenitic stainless steels, a new type austenitic stainless steel was designed based on Fe–Mn–Si–Cr–Ni shape memory alloys in this article. Studies on its wear resistance and wear mechanism have been carried out by comparison with that of AISI 321 stainless steel using friction wear tests, X-ray diffraction, scanning electron microscope. Results showed that the wear resistance of Fe–14Mn–5.5Si–12Cr–5Ni–0.10C alloy was better than that of AISI 321 stainless steel both in dry and oily friction conditions owing to the occurrence of the stress-induced γ → ε martensitic phase transformation during friction process. This article also compared the corrosion performance of the two stainless steels by testing the corrosion rate. Results showed that the corrosion rate of Fe–14Mn–5.5Si–12Cr–5Ni–0.10C alloy was notably lower in NaOH solution and higher in NaCl solution than that of AISI 321 stainless steel.     

Deformation behavior of an Al–Cu–Mg–Mn–Zr alloy during hot compression

Deformation behavior of an Al–Cu–Mg–Mn–Zr alloy during hot compression was characterized in present work by high-temperature testing and transmission electron microscope (TEM) studies. The true stress–true strain curves exhibited a peak stress at a critical stain. The peak stress decreased with increasing deformation temperature and decreasing strain rate, which can be described by Zener–Hollomon (Z) parameter in hyperbolic sine function with the deformation activation energy 277.8 kJ/mol. The processing map revealed the existence of an optimum hot-working regime between 390 and 420 °C, under strain rates ranging from 0.1 to 1 s⁻¹. The main softening mechanism of the alloy was dynamic recovery at high lnZ value; continuous dynamic recrystallization (DRX) occurred as deformed at low lnZ value. The dynamic precipitation of Al₃Zr and Al₂₀Cu₂Mn₃ dispersoids during hot deformation restrained DRX and increased the hot deformation activation energy of the alloy.     

Corrosion study of single crystal Ni–Mn–Ga alloy and Tb<Subscript>0.27</Subscript>Dy<Subscript>0.73</Subscript>Fe<Subscript>1.95</Subscript> alloy for the design of new medical microdevices

Once placed in a magnetic field, smart magnetic materials (SMM) change their shape, which could be use for the development of smaller minimally invasive surgery devices activated by magnetic field. However, the potential degradation and release of cytotoxic ions by SMM corrosion has to be determined. This paper evaluates the corrosion resistance of two SMM: a single crystal Ni–Mn–Ga alloy and Tb_0.27Dy_0.73Fe_1.95 alloy. Ni–Mn–Ga alloy displayed a corrosion potential (E _corr) of −0.58 V/SCE and a corrosion current density (i _corr) of 0.43 μA/cm². During the corrosion assay, Ni–Mn–Ga sample surface was partially protected; local pits were formed on 20% of the surface and nickel ions were mainly found in the electrolyte. Tb_0.27Dy_0.73Fe_1.95 alloy exhibited poor corrosion properties such as E _corr of −0.87 V/SCE and i _corr of 5.90 μA/cm². During the corrosion test, this alloy was continuously degraded, its surface was impaired by pits and cracks extensively and a high amount of iron ions was measured in the electrolyte. These alloys exhibited low corrosion parameters and a selective degradation in the electrolyte. They could only be used for medical applications if they are coated with high strain biocompatible materials or embedded in composites to prevent direct contact with physiological fluids.     

The enhancement of creep in Al–22?wt% Ag alloy by cyclic stressing

The plastic deformation behavior of Al–22 wt% Ag alloy during phase transformation was investigated by studying the creep behavior under cyclic stress reduction of low frequencies. The cyclic creep curves obtained describe clearly the cyclic stress acceleration behavior. Increasing frequency of cyclic stress reduction enhanced the creep deformation depending upon the combination of the experimental variables as testing temperature, aging temperature and static creep rate. The irregularity in the creep parameters, n, β and ε_st with increasing aging temperatures, has been explained on the basis of structure transformations occurring in Al–Ag system and their mode of interaction with mobile dislocations.     

Nanoscale precipitation and mechanical properties of Al-0.06 at.% Sc alloys microalloyed with Yb or Gd

Dilute Al-0.06 at.% Sc alloys with microalloying additions of 50 at. ppm of ytterbium (Yb) or gadolinium (Gd) are studied with 3D local-electrode atom-probe (LEAP) tomography for different aging times at 300 °C. Peak-aged alloys exhibit Al₃(Sc_1−x Yb _x ) or Al₃(Sc_1−x Gd _x ) precipitates (L1₂ structure) with a higher number density (and therefore higher peak hardness) than a binary Al-0.06 at.% Sc alloy. The Al–Sc–Gd alloy exhibits a higher number density of precipitates with a smaller average radius than the Al–Sc–Yb alloy, leading to a higher hardness. In the Al–Sc–Gd alloy, only a small amount of the Sc is replaced by Gd in the Al₃(Sc_1−x Gd _x ) precipitates, where x = 0.08. By contrast, the hardness incubation time is significantly shorter in the Al–Sc–Yb alloy, due to the formation of Yb-rich Al₃(Yb_1−x Sc _x ) precipitates to which Sc subsequently diffuses, eventually forming Sc-rich Al₃(Sc_1−x Yb _x ) precipitates. For both alloys, the precipitate radii are found to be almost constant to an aging time of 24 h, although the concentration and distribution of the RE elements in the precipitates continues to evolve temporally. Similar to microhardness at ambient temperature, the creep resistance at 300 °C is significantly improved by RE microalloying of the binary Al-0.06 at.% Sc alloy.     

Effects of minor Zr and Sr on as-cast microstructure and mechanical properties of Mg–3Ce–1.2Mn–1Zn (wt%) magnesium alloy

The effects of minor Zr and Sr on the as-cast microstructure and mechanical properties of the Mg–3Ce–1.2Mn–1Zn (wt%) alloy were investigated. The results indicate that adding minor Zr and/or Sr to the Mg–3Ce–1.2Mn–1Zn alloy does not cause an obvious change in the morphology and distribution of the Mg₁₂Ce phase. However, the grains of the Zr- and/or Sr-containing alloys are effectively refined. Among the Zr- and/or Sr-containing alloys, the grains of the alloy with the addition of 0.5 wt%Zr + 0.1 wt%Sr are the finest. Furthermore, adding minor Zr and/or Sr to the Mg–3Ce–1.2Mn–1Zn alloy can improve the tensile properties. Among the Zr- and/or Sr-containing alloys, the alloy with the addition of 0.5 wt%Zr + 0.1 wt%Sr obtains the optimum tensile properties. In addition, adding minor Zr and/or Sr to the Mg–3Ce–1.2Mn–1Zn alloy also can improve the creep properties, and the creep properties of the three alloys with the additions of 0.5 wt%Zr + 0.1 wt%Sr, 0.5 wt%Zr, and 0.1 wt%Sr are similar.     

Microwave dielectric properties and low-temperature sintering of (Zn0.8Mg0.2)2SiO4–TiO2 ceramics with Li2O–B2O3

Effects of Li₂O–B₂O₃ on the sintering behavior and the microwave dielectric properties of (Zn_0.8Mg_0.2)₂SiO₄–TiO₂ ceramics were investigated as a function of Li₂O–B₂O₃ content and sintering temperature. The Li₂O–B₂O₃ combined additives successfully reduced the sintering temperature of (Zn_0.8Mg_0.2)₂SiO₄–TiO₂ ceramics from 1,250 °C to 900 °C. With the increase of Li₂O–B₂O₃ content, the TiO₂ phase decreased and the unknown second phase increased, which led to the dielectric constant (ε _r ) and the maximum Q × f value decrease, and the temperature coefficient of resonant frequency (τ _f ) shift to a negative value. The specimens with 3 wt%Li₂O–B₂O₃ sintered at 900 °C for 2 h showed ε _r of 8.84, Q × f value of 15,500 GHz, and τ _f of 17.8 ppm/°C. And the material was compatible with Ag electrodes, which made it a promising ceramic for low temperature co-fired ceramics technology application.     

Phase, microstructure and microwave dielectric properties of Zr-doped SrLa4Ti5?xZrxO17

The effect of Zr⁺⁴ substitution for Ti⁺⁴ on the phase, microstructure and microwave dielectric properties of compositions in the SrLa₄Ti_5−xZr_xO₁₇ (0 ≤ x ≤ 0.1) series was investigated. All the compositions formed single phase ceramics within the detection limit of in-house X-ray diffractometer when sintered in the temperature range 1,450–1,580 °C for 4 h in air. The substitution of Zr⁺⁴ for Ti⁺⁴ enabled processing of highly dense ceramics with a decrease in temperature coefficient of resonant frequency (τ_f) from ~117 to 70 ppm/°C, dielectric constant (ε_r) from 60.8 to 57.3, with no significant effect on the quality factor. In the present study, optimum properties i.e. ε_r ~ 57.3, τ_f ~ 70 ppm/°C and quality factor (Q _u  × f _o ) ~ 9,841 GHz, were achieved for SrLa₄Ti_4.9Zr_0.1O₁₇. The trend of the results demonstrates that further studies with increased Zr⁺⁴ content are required to achieve ultra low loss SrLa₄Ti_5−xZr_xO₁₇ ceramics.     

Microstructure modification of Al–17%Si alloy by addition of Mg

The calculated phase diagrams of Al–17%Si alloy with additions of Mg up to 10 wt% Mg have indicated two critical compositions at 4.6 and 6.8% Mg where the liquidus, the binary reaction temperatures as well as the temperature range of the formation of Mg₂Si particles are changed. The eutectic formation temperature is decreased with the addition of Mg up to 4.6% Mg, followed by constant value due to the change of the eutectic formation reaction from the binary (Liq. → Al ± Si) to the ternary (Liq. → Al + Si + Mg₂Si) reaction. This change contributes to the transformation of the eutectic silicon in matrix from long platelet, to fine and compact particles resulting in overall increased hardness of the high Mg alloys even though the hardness of the primary Mg₂Si particles is much smaller than that of the primary silicon particles. A pronounced decrease of both primary and eutectic silicon particles size was also observed for 0.4% Mg alloy when compared to the base alloy showing significant microstructural modification.     

Influence of Sm substitution on the phase,microstructure and microwave dielectric properties of SrLa<Subscript>4</Subscript>Ti<Subscript>5</Subscript>O<Subscript>17</Subscript>

New dielectric ceramics in the SrLa_4−xSm_xTi₅O₁₇ (0 ≤ x ≤ 4) composition series were prepared through a solid state mixed oxide route to investigate the effect of Sm⁺³ substitution for La⁺³ on the phase, microstructure and microwave dielectric properties. At x = 0–3, all the compositions formed single phase ceramics within the detection limit of in-house X-ray diffraction when sintered in the temperature range 1500–1580 °C. At x = 4, a mixture of Sm₂Ti₂O₇ and SrTiO₃ formed. The maximum Sm⁺³-containing single phase ceramics, SrLaSm₃Ti₅O₁₇, exhibited relative permittivity (ε_r) = 42.6, temperature coefficient of resonant frequency (τ _f ) = −96 ppm/^oC and quality factor (Q _u f _o ) = 7332 GHz. An analysis of results presented here indicates that SrLa_4−xSm_xTi₅O₁₇ ceramics, exhibiting τ _f  ~ 0 and ε_r ~ 53 could be achieved at x ~ 1.4 but at the cost of decrease in Q _u f _o .     

Effect of sintering conditions on resistivity of nanoparticle Mn–Zn ferrite prepared by nitrilotriacetate precursor method

Mn–Zn spinel ferrites are most important class of magnetic materials owing to their high saturation magnetization, high permeability, low loss and interesting applications in various fields. The magnetic as well as electrical properties of these ferrites depend on relative distribution of cations at different sites, grain size, sintering conditions as well as preparative conditions. Nanoparticle Mn–Zn ferrite material having general formula Mn _x Zn_1 − x Fe₂O₄ with x = 0.35/0.4/0.45/0.5/0.55/0.6/0.65 were synthesized using nitrilotriacetate precursor method and characterized using standard techniques. The resistivity measurements of all these samples were carried out after sintering the same in nitrogen atmosphere at 1,050 °C/1,150 °C/1,250 °C/1,350 °C, respectively. High resistivity values obtained for the system of materials would provide a low eddy current loss material for wide ranging applications in electronics and telecommunications. Semiconductor like behavior of the material with resistivity variation over large range of temperature is ideal characteristic essential for materials in sensor applications.     

FCC to HCP transformation kinetics in a Co–27Cr–5Mo–0.23C alloy

In this work the kinetic aspects associated with the FCC → HCP martensitic transformation in a Co–27Cr–5Mo–0.23C alloy processed by powder metallurgy were investigated. In situ X-ray diffraction during isochronous heat treatments in a hot stage indicated that a fully metastable FCC matrix transforms rather fast at temperatures above 725 °C and reaches a maximum transformation into the HCP phase at 940 °C. Alternatively, when the matrix is HCP, some HCP martensite reverts to metastable FCC. Apparently, at low temperatures carbon excess in the HCP martensite promotes the reversal to metastable FCC. In addition, the volume percent of ε-martensite precipitated from stable FCC was determined as a function of time and temperature during isothermal aging between 675 and 900 °C. From these results, TTT diagrams were plotted for a 1% HCP transformed martensite. Maximum transformation rates were found to occur between 825 and 850 °C and activation energies, Q _s of 41–52 kcal/mol were estimated from the experimental outcome. The aged microstructures indicated that below 800 °C, the isothermal transformation was dominated by a lamellar morphology. Nevertheless, aging above 800 °C promoted carbide nucleation and coarsening along the grain boundaries independently of the FCC → HCP martensitic transformation.     

Role of some rare earth (RE) ions (RE = La, Pr, Nd, Sm, Gd and Dy) in crystal and mechanical behaviours of sol-gel derived ZrO2-2 mol% REO2O3 spun fibres calcined at 1300°C

Crystal behaviours such as crystallization temperature (amorphous to tetragonal (t) zirconia), tendency of phase transformation (tetragonal to monoclinic (m) zirconia) and lattice strain were studied with mechanical property e.g. tensile strength of sol-gel derived ZrO₂-2 mol% RE₂O₃ (RE = La, Pr, Nd, Sm, Gd and Dy) spun fibres. Rare earth cations of varying sizes played a significant role in changing the above mentioned properties of ZrO₂-2 mol% RE₂O₃ fibres. It was found that with decreasing the ionic size difference between the zirconium and RE ions, crystallization temperature (amorphous →)t-ZrO₂) decreased, the probability of phase transformation (t→m) decreased, lattice strain which is related to lattice distortion decreased and tensile strength increased.     

Microwave dielectric properties of Ca4La2Ti5O17–LaAlO3 system ceramic materials

Compositions in the (1 − x) Ca₄La₂Ti₅O₁₇–xLaAlO₃ system were prepared in order to modify the positive temperature coefficient of the resonant frequency (τ _f ) of Ca₄La₂Ti₅O₁₇. The microwave dielectric properties and phase composition of this system ceramics were investigated. X-ray powder diffraction results showed that Ca₄La₂Ti₅O₁₇ and LaAlO₃ formed a solid solution only when x ≤ 0.2. The τ _f values showed a near linear decrease with increasing additions of LaAlO₃. It was observed that near zero τ _f value can be achieved as x = 0.64. The permittivity (ε_r) and the quality factor (Q × f) values exhibited a non-linear behavior with LaAlO₃ additions. The microwave dielectric properties were strongly correlated with composition, secondary phases and grain sizes. For practical application, a permittivity (ε_r) of 42, a quality factor (Q × f value) of 12,450 GHz and a temperature coefficient of resonant frequency (τ _f ) of ~0 ppm/°C for 0.36 Ca₄La₂Ti₅O₁₇−0.64 LaAlO₃ were proposed.     

Microwave dielectric properties of Ba<Subscript>5</Subscript>Nb<Subscript>4</Subscript>O<Subscript>15</Subscript> ceramic by molten salt method

Ba₅Nb₄O₁₅ powders were synthesized by molten-salt method in NaCl–KCl flux at a low temperature of 650–900 °C for 2 h, which is lower than that of the conventional solid-state reaction. This simple process involved mixing of the raw materials and salts in a certain proportion. Subsequent calcination of the mixtures led to Ba₅Nb₄O₁₅ powders at 650–900 °C. XRD and SEM techniques were used to characterize the phase and morphology of the fabricated Ba₅Nb₄O₁₅ powders, respectively. After sintering at 1,300 °C for 2 h, the densified Ba₅Nb₄O₁₅ ceramics with good microwave dielectric properties of ε_r = 39.2, Q × f approximated as 27,200 GHz and τ _f  = 72 ppm/°C have been obtained.