Preparation and characterization of As-cast and hot-rolled Mg–3Al–0.5Mn–0.5Zn–1MM alloy

Mg–3Al–0.5Mn–0.5Zn–1MM alloy was prepared by metal mould casting method. The as-cast ingot was homogenized and then hot-rolled at 673 K with total thickness reduction of 65%. Microstructure and mechanical properties of the as-cast and hot-rolled samples were investigated. The results showed that the as-cast sample mainly consisted of α-Mg, β-Mg₁₇Al₁₂, Al₁₀Ce₂Mn₇, and Al₁₁RE₃ (RE = La and Ce) phases. The average grain size of the sample homogenized at 673 K was about 240 μm, and it was greatly refined to about 7 μm by dynamic recrystallization for the hot-rolled sample. The ultimate tensile strength and 0.2% yield strength of the hot-rolled sample were 300 MPa and 230 MPa, respectively. They were enhanced by 55% and 400% correspondingly compared with those of the as-cast sample. The improvement of the strengths was attributed to the refined grains, breakup of the precipitates and increase of the dislocation density.     

Structure and piezoelectric properties of K0.5Na0.5NbO3–Bi0.5Li0.5TiO3 lead-free ceramics

Lead-free (1−x) K_0.5Na_0.5NbO₃–xBi_0.5Li_0.5TiO₃ + 1 mol% MnO₂ piezoelectric ceramics have been prepared by a conventional ceramic technique and their structure and piezoelectric properties have been studied. Our results reveal that Bi_0.5Li_0.5TiO₃ diffuse into K_0.5Na_0.5NbO₃ lattices to form a solid solution with a perovskite structure. The addition of Bi_0.5Li_0.5TiO₃ to the K_0.5Na_0.5NbO₃ solid solution decreases the paraelectric cubic-ferroelectric tetragonal phase transition temperature (T _C) slightly, but shifts the ferroelectric tetragonal-ferroelectric orthorhombic phase transition temperature (T _O−T) significantly to low temperatures. As a result, coexistence of the orthorhombic and tetragonal phases is formed at 0.01 < x < 0.03 near room temperature, leading to a significant improvement in the piezoelectric properties of the ceramics. The ceramic with x = 0.025 exhibits a relatively high T _C (392 °C) and optimum piezoelectric properties: d ₃₃ = 191 pC/N, k _p = 51.5% and k _t = 45.5%. The ceramic also exhibit a good thermal stability of piezoelectric properties.     

Piezoelectric and dielectric properties of Bi0.5Na0.5TiO3–Bi0.5Li0.5TiO3 lead-free ceramics

(1 − x)Bi_0.5Na_0.5TiO₃–xBi_0.5Li_0.5TiO₃ lead-free ceramics have been prepared by a conventional solid-state reaction method, and their piezoelectric and dielectric properties have been studied. X-ray diffraction studies reveal that Li⁺ diffuses into the Bi_0.5Na_0.5TiO₃ lattices to form a solid solution with a pure perovskite structure. The addition of Bi_0.5Li_0.5TiO₃ effectively lowers the sintering temperature of the ceramics and greatly assists in the densification of the ceramics. The ceramic with x = 0.075 possesses the optimum piezoelectric properties: piezoelectric coefficient d ₃₃ = 121 pC/N and planar electromechanical coupling factor k _P = 18.3%. After the partial substitution of Li⁺ for Na⁺ in the A-sites of Bi_0.5Na_0.5TiO₃, the ceramics exhibit more relaxor characteristic, which is probably resulted from the cation disordering in the 12-fold coordination sites. The depolarization temperature T _d shifts to low temperature with the substitution level x of Li⁺ for Na⁺ increasing.     

Hydrogen-induced intergranular stress corrosion cracking (HI-IGSCC) of 0.35C–3.5Ni–1.5Cr–0.5Mo steel fastener

This paper brings a failure case study of high strength 0.35C–3.5Ni–1.5Cr–0.5Mo steel fastener, which failed due to hydrogen-induced intergranular stress corrosion cracking (HI-IGSCC). 0.35C–3.5Ni–1.5Cr–0.5Mo steel in hardened and tempered condition, meeting the specified axial tensile stress rating of 1250 MPa is widely used as fasteners in space programmes.In the course of assembly of the structural parts of a satellite launch vehicle, 10 nos of fasteners developed cracks on tightening using a torque wrench set to 6 N m torque surprisingly.Also some fasteners, which were under assembly load of more than 6 months in the same vehicle assembly, were found to be cracked.The failure was attributed to hydrogen-induced intergranular stress corrosion cracking (HI-IGSCC). The details of the analysis and mechanism involved in the HI-IGSCC are presented in detail.Detailed metallurgical analyses of the cracked fasteners support the successive steps of the corrosion enhanced plasticity model, which is based on a local softening in the SCC crack region. The mechanism of a dislocation pileup ahead of a crack under corrosion and stress due to diffusing hydrogen promotes stress concentration against micro-obstacle and caused failure.     

Na0.5K0.5NbO3 and 0.9Na0.5K0.5NbO3–0.1Bi0.5Na0.5TiO3 nanocrystalline powders synthesized by low-temperature solid-state reaction

Nanocrystalline powders of K_0.5Na_0.5NbO₃ (KNN) and 0.9Na_0.5K_0.5NbO₃–0.1Bi_0.5Na_0.5TiO₃ (KNN–BNT) have been prepared using a low-temperature solid-state reaction. Phase development of the powders incurred during various calcination temperatures was examined by X-ray diffraction (XRD). Crystallite size and particle morphology of KNN powders were examined by XRD and transmission electron microscopy, respectively. Perovskite phase was formed at the temperature as low as 500 °C, and the average crystallite size of KNN powders depended on calcination temperature. In addition, the crystalline structure of KNN powders tended to change from tetragonal symmetry to orthorhombic symmetry with increase in crystallite size. Similar results were obtained in KNN–BNT system. The developed method is well suited for the mass production of niobate nanocrystalline powders due to its simplicity and low cost.     

Crystallographic textured evolution in 0.85Na0.5Bi0.5TiO3–0.04BaTiO3–0.11K0.5Bi0.5TiO3 ceramics prepared by reactive-templated grain growth method

0.85Na_0.5Bi_0.5TiO₃–0.04BaTiO₃–0.11K_0.5Bi_0.5TiO₃ (BNBK) lead-free piezoelectric ceramics with extensive [001]_pc (pc: pseudo cubic) texture were fabricated by the reactive-templated grain growth method using anisotropic Bi₄Ti₃O₁₂ (BIT) particles as templates. The degree of grain orientation increased with increasing heat treatment temperature (600–1,200 °C). The obtained textured ceramics showed dense and brick-wall like microstructure, giving a Lotgering factor of 0.6. A physical understanding of interaction between BIT templates and matrix powders and the mechanism of texture evolution were proposed and confirmed by experimental evidences of X-ray diffraction patterns, scanning electron microscope images and density measurements. The piezoelectric response was enhanced by the grain orientation, and the piezoelectric constant (d₃₃) of the textured ceramics sintered at 1,170 °C attained a value of 254 pC/N, which was 41 % higher than random ceramics (180 pC/N).     

Pearlite growth rate in Fe–C and Fe–Mn–C steels

The kinetic theory for the growth of pearlite in binary and ternary steels is implemented to ensure local equilibrium at the transformation front with austenite, while accounting for both boundary and volume diffusion of solutes. Good agreement is on the whole observed with published experimental data, although the reported growth rate at the lowest of temperatures is much smaller than predicted. To investigate this, experiments were conducted to replicate the published data. It is found that the cooperation between cementite and ferrite breaks down at these temperatures, and surface relief experiments are reported to verify that the resulting transformation product is not bainite.     

Microstructure and mechanical properties of Mg–8Li–xAl–0.5Ca alloys

The effect of the Al content on the microstructure and mechanical behaviour of Mg–8Li–xAl–0.5Ca alloys is investigated. The experimental results show that an as-cast Mg–8Li–0.5Ca alloy is mainly composed of α-Mg, β-Li and granular Mg₂Ca phases. With the addition of Al, the amount of α-Mg phase first increases and then decreases. In addition, the intermetallic compounds also obviously change. The microstructure of the test alloys is refined due to dynamic recrystallisation that occurs during extrusion. The mechanical properties of extruded alloys are much more desirable than the properties of as-cast alloys. The as-extruded Mg–8Li–6Al–0.5Ca alloy exhibits good comprehensive mechanical properties with an ultimate tensile strength of 251.2?MPa, a yield strength of 220.6?MPa and an elongation of 23.5%.     

Solid state phase transformations in Ti–Al–C and Ti–Al–Nb–C alloys

Abstract

Results are reported of an investigation of solid state transformations in a series of α₂ based alloys having an aluminium content of 26 at.-% with carbon up to 3 at.-%; two α₂ basedquaternary Ti–Al–Nb–C alloys with 5 and 12 at.-%Nb and 3 at.-%C were also studied. Ordering occurs in the ternary Ti–Al–C alloys and also in the 23Al–5Nb–3C alloy on quenchingfrom 1250°C. Additional carbide precipitation was not observed in the ternary Ti–Al–C alloys on reheating to 750°C. Additions of niobium resulted in the presence of the β phase at 1050°C in the 5%Nb alloy and at 1050 and 750°C in the 12%Nb alloy. In the quaternary Ti–Al–Nb–C alloys, (Ti, Nb)₃AlC was found to be the primary phase and was present in the microstructure over the temperature range studied. In the 21Al–12Nb–3C alloy, the ordered β phase transformed to α″₂ martensite on quenching from 1250;amp;#x00B0;C.

MST/1306     

Al–Ti–C phase diagram

Abstract

Equilibrium experiments have been performed at 1373, 1173, and 973 K, with alloys of compositions within the aluminium rich corner of the Al–Ti–C phase diagram. The samples have been metallographically investigated using light optical microscopy and a scanning electron microscope equipped with a system for energy dispersive spectrometry. Equilibrium phases, as well as effects of cooling, have been identified. Dynamic effects originating from cooling are discussed and a tentative phase diagram is proposed. It was predicted theoretically and confirmed experimentally that a class II reaction involving four phases occurs, i.e. Al(l) + TiC(s)?Al₃Ti(s) + Al₄C₃(s), below 1100 K.

MST/1807     

Sol–gel synthesis and spark plasma sintering of Ba0.5Sr0.5TiO3

Ultrafine Ba_0.5Sr_0.5TiO₃ powders were prepared by using barium nitrate, strontium nitrate, tetrabutyl titanate, and ammonia via citrate–nitrate combustion process at low temperature (500 °C), along with the X-ray diffraction (XRD), differential scanning calorimetry (DSC)/thermogravimetry analysis (TGA) and scanning electron microscopy (SEM) analytic reports. Spark plasma sintering was carried out to obtain the ultrafine crystalline BST and to improve the dielectric properity. It was found that the sintered BST showed ultrafine crystalline microstructure. At 25 °C, the dielectric constant and dissipation factor of the sintered sample were 1533 and 0.0063 at 10 kHz.     

挤压态Mg–9Al–Zn–0.5RE–Ca–Si合金微观组织和力学性能

目的 改善AZ91镁合金在温度超过120℃时的力学性能。方法 在AZ91合金中添加Ca、Si和La/Ce混合稀土元素。在360℃下等温挤压,平均挤压速度为1.2 mm/s,挤压比为30︰1,以探究Ca、Si和La/Ce混合稀土元素对AZ91合金力学性能、物相组成和显微组织等的影响。结果 在AZ91挤压态合金中,与添加Si元素相比,Ca元素对挤压态合金的力学性能影响更大。在室温时,Mg–9Al–Zn–0.5RE–Si挤压态合金的屈服强度、极限抗拉强度和伸长率分别是254 MPa、306MPa、7.0%,而Mg–9Al–Zn–0.5RE–0.5Ca挤压态合金的屈服强度、极限抗拉强度和伸长率分别是308 MPa、330 Ma、7.1%。Mg–9Al–Zn–0.5RE–0.5Ca–0.5Si挤压态合金室温力学性能最佳,其屈服强度、极限抗拉强度和伸长率分别是351 MPa、383 MPa、7.4%,说明Ca、Si这2种元素的协同作用可同时提高室温下AZ91合金的强度和塑性。在150℃和200℃下,Mg–9Al–Zn–0.5RE–0.5Ca–0.5Si合金仍然具有最佳的力学性能。在150℃下,其屈...     

Superplasticity and production of mechanically milled Ti–6Al–4V–0.5B

Abstract

Superplastic forming of conventional titanium alloy sheet is limited commercially by the relatively long cycle times imposed by the high temperatures and slow strain rates required. In order to minimise cycle times material with a fine grain size is required to allow either, an increase in the forming rate or a reduction in the deformation temperature. This study details the manufacture of Ti–6Al–4V–0.5B powder with a nanocrystalline grain size, which was produced by mechanical milling. The material was consolidated by hot isostatic pressing at a range of temperatures during which ～2.5 vol.-%TiB was formed by an in situ reaction between the titanium and boron. The TiB particles limited the growth of the grain size in the titanium from the nanocrystalline structure in the powder to sizes in the range 600 nm–4 µm after consolidation. The consolidated material was hot tensile tested at a range of temperatures and strain rates. A superplastic elongation of 310%was achieved when testing at 900°C at a strain rate of 6×10^-2 s^-1 compared with 220% for conventional Ti–6Al–4V sheet. However, extensive cavitation, induced by the presence of argon, occurred during high temperature deformation and limited the superplastic extensions achieved.     

Structures and electrical properties of (Na0.5K0.5)NbO3–Li(Ta0.5Nb0.5)O3 lead-free piezoelectric ceramics

Solid solutions of (Na_0.5K_0.5)NbO₃ (NKN) and Li(Ta_0.5Nb_0.5)O₃ (LTN) were investigated as a potential candidate of lead-free piezoelectric ceramics. It was found that the Curie temperature of solid solutions increases slightly with increasing the LTN content and simultaneously the polymorphic phase transition temperature linearly decrease till below room temperature. An orthorhombic to tetragonal phase transformation at room temperature, or a morphotropic phase boundary, in NKN is induced by ~7 at% LTN addition, where the best dielectric, piezoelectric and electromechanical properties are achieved. The 0.94NKN–0.07LTN ceramics possess a dielectric constant of 765, a loss tangent of 0.04 at 1 kHz, a piezoelectric constant d₃₃ of 253 pC/N and an electromechanical coupling factor k_p of 48%.     

Constitution of Ti–Al–C alloys in temperature range 1250–750°C

Abstract

An investigation has been made of the solid state constitution of the titanium rich portion of the Ti–Al–C system; partial isothermal sections have been established at 1250, 1050, and 750°C by means of electron microscopy (including energy dispersive X-ray analysis) and X-ray diffraction. In addition, a schematic liquidus projection has been deduced based on the solid state and as cast structures. The carbide phases present in the range studied are TiC, Ti₃AlC, and Ti₂AlC.

MST/1305