Effect of Al addition on microstructure and mechanical properties of Mg−Zn−Sn−Mn alloy

The microstructure and properties of the as-cast, as-homogenized and as-extruded Mg−6Zn−4Sn−1Mn (ZTM641) alloy with various Al contents (0, 0.5, 1, 2, 3 and 4 wt.%) were investigated by OM, XRD, DSC, SEM, TEM and uniaxial tensile tests. The results show that when the Al content is not higher than 0.5%, the alloys are mainly composed of α-Mg, Mg₂Sn, Al₈Mn₅ and Mg₇Zn₃ phases_. When the Al content is higher than 0.5%, the alloys mainly consist of α-Mg, Mg₂Sn, MgZn, Mg₃₂(Al,Zn)₄₉, Al₂Mg₅Zn₂, Al₁₁Mn₄ and Al₈Mn₅ phases. A small amount of Al (≤1%) can increase the proportion of fine dynamic recrystallized (DRXed) grains during hot-extrusion process. The room- temperature tensile test results show that the ZTM641−1Al alloy has the best comprehensive mechanical properties, in which the ultimate tensile strength is 332 MPa, yield strength is 221 MPa and the elongation is 15%. Elevated- temperature tensile test results at 150 and 200 °C show that ZTM641−2Al alloy has the best comprehensive mechanical properties.     

Dilatometric analysis on phase transformations of intercritical annealing of Fe−Mn−Si and Fe−Mn−Si−Cu low carbon TRIP steels

Evaluations of austenite fraction and transformation kinetics upon intercritical annealing of low carbon TRIP steels were attempted using quantitative dilatometric analysis. The measured dilation curves were analyzed by taking the carbon distribution between austenite and its decomposed phases into account. The amount of austenite formed during intercritical annealing and its carbon content obtained by dilatometric measurement was compared with the values predicted by thermodynamic calculations under the ortho-equilibrium and para-equilibrium conditions. The kinetics of the reaustenization process including pearlite dissolution and non-isothermal and isothermal formation of austenite could be quantitatively characterized by means of a modified JMAK (Johnson-Mehl-Avrami-Kolmogrov) equation.     

Hydrogen permeation properties of Pd-coated Ni−Nb−Ti−Zr amorphous alloys

Hydrogen permeability of Pd-coated Ni₆₀Nb₁₅Ti₁₅Zr₁₀ and Ni₆₀Nb₂₀Ti₁₅Zr₅ amorphous alloys was measured in the temperature range of 673 K to 773 K and was compared with the results obtained using Ni₆₀Nb₄₀, a binary amorphous alloy. The permeability thus measured was found to increase moderately increasing temperature. A long-term permeation test for the Pd-coated Ni₆₀Nb₁₅Ti₁₅Zr₁₀ amorphous alloy revealed high permeation stability up to 16.6 h.     

Effect of welding speed on microstructure and mechanical properties of Al−Mg−Mn−Zr−Tialloy sheet during friction stir welding

Effects of welding speed on the microstructure evolution in the stir zone (SZ) and mechanical properties of the friction stir welding (FSW) joints were studied by OM, XRD, SEM, TEM, EBSD and tensile testing. Compared with the base metal (BM), an obviously fine dynamic recrystallization (DRX) microstructure occurs in the SZ and the DRX grain size decreases from 5.6 to 4.4 μm with the increasing of welding speed. Fine DRX microstructure is mainly achieved by continuous dynamic recrystallization (CDRX) mechanism, strain induced boundary migration (SIBM) mechanism and particle stimulated nucleation (PSN) mechanism. Meanwhile, the geometric coalescence and the Burke−Turnbull mechanism are the main DRX grain growth mechanisms. Among all the welding speeds, the joint welded at rotation speed of 1500 r/min and welding speed of 75 mm/min has the greatest tensile properties, i.e. ultimate tensile strength (UTS) of (509±2) MPa, yield strength (YS) of (282±4) MPa, elongation (El) of (23±1)%, and the joint efficiency of 73%.     

Effects of Li content on microstructure and mechanical properties of as-cast Mg−xLi−3Al−2Zn−0.5Y alloys

The effects of Li content on the microstructure and mechanical properties of the as-cast Mg?xLi?3Al?2Zn? 0.5Y (LAZx32-0.5Y) alloys were investigated by XRD, SEM, TEM, hardness tester and universal testing machine. The results show that the matrix of the alloy transforms from α-Mg to α-Mg+β-Li and then to β-Li when the Li content increases from 4% to 14% (mass fraction). All LAZx32-0.5Y alloys contain AlLi and Al₂Y, while MgLi₂Al appears only in the alloy containing the β-Li matrix. As the Li content increases, the content of AlLi and MgLi₂Al gradually increases, while the content of Al₂Y does not change much. As the Li content increases from 4% to 10%, the ultimate tensile strength and hardness of the as-cast LAZx32-0.5Y alloys gradually decrease while the elongation gradually increases. The corresponding fracture mechanism changes from cleavage fracture to quasi-cleavage fracture and then to microporous aggregation fracture. This is mainly attributed to the decrease of α-Mg and the increase of β-Li in the alloy. When the Li content continues to increase to 10% and 14%, the yield strength, ultimate tensile strength and hardness of the as-cast LAZx32-0.5Y alloys gradually increase, while the elongation decreases sharply, which is mainly attributed to the nano-scale MgLi₂Al uniformly distributed in the β-Li matrix.     

Correlation between the oxide impedance and corrosion behavior of Zr−Nb−Sn−Fe−Cu alloys

The correlation between the oxide impedance and corrosion behavior of two series of Zr−Nb−Sn−Fe−Cu alloys was evaluated. Corrosion tests were performed in a 70 ppm LiOH aqueous solution at 360°C for 300 days. The results of the corrosion tests revealed that the corrosion behavior of the alloys depended on the Nb and Sn content. The impedance characteristics for the pre- and post-transition oxide layers formed on the surface of the alloys were investigated in sulfuric acid at room temperature. From the results, a pertinent equivalent circuit model was preferably established, explaining the properties of double oxide layers. The impedance of the oxide layers correlated with the corrosion behavior; better corrosion resistance always showed higher electric resistance for the inner layers. It is thus concluded that a pertinent equivalent circuit model would be useful for evaluating the long-term corrosion behavior of Zr−Nb−Sn−Fe−Cu alloys.     

Formation of reversed austenite during tempering of 14Cr−7Ni−0.3Nb−0.7Mo−0.03C super martensitic stainless steel

Reversed austenite transformation and existence of retained austenite during tempering of the super martensitic stainless steel of Fe-14Cr-7Ni-0.3Nb-0.7Mo-0.03C were studied by means of experiments on microstructure and X-ray diffraction, and tensile and hardness tests. Acicular type retained austenite at the lath boundary of martensite with the interior appeared after solution annealing. This retained austenite still existed, and reversed austenite was not formed, with tempering up to 24h at 450°C. The reversed austenite began to form above 550°C, and the volume fraction of reversed austenite decreased with increasing tempering temperature after showing a maximum value of 19.2% at 650°C. This maximum volume fraction of reversed austenite was responsible for the lowest value of strength and hardness. The Ni contents of plate type reversed austenite and the surrounding matrix increased and decreased respectively, implying that the reversed austenite was formed as a result of nickel diffusion. The orientation relationships between reversed austenite and the surrounding matrix showed a K-S relationship of and.     

Cold resistance of low-carbon Mn−Ni−Mo−V steels for pressure vessels

Conclusions  

Evaluation of residual stress and corrosion behaviour of electroless plated Ni−P/Ni−Mo−P coatings

Single Ni?P and Ni?Mo?P coatings as well as duplex Ni?P/Ni?Mo?P coatings with the same compositions were prepared by electroless plating. The residual stresses of the coatings on the surface and cross sections were measured by nanoindentation and AFM analysis, and the corrosion behaviour of the coatings in 10% HCl solution was evaluated by electrochemical methods, to establish the correlation between the residual stresses and corrosion behaviour of the coatings. The results showed that the single Ni?P and duplex Ni?P/Ni?Mo?P coatings presented residual compressive stresses of 241 and 206 MPa respectively, while the single Ni?Mo?P coating exhibited a residual tensile stress of 257 MPa. The residual compressive stress impeded the growth of the pre-existing porosity in the coatings, protecting the integrity of the coating. The duplex Ni?P/Ni?Mo?P coatings had better corrosion resistance than their respective single coating. In addition, the stress states affect the corrosive form of coatings.     

Embrittlement of Pd-coated Ni−Nb−Ti−Zr amorphous alloys during hydrogen permeation

In this study, we report results of an investigation into the failure of Ni₆₀Nb₁₅Ti₁₅Zr₁₀ amorphous alloys occurring during hydrogen permeation performed at 473 K and 573 K. However, the amorphous membrane did not fail during test performed at higher temperatures (673 K and 773 K). The failure of the Pd-coated Ni₆₀Nb₁₅Ti₁₅Zr₁₀ amorphous ribbon is attributed to the cracking of the hydrogenated Pd coating induced by the formation of α′ hydride phase in the low temperature range. 0     

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.     

Effects of trace Ag on precipitation behavior and mechanical properties of extruded Mg−Gd−Y−Zr alloys

The effects of trace Ag element on the precipitation behaviors and mechanical properties of the Mg−7.5Gd− 1.5Y−0.4Zr (wt.%) alloy by means of tensile test, X-ray diffractometry, scanning electron microscopy, electron backscattered diffractometry, and scanning transmission electron microscopy. There is an unusual texture (〈0001〉//extrusion direction) in the extruded Mg−Gd−Y−Zr alloys containing 0.5 wt.% Ag. During the aging periods at 225 °C, the addition of the trace Ag does not form new precipitates, just accelerates aging kinetics, and refines β′ precipitates, thereby increasing the number density of the β′ precipitates by Ag-clusters. Moreover, the Mg−Gd−Y−Zr alloy containing 0.5 wt.% Ag shows the most excellent synergy of strength and plasticity (408 MPa of ultimate tensile strength, 265 MPa of yield strength, and 12.9% of elongation to failure) after peak-aging.