Microstructural refinement and tensile properties enhancement of Mg-3Al alloy using charcoal additions

Significant grain refinement in Mg-3Al alloy is achieved with the addition of charcoal due to the formation of Al₄C₃ particles, which act as effective nuclei for magnesium grains. Addition of 0.5 wt% charcoal has lead to reduced grain size of Mg-3Al alloy from 500 to 80 μm and no substantial grain refinement is obtained on further addition of charcoal. The results further reveal that the prolonged holding of the melt after the addition of charcoal has not affected the grain refining efficiency of Al₄C₃. Steady increase in tensile properties observed with increasing amount of charcoal addition has been attributed to the grain refinement and the presence of fine Al₄C₃ particles. The strengthening mechanisms due to charcoal addition are discussed in terms of Hall-Petch relation and dispersion strengthening. The predicted values are in good agreement with experimental results.     

Effect of Pd on microstructures and tensile properties of as-cast Mg-6Al-1Zn alloys

The effects of Pd on the microstructure and mechanical properties of Mg-6Al-1Zn alloys were investigated. Mg-6Al-1Zn-xPd (x = 0-6 wt.%) alloys were prepared using a permanent mould casting method. The microstructure of the as-cast alloys was characterized by the presence of Mg₁₇Al₁₂ and Al₄Pd phases. The volume fraction of the Al₄Pd phase was increased by the addition of 1-6 wt.%Pd but the volume fraction of the Mg₁₇Al₁₂ phases decreased. At room temperature, the tensile strength increased with increasing Pd addition up to 2 wt.%Pd, and the elongation to fracture decreased with a concomitant increase in the aggregation of the coarse Al₄Pd phase. At 150 °C, the tensile strength increased with the addition of Pd. Therefore, the room and elevated temperature tensile properties of as-cast Mg-6Al-1Zn alloys can be improved by Pd addition.     

Performance of AlTi5B1, AlTi3B3 and AlB3 master alloys in refining grain structure of aluminium foundry alloys

Abstract

The capacity of AlTi5B1, AlTi3B3 and AlB3 grain refiners to refine the grain structures of AlSi7Mg and AlSi11Cu2 foundry alloys was investigated. The performance of AlTi5B1, well established to be the best grain refiner for wrought aluminium alloys, is not nearly as good with the AlSi7Mg and AlSi11Cu2 alloys. Relatively smaller grains are obtained with the AlTi3B3 grain refiner in both alloys. The AlB3 grain refiner, on the other hand, improves the grain structure only as much as the AlTi5B1 grain refiner does. With as much as 0·04–0·1 wt-%Ti, the commercial alloys cannot enjoy the outstanding potency of the AlB₂ particles since the B supply is readily transformed to TiB₂ particles. However, the grains of the Ti free AlSi7Mg and AlSi11Cu2 alloys (～0·005 wt-%Ti) are very small and nearly globular for the entire range of holding times when inoculated with AlB3, implying not only a remarkable grain refining capacity but also a strong resistance to fading of the grain refinement effect. The lack of Ti in the melt allows the entire B to form AlB₂ particles, the perfect substrates to promote the nucleation of α-Al crystals. Aluminium castings can enjoy grain sizes well below 200 μm, with an addition of 0·02 wt-%B, provided that they are Ti free. That the potent substrates are made available just before the nucleation of α-Al crystals avoids fading effects and is a further advantage of the AlB3 grain refiner in recycling operations.     

Microstructure and mechanical properties of Mg-5Li-1Al sheets prepared by accumulative roll bonding

Ultrafine-grain and high-strength Mg-5Li-1Al sheets were prepared by accumulative roll bonding (ARB) process. Evolution of microstructure and mechanical properties of ARB-processed Mg-5Li-1Al sheets was investigated.Results show that, during ARB process, the evolution of deformation mechanism of t Mg-5Li-1Al alloy is as follows: twinning deformation, shear deformation, forming macro shear zone, and finally dynamic recrystallization (DRX). The grain refining mechanism changes from twin DRX to rotation DRX. With the increase in ARB cycles, strength of the Mg-5Li-1Al sheets is enhanced, whilst elongation varies slightly. With the increase in rolling cycles, anisotropy of mechanical properties decreases. It is conclusive that strain hardening and grain refinement dominate the strengthening mechanism of Mg-5Li-1Al alloy.     

Mechanical,tribological and anti-corrosive properties of polyaniline/graphene coated Mg-9Li-7Al-1Sn and Mg-9Li-5Al-3Sn-1Zn alloys

The mechanical, tribological and corrosion protection offered to Mg-9Li-7Al-1Sn and Mg-9Li-5Al-3Sn-1Zn alloys by the epoxy coating containing polyaniline/graphene (PANI/Gr) pigments is undertaken in the current work. PANI/Gr containing coatings were observed to be strongly adherent with a higher scratch hardness (H_s) and plowing hardness (H_p), i.e. H_s of 0.43 GPa, and H_p of 0.61 GPa, respectively when compared to that of neat epoxy coating (H_s of 0.17 GPa, and H_p of 0.40 GPa, respectively). Due to their higher H_s and H_p values, PANI/Gr based coatings displayed an enhanced wear resistance (Wear volume, W_v = 4.53 × 10^-3 m³) than that of neat epoxy coating (W_v = 5.15 × 10⁻³ m³). The corrosion protection efficiency in corrosive environment of 3.5 wt% NaCl solution was obtained to be >99% for PANI/Gr containing coatings when compared to that of neat epoxy coating. The charge-transfer resistance (R_ct) of the PANI/Gr containing coatings were estimated to be >10⁶ Ω cm², which indicates their highly protective nature when compared to that of neat epoxy coating (R_ct ˜10⁵ Ω cm²). Hence, PANI/Gr containing coatings can be potentially used for wear resistance and corrosion protection applications in marine environments.     

Effects of Nd on microstructures and properties of extruded Mg-2Zn-0.46Y-xNd alloys for stent application

The microstructures, mechanical and corrosion properties of three extruded Mg-2Zn-0.46Y-xNd alloys (x = 0.0, 0.5, 1.0 wt%) were studied by optical microscopy, scanning electronic microscopy (SEM), electrochemical measurements and tensile tests. Microstructural observations indicated that Nd led to the uniformity and the variation of morphology of major second phase; tensile tests showed that Nd can improve the ductility at moderate amount (0.5 wt%) and will be detrimental up to 1.0%; Mg-2Zn-0.46Y-0.5Nd alloy exhibited excellent mechanical properties (σ_b, 269.0 MPa, σ_0.2, 165.6 MPa and elongation, 24%); electrochemical tests revealed that Nd can enhance the corrosion resistance and Mg-2Zn-0.46Y-1.0Nd alloy had lowest corrosion current density, which was reasoned that the line-shape and rodlike NdZn₂ phase might serve as corrosion barriers and the dissolved Nd can raise the electrode potential of the matrix.     

Effects of samarium on microstructures and tensile properties of Mg-5Al-0.3Mn alloy

Microstructures and tensile properties of Mg-5Al-0.3Mn-xSm (x = 0, 1, 2 and 3 wt.%) alloys prepared by metal mould casting method were investigated. It was demonstrated that Mg-5Al-0.3Mn alloy was mainly composed of α-Mg and β-Mg₁₇Al₁₂ phases. However, the other two precipitates (Al₁₁Sm₃ and Al₂Sm) were observed along grain boundaries in the alloys containing Sm. The amount of Al₁₁Sm₃ and Al₂Sm precipitates was increased with the increment of Sm content. Meanwhile, volume fraction of β-Mg₁₇Al₁₂ phase was decreased. Moreover, the morphology of β-Mg₁₇Al₁₂ was altered from bulk bone-like shape to spherical one. Tensile results showed that Mg-5Al-0.3Mn-2Sm alloy exhibited the highest tensile properties both at room temperature and 150 °C. Compared with ultimate tensile strength (UTS), yield strength (YS) and elongation (?) of Mg-5Al-0.3Mn alloy, UTS, YS and ? of Mg-5Al-0.3Mn-2Sm alloy were enhanced by 30%, 45% and 35% at room temperature, and by 17%, 48% and 96% at 150 °C, respectively. The improvement of tensile properties was attributed to the decreased amount of β-Mg₁₇Al₁₂ and its refined morphology, and high thermal stable Al₁₁Sm₃ and Al₂Sm precipitates which effectively prohibited dislocation movement and grain boundary sliding during deformation process.     

Effects of Oxygen on the Microstructure of Ti_(47)Al_(0.7)B Alloys

The effects of oxygen on the microstructure of Ti-47Al-0.7B (at. pct) alloy for as-cast automotive valves were investigated. Six alloys with oxygen content from 0.4 to 1.4 at. pct were prepared by induction melting and centrifugal casting in CaO crucible under protective atmosphere. The microstructures were observed by optical microscopy (OM) and scanning electron microscope (SEM). The results show that the increase of oxygen content led to grain refinement and enhanced the microhardness as well as the α 2 ...     

Micro-mechanical properties of calcium sulfoaluminate cement and the correlation with microstructures

To investigate the micro-mechanical properties of calcium sulfoaluminate cement and the correlation with the microstructures, we apply a variety of advanced techniques of microstructural and micro-mechanical characterization, including scanning electron microscopy with backscattered electron and energy-dispersive X-ray spectroscopy detectors, X-ray fluorescence, X-ray diffraction and nanoindentation. For the first time, the micro-mechanical properties of material microstructures present in a calcium sulfoaluminate cement are estimated. In the calcium sulfoaluminate cement used in this research, two type of hydration product microstructures with the differentiable microstructural morphologies, compositions and micro-mechanical properties are identified and investigated. The correlation of the micro-mechanical properties with the microstructures shows that the hydration product microstructure containing more ettringite has lower indentation modulus and hardness than that containing more aluminum hydroxide.     

Texture and mechanical properties of ultrafine-grained Mg-3Al-1Zn alloy sheets prepared by high-ratio differential speed rolling

Mechanical properties and textures of the ultrafine grained (UFG) Mg-3Al-1Zn (AZ31) alloy with a mean grain size of 1 μm produced by high-ratio differential speed rolling were investigated. The resulting material exhibited high strength and relatively high ductility at ambient temperature. The high strength was attributed to grain-size and texture strengthening, while the high ductility was attributed to suppression of inhomogeneous twinning and increased strain-rate-sensitivity. The rolling temperature and the amount of shear strain accumulated during HRDSR affected the basal texture intensity and the rotation angle of the basal poles. Bimodal grain-size distribution obtained by annealing the UFG AZ31 at 573 K for a short time period resulted in considerable improvement of uniform elongation.     

Microstructure and superplasticity of Mg–Al–Ca electromagnetic casting alloys after hot extrusion

Microstructure and superplastic properties of the plates extruded from the Ca containing Mg alloy (1 wt.% Ca–AZ31) billets fabricated by electromagnetic casting (EMC) without and with electromagnetic stirring (EMS) were examined. The linear intercept grain sizes of the extruded materials were 3.7 μm and 2.1 μm, respectively. The material extruded from the EMC + EMS billet exhibited good superplasticity at low temperatures as well as at high strain rates, including the tensile elongations of 370% at 1 × 10⁻³ s⁻¹, −523 K and 550% at 1 × 10⁻² s⁻¹, −673 K. These values largely exceeded those of the AZ31 alloys with the similar grain sizes. The superior superplasticity of the extruded EMC + EMS billet could be attributed to fine grains and high grain stability at elevated temperatures by the presence of finely dispersed particles of thermally stable (Al,Mg)₂Ca phase. The constitutive equations were developed for describing the high-temperature deformation behavior of the fine-grained 1 wt.% Ca–AZ31 alloys with different grain sizes in wide range of temperature and strain rate.     

Effects of Zr,Ti and Sc additions on the microstructure and mechanical properties of Al-0.4Cu-0.14Si-0.05Mg-0.2Fe alloys

The evolution of microstructure and mechanical properties of Al-0.4Cu-0.14Si-0.05Mg-0.2Fe (wt.%) alloys, micro-alloyed with Zr, Ti and Sc, were investigated. The addition of 0.2%Zr to base alloy accelerates the precipitation of Si-rich nano-phase in α-Al matrix, which plays an important role in improving the mechanical properties of an alloy. The tensile strength increases from 102 MPa for the base alloy to 113 MPa for the Zr-modified alloy. Adding 0.2%Zr + 0.2%Ti to base alloy effectively refines α-Al grain size and accelerates the precipitation of Si and Cu elements, leading to heavy segregation at grain boundary. By further adding 0.2%Sc to Zr + Ti modified alloy, the segregation of Si and Cu elements is suppressed and more Si and Cu precipitates appeared in α-Al matrix. Accompanied with the formation of coherent Al₃Sc phase, the tensile strength increases from 108 MPa for the Zr + Ti modified alloy to 152 MPa for the Sc-modified alloy. Due to excellent thermal stability of Al₃Sc phase, the Sc-modified alloy exhibits obvious precipitation hardening behavior at 350 °C, and the tensile strength increases to 203 MPa after holding at 350 °C for 200 h.     

Mechanical properties and microstructure of new magnesium—lithium base alloys

Magnesium-lithium base alloys containing aluminum, silicon and rare-earth additions have been prepared by melting and solidification in a low-carbon steel crucible, homogenized at 350°C and extruded at the same temperature. The distribution of alloying elements in homogenized specimens was determined and the microstructure of these alloys characterized. The mechanical properties were measured and compared with those of similar alloys.     

Effect of melt treatment on microstructure and impact properties of Al-7Si and Al-7Si-2.5Cu cast alloys

The microstructures and impact toughness of Al-7Si and Al-7Si-2.5Cu cast alloys were studied after various melt treatments like grain refinement and modification. The results indicate that combined grain refined and modified Al-7Si-2.5Cu alloys have microstructures consisting of uniformly distributed α-Al grains, interdendritic network of fine eutectic silicon and fine CuAl₂ particles in the interdendritic region. These alloys exhibited improved impact toughness in as cast condition when compared to those treated by individual addition of grain refiner or modifier. The improved impact toughness of Al-7Si-2.5Cu alloys are related to breakage of the large aluminum grains and uniform distribution of eutectic silicon and fine CuAl₂ particles in the interdendritic region resulting from combined refinement and modification. This paper attempts to investigate the influence of microstructural changes in the Al-7Si and Al-7Si-2.5Cu cast alloys by grain refinement, modification and combined action of both on the impact toughness.     

Effect of Cu addition on microstructures and tensile properties of high-pressure die-casting Al-5.5Mg-0.7Mn alloy

In this study, Cu was added into the high-pressure die-casting Al-5.5Mg-0.7Mn (wt%) alloy to improve the tensile properties. The effects of Cu addition on the microstructures, mechanical properties of the Al-5.5Mg-0.7Mn alloys under both as-cast and T5 treatment conditions have been investigated. Additions of 0.5 wt%, 0.8 wt% and 1.5 wt% Cu can lead to the formation of irregular-shaped Al₂CuMg particles distributed along the grain boundaries in the as-cast alloys. Furthermore, the rest of Cu can dissolve into the matrixes. The lath-shaped Al₂CuMg precipitates with a size of 15–20 nm × 2–4 nm were generated in the T5-treated Al-5.5Mg-0.7Mn-xCu (x = 0.5, 0.8, 1.5 wt%) alloys. The room temperature tensile and yield strengths of alloys increase with increasing the content of Cu. Increasing Cu content results in more Al₂CuMg phase formation along the grain boundaries, which causes more cracks during tensile deformation and lower ductility. Al-5.5Mg-0.7Mn-0.8Cu alloy exhibits excellent comprehensive tensile properties under both as-cast and T5-treated conditions. The yield strength of 179 MPa, the ultimate tensile strength of 303 MPa and the elongation of 8.7% were achieved in the as-cast Al-5.5Mg-0.7Mn-0.8Cu alloy, while the yield strength significantly was improved to 198 MPa after T5 treatment.     

Microstructural evolution and superplasticity of rolled Mg-9Al-1Zn

Microstructural evolution and superplasticity of a Mg-9Al-1Zn alloy rolled at 673 K were investigated at 573 K and 1.5×10⁻³ s⁻¹. The grain size of the as-rolled Mg alloy was 39.5 μm. However, the grain size of the specimen deformed to a true strain of 0.6 was 9.1 μm. The grain refinement was attributed to dynamically continuous recrystallization during an initial stage of tensile test. Stabilization of subgrain boundaries by fine particles and stimulation of continuous recrystallization by prior warm-deformation were not needed to attain dynamically continuous recrystallization in the Mg alloy. As a result of the grain refinement, the rolled Mg alloy exhibited superplastic behavior.     

Mechanical properties and microstructural evolution of Al 6061 alloy processed by multidirectional forging at liquid nitrogen temperature

Al–Mg–Si alloy was subjected to multidirectional forging (MDF) at liquid nitrogen temperature (LNT), to cumulative strains of 1.8, 3.6 and 5.4. The deformed microstructures were examined by optical microscopy under polarized light and transmission electron microscopy (TEM). The deformed samples showed the formation of dislocation cells structure with high dislocation density at lower strains. Composite structure consisting of lamellar morphology at deformation bands and equiaxed grain morphology was observed. Significant differences in microstructure of the deformed samples were observed with increasing strain at LNT. At cumulative strain of 5.4, the microstructure shows nearly equiaxed subgrain structure with an average size of 250 nm with high angle grain boundaries. The mechanical properties were studied through Vickers hardness testing machine and tensile tester. The hardness value of MDFed alloy at LNT has increased from 50 Hv to 115 Hv for cumulative strain of 5.4. Tensile strength has increased from 180 MPa to 388 MPa with 4.5% percentage of elongation to failure. The improvement in hardness and tensile strength of forged alloy is attributed to the formation of equiaxed sub-grain structures and the presence of high dislocation density.     

Fatigue crack propagation in Ti3 Al based alloys

Abstract

Effects of microstructure, stress ratio, and environment on the fatigue crack growth resistance of Ti–23Al–9Nb–2Mo–1Zr–1·2Si and Ti–23Al–11Nb–0·9Si (at.-%) Ti₃ Al based alloys have been studied at room and elevated temperatures. Only modest effects of microstructure on fatigue crack growth resistance have been obtained at room temperature, and these tend to reduce further at the elevated temperatures of 600 and 700°C both in air and in vacuum. At room temperature the fatigue crack growth resistance of Ti₃ Al based alloys is controlled primarily by the thickness of the retained βphase rather than by its volume fraction and the microstructure with a larger average thickness of retained β laths shows improved fatigue crack growth resistance. However, in some microstructures, the spatial distribution of the β phase can also be deduced to be important. A marked difference on crack growth resistance is obtained for stress ratios of 0·1 and 0·5 both at room temperature and at a temperature of 600°C. The mechanisms of fatigue crack growth in air and vacuum are discussed.     

Friction stir welding of dissimilar materials between AA6061 and AA7075 Al alloys effects of process parameters

Dissimilar AA6061 and AA7075 alloy have been friction stir welded with a variety of different process parameters. In particular, the effects of materials position and welding speed on the material flow, microstructure, microhardness distribution and tensile property of the joints were investigated. It was revealed that the material mixing is much more effective when AA6061 alloy was located on the advancing side and multiple vortexes centers formed vertically in the nugget. Three distinct zones with different extents of materials intercalations were identified and the formation mechanism of the three zones was then discussed. Grain refinement was observed in all three layers across the nugget zone with smaller grains in AA7075 Al layers. All the obtained joints fractured in the heat-affected zone on the AA6061 Al side during tensile testing, which corresponds very well to the minimum values in microhardness profiles. It was found that the tensile strength of the dissimilar joints increases with decreasing heat input. The highest joint strength was obtained when welding was conducted with highest welding speed and AA6061 Al plates were fixed on the advancing side. To facilitate the interpretation, the temperature history profiles in the HAZ and at zones close to TMAZ were also measured using thermocouple and simulated using a three-dimensional computational model.