β-Mg17Al12 Phase Research Articles

Investigation of rapidly decomposable AZ91–RE–xCu (x=0, 1, 2, 3, 4) alloys for petroleum fracturing balls

AZ91–RE–xCu (x = 0%, 1%, 2%, 3%, and 4%) alloys have been prepared by a semi-continuous casting followed by hot-extrusion technique, in order to find suitable degradable materials for use in petroleum fracturing balls. Results indicate that the Cu-free AZ91–RE alloy mainly consists of dynamically recrystallized α-Mg grains and the β-Mg17Al12 phase. When Cu is added to AZ91–RE, a new T-AlCuMgZn phase is formed. The grains are also dramatically refined for Cu contents of 0–3%. When the Cu content is increased to 4%, however, the grain size is slightly coarsened to 8.9 μm. Consequently, both the σf and σ0.2 reach maximum values of 244 and 405 MPa, respectively, for AZ91–RE–3Cu. The corrosion rate of AZ91–RE–xCu alloys increases with increasing Cu concentration. The addition of 4% Cu increases the decomposition rate of the AZ91–RE alloy, but slightly reduces its compressive strength.

Corrosion and microstructure of as-cast magnesium alloy AM60-based hybrid nanocomposite

ABSTRACT Two types of Mg alloy AM60-based composites containing (1) only 7 vol.% Al2O3 Fibre and (2) both 7 vol.% Al2O3 Fibre + 3 vol.% Al2O3 nano-Particle, named 7FC and MHNC-7F3NP, respectively, as well as the unreinforced matrix alloy AM60 were prepared by using the preform-squeeze casting technique. The microstructure of the matrix alloy AM60 characterised by optical microscopy (OM), scanning electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDS) consisted of primary α-Mg grains, eutectic β-Mg17Al12 phases and Al-Mn intermetallics, of which distribution were different from those in the composites. The reinforcement introduction refined the matrix grain structure of the composites significantly. The corrosion behaviours of the 7FC and MHNC-7F3NP composites and the matrix alloy were investigated by using the potential-dynamic polarisation test in 3.5 wt.% NaCl aqueous solution. Compared with the matrix alloy, the introduction of micron-sized alumina fibres decreased the corrosion resistance of the matrix alloy AM60 considerably due to the presence of excessive interfaces, while the high density of grain boundaries and the absence of noble precipitates such as β-Mg17Al12 phases and Al-Mn intermetallics at the grain boundaries in the composites should also be somewhat responsible for their poor corrosion resistance. The addition of the nano-sized particles led to almost no further reduction in the corrosion resistance of the MHNC-7F3NP.

Simulation and Prediction of the Vickers Hardness of AZ91 Magnesium Alloy Using Artificial Neural Network Model

In this study, an artificial neural network (ANN) model was used to simulate and predict the Vickers hardness of AZ91 magnesium alloy. The samples of AZ91 alloy were aged at different temperatures (Ta = 100 to 300 °C) for different durations (ta = 4 to 192 h) followed by water quenching at 25 °C. The age-hardening response of the samples was investigated by hardness measurements. The microstructure investigations showed that only discontinuous precipitates formed at low aging temperatures (100 and 150 °C), while continuous precipitates invaded all the samples at a high aging temperature (300 °C). Both discontinuous and continuous precipitates formed at the intermediate aging temperatures (200 and 250 °C). X-ray diffraction (XRD) analysis revealed that the microstructure comprised two phases: The α-Mg matrix and intermetallic β-Mg17Al12 phase. The alteration of the crystalline lattice parameters a, c, and c/a ratio with the aging time at various aging temperatures was also investigated. Both c and c/a ratio had the same behavior with aging time while a had an inverse trend. The observed variations of the lattice parameters were attributed to the mode of precipitation in AZ91 alloy. The ANN findings for the simulation and prediction perfectly conformed to the experimental data.

An analysis of microstructure and impression creep response of squeeze-cast AZ91–xBi–ySr alloys

ABSTRACTThe present investigation deals with the microstructural modification following the Bi + Sr additions to the squeeze-cast AZ91 alloy and its effect on impression creep response. The Bi + Sr additions form the Al4Sr and Sr2Bi phases besides the α-Mg and β-Mg17Al12 phases, and improves creep resistance of the AZ91 alloy. The AZ91 + 1.0Bi + 0.5Sr alloy reveals the best creep resistance among the alloys. The stress exponent and the activation energy values of all the alloys are in the range of 4–7 and 100.2–112.7 kJ mol−1, respectively, depicting the pipe diffusion-controlled dislocation creep is the governing creep mechanism. The post-creep microstructural study confirms several dislocations pile-ups around the Al4Sr and Sr2Bi phases resulting in improved creep resistance of the modified AZ91 alloys.

Effect of near-liquidus squeeze casting temperature on microstructure and mechanical property of AZ91D alloy differential support

The effect of near-liquidus squeeze casting temperature on microstructure and mechanical property of AZ91D alloy differential support was studied. By increasing the pouring temperature, the grain size of die casting AZ91D alloy differential support increased, the grain morphology changed from spherical equiaxed grain to equiaxed dendrite, and the β-Mg17Al12 phase changed from fine uniform distribution to large-area segregation. Furthermore, with the increase of pouring temperature, the tensile strength and elongation of the AZ91D alloy differential support both decreases.

Influence of trace As content on the microstructure and corrosion behavior of the AZ91 alloy in different metallurgical conditions

Abstract The influence of trace As content (0.04, 0.05, 0.06 wt%) on the microstructure and corrosion behavior of AZ91 alloy in different metallurgical conditions (as-cast, as-quenched, peak-aged) was firstly investigated. It is found that the corrosion resistance of AZ91 alloy in all conditions is significantly enhanced by As addition, mainly due to cathodic toxication, melt purification and incidence reduction of local corrosion. As alloying leads to the formation of Mg3As2 and makes the β-Mg17Al12 phase less continuous in the as-cast alloys. The alloy containing 0.06 wt% As obtains a higher corrosion rate than those of the alloy containing 0.05 wt% As owing to a more discontinuity of β phase. The β phase is dissolved into the matrix in the as-quenched alloys and reprecipitates along the grain boundaries after aging. The more continuous β phase distribution in the peak-aged alloys contributes to corrosion resistance. The corrosion rates are in the order of as-quenched>as-cast>peak-aged. The lowest corrosion rate (0.67 mm/y) in 3.5 wt% NaCl solution is obtained in the peak-aged AZ91-0.05As alloy, which is over 70% lower than that of as-cast AZ91 alloy (2.22 mm/y).

Effect of La/Nd ratio on microstructure and mechanical properties of as-cast AZ91-xLa/Nd alloy

The effect of La/Nd ratio on microstructure and mechanical properties of as-cast AZ91D alloys were investigated. The alloys were produced by casting the molten metal into a preheated steel mould. Microstructures were investigated by XRD and SEM (equipped with EDS). The hardness measurement was carried out by Brinell hardness (HB) digital display tester. Compression tests were conducted at both room and high temperatures (20 °C and 150 °C). Microstructural characterizations reveal that the area fraction of the β-Mg17Al12 phase changed with the variation of La/Nd ratio. The area fraction of the β-Mg17Al12 phase decrease significantly and then increased slightly with the increase of La/Nd ratio. Besides, the results show that the addition of La and Nd contributed to the formation of needle-like phase Al11RE3. Meanwhile, the Brinell hardness and high-temperature compression properties of this alloy reach the maximum values (67 HB and 330 MPa, respectively) when the La/Nd ratio is 2/3. In this alloy, the amount of Al11RE3 phase is 51/TA and the average length is 69.55 μm. Moreover, a generation model of Al11RE3 is established and the growth mechanism of Al11RE3 is analyzed. This work thus has proved that La/Nd ratio has a great effect on the microstructure and mechanical properties of AZ91.

The significant effect of trace yttrium level on the mechanical properties of cast Mg–6Al alloy through a refinement mechanism

It was observed that low level of yttrium (Y) has a modifying effect on the Mg17Al12 phase in the binary Mg-6wt%Al casting alloy. Scanning/transmission electron microscopy, mechanical testing and thermodynamic calculations were employed to understand the modification mechanism. It is found that Y at trace level refines the β-Mg17Al12 phase leading to increased tensile elongation and compressive strain to fracture. The refinement effect is lost with further increase in the Y level. Thermodynamic calculations (Scheil cooling) indicate that the formation temperature of the Al4MgY phase changes with the Y level. At trace levels, its nucleation temperature is the same as that of Mg17Al12, producing a co-precipitation/nucleation effect. As Y increases, Al4MgY forms at increasingly higher temperatures than β-Mg17Al12 with a loss of the refinement due to the disappearance of co-precipitation.

Improved corrosion response of squeeze-cast SiC nanoparticles reinforced AZ91-2.0Ca-0.3Sb alloy

The present work investigates the effect of SiC nanoparticle additions on corrosion response of the squeeze-cast AZ91 + 2.0Ca+0.3Sb (wt.%) alloy subjected to immersion, hydrogen evolution, and potentiodynamic polarization scan in a 0.1 M NaCl solution. All the AZ91 + 2.0Ca+0.3Sb + xSiCnp (x = 0.5, 1.0, 2.0 (wt.%)) nanocomposites demonstrate a superior corrosion resistance than the alloy, and the nanocomposite reinforced with 2.0SiCnp exhibits the highest corrosion resistance. The improved corrosion performance of the nanocomposites is attributed to the decrease in the potential difference between α-Mg and β-Mg17Al12 phases, reduced quantity of β-Mg17Al12 phase, and an increased amount of Al2Ca phase following SiC nanoparticles additions.

Microstructural Evolution and Mechanical Properties of As-Cast Mg-12Zn Alloys with Different Al Additions

Abstract In this study, Mg-12Zn magnesium alloys alloyed with Al additions (0, 2, 4, 6, 8 and 10, wt.%) were fabricated by permanent mould casting. The Al content on their microstructure and mechanical properties were systematically examined with an optical microscope (OM), a scanning electron microscope (SEM), an X-ray diffractometer (XRD) and mechanical tests at room temperature. The experimental results indicate that the microstructure of the alloys is mainly composed of α-Mg and semi-continuous or continuous eutectic phases. A higher addition of Al (≥6%) causes the generation of the Mg17Al12 phases. Notably, the grain sizes of the alloys gradually decrease, whilst the partial morphology of some eutectic phases is modified into lamellar structure with increasing of Al addition. Mechanical properties characterization manifested that, the alloys with different Al additions reveal distinguishing tensile properties. Among them, the alloy with 4% Al provides an excellent mechanical properties, i.e., a UTS of 206 MPa and an EL of 7.92%, which is respectively higher 28 MPa and 1.08% than that of ZA120 alloy. The deterioration in the tensile properties for the higher Al-bearing alloys is possibly related to the lamellar structure, coarse and continuous net-work morphology and β-Mg17Al12 phases, respectively.

Microstructure Evolution and Mechanical Properties of AZ80 Mg Alloy during Annular Channel Angular Extrusion Process and Heat Treatment.

Microstructure evolution and mechanical properties of AZ80 Mg alloy during annular channel angular extrusion (350 °C) and heat treatment with varying parameters were investigated, respectively. The results showed that dynamic recrystallization of Mg grains was developed and the dendritic eutectic β-Mg17Al12 phases formed during the solidification were broken into small β-phase particles after hot extrusion. Moreover, a weak texture with two dominant peaks formed owing to the significant grain refinement and the enhanced activation of pyramidal <c + a> slip at relative high temperature. The tension tests showed that both the yield strength and ultimate tensile strength of the extruded alloy were dramatically improved owing to the joint strengthening effect of fine grain and β-phase particles as compared with the homogenized sample. The solution treatment achieved the good plasticity of the alloy resulting from the dissolution of β-phases and the development of more equiaxed grains, while the direct-aging process led to poor alloy elongation as a result of residual eutectic β-phases. After solution and aging treatment, simultaneous bonding strength and plasticity of the alloy were achieved, as a consequence of dissolution of coarse eutectic β-phases and heterogeneous precipitation of a large quantity of newly formed β-phases with both the morphologies of continuous and discontinuous precipitates.

Effect of Solutionizing Time on Improving the Microstructure and Mechanical Properties of Aged AZ80 Mg Alloy

The effects of solutionizing soak time and aging on the microstructure and mechanical properties of cast AZ80 Mg alloy were investigated. Optical microscopy, scanning electron microscopy and x-ray diffraction revealed the dissolution of β-Mg17Al12 phase in the solution-treated samples which was later detected in the aged condition. Solution treatments at 420 °C for 4-16 h with water quenching followed by aging at 200 °C for 10 h were carried out, causing discontinuous precipitates to be formed with different precipitate morphologies at the various soak times. The discontinuous precipitates were observed to transform from the interdendritic segregation to parallel lamellae with increasing soak time to 8-10 h. As the soak time increased to 16 h, parallel lamellae transformed into tree-like precipitate morphology. The results showed significant changes in the interlamellar spacing at different soak times, and such precipitate morphology could influence the microstructure and mechanical properties after the aging process. A good combination of strength and ductility was obtained when the cast AZ80 Mg alloy was solution-treated at 420 °C for 10 h and aged at 200 °C for 10 h. Compared to the as-cast condition, a significant increase in ultimate tensile strength (50%), yield strength (47%) and elongation (43%) were observed.

Improved Mg–Al–Zn Magnesium Alloys Produced by High Energy Milling and Hot Sintering

Powders of commercially pure magnesium (c.p. Mg), AZ91 magnesium alloy and zinc were milled using a high-energy mill. The effect of high energy milling (HEM) on powders morphology, chemical composition, crystallite size and compaction of different powders mixtures were studied. After compaction, samples were thermally treated at 450 °C and both density and hardness were evaluated. It was found that as milling speed and time increases, the AZ91 alloy and c.p. Mg particles were deformed and fractured up to sizes below 10 μm. X-ray diffraction patterns for both the c.p. Mg and the AZ91 powders revealed that the milling process induced changes in both the α-Mg and the β-Mg17Al12 phases. By increasing the milling speed, the crystallite size decreases by up to 70% for AZ91 powders and by 80% for magnesium powders. The relative densities of the compacted AZ samples were greater than 85% and this parameter increased for all samples after thermal treatment at 450 °C, obtaining densities higher than 88%. Hardness measurements disclosed values as high as 84.3 HR15T. Theoretical calculations of mechanical strength were obtained for all samples based on the hardness values measured, finding very encouraging results for the three Mg alloys.

Microstructure and mechanical properties of AZ91 magnesium alloy with minor additions of Sm, Si and Ca elements

The effects of Sm, Si and Ca on the microstructure and mechanical property of AZ91 magnesium alloy were investigated by means of optical microscopy (OM), differential scanning calorimetry (DSC), scanning electronic microscopy (SEM), X-ray diffraction (XRD) and tensile testing. The results indicated that the addition of 1.5 wt.% Sm with or without 0.8 Si/Ca led to a decrease in the volume fraction of the β-Mg17Al12 phase and the formation of the intermetallic compounds of Al-Sm, Mg2Si, MgAlCa and Al2Ca. The microstructure of AZ91 alloy was significantly refined and distribution became discrete with additions of Sm and Ca; the average grain size of the α-Mg matrix was reduced from 239.7 ± 16.9 µm to 66.34 ± 5.10 µm. The AZ91-Sm-Ca alloy exhibited a good combination of yield strength at 135 MPa, ultimate tensile strength at 199 MPa and elongation at 4.32%, which was ascribed to grain refinement strengthening. Furthermore, the T6 treated AZ91-Sm-Ca alloy possessed yield strength of 154 MPa and elongation of 7.1%, which was due to grain refinement strengthening and reduction in discontinuous precipitates.

A Comparative Study on Microstructure, Mechanical and Tribological Properties of A4, AE41, AS41 and AJ41 Magnesium Alloys

Microstructure, tensile and wear properties of as-cast A4 (Mg-4Al), AE41 (Mg-4Al-0.5Ce-0.5La), AS41 (Mg-4Al-1Si) and AJ41 (Mg-4Al-1Sr) alloys were investigated, and the results were compared with each other in this study. Microstructures were investigated by XRD, optical and scanning electron microscopes. Tensile tests were conducted at both room and elevated temperatures. Tribological properties were examined by pin-on-disk wear tests under different applied loads. Microstructure characterizations revealed that the volume fraction of second phases considerably increased by alloying additions of 1 wt.% Ce/La, Si and Sr. The microstructure of A4 alloy consisted of α-Mg grains and divorced β-Mg17Al12 phases. After individual alloying additions of 1 wt.% Ce/La, Si and Sr, the secondary phases were primarily replaced by needle-shaped and massive blocky-shaped Al11(Ce,La)3 phases in AE41 alloy, Chinese-script-type Mg2Si phases in AS41 alloy and divorced globular-like and massive blocky-shaped Al4Sr and (Mg,Al)17Sr2 phases in AJ41 alloy. The tensile tests showed that at both room and elevated temperatures alloying additions of 1 wt.% Ce/La, Si and Sr resulted in an increase in the strength but a decrease in the ductility. Among the studied alloys, AS41 alloy exhibited the best strength. Wear test results showed that AE41 and AJ41 alloys similarly exhibited the best wear resistance owing to the presence of hard and dense intermetallics. Abrasion was the main wear mechanism under low applied loads while delamination, adhesion and oxidation mechanisms were majorly observed under high applied loads.

A continuous net-like eutectic structure enhances the corrosion resistance of Mg alloys.

Mg alloys degrade rather rapidly in a physiological environment, although they have good biocompatibility and favorable mechanical properties. In this study, Ti was introduced into AZ61 alloy fabricated by selective laser melting, aiming to improve the corrosion resistance. Results indicated that Ti promoted the formation of Al-enriched eutectic α phase and reduced the formation of β-Mg17Al12 phase. With Ti content reaching to 0.5 wt.%, the Al-enriched eutectic α phase constructed a continuous net-like structure along the grain boundaries, which could act as a barrier to prevent the Mg matrix from corrosion progression. On the other hand, the Al-enriched eutectic α phase was less cathodic than β-Mg17Al12 phase in AZ61, thus alleviating the corrosion progress due to the decreased potential difference. As a consequence, the degradation rate dramatically decreased from 0.74 to 0.24 mg·cm-2·d-1. Meanwhile, the compressive strength and microhardness were increased by 59.4% and 15.6%, respectively. Moreover, the Ti-contained AZ61 alloy exhibited improved cytocompatibility. It was suggested that Ti-contained AZ61 alloy was a promising material for bone implants application.

Influence of Heat Treatment on Microstructures and Impact Toughness of Mg-Al-Zn Alloy

Microstructures and impact properties of the rolled Mg-8.10Al-0.46Zn-0.18Mn-0.18Ag (wt.%) alloy were investigated under various thermal conditions including as-rolled, solid solution (T4) and solid solution plus aging (T6) states. The results show that the impact toughness of the as-rolled alloy is slightly enhanced after solution treatment but remarkably reduced by the subsequent aging process, exhibiting a similar trend with the elongation of the alloy. Fractography analysis suggests that the impact toughness variations are closely associated with the changes in the deformation mode: the superior impact toughness of the as-rolled and T4 samples are mainly attributed to the occurrence of profuse twins during impact testing, while, in the T6 sample, the twinning activity is significantly suppressed due to the presence of β-Mg17Al12 phases. In addition, the lamellar-shaped β-Mg17Al12 phases are easy to fracture, which accelerates the crack propagation and thus degrades the impact property.

Solute clustering and precipitation of Al-5.1Mg-0.15Cu-xZn alloy

The AlMgZn phase (T-phase)-an interesting strengthening phase in precipitation-hardened aluminum alloys has not received sufficient attention and is very different from the commonly studied η-MgZn2 phase. This study focuses on Al-5.1Mg-0.15Cu-xZn alloys with a Zn/Mg ratio below 1.0. This is different from commonly known 7xxx series alloys whose Zn/Mg ratios are above 2.0. The relations among the age-hardening response behavior, clustering of Mg-Zn or Mg-Cu atoms, and precipitation of different hardening phases in Al-5.1Mg-0.15Cu-xZn alloys are investigated via atom probe tomography and transmission electron microscopy. According to the results, the Zn addition stimulates the precipitation of coherent T″ precipitates, but suppresses S-Al2CuMg and β-Al3Mg2 phases. This results in an enhanced and accelerated age-hardening response in alloys with high Zn contents. The 3.0Zn (wt.%) alloy with finer T″ precipitates exhibits the best age-hardening response, followed by the 2.0Zn (wt.%) alloy with T′ precipitates and the 1.0Zn (wt.%) alloy with T precipitates. Clusters with high Mg/(Al + Zn + Cu) ratios cannot act as effective precursors for the transformation of Mg-Zn clusters into T″ precipitates, thereby resulting in a weaker age-hardening response and lower hardness in the 1.0Zn (wt.%) alloy. Furthermore, although T-phases originate from different alloys, their Mg/(Al + Zn + Cu) ratios reach the constant value 3/7 for larger particle sizes. This challenges the chemical composition of traditional equilibrium T precipitates.

Mechanical properties and corrosion behaviour of AZ91D-HAP surface composites fabricated by friction stir processing

The biomedical applications of magnesium alloy AZ91D are limited because of the dendritic β-Mg17Al12 phase, which degrades the mechanical properties and corrosion resistance. To overcome this, friction stir processing is implemented to fabricate surface composite of AZ91D with hydroxyapatite as reinforcement. This results in refinement of grains and fragmentation of the β phase with homogeneous dispersion of hydroxyapatite in the composite. The combined effect of reinforcement of hydroxyapatite and fragmentation of the β phase resulted in simultaneous improvement in mechanical and corrosion properties. The various phases, surface morphology, and composition of the developed composite are analyzed using a transmission electron microscope and a scanning electron microscope before and after corrosion studies. The mechanism behind the improvement in the property of the developed composite is correlated with the characterization results.

Aging behavior of nano SiC particles reinforced AZ91D composite fabricated via friction stir processing

AZ91D magnesium alloy matrix composites reinforced with nano SiC particles (n-SiCp) was fabricated by Friction Stir Processing, and the effects of n-SiCp on the β-Mg17Al12 phase precipitation at 180 °C as well as the age hardening response of AZ91D were investigated by X-ray diffraction, optical microscopy, scanning electron microscopy, transmission electron microscopy, and micro Vickers hardness measurements. The results show that, with the addition of n-SiCp, the microstructure of AZ91D after FSP was greatly refined, the texture strength was decreased, and the age hardening efficiency became inferior. As n-SiCp can efficiently block the migration of grain boundaries and grain growth, the composite exhibited a higher microstructure stability at elevated temperatures, and the discontinuous precipitation of β-Mg17Al12 phase that dominated the age process of AZ91D at 180 °C was remarkably restricted in the composite. Besides, n-SiCp promoted the continuous precipitation of β-Mg17Al12 phase and the precipitation rate in the composite was faster at the initial aging stage, but on further aging the precipitation rate became gradually slower than that in the AZ91D. The underlying mechanisms for the different age precipitation behavior and age hardening response were also discussed.