Al2Ca Particles Research Articles

Effects of Ca contents on microstructures and mechanical properties of friction stir processed Mg alloys

Ca alloying is beneficial for improving the uniform elongation for friction stir processed (FSPed) Mg-4Al-2Sn-xCa (0, 0.5, 1.0 and 1.5 wt.%) (AT42-xCa) alloys. Coarse CaMgSn and Al2Ca particles in as-cast AT42-xCa alloys lead to a sharp decrease in elongation. Friction stir processing (FSP) is effective in improving strength and elongation by refining grain structure along with CaMgSn and Al2Ca particles, and inducing grain orientations with high Schmid factors for basal slip. Consequently, FSPed AT42-0.5Ca alloy exhibits simultaneously enhanced ultimate tensile strength (∼224 MPa) and elongation (∼30.9%). Compared to the Ca-free alloy, 0.5 wt.% Ca alloying increases the heat input and promotes the dislocation recovery, reducing residual dislocations. Nano-sized CaMgSn precipitates in grain interiors inhibit the dynamic recovery of dislocations during tensile, facilitating the work-hardening ability. Besides, high Schmid factors facilitate basal dislocation slips. Therefore, high EL is primarily due to the dislocation-free fine grain (∼2.7 μm), refined CaMgSn particles (micron-sized particles (∼1.3 μm) and nano-sized precipitates (∼55 nm)) and high Schmid factors (∼0.446) for basal slips.

Effect of extrusion conditions on microstructural characteristics and mechanical properties of SEN6 magnesium alloy

This study investigates the influence of extrusion conditions on the microstructure and tensile properties of Mg–6Al–0.3Mn–0.3Ca–0.2Y (SEN6) alloy, which is processed at various extrusion temperatures and ratios. An increase in both the extrusion temperature and ratio facilitates dynamic recrystallization during hot extrusion by enhancing thermal and strain energies, increasing the area fraction of recrystallized grains within the extruded material. As either the extrusion temperature or ratio increases, lattice rotation within the recrystallized grains intensifies, resulting in a stronger basal texture in the extruded material. During extrusion, the undissolved Al2Y, Al8Mn4Y, and Al2Ca particles within the billets are partially fragmented and realigned along the extrusion direction. However, the quantity, size, and dispersion of these particles are largely unaffected by variations in extrusion temperature and ratio. The tensile yield strength of the extruded material shows a positive correlation with increasing extrusion temperature and ratio, predominantly due to enhanced texture hardening. Specifically, the tensile yield strength of the material extruded at 400 °C with an extrusion ratio of 25 is 23.2 MPa higher than that of the material extruded at 300 °C with an extrusion ratio of 10. Despite the presence of non-recrystallized grains in materials extruded at the lower extrusion ratio of 10, compression and double twins form uniformly throughout the material during tension, as the widths of non-recrystallized grains are similar to those of recrystallized grains. Consequently, all extruded materials exhibit high tensile elongations, exceeding 20%.

The role of Ca on the microstructure and tensile properties of Mg-Al-Zn-Ca alloys

Synergy between Ca and other alloying elements such as Al and Zn can modify the microstructure and mechanical properties of Mg alloys. In this work, we compared four extrusion alloys: Mg-6Al-2 Zn, Mg-6Al-2Zn-0.5Ca, Mg-6Al-2Zn-1.0Ca, and Mg-6Al-2Zn-2.0Ca (all in wt.%). Tensile properties were remarkably not improved with Ca addition. The underlying reasons were explored using in situ synchrotron X-ray diffraction, electron microscopy, and cyclic loading tests. It is found that solute Ca imposes a softening effect on Mg's slip systems. The slight increase of the yield strength by the formation of Al2Ca particles is fully offset by the reduced Al solute strengthening. Texture weakening and resultant ductility enhancement reported in Mg-Zn-Ca ternary alloys is largely compromised by the high amount of Al. Nevertheless, Al2Ca particles can enhance the materials’ strain hardening by inducing back stress. An elastic-viscoplastic self-consistent (EVPSC) model was employed to simulate the deformation of the four alloys. Discrepancies between the simulation results and the experimental measurements mostly occur in the microscopic level (e.g. lattice strains and dislocation density evolution), and the reasons will be discussed.

Atomistic simulation of the dislocation interactions with the Al2Ca Laves phase in Mg–Al–Ca alloy

The mechanical properties of Mg–Al–Ca alloys are significantly affected by their Laves phases, including the Al2Ca phase. Laves phases are generally considered to be brittle and have a detrimental effect on the ductility of Mg. Recently, the Al2Ca phase was shown to undergo plastic deformation in a dilute Mg-Al-Ca alloy to increase the ductility and work hardening of the alloy. In the present study, we investigated the extent to which the deformation of Al2Ca is driven by dislocations in the Mg matrix by simulating the interactions between the basal edge dislocations and Al2Ca particles. In particular, the effects of the interparticle spacing, particle orientation, and particle size were considered. Shearing of small particles and dislocation cross-slips near large particles were observed. Both events contribute to strengthening, and accommodate to plasticity. The shear resistance of the dislocation to bypass the particles increased as the particle size increased. The critical resolved shear stress (CRSS) for activating dislocations and stacking faults was easier to reach for small Al2Ca particles owing to the higher local shear stress, which is consistent with the experimental observations. Overall, this work elucidates the driving force for Al2Ca particles in Mg–Al–Ca alloys to undergo plastic deformation.

Bimodal grain structure formation and strengthening mechanisms in Mg-Mn-Al-Ca extrusion alloys

The effects of small additions of calcium (0.1% and 0.5%11All compositions in weight percentage.) on the dynamic recrystallization behavior and mechanical properties of as-extruded Mg-1Mn-0.5Al alloys were investigated. Calcium microalloying led to the formation of Al2Ca in as-cast Mg-1Mn-0.5Al-0.1Ca alloy and both Mg2Ca and Al2Ca phases in Mg-1Mn-0.5Al-0.5Ca alloy. The formed Al2Ca particles were fractured during extrusion process and distributed at grain boundary along extrusion direction (ED). The Mg2Ca phase was dynamically precipitated during extrusion process, hindering dislocation movement and reducing dislocation accumulation in low angle grain boundaries (LAGBs) and hindering the transformation of high density of LAGBs into high angle grain boundaries (HAGBs). Therefore, a bimodal structure composed of fine dynamically recrystallized (DRXed) grains and coarse unDRXed regions was formed in Ca-microalloyed Mg-1Mn-0.5Al alloys. The bimodal structure resulted in effective hetero-deformation-induced (HDI) strengthening. Additionally, the fine grains in DRXed regions and the coarse grains in unDRXed regions and the dynamically precipitated Mg2Ca phase significantly enhanced the tensile yield strength from 224 MPa in Mg-1Mn-0.5Al to 335 MPa and 352 MPa in Mg-1Mn-0.5Al-0.1Ca and Mg-1Mn-0.5Al-0.5Ca, respectively. Finally, a yield point phenomenon was observed in as-extruded Mg-1Mn-0.5Al-xCa alloys, more profound with 0.5% Ca addition, which was due to the formation of (101¯2)extension twins in unDRXed regions.

Multiscale precipitate evolution and its implications on the tensile deformation behavior in thermomechanically processed and peak-aged lean Mg-Al-Ca-Mn alloy

A cast and solution-annealed Mg-1Al-0.28Ca-0.13Mn-0.05Si (wt. %) alloy is thermomechanically processed using hard plate hot forging (HPHF) at 723 K and subsequently aged at 473 K. Significant grain refinement (∼6 µm) and strong basal fiber texture (∼35 MRD) have been attained through HPHF, which remain unchanged (∼6 µm and ∼32 MRD, respectively) after the peak aging treatment as the grain boundaries remain pinned by nano-sized particles. Some of the coarse (Mg,Al)2Ca particles from the solution-annealed condition are fragmented during HPHF leading to much finer (∼9 µm) particles, while some evolve into eutectic-like clusters of (Mg+Al2Ca). During peak-aging, the (Mg+Al2Ca) clusters transform into fine Al2Ca precipitates, while a few of the fine (Mg,Al)2Ca particles still remains unaltered. Presence of nano-sized Al8Mn5 precipitates (∼29 nm) is also detected in the HPHF specimen, whereas new evolution of finer Al8Mn5 (∼10 nm) particles is observed in the peak-aged specimen. During the tensile tests, the HPHF specimen exhibits relatively better ductility (∼5%) and work hardening than the peak-aged specimen (∼3.7%) due to higher slip and twinning activity. In contrast, finer size and larger fraction of the Al8Mn5 precipitates results in considerably greater resistance to twinning as well as basal-slip in the peak-aged specimen, thereby reducing its ductility. On the other hand, the same precipitates are effective in pinning the dislocations and thus, primarily accountable for the considerably greater strength in the peak-aged specimen.

Development of microstructure in solution-heat-treated Mg-5Al-xCa alloys

AbstractThe development of the microstructure in Mg–Al –Ca alloys was investigated both in the as-cast condition and after solution heat treatment at 415 °C for 0.25 – 120 h, using secondary electron imaging, electron probe microanalysis and X-ray diffraction analysis in combination with light microscopy and image analysis. The main intermetallic compound in the Ca-containing alloys is Al2Ca, and the formation of the Mg17Al12phase is suppressed in the presence of Ca. The decomposition of Al2Ca during the solution treatment is more difficult than that of Mg17Al12, and undissolved Ca was still present in the form of stable Al2Ca even after 120 h. With increasing treatment time, the coarse Al2Ca phase, which was continuously distributed like a network around the α-Mg grains in the as-cast state, underwent thinning and necking in the initial stage. Then it became disconnected and fragmentized, and finally the fragmentized Al2Ca particles were spheroidized and fined due to the diffusion of Ca and Al along the Al2Ca/Mg interface. As a result, the continuous network-like Al2Ca phase turned into a dispersion of spherical particles. The fine Al2Ca particles were homogeneously distributed in the matrix, which can be very beneficial to creep resistance and mechanical properties of Ca-containing alloys.

Comparative study of tensile and high-cycle fatigue properties of extruded AZ91 and AZ91–0.3Ca–0.2Y alloys

Mg–Al–Zn–Ca–Y alloys with excellent ignition and corrosion resistances—termed SEN alloys (where the letters “S,” “E,” and “N” stand for stainless, environmentally friendly, and non-flammable, respectively)—have been developed recently. In this study, the microstructure, tensile properties, and high-cycle fatigue properties of an extruded Mg–9.0Al–0.8Zn–0.1Mn–0.3Ca–0.2Y (SEN9) alloy are investigated and compared with those of a commercial Mg–9.0Al–0.8Zn–0.1Mn (AZ91) alloy extruded under the same conditions. Both the extruded SEN9 alloy and the extruded AZ91 alloy have a fully recrystallized structure comprising equiaxed grains, but the former has a smaller average grain size owing to the promoted dynamic recrystallization during extrusion. The extruded AZ91 alloy contains coarse Mg17Al12 discontinuous precipitate (DP) bands parallel to the extrusion direction, which are formed during its cool down after extrusion. In contrast, the extruded SEN9 alloy contains relatively fine undissolved Al2Ca, Al8Mn4Y, and Al2Y second-phase particles, which are formed during the solidification stage of the casting process. The tensile strength of the extruded SEN9 alloy, which has finer grains and more abundant particles, is slightly higher than that of the extruded AZ91 alloy. However, the difference in their strengths is relatively small because the stronger solid-solution hardening and precipitation hardening effects in the extruded AZ91 alloy offset the stronger grain-boundary hardening and dispersion hardening effects in the extruded SEN9 alloy to some extent. The tensile elongation of the extruded AZ91 alloy is significantly lower than that of the extruded SEN9 alloy because the large cracks formed in the DP bands in the former cause its premature fracture. Although the extruded SEN9 alloy has higher tensile properties than the extruded AZ91 alloy, the high-cycle fatigue life and fatigue strength of the former are shorter and lower, respectively, than those of the latter. The DP bands in the extruded AZ91 alloy do not act as fatigue crack initiation sites, and therefore, fatigue cracks initiate on the specimen surface at all stress amplitude levels. In contrast, in most of the fatigue-fractured specimens of the extruded SEN9 alloy, fatigue cracks initiate on the undissolved Al2Ca and Al2Y particles present on the surface or subsurface of the specimens because of the high local stress concentration on the particles during cyclic loading. This particle-initiated fatigue fracture eventually decreases the high-cycle fatigue resistance of the extruded SEN9 alloy.

Discharge properties of ECAP processed AZ31－Ca alloys as anodes for seawater-activated battery

The effects of Ca addition and equal channel angular pressing (ECAP) on the microstructure evolution and discharge properties of AZ31-1.0Ca alloy were investigated to develop high-performance anodes for seawater-activated battery. Microscopic observations show that the net-like (Mg, Al)2Ca phase replaces the Mg17Al12 phase and distributes along grain boundaries in AZ31-1.0Ca alloy. 12-pass ECAP effectively refines the grains and crushes the (Mg, Al)2Ca phase into fine particles. The discharge measurements reveal that the 12-pass ECAP processed AZ31-1.0Ca anode exhibits desirable discharge properties with the largest power density of 88.1 mW cm−2 and the highest anodic efficiency of 63.3% at 100 mA cm−2, which are 25.0% and 13.6% higher than that of AZ31 anode, respectively. The enhanced discharge performances can be attributed to the modified microstructure with refined grains, fragmented and dynamically precipitated (Mg, Al)2Ca particles.

Microstructure and mechanical properties of high-strength high-pressure die-cast Mg–4Al–3La–1Ca–0.3Mn alloy

A new high-pressure die-cast (HPDC) Mg–4Al–3La–1Ca–0.3Mn (ALaX431) alloy with high strength has successfully been fabricated. This HPDC alloy in peak-aged state exhibits tensile yield strength (TYS) of 220 MPa at room temperature and TYS of 145 MPa at 250 °C, higher than the corresponding strength of HPDC Mg alloys reported so far. These high strengths are mainly due to the formation of fine grained structure, semi-continuous reticular structure consisting of stable Al3La and (Mg,Al)2Ca particles along grain boundaries and numerous nanoscale Al2Ca precipitates within grains. Due to its higher strength than existing HPDC Mg alloys, the new developed alloy has great application potential.

Unusual relationship between extrusion temperature and tensile strength of extruded Mg–Al–Zn–Ca–Y–MM alloy

This study investigates the effects of extrusion temperature on the microstructural characteristics and mechanical properties of an extruded Mg–9Al–0.8Zn–0.9Ca–0.6Y–0.5MM (AZXWMM91100) alloy and the resulting unusual relationship between the extrusion temperature and the tensile strength of the extruded material. All the extruded materials, i.e., alloy samples extruded at 325 °C, 350 °C, and 400 °C, have a fully recrystallized grain structure and contain abundant undissolved Al8Mn4Y, Al2Ca, and Al2Y particles. A small number of spherical nanosized Mg17Al12 precipitates are formed during extrusion, and their number density decreases with increasing extrusion temperature. The undissolved-particle bands, which are distributed parallel to the extrusion direction, act as obstacles to the growth of recrystallized grains during and after extrusion, which prevents excessive coarsening of grains at high extrusion temperatures. As a result, despite a large increase in the extrusion temperature from 325 °C to 400 °C, the average grain size of the extruded material increases only slightly, from 17.6 µm to 23.0 µm. Concurrently, the tensile strength of the extruded material increases from 183 MPa to 224 MPa; this directly proportional relationship between temperature and strength contrasts with the general inverse relationship between extrusion temperature and tensile strength. An increase in the extrusion temperature leads to an increase in the texture intensity of the extruded material; this increase is attributed to the following phenomena: (i) transition of the dominant dynamic recrystallization (DRX) mechanism from twinning-induced DRX and continuous DRX to discontinuous DRX, (ii) weakening of DRX through particle-stimulated nucleation, and (iii) promotion of the activation of prismatic slip. Therefore, the unusual extrusion temperature–tensile strength relationship for the AZXWMM91100 alloy is a combined result of the grain coarsening suppressed by the undissolved particles and strengthening of the texture hardening effect with the variation in the DRX behavior during extrusion.

Study of Mg–Al–Ca magnesium alloy ameliorated with designed Al8Mn4Gd phase

To investigate the effect of Al8Mn4Gd phase on microstructural and mechanical properties of Mg–Al–Ca magnesium alloy, two Mg–2.5Al2Ca and Mg–2.5Al2Ca-0.1Al8Mn4Gd alloys were designed and compared in this work. The results show that a small amount of Gd can significantly refine α-Mg grains and change the morphology of Al2Ca particles. Indeed, the formed Al8Mn4Gd phase could serve as a heterogeneous nucleation site for the α-Mg grains and Al2Ca particles. Furthermore, the introduction of Gd not only optimized the mechanical properties of Mg–Al–Ca alloy, but also facilitated the thermal deformation (such as hot rolling).

Effect of Ca addition on the microstructure and the mechanical properties of asymmetric double-sided friction stir welded AZ61 magnesium alloy

Abstract The effects of 2% Ca addition on the microstructure and the mechanical properties of the asymmetric double-sided friction stir welded magnesium alloy AZ61 were investigated. Compared to the Ca-free AZ61magnesium alloy, the addition of 2% Ca significantly refines the grains in the stir zone after welding. Due to the existence of thermally stable Al2Ca particles, there is no noticeable change in the grain size in the stir zone regardless of the welding heat input. The electron backscattered diffraction analysis at the center of the stir zone confirmed the significant deterioration of the (0001) texture strength from the addition of 2% Ca. The tensile test of the joint suggested that the addition of 2% Ca provides stable tensile properties to the joint regardless of the welding parameters. The critical (0001) texture strength that determines the ultimate tensile strength decreased from 20 to 15 by the Ca addition. The addition of Ca enhanced the deterioration of the plastic elongation when the (0001) texture strength increased.

Improvement of mechanical properties of extruded AZX912 magnesium alloy using high-temperature solution treatment

Abstract

Improving Strength and Ductility of a Mg-3.7Al-1.8Ca-0.4Mn Alloy with Refined and Dispersed Al2Ca Particles by Industrial-Scale ECAP Processing

Tailoring the morphology and distribution of the Al2Ca second phase is important for improving mechanical properties of Al2Ca-containing Mg-Al-Ca based alloys. This work employed the industrial-scale multi-pass rotary-die equal channel angular pressing (RD-ECAP) on an as-cast Mg-3.7Al-1.8Ca-0.4Mn (wt %) alloy and investigated its microstructure evolution and mechanical properties under three different processing parameters. The obtained results showed that RD-ECAP was effective for refining the microstructure and breaking the network-shaped Al2Ca phase. With the increase of the ECAP number and decrease of the processing temperature, the average sizes of Al2Ca particles decreased obviously, and the dispersion of the Al2Ca phase became more uniform. In addition, more ECAP passes and lower processing temperature resulted in finer α-Mg grains. Tensile test results indicated that the 573 K-12p alloy with the finest and most dispersed Al2Ca particles exhibited superior mechanical properties with tensile yield strength of 304 MPa, ultimate tensile strength of 354 MPa and elongation of 10.3%. The improved comprehensive mechanical performance could be attributed to refined DRX grains, nano-sized Mg17Al12 precipitates and dispersed Al2Ca particles, where the refined and dispersed Al2Ca particles played a more dominant role in strengthening the alloys.

The role of Al 2 Ca and Al 2 (Sm,Ca,La) particles in the microstructures and electrochemical discharge performance of as‐extruded Mg‐3wt.%Al‐1wt.%Zn‐based alloys for primary Mg‐air batteries

In this work, the role of Al2Ca and Al2(Sm,Ca,La) particles in the microstructures and electrochemical discharge performance of the as-extruded Mg-3wt.%Al-1wt.%Zn-based alloys has been reported and discussed for the anode design of Mg-air batteries. The Al2Ca and Al2(Sm,Ca,La) particles strongly refine the grains of the as-extruded AZ31 alloy from 9.1 ± 4.1 μm down to 5.1 ± 3.3 μm. The Al2Ca and Al2(Sm,Ca,La) particles increase the outputting cell voltage and discharge capacity of the modified AZ31 alloy. The AZ31-Ca alloy exhibits the highest discharge capacity and anodic efficiency of 1153 mAh/g and 52.5%, respectively, at 10 mA/cm2. The promoted discharge performance should be mainly attributed to the grain refinement (improving the corrosion resistance) and fine Al2Ca phase throughout the matrix (beneficial for uniform dissolution of Mg phase). Additional Al2(Sm,Ca,La) cubic particles further stimulate the anodic kinetics and aggravate the local dissolution of Mg phase near around, resulting in the deterioration of discharge performance.

Nucleation of Al2Ca particles on Al2Nd phase in an Mg-Al-Ca-Nd alloy

The distribution and structure of second phases in an Mg-12Al-6.5Ca-1Nd alloy prepared by spray deposition technique were studied using scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) combination of energy dispersion spectrum (EDS) mapping. Results show that several Al2Ca particles nucleated on the Al2Nd phase, revealing the initial liquid appearance of Al2Nd which finally led to the formation of heterogeneous nucleation sites. Low interfacial energy between the phases caused by closed lattice parameters is responsible for the heterogeneous nucleation. The crystallization sequence can be explained satisfactorily by the phase stability obtained from first-principles calculations.

Tailoring the Rolling Texture of AZ31 Mg Alloy with Calcium and Tin Addition

A small amount of Ca and Sn is added into AZ31 alloy separately to modify its deformation behavior and optimized rolling texture. Results show that addition of Sn increases the activation of twinning in AZ31. Some unusual double twins, inducing and double twins, are activated in AZ31–0.5Sn alloy. Refined grains with similar orientation as their parents are produced by continuous static recrystallization, which limits texture modification in AZT310. Ca gives rise to a remarkable grain refinement during hot rolling. Recrystallized grains in AZX310 exhibit a weaker double peak texture with a wide orientation spread. It is found that Al2Ca particles simulate recrystallized nucleation and some of these refined grains exhibit non‐basal orientations. Primary compression twins are favorable nucleation sites for recrystallization and they are more effective in texture modification compared to other double twins.

On the High Formability of AZ31-0.5Ca Magnesium Alloy.

In this work, we investigated the effect of Ca on the formability of the AZ31 Mg alloy. For this purpose, the microstructure, texture, mechanical properties and formability of AZ31 Mg alloy samples containing 0.5 wt. % Ca (AZ31-0.5Ca) were studied. For comparison, the performance of Ca-Free AZ31 alloy samples with similar grain size was also investigated. In addition, formability of this alloy was reached at a high punch speed. The results of this work showed that the addition of 0.5 wt. % Ca can enhance the formability of the AZ31 alloy, which was three times greater than that of the Ca-Free AZ31 alloy. The improved formability was attributed to the formation of (Mg,Al)2Ca particles (~1 μm), which, in turn, contribute to reducing the intensity of the strong basal texture during the primary processing of the alloy. The in-grain misorientation axis analysis determined by electron back-scattered diffraction and critical resolved shear stress calculations carried out by the viscoplastic self-consistent model showed that the non-basal slip systems could be activated in the AZ31-0.5Ca alloy.

Enhancement of the microstructure and elevated temperature mechanical properties of as-cast Mg‑Al2Ca‑Mg2Ca in-situ composite by hot extrusion

Mechanical properties of Mg‑Al‑Ca in-situ composite in the as-cast state and after hot deformation (by extrusion process) were evaluated and discussed based on the detailed microstructural analysis. The as-cast microstructure, containing α‑Mg, primary Al2Ca particles, and eutectic cells (α‑Mg + Mg2Ca and α‑Mg + Al2Ca), showed poor room temperature mechanical properties due to the presence of a large fraction of intermetallic phases with unfavorable morphology. The extrusion process resulted in a grain-refined microstructure introduced by dynamic recrystallization (DRX) accompanied with homogeneously distributed fragmented intermetallic particles. It was shown that the grain size of extruded specimens decreases with decreasing deformation temperature (decreasing the Zener-Hollomon parameter) and increasing extrusion ratio. Based on the electron backscattered diffraction (EBSD) analysis, the particle-stimulated nucleation (PSN) was characterized to be a major recrystallization mechanism during hot extrusion of Mg‑Al2Ca‑Mg2Ca composite. The room temperature, tensile properties of the as-cast composite were comparable to those obtained for the as-cast AZ91 (Mg‑9Al‑1Zn) alloy. The mechanical properties of the composite were significantly enhanced by the hot extrusion process. Moreover, the results of the tensile tests at 300 °C revealed that the tensile strength of the extruded Mg‑Al2Ca‑Mg2Ca composite is much higher than that of the extruded AZ91 alloy. Such improvement was related to the presence of thermally stable intermetallic phases in the composite microstructure. As a result, the designed Mg‑Al2Ca‑Mg2Ca composite might be suitable for use in high-temperature structures, where many commercial alloys are not applicable.