Al2Ca Phase Research Articles

Microstructural refinement and mechanical properties improvement of Mg–Al–Mn–Zn–Ca alloy processed by rotary swaging at room temperature

Mg-4.1Al-0.6Mn-0.5Zn-0.4Ca alloy with high strength was prepared through hot extrusion at 330 °C followed by room temperature rotary swaging. The changes of microstructure and mechanical behaviors of extruded and swaged alloys after undergoing different rotary waging passes were investigated. The results reveal significant microstructural refinement, fragmentation of the Al8Mn5 phases, and precipitation of the Al2Ca phases after rotary swaging. With the increase of rotary swaging passes, a noticeable rise in the fraction of LAGBs and a more uniform distribution and refinement of second phases occurred. The texture with <10–10> parallel to processing direction was formed after rotary swaging. The rotary swaging largely improves the alloy's strength, with the ultimate tensile strength and yield strength increasing from 298 MPa to 198 MPa for the as-extruded alloy to 372 MPa and 359 MPa for the as-swaged alloy after 13 passes, respectively. The improvement of mechanical properties is a result of grain boundary strengthening, dislocation strengthening, and the strengthening of second phases. This result may provide valuable insights and guidance in the design of high-strength magnesium alloys as well as their plastic processing methods.

Modification of Microstructure and Mechanical Properties of Extruded AZ91-0.4Ce Magnesium Alloy through Addition of Ca.

The effect of the addition of alkali earth element Ca on the microstructure and mechanical properties of extruded AZ91-0.4Ce-xCa (x = 0, 0.4, 0.8, 1.2 wt.%) alloys was studied by using scanning electron microscopy, transmission electron microscopy, and tensile tests. The results showed that the addition of Ca could significantly refine the second phase and grain size of the extruded AZ91-0.4Ce alloy. The refinement effect was most obvious when 0.8 wt.% of Ca was added, and the recrystallized grain size was 4.75 μm after extrusion. The addition of Ca resulted in the formation of a spherical Al2Ca phase, which effectively suppressed the precipitation of the β-Mg17Al12 phase, promoted dynamic recrystallization and grain refinement, impeded dislocation motion, and exerted a positive influence on the mechanical properties of the alloy. The yield strength (YS), ultimate tensile strength (UTS), and elongation (EL) of the AZ91-0.4Ce-0.8Ca alloy were 238.7 MPa, 338.3 MPa, and 10.8%, respectively.

Mitigating segregation and synergistically improving corrosion resistance via Ca-Y microalloying in twin-roll cast Mg-Al-Mn dilute alloys

This study investigates Ca-Y microalloying to address macro-segregation and enhance corrosion resistance in twin-roll cast Mg-Al-Mn alloys. Ca-Y microalloying alleviates macro-segregation by transforming Al-Al pairs into Al-Ca pairs and refines eutectic Al2Ca via heterogeneous nucleation. In 3.5 wt% NaCl solution, Ca-Y microalloying promotes the formation of protective Al2O3, Y2O3, and Ca-oxides on the eutectic Al2Ca, thus reducing pitting corrosion. More importantly, Ca-Y microalloying prevents the formation of Al-poor regions, which are prone to micro-galvanic corrosion and oxide film degradation. Overall, Ca-Y microalloying significantly improves corrosion resistance via a synergistic effect in refining the eutectic Al2Ca phase and decreasing Al-solute segregation.

Effect of Co-addition of Sn, Traces of RE and Ca on the Mechanical and Micro Structural Properties of As-cast AZ80 Alloy

In this study, the effect of co-addition of tin (0, 6 and 9 wt.%), cerium (0, 0.3 and 0.6 wt.%), gadolinium (0, 0.6 and 0.9 wt.%) and calcium (0, 0.6 and 0.9 wt.%) on the mechanical and microstructural properties of AZ80 magnesium alloy is investigated. The results indicate that the mechanical properties are improved for all the as-cast alloys when compared to base alloy except for one alloy which is due to the presence of Mg-Sn-Ca phase in high volume fraction. Also, it is found that the existence of Al2Ca, Mg2Ca, Al2RE, MgSnRE and Mg2Sn phases have a significant impact on the mechanical and microstructural characteristics of the as-cast AZ80 alloys. It is observed that without the addition of Sn content, the best room temperature tensile properties are obtained for the alloy containing 1.5 wt.% of RE and 0.9 wt.% of Ca. With the addition of Sn, it is found that the best room temperature tensile properties are obtained for the alloy containing 6 wt.% of Sn, 0.9 wt.% of RE and 0.5 wt.% of Ca.

Role of Ca content on microstructure, mechanical properties and strain evolution of as-rolled Mg-Al-Ca-Zn-Mn alloy

The role of Ca content (0.5, 1.0, 2.0 wt.%) on microstructure, mechanical properties and strain evolution of as-rolled Mg-Al-Ca-Zn-Mn alloy was thoroughly investigated in this work. The results indicate that the primary second phase transformed from the Mg17Al12 phase to the Al2Ca phase after homogenization, and the amount of Al2Ca phase increased significantly with increasing Ca content. After hot rolling, the alloys exhibited the typical bimodal microstructure composed of fine dynamic recrystallized (DRXed) grains and coarse elongated un-DRXed grains, the area fraction of the DRXed regions increased with increasing Ca content. Besides, a large number of submicron-sized as well as nano-scaled spherical Mg17Al12 phases dynamically precipitated along the DRXed grain boundaries in all alloys, which promoted the DRX and restricted the grain growth. During rolling deformation, DRX preferentially occurred near the primary second phases and shear bands by the particle stimulated nucleation (PSN) and shear band induced nucleation (SBIN) mechanism in the alloys. The ultimate tensile strength (UTS), yield strength (YS), and elongation to failure (EF) along the rolling direction (RD) of the Mg-8.0Al-1.0Ca-1.0Zn-0.4Mn (wt.%) sheet were 393 MPa, 334 MPa and 8.7 %, respectively. Such high strength was mainly attributed to fine DRXed grains, high number density of dynamically precipitated Mg17Al12 phases and strongly textured un-DRXed grains with numerous sub-structures. The reasonable DRX ratio moderated strain localization and thus stabilized tensile deformation, leading to moderate plasticity of the alloy.

The semi-continuous (Mg,Al)2Ca second phase on Mg-Al-Ca-Mn alloys as an efficient anti-corrosion anode for Mg-air batteries

Developing high anti-corrosion and good discharge properties of magnesium based alloys for Mg-air batteries is still in challenge. In this work, a series of Mg-Al-Ca-Mn (AMX) alloys are prepared, and the second phases on AMX alloys are adjusting by varying the Al content. The AMX alloys with 3 wt% Al (denoted as AMX311) exhibits the excellent anti-corrosion behavior with a corrosion current density of 85.7 μA cm−2 and a low corrosion rate (4.1±0.21 mm y−1) in the 3.5 wt% NaCl solution. Furthermore, AMX311 shows lower hydrogen evolution volume (20.9±1 ml/cm2) and weight loss (5.8±0.3 mg) compared to other contrast samples. The outstanding electrochemical corrosion performance of AMX311 is attributed to the Al-rich oxide film which slowing corrosion at the initial stage and the formation of semi-continuous (Mg,Al)2Ca phase which can acts as a physical barrier to prevent the electrolyte from further corroding the magnesium matrix. Moreover, the AMX311 exhibits the highest discharge voltage (-1.471 V) and a peak specific energy of 1684.7±30.0 mW h g−1 at 20 mA/cm2, due to its superior corrosion resistance and reduced chunk effect. This work provides a good guide for designing advanced Mg-based alloys and an excellent insight into the effect of second phase on the corrosion and discharge performance of Mg-air batteries anode materials.

Anticorrosion and discharge performance of calcium and neodymium co-doped AZ61 alloy anodes for Mg-air batteries

Calcium (Ca) and neodymium (Nd) were introduced in the AZ61 alloy as alloying elements. The microstructure, corrosion behavior, and discharge properties of AZ61-1Nd-xCa (x = 0, 0.5 wt.%, 1 wt.%, 2 wt.%) alloys as anodes for Mg-air batteries were systematically investigated. The results indicated that the AZ61-1Nd-1Ca alloy exhibits the best corrosion resistance during electrochemical experiments and hydrogen evolution tests. Discharge performance tests showed that the AZ61-1Nd-1Ca alloy exhibits the best specific capacity (1193.6 mAh g−1), energy density (1893.7 mWh g−1), anode efficiency (60.3 %), and cell voltage (1.246 V) at higher current densities. This is mainly attributed to the addition of Ca element, which refines the grain size of the alloy and increases the grain boundary area. In addition, Al2Nd and Al2Ca phases have similar corrosion mechanisms in the cross-section of the extruded alloy. The precipitated granular Al2Ca phase is uniformly dispersed on the substrate and acts as a physical barrier. This not only enhances the corrosion resistance of the alloy but also improves the anode efficiency of the alloy during discharge.

Effect of Individual Alloying Addition on the Microstructure and Creep Behavior of Squeeze Cast AZ91 Magnesium Alloy

The microstructure of AZ91 (Mg-Al) alloy is comprised of α-Mg and β-Mg17Al12 massive phase. The lower melting point associated with the β-Mg17Al12 phase results in poor creep resistance of the alloy. In the present study, the AZ91 alloy with the addition of calcium (Ca, 1wt%) and cerium (Ce, 1wt%) is cast, and their effect on the microstructure and creep behavior of AZ91 alloy have been investigated. Thermally stable phases such as Al2Ca and Al11Ce3 are introduced in the AZ91 alloy through the addition of Ca and Ce elements. Energy dispersive spectroscopy (EDS) and x-ray diffraction analysis confirmed the presence of these intermetallic phases in the microstructure. Tensile creep tests on the as-cast samples were performed at 175°C temperature under 50 MPa stress. The study shows that the creep resistance of AZ91 alloy is greatly improved with the presence of Al2Ca and Al11Ce3 intermetallic phases because of their better thermal stability than β-Mg17Al12.

Effect of rolling reduction on the microstructure and mechanical properties of hot-rolled Mg-Li-Al-Ca alloys

To improve the strength of magnesium (Mg)-lithium (Li) alloy without sacrificing plasticity, Mg-7.3Li-4Al-3Ca alloy is designed and subsequently rolled at 350 °C. The microstructure evolution and mechanical properties of the alloy under different rolling reductions are studied. The results show that the as-cast and hot rolled alloy is composed of α-Mg, β-Li matrix phases as well as Al2Ca and (Mg, Al)2Ca precipitated phases. After hot rolling, the agglomerated Al2Ca precipitates are broken and dispersed in the matrix. A large amount of continuous filamentous (Mg, Al)2Ca phase becomes discontinuous and the phase content decreases monotonously with increasing rolling reduction due to the decomposition of (Mg, Al)2Ca phase and the corresponding constituent elements dissolve in the matrix. During the hot-rolling process, some fine equiaxed grains appear in the alloy matrix due to dynamic recrystallization. Both the strength and elongation of Mg-7.3Li-4Al-3Ca alloy increases monotonously with increasing rolling reduction. For the alloy with rolling reduction of 60%, the ultimate tensile strength and elongation are 218Mpa and 24.8%, respectively, which are 52.4% and 153% higher than those for the as-cast alloy. The strengthening mechanism is attributed to fine grain strengthening, precipitation strengthening and solid solution strengthening. Grain refinement and the appropriate distribution of Al2Ca and (Mg, Al)2Ca phases causes the obvious increase of the plasticity.

Microstructure and Mechanical Properties in a Gd-Modified Extruded Mg-4Al-3.5Ca Alloy

In the present study, the microstructure and mechanical properties of a new Mg-4Al-3.5Ca-2Gd (AXE432) alloy are investigated. The microstructure of the as-cast AXE432 alloy consists of α-Mg, C14 (Mg2Ca), and C36((Mg, Al)2Ca) phases. After the heat treatment at 480 °C for 8 h, the C14 with fine lamellar structure changes from narrow stripes to micro-scale particles, and part of the C36 and the C14 dissolve into the α-Mg matrix, with many short needle-shaped C15 (Al2Ca) phase precipitating in the primary a-Mg grains. The AXE432 alloy extruded at a temperature as high as 420 °C exhibits a refined dynamically recrystallized (DRXed) microstructure with grain sizes less than 1.5 ± 0.5 μm and a strong {0001}&lt;101¯0&gt; basal texture with a maximum intensity of 5.62. A complex combination of the effects from grain size, texture, second-phase particles, and strain hardening results in balanced mechanical properties, with the tensile yield strength (TYS), ultimate tensile strength (UTS), elongation (El), compressive yield strength (CYS), and ultimate compressive strength (UCS) of 331.4 ± 2.1 MPa, 336.9 ± 3.8 MPa, 16.1 ± 2.3%, 270.4 ± 1.6 MPa and 574.5 ± 12.4 MPa, respectively.

Effect of friction stir back extrusion rotational speed on microstructure, mechanical properties, and corrosion behaviour of AZ91-Ca alloy

In this study, the effect of severe plastic deformation via friction stir back extrusion on microstructure, mechanical properties, and corrosion behavior of AZ91-Ca alloy were investigated. The results show that due to the high temperature during the process at a rotation speed of 1200 rpm, all the β-Mg17Al12, and Al2Ca phases are dissolved, and next to the manganese-rich compounds, re-precipitation of the β-Mg17Al12 and Al2Ca phases is observed. Also, increasing the rotation speed from 800 to 1200 rpm increases the accumulated strain from 0.186 to 0.351. With decreasing rotational speed from 1200 to 800 rpm, the texture parameter of the basal and prismatic planes is reduced, and on the contrary, the texture parameter of the pyramidal planes is increased. The ultimate tensile strength value increases from 332.14 ± 8.71 MPa to 375.13 ± 7.45 MPa by decreasing the rotational speed from 1200 to 800 rpm. The ultimate tensile strength, yield strength, elongation percent, and corrosion resistance in the sample processed at 800 rpm increased by 78, 49, and 32%, respectively, when compared to the as-cast base metal.

Improvement of Hot Tearing Resistance of AZ91 Alloy with the Addition of Trace Ca.

Hot tearing is the most common and serious casting defect that restricts the light weight and integration of magnesium alloy components. In the present study, trace Ca (0-1.0 wt.%) was added to improve the resistance of AZ91 alloy to hot tearing. The hot tearing susceptivity (HTS) of alloys was experimentally measured by a constraint rod casting method. The results indicate that the HTS presents a ν-shaped tendency with the increase in Ca content, and reaches its minimum value in AZ91-0.1Ca alloy. Ca is well dissolved into α-Mg matrix and Mg17Al12 phase at an addition not exceeding 0.1 wt.%. The solid-solution behavior of Ca increases eutectic content and its corresponding liquid film thickness, improves the strength of dendrites at high temperature, and thereby promotes the hot tearing resistance of the alloy. Al2Ca phases appear and aggregate at dendrite boundaries with further increases in Ca above 0.1 wt.%. The coarsened Al2Ca phase hinders the feeding channel and causes stress concentration during the solidification shrinkage, thereby deteriorating the hot tearing resistance of the alloy. These findings were further verified by fracture morphology observations and microscopic strain analysis near the fracture surface based on kernel average misorientation (KAM).

Effect of Ca content on microstructure and strength of as-extruded Mg-1.0Al-xCa alloy

In this work, the effects of Ca content and extrusion temperature on the microstructures and mechanical properties of Mg–1Al–xCa alloys have been investigated. For Mg–1.0Al–0.4Ca (wt-%, AX10) dilute alloy, decreasing extrusion temperature from 290°C to 260°C can inhibit the dynamic recrystallisation (DRX) process, and the yield strength (YS) is also increased from ∼266 ± 6 MPa to ∼309 ± 3 MPa. When increasing the Ca content, Mg–1.0Al–0.8Ca (wt-%, AX11), a higher density of the Al2Ca phase is dispersed at the grain boundaries, which can further restrict the growth of DRXed grains, only ∼1.0 µm. Consequently, the higher YS of ∼330 ± 5 MPa can be obtained. The relevant results can shed light on designing other high-strength and low-alloyed Mg wrought alloys.

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.

Microstructures, tensile properties, and strengthening mechanisms of novel Al-Mg alloys with high Mg content

This study investigated the microstructures, mechanical properties, tensile deformations, and strengthening mechanisms of the Al-7-mass%-Mg (7Mg) and Al-9-mass%-Mg (9Mg) alloys developed from the Mg+Al2Ca master alloy. The microstructures of both as-extruded alloys comprised an Al matrix and eutectic Mg2Al3, Al2Ca (C15), and Al6(Mn,Fe) phases. The 7Mg alloy mainly consisted of elongated grains surrounded by small equiaxed grains, whereas the 9Mg alloy contained only fine equiaxed grains. Higher Mg content resulted in higher fractions of evenly distributed C15 phases. Tensile tests revealed yield strengths and maximum-tensile strengths of 234 and 429 MPa, respectively, for the 7Mg alloy, and these values increased to 276 and 479 MPa, respectively, for 9% Mg content. The characteristic serrated flow in the 7Mg alloy started at approximately 10% of the engineering strain. In contrast, in the 9Mg alloy, the development of the serrated flow from the beginning of tensile deformation, immediately after the yield point, is considered to be facilitated by the fine equiaxed grains and higher Mg content and number densities of obstacles (owing to the formation of secondary phases). Solid-solution and grain-boundary strengthening mainly contributed to the overall yield strength of the aluminum-magnesium (Al-Mg)-based alloys. Three microstructural factors, higher Mg solid solubility, smaller grain size, and a higher fraction of secondary phases, were responsible for the higher yield strength of the alloy with higher Mg content. Theoretical calculations based on conventional strength-prediction models predicted yield-strength values almost identical to those obtained experimentally, demonstrating the potential of the conventional models to accurately predict the yield strengths of Al-Mg alloys with high Mg content.

Atomistic insight into impact of solute segregation on α-Mg/FCC-Al2Ca interface stability

Solute segregation at phase boundary (PB) is critical for stabilizing nano-precipitates, which plays a decisive role in designing Mg alloys with high thermal stability. To clarify the fundamental factors that govern the PB stability, impacts of solute Ag, Sn, Bi, and In on interfacial properties of coherent α-Mg (0001)/Al2Ca (111) were systematically investigated via first-principles calculations. Solute segregation was proven to affect the PB stability by both chemical and mechanical contributions. Feasible alloying elements that could stabilize the nano-precipitates should form strong bonding and/or alleviate the distortion near PB by segregation, wherein the former plays a more dominant role. Accordingly, Ag segregation at PB was deemed to improve the thermal stability of the Al2Ca phase. Our work aims to provide a new perspective for the development of Mg alloys for potential high-temperature applications via tailoring interfacial properties.

Influence of aluminum content on microstructure and performance of Mg-Zn-Ca-Al-Mn magnesium alloys

The influences of Al content on microstructure, mechanical properties, stretch formability and thermal conductivity of Mg-1.5Zn-0.3Ca-xAl-0.2Mn (x: 0.2, 0.5, 1, 1.25, 1.5, 2, wt%) alloys were investigated. With increasing the Al content from 0.2 wt% to 2 wt%, the texture changed from the TD-split texture to the quadrupolar texture at the Al content of 1.25 wt%, and finally became the RD-split texture with an increased texture intensity. The change of texture can be attributed to the formation of Al2Ca phase, which weakens the segregation of Ca at grain boundaries due to the decreased amount of Ca solute atoms in the α-Mg matrix. In addition, Al-Mn precipitates changed from a spherical shape to a rod shape accompanied by a largely increased Al/Mn ratio and a decreased amount of precipitates, when the Al content was increased to be larger than 1 wt%. The 0.2–1.5 wt% Al added alloys exhibited excellent stretch formability with the Erichsen values of 8.4–9.1 mm, which were much larger than that (6.8 mm) of the 2 wt% Al added alloy due to texture effect. The 1 wt% Al added alloy had the largest Erichsen value of 9.1 mm, while the 0.2 wt% and 2 wt% Al added alloys exhibited much higher yield strength (165 and 171 vs. 144 MPa). The thermal conductivity continuously decreased with increasing the Al content. Compared to the 2 wt% Al added alloy, the 0.2 wt% Al added alloy exhibited a much higher thermal conductivity (140 vs. 105 W/(m·K)) benefiting from the low concentration of Al solute atoms in the α-Mg matrix. Consequently, a combination of strong mechanical strength, excellent stretch formability and high thermal conductivity was achieved in the low Al-containing alloys with 0.2–0.5 wt% Al.

Investigation on microstructure evolution and mechanical properties of Mg–Li–Al–Ca alloy after a series of heat treatments

Four heat treatment process routes were designed to study the microstructure evolution and mechanical properties of Mg–Li–Al–Ca alloy in relation to the heat treatment process. After heat treatment (Mg,Al)2Ca phase decomposed, and Li element volatilized. A large number of fine α-Mg precipitated phases appeared in β-Li matrix. The optimum mechanical properties of heat-treated alloy were obtained at 350 °C for 120 min, which are 190 MPa and 12% for tensile properties and 507 MPa and 51.9% for compression properties. Segregation of Al and Ca element was observed at grain boundaries. The phase boundaries between newly precipitated α-Mg phase and β-Li matrix impeded the slip of dislocations. It is believed that both the precipitation strengthening due to the formation of α-phase and the solid solution strengthening enhanced by the decomposition of (Mg,Al)2Ca phase play crucial roles in the improvement of mechanical properties.

Effect of Ca addition on the microstructure and creep behaviour of AZ91 Mg alloy

The lower density and higher specific mechanical properties of the magnesium and its alloys are attracting the great attention of the automakers from the perspective of automobile fuel economy and reducing the environmental impact. The AZ91 magnesium alloy has excellent mechanical properties; however, its application is limited by its poor creep resistance. In the present study, the effect of Ca addition and squeeze casting on the microstructure and creep behaviour of AZ91 Mg alloy has been investigated. Tensile creep test is conducted at temperatures of 150 and 175 °C under 50 MPa stress. The study shows an increase in creep resistance with Ca addition. Microstructural investigations with scanning electron microscopy confirm that the precipitation of thermally stable Al2Ca phase with skeleton morphology leads to increased creep resistance.

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.