Ca-containing Alloys Research Articles

Effect of Ca addition on structure, phase composition and hardness of Al–6 %Cu–2 %Mn sheet alloy

The Al–Cu–Mn–Ca system was proposed as a bases for the design of new group of heat-resistant alloys which can be used for high-temperature applications instead of the 2219 types industrial alloys. Unlike the latter ones, the new alloys also do not require a full cycle of strengthening heat treatment including solid solution treatment, quenching and aging. The effect of calcium addition in the 1–4 wt% range on the structure, phase composition and hardness after high temperature annealing of the Al–6 %Cu–2 %Mn base sheet alloy has been studied. It has been shown that calcium addition leads to the formation of high-temperature eutectics (614–617 °С) with the participation of the Al27Ca3Cu7 and (Al,Cu)4Ca phases that are capable of spheroidization at high temperature annealing. The ingots of Ca-containing alloys do not require homogenization but have sufficient deformation plasticity at both hot and cold rolling. The structure of the previously unstudied quaternary Al–Cu–Mn–Ca phase diagram in the region of the aluminum corner has been also proposed, according to which it can contain 5 four-phase fields in the solid state with the participation of (Al), binary (Al2Cu, Al4Ca, Al6Mn) and ternary (Al8CaCu4, Al27Ca3Cu7, Al10CaMn2 and Al20Cu2Mn3) phases. Based on the obtained data, the composition Al–6 %Cu–2 %Mn–1 %Ca has been proposed as a basis for the development of a new heat-resistant alloy. It is shown that this alloy superior to industrial 2219 alloy in terms of heat resistance, especially after annealing at 400 °C, when the difference in hardness reaches 32 HV. The high heat resistance of the new wrought alloy originates from the formation of a specific microstructure consisting of fine Ca-containing eutectic inclusions and Al20Cu2Mn3 phase dispersoids with a size of about 100 nm which prevent recrystallization and allow maintaining the fine sub-grained structure.

Orthotropic Behavior of Twin-Roll-Cast and Hot-Rolled Magnesium ZAX210 Sheet.

Magnesium sheet metal alloys offer a deformation asymmetry, which is strongly related to grain size and texture. In order to predict deformation behavior as well as to provide methods to eliminate anisotropy and yield asymmetry, a lot of effort is invested in studying the tension–compression asymmetry of magnesium alloys. However, only a few studies deal with the characterization of the yield asymmetry of magnesium wrought alloys, especially Ca-containing alloys, related to temperature and strain. In this study, the orthotropic behavior of a twin-roll-cast, homogenized, rolled and finish-annealed Mg-2Zn-1Al-0.3Ca (ZAX210) magnesium alloy was investigated by tensile testing at room temperature, 150 °C and 250 °C. The r-values were determined and the Hill’48 yield criterion was used for the constitutive formulation of the plastic yielding and deformation. The yield loci calculated using Mises and Hill’48 as well as the determined r-values reveal an almost isotropic behavior of the ZAX210 alloy. The r-value increases with increasing logarithmic strain. At 0.16 logarithmic strain the r-values at room temperature vary between 1 (0°) and 1.5 (45° and 90°). At higher temperatures (250 °C), r-values close to 1 at all tested directions are attained. The enhanced yield asymmetry can be attributed to the weaker basal texture that arises during hot rolling and final annealing of the twin-roll-cast ZAX210 magnesium alloy. In comparison to AZ31, the ZAX210 alloy shows a yield behavior close to transversal isotropy. Finally, responsible mechanisms for this behavior are discussed.

Role of micro-Ca/In alloying in tailoring the microstructural characteristics and discharge performance of dilute Mg-Bi-Sn-based alloys as anodes for Mg-air batteries

The influence of micro-Ca/In alloying on the microstructural characteristics, electrochemical behaviors and discharge properties of extruded dilute Mg-0.5Bi-0.5Sn-based (wt.%) alloys as anodes for Mg-air batteries are evaluated. The grain size and texture intensity of the Mg-Bi-Sn-based alloys are significantly decreased after the Ca/In alloying, particularly for the In-containing alloy. Note that, in addition to nanoscale Mg3Bi2 phase, a new microscale Mg2Bi2Ca phase forms in the Ca-containing alloy. The electrochemical test results demonstrate that Ca/In micro-alloying can enhance the electrochemical activity. Using In to alloy the Mg-Bi-Sn-based alloy is effective in restricting the cathodic hydrogen evolution (CHE) kinetics, leading to a low self-corrosion rate, while severe CHE occurred after Ca alloying. The micro-alloying of Ca/In to Mg-Bi-Sn-based alloy strongly deteriorates the compactness of discharge products film and mitigates the “chunk effect” (CE), hence the cell voltage, anodic efficiency as well as discharge capacity are greatly improved. The In-containing alloy exhibits outstanding discharge performance under the combined effect of the modified microstructure and discharge products, thus making it a potential anode material for primary Mg-air battery.

Microstructure of die-cast alloys Mg–Zn–Al(–Ca): a study by electron microscopy and small-angle neutron scattering

AbstractThe microstructure of a magnesium die-cast alloy with 8 wt.% Zn and 5 wt.% Al (ZA85) and of two alloy modifications with 0.3 and 0.9 wt.% Ca (ZACa8503 and ZACa8509) are compared. Optical, scanning electron and transmission electron microscopy (TEM) were used to study the microstructure in as-cast and annealed conditions. Small-angle neutron scattering (SANS) measurements were carried out to characterize the early stages of the precipitation process. The die-cast microstructure of ZA85, ZACa8503 and ZACa8509 consists of dendritic α-Mg grains and coarse intermetallic grain-boundary phases. In the Ca-free ZA85 alloy, grain boundaries are decorated with a quasi-crystalline phase with icosahedral point-group symmetry (I phase), a metastable modification of the equilibriumτphase (Mg32(Al, Zn)49). At the grain boundaries of the Ca-containing ZA85 alloys, small amounts of the stable cubicτphase Mg32(Al, Zn, Ca)49as well as Al2(Ca, Zn) phase, were detected in addition to the I phase. In the as-cast condition, regions supersaturated with Al and Zn were found to surround the α-Mg grains and to becomeτphase precipitation sites during high-temperature annealing in all alloys investigated. TEM and SANS measurements revealed no influence of Ca on the size distribution and the coarsening rate of the precipitates, which are formed by a continuous precipitation process.

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.

Effect of microalloyed Ca on microstructure and mechanical properties of Mg–6Zn–1Mn–4Sn (wt.%) alloy

The effect of microalloyed Ca on the microstructure, mechanical properties and aging precipitation of Mg–6Zn–1Mn–4Sn (wt.%, ZMT614) alloy were investigated. The as-cast ZMT614 alloy was mainly composed of α-Mg, Mg–Zn and Mg2Sn phase, and a new CaMgSn phase with high melting point was formed with Ca addition. The CaMgSn phase was easier to form than Mg2Sn during solidification, so the formation of Mg2Sn was suppressed. Undissolved CaMgSn phases after solution treatment played an important role in pinning grain boundary migration and grain growth, and can act as nucleation sites promoting precipitation during aging process. CaMgSn particles also retarded the recovery process and the retained dislocations could act as additional heterogeneous nucleation sites for precipitation. β′ precipitates along [0001]α were finer and disc-like precipitates β'' on (0001)α were denser for Ca-containing alloys after aging treatment. The peak-aged ZMT614-0.5Ca alloy exhibited the optimum mechanical properties and its yield strength, ultimate tensile strength and elongation were 345 MPa, 378 MPa and 6.5%, respectively. The strengthening mechanism was mainly attributed to the grain refinement and precipitation strengthening.

Enhanced ductility of Mg–1Zn–0.2Zr alloy with dilute Ca addition achieved by activation of non-basal slip and twinning

Wrought Mg–Zn–Zr alloys with a strong basal texture usually exhibit highly anisotropic plastic behavior, resulting in premature fracture during deformation at room temperature. In the present study, we prepared a Mg–1Zn–0.2Zr (ZK10) alloy sheet by multi-pass hot rolling and annealing, tailoring the basal texture and reducing the plastic anisotropy with the addition of dilute Ca. Microstructural characterizations of the tensile-deformed alloys show that pyramidal <c+a> slip and 101¯2 twinning are more easily formed in the Ca-containing alloy compared with those in the ZK10 alloy. The weakened basal texture involving Ca addition leads to promoted activation of 101¯2 twinning, without significant impact on the Schmid factor distribution of pyramidal <c+a> slip. Simulation results of the visco-plasticity self-consistent model further indicate that Ca reduces the ratio of critical resolved shear stress for pyramidal <c+a> slip and 101¯2 twinning versus basal <a> slip, leading to a significant improvement in ductility. This work may provide a new strategy to develop wrought Mg alloys with enhanced ductility.

Combined Effect of Mg and Ca on Oxidation Behaviors of Zn-Mg Based Alloys Containing a Trace of Ca at High Temperature.

In this study, the combined effect of Mg and Ca on the high temperature oxidation behaviors of Zn-Mg based alloys containing trace Ca was investigated. Phase diagrams for the oxygen partial pressure versus contents of constituent elements were conducted on the basis of thermodynamic calculations to predict oxide scale behavior. Observation of Zn-1/3/5 mass%Mg alloys showed the distribution of a Zn-Zn11Mg₂ eutectic phase after primary formation. As-cast microstructures of the Ca-containing alloys included the formation of a Ca-based intermetallic phase. The change in oxidation resistance with variation of the Mg and Ca contents was experimentally examined via thermogravimetric analysis (TGA). The addition of trace Ca led to the formation of Zn13Ca and a CaO/MgO mixed oxide layer on the surface at 460 °C. After TGA at 460 °C under air atmosphere for 1 h, the Ca-free alloys showed rapid weight gain by oxidation, whereas the oxidation resistance of the Ca-added alloys was substantially increased.

High-Temperature Oxidation of NiAlCr\u2013Ca and NiAlCr\u2013Sr Alloys in Air

In this study, interrupted oxidation behavior of synthetic NiAlCr–Ca (Ca = 0.3, 1.4, 2 at.%) and NiAlCr–Sr (Sr = 0.4 at.%) alloys in the air at 1027 °C for 192 h was investigated. Parabolic rate constants (kp) showed that the Sr-containing alloy exhibited the best oxidation resistance among the alloys investigated in this study. The oxide scale formed on the Sr-containing alloy was composed of α-Al2O3 phase with Sr-rich nodules. Increasing the Ca concentration in the alloys was found to reduce the oxidation resistance due to the formation of non-protective Ca-rich complex oxides and consumption of α-Al2O3 scale by the reaction between Al2O3 and CaO. The Ca-rich complex oxides were initially formed on the Ca-rich interdendritic region and grew with time. Very little scale spallation was observed for the Sr-containing alloy, while it was notable for 0.3 at.% Ca-containing alloy. Spallation was attributed to the coefficient of thermal expansion (CTE) mismatch arisen from the formation of CaAl4O7 phase, a compound with a very low CTE.

Microstructure and mechanical properties of Mg-Zn-Ca-Zr alloy fabricated by hot extrusion-shearing process

In this study, ultra-fine grain Mg-3Zn-xCa-0.6Zr (x = 0, 0.6, 1.2, 1.8) alloy was obtained by a novel extrusion-shearing (ES) deformation process combining initial forward extrusion and subsequent equal channel angular extrusion (ECAP) shearing. The purpose of this paper is to improve the strength and plasticity of the alloy by ES process. The results of as-cast and homogenized alloy show that the ternary Ca2Mg6Zn3 phase with excellent thermal stability and refined grain size was formed in the alloy by adding Ca. In the ES process, the ternary phase was broken into ultra-fine particles pinned to the dynamic recrystallization (DRX) grain boundary, which introduced strong particle stimulated nucleation (PSN). Moreover, large number of MgZn phases in Ca-containing alloys were dynamically precipitated at DRX grain boundaries and within grains. In addition, in the early stage of the ES process, Ca2Mg6Zn3 phase promoted the accumulation of dislocations at the grain boundaries, and part of the grain boundary was transformed into low-angle grain boundaries (LAGBs) and high-angle grain boundaries (HAGBs). After the die corner shearing, the accumulated LAGBs gradually transformed into HAGBs due to continuous dynamic recrystallization (CDRX). Through the novel ES process and the addition of Ca element, the basal plane texture was significantly weakened and the DRX grain was refined. With the increase in Ca content, the strength and toughness of ES alloy increased obviously, and the yield asymmetry decreased gradually. When the content of Ca was 1.2 wt%, ES alloy exhibited the best combination of strength and toughness (UTS = 342 MPa, δ = 20.9%) due to nano-scale Ca2Mg6Zn3 phase, grain refinement and texture weakening. This property is of great significance for the development of magnesium alloys for biological applications.

Improving flame resistance and mechanical properties of magnesium–silver–calcium sheet alloys by optimization of calcium content

Abstract Low flame resistance and inferior mechanical properties are critical issues in widening applications of wrought magnesium alloys as a structural component. To address these issues, the present authors attempted to develop a flame-resistant magnesium–silver-based sheet alloy with excellent room temperature (RT) formability. It was found that the ignition temperature of Mg–1.5 wt%Ag–x wt.%Ca alloy was significantly increased with increasing the Ca content up to 2 wt%. The 2 wt% Ca-containing alloy exhibited a non-flammability characteristic upon 1000 °C due to the formation of compact and dense CaO film on the surface. Strength and in-plane yield anisotropy were enhanced with increasing the Ca content, which were mainly associated with a finer microstructure and a denser distribution of Mg2Ca second phase particles. In contrast, stretch formability was decreased with increasing the Ca content due to an increased number of the Mg2Ca particles. A compositionally optimized Mg–1.5Ag–1Ca (wt.%) alloy showed a high ignition temperature of 912.5 °C with a large index Erichsen value of 7.8 mm at RT.

Effect of micro-alloying Ca on microstructure, texture and mechanical properties of Mg–Zn–Y–Ce alloys

Microstructure, texture and mechanical properties of Mg–5Zn-0.3Y-0.2Ce alloys with the addition of trace xCa (x = 0, 0.3, 0.6 wt%) were systematically investigated in this work. The results revealed that more secondary eutectic phases and smaller grain size of as-cast microstructure could be found with increasing Ca content. After hot extrusion, the Ca-free alloy showed a uniformly recrystallized grain structure, while the Ca-containing alloys possessed a bimodal grain structure composed of fine dynamic recrystallized (DRXed) grains with a size of several microns and un-recrystallized coarse grains. EBSD analysis showed that the three extruded alloys had a fiber texture of (0001) basal plane aligned with the extrusion direction. Texture intensity of the DRXed region was weaker than the deformed region. The extruded alloy with the addition of 0.6 wt% Ca exhibited the highest yield strength of 321MPa due to the smallest DRXed grain size, the deformed region with strong basal texture and dense nanosized precipitates.

Effect of alloyed Ca on the microstructure and corrosion behavior of extruded Mg-Bi-Al-based alloys

Microstructural characteristics and corrosion behavior of extruded Mg-1Bi-1Al-xCa (x = 0, 0.6 and 1.2 wt%) alloys were investigated. It was found that the average grain size, texture intensity and corrosion resistance of the present low-alloyed alloy system initially increased and then decreased with increasing Ca content. Intergranular corrosion was observed in the present alloy system, which was related to the higher energy of the grain boundaries. Furthermore, the corrosion mode changed from pitting corrosion in the Ca-free alloy to the mixed pitting and filiform corrosion in the Ca-containing alloys in the initial stage of immersion based on the analysis in terms of micro-galvanic corrosion and the formation of protective film. As the immersion time increased, the filiform corrosion dominated in the Mg-1Bi-1Al-0.6Ca alloy. However, the corrosion product film of the Mg-1Bi-1Al-1.2Ca alloy was quickly destroyed due to the shedding of the coarse Mg2Bi2Ca2 phase, which resulted in severe corrosion.

Effects of Ca and Sr additions on microstructure, mechanical properties, and ignition temperature of hot-rolled Mg–Zn alloy

In this study, the microstructures, mechanical properties, and ignition temperatures of hot-rolled Mg-1.5Zn-X (X: Ca and/or Sr) alloys were investigated. The Mg-1.5Zn, Mg-1.5Zn–1Ca and Mg-1.5Zn–1Sr alloys had the mean grain sizes of 56.1 μm, 8.7 μm, and 18.5 μm, respectively. This indicates that Ca addition exerts a stronger effect on grain refinement than Sr addition. Both Ca and Sr additions significantly weakened the basal texture to approximately one-quarter in intensity for the annealed sheets. Ca-containing alloys exhibited texture splitting in the transverse direction, while Sr-containing alloys tended to maintain it in the rolling direction after annealing. The Ca and Sr additions improved the mechanical strength considerably due to grain refinement, which overcompensated for the influence of texture softening. Meanwhile, benefiting from the weakened textures, the 1% Ca and Sr additions remarkably increased the Erichsen values from 3.8 to 8.0 and 7.1, respectively. However, further addition resulted in the deterioration of both tensile ductility and stretch formability due to the cracking of secondary-phase particles. Ca addition had a better effect on enhancing the flame-retardant property than Sr addition, and 2% Ca and Sr additions increased the ignition temperature from 629 °C to 800 °C and 720 °C, respectively.

Effect of Ca addition on microstructures of aged Mg-15Gd-0.5Zr alloy

Effect of Ca addition on the microstructures of Mg-15Gd-0.5Zr (wt%) alloy during isothermal ageing at 250 °C have been investigated using high-angle annular dark-field scanning transmission electron microscopy and X-ray mapping. The results show that precipitation in the Ca-containing alloy involves the formation of ordered solute clusters, G.P. zones, β′, β1, β, Mg2Ca, and a novel ternary phase containing Mg, Gd and Ca. The novel precipitate phase has a plate morphology with its habit plane parallel to the basal plane of the matrix. It has a hexagonal structure (with lattice parameters a = 0.59 nm and c = 1.01 nm) and an orientation relationship with the matrix phase: (0001)P//(0001)α, 1¯21¯0P//011¯0α. Ca segregation inside β1 and β phases is observed and interpreted based on density functional theory calculations.

Influence of Nd or Ca addition on the dislocation activity and texture changes of Mg–Zn alloy sheets under uniaxial tensile loading

In situ hard X-ray diffraction experiments were carried out to investigate the dislocation slip activity of Mg-1 wt.% Zn-based alloys containing Nd or Ca during tensile loading. Diffraction patterns collected during tensile loading at 3 temperatures were analyzed using a convolutional multiple whole profile fitting procedure. High activation of nonbasal <a> and pyramidal <c+a> dislocations was found in the Nd- and Ca-containing alloys. The microstructure evolution after 10% deformation was examined by complementary EBSD measurements. The microstructure evolution was related to the differences in the initial texture and active deformation modes, according to the alloying and temperature. In-grain misorientation axes analysis obtained from the EBSD measurements confirms that the addition of Nd or Ca contributes to the higher activity of prismatic <a> slip. The high activity of prismatic <a> slip leads to a broadening of the basal poles perpendicular to the loading direction and a strengthening of the <101‾0> pole along the loading direction. The overall dislocation density evolution at elevated temperatures is controlled by dynamic recovery and dynamic recrystallization. These thermally activated mechanisms are retarded in the Nd- or Ca-containing alloys.

Effects of Calcium on Strength and Microstructural Evolution of Extruded Alloys Based on Mg-3Al-1Zn-0.3Mn

A family of alloys based on the Mg-Al-Zn-Ca-Mn system (Mg-3Al-1Zn-1Ca-0.3Mn, Mg-3Al-1.5Zn-0.5Ca-0.3Mn, and Mg-3Al-1Ca-0.3Mn, wt pct) was developed for extrusion. Their mechanical properties were evaluated by tensile testing at room temperature, and compared to those of the benchmark Mg-alloy Mg-3Al-1Zn-0.3Mn (AZ31). The microstructures of the extruded alloys were characterized in detail in order to reveal the effect of Ca on microstructural evolution, and consequently the alloy strength. The addition of Ca to the AZ31 stifles dynamic recrystallization and grain growth, with only ~30 pct recrystallization and a recrystallized grain size of ~480 nm. In contrast, the benchmark alloy is essentially completely recrystallized with an average grain size of ~2.3 μm. A high density of low-angle grain boundaries (LAGBs) and dislocations were observed in Ca-containing alloys, and were identified as a major factor in the observed strengthening. Such LAGBs form cellular subgrains predominantly along initial grain boundaries, or newly formed boundaries that are closely spaced (~ 600 nm) and nearly parallel to the extrusion direction. The subgrains have an ultrafine size of 100 to 400 nm, and difficult to convert to recrystallized grains. Solute segregation to grain boundaries was also observed. It is hypothesized that it is the Ca segregation to dislocation cores along LAGBs that decreases the dislocation mobility and stabilizes LAGBs, by thermodynamically decreasing the dislocation energy and/or kinetically imposing a solute drag effect.

ZnMg0.8Ca/Sr0.2 ternary alloys – the influence of the third element on material properties

Zinc-based materials alloyed with the elements of the 2nd group of the periodic table have been studied as potential materials for the fabrication of various biodegradable implants. In this study, we prepared two ternary alloys: ZnMg0.8Ca0.2 (wt.%) and ZnMg0.8Sr0.2 by melting a mixture of pure elements and subsequent gravity casting. We studied the influence of the third element (Ca or Sr) on the microstructural and mechanical characteristics of the alloys in the as-cast states and after various times of annealing at 350 °C. The microstructure of both ternary alloys was similar, the main difference was in the size and morphology of the Ca/SrZn13 phase. The SrZn13 phase formed fine particles with a submicron size and had more significant hardening effect in the as-cast state compared to the CaZn13 phase. The annealing led to a transformation of the eutectic structure into the “massive” Mg2Zn11 phase which caused a significant increase of both hardness and compressive yield stress. In the annealed states, comparable hardness was observed for both alloys and higher compressive yield strength for the Ca-containing alloy.

Influence of Ca addition on microstructure, mechanical properties and corrosion behavior of Mg-2Zn alloy

The microstructure, mechanical properties and corrosion behaviors of as-cast ternary Mg-2Zn-xCa (x=0, 0.2, 0.4, 0.8) alloys have been investigated in this study. Results indicate that the microstructure of Mg-Zn-Ca alloys can be significantly refined with increasing Ca concentration. Moreover, the alloys with different contents of Ca exhibit the different phases formation behaviors, i.e. α-Mg + Ca2Mg6Zn3 phases for Mg-2Zn-0.2Ca and Mg-2Zn-0.4Ca alloys, and α-Mg + Ca2Mg6Zn3 + Mg2Ca phases for Mg-2Zn-0.8Ca alloy, respectively. Among all the alloys, the maximum ultimate tensile strength and elongation (161 MPa and 9.1%) can be attained for the Mg-2Zn-0.2Ca alloy. Corrosion tests in Hanks’ balanced salt solution indicated that Ca addition is detrimental to corrosion resistance of Mg-2Zn alloy. The relationship between as-cast microstructure and properties for different Ca-containing alloys is also discussed in detail.

Effects of tensile twinning on the stretch formability of Mg

This study examined the influences of twinning during the Erichsen test. {10-12} tensile twinning plays the critical role in Mg alloys. Alloy elements and grain size are important factors that determine the formation of {10-12} tensile twins at room temperature. Mg-6.0Zn(Z6) and Mg-6.0Zn-0.3Ca(ZX60) alloys were fabricated and their grain size was varied under different annealing conditions. Tensile twinning is promoted by the addition of Ca, as assessed from measurements of the microstructure and the viscoplastic self-consistent calculations. The coarse-grain Ca-containing alloy showed the largest amount of tensile twinning. However, the stretch formability increased with grain size up to a certain point and then decreased. It can be inferred that microstructures with large grains activated the tensile twin, which became origins of cracks. The results of the small Erichsen test showed that tensile twinning contributes to high stretch formability releasing the stress concentration in the grain boundaries but the interaction between twin and slip causes cracks as the grain size increases.