Basal Texture Research Articles

Comparison of the Strain Rate Sensitivity in AZ31 and WE43 Magnesium Alloys under Different Loading Conditions

The initial texture effect of strain rate sensitivity response in magnesium alloys was studied using the quasi in situ electron backscatter diffraction technique. The strain accumulations, under uniaxial loading of both tensile and compression along the rolling direction, showed strain rate dependency. For the AZ31 magnesium sheets with strong basal texture (max intensity to 11.2), twinning first occurs at the onset of plastic deformation, resulting in macroscopic strain rate insensitivity. With further loading, the strain rate sensitivity significantly increases (with the strain rate sensitivity coefficient m increased from 0.0025 to 0.012), arising from the initiation of slip. For the WE43 alloy sheets with weak basal texture (max intensity to 2.6), the overall deformation is sensitive to strain rate under both tensile and compression. The twinning growth rate and twinned volume fraction have been used to identify whether the process is twin-dominated or slip-dominated during the plastic deformation of a magnesium alloy.

Effect of Crystallographic Orientations on Bendability in a Strongly Textured Mg-9Al Extrusion Plate and Texture Evolution during Three-Point Bending

The dependence of bendability on crystallographic orientations and texture evolution was investigated in a strongly textured Mg-9Al extrusion plate by bending along four directions. Results show that the bars have relatively small and reasonably close bendability when bent along the extrusion direction, transverse direction, and through-thickness direction. In contrast, the bendability of the 45° bar is much larger. Microstructure examination indicates that twins are prevalent in all bars. Furthermore, a detailed analysis of deformation mechanisms suggests that the initial texture transforms towards a basal texture during bending. Nevertheless, the texture transformation efficiency is drastically lower when basal slip-in contrast to tensile twinning-is the dominant deformation mechanism. The difference in texture evolution efficiency was used to rationalize the varied bendability along different directions. The findings of this provide insights into improving the bendability of magnesium alloys.

Mechanical and functional properties of ultra-thin Mg foils

Wide and thin Mg foils with thicknesses between 35 μm and 400 μm without any edge cracks are successfully produced by using commercial grade Mg (99.9% purity), in this study. Control of the initial microstructure in the billet (before rolling) is effective and essential from a processing viewpoint. All of the produced rolled foils have similar microstructures, i.e., grain sizes of approximately 10 μm, and a basal texture, regardless of the foil thickness. The results obtained from hardness (indentation) and tensile tests demonstrate the effect of foil thickness on mechanical properties. Hardness is independent of the indentation plane and foil thickness; on the contrary, the decrease in tensile strength is observed at the critical value of t/d, where t is the foil thickness and d is the grain size, owing to the lack of grain boundaries. This trend is the same as that shown by other thin foils of conventional metallic materials. In addition to the foil thickness effect, these rolled Mg foils exhibit good damping property in association with their crystal structure. This provides the possibility for Mg foils to be employed in electric devices and other applications.

Effect of Al Concentration on Basal Texture Formation Behavior of AZ-Series Magnesium Alloys during High-Temperature Deformation

Magnesium and its alloys have been restricted in their industrial applications due to problems related to their formability. To overcome this issue, controlling the crystallographic texture is important, and the texture formation mechanism should be investigated in relation to factors including deformation conditions and solute atoms. In particular, the effects of solute atoms on the texture formation behavior should be further analyzed because they can considerably affect the deformation behavior. Thus, in this study, to clarify the effect of aluminum concentration on the texture formation behavior and microstructure, high-temperature uniaxial compression tests were conducted on three types of AZ-series magnesium alloys (AZ31, AZ61, and AZ91). Compression was conducted at 673 K and 723 K, with strain rates of 0.05 s-1 and 0.005 s-1, up to a true strain of -1.0. Cylindrical specimens were prepared from a rolled plate that had a (0001) basal texture and was compressed parallel to the c-axis of the grains. Consequently, work softening and fiber texture formation were observed in all the specimens. During the deformation, the development of grain boundaries, which is a typical characteristic of continuous dynamic recrystallization (CDRX), was observed, and the (0001) texture was highly developed with increasing Al content. Although each alloy was associated with the same deformation conditions and mechanisms, the AZ31 alloy exhibited a non-basal texture component. The stacking fault energy contributed to the generation of slip systems and gliding, and it was seen as the main reason for texture variation.

In situ analysis of phase constituents evolution upon hydrogen cycling of cold-forged Mg-Ni powders

Development of hydrogen storage in metal hydrides requests efficient, simple and safe processes such as room temperature forging. Samples elaborated in this way from Mg and Ni powders result in bulk pellets whose only identified phases are Mg and Ni. Deuterium absorption occurs from 250 °C in these samples due to the co-existence of basal fiber texture and structural defects, both induced by forging. In situ neutrons diffraction analysis shows the progressive formation of the Mg2NiD0.3 / Mg2NiD4 phases upon deuterium cycling and allows to identify the multi-step absorption and desorption mechanisms at temperatures as low as 270 °C. The growth of the Mg2NiD0.3 / Mg2NiD4 phases occurs in a short time during the second step of the deuterium absorption – as Mg2NiD0.3 transformed into Mg2NiD4 – and progresses from the periphery of the initial Ni particles towards their center. The mechanism is controlled simultaneously by deuterium diffusion and migration of Mg atoms through the already formed Mg2NiD4 deuteride. The reaction between Mg and Ni is not observed in the absence of deuterium, even after annealing at 285 °C for 48 h, thus we evidence a deuterium-induced mobility of the metal atoms.

Elimination of in-plane mechanical anisotropy and enhancement of strength-ductility of rolled Mg sheets via electric-associated twinning and recrystallization

A novel method referred as electric-associated corrugated wide limit alignment (E-CWLA) was proposed to address the contradiction between strength and plasticity of hot-rolled Mg sheets. The counterpart without electricity was known as CWLA. A huge number of tensile twins (TTs) was induced in CWLAed sample R300, and dynamic recrystallization (DRX) occurs. Strong transverse direction (TD)-titled texture accounts for the huge difference in yielding strength between different loading directions, which are closely related to Schmid factors of prismatic <a> and pyramidal <c+a> slips, resulting in different ultimate tensile strength (UTS) and ductility of R300 along different directions. E-CWLAed sample E300 presents good in-plane mechanical isotropy with the maximum UTS of 272.1 MPa, yielding strength of 213.6 MPa and elongation of 22.7%, increasing by 32%, 53% and 83% compared with the as-received sheets, respectively. Regardless loading direction, basal and non-basal slips possess similar SFs and greatly facilitates uniform plastic deformation and mechanical isotropy of E300. The special microstructure including lots of fine grains, some large grains and TTs results in a bimodal basal texture with very weak TD-titled component promotes its good deformation compatibility. Electricity contributes little to TTs formation of E300, and its thermal effect promoting dislocation mobility and DRX and the athermal effect increasing DRX nucleation rate are responsible for final grain size distribution.

Effects of bimodal basal texture on bending stress and microstructure evolution of Mg−2.6Nd−0.55Zn−0.5Zr alloys

AbstractThe extrusion direction (ED) and transverse direction (TD) samples were cut from Mg−2.6Nd− 0.55Zn−0.5Zr alloy sheet, and the effects of bimodal basal texture on the bending stress and microstructure evolution were investigated. Results showed that the ED sample with weak basal texture possessed much lower bending stress than the TD sample. This anisotropy is attributed to the role of different textures on the evolution of deformation mechanisms and microstructures. For the ED sample with weak basal texture, the dominant deformation mechanisms were basal slip in the majority with minor supplement of tension twin in the inner region while basal slip and prismatic slip in the outer region. In comparison, the TD sample with strong basal texture activated more tension twin in the inner region and more prismatic slip in the outer region. The activation of tension twin promoted the development of 〈0002〉//compression direction type texture with the confined ±30° angle range in the inner region and reduced the emergence propensity of high dislocation density and low angle grain boundaries. The massive activation of tension twin allowed the TD sample to maintain a high strain-hardening capacity, which was the fundamental reason for the high bending stress in this sample.

Fracture behaviors of commercially pure titanium under biaxial tension: Experiment and modeling

• The effect of typical basal texture on the fracture behaviors and associated mechanisms of Ti under biaxial tension was for the first time elucidated by experiments and Finite Element modeling. • The presence of typical basal texture induced significant mechanical anisotropy in Ti, contributing to a two-stage fracture behavior consisting of a first normal fracture perpendicular to the rolling direction (RD) and a secondary shear fracture along the 45° direction relative to the RD. • An anisotropy parameter K , defined as the ratio of the equivalent effective traction stress to the yield strength, was proposed for the first time to predict the fracture path with the impact of crystallographic preferred orientation. The fracture behaviors and associated mechanisms of metallic materials under biaxial stress are vital for their manufacturability and service performance. In this work, the fracture behaviors of commercially pure titanium (CP-Ti) under quasi-uniaxial and equi-biaxial tension were investigated by using the digital image correlation technique and finite element modeling. The fracture behaviors under quasi-uniaxial tension were characterized by a general normal fracture. In contrast, normal fracture firstly occurred perpendicular to the rolling direction (RD) under equi-biaxial tension, followed by secondary shear fracture along the 45° direction relative to the RD. The normal fracture was attributed to the lower strain hardening ability in RD compared to the transverse direction (TD) induced by the TD-split type basal texture. The different hardening abilities introduced large shear stress in the 45° direction, which contributed significantly to the secondary shear fracture. An anisotropy parameter K ( Δ S s / σ s ), defined as the ratio of the equivalent effective traction stress to the yield strength, was proposed for the first time, to predict the fracture path with the impact of crystallographic preferred orientation.

Evolution of microstructure and texture of AZ80 magnesium alloy under hot torsion with constant decreasing temperature rate

Hot torsion tests for AZ80 magnesium alloy were carried out in the temperature range of 380 °C-260 °C, with a constant decreasing temperature rate of 10 °C/s in order to weaken the basal texture and refine the grains. The results indicated that the average grain sizes were refined forming gradient structure with increasing specimen radial position from center (12.2–5.4 μm), and that the initial basal texture intensity of the extruded magnesium alloy was weakened from 46.2 to 8.3. Furthermore, the extension twins (ETs) could be disintegrated from the twins forming separated twins with smaller sizes. Interestingly, ETs with the same twin variant intersecting with each other could be coalesced forming grains with similar orientation, while ETs with different twin variants were separated by twins boundaries contributing to grain refinement. Moreover, in addition to the conventional continuous dynamic recrystallized (CDRX) grains with 30˚ orientation rotated around C-axis of the parent grains, CDRXed grains with 30˚ rotation around a-axis and random rotation axis were also discerned. Besides, the CDRX evolution induced twins were also elaborated, exhibiting the complex competition between CDRX and twining. Hot torsion deformation with constant decreasing temperatures rate is an effective way of grain refinement and texture modification.

The influence of the LPSO on the deformation mechanisms and tensile properties at elevated temperatures of the Mg-Gd-Zn-Mn alloys

The introduction of long period stacking ordered (LPSO) phase is an effective way to improve the performance of the magnesium alloys at elevated temperatures. In this paper, the hot tensile behaviors of two Mg-4Gd-1Zn-0.5Mn (wt.%) alloys with different lamellar LPSO volume fractions under the strain rate of 2.2 × 10–4 s –1 and at varied temperatures (150 °C–250 °C) were investigated. With the increasing of the deformation temperatures, the peak stresses of the alloy with lower volume fraction of the LPSO decreased (denoted as LL alloy). The hot tensile mechanical properties of the alloy with higher volume fraction of the LPSO (denoted as HL alloy) are significantly higher than those of LL alloy. When the deformation temperature was 250 °C, the LL alloys significantly underwent dynamic recrystallization (DRX) and the DRX dominated by discontinuous dynamic recrystallization (DDRX). However, the HL alloys did not have obvious DRX due to the lamellar LPSO hindering the grain boundary migration. The basal fiber texture of the alloy was produced after hot tensile. The deformation of the LL alloys was dominated by prismatic <a> slip during the hot deformation while the deformation of the HL alloys was dominated by the prismatic <a> slip and pyramidal <c+a> slip. The pyramidal <c+a> slip was activated by the Young's moduli mismatch between the α-Mg matrix and LPSO. Lamellar LPSO has an positive effect on strengthening the mechanical properties of the alloy which is attributed to the hinder of LPSO to non-basal slip, the prick function of kink to basal slip and the inhibition of LPSO to DDRX.

Effect of rolling routes on the microstructure, texture and mechanical properties of Mg-5Y binary alloy

ABSTRACT The present study examined the influence of rolling routes and strains on the microstructure, texture, and mechanical properties of Mg-5Y binary alloy. The specimens were hot-rolled at 420°C with varying rolling routes (i.e. unidirectional rolling (UR) and cross rolling (CR)) and rolling strain (10%–60%). The microstructural and textural evolution was studied using EBSD maps and X-ray pole figures. Further, the mechanical behaviour was examined using room temperature tensile tests. The results suggested that rolling strain increment leads to the increased recrystallized fraction in both UR and CR specimens. Alternatively, CR samples showed finer grain size with a higher fraction of recrystallized grains than UR samples at fixed strain. The texture of the former was found to be different as compared to the latter. Further, the mechanical properties of CR samples showed significant improvement than UR samples. This was attributed to fine grain size and altered basal texture.

Multi-mechanism constitutive model for uniaxial ratchetting of extruded AZ31 magnesium alloy at room temperature

A multi-mechanism macroscopic phenomenological constitutive model is established to characterize the uniaxial ratchetting of extruded AZ31 magnesium (Mg) alloy at room temperature in the framework of small deformation. Because the uniaxial ratchetting of the alloy depends greatly on the strong basal texture and different deformation mechanisms (i.e., dislocation slipping, twinning and detwinning) and presents apparent tension-compression asymmetry, two separated yield functions and different hardening rules are used in the proposed model regarding to different deformation mechanisms. That is, the traditional von Mises yield criterion and a flow rule in power-law form are used for the plasticity contributed by dislocation slipping, while the Cazacu–Barlat–Plunkett (CPB) yield criterion extended with a back stress tensor and an internal variable f (i.e., twin volume fraction) is employed to express the plastic deformation induced by twinning/detwinning. In addition, the modified Armstrong-Frederick (A-F) kinematic hardening models are adopted to reflect the strain hardening features contributed by different plastic mechanisms, and the interaction among different plastic mechanisms is considered in the developed constitutive model. The reproduced uniaxial ratchetting of extruded AZ31 Mg alloy by the proposed multi-mechanism constitutive model is in a good agreement with the corresponding experimental results obtained at room temperature, which validates the reasonability and capability of the proposed model.

On rate dependence and anisotropy in phase field modeling of polycrystalline fracture

A phase field theory of fracture incorporating rate dependence (i.e., kinetic viscosity) and anisotropic elasticity is implemented to study strength and failure modes in polycrystalline materials. After presentation of the constitutive framework, a new analytical solution is derived for one-dimensional (1D) homogeneous loading at constant strain rate, wherein crack viscosity is enabled. The three-dimensional (3D) theory is implemented in a finite element (FE) context, with static and dynamic solutions verified up to peak loading for homogeneous microstructures. Extensive simulations of heterogeneous polycrystalline microstructures follow, where baseline material properties correspond to either boron carbide (B4C) or titanium diboride (TiB2). Results quantify effects of loading rate, elastic anisotropy, cleavage energy anisotropy, and grain boundary surface energy on tensile strength. Notable are effects of strong basal texture, which influences average strength via both elastic anisotropy and basal cleavage plane orientation. Effects of loading rate are naturally more prominent for materials that fail mostly intergranularly rather than transgranularly, in agreement with experimental observations.

Simultaneous enhancement of strength and ductility in friction stir welded AZ31 alloy via multi-pass hot-rolling and subsequent annealing

Friction stir welding (FSW) is a new solid-state joining process for joining light metals, such as magnesium (Mg) alloys. However, the texture-induced softening mechanism (in stir zone, SZ) tends to cause the worsening of mechanical properties in FSW Mg alloys. In this work, multi-pass hot-rolling and subsequent annealing is used to ameliorate the FSW AZ31 alloy plate. The results show that multi-pass hot-rolling leads to the c-axes of most grains in the SZ approximately perpendicular to the normal direction (ND) transforming into these almost parallel to the ND, accompanied with multiple twin types. After subsequent annealing, twins, as nucleation sites, promote the formation of new grains with off-basal texture, resulting in the weak basal texture. Compared with the FSW plate and the initial plate, the rolled-annealed FSW plate exhibits a relatively uniform hardness distribution and a “strength-ductility” synergy, which are mainly attributed to an appropriate texture feature and a homogeneous grain structure. This work provides a new idea for the development of new high-performance FSW Mg alloys.

Microstructure evolution and deformation mechanism of AZ80 alloy during die forging

Die forging is a processing method that can produce a size and shape close to the finished product and improve the mechanical properties of the alloy. In this paper, Mg–8Al-0.5Zn magnesium alloy was used to prepare stepped shaft by isothermal die forging. The microstructure and stress-strain state of different parts of stepped shaft forging were analyzed. The metal rheological law and microstructure evolution mechanism of AZ80 alloy during die forging were discussed. At the position of low stress and high strain state of the die forging, continuous dynamic recrystallization (CDRX) is the main deformation mechanism, and the microstructure is mainly composed of coarse un-dynamic recrystallized (unDRXed) grains and fine equiaxed dynamic recrystallized (DRXed) grains. The unDRXed grains have a strong basal texture, while the DRXed grains exhibit random orientations. At the position of high stress and low strain state of the die forging, {10–12} twinning is the main deformation mechanism, the parent grains show a strong basal texture, while {10–12} twins present [-12-10]-[01–10] double fiber texture parallel to forging direction (FD). The compressive strength and yield strength of the part in the low stress and high strain state are higher than those in the high stress and low strain state, but the fracture strain of the former is lower than that of the latter. The results of this study can provide a reference for the design of die forging die together with the blank shape and size, so as to meet the corresponding performance requirements of different parts of the die forging.

Plastic deformation behavior of a Mg-1.5Zn-0.1Ca (mass%) alloy sheet under different strain paths

Mg–Zn–Ca alloy sheets with weak basal texture are expected to improve the room-temperature press formability of Mg-alloy sheets. The plastic deformation behavior of Mg–Zn–Ca alloy sheets should differ significantly from that of typical Mg-alloy sheets with strong basal texture. However, this has not yet been substantially studied. In this study, the plastic deformation behavior of a rolled Mg-1.5Zn-0.1Ca (mass%) alloy sheet with a thickness of 1 mm is studied experimentally. Tests of uniaxial tension, compression, reverse loadings from tension to compression, and from compression to tension are conducted in the rolling and transverse directions, and the work-hardening behavior and evolution of the Lankford value are investigated. Electron backscattered diffraction measurements are also conducted to examine the texture evolution. Under tension, the stress was less in the transverse direction than that in the rolling direction, and an apparent plateau region appeared only in the transverse direction. Under compression, the stress-strain curves exhibited plateau regions at the initial stage and were similar in the rolling and transverse directions. The Lankford values were less under compression than under tension in both directions. Under reverse loading from compression to tension, sigmoidal curves appeared, irrespective of the loading direction; however, they were more pronounced in the rolling direction than in the transverse direction. Under reverse loading from tension to compression, sigmoidal trends did not occur, irrespective of the loading direction, and the in-plane anisotropy in the work-hardening behavior was small. The mechanisms yielding these macroscopic deformation behaviors are discussed in terms of twinning and slip activities.

Effect of a new upsetting process with large height-to-diameter ratio on the microstructure of Mg-Gd-Y-Zn-Zr alloy

In this study, a new upsetting process with large height-to-diameter ratio is proposed, which breaks the limitation of the traditional upsetting process on the height-to-diameter ratio of the billet. The Mg-9.5Gd-4Y-2.2Zn-0.5Zr (wt.%) alloy bar with a height-to-diameter ratio of 6.5 was successfully upset and its microstructure evolution was investigated. The alloy underwent dynamic recrystallization (DRX) during upsetting, and the microstructure refinement was obvious, with the highest recrystallization ratio of 46.3 % in the center of the billet and the average grain size of 20.2 μm. The basal texture was roughly parallel to the metal flow lines in each region, and weakened as the recrystallization rate increases.

The microstructural features and strengthening mechanisms of Mg-Al-Zn-Ca-(RE) extruded alloys having high-volume fractions of second-phase particles

The microstructures and tensile properties of Mg-4∼6Al-2∼4Zn-2∼3Ca-0∼1RE (in wt.%) extruded alloys have been investigated. C15 and C36 Laves phases, Ca2Mg6Zn3, Al2REZn2, and Al11RE3 phases were formed as networks during solidification in different alloys. After extrusion, coarse dendritic were significantly refined due to dynamic recrystallization, second phases were fragmented into pieces arranged as stripes, and typical basal fiber texture {0002} <10-10> with the extrusive direction paralleled to (0001) were formed in all studied alloys. Laves phase particles were prone to be crushed into nanoparticles, which appeared more easily to hinder the recrystallized grain growth and could bring about more than 30 vol % ultra-fine grains after extrusion. During extrusion, twinning-induced dynamic recrystallization occurred at first, and both discontinuous and continuous dynamic recrystallization dominated the microstructural evolution. The volume fraction of second phases had critical roles in recrystallization process, and a nearly complete recrystallization degree could be obtained in alloys containing more than 9 vol% second phases. The strengthening mechanisms were evaluated based on classic strengthening models, which agreed well with the experimental results. It was found that grain boundary strengthening, second phase strengthening, and texture strengthening contributed around 60%, 30%, and 10%, respectively, to yield strengthening, except for Mg-4Al-4Zn-2Ca alloy because of the high percentage of unrecrystallized region. Moreover, the overestimate of second phase strengthening happened in alloys having more than 9 vol% second phases, which revealed that too many second phases had limited influence on microstructures and the resulting mechanical properties.

The effect of Gd on the microstructure evolution and mechanical properties of Mg-4Zn-0.6Ca alloy

In this work, the effect of Gd on the microstructure evolution and mechanical properties of Mg–4Zn–0.6Ca–xGd (x = 0, 0.3, 0.6 and 1, wt%) alloys were investigated systematically. The results show that Mg-4Zn-0.6Ca alloy was composed of α-Mg, Mg7Zn3 and Ca2Mg6Zn3 phase. The W-phase (Mg3Zn3Gd2) was synthesized after addition of Gd. The dynamic recrystallization was restrained and the average grain size decreased after 0.6 wt% Gd addition, causing the increase of unrecrystallized region. In addition, the volume fraction of second phase increased with increasing Gd content up to 1 wt%, and the effect of particles stimulated nucleation (PSN) was enhanced, resulting in a decrease in unrecrystallized region. The addition of Gd promoted the occurrence of the overall texture split and TD-split texture component, causing decreased basal texture intensity. An optimal mechanical property of Mg–4Zn–0.6Ca–1Gd alloy, with the yield and ultimate tensile strength of 240 MPa and 297 MPa, respectively, was achieved. The values were increased by 82 MPa and 37 MPa, respectively, compared with that of Mg–4Zn–0.6Ca alloy. In initial deformation stage, strain-hardening rate of alloys decreased with increase of Gd content. Mg-4Zn-0.6Ca-0.6Gd alloy possessed the lowest strain-hardening rate because of fine grain size. However, strain-hardening rate was increased when Gd content was increased to 1 wt%, which may be related to decreased texture intensity and presence of many second phases. This work reveals the effect of co-addition of minor Ca and Gd on the microstructure evolution and mechanical properties of Mg-Zn alloys, which provides theoretical basis for the development of high strength Mg alloy.

An experimental and crystal plasticity investigation of anisotropic compression behaviour of Mg-Sn-Ca alloy

The ductility of Magnesium alloys is often limited due to the strong basal texture. Initial attempts were made to reduce the intensity of the basal textures by adding rare earth(RE) elements. However, owing to the cost and scarcity of RE elements, alloys with calcium and tin were recently introduced. Among them, Mg-2Sn-2Ca alloys are the most promising, with high strength reported in the extruded condition. In this work, we report, for the first time, the mechanical properties of the alloy in the sheet form. In plane mechanical anisotropy in compressive behaviour is studied along with the detailed characterisation of texture, microstructure and the deformation twins in the material. Unlike strongly basal textured Mg alloys, deformation twinning was also observed in samples loaded along the normal direction. Crystal plasticity simulations were performed to confirm the slip and twin activity. Furthermore, a detailed analysis of deformation twins revealed that the so called Schmid twins accommodate strain by both twin growth and slip, whereas the non-Schmid twins accommodate the strain exclusively by crystallographic slip.