Zr Alloy Sheets Research Articles

Texture evolution, deformation mechanism and mechanical properties of the hot rolled Mg-Gd-Y-Zn-Zr alloy containing LPSO phase

The texture evolution, crystal orientation-dependent deformation mechanism and mechanical properties of the Mg-5.5Gd-4.4Y-1.1Zn-0.5Zr (wt%) alloy sheet with bimodal grain structure during hot rolling were investigated in this study. The pre-rolled sheet possesses the texture with c-axis randomly distributed in rolling-transverse plane. During the hot rolling deformation, the initial texture is gradually changed to an uncommon texture with c-axis continuously distributed between transverse direction (TD) and normal direction (ND) of the rolled sheet. The formation of the texture is related to the orientation-dependent deformation mechanism of coarse grains. For the coarse grains with c-axis close to rolling direction (RD), {10-12} twinning was identified to be the dominated deformation mechanism. With the activation of twinning, the c-axis of the matrix is reoriented towards ND. For the coarse grains with c-axis near TD, prism <a> slip and kink deformation were figured out as the dominated deformation modes. In early stage of the hot rolling deformation, the c-axis of the grains remains in TD with the activation of prism <a> slip. Afterwards, kink deformation activates and the c-axis of the grains is rotated away from TD towards ND with different degrees. In addition, the mechanical properties of rolled sheets were examined and the results demonstrate that yield strength and tensile strength are gradually improved by rolling deformation. Nevertheless, the improvement of yield strength in RD is much greater than that in TD and a serious yield strength anisotropy is shown in the rolled sheet. The strong yield strength anisotropy derives from the variation of texture strengthening in the RD and TD due to the formation the texture with c-axis continuously distributed between TD and ND.

Evolution of LPSO phases in a Mg-Zn-Y-Gd-Zr alloy during semi-continuous casting, homogenization and hot extrusion

A large size Mg-1.2Zn-3.4Y-4.7Gd-0.5Zr (wt%) alloy sheet was manufactured by semi-continuous casting, homogenization and hot extrusion, in which the microstructures were systematically examined. The results showed that: (i) During homogenization, the generation of 14H LPSO phase was mainly achieved by two phase transition processes of “Mg5(Gd,Y,Zn) phase → 14H LPSO phase” and “18R LPSO phase → 14H LPSO phase”. (ii) During extrusion, the evolution of LPSO phases with various morphologies was driven by severe deformation and dynamic recrystallization (DRX). Under severe deformation, the 14H LPSO phases in homogenized alloy were broken down to fragments with various sizes. Under the influence of DRX, the deformation defects of small LPSO fragments were fully eliminated and formed the “sandwich-like” LPSO phase. While the deformation defects of coarse LPSO fragments were less eliminated and formed the “irregular-shaped” LPSO phase. Meanwhile, due to the decreased solubility of Zn, Y and Gd in the extruded alloy matrix, the solution elements of Zn, Y and Gd were impelled to precipitate at the scattering arranged stacking faults (SFs) and formed the “needle-like” LPSO phase. It is found that the gliding of Shockley partial dislocation(s) on basal plane was structurally needed during the precipitation of needle-like LPSO phase.

Strengthening or weakening texture intensity of Zr alloy by modifying cooling rates from α + β region

A typically textured Zr alloy sheet was heat-treated in an α + β region (at 950 °C) and then cooled at three different rates (in water, air and furnace). Microstructures and textures of various specimens were carefully characterized and analyzed by electron channeling contrast (ECC) imaging and electron backscatter diffraction (EBSD) techniques. Results show that rapid cooling in water leads to considerably weakened texture while slower cooling produces perfectly inherited (air cooling) or even intensified texture (furnace cooling). Microstructural examinations reveal that duplex microstructures consisting of fine intersecting plates (martensites) and bulk prior α grains are produced in the water-cooled specimen. Air cooling is also able to induce duplex microstructures with the plates corresponding to coarse parallel-plate Widmanstätten structures. Uniform microstructures of fully equiaxed grains are obtained in the slowest cooled specimen (in furnace). EBSD orientation analyses show that multiple α variants with scattered orientations can be produced inside each β phase during water cooling, resulting in the weakened transformation texture. At slower cooling, however, α/β phase boundaries act as preferred nucleation sites and then strong variant selection occurs, leading to textural inheritance or strengthening.

Effect of Heat Treatments on Microstructures and Tensile Properties of Cu–3 wt%Ag–0.5 wt%Zr Alloy

The microstructures and tensile properties of Cu–3 wt%Ag–0.5 wt%Zr alloy sheets under different aging treatments are investigated in this research. As one kind of precipitate, Ag nanoparticles with coherent orientation relationship with matrix precipitate. However, after the peak-age point, most of Ag nanoparticles grow into short rod shape with the interface translating to semi-coherent, which leads to the lower strength of over-aging sample. The yield strength is estimated by considering solid solute, grain boundary and precipitation strengthening mechanisms. The result shows that the Ag precipitates provide the main strengthening role. Then a constitutive equation representing the evolution of dislocation density with plastic strain is built by considering work-hardening behavior coming from shearable and non-shearable precipitates which is mainly the particles containing Zr. The flow stress contributed by shearable particle hardening is higher than that of non-shearable one. Due to the coarsening of grain boundary precipitates and low rate of damage accumulation of these non-shearable particles, the micro-cracks nucleate easily at grain boundary which leads to intergranular fracture.

Strength and substructure of Al–4.7Mg–0.32Mn–0.21Sc–0.09Zr alloy sheets

Laws of the formation of substructure and of changes in the hardness and in the mechanical properties have been established for sheets of 1545K alloy obtained by tension according to different technologies at various accumulated strains. With an increase in cold deformation (e cold) from 0 to 2.64, the yield stress of cold-worked sheets increases from 355 to 466 МPа and the relative elongation decreases insignificantly from 4 to 3.5%. The maximum strength with σ0.2 = 410 МPа, σu = 460 МPа, and δ = 6.5% is provided by annealing at 150°C for 1 h of the sheets obtained via the technology with the maximum fraction of cold deformation (e cold = 2.64). After annealing at 300°C for 30 min, a twofold increase in the plasticity is observed without a significant reduction in the strength characteristics a follows: σ0.2 = 385 МPа, σu = 436 МPа and δ = 13%. It has been shown that the level of mechanical properties is determined by the substructure that is formed inside deformed grains during annealing.

Microstructure and texture evolution of Mg–7Y–1Nd–0.5Zr alloy sheets with different rolling temperatures

The extruded Mg–7Y–1Nd–0.5Zr (wt%) alloy were performed to the same strain hot rolling with different temperatures. The microstructure and texture evolution of the sheets were investigated by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and electron back-scattered diffraction (EBSD). The results indicate that the microstructure becomes homogeneous after hot rolling process and precipitated phase distributes at grain boundaries along rolling direction. With the increase in rolling temperature, the grains of sheet grow up. The sheet rolling at 400 °C is composed of recrystallization grains, the necklace of the precipitated phase in the grain boundaries and excessive dislocations. The structure of necklace of the precipitated phase is fcc structure with lattice constant of a = 0.75 nm. With rolling temperatures increasing from 400 to 450 °C, the content of recrystallized grains in volume fraction with relatively random orientations increases significantly. Compared with the rolling process at 400 °C, the amount of precipitated phases is reduced at the grain boundary, and the precipitated phase begins to appear in the grain interior when rolling at 450 °C. The structure of the precipitated phase is fcc with lattice constant of a = 2.22 nm. The recrystallization grains begin to grow in the rolling process at 500 °C. The basal texture is obviously produced during the rolling process at 400 and 450 °C; however, the basal texture is weak in the rolling process at 500 °C. The pole point of the (0002) pole figure is concentrated in the center. It can be seen that the basal surface is parallel to RD–TD surface of the rolling sheets. Compared with as-extruded alloy, the basal texture is significantly enhanced with a small increase in basal texture intensity from 6.127 to 7.175, and $$(10\bar{1}0)$$ and $$(11\bar{2}0)$$ textures change to random textures.

Superplastic behavior and microstructure evolution of a new Al-Mg-Sc-Zr alloy subjected to a simple thermomechanical processing

A new Al-Mg-0.15% Sc-0.10% Zr (wt%) alloy sheet with an average (sub)grain size of ~2.25µm was processed by a simple thermomechanical processing. Excellent superplastic (elongations of ≥800%) can be achieved at a temperature range of 450–500°C and a high strain rate range of 1×10−2–1×10−1s−1. A maximum elongation of ~1579% was obtained at 475°C and a high strain rate of 5×10−2s−1. Electron back scatter diffraction analysis and transmission electron microscopy results showed that superior superplastic ductility of the Al-Mg-Sc-Zr alloy can be ascribed to the complete transformation of low angle grain boundaries to high angle grain boundaries due to the occurrence of continuous dynamic recrystallization and the presence of stable coherent Al3ScxZr1−x particles that effectively impede the growth of the grains during superplastic deformation. Besides, strong β-fiber rolling textures gradually weakened, and random textures were predominant in the superplastic deformed alloy. Analyses on the superplastic data revealed that the average strain rate sensitivity parameter and the average activation energy of the Al-Mg-Sc-Zr alloy were ~0.48 and ~84.4kJ/mol–1, respectively. All results indicated that the main superplastic deformation mechanism was grain boundary sliding.

Synergetic effects of Sc and Zr microalloying and heat treatment on mechanical properties and exfoliation corrosion behavior of Al-Mg-Mn alloys

Mechanical properties, exfoliation corrosion behavior and microstructure of Al-5.98Mg-0.47Mn and Al-6.01Mg-0.45Mn-0.25Sc-0.10Zr (wt%) alloy sheets under various homogenizing and annealing processes were investigated comparatively by tensile tests, electrochemical measurements, X-ray diffraction technique and microscopy methods. The as-cast alloys mainly consist of Fe and Mn enriched impurity phases, Mg and Mn enriched non-equilibrium aluminides and Mg3Al2 phases. During homogenization treatment, solvable intermetallics firstly precipitate and then dissolve into matrix. The optimized homogenization processes for removing micro-segregation and obtaining maximum precipitation strengthening of secondary Al3(Sc, Zr) particles are 440°C×8h and 300°C×8h, respectively. Sc and Zr additions can make the yield strength of Al-Mg-Mn alloy increase by 21MPa (6.9%), 120MPa (61.2%) and 127MPa (68.3%), when annealed at 270°C, 300°C and 330°C, respectively, indicating that Orowan precipitation strengthening caused by secondary Al3(Sc, Zr) nano-particles is much greater than grain boundary strengthening from primary Al3(Sc, Zr) micro-particles. Increasing homogenization and annealing degrees and adding Sc and Zr all can decrease corrosion current density and improve exfoliation corrosion resistance. The exfoliation corrosion behavior is dominant by anodic dissolution occurring at the interface between intermetallics and ɑ(Al) matrix. After homogenizing at 440°C for 8h and annealing at 300°C for 1h, yield strength, ultimate strength, elongation to failure and exfoliation corrosion rank are 196MPa, 360MPa, 20.2% and PA (slight pitting corrosion) in Al-Mg-Mn alloy, and reach to 316MPa, 440MPa, 17.0% and PA in Al-Mg-Mn-Sc-Zr alloy, respectively, revealing that high strength, high ductility and admirable corrosion resistance of Al-Mg-Mn alloys can be achieved by the synergetic effects of Sc and Zr microalloying and heat treatment.

Achieving high superplasticity of a new Al–Mg–Sc–Zr alloy sheet prepared by a simple thermal–mechanical process

A new fine-grained Al–Mg–Sc–Zr alloy sheet with the grain size of 2.5μm was prepared by simple rolling and thermal treatment process. Superplastic investigations in the temperature range of 450–550°C and initial strain rate range of 1×10−3s−1–1×10−1s−1 show that a maximum elongation of 3250% is achieved at 525°C and 5×10−3s−1. The alloy also exhibits excellent superplastic ductility (>2390%) in the temperature interval 475–500°C at 5×10−2s−1. Analyses on the superplastic data reveal that the average values of the strain rate sensitivity and activation energy of the Al–Mg–Sc–Zr alloy are about 0.45 and 83KJ/mol, respectively. The microstructure results show that the studied alloy is characterized by a high fraction of low angle grain boundaries and strong Copper, Brass and Cube texture. During superplastic deformation, low angle grain boundaries gradually transfer into high angle grain boundaries and the grains gradually orientate randomly. The fine coherent Al3(Sc, Zr) particles play an important role in obtaining excellent superplasticity. Grain boundary sliding is the predominant deformation mechanism.

Strengthening effect of incremental shear deformation on Zr alloy sheets

To improve the intensity of Zr alloy sheets, the structural evolution and mechanical properties of 47Zr–45Ti–5Al–3V alloys treated with incremental shear deformation were investigated by X-ray diffraction, microhardness tester, nanoindentation, and uniaxial tensile testing. Results indicate that the strengthening effect of incremental shear deformation is significant. The hardness and friction coefficient of sheet specimens increase with increased deformation degree, but considerable deformation reduces their ductility and produces internal microcracks. Strengthening effect is caused not only by cold deformation strengthening, which leads to increased dislocation density, but also by shear deformation, which can transform the α and β phases in the sheet into a high-intensity ω phase (i.e., dispersion-strengthening effect). In the process of shear deformation, β phase transformation is more preferable because of its softness compared with α phase.

Microstructure, mechanical properties and stress corrosion cracking of Al–Zn–Mg–Zr alloy sheet with trace amount of Sc

Microstructural and property evolution of the Al–Zn–Mg–0.10%Sc–0.10%Zr alloy sheet during its preparation were investigated in detail by means of optical microscopy (OM), scanning electron microscope (SEM), energy dispersive X-ray (EDX), transmission electron microscopy (TEM), Vickers micro-hardness test and room temperature tensile test. Stress corrosion cracking (SCC) behavior of the Al–Zn–Mg–0.10%Sc–0.10%Zr alloy under different heat treatments was studied using slow strain rate test. The results showed that serious dendritic segregation existed in as-cast condition. The suitable homogenization treatment for Al–Zn–Mg–0.10%Sc–0.10%Zr alloy was 470°C/24 h. After homogenization treatment, dissoluble Zn and Mg enriched non-equilibrium phases dissolved into α-Al matrix completely. The suitable solid solution-aging treatment for Al–Zn–Mg–0.10%Sc–0.10%Zr alloy was solution treated at 470 °C for 60 min, followed by water quenching and then aged at 120 °C for 24 h. Under this aging temper, the grain structures were composed of sub-grains, ηʹ phases and nanometer-sized, spherical Al3(Sc, Zr) particles. Grain boundary precipitates (GBPs) area fraction was found to be an important parameter to evaluate the SCC susceptibility. The improved corrosion resistance from increasing aging temperature or prolonging aging time was due to the discontinuous η precipitates along the grain boundary and the high area fraction of GBPs. The main strengthening mechanisms of Al–Zn–Mg–0.10%Sc–0.10%Zr alloy are precipitation strengthening derived from ηʹ precipitates, dispersion strengthening, sub-grain strengthening and grain refinement caused by coherent Al3(Sc, Zr) particles.

Effect of Y and Ce additions on microstructure and mechanical properties of Mg–Zn–Zr alloys

The effects of trace Y and Ce additions on the microstructure and mechanical properties of extruded Mg–Zn–Zr alloy sheets were investigated. Results indicated that Y and Ce additions led to significant grain refinement and to the formation of Mg–Zn–Ce, Mg3Zn6Y (I-phase) and Mg3Zn3Y2 (W-phase). Mg–Zn–Ce and W-phase were found to possess an orthorhombic and a face centered cubic structure, respectively. Ultimate tensile strength, particularly yield strength, significantly increased after Y and Ce additions, and good elongation was also achieved. The extruded alloy with 0.28wt% Y and 0.52wt% Ce exhibited optimal yield strength (313MPa), ultimate tensile strength (356MPa) and elongation (12.1%). With increasing Y and Ce additions, strain hardening rate decreased, and the yield-to-ultimate tensile strength ratio (σs/σb) increased. Variations in mechanical properties were discussed based on microstructure observation.

Property improvements in fine–grained Mg–Zn–Zr alloy sheets produced by temperature–step–down multi–pass rolling

The microstructure and mechanical properties of Mg–6.63wt.%Zn–0.56wt.%Zr (ZK61) Mg alloy sheets processed by temperature–step–down multi–pass rolling were investigated. A uniform fine–grained structure with the average grain size of about 3.5 μm, which is enriched with rod–shaped β1′(MgZn2) and blocky β1′(Mg2Zn3) precipitates, is attained by multiple dynamical recrystallization. An intense {0002} basal texture with a–axis randomly rotated in the plane is developed. As a consequence, a yield strength of 219 MPa, an ultimate tensile strength of 313 MPa, and a fracture elongation larger than 32% can be obtained. This enhancement in strength during rolling is mainly due to grain refinement and basal texture strengthening in the early stage, and is further contributed by precipitation hardening, simultaneously. Ductility improvement is attributed to the enhanced uniform elongation correlated with twinning suppression, and the enhanced post–necking elongation controlled by grain refinement, especially at the fine grain sizes. This can provide an essential reference for designing high-performance wrought Mg alloy sheets suitable for industrial fabrication.

Effect of rolling passes on the microstructures and mechanical properties of Mg–Gd–Y–Zr alloy sheets

Abstract Microstructures and mechanical properties of the pre-extruded Mg–8Gd–3Y–0.4Zr (wt%) alloy in different rolling passes were investigated. The microstructure became homogeneous, which was composed of almost ultrafine dynamically recrystallized grains (6.5 μm) after 5 hot-rolling passes, i.e., total thickness reduction of ~80%. Dynamic precipitation happened in the hot rolling of the alloy and β′ phase precipitates were observed and studied in the peak-aged alloy. With the increase of rolling passes, the equilibrium Mg5(Gd,Y) dynamic precipitates increased while the grain size and the (0002) basal texture followed the opposite trend, and the dynamic precipitation may weaken the ageing precipitation strengthening. The mechanical properties of the as-rolled sample after 5 hot-rolling passes were improved, and the ultimate tensile strength, tensile yield strength and elongation to failure reached 326 MPa, 249 MP and 12%, respectively.

Influence of aging temperature on corrosion behavior of Al–Zn–Mg–Sc–Zr alloy

Al–Zn–Mg–Sc–Zr alloy sheets were prepared using water chilling copper mould ingot metallurgy processing which was protected by active flux. The effect of aging temperature on the corrosion characteristics of Al–Zn–Mg–Sc–Zr alloy was investigated by means of exfoliation corrosion testing, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) combined with transmission electron microscopy, scanning electron microscopy and optical microscopy observation. It is found that with increasing the aging temperature the susceptibility to exfoliation corrosion decreases. Electrochemical measurements reveal that at early stage of immersion in testing solution, EIS plots of the samples are composed of a capacitive arc and an inductive loop. Inductive loop disappears with the increasing of immersion time and two time constants in impedance diagrams appear. Moreover, the trends of corrosion resistance are further confirmed by polarization curve and EIS test. In addition, transmission electron microscopy observations show that the improved corrosion resistance from increasing aging temperature is duo to the coarsening of matrix and separated precipitates at the grain boundary, and the increased spacing of grain boundary precipitates.

Microstructure and mechanical properties of Mg–Gd–Y–Zn–Zr alloy sheets processed by combined processes of extrusion, hot rolling and ageing

The microstructure and mechanical properties of Mg–8.2Gd–3.8Y–1.0Zn–0.4Zr (wt%) alloy sheets produced by combined processes of extrusion, large-strain hot rolling and ageing treatment were investigated. The sheet rolled at 300°C was mainly composed of deformed grains while that rolled at 400°C was fully recrystallized, and the LPSO phase distributed at the grain boundaries in both sheets. The sheet rolled at 300°C and then peak-aged at 200°C exhibits 0.2% proof stress (PS) of 454MPa, ultimate tensile strength (UTS) of 469MPa and elongation to failure of 1.3% at ambient temperature, and 0.2% PS of 413MPa, UTS of 457MPa and elongation to failure of 6.8% at 200°C, respectively. While the sheet rolled at 400°C and peak aged at 200°C had lower 0.2% PS and superior ductility. The improvement of strength is ascribed to the fine β′ phase precipitated within the grains, the LPSO phase and dispersed cuboid Mg–Gd–Y containing phase located at the grain boundaries.

Influence of rolling temperature on the microstructure and mechanical properties of Mg–Gd–Y–Zn–Zr alloy sheets

The extruded Mg–8.2Gd–3.8Y–1.0Zn–0.4Zr (wt%) alloy were subjected to large-strain hot rolling with different final rolling temperatures. The microstructural evolution and mechanical properties of the sheets were investigated. The microstructure became homogeneous after hot rolling process and long period stacking ordered (LPSO) phase distributed at grain boundaries along rolling direction. The sheet rolled at 300°C was composed of deformed grains, substructures and excessive dislocations. With increasing final rolling temperatures from 300°C to 400°C, the volume fraction of recrystallized grains with relatively random orientations increased significantly. The strength of the rolled sheets was improved while the ductility was deteriorated with decreasing final rolling temperatures. The tensile yield strength and ultimate tensile strength of the sheet rolled at 300°C reached 320MPa and 416MPa, respectively with the elongation to failure of 5.3%. The mechanical anisotropy of the rolling sheet became unity when rolled at higher temperature of 400°C due to the formation of weaker basal texture.

Effect of final rolling reduction on the microstructure and mechanical properties of Mg–Gd–Y–Zn–Zr alloy sheets

The Mg–8.2Gd–3.8Y–1.0Zn–0.4Zr (wt.%) alloy was hot rolled with different final rolling reductions at 400°C. The microstructure evolution and texture of the sheets were analyzed, and their effect on the mechanical properties was discussed. All of the rolled sheets consist of fine dynamically recrystallized grains with almost random orientation, large deformed grains with strong basal texture. In addition, bent lamellar-shaped long period stacking ordered (LPSO) phases due to kink deformation were observed in the deformed grains. The volume fraction of the dynamic recrystallization increased with increasing final rolling reduction and the basal texture weakened gradually. The strength of the alloy sheets changed a little with increasing final rolling reduction, while the yield anisotropy and the ductility were significantly improved. The sheet final-rolled with thickness reduction of 60% exhibits tensile yield strength of 306MPa, ultimate tensile strength of 393MPa and elongation to failure of 14.6% at ambient temperature, and tensile yield strength of 264MPa, ultimate tensile strength of 345MPa and elongation to failure of 19.4% at 250°C.

Microstructure and mechanical properties of Al-5.8Mg-Mn-Sc-Zr alloy after annealing treatment

An Al-5.8Mg-0.4Mn-0.35(Sc+Zr) (mass fraction, %) alloy sheet was prepared using water chilling copper mould ingot metallurgy processing which was protected by active flux. The influence of stabilizing annealing on mechanical properties and microstructure of the cold rolling sheet was studied. The results show that the strength and hardness of the alloy decrease, while the elongation increases with increasing the stabilizing annealing temperature. With the increase of stabilizing annealing time, the strength and hardness of the alloy drop slightly but its ductility exhibits no change. Partial recovery and recrystallization orderly occur with the increase of annealing temperature during stabilizing treatment. Only different degrees of recovery occur in the alloys annealed below 400 °C for 1 h. Partial recrystallization occurs after annealed at 450 °C for 1 h. By annealing at 300 °C for 1 h, the alloy can obtain the optimum application values of σb, σ0.2 and δ, which are 436 MPa, 327 MPa and 16.7%, respectively.

Ultra high-strength Mg–Gd–Y–Zn–Zr alloy sheets processed by large-strain hot rolling and ageing

Abstract Ultra high-strength Mg–8.2Gd–3.8Y–1.0Zn–0.4Zr alloy sheet was prepared by large-strain hot rolling and subsequent ageing process. The sheet exhibits excellent tensile properties at ambient temperature with ultimate tensile strength of 517 MPa, 0.2% proof stress of 426 MPa and elongation to failure of 4.5%. The notable improvement in strength is attributed to the dense distribution of the fine precipitates inside the grains, scattered precipitates at the grain boundaries, bimodal grain size distribution with fine recrystallized grains and large deformed grains with intense basal texture.