Hot deformation behavior of spray-deposited Al–Zn–Mg–Cu alloy

The flow behavior of spray-deposited Al–10.21Zn–2.76Mg–1.45Cu–0.16Zr (wt.%) alloy has been systematically investigated by thermal compression tests with temperature and strain rate ranging from 613 K to 733 K and 0.001–1 s⁻¹, respectively. Microstructural observations revealed that the average grain size of spray-deposited alloy was below 25 μm due to the high cooling rate. Both relatively high temperature and low strain rate could promote the formation of dynamic recrystallization (DRX). The stress level of the alloy decreased with increasing deformation temperature and decreasing strain rate, which could be characterized by a Zener–Hollomon parameter in the hyperbolic-sine equation. Furthermore, the strain-dependent constitutive equation could lead to a good agreement between the calculated and measured flow stresses in the elevated temperature range for spray-deposited alloy. The deformation activation energy for spray-deposited alloy was relatively lower than that of the as-cast alloy owing to ultrafine grains and high supersaturated solid solubility.     

Modified twinning behaviour induced by texture evolution in hot rolled Mg–3Al–1Zn alloy

Abstract

Compressive deformation along the rolling direction (RD) of a hot rolled Mg–3Al–1Zn alloy is applied to investigate the texture evolution and the recompressive yield strength (RYST) along the transverse direction (TD). Preferential orientation of the basal and prismatic planes is generated by the plastic deformation. Precompression along RD results in one plane of {10–10} aligned nearly perpendicular to the normal direction to the rolling plane. As the compressive strain along RD increases, the RYST shows an earlier raised and later decreased trend. The modified twinning mechanism is investigated using X-ray diffraction and electron backscattered diffraction observations. The results reveal that {10–12} twinning in the matrix dominates the recompression along TD, while the formation of {10–12}–{10–12} twins becomes comparatively easier to occur in the previous {10–12} twins for large precompressed samples.     

Effects of germanium on quench sensitivity in Al–Zn–Mg–Zr alloy

Effects of the trace element germanium (Ge) on the quench sensitivity of an Al–Zn–Mg–Zr alloy were investigated in the present work. The results showed that the Ge-bearing alloy exhibited lower quench sensitivity as compared to the Ge-free alloy. This phenomenon could be reasonably interpreted in terms of the stability of supersaturated solid solution of alloys after quenching from an elevated temperature. The apparent vacancy formation energies for the Ge-free and Ge-bearing alloys were determined to be 0.49 and 0.58 eV respectively. This suggested that the addition of a small amount of Ge was able to trap excess vacancies, leading to a decrease in the amount of coarse dispersoids and resultant low quench sensitivity in Ge-bearing alloys. Therefore, Ge could be used in alloy productions, which require a slow cooling rate to reduce the residual stresses and distortions.     

Effects of grain size on high temperature deformation behavior of Al–4Mg–0.4Sc alloy

A high temperature tensile test has been performed at 450 °C with various strain rates and grain sizes for an Al–4Mg–0.4Sc alloy. The experimental results show that the total elongation is strongly sensitive to the grain size and is approximately a linear function of an inverse average grain size. The Portevin–Lechatellier (P-L) effect with significant serrations on the stress–strain curves during tensile deformation at high temperature has been shown to be usual for the Al–4Mg–0.4Sc alloy. The average amplitude of the stress oscillation decreases with increasing strain rate at high temperature.     

Microstructure evolution in abrasion-induced surface layer on an Al–Zn–Mg–Cu alloy

An altered surface layer forms on an Al–Zn–Mg–Cu alloy during surface preparation by abrasion with grinding paper. Strain-induced dissolution of η′/η precipitates and formation of nano-sized subgrains were observed in the surface layer with thickness of several hundred nanometers. The segregation of solute elements along dislocations and subgrain boundaries and the precipitation of Al₂Cu phase at the sub-boundaries and the free surface were related to enhanced diffusion accelerated by deformation-induced vacancies, dislocations and subgrain boundaries. The microstructure evolution in this layer is mainly attributed to the shear strain and is modified by temperature rise during surface abrasion. The unique surface microstructural changes produced by abrasion might alter the surface properties.     

EBSD analysis of {10–12} twinning activity in Mg–3Al–1Zn alloy during compression

The {10–12} twinning activity of Mg–3Al–1Zn magnesium alloy during uniaxial compression at room temperature has been investigated by electron backscatter diffraction. The results indicated that the twinning activity was closely related with two angles: one was the angle between the c-axis and the compression direction and the other was the angle between the a-axis and the titling direction in the basal plane for a given relation between the c-axis and the compression direction. These two parameters can be used to explain which twinning variant will operate under the given strain path. For the grains containing a single {10–12} twinning variant, the {10–12} twinning variant occurred in a wide range of Schmid factor values (0 < Schmid factor < 0.5) and the Schmid factor rank of 1 or 2 was the most commonly observed. By contrast, for the grains containing two {10–12} twinning variants, the {10–12} twinning activity exhibited a stronger orientation dependence and the combinations of Schmid factor ranks 1–3 and 1–2 were the most commonly observed.     

Plastic deformation behavior of ultrafine-grained Al–Mg–Sc alloy

Ultrafine-grained (UFG) Al–Mg–Sc alloy was obtained by friction stir processing. The UFG alloy was subjected to uniaxial tensile testing to study the tensile deformation behavior of the alloy. An inhomogeneous yielding (Lüdering phenomenon) was observed in the stress–strain curves of UFG alloy. This deformation behavior was absent in the coarse-grained alloy. The Lüdering phenomenon in UFG alloy was attributed to the lack of dislocations in UFG microstructure. A strong dependence of uniform ductility on the average grain size was exhibited by the UFG alloy. Below a critical grain size (0.5 μm), ductility was very limited. Also, with the decrease in grain size, most of the plastic deformation was observed to be localized in necked region of the tensile samples. The negative strain rate sensitivity (SRS) observed for the UFG alloy was opposite of the SRS values reported for UFG alloys in the literature. Based on activation volume measurement, grain boundary mediated dislocation-based plasticity was concluded to be the micro-mechanism operative during plastic deformation of UFG Al–Mg–Sc alloy.     

Influence of high-pressure torsion on microstructural evolution in an Al–Zn–Mg–Cu alloy

A commercial age-hardenable Al-7136 alloy was successfully processed by high-pressure torsion (HPT) at room temperature through 1/8 to 4 turns. Microhardness measurements showed significant hardening even after 1/8 turn with the average hardness value reaching a maximum after 1 turn and then slowly decreasing. Higher hardness values were attained by processing the alloy through one pass of equal-channel angular pressing in a supersaturated condition at room temperature and then applying HPT for 1 or 2 turns. Microstructural observations revealed the possibility of achieving true nanometer grain sizes of <100 nm after processing at room temperature. There were variations in hardness with imposed strain due to the fragmentation and subsequent growth of precipitates during processing.     

Hot tensile deformation behaviors and constitutive model of an Al–Zn–Mg–Cu alloy

The hot tensile deformation behaviors of an Al–Zn–Mg–Cu alloy are studied by uniaxial tensile tests under the deformation temperature of 340–460 °C and strain rate of 0.01–0.001 s⁻¹. The effects of deformation temperature and strain rate on the hot tensile deformation behaviors and fracture characteristics are discussed in detail. The Arrhenius-type constitutive model is developed to predict the peak stress under the tested deformation condition. The results show that: (1) The true stress–true strain curves under all the tested deformation conditions are composed of four distinct stages, i.e., elastic stage, uniform deformation stage, diffusion necking stage and localized necking stage. The flow stress decreases with the increase of deformation temperature or the decrease of strain rate. (2) The elongation to fracture increases with the increase of deformation temperature. Under the tested conditions, the strain rate sensitivity coefficient varies between 0.1248 and 0.2059, which indicates that the main deformation mechanism is the lattice diffusion-controlled dislocation climb. (3) The localized necking causes the final fracture of specimens under all the deformation conditions. Microvoids coalescence is the main fracture mechanism under relatively low deformation temperatures. With the increase of deformation temperature, the intergranular fracture occurs. (4) The peak stresses predicted by the developed model well agree with the experimental results, which indicate the validity of the developed model.     

Solute cluster size effect on the fatigue crack propagation resistance of an underaged Al–Cu–Mg alloy

The effect of Cu–Mg cluster size and number density on the fatigue fracture behavior of Al–Cu–Mg alloy with various aging conditions was investigated by means of transmission electron microscopy (TEM), atom probe tomography (APT), scanning electron microscopy (SEM) and fatigue testing. Results showed that the fatigue crack propagation (FCP) resistances of 170 °C/1 h and 170 °C/8 h samples were higher than that of 170 °C/0.5 h sample due to increased number density of great size Cu–Mg co-clusters (>50 atoms). These large clusters were harder to dissolve during cycle deformation, thus reduced the cyclic softening effect and enhanced the FCP resistance. Moreover, as aging prolonged, the critical shear stress (τ_m) of co-clusters by modulus hardening increased from 10.2 (MPa) in 170 °C/0.5 h sample to 12.4 in 170 °C/1 h sample and 12.1 in 170 °C/8 h sample. Thus the force required for the movement of dislocations impeded by co-clusters, as well as the resistance of FCP caused by co-clusters, in 170 °C/1 h and 170 °C/8 h sample was higher than that in 170 °C/0.5 h sample. The 170 °C/8 h sample possessed the lower FCP resistance than 170 °C/1 h sample because of the existence of S′ phase. S′ phase was a kind of semi-coherent unshearable precipitate and hence reduced the planar-reversible slip.     

Phase-field modelling on effect of pressure on growth kinetics of Mg–Al–Sn alloy

The effects of pressure on dendritic growth kinetics of Mg–Al–Sn alloy were investigated using a ternary phase-field model coupled with thermodynamics with pressure effects and experiments. The results showed that the improved growth velocity and nucleation rate caused by pressure had the opposite effects on grain size. For one-grain growth, the dendrite was larger and more developed under pressure of 85?MPa than that under ambient pressure owing to larger thermodynamic driving force, and thus higher growth velocity. However, when nucleation was considered in multigrain growth case, the average grain size under pressure was smaller owing to less growth space. Growth velocity decreased with the increase in Sn content, on which pressure had no great influence.     

Superplasticity and grain boundary sliding characteristics in two stage deformation of Mg–3Al–1Zn alloy sheet

A ‘Two-Stage Deformation Method’ was proposed to enhance the superplasticity of Mg–3Al–1Zn (AZ31) alloy sheet. This method exploited the capability of the material to undergo dynamic recrystallization (DRX) at optimum DRX conditions of 250 °C and constant strain rate of 1×10⁻⁴ s⁻¹. Stage I was aimed at refining the coarse microstructure of the as-received alloy to result in fine equiaxial grains measuring less than 10 μm, which deformed by grain boundary sliding accommodated by intragranular slip. Subsequently, Stage II was performed at a higher deformation temperature, whereby viscous glide mechanism accommodated by lattice diffusion was predominant. By altering the deformation mechanisms at different strain levels, elongation-to-failure of 320 and 360% was attained at 400 and 450 °C, respectively.     

Characterisation of interfaces in Al–Zn–Mg alloy reinforced with Sicabo fibres

Abstract

The microstructure of a metal matrix composite consisting of an Al–Zn–Mg alloy reinforced with SiC coated boron fibres has been examined by electron microscopy, electron probe microanalysis, and by optical microscopy. Considerable amounts of Mg₂Si phase were found to be segregated at the fibre/matrix interface. This intermetallic was not formed by a reaction between the fibre and matrix during the fabrication process, a liquid infiltration technique, but as a result of silicon impurities present as contaminants in the melt. It was concluded that the interface phase was precipitated from the metal matrix in the later stages of solidification without any nucleation role being played by the fibre. The Mg₂Si phase appears to be brittle and was present in amounts likely to have a deleterious effect on the strength of the composite.

MST/871     

Effect of grain size on strain rate sensitivity of cryomilled Al–Mg alloy

Al–Mg alloy powder was cryomilled to achieve a nanocrystalline (NC) structure having an average grain size of 50 nm with high thermal stability, and then consolidated by quasi-isostatic forging. The consolidation resulted in a bulk material with ultrafine grains of about 250 nm, and the material exhibited enhanced strength compared to conventionally processed Al–Mg alloy. The hardness of as-cryomilled powder, the forged ultrafine-grained (UFG) material, and the conventional coarse-grained (CG) alloy were measured by nanoindentation using various loading rates, and the results were compared with strain rate sensitivity (SRS) from uniaxial compression tests. Negative SRS was observed in the cryomilled NC powder and the forged UFG material, while the conventional alloy was relatively insensitive to strain rate. The dependence on loading rate was stronger in the NC powders than in the UFG material.     

Grain refinement of Mg–Al alloys with Al4C3–SiC/Al master alloy

A novel Al₄C₃–SiC/Al master alloy for grain refinement of Mg–Al–Zn alloys has been developed in the present work. X-ray diffraction (XRD) and electron probe microanalysis (EPMA) results show the existence of Al₄C₃ and SiC particles in this master alloy. The master alloy presents good grain refining efficiency on both AZ31 and AZ63 alloys, but little effect on AZ91 alloy. After addition of 0.5 wt.% Al₄C₃–SiC/Al master alloy, the average grain size of AZ31 and AZ63 decreased dramatically from 1300 to 225 μm, and from 300 to 200 μm, respectively. However, no further refinement of grain size was achieved with additional amount of Al₄C₃–SiC/Al master alloy exceeding 0.5 wt.% for both AZ31 and AZ63 alloys in the present investigation. Duplex phase of Al₄C₃ and SiC was found to be located at the grain center of α-Mg and is proposed to be the nucleating agent during solidification of α-Mg.     

Microstructural evolution of Al–Zn–Mg–Cu alloy during homogenization

In this study, the microstructural evolution of an as-cast Al–Zn–Mg–Cu alloy (AA7085) during various homogenization schemes is investigated. It is found that in a single-stage homogenization scheme, some of the primary eutectic gets transformed into the Al₂CuMg phase at 400 °C, and the primary eutectic and Al₂Cu phase gradually dissolve into the alloy matrix at 450 °C. The Al₃Zr particles are mainly precipitated at the center of the grain because Zr is peritectic. However, the homogeneous distribution of the Al₃Zr particles improves and the fraction of Al₃Zr particles increases in two-stage homogenization scheme. At the first low-temperature (e.g., 400 °C) stage, the Al₃Zr particles are homogeneously precipitated at the center of the grain by homogeneous nucleation and may be heterogeneously nucleated on the residual second-phase particles at the grain boundary regions. At the second elevated-temperature (e.g., 470 °C) stage, the Al₃Zr nuclei become larger. A suitable two-stage homogenization scheme for the present 7085-type Al alloy is 400 °C/12 h + 470 °C/12 h.     

High-pressure homogenization treatment of Al–Zn–Mg–Cu aluminum alloy

Absract The microstructures and aging hardening response of Al–12Zn–3.5Mg–3.0Cu–0.14Zr aluminum alloy after a high-pressure homogenization treatment at 750 °C for 45 min under 5 GPa were investigated. The results showed that the constituent phases dissolved completely and formed α-Al single-phase solid solution comparable to that formed after ambient-pressure homogenization at 450 °C/96 h + 460 °C/128 h. The complete dissolution of the constituent phases increased the solubility of the alloying elements, as well with the over-burning temperature and aging hardness.     

Effect of creep-aging processing on corrosion resistance of an Al–Zn–Mg–Cu alloy

Creep-aging forming, combining both the aging treatment and forming process, has recently drawn much attention of researchers. In this study, the effects of creep-aging processing on the corrosion resistance of an Al–Zn–Mg–Cu alloy are studied. Results show that the corrosion resistance of the studied Al–Zn–Mg–Cu alloy is sensitive to creep-aging processing parameters (creep-aging temperature and applied stress). With the increase of creep-aging temperature, the corrosion resistance first increases and then decreases. Increasing the applied stress can deteriorate the electrochemical corrosion resistance and improve the exfoliation corrosion resistance. The creep-aging processing can change the size and distribution of precipitates in the aluminum matrix, which significantly affects the corrosion resistance. The discontinuous grain boundary precipitates and narrow precipitate-free zones can enhance the corrosion resistance.     

Influence of second phases on mechanical properties of spray-deposited Al–Zn–Mg–Cu alloy

Al–10.66Zn–2.48Mg–1.41Cu–0.17Zr–0.17Sc (wt.%) alloy prepared by spray deposition was processed with different hot deformation followed by heat treatment. The mechanical properties and microstructure evolution were investigated. The results indicate that uniform ultimate tensile strength of 774 MPa, yield strength of 734 MPa and elongation of 13.7% are obtained with two-step hot deformation, which increase by 2.7%, 3.82% and 95% compared with one-step hot deformation. Microstructural observations show that increase of elongation is mainly ascribed to high volume fraction of smaller precipitates and reduction of stress concentration areas as a result of disappearance of the coarse second phases. The fractured tensile specimens with two-step hot deformation exhibit dimple fractographic features. Improvement of strength is attributed to the precipitation strengthening and dispersed strengthening.     

Constitutive analysis of Mg–Al–Zn magnesium alloys during hot deformation

The constitutive behaviors of Mg–Al–Zn magnesium alloys during hot deformation were studied over a wide range of Zener–Hollomon parameters by consideration of physically-based material’s parameters. It was demonstrated that the theoretical exponent of 5 and the lattice self-diffusion activation energy of magnesium (135 kJ/mol) can be used in the hyperbolic sine law to describe the flow stress of AZ31, AZ61, AZ80, and AZ91 alloys. The apparent hyperbolic sine exponents of 5.18, 5.06, 5.17, and 5.12, respectively for the AZ31, AZ61, AZ80, and AZ91 alloys by consideration of deformation activation energy of 135 kJ/mol were consistent with the considered theoretical exponent of 5. The influence of Al upon the hot flow stress of Mg–Al–Zn alloys was characterized by the proposed approach, which can be considered as a versatile tool in comparative hot working and alloy development studies. It was also shown that while the consideration of the apparent material’s parameters may result in a better fit to experimental data, but the possibility of elucidating the effects of alloying elements on the hot working behavior based on the constitutive equations will be lost.