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.     

Dynamic recrystallization and texture evolution of Mg–Y–Zn alloy during hot extrusion process

The microstructure and texture evolution of Mg_98.5Y₁Zn_0.5 and Mg_92.5Y₅Zn_2.5 (atomic percent) alloys during hot extrusion were systematically investigated. The coarse LPSO phases with higher volume fraction (~ 57%) suppressed the twinning generation in the initial stage of extrusion, and accelerated the dynamic recrystallization through the particle deformation zones. Therefore, the volume fraction of DRXed grains in as-extruded Mg_92.5Y₅Zn_2.5 alloy was much higher than that of Mg_98.5Y₁Zn_0.5 alloy. The intensive recrystallization process resulted in the conventional basal texture weakening, although the texture evolution was mainly dominated by flow behavior. The dynamic recrystallization behavior in Mg_92.5Y₅Zn_2.5 alloy restricted the formation of deformation texture, and thus the more random texture was observed during the whole extrusion process.     

Microstructural evolution during extrusion and ECAP of a spray-deposited Al–Zn–Mg–Cu–Sc–Zr alloy

The microstructural evolution of an Al–Zn–Mg–Cu–Sc–Zr alloy prepared by spray deposition via extrusion and equal-channel angular pressing (ECAP) was investigated in this study. Deformation route A for Al–11.5 wt% Zn–2 wt% Mg–1.5 wt% Cu–0.2 wt% Sc–0.15% Zr super-strength alloy was carried out at 573 K by ECAP. The microstructures of extruded and ECAP samples were investigated by means of Electron Backscatter Diffraction (EBSD), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). A large amount of dislocation tangles were formed inside grains during ECAP, which further evolved into sub-boundaries and high angle grain boundaries. Microstructure analyses showed that the grain size was refined to 800 nm after 8 passes ECAP from earlier 3.5 μm of sprayed and extruded alloy. A few finer MgZn₂ and Al₃(Sc,Zr) were dispersed uniformly after ECAP. The textures of 8 passes ECAPed sample were dominated by the strong Cu orientation and relatively weak S orientation.     

Influences of heat treatment on microstructural evolution and tensile behavior of squeeze-cast Mg–Gd–Y–Zr alloy

The effects of solution heat treatment and aging on the microstructural evolution and mechanical behavior of a squeeze-cast (SC) Mg–10Gd–3Y–0.5Zr (GW103K) alloy, processed using various applied pressures (e.g., 0.1, 40, 80 and 160 MPa) were systematically investigated. Our results show that, after solution heat treatment, secondary phases and pressure-induced dislocations are dissolved in the matrix of the squeeze-cast alloys. Moreover, subsequent aging heat treatment leads to an increased age-hardening response relative to that in squeeze-cast GW103K and this trend increases with increasing applied pressure. The room temperature tensile test results show that the yield strength (YS) for the squeeze-cast alloy in the as-cast, the as-T4 heat-treated and the as-T6 heat-treated states increases with increasing applied pressure, from 0.1 to 80 MPa, and remains relatively constant when the applied pressure is increased to 160 MPa, whereas the ultimate tensile strength (UTS) and elongation-to-failure (E _f) increases continuously with increasing applied pressure. The measured increases in YS and UTS (or E _f), are discussed in terms of the mechanisms that govern the evolution of microstructure in squeeze-cast GW103K, paying particular attention to gain size and porosity.     

Static recrystallisation and recrystallisation during hot deformation of Al–1Mg–1Mn alloy

Abstract

Plane strain compression tests at 5 s^?1 and at temperatures of 270–480°C have been carried out on an Al–1Mg–1Mn alloy containing a bimodal distribution of intermetallic particles and after a prior heat treatment to coarsen all particles to greater than 1 μm in size. During the heat treatment, recrystallisation of the initially hot worked material only proceeded with coarsening of the fine particles. During subsequent hot deformation, thin foil electron microscopy revealed that identical subgrain structures were developed in the two materials by dynamic recovery at temperatures below 450°C. At higher temperatures, the initially recrystallised material showed localised particle stimulated dynamic recrystallisation. The subsequent static recrystallisation rate was more than 10³ times faster in the material free from small particles.

MST/751     

Hot shear deformation constitutive analysis and processing map of extruded Mg–12Li–1Zn bcc alloy

The hot shear deformation behavior of an extruded Mg–12Li–1Zn alloy was studied by shear punch test (SPT) in the temperature range 200–300 °C, and in the shear strain rate range 1.2 × 10⁻³–6.0 × 10⁻² s⁻¹. Based on the constitutive analysis of the SPT data, it was found that a sine hyperbolic function could properly describe the hot shear deformation behavior of the material. The activation energy of 108 kJ mol⁻¹ calculated from sine hyperbolic function together with the power-law stress exponents of 3.6–4.7 is indicative of lattice-diffusion-controlled dislocation climb mechanism as an operative deformation mechanism. As a new approach, the shear processing map was developed in order to determine the optimum processing condition, which was found to be 300 °C and 1.2 × 10⁻³ s⁻¹. Domains of the processing map are also interpreted on the basis of the associated microstructural observations. It was found that the post-deformation microstructure is sensitive to the Zener–Hollomon parameter, so that DRX was encouraged with decreasing Z-value.     

An analysis of hot deformation of an Al–Cu–Mg alloy produced by powder metallurgy

The high-temperature plasticity of a 2014 aluminium alloy produced by powder metallurgy was investigated in a wide range of temperatures and strain rates. When the strain rate was plotted as a function of stress (either peak flow stress in torsion, or applied stress in tensile creep), the alloy exhibited the same threshold-like behaviour observed in similar materials. The microstructure of representative torsioned samples was analysed in a transmission electron microscope (TEM) and the characteristics of particles and precipitate distribution were estimated. The dependence on stress and temperature was analysed by means of the conventional constitutive equations used for describing the hot-working behaviour and by means of a modified form of the sinh-equation, where the stress was substituted by an effective stress i.e. by the difference between the actual stress and a threshold stress. This temperature-dependent threshold stress was found to be a constant fraction (15%) of the Orowan stress generated by the dispersion of alumina particles and of precipitated intermetallic phases.     

Microstructure evolution and mechanical responses of Al–Zn–Mg–Cu alloys during hot deformation process

Al–Zn–Mg–Cu alloys can be fabricated by a series of thermo-mechanical processing methods (e.g., hot rolling, forging and extrusion), which is able to serve in aeronautic, automobile, and marine industries because of its excellent physical properties. However, reaching the balance between high strength and favorable ductility to present its high performance is still in progress, during which temperature and strain rate are two very important external variables. More importantly, the core lies in sophisticated microstructure evolution paths involved in hot deformation, which consists of different microstructure mechanisms and behaviors and can be expressed as various mechanical responses. Therefore, a fundamental review of microstructure mechanisms and behaviors, microstructure evolution and relevant mechanical responses of Al–Zn–Mg–Cu alloys during high-temperature deformation is of great significance. In present paper, first, various experimental methods have been introduced. Second, general trends of mechanical properties changing with temperature and strain rate have been summarized. Third, major microstructure mechanisms and behaviors have been discussed. Then, a schematic illustration originating from dislocations’ movement has been depicted, which succeeding microstructure evolution and mechanical responses (including superplasticity) have been reviewed accordingly. Finally, further suggestions of hot deformation of Al–Zn–Mg–Cu alloys have been given.

     

Low cycle fatigue of an extruded Mg–3Nd–0.2Zn–0.5Zr magnesium alloy

The purpose of this study was to evaluate strain-controlled cyclic deformation behavior of an extruded Mg–3Nd–0.2Zn–0.5Zr (NZ30K) magnesium alloy. The microstructure of this alloy consisted of a bimodal microstructure with equiaxed recrystallized grains and unrecrystallized coarse grains along with a large number of smaller second-phase particles present inside the grains and larger particles along the grain boundaries alongside a characteristic precipitate free zone (PFZ). The average grain size was about approximately 5–7 μm. It was observed that unlike the higher RE-containing Mg–10Gd–3Y–0.5Zr (GW103K) magnesium alloy, the NZ30K alloy exhibited asymmetrical hysteresis loops in tension and compression in the fully reversed strain-control tests at a strain ratio of R_ε = −1. This was mainly due to the presence of relatively stronger crystallographic texture, PFZ, and the resultant twinning–detwinning activities during cyclic deformation. While this alloy exhibited cyclic softening at lower strain amplitudes and cyclic hardening at higher strain amplitudes, it had an equivalent fatigue life to that of other extruded Mg alloys. Fatigue crack was observed to initiate from the specimen surface with some isolated facets of the cleavage-like planes near the initiation site. Crack propagation was basically characterized by serrated fatigue striations.     

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.     

Investigation of high-strength and superplastic Mg–Y–Gd–Zn alloy

The Mg–7Y–4Gd–1Zn (wt.%) alloy was prepared by hot extrusion technology, and the microstructure, tensile properties and superplastic behavior have been investigated. The extruded alloy possesses high tensile strength both at room temperature and 250 °C, and especially the yield strength can remain above 300 MPa at 250 °C. The outstanding microstructure, i.e. bent 18R long period stacking ordered (LPSO) strips and dynamic recrystallization (DRX) Mg grains containing fine lamellae with 14H LPSO or stacking fault structures, is responsible for the excellent mechanical properties, and it is considered that the integrated performance can be further improved by controlling the size of LPSO phase. The alloy shows the maximum elongation of 700% at 470 °C and 1.7 × 10⁻⁴ s⁻¹. The predominant superplastic mechanism is considered to be grain boundary sliding assisted by lattice diffusion. The fracture of superplastic deformation is related to the microstructure evolution, i.e. the disappearance of LPSO phase and the formation of cubic phase. Both high temperature and stress contribute to the phase transformation.     

Microstructure and mechanical properties of high-performance Mg–Y–Er–Zn extruded alloy

The Mg–8Y–1Er–2Zn (wt.%) alloy with high strength, plasticity and heat-resistance was prepared by the hot extrusion technique and the following aging treatment. The microstructure and mechanical properties were investigated. The results show that long period stacking ordered (LPSO) phase is different from common inermetallics, and the former can be bent by plastic deformation and presents good combination with the Mg matrix. The good mechanical properties of as-extruded alloy are mainly attributed to the lamellar strips with 18R LPSO structure as well as the microstructure refinement. Aging treatment at 220 °C can further improve the strength but not at the expense of plasticity. The ultimate tensile strength (UTS) and elongation to failure (ε) of as-extruded alloy at peak hardness are 390 MPa and 18% at room temperature, and 322 MPa and 30% at 250 °C, respectively. The formation of fine α-Mg recrystallization grains with high number density of 14H LPSO structure is mainly responsible for the superior mechanical properties of extruded alloy after peak-aging.     

Tensile strength improvement of an Mg–12Gd–3Y (wt%) alloy processed by hot extrusion and free forging

An Mg–12Gd–3Y (wt%) alloy was prepared by conventional casting method using permanent steel mold. Then this alloy was subjected to hot processing, involving hot extrusion and free forging. Tensile strength at room temperature can be improved, with the highest ultimate tensile strength (UTS) value of 390.2 MPa achieved by hot extrusion in comparison to that of as-cast alloy. Temperature dependence of tensile strength is distinguishable for the as-extruded alloy, while the relative stability in UTS values of the alloy after being freely forged should be ascribed to the inter-crossing among deformation bands located at various orientations and the accommodation effect of twining lamellas resulting from forging process on plastic deformation during tensile test at elevated temperatures. Further annealing after hot processing can only have adequate influence on the tensile strength of as-forged alloy. For the alloy freely forged and annealed at 523 K for 4 h, the highest UTS (441.1 MPa) at room temperature is found, which should be mainly related to an evolution from the original as-forged microstructure with subgrains to a more stable combination of large and refined grains through dynamic recrystallization during free forging, and the stress at offset yield YS (384.3 MPa) is also comparable to that relatively high value of 396.9 MPa after solution treatment and isothermal aging of the as-cast alloy.     

Tribological behaviour of hot extruded Al–Cu–Mg–Ag alloy reinforced by TiB2 particles

In the present investigation, tribological behaviour of the hot extruded Al–Cu–Mg–Ag (matrix) alloy and the effect of Ti and TiB₂ addition in matrix alloy have been studied. Hot extrusion was introduced to eliminate cast defects like porosity, voids and micro cracks. Addition of Ti and TiB₂ particles increased the hardness of the matrix by grain refinement and dispersion hardening, respectively. It has been observed that the increase in hardness had significantly improved the wear resistance of the material. Detail study of the wear surfaces and debris were carried out to understand the wear mechanism of the samples. It revealed a complex mechanism of micro-cutting, plastic deformation, abrasion and delamination of the wear samples.     

Effect of Zn concentration on the microstructures and mechanical properties of extruded Mg–7Y–4Gd–0.4Zr alloys

Microstructures and mechanical properties of the Mg–7Y–4Gd–xZn–0.4Zr (x = 0.5, 1.5, 3, and 5 wt.%) alloys in the as-cast, as-extruded, and peak-aged conditions have been investigated by using optical microscopy, scanning electron microscope, X-ray diffraction, and transmission electron microscopy. It is found that the peak-aged Mg–7Y–4Gd–1.5Zn–0.4Zr alloys have the highest strength after aging at 220 °C. The highest ultimate tensile strength and yield tensile strength are 418 and 320 MPa, respectively. The addition of 1.5 wt.% Zn to the based alloys results in a greater aging effect and better mechanical properties at both room and elevated temperatures. The improved mechanical properties are mainly ascribed to both a fine β′ phase and a long periodic stacking-ordered structure, which coexist together in the peak-aged alloys.