Peak-aged Condition Research Articles

Equal-channel angular pressing influencing the mean stress sensitivity in the high cycle fatigue regime of the 6082 aluminum alloy

The high cycle fatigue (HCF) behavior is investigated for a 6082 aluminum alloy in an as-extruded peak-aged condition and after single-pass equal-channel angular pressing (ECAP). The focus of this study is on the effect, that ECAP processing exerts on the mean stress sensitivity of the fatigue strength. The fatigue strength at 107 cycles is compared for alternating tension-compression (R = −1), tension (R = 0) and tension-tension loading (R = 0.1) and related to the microstructural features examined by electron backscatter diffraction (EBSD) measurements and the mechanical properties determined by quasi-static tensile and compression tests.For the as-extruded peak-aged alloy, the fatigue strength depends on the stress ratio. An increase in mean stress due to a higher applied stress ratio leads to decreased fatigue strength. Single-pass ECAP enhances the fatigue strength for all stress ratios when compared to the as-extruded peak-aged alloy. Furthermore, the mean stress effect is diminished and the fatigue strength is almost the same for the tested stress ratios. The mean stress insensitive behavior of the ECAP processed condition is most likely a result of an increased sensitivity to compression loading, which is caused by the ECAP induced shear-deformation and expresses further in a minor yield strength under compression loading when compared to the as-extruded peak-aged alloy.

Influence of heat treatment on microstructure and mechanical behaviour of hot-rolled Al-Mg-Si alloy

ABSTRACT As cast Al-Mg-Si alloy was hot-rolled at a temperature of 400 ° C. After rolling, the alloy was subjected to T6 heat treatment by solutionising at 530 ° C for 2 hours followed by quenching in water media. Heat-treated rolled Al-Mg-Si alloy was subjected to microstructural studies, microhardness test and tensile properties (In rolling direction, 45 ° and 90 ° to rolling direction). It is observed that heat treatment of rolled Al-Mg-Si alloy increases microhardness up to a certain duration of ageing and further drops to lower hardness. On heat treatment, the rolled alloy exhibited maximum tensile strength in rolling direction in comparison with 45 ° and 90 ° directions to rolling. It is observed that hot rolling followed by T6 heat treatment of Al-Mg-Si alloy tends to impart higher tensile strength compared to previously reported experimental values or commercially available alloys for developing high strength lightweight material. Under peak-aged condition, yield strength of 410 MPa and UTS of 440 MPa was achieved in the present work which is significantly higher than values reported in previous works.

Aging precipitation characteristics and tensile properties of Al–Zn–Mg–Cu alloys with different additional Zn contents

The increase in zinc content in Al–Zn–Mg–Cu alloys provides an effective method to enhance the strength. Transmission electron microscopy (TEM) and tensile test were employed to analyze the microstructure and tensile properties, respectively, of Al–9.3Zn–2.0Mg–1.8Cu alloy (9.3Zn alloy) and Al–9.8Zn–2.0Mg–1.8Cu alloy (9.8Zn alloy) with single- and double-stage aging states. The results showed that the two alloys possessed a closed strength under single-stage peak-aging treatment. As for double-stage aging treatment, 9.8Zn alloy had a higher strength values than 9.3Zn alloy under the same aging regimes. With the second step aging time prolonging, the strength gap was extended. The main precipitates for the two alloys with single-stage peak-aging state were GP zones and η′ phase, while there were a majority of η′ phases and η phases for the two alloys under typical double-stage over-aging state. Under double-stage over-aging state, the proportion of precipitate with large size for 9.3Zn alloy was larger than that for 9.8Zn alloy. Besides, a more obvious trend was revealed for the double-stage over-aging state. The gap of strength between the two alloys was explained by the difference of precipitation characteristics via interaction mechanism between precipitates and dislocations.

Microstructure evolution and fracture behavior of Mg-9.5Gd-0.9Zn-0.5Zr alloy subjected to different heat treatments

Microstructure evolution and fracture behavior of Mg-9.5Gd-0.9Zn-0.5Zr alloy subjected to different heat treatments were systematically investigated using SEM and TEM as well as tensile testing. It was found that the microstructure of the as-cast alloy consisted of α-Mg matrix, net-like eutectic compounds (α-Mg + Mg3 (Gd, Zn)), cubic GdH2 phases and lamellar 14H LPSO phases. After solution treated at 515 °C for 24 h, two different cooling processes were used to elucidate the effect of cooling rates on the precipitation microstructure. With a hot water quenching, those secondary phases were completely dissolved. Instead, grey-like patches were observed within the α-Mg matrix, which were proposed to be rod-like Zn2Zr3 phases around the α-Zr particle. In contrast, with a furnace cooling, the formation of 14H LPSO and several cubic Mg3 (Gd, Zn) phases was observed. Furthermore, after hot water quenching, the subsequent ageing treatment parameters were also optimized to be 225 °C for 48 h. In the peak-aged condition, a denser and uniform distribution of basal precipitates γ″ and several basal precipitates γ′ together with Zn2Zr3 and ZnZr2 phases were observed. The samples after the solution treatment (for both hot water quenching and furnace cooling) showed a much higher ductility than the as-cast alloy, while the tensile yield strength (TYS) and ultimate tensile strength (UTS) remained unchanged. After the peak-ageing, a significant increase in the TYS and UTS but a great loss in ductility was observed. In the as-cast alloy, the initiation of microcracks occurred from the net-like eutectic compounds, which was believed to be one of the most important reasons for the tensile fracture, and showed a co-existence of intergranular and transgranular fracture behavior. After the solution treatment, with a hot water quenching, the fracture can be mainly related with failures along contraction twins, and then showed a transgranular fracture behavior. While, with a furnace cooling, due to the presence of the kinked 14H LPSO phases, the fracture was caused by the broken of 14H LPSO phases, and then lead to a transgranular fracture. The peak-aged alloy exhibited a brittle intergranular fracture, which can be related with failures along soft precipitation free zones (PFZs).

Dependence of microstructure, mechanical properties, and inter-granular corrosion behavior of Al-5.1Mg-3.0Zn-0.15Cu alloys with high temperature pre-treatment

The age-hardenable Al-5.1Mg-3.0Zn-0.15Cu (wt%) alloy in peak-aged condition has high strength but poor inter-granular corrosion resistance. The effect of high temperature pre-treatment on mechanical properties and inter-granular corrosion of the alloy has been investigated via hardness, tensile, inter-granular corrosion depth, and electrochemical testing. The morphology, distribution, and composition evolution of precipitates in the grain and along grain boundaries are characterized and analyzed using transmission electron microscopy. The relationships between mechanical properties, inter-granular corrosion resistance, and microstructure evolution during the high temperature pre-treatment and the subsequent aging process have been established. The inter-granular corrosion resistance is enhanced with decreasing pre-treatment temperature. While the strength increases with increasing of pre-treatment temperature, and it maintains a similar value to that of the T6 temper, when the pre-treatment temperature reaches 410 °C. The 410 °C/1 h + T6 temper can improve the inter-granular corrosion resistance approximately 55% and while simultaneously maintaining a similar strength to that of the T6 temper, which mainly results from preferentially segregated Mg and Zn atoms along grain boundaries and the preservation of the maintained solid solution state in grains after the 410 °C/1 h treatment, respectively. The discontinuity degree of grain boundary precipitates and the concentration difference of Cu element between grain boundary precipitates and adjacent zones have been found to be the key factors to improve the inter-granular corrosion resistance.

Mechanical and microstructural behaviour of AA7075 aluminium alloy for sub-zero temperature sheet stamping process

Age-hardenable aluminium 7XXX alloy sheets represents a very attractive material for the aerospace field; however, their limited formability at room temperature restricts their adoption in the industrial context.In the present study, deformation of AA7075 sheets at temperatures lower than the room one is presented as a possible strategy to overcome this limitation.To this aim, a comprehensive experimental campaign was carried out on AA7075 sheets in peak aged condition at varying temperature and triaxiality states. Specifically, the temperature was changed from −100 °C to 300 °C whereas different stress triaxialities states were achieved by adopting different specimen configurations, namely smooth, notched and shear. A necking locus curve was presented and modelled. Results show that, regardless of the stress state, the uniform elongation monotonically increased as the testing temperature was reduced, favouring material formability. Microstructural and mechanical investigations carried out on the deformed samples revealed that deforming at temperatures higher than the room one drastically increased the number of coarse intermetallic particles. On the contrary, sub-zero deforming temperatures favoured the intermetallic particle fragmentation and the formation of high density of precipitates and dislocations.

Novel Mg–Bi–Al alloy with extraordinary extrudability and high strength

A novel high-alloyed Mg–5Bi–3Al (BA53, wt%) alloy with extraordinary extrudability and high strength is developed. It is successfully extruded at a die-exit speed of 67 m/min without any hot cracking, which is over 10 times the maximum extrusion speeds of commercial high-alloyed Mg alloys, e.g., AZ80 and ZK60. The BA53 alloy extruded at 67 m/min shows high tensile yield strengths in the as-extruded and peak-aged states (188 and 214 MPa, respectively). These excellent extrudability and high strength are mainly attributed to the formation of thermally stable Mg3Bi2 phase. Undissolved coarse and fine Mg3Bi2 particles effectively suppress grain growth during and after extrusion through grain-boundary pinning, leading to the formation of a relatively fine grain structure. Fine rod-type Mg3Bi2 precipitates formed on the prismatic plane along the <0001>Mg direction during air-cooling after existing the die lead to precipitation hardening. The formation of numerous nanosized Mg3Bi2 precipitates along the <01¯10>Mg direction during subsequent aging improves the material strength without ductility loss. The developed BA53 alloy, which comprises inexpensive alloying elements Bi and Al, can be extensively used for manufacturing extruded Mg products because of its high cost-competitiveness, processing efficiency, and mechanical properties.

Age-hardening response of AlMgZn alloys with Cu and Ag additions

A recurrent challenge with aluminum alloys is their longstanding trade-off between mechanical strength and formability. Recently recyclability has put further pressure on the development of single-alloy concepts for solving this challenge. This study addresses an AlMg-based system featuring additional elements to facilitate age-hardening but retaining a high Mg content for inherent pronounced strain hardening as a potential candidate. Age-hardening was enabled by T-phase based precipitation in the commercial alloy EN AW-5182 via the addition of 3.5 wt.% of Zn. The investigation shows that minor additions of Cu and Ag enhance and accelerate it. The study also compares single-step and double-step artificial aging. Hardness and tensile testing and scanning transmission electron microscopy methods were deployed to characterize the alloys investigated, mechanically and microstructurally. An alloy with added Zn, Cu and Ag showed improved strain hardening and reduced serrated flow in the soft state, while exhibiting an age-hardening response of up to 326 MPa in yield strength leading to an ultimate tensile strength of 550 MPa in peak-aged condition. The study discusses the evolution of the microstructure during artificial aging in the light of Zn, Cu and Ag additions and their effect on the precipitation process.

Environmentally assisted cracking behavior of U-bend specimens of Mg–RE alloys in chloride containing basic solution

Abstract Environmentally assisted cracking (EAC) behavior of two Mg-rare earth (RE) alloys such as Mg–Zn–Gd–Nd–Zr (EV31A) and Mg–Y–Nd (WE43C) alloys was investigated by using U-bend specimens. Open circuit potentials (OCP) of the U-bend specimens were monitored during the EAC tests in 0.1 M NaOH solution with different chloride concentrations at room temperature. EV31A (as-received, and peak aged) and WE43C (peak aged) specimens failed by SCC in 80 ppm chloride containing 0.1 M NaOH solution at OCP. When the EAC initiation occurred, the OCP decreased continuously. Irregular fluctuations of the OCP were observed in the absence of EAC. The OCP versus time profile could be used for monitoring EAC failure of the Mg–RE alloy components in real life service. Applied potentials did not cause cracking of the EV31A alloy in 80 ppm Cl− containing 0.1 M NaOH. Accelerated cracking was observed on the WE43C alloy in peak-aged condition under the applied potentials in the transpassive region when compared to that of OCP condition. Overaging decreased the susceptibility to cracking.

Effect of Precipitate Characteristics on the Corrosion Behavior of a AZ80 Magnesium Alloy

In this study, a systematic investigation on the effect of both aging temperature (250 °C and 330 °C) and time (chosen based on aging curves) on the distribution and volume fraction of the Mg17Al12 precipitates and subsequently on the corrosion behavior of the AZ80 magnesium alloy was carried out by conducting both immersion and dynamic polarization test in a freely aerated 3.5% NaCl solution. The analysis of corrosion products using Raman spectroscopy and X-ray diffraction and investigation of corrosion morphologies using scanning electron microscopy suggested galvanic corrosion of the anodic α-Mg matrix in contact with the cathodic Mg17Al12 precipitates. The higher corrosion resistance of the alloy aged at lower temperature (250 °C) was attributed to the higher number density of the Mg17Al12 precipitates than that at higher temperature (330 °C). In the peak aged condition, the average number densities of precipitates (1/µm2) were measured to be ~ 0.74 and ~ 0.32 and average corrosion rates (mm/y) were measured to be ~ 3.0 and ~ 5.0 at the aging temperatures of 250 °C and 330 °C, respectively. At a particular aging temperature, the corrosion resistance increased to a maximum at peak-aged condition followed by a decline in the over-aged condition and further increase with prolong aging, which was correlated with the number density, size and distribution of the Mg17Al12 precipitates.

Microstructure and Mechanical Properties of Squeeze Cast Al-5 Mg-3Zn-1Cu-1Si Alloy Along Cross Section

The variation of microstructure and mechanical properties along the cross section of squeeze cast Al-5Mg-3Zn-1Cu-1Si alloy is studied under both squeeze-cast and peak-aged condition (470 °C × 24 h + 125 °C × 24 h). For squeeze-cast alloy, along the depth direction, second dendrite arm space and the percentage of $${\alpha }$$-Al increase significantly, while the volume fraction of T-Mg32(AlZnCu)49 and Mg2Si phase showed a dramatic decrease. The ultimate tensile strength of squeeze-cast alloy slightly decreased from 357.04 MPa for the surface layer to 344.21 MPa for the central layer, yield strength dropped from 296.86 MPa to 241.03 MPa, but the elongation almost tripled from 1.42% to 3.78%. After peak aging, the ultimate tensile strength of surface layer and central layer reached 538.45 MPa and 504.03 MPa, yield strength reached 447.46 MPa and 442.65 MPa, and the elongation reached 5.23% and 3.97% respectively. Meanwhile the difference of ultimate tensile strength and elongation was increased, but that of yield strength reduced after aging treatment. Precipitation strengthening is the main strengthening mechanism while the improvement in ductility is due to the dissolution of T-Mg32(AlZnCu)49 and the morphological change of eutectic Mg2Si.

The effect of heat treatment on the mechanical and tribological properties of dual size SiC reinforced A357 matrix composites

In the present work, the effect of aging temperature and particle size ratio of SiC particles on the mechanical and tribological properties of A357 composites reinforced with dual particle size SiC were investigated. The composites were prepared by melt-stirring assisted permanent mold casting technique with different weight fractions (3% coarse +3% fine, 4% coarse +2% fine, and 2% coarse +4% fine) of large and small size SiC particles. These three prepared composites are referred as DPS1, DPS2 and DPS3 composites. The solutionizing temperature was maintained constant at 540°C for 9h while the aging was done at 160°C, 180°C and 200°C (T6 treatment) for 6h. Optical and scanning electron microscopy studies showed fairly uniform dispersion of dual size SiC particles in A357 matrix with good interfacial bonding. High-resolution transmission electron microscopy images showed formation of uniformly dispersed needle-like β″ phase and spherical shaped β-Mg2Si precipitates under peak aging conditions. Compared to T6 treated A357 alloy, the T6 treated DPS A357 composites showed improved yield strength, tensile strength, hardness and wear resistance. Among the three composites, hardness and wear resistance of T6 treated DPS2 composite was found to be significantly higher when compared to the other two composites (DPS1 and DPS3). Ratio of large particles to small particles also seems to effect the mechanical and tribological properties. Presence of more small particles was found to be good for strength and ductility whereas more large particles were found to be good for hardness and wear resistance.

Ageing condition of tensile specimens: fracture behavior of notched Al2024 sheet under tensile loading

The effect of different (T6 and T8) heat treatments on tensile properties and fracture behavior of Al2024 in presence and absence of notch by means of microhardness, tensile tests and scanning electron microscopy were investigated. Tensile and hardness specimens were subjected to two different artificial ageing (T6 and T8 heat treatment) for various times. These included under ageing (UA), peak ageing (PA) and over ageing (OA). T8 heat treatment, which has a cold rolling between solutionizing and ageing in its steps, showed a higher value of hardness and yield strength in comparison with common artificial ageing of T6 heat treatment. In notched-tensile specimens, yield stress was found to increase up to the peak ageing condition with a simultaneously decrease in elongation at fracture. This behavior was converse at OA condition. Although introducing of notch increase the yield stress of samples under T6 and T8 conditions in comparison with un-notched samples, the notch strengthening phenomenon was observed only under T8 treatment. Despite of an enhancement in strengthening by applying notch on tensile samples, the elongation to failure was notably lessen in both notched-heat treated samples in comparison with un-notches ones. Also, it was confirmed that the toughness of notched samples of both heat treatments at PA condition were significantly lower than un-notched ones. Consequently, toughness decrement was considerably dominated by the role of deformability compared to strengthening factor, however, the presence of cold rolling in the process of heat treatment (T8) could reduce the harmful effects of notch by increasing the stress bearing capacity in contrary with T6 heat treatment. Moreover, inserting the mechanical properties of peak aged samples from the un-notched tensile test in Abaqus finite element software; the V-shaped notch tensile test was simulated and confirmed the experimental results. It was shown in SEM results that the presence of notch enhanced the contribution of cracked particles, compared to particle/matrix deboning and matrix deformation, therefore, the non-homogeneous distribution of fracture features confirmed the harmful effect of notch. In the following, the distribution of three fracture micro-mechanisms were homogeneous in un-notched samples, which demonstrated the superior values of toughness in smooth samples. The present finding sheds light on development of processing techniques to optimize the mechanical properties of Al 2024 alloy.

Kinetics and thermodynamics associated with age hardening of Cu–4at%Ti alloy

In this study, age-hardened Cu–4at%Ti alloys were studied using high-resolution scanning transmission electron microscopy and high-power X-ray diffraction to identify the hardening structures and their formation mechanisms. The formation of ordered α-Cu4Ti phase (D1a, MoNi4-type) D1a domains during quenching following solution treatment was attributed to the isotropic random diffusion of Ti in the solution via a so-called reaction–diffusion mechanism. This first nucleation stage of the domains forms a Ti body-centered cubic sublattice on a Cu face-centered cubic lattice, thereby generating a fine D1a domains with a short-range-ordered superlattice. Subsequent anisotropic diffusion occurs because of variations in the radii of the fine domains, leading to Ostwald ripening during the second coarsening stage of the D1a domain formation process. During the early stage of aging, the number of domains sharply decreases until the peak aging condition. Ostwald ripening is considered to progress because D1a is a metastable and substitutional-solid-solution phase. As a result, diffusional transformation comprising reaction–diffusion and subsequent Ostwald ripening proceeded with aging to form a hardened structure. The domain diameters at peak aging ranged from 10 to 20 nm. It is proposed that this hardening mechanism, for which no spinodal decomposition is observed, be called ordered-domain dispersion hardening.

Hardness, quench sensitivity, and electrical conductivity of 7xxx Al alloys with high Zn concentrations

In this study, 7xxx (Al-Zn-Mg-Cu) alloys with high Zn concentrations were fabricated by increasing the Zn contents of 7085 Al alloys. The hardness, quench sensitivity, and electrical conductivity of the alloys and a 7085 Al alloy under peak-aged state were studied. The high Zn concentration alloys had higher η′ phase densities and lower quantities of Mg solid atoms than the 7085 Al alloy and thus exhibited higher hardness and electrical conductivity. The high Zn concentration alloys also exhibited slightly higher quench sensitivity than 7085 Al alloy, and the depth of the quenching layer of the alloy with 11.27 wt% Zn content reached 100 mm. This work confirmed that the novel 7xxx alloy with high Zn concentration showed balanced performance, exhibiting enhanced hardness and conductivity and reasonable quench sensitivity compared with the 7085 commercial Al alloy.

Age-Hardening Response of AlMgZn Alloys with Cu and Ag Additions

A recurrent challenge with aluminum alloys is their long-standing trade-off between mechanical strength and formability. Recently recyclability has put further pressure on the development of single-alloy concepts for solving this challenge. This study addresses an AlMg-based system featuring additional elements to facilitate age-hardening but retaining a high Mg content for inherent pronounced strain hardening as a potential candidate. Age-hardening was enabled by T-phase based precipitation in the commercial alloy EN AW-5182 via the addition of 3.5 wt.% of Zn. The investigation shows that minor additions of Cu and Ag enhance and accelerate it. The study also compares single-step and double-step artificial aging. Hardness and tensile testing and scanning transmission electron microscopy methods were deployed to characterize the alloys investigated, mechanically and microstructurally. An alloy with added Zn, Cu and Ag showed improved strain hardening and reduced serrated flow in the soft state, while exhibiting an age-hardening response of up to 326 MPa in yield strength leading to an ultimate tensile strength of 550 MPa in peak-aged condition. The study discusses the evolution of the microstructure during artificial aging in the light of Zn, Cu and Ag additions and their effect on the precipitation process.

Quantitative analysis of the effects of particle content and aging temperature on aging behavior in B4C/6061Al composites

This work gives a quantitative analysis of precipitations in B4C/6061Al composites with B4C volume fraction of 3% and 12% and the impacts of B4C particles volume fraction and aging temperatures on aging behavior. These aspects of microstructural evolution were unveiled through differential scanning calorimetry (DSC), transmission electron microscopy (TEM) and Brinell hardness measurements. It is shown that the aging kinetic is considerably accelerated with the increase of ceramic particle volume fraction. With the aging temperature increasing from 130 °C to 190 °C for B4C/6061Al composites when the particle volume fraction is 3%, the time to reach peak-aged state decreases from 60 h to 5 h. Compared with commercial 6061Al alloys, the addition of B4C particles inhibits the formation of G.P. zone, and changes the order of precipitations in B4C/6061Al composites. Importantly, the number density, cross-sectional area and volume fraction of precipitations in the matrix were quantified in order to characterize the effect of B4C particle volume fraction and aging temperatures on the aging behaviors in B4C/6061Al composites.

Microstructure evolution, plastic anisotropy, and intergranular corrosion of Al-Mg-Si sheet processed through a combination of hot extrusion and cold rolling

Peak-aged Al-Mg-Si alloys fabricated through a combination of hot extrusion and cold rolling were studied, and the microstructure evolution, mechanical properties, plastic anisotropy, and intergranular corrosion (IGC) were clarified. The results show that fully dynamic recrystallization and grain growth occurred during the hot extrusion, resulting in a coarse and equiaxed grain structure. The cold rolling caused an elongation of the coarse grains and the formation of some fine grains. After solution and aging, the as-extruded grains were unchanged, whereas the as-rolled specimens exhibited a finer grain structure with a large amount of high-angle grain boundaries owing to the occurrence of static recrystallization. All as-extruded/rolled specimens mainly showed a cube and Goss texture. In the peak-aged state, the deformation textures of the rolled specimens decreased to a low level, whereas the fraction of Q and R significantly increased. A fine high-density needle-like β″ phase was uniformly distributed in all aged specimens. If the thickness of the peak-aged Al-Mg-Si sheet is uniform, the combination of hot extrusion and cold rolling, or an increase in the rolling reduction, can enhance the tensile strength and elongation. Although the combined process enhanced the plastic anisotropy, the variation in the r-values along the different directions was decreased. The IGC resistance of the Al sheet experienced hot extrusion and cold rolling, and the peak aging was slightly reduced owing to a high fraction of high-angle grain boundaries.

Bimodal microstructure dispersed with nanosized precipitates makes strong aluminum alloy with large ductility

A bimodal microstructure composed of large primary α-Al phases (PPs) and small secondarily solidified grains (SSGs) is fabricated in 6061 aluminum alloy by powder thixoforming (PTF) method after solution treatment. Artificial aging treatment introduces nanosized precipitates into the bimodal microstructure, in which the number and growth rate of precipitates are different: the number in PPs is larger than that in SSGs due to the higher density of dislocations and lattice distortions in PPs, while the growth rate in SSGs is higher than that in PPs owing to the higher solute concentration in SSGs. As the number and size of precipitates are optimized in the bimodal microstructure, i.e., in peak-aged state, remarkable increments of 56.1% and 130.3% in ultimate tensile strength and yield strength while a moderate increase of 6.3% in ductility are achieved compared with the as-fabricated state, which overcomes the trade-off in strength and ductility of conventional aging-treated 6061 alloys with unimodal microstructure. The strengthening comes from the geometrically necessary dislocations induced by the deformation incompatibility of bimodal microstructure and from precipitate strengthening by nanosized precipitates, while the toughening is from the enhanced dislocation accumulation in this alloy.

Effect of dynamic plastic deformation on the microstructure and mechanical properties of an Al–Zn–Mg alloy

A supersaturated Al–Zn–Mg alloy was processed by dynamic plastic deformation (DPD) using a Split-Hopkinson pressure bar (SHPB) facility with strain range from 0.5 to 1.5 and then subjected to compressive testing at strain rates of 1 × 10−3–4 × 103 s−1. The results show that there is concurrent occurrence of grain refinement, increase in dislocation density and precipitation of fine precipitates in different morphologies during DPD at strain over 1. This enhances the yield strength of the Al alloy after DPD over the as-received material in peak-aging state (T6). Grains are refined through the formation of low angle grain boundaries (LAGBs) by rearrangement of dislocations which leads to the subdivision of original grains. Heterogeneous coarse precipitates mainly of T (Al20Cu2Mn3) and E (Al18Mg3Cr2) phases distribute along dislocations and grain boundaries after DPD. Dynamic precipitation of very fine η′ and η during DPD occurs where the volume fraction of precipitates increases with increasing the number of presses. The influence of precipitates on the strain rate sensitivity is also discussed.