Tensile Behavior of Ultrafine-Grained Al-4Zn-2Mg Alloy Produced by Cryorolling
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- Gopala Krishna, K., Singh, N., Venkateswarlu, K. et al. J. of Materi Eng and Perform (2011) 20: 1569. doi:10.1007/s11665-011-9843-1
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An Al-4Zn-2Mg alloy was subjected to cryorolling (CR) followed by short annealing. An average grain size of ~100 nm was achieved. Cryorolled samples showed large reduction in grain size due to suppression of dynamic recovery and absence of annihilation of dislocations, as compared to room temperature rolled samples. Further, the ultrafine-grained (UFG) Al-4Zn-2Mg alloy when subjected to natural aging showed an improved strength of ~413 MPa with ductility of ~25%, as compared to ~360 MPa and 22% ductility in peak aged condition of coarse-grained alloy. However, UFG alloy in peak aging condition, exhibited a relatively strength (~375 MPa) and 24% ductility combinations than the natural aging condition. The latter is attributed to dynamic precipitation and stored energy. In the present study, it is demonstrated that simultaneous improvement in strength as well as ductility can be achieved for the Al-4Zn-2Mg alloy through CR and controlled heat treatment combinations.
KeywordsagingAl-4Zn-2Mg alloycryorollingsecondary precipitationtensile behaviorultrafine-grain structure
Aluminum and its alloys are extensively used for various design engineering applications, where the high strength to weight ratio is one of the basic criteria. In the recent past, ultrafine-grain (UFG) and nano-crystalline materials obtained by severe plastic deformation (SPD) have attained significant importance due to their superior mechanical properties. Formation of UFG structure in materials is a technologically interesting phenomenon as it enables an increase in strength without any significant degradation in toughness, elastic moduli, diffusivity of solute, etc. (Ref 1-3). Refinement of the microstructure can be achieved through SPD techniques, which include high-pressure torsion, reciprocal extrusion, equal-channel angular pressing (ECAP) (Ref 4-6), accumulative roll bonding (Ref 7-10), repetitive corrugation and straightening (Ref 11-13), constrained groove pressing (CGP) (Ref 14), and constrained groove rolling (CGR) (Ref 15), etc. These processes generally result in materials with UFG (<1 μm) structure. Though such materials exhibit higher strength, they lack good ductility. Several efforts have been made in the past to achieve both, high strength and good ductility, simultaneously.
Cryorolling (CR) was proved to be an effective method for enhancing both tensile strength and yield strength of the AA5083 Al alloy (Ref 16). About 10% increase in yield strength was reported for cryorolled AA5083 Al alloy, as compared to the same alloy, rolled at room temperature with the same reduction ratio. Suppression of dynamic recovery during deformation at cryogenic temperature was found to be the reason (Ref 4) for enhanced strength due to high density of defects generated by deformation, which act as potent recrystallization sites. It is also reported that rolling of commercial purity Cu at cryogenic temperature, resulted in UFG structure at lower plastic deformation when compared to other SPD processes, at ambient or elevated temperatures (Ref 5).
One of the successful methods for increasing the strength while retaining the ductility, with respect to heat treatable Al alloys, was CR involving several steps of processing (Ref 6), i.e., (1) solutionizing the alloy to dissolve all second-phase particles and then quenching to produce a supersaturated solid solution; (2) CR at liquid nitrogen temperature to produce heavily deformed structure; (3) short annealing to produce UFG microstructure; and (4) aging to produce homogeneously distributed strengthening precipitates. An AA 2219 Al alloy, that was subjected to CR and subsequent annealing and aging, was reported to have a grain size in the range of 500 nm to 1 μm with UTS of 540 MPa and while retaining the ductility at 11% (Ref 17).
Both high strength and high ductility in a AA 2024 Aluminum alloy was obtained by CR with an approach involving (i) solution treatment, (ii) CR to produce a fine-grain structure with high density of dislocations, and (iii) aging to generate highly dispersed nano-sized precipitates (Ref 18). The objective of the present work was to obtain UFG structure in an Al-4Zn-2Mg alloy using CR technique followed by short annealing and to study its tensile behavior after aging. This alloy is currently used in automobile and aerospace industries and other structural applications.
Experimental Material and Procedure
Chemical composition of the alloy
Tensile properties of CG and UFG alloy after different aging treatments
Sample description (code)
Coarse-grained peak aged (CGPA)
[100 °C/5 h + 150 °C/6 h]
Cryorolled peak aged (CRPA)
[100 °C/5 h + 150 °C/90 min]
Cryorolled naturally aged (CRNA)
[At room temperature for 180 days]
Room temperature rolled, peak aged (RRPA)
[100 °C/5 h + 150 °C/90 min]
Room temperature rolled, naturally aged (RRNA)
[At room temperature for 180 days]
Hardness was measured using the Indentec® Vickers hardness with a load of 5 kg. Miniature-sized flat tensile specimens; with 5 mm gage length and thickness of ~1 mm were prepared from the strips, along the rolling direction. Tensile tests were carried out at a nominal strain rate of 3 × 10−3/s on an Instron machine of 10 kN capacities. All the tensile tests were carried out at ambient temperature. Microstructural characterization was carried out using Leica Optical and Philips CM-12 transmission electron microscope (TEM). For metallographic examination, samples were prepared by electrolytic etching, using Barker’s reagent (4-5 mL fluoroboric acid in 200 mL water) on mirror polished samples. For TEM study, thin discs were sliced from the strips using slow speed cutter which were mechanically thinned down to 100 μm. These 3 mm discs were further thinned down using twin-jet electropolishing technique at 15 V in a mixture of 30% nitric acid and 70% methanol maintained at ~20 °C.
Results and Discussion
The UTS values for the CRPA and CRNA, 390 and 415 MPa, respectively, are higher than the coarse-grained peak aged (CGPA) alloy. There has been ~10% improvement in the strength values, while the ductility level is also increased by ~10%. The UTS values for RRPA and RRNA showed 375 and 400 MPa, respectively. However, the ductility levels are reduced to 11 and 14%, respectively, compared to 21.5% for the CGPA sample (Fig. 4a).
The work hardening characteristics of the experimental alloy under different heat-treatment conditions was understood by plotting work hardening rate (θ), against flow stress (σ − σy) as shown in Fig. 4(b). It is well known that the work hardening characteristics of the aluminum alloy affect the yield strength and fracture toughness properties. Decreasing the work hardening temperature in case of cryorolled Al and Al-Mg alloy resulted in improved ductility without much loss in tensile strength (Ref 19). It was also shown that work hardening at lower temperature enables greater slip homogeneity and decreases the dynamic recrystallization effects. From the Fig. 4(b), it is evident that the work hardening behavior of cryorolled samples is superior to room temperature rolled samples. Apart from this, the alloy in naturally aged condition, showed better work hardening behavior as compared to that of artificially aged condition. The nature of work hardening is very much dependent on the dislocation storage mechanism rather than precipitate-dislocation interactions. Once the precipitates start forming, the work hardening behavior is dependent on precipitation hardening characteristics of the alloy. During artificial aging, large scale precipitation ensures that the dislocation storage mechanism is solely dependent on the nature, size, and distribution of precipitates rather than the grain size. This will result in higher yield strength in the case of artificially aged sample due to large scale precipitation and pinning the mobile dislocations that delays the yielding, which is also evident from the tensile curve of CRPA sample (Fig. 4a). However, in case of naturally aged sample, the yield strength was less as the number of precipitates available for pinning the dislocations, is less and thus dislocations become mobile at a lower applied strain.
Grain growth during artificial aging, results in a decrease of the strength.
High density of dislocations generated during rolling mostly disappears due to recovery and recrystallization, resulting in decrease in strength and restoring ductility to some extent.
High density of G.P. zone and η′ precipitates, generated during aging, results in increased strength.
Hence, it is assumed that two opposing mechanisms are operative during aging subsequent to rolling and short annealing, i.e., (i) and (ii) promoting ductility and (iii) promoting strength. Net result of all these, should lead to an increased strength without compromising the ductility. It is evident from Fig. 3 that strength of cryorolled as well as room temperature rolled material is improved when compared to CGPA. This increase in strength could be attributed to smaller grain size, finer precipitates, and absence of PFZ. On rolling, dislocation density is increased that eventually act as nucleation sites for strengthening precipitates. This leads to finer precipitates in rolled alloys as compared to the CG alloy.
The strength of the sample is increased when its grain size is reduced. At the same time, by reducing the grain size to sub-micron level, slip distance is also reduced and hence the stress concentration lowers across the grain boundaries and grain boundary triple points. This resulted in improving the ductility levels (Ref 18, 25). Although rolling resulted in heavily deformed structure, further short annealing and aging resulted in a structure, having low-dislocation density with much scope for dislocation accumulation before saturation, which should increase the work hardening and hence improve ductility. Further improvement in ductility can be achieved through high density of fine precipitates, which provides effective sites for trapping and accumulation of dislocations.
Higher level of dislocation density can be uniformly stored in the material rolled at cryogenic temperatures as a result of the suppressed annihilation of dislocations, which if present, should occur at grain boundary and in the lattice through thermally activated cross-slip and climb. Hence, ductility of room temperature rolled material was found to be much lower, compared to cryorolled samples, which was comparable to that of coarse-grained material.
Hence, it can be concluded from the observation that for simultaneously achieving increased strength as well as good ductility, the microstructure should have high density of finer second-phase precipitates and adequate dislocation density. In the present study it is demonstrated, that simultaneous improvement in strength as well as ductility can be achieved in the base Al-4Zn-2Mg alloy, through CR and heat-treatment combinations.
The aging and tensile behavior of an Al-4Zn-2Mg alloy subjected to RT and CR was investigated. Following conclusions are drawn from the results.
Peak aging, after CR and short annealing showed an increased strength while retaining ductility as compared to CGPA. However, peak aging after RT rolling resulted in increased strength with loss in ductility, as compared to CGPA. An improved strength and ductility of Al-4Zn-2Mg alloys, subjected to CR, is attributed mainly due to ultrafine-grain structure combined with controlled aging, resulting in finer precipitates and presence of dislocations. High amount of work hardening behavior for CRNA is attributed to dynamic precipitation during tensile deformation. The results also demonstrated, a successful method for increasing the strength while retaining the ductility, with respect to heat treatable Al-4Zn-2Mg alloy, by employing CR.
The authors are grateful to Dr. S. Srikanth, Director, National Metallurgical Laboratory, Jamshedpur, India for his encouragement and extending the necessary support for the work.