Experiment and multiscale modeling of the coupled influence of constituents and precipitates on the ductile fracture of heat-treatable aluminum alloys

doi:10.1016/j.actamat.2005.04.002

Acta Materialia

Volume 53, Issue 12, July 2005, Pages 3459-3468

https://doi.org/10.1016/j.actamat.2005.04.002 Get rights and content

Abstract

Solution and quench treatments are very important for the heat-treatable aluminum alloys because any change in these treatments will induce a trade-off in volume fraction between the constituents and precipitates and so will cause a change in the mechanical properties. In this paper, three types of solution + quench treatments were applied to two kinds of aluminum alloys, i.e., the Al–Cu–Mg alloy that contains disc/plate-shaped precipitates and the Al–Mg–Si alloy that contains rod/needle-shaped precipitates, to change the relative content between the constituents and the precipitates and to develop different coupling of the constituent and precipitates. The specimen treated with an enhanced solution or a stepped solution is found to exhibit a significant increase in yield strength, ductility, and fracture toughness. A multiscale model is presented to quantitatively estimate the coupled influence of the constituents and precipitates on the mechanical properties by combining with a strengthening model. The experimentally observed non-monotonic dependence of ductility on the trade-off in volume fraction between the constituents and precipitates is reasonably explained by using this multiscale model. In addition, the influence of stress triaxiality level on the ductility and fracture toughness is also calculated. All the calculations are in quite good agreement with the experimental results.

Introduction

Low density combined with high strength have made heat-treatable aluminum alloys the primary materials of choice for applications such as in aircraft, where the specific strength (strength-to-weight ratio) is a major design consideration. However, the low fracture toughness limits the extensive application of commercial heat-treatable aluminum alloys. The coarse second phase particles, termed constituents in aluminum alloys, are mainly responsible for the low fracture resistance because the constituents are usually the void/crack initiators or the preferential crack paths [1]. Therefore, lowering the volume fraction of constituents has been regarded as the most important approach to improve the fracture toughness for the aged aluminum alloys [2], [3], [4].

The formation of constituents in aged aluminum alloys primarily results from the presence of impurities, e.g., Fe and Si, and excessive amounts of major alloying elements such as Mg, Zn, and Cu. This means that the decrease of the iron and silicon levels and hence the volume fraction of constituents would improve the fracture toughness [5], [6]; this approach has been employed in the development of 2124, 7050, 7175, and 7475 aluminum alloys. However, the improvement of fracture toughness by purification is limited because the complete removal of impurities is hard to achieve especially in commercial aluminum alloys. Besides the constituents resulting from the presence of impurities and excessive alloying elements, another kind of constituent is present in the commercial aluminum alloys, which contains alloying elements as well but results from the non-equilibrium solidification due to the lower solvus limit of the alloying elements in aluminum. In principle, this kind of constituent is soluble and could be dissolved into the solid solution reversibly through solution treatment. It is then possible to develop another effective approach to improving the fracture toughness by solubilizing these soluble constituents by applying an enhanced solution treatment.

The enhanced solution treatment is a stepped solution treatment, in which the solution treatment will be performed at temperatures which is gradually elevated with time. In comparison, the traditional solution treatment is usually performed at a stationary temperature and, in order to avoid the presence of transient liquid phase, the stationary temperature will be greatly limited so as not to exceed the melting point of multiphase eutectic. Since each of eutectic phases dissolves into the matrix in a certain sequence during solution treatment, the eutectic temperature will be elevated with the complete solution of one of the eutectic phases. This indicates that the upper temperature limit for solution will gradually increase during the treatment and so the application of traditional solution treatment with a stationary lower temperature could not dissolve as much soluble constituents as possible. In contrast, the application of stepped solution treatment with a gradual increase in temperature could both avoid exceeding the eutectic melting and realize the complete solution of dissolvable constituents. So the enhanced solution treatment or stepped solution treatment has a potential for industrial application. However, little work has been carried out on the application of stepped solution treatment and some limited results showed that the aged aluminum alloys treated by the stepped solution treatment exhibited greater strength [7], [8]. But most importantly, the enhanced solution treatment should have a significant influence on the ductile fracture of aluminum alloys, which has not been studied up to now.

The application of enhanced solution treatment is believed to decrease the volume fraction of constituents. At the same time, the solution of dissoluble constituents will make more alloying elements dissolve into the matrix and so will induce more strengthening second phase particles to be precipitated. This indicates that a trade-off in volume fraction between the constituents and precipitates will be achieved by applying the enhanced solution treatment. Because the independent decrease of coarse constituents should be favorable for enhancing the fracture resistance while the independent increase of strengthening precipitates should induce somewhat contrary results, the trade-off in volume fraction between the constituents and the precipitates will be possible to create a non-monotonic evolution trend of the ductility or of the fracture toughness. For analysis purposes, there is an urgent need to understand the combined influence of both the constituents and the precipitates on the ductile fracture. It has been well known that the coarse constituents in aluminum alloys are from several to tens of micrometers in diameter, depending on the fabrication procedure, and the fine strengthening precipitates are tens of nanometers in size, hundreds of times less than that of constituents [9]. In addition, there exists another kind of second phase particle of intermediate size between the constituents and precipitates, termed dispersoids, in the commercial aluminum alloys (see Fig. 1) and all three kinds of the second phase particles contribute to the fracture behaviors simultaneously. Therefore, the combined influence of both the constituents and the precipitates is essentially a multiscale coupling that should connect the microstructural features from micrometer-scale length to nanometer-scale length [10], [11], [12], [13]. There have been rare studies on the multiscale coupling of second phase particles intrinsically contained in aged aluminum alloys, so the application of enhanced solution treatment should motivate not only some technological attention but also some theoretical attention especially on the modeling and simulation of the combined influence of multiscale-sized second phase particles on the ductile fracture.

In this paper, we focus on the experimental and theoretical investigations of the combined influence of coarse constituents and fine precipitates on the ductile fracture in heat-treatable aluminum alloys. Three types of solution + quench treatments, including an enhanced solution treatment, have been applied to two typical heat-treatable aluminum alloys, i.e., Al–Cu–Mg alloy containing disc- or plate-shaped strengthening precipitates and Al–Mg–Si alloy containing rod- or needle-shaped strengthening precipitates, respectively, to study the coupled influence of constituents and precipitates on the ductile fracture and to study the improvement of ductility and fracture toughness by applying the enhanced solution treatment. The changes in volume fractions of second phase particles as well as the changes in quasi-stationary mechanical properties were experimentally measured for comparison. With consideration of the significant influence of stress triaxiality level on the constituent cracking or decohesion, the dependence of strain to fracture or tensile ductility on the stress triaxiality level was investigated as well. A multiscale fracture model [14], [15] was presented to estimate the coupled influence of second phase particles on the ductility and fracture toughness of aged aluminum alloys and the calculations were found to fit well with the experimental results.

Section snippets

Materials and processing

The aluminum alloys used in present experiment were hot-rolled Al–Cu–Mg plate of 16 mm thickness and extruded Al–Cu–Mg and Al–Mg–Si rods of 18 mm diameter, both supplied by the research laboratory of Xi’an Aircraft Industry Ltd. The Al–Cu–Mg plate and the Al–Cu–Mg rod were from the same ingot. The composition in weight percentage is 4.62% Cu, 0.65% Mg, 0.22% Mn, 0.08% Si, 0.1% Fe, 0.1% Zn, and balance Al for Al–Cu–Mg alloy and 1.12% Mg, 0.57% Si, 0.25% Cu, 0.22% Cr, and balance Al for Al–Mg–Si

Microstructures

Fig. 3, Fig. 4 show the optical micrographs of sectioned Al–Cu–Mg and Al–Mg–Si rods, respectively, with different treatments. It is found that the change in grain size with different treatment is not noticeable, as shown at Table 1 where L and w are the averaged grain length and grain width, respectively, and the values in parentheses are the corresponding standard deviations. However, the EF-treated specimens (Fig. 3, Fig. 4(c)) contain fewer constituents while the TS-treated specimens (Fig. 3

Coupled influence of both constituents and precipitates

Microstructural observations revealed that both the present two types of aluminum alloys, even after EF-treatment, fracture in a transgranular mode predominantly. So for simplicity, we neglect the influence of intergranular fracture. A multiscale model has been established by the authors [14], [15] to describe the dependence of strain to fracture and fracture toughness on the three kinds of second phase particles intrinsically contained in commercial aged aluminum alloys. Some unified

Conclusions

(1)
The application of enhanced solution treatment is found to be effective in decreasing the volume fraction of coarse constituents and hence in comprehensively improving the mechanical properties of heat-treatable aluminum alloys.
(2)
An unusual non-monotonic dependence of ductility or strain to fracture on the trade-off in volume fraction between the constituents and precipitates is experimentally observed. A multiscale model is presented to explain this dependence reasonably by revealing the coupled

Acknowledgments

This work was supported by the National Basic Research Program of China (Grant No. 2004CB619303, 2002CB613303). The authors also wish to express their special thanks for the support from the National Natural Science Foundation of China and the National Outstanding Young Investigator Grant of China. Valuable comments and kind suggestions from reviewers are sincerely appreciated too. This work was supported by Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT).

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Experiment and multiscale modeling of the coupled influence of constituents and precipitates on the ductile fracture of heat-treatable aluminum alloys

Abstract

Introduction

Section snippets

Materials and processing

Microstructures

Coupled influence of both constituents and precipitates

Conclusions

Acknowledgments

Eng Fract Mech

Acta Mater

Acta Mater

Acta Mater

Mater Sci Eng A

Eng Fract Mech

Acta Mater

Acta Mater

Acta Mater

Scrip Mater

Eng Fract Mech

Acta Mater

Metall Trans A

Metall Trans A

Metall Trans A