Effect of hot deformation on microstructure and quenching-induced precipitation behavior of Al-Zn-Mg-Cu alloy

https://doi.org/10.1016/j.matchar.2020.110861Get rights and content

Highlights

  • Major quenching-induced η and minor T phase appear in the slowly-quenched alloy without hot rolling.

  • Quenching-induced η, T, S and Y phase appear in the slowly-quenched alloy after hot rolling.

  • η and T phase tend to form on (sub) grain boundaries and Al3Zr dispersoids inside grains.

  • S and Y phase tend to associate with dislocations inside subgrains.

Abstract

The effect of hot deformation on microstructure and quenching-induced precipitation behavior of an Al-Zn-Mg-Cu alloy 7055 was investigated by electron backscattered diffraction (EBSD), transmission electron microscope (TEM), high resolution transmission electron microscope (HRTEM) and scanning transmission electron microscope (STEM). In the solution heat treated and slowly-quenched alloy without hot deformation, there are equiaxed grains with low dislocation density, and there is major quenching-induced η (MgZn2) phase located at grain boundaries and on some Al3Zr dispersoids in the interior of grains and minor T(Al2Zn3Mg3) phase on Al3Zr dispersoids inside grains. While in the solution heat treated and slowly-quenched alloy after hot rolling, there are recrystallized grains and subgrains with higher dislocation density, and there is a larger amount of quenching-induced η and T phase on incoherent Al3Zr dispersoids inside recrystallized grains and at (sub) grain boundaries; moreover, quenching-induced S (Al2CuMg) phase and Zn-Cu rich Y phase are observed in the interior of subgrains. The precipitation behavior of these quenching-induced phases has been discussed primarily based on their chemical compositions, nucleation sites and different microstructure features in the solution heat treated alloy with and without hot deformation.

Introduction

Al-Zn-Mg-Cu alloys exhibit high strength after solution heat treatment, quenching and ageing, and therefore they have been used as structural materials especially in the aerospace industry [[1], [2], [3], [4]]. Quenching is a critical step in the production of these alloys. With the decrease of quenching rate, the mechanical properties of Al-Zn-Mg-Cu alloys tend to decrease after ageing, and this phenomenon is called quenching sensitivity [[5], [6], [7], [8]]. It is known that in the slowly-quenched Al-Zn-Mg-Cu alloys, a large amount of quenching-induced phase form, resulting in fewer solutes in the solid solution and then fewer metastable η' strengthening precipitates after ageing; as a result, the strength decreases [[9], [10], [11]].

Quenching-induced η phase (MgZn2), T phase (Al2Zn3Mg3), S phase (Al2CuMg) or Zn-Cu rich Y phase can be found in Al-Zn-Mg-Cu alloys, depending on the chemical compositions and microstructure of the alloy, and quenching rate, as recently summarized by Liu et al. [12]. Among them, Y phase is a new platelet phase recently found and named by Zhang et al. [13]. There is limited information about this phase. Y phase, of which the structure is similar to that of T1(Al2CuLi) phase observed in the Al-Cu-Li alloys, exhibits a high aspect ratio, and mainly contains Al, Zn, Cu and a small amount of Mg [13]. In some investigations on 7175, 7050, 7085 aluminum alloys, only quenching-induced η phase was identified under the studied quenching conditions [[14], [15], [16], [17]]. While in investigations on slowly-quenched aluminum alloys like 7010, 7055, 7085, 7136 etc. [13,[18], [19], [20], [21], [22], [23], [24]], not only η phase but also other quenching-induced phases were found. For instance, Godard et al. [18] found η, S and T phases in 7010 aluminum alloy during step quenching. Tang et al. [19] found that in slowly-quenched 7055 aluminum alloy, in addition to η phase located at (sub)grain boundaries and inside grains, there were S and T phases in the substructure. Zhang [20] and Starink et al. [13,21,22] observed η, S and Y phases in 7150 aluminum alloy during step quenching and continuous cooling. More recently, Liu et al. [12] found η phase and Y phase in a slowly-quenched 7085 aluminum alloy. Tang et al. [23] found η, T and Y phases in a slowly-quenched 7136 aluminum alloy.

The precipitation of quenching-induced η phase seems to be well understood. It is known that this phase is closely related to heterogeneous nucleation sites such as grain boundaries (GBs), subgrain boundaries (SGBs), dispersoids, etc. [[12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]]. These nucleation sites are often affected by hot deformation, which is an indispensable step in the production of these alloys. For instance, Zhang et al. [25] found that for hot-rolled and solution heat treated 7050 aluminum alloy, with the increase of deformation rate, subgrains coarsen and the fraction of SGBs with high misorientation angle increased, resulting in a large amount of quenching-induced η phase inside subgrains and at SGBs. Li et al. [26] showed Zener-Hollomon parameter can change the amount of GBs, SGBs and Al3Zr dispersoids available for nucleation sites of quenching-induced η phase, and then change quenching sensitivity of 7085 aluminum alloy. By contrast, little is known on the precipitation behavior of quenching-induced T, S and Y phases though they have been observed in some Al-Zn-Mg-Cu alloys [[19], [20], [21], [22], [23], [24]].

In this work, only quenching-induced η and T phases were detected in a slowly-quenched 7055 aluminum alloy without hot rolling, while quenching-induced η, T, S and Y phases were detected in the slowly-quenched alloy after hot rolling and solution heat treatment. By means of electron backscattered diffraction (EBSD), transmission electron microscope (TEM), high resolution transmission electron microscope (HRTEM) and scanning transmission electron microscope (STEM), the precipitation behavior of these quenching-induced phases has been discussed based on their chemical compositions, nucleation sites and the microstructure features in different alloys. It can help to have better understanding of quenching sensitivity of Al-Zn-Mg-Cu alloys.

Section snippets

Experimental

A 7055 aluminum alloy ingot with a thickness of 400 mm was prepared by direct-chilling technique, and the chemical compositions (wt%) are Al-8.10Zn-2.08 Mg-2.25Cu-0.11Zr, Fe<0.07, Si<0.07. The ingot was fully-homogenized by heating slowly to 465 °C and holding for 24 h in an air furnace and then cooled in still air. After pre-heating at 440 °C for 2 h, the ingot was rolled to a 60 mm thick plate by multi-passes with a total reduction rate of 85%. Samples with a size of 25 mm in length, 25 mm in

Phase diagram and CCT curves

Fig. 1 shows the vertical section phase diagram based on 7055 aluminum alloy composition (Zn=8.10 wt%, Mg=2.08 wt%) with variable Cu values. The black dotted line shows the approximate location of the studied 7055 aluminum alloy in this work. There are α(Al)+ S(Al2CuMg) phases at 470 °C, which is the solution heat treatment temperature of the studied alloy. With the decrease of temperature, the α(Al) phase starts to decompose. The phase regions appear in the order of α(Al) + S(Al2CuMg) + η(MgZn2

Discussion

The major quenching-induced phases and their nucleation sites in slowly-quenched HS and RS samples are summarized in Table 1. There are only η phase and T phase in the HS sample of 7055 aluminum alloy (Fig. 5, Fig. 6); while in the RS sample (Fig. 7, Fig. 8, Fig. 9, Fig. 10), not only η and T phases, but also S and Y phases have been observed. These results are not always in accordance with the simulated results in Fig. 1, Fig. 2. The effect of hot rolling need to be considered during

Conclusions

  • (1)

    Hot rolling changes the features of Al3Zr dispersoids and results in subgrains with higher dislocation density in 7055 aluminum alloy after solution heat treatment, and therefore leads to increased number and variety of quenching-induced phases.

  • (2)

    In the solution heat treated and slowly-quenched alloy without hot rolling, there are equiaxed grains with low dislocation density, and there is major quenching-induced η (MgZn2) phase located at grain boundaries and on some Al3Zr dispersoids in the

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

Declaration of Competing Interest

None.

Acknowledgments

This work is supported by the National Key Research and Development Program of China (2016YFB0300901). The authors would like to thank Mr. Xingtao Liu from Purdue University who helped in calculating the phase diagram.

References (56)

  • P. Li et al.

    Quench sensitivity and microstructure character of high strength AA7050

    Trans. Nonferrous Metals Soc. China

    (2012)
  • Y. Zheng et al.

    Effect of homogenization time on quench sensitivity of 7085 aluminum alloy

    Trans. Nonferrous Metals Soc. China

    (2014)
  • S.T. Lim et al.

    Improved quench sensitivity in modified aluminum alloy 7175 for thick forging applications

    Mater. Sci. Eng. A

    (2004)
  • D. Godard et al.

    Precipitation sequences during quenching of the AA7010 alloy

    Acta Mater.

    (2002)
  • J. Tang et al.

    Influence of quench-induced precipitation on aging behavior of Al-Zn-Mg-Cu alloy

    Trans. Nonferrous Metals Soc. China

    (2012)
  • M.J. Starink et al.

    Predicting the quench sensitivity of Al-Zn-Mg-Cu alloys: a model for linear cooling and strengthening

    Mater. Des.

    (2015)
  • B. Yang et al.

    Continuous cooling precipitation diagram of aluminium alloy AA7150 based on a new fast scanning calorimetry and interrupted quenching method

    Mater. Charact.

    (2016)
  • P. Priya et al.

    Precipitation during cooling of 7XXX aluminum alloys

    Comput. Mater. Sci.

    (2017)
  • X. Zhang et al.

    Effect of processing parameters on quench sensitivity of an AA7050 sheet

    Mater. Sci. Eng. A

    (2011)
  • C. Li et al.

    Effect of Zener-Hollomon parameter on quench sensitivity of 7085 aluminum alloy

    J. Alloys Compd.

    (2016)
  • S. Liu et al.

    Influence of grain structure on quench sensitivity relative to localized corrosion of high strength aluminum alloy

    Mater. Chem. Phys.

    (2015)
  • Y. He et al.

    Effect of minor Sc and Zr on microstructure and mechanical properties of Al-Zn-Mg-Cu alloy

    Trans. Nonferrous Metals Soc. China

    (2006)
  • J. Liu et al.

    Effect of minor Sc and Zr on recrystallization behavior and mechanical properties of novel Al-Zn-Mg-Cu alloys

    J. Alloys Compd.

    (2016)
  • W. Pantleon

    Resolving the geometrically necessary dislocation content by conventional electron backscattering diffraction

    Scr. Mater.

    (2008)
  • Y. Wang et al.

    Recrystallization of Al-5.8Mg-Mn-Sc-Zr alloy

    Trans. Nonferrous Metals Soc. China

    (2013)
  • J.D. Robson

    A new model for prediction of dispersoid precipitation in aluminium alloys containing zirconium and scandium

    Acta Mater.

    (2004)
  • Y. Deng et al.

    Influence of Mg content on quench sensitivity of Al-Zn-Mg-Cu aluminum alloys

    J. Alloys Compd.

    (2011)
  • M. Tiryakioğlu et al.

    On the quench sensitivity of 7010 aluminum alloy forgings in the overaged condition

    Mater. Sci. Eng. A

    (2014)
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